technical summariesspie.org/documents/conferencesexhibitions/pw11l...1-um region. this technology...

TEL: +1 360 676 3290 · +1 888 504 8171 · [email protected] 1Return to Contents

spie.org/pw

Technical Summaries

Connecting minds for global solutions

Conferences/Courses: 22-27 January 2011BiOS Exhibition: 22-23 January 2011Photonics West Exhibition: 25-27 January 2011

The Moscone CenterSan Francisco, California, USA

Contents7912: Solid State Lasers XX: Technology and Devices 2

7913: Laser Resonators and Beam Control XIII 23

7914: Fiber Lasers VIII: Technology, Systems, and Applications 33

7915: High Energy/Average Power Lasers and Intense Beam Applications VI 59

7916: High Power Lasers for Fusion Research 63

7917: Nonlinear Frequency Generation and Conversion: Materials, Devices, and Applications X 73

7918: High-Power Diode Laser Technology and Applications IX 91

7919: Vertical External Cavity Surface Emitting Lasers (VECSELs) 101

7920: Laser Applications in Microelectronic and Optoelectronic Manufacturing (LAMOM) XVI 110

7921: Laser-based Micro- and Nanopackaging and Assembly V 123

7922: Synthesis and Photonics of Nanoscale Materials VIII 130

7923: Free-Space Laser Communication Technologies XXIII 136

7924: Atmospheric and Oceanic Propagation of Electromagnetic Waves V 142

7925: Frontiers in Ultrafast Optics: Biomedical, Scientific, and Industrial Applications XI 147

SPIE Photonics West 2011 · spie.org/pw2 Return to Contents

Conference 7912: Solid State Lasers XX: Technology and DevicesSunday-Thursday 23-27 January 2011 • Part of Proceedings of SPIE Vol. 7912 Solid State Lasers XX: Technology and Devices

7912-01, Session 1

Optical and thermal design of a compact passively Q-switched laser system for the Mars Organic Molecule Analyzer (MOMA)C. Kolleck, A. Buettner, M. Ernst, T. Huelsenbusch, T. Lang, R. Marwah, M. Priehs, D. Kracht, J. Neumann, Laser Zentrum Hannover e.V. (Germany)

The Mars Organic Molecule Analyzer (MOMA) will be a laser-desorption mass spectrometer for the rover’s Pasteur Payload of the joint ESA/NASA mission searching for organic molecules on Mars. As excitation source, a lightweight q-switched solid-state laser system at a wavelength of 266nm with pulse energies above 250µJ at 1 Hz repetition rate is required. The operating temperature spans an interval of 75K. Near-flight prototype laser systems, which fit into the mass budget of <700g, have been developed. The passively q-switched oscillator is based on Nd:YAG and emits pulses with an energy of about 2mJ at a wavelength of 1064nm and a duration of ~2ns. Frequency conversion with KTP and subsequently with BBO results -depending on crystal configuration- in pulse energies of 300 to 500µJ at 266nm. A subsequent stage contains photo diodes for the monitoring of the green and the UV pulses and removes the remaining IR and green light from the output signal.

Aspects of the optical design and the optomechanics including alignment of optical elements are presented. During further design optimization, special emphasis was laid on the oscillator’s energy stability and its mode behaviour with varying temperature. A thermal design is developed and being tested in thermal vacuum tests. The design consists of three controlled heaters, which are placed on the oscillator, the frequency conversion stage, and the housing of the UV laser. The thermal concept enables energy variation by temperature tuning of the frequency conversion crystals with minimum thermal influence on the directly neighboring oscillator.

7912-02, Session 1

Wavelength and time-multiplexed multi-channel lidar transmitter for topographic mapping missionY. Chen, F. Kimpel, J. Fouron, S. Gupta, Fibertek, Inc. (United States); J. R. Chen, NASA Goddard Space Flight Ctr. (United States)

We demonstrate an innovative architecture for a nsec pulsed lidar transmitter, scalable to large channel counts, via wavelength- and time-multiplexing in a multi-stage Yb-fiber amplifier, operating in the 1-um region. This technology enables lidar systems with much greater spatial coverage and range resolution while substantially reducing the mass, size, power consumption, complexity, risk, and cost of the laser transmitter. This has the potential to benefit a number of NASA missions requiring detailed topographic mapping, including the National Lidar Mapping Initiative (NLMI), Lidar Surface Topography (LIST) mission, and planetary missions to icy moons (Europa, Enceladus) of outer planets.

For our specific demonstration hardware prototype, we have multiplexed three(3) wavelengths at 1060nm, 1061nm and 1064nm. The seed lasers are directly modulated with 50nsec pulses, with a 1.3nsec pulse carved out from a suitable time-window, via high-speed LiNbO3 EO-modulators. The pulse repetition rate is programmable, and is set at 333.3kHz, with equal time interleaved duration for the wavelengths. The combined repetition rate is 1MHz, and the corresponding duty cycle is 0.13%. The multiplexed pulses are seeded into a 3-stage Yb-fiber amplifier system, optimized for mitigating ASE and nonlinear

effects. Average power of >20W is achieved at 60% optical conversion efficiency, with OSNR>20dB. A special filter is used to demultiplex one of the wavelengths (1064nm), and frequency-doubled in single-pass configuration in a non-critically phase-matched LBO crystal, producing 1.2W at 532nm (35% conversion efficiency). High-speed FPGA based control provides programmable and independent control of the pulse rate, timing trigger, pulsewidth, and also the intra-pulse-pattern for improved detection schemes. We discuss various design trade-offs and the scaling potential to larger effective lidar channel counts.

7912-03, Session 1

Recent progress made in testing of space exposed laser componentsN. S. Prasad, NASA Langley Research Ctr. (United States)

The objective of the Materials International Space Station Experiment (MISSE) is to study the performance of novel materials when subjected to the synergistic effects of the harsh space environment elements such as cosmic radiation, atomic oxygen, UV radiation and temperature cycling in vacuum condition. Solid-state laser components including 808 nm laser diode, gain medium, and mirrors were among more than 400 specimens that were transported to the international space station (ISS) by STS-123 mission on March 11, 2008 for the MISSE 6 effort. The MISSE 6 experimental package was installed on the Columbus module via extra vehicular activity (EVA). It had two containers namely MISSE 6A and MISSE 6B. The laser components from NASA Langley Research Center (LaRC) were located on the MISSE 6B container and were exposed primarily to UV radiation. The MISSE 6 package was transported back to the Earth via STS-128 mission that was launched on August 28, 2009. Upon unpacking the MISSE 6 package at NASA LaRC, the laser components did not appear to have any visible damage. Performance tests of these elements to understand deviations from pre-exposure characteristics for durability and long term survivability are progressing. In this paper, the results of ongoing performance tests will be discussed.

7912-04, Session 2

Non-linear optical frequency conversion crystals for space applicationsA. Ciapponi, W. Riede, H. B. Schröder, P. Mahnke, Deutsches Zentrum für Luft- und Raumfahrt e.V. (Germany)

Accessing space with high-power laser systems is not a straightforward task as the space environment entails various risks for optical surfaces and even bulk materials. The DLR laser optics qualification laboratory has specialized on qualifying of optical coatings and nonlinear optical crystals on their suitability for space applications.

An important task is to optimize the frequency conversion efficiency during long term operation of second or third harmonic generation (SHG/THG) crystals, as these are necessary to access the desired wavelength range, e.g. for LIDAR applications. High conversion efficiency especially under ultra-high vacuum conditions is of uttermost importance due to the limitation of available electrical power on the satellite. As candidate crystals, borates (LBO, BBO and BiBO) and phosphates (KTP) are considered, under various combinations, with the goal to reach > 30% energy conversion efficiency from 1.06 µm to 355 nm.

In addition, the frequency converter crystals can also suffer from bulk absorption induced by high-energy radiation (so called gray tracking). To identify these effects, crystals were exposed to typical 3 year orbital doses of proton and gamma radiation and the resulting transmission losses were monitored.


7912-05, Session 2

Development of a low SWaP laser transmitter for atmospheric lidar applicationsN. S. Prasad, NASA Langley Research Ctr. (United States); A. Rosiewicz, S. M. Coleman, EM4, Inc. (United States)

NASA LaRC is working on advanced lidars operating in the 1.57 micron spectral band for CO2 sensing and 1.26 micron spectral band for oxygen sensing for Active Sensing of CO2 Emissions over Nights, Days, and Seasons (ASCENDS) mission application. Oxygen sensing is required for surface pressure measurements to determine CO2 mixing ratios with <0.3% accuracies. In this paper, we discuss recent progress made in the development of compact, single mode transmitter technologies for CO2 and O2 lidars. In particular, the development of advanced DFB laser diode module master oscillator that is efficiently coupled to integrated electronics and nano-cooling scheme in a single package for use in a CO2 lidar is presented. The current prototype device operates at 1.571 m with an output power of 40 mW. It has coarse and fine wavelength tuning capabilities using external 0-2 V adjustments. Coarse adjustment is provided by altering the chip operating temperature. Linear interpolation of the measured data indicates that the actual tuning rate is -140 MHz/mV input which corresponds to a wavelength tuning rate of 1.15 pm/mV. In units of temperature, the set point tunes at a rate of -11 mK/mV. Fine wavelength adjustment is accomplished by varying the drive current. Measured tuning rate is 0.2 pm/mV with a stability of 0.8 pm over 100 seconds. The close proximity of the laser and drive circuitry ensures tighter current control and lower noise. The overall volume of the current package is less than 2” x 2” x 0.5”.

7912-06, Session 2

High repetition rate pulsed 2-micron laser transmitter for CO2 measurementU. N. Singh, J. Yu, Y. Bai, M. Petros, NASA Langley Research Ctr. (United States); S. Chen, Science Systems and Applications, Inc. (United States)

Under the NASA Laser Risk Reduction Program (LRRP), funded by Earth Science Technology Office (ESTO), an efficient, injection seeded, high repetition rate Tm:fiber laser pumped Ho:YLF laser has been developed and operation between 100 Hz to 10 kHz has been demonstrated. The work is underway to develop an efficient, high-repetition-rate, pulsed, 2-micron, coherent Differential Absorption Lidar (DIAL) / Integrated Path Differential Absorption (IPDA) instrument for measuring the atmospheric CO2 profiles (DIAL) and column densities (IPDA) from an airborne platform. In DIAL mode, this instrument will provide the first ever range-resolved, high-precision, remote measurements of the CO2 content of the atmospheric boundary layer (ABL) and lower troposphere.

7912-07, Session 3

Efficient Yb-doped laser materials for high power applicationsC. Kränkel, K. Beil, K. Petermann, G. Huber, Univ. Hamburg (Germany)

We review Yb3+-doped crystals suitable for efficient high power laser operation. Novel materials exceed the standard host material Y3Al5O12 (YAG) in terms of laser efficiency, thermal conductivity, or emission bandwidth. For kW-power generation Yb:Lu3Al5O12 (Yb:LuAG) is a competing garnet crystal with improved thermal conductivity. HEM-grown sesquioxide crystals Yb:Lu2O3, Yb:Sc2O3, and the mixed crystal Yb:LuScO3, deliver up to 300 W of laser output power at nearly 75% optical-to-optical efficiency. At lower power levels, tuning ranges exceeding 100 nm have been demonstrated with Yb:sesquioxides. Modelocked thin-disk laser operation of these materials feature record

high average powers (141 W, Yb:Lu2O3) and very short pulse durations (227 fs, Yb:LuScO3).

7912-08, Session 3

From multi kW continuous wave to multi MW femtosecond pulses: recent developments exploiting disk laser technologyS. Weiler, M. Holzer, TRUMPF Laser- und Systemtechnik GmbH (Germany)

Disk lasers combine high efficiency, excellent beam quality, high average and/or peak power with moderate cost and high reliability at multiple wavelengths, ranging from the infrared over the green to the ultraviolet.

The demonstrated infrared average powers range from tens of kW in CW operation over >1 kW in ns pulses to >100 W in ps pulses and > 70 W in fs pulses.

Wavelength conversion for nearly all modes of operation, e.g. 700 W@515nm in ns pulses, enlarges the fields of applications, making the disk technology today’s most versatile laser platform.

7912-09, Session 3

50 W thin-disk laser with variable pulse durationM. A. Larionov, F. Dausinger, Dausinger + Giesen GmbH (Germany)

Since many years shortening of pulse duration down to femtosecond regime is propagated as a recipe for achieving higher accuracy in micro machining with lasers. The drawback of this approach is a loss in productivity which cannot be accepted when large work pieces have to be treated. A novel development tool based on disk technology will be presented allowing finding application specific optima for large scale micro machining. It has a power of 50 W, the pulse duration is variable between sub-picosends and microseconds at repetition rates up to 1 MHz.

The laser system is based on an Yb:YAG thin-disk regenerative amplifier, which is operated with different seed sources and in different operation modes in order to address broad spectrum of pulse durations. For sub-picosecond pulse duration a the dispersion of the regenerative amplifier output is compensated with a pair of diffraction gratings. Tuning of the distance between gratings allows for output pulse durations from 500 fs to several picoseconds. Using a second seed source allows for pulse durations up to several nanoseconds. Further the amplifier can be operated in cavity-dumped or in Q-switched mode just by changing of the electrical control of the Pockels cell in the amplifier. The pulse durations range is extended, correspondingly, to several 100 ns and 2 microseconds.

In all discussed operation modes the output beam is defined by the same amplifier cavity. Consequently, the beam parameters at the output are the same for all pulse durations.

7912-11, Session 3

Thermal and stress characterization of various thin disk laser configurations at room temperatureN. Vretenar, T. Carson, T. Lucas, T. Newell, P. Peterson, W. P. Latham, Air Force Research Lab. (United States)

Operational performance of kilowatt-class thin disk ceramic and single crystal Yb:YAG lasers is presented. High pump power is applied to various thin disk assemblies. The assemblies are composed of ASE caps,

Conference 7912: Solid State Lasers XX: Technology and Devices


200µm gain media, and heat sinks made of SiC, sapphire, or diamond.

FEA modeling of the assemblies is performed using COMSOL stress and thermal computations to understand and quantify thermal and stress effects on beam quality and laser output power. Under increased pump power, the thin disk can deform 5-10 µm in the center, destroying cavity stability. This is observed experimentally. Extensive thermal analysis includes an Amplified Spontaneous Emission (ASE) computation. ASE is numerically modeled using a Monte-Carlo ray tracing approach and ZEMAX modeling. Ultimately the cavity stability domain is calculated and compared to the experimental findings.

The results of this work indicate that a single thin disk laser could simultaneously produce high beam quality and power exceeding 10 kW.

7912-12, Session 4

High power thin disk Ho:YAG laserJ. Speiser, G. Renz, A. Giesen, Deutsches Zentrum für Luft- und Raumfahrt e.V. (Germany)

The thin-disk laser has already proven itself in the 1 µm wavelength range as a successful concept for achieving output powers of several kilowatts with high conversion efficiency and good beam quality. The use of different active media allows the exploration of new wavelengths and application areas.

Holmium-doped materials are a promising approach to obtain high power and/or high pulse energy with good beam quality with the thin-disk laser design in the 2 µm range. Ho:YAG is especially interesting for high pulse energies due to the long fluorescence lifetime (~ 8 ms) which provides good energy storage capabilities. Possible pump sources in this case are either laser diodes at 1.91 µm or Tm-fiber lasers, which results in little thermal heating due to the small quantum defect

In first experiments, we have realized a thin-disk laser based on Ho:YAG with a cw output power of 15 W at 2.09 µm and a maximum optical-to-optical conversion efficiency of 37%. The laser was pumped with a Tm-fiber laser. The output power was limited by the available pump power.

Further experiments to reach higher output powers as well as experiments on pulsed operation are in preparation. The experimental work will be complemented by numerical simulations.

7912-14, Session 4

Optical extraction model and optimal outcoupling for a CW quasi-three level thin disk laserD. A. Copeland, Aqwest, LLC (United States)

Using the well-known, but simplified, quasi-three level kinetics model of Beach [1] and Bourdet [2] an exact analytical solution of the coupled medium and geometric, plane-wave optical propagation equations for a longitudinally or face-pumped CW laser is obtained. Although the quasi-three level kinetics model ignores all medium losses, e.g. amplified spontaneous emission, up-conversion, and excited state absorption, it is applicable to Yb:YAG devices. The optical extraction model, which accounts for both laser wave amplification and pump wave absorption saturation coupling, treats both one- and two-face pumping as well as single-, double-, and multiple-reflections of the pump wave between the faces of the disk. Analytical expressions for the laser output power, the absorbed pump power, the threshold pump power, as well as the pump absorption, optical-to-optical, optical extraction, and slope efficiency are obtained. With a suitable and obvious modification of the pump absorption efficiency multi-pass pumping via pump beam re-injection as achieved with a parabolic reflector by Stewen et al [3] can also be treated. Explicit equations for determining the spatial distributions of the pump and laser intensities along the optic axis of the resonator are presented. Finally, explicit transcendental equations to determine the outcoupling which maximizes either the optical-to-optical or the optical extraction efficiency as a function of mirror loss and pump power are

derived. As an example the theory is applied to the Yb:YAG gain medium.

References:

(1) R. J. Beach, “Optimization of Quasi-Three Level End-Pumped Q- Switched Lasers,” IEEE Journal of Quantum Elec¬tronics, Vol. 31, No. 9, September 1995, 1606-1613.

(2) G. L. Bourdet, “Theoretical Investigation of Quasi-Three-Level Longitudinally Pumped Continuous Wave Lasers,” Applied Optics, Vol. 39, No. 6, 20 February 2000, pages 966-971.

(3) C. Stewen, K. Contag, M. Larionov, A. Giesen, and H. Hügel, “A 1-kW CW Thin disk Laser,” IEEE J. Selected Topics in Quantum Electronics, Vol. 6, No. 4, July/August, 2000, pages 650-657.

7912-15, Session 4

Fast wavelength switching of a single-frequency disk laser amplifierW. Paa, V. Wagner, T. Zeuner, M. Franke, IPHT Jena (Germany)

For special applications in spectroscopy, e.g. combustion diagnostics, tuneable single-frequency lasers are required to excite selectively relevant molecules. Amplified disk lasers based on Yb:YAG provide a unique opportunity for such lasers with a large range of advantageous properties (diode pumped, solid state, compact, tunable, controllable pulse duration, single-frequency operation, excellent beam profile M^2, efficient frequency conversion). Nevertheless, changing the wavelength from shot to shot at kHz repetition rates - desired e.g. for background subtraction or two-wavelength methods - remains challenging.

We present results from two approaches which - in combination - allow for fast wavelength switching of the oscillator and for extension of the tunability range of the system. For wavelength switching we apply a high voltage (typ. some kV) to a specially designed birefringent filter (Lyot filter). The resulting polarization rotation induced by the electric field yields losses at the wavelength emitted without voltage: the laser emits at the “new” wavelength with the highest gain. This new wavelength is shifted by one or more multiples of the free spectral range of the intra-cavity etalon used for ensuring the single-frequency regime.

Since the second stage of the laser system comprises an Yb:YAG regenerative amplifier, one also has to ensure that parasitic lasing of this laser at the gain maximum (known as ASE without a resonator) is suppressed effectively. This is done by inserting an additional birefringent filter into the amplifier resonator. Adjusting this filter leads to effective suppression of parasitic lasing and extends the tuneability range of the system by a factor of more than 4 (1016-1051 nm).

7912-16, Session 4

High power disk lasers: advances and applicationsD. L. Havrilla, TRUMPF Inc. (United States); M. Holzer, TRUMPF Laser- und Systemtechnik GmbH (Germany)

Though the genesis of the disk laser concept dates to the early 90’s, the disk laser continues to demonstrate the flexibility and the certain future of a breakthrough technology. On-going increases in power per disk, and improvements in beam quality and efficiency continue to validate the genius of the disk laser concept. As of today, the disk principle has not reached any fundamental limits regarding output power per disk or beam quality, and offers numerous advantages over other high power resonator concepts, especially over monolithic architectures.

Fast approaching 1000 high power disk lasers installations, the disk laser has proven to be a robust and reliable industrial tool. With advancements in running cost, investment cost and footprint, manufacturers continue to implement disk laser technology with more vigor than ever.

This paper will explain important details of the TruDisk laser series and process relevant features of the system, like pump diode arrangement, resonator design and integrated beam guidance. In addition, advances



in applications in the thick sheet area and very cost efficient high productivity applications like remote welding, remote cutting and cutting of thin sheets will be discussed.

7912-01, Session 5

Multi-watt orange light generation by intracavity frequency doubling in a dual-gain quantum dot semiconductor disk laserJ. Rautiainen, Tampere Univ. of Technology (Finland); I. L. Krestnikov, Innolume GmbH (Germany); J. Nikkinen, O. G. Okhotnikov, Tampere Univ. of Technology (Finland)

Various operation wavelengths can be achieved from semiconductor disk lasers based on quantum well material by band-gap engineering. The wavelength tailoring with this quantum-confined system, however, suffers from certain constrains related to critical growth conditions for specific spectral ranges. Quantum dot media allow expanding further the wavelength coverage of semiconductor disk lasers due to alleviated strain impact to the epitaxial structure. In this talk we present a frequency doubled dual-gain quantum-dot semiconductor disk laser producing 590 nm orange light. The multiple-gain concept is particularly relevant to quantum-dot systems whose single-pass gain is typically lower than the gain of quantum-well-based materials. This geometry can be applied to a number of gain elements and exhibit superior gain by consuming more pump power and sharing the thermal load among numerous active media, thus preventing excessive heating and rollover.

The structures grown by molecular beam epitaxy on a GaAs substrate comprise GaAs/AlAs distributed Bragg reflector and active region composed of 39 layers of InGaAs Stranski-Krastanov quantum dots designed for excited state emission at 1180 nm. The gain mirrors each assembled with intracavity diamond heat spreader produce individually 3 and 4 W of output power while the laser with both elements in a single cavity reveals 6 W at 1180 nm with beam quality factor M2<1.2. A BBO nonlinear crystal was employed for frequency doubling of the laser radiation. The loss induced by the nonlinear crystal is compensated by gain boosting in the dual-gain laser and 2.5 W of output power at 590 nm was achieved after frequency conversion.

7912-02, Session 5

1 W at 531 nm generated in a ppMgO:LN planar waveguide by means of frequency doubling of a DBR tapered diode laserD. Jedrzejczyk, R. Güther, K. Paschke, G. Erbert, Ferdinand-Braun-Institut (Germany)

Visible laser light sources with a moderate output power and spatially and spectrally single-mode operation are desired for many applications, such as spectroscopy or biotechnology. Frequency doubling of near-infrared (NIR) diode laser radiation in nonlinear crystals allows to realize such devices in a compact manner. One of the concepts to increase the conversion efficiency and the maximal output power at the same time is the application of a nonlinear planar waveguide.

In this work, we investigated experimentally second-harmonic generation (SHG) in a periodically poled 5 %mol MgO doped LiNbO3 (ppMgO:LN) planar waveguide. As a pump source a 6 mm long distributed Bragg reflector (DBR) tapered diode laser was applied. The DBR tapered laser emits nearly diffraction limited, spectrally single-mode CW radiation at 1063 nm and is therefore well suited for the SHG process. With the applied lens system in a bench-top experiment a coupling efficiency into the planar waveguide of 73 % was reached. A maximal SH power of 1.07 W was generated at an optical and electro-optical conversion efficiency of 26 % and 8.4 %, respectively. This is, to the best of our knowledge, the highest power level generated in a nonlinear waveguide by means of frequency doubling of diode laser radiation.

At the conference, we will present the experimental setup with the

corresponding optimal focusing parameters. Furthermore, the SH power dependence on the NIR power and on the crystal temperature, as well as the spectral characteristics of the SH radiation will be shown.

7912-03, Session 5

High-power (1 1W) green (532nm) laser source based on single-pass second harmonic generation on a compact micro-optical benchP. Q. Liu, Princeton Univ. (United States) and Ferdinand-Braun-Institut (Germany); C. Fiebig, M. Uebernickel, G. Blume, D. Feise, A. Sahm, D. Jedrzejczyk, K. Paschke, G. Erbert, Ferdinand-Braun-Institut (Germany)

Compact, high-power lasers emitting green light with good beam quality are indispensable for many applications such as high-brilliance display technology [1] and certain spectroscopy techniques. Second harmonic generation (SHG) of green light using a high-performance edge-emitting diode laser and a nonlinear bulk crystal in a single-pass configuration is a straightforward and effective way to generate green light with >1 W output power and good beam quality, and offers the possibility of compact system integration. Several groups have already demonstrated such systems using a macroscopic setup with distributed Bragg reflector (DBR) tapered diode laser and periodically poled MgO:LiNbO3 crystal [2,3].

Here, we demonstrate results on a highly compact single-pass SHG system for green light generation. The whole system is integrated on a compact micro-optical bench (MIOB), which has a dimension of 50 mm x 10 mm x 5 mm. As pump source we used a DBR tapered diode laser with >8 W maximum optical output power. Using a 2.5 cm long periodically poled MgO:LiNbO3 crystal we achieved an optical output power of 1.1 W at ~532 nm with ~7.6 W pumping power. The maximum achieved optical conversion efficiency is nearly 15%, the corresponding wall-plug efficiency is 4.3%. The green laser beam shows a relatively good beam quality (measured at 0.9 W of green light power) of M²=1.8 in vertical and M²=4.9 in lateral direction, respectively. The details of the system design as well as the properties of the devices will be given at the conference.

[1] G. Hollemann, B. Braun, P. Heist, J. Symanowski, U. Krause, J. Kraenert and C. Deter, “High-power laser projection displays,” Proc. SPIE 4294, 36 (2001).

[2] O.B. Jensen, P.E. Andersen, B. Sumpf, K.H. Hasler, G. Erbert and P.M. Petersen, “1.5W green light generation by single-pass second harmonic generation of a single-frequency tapered diode laser,” Optics Express, Vol. 17, Issue 8, pp. 6532-6539 (2009).

[3] A. Jechow, R. Menzel, K. Paschke and G. Erbert, “Blue-green light generation using high brilliance edge emitting diode lasers,” Lasers and Photonics Reviews, published online Dec. 2009.

7912-04, Session 5

Modulation and efficiency characteristics of miniature microchip green laser sources based on PPMgOLN nonlinear materialJ. Khaydarov, A. V. Shchegrov, S. Essaian, S. Slavov, Spectralus Corp. (United States); H. Danielyan, G. Gabrielyan, A. Poghosyan, S. Soghomonyan, Spectralus CJSC (Armenia)

We developed a highly efficient diode-pumped solid-state (DPSS) green laser source, based on a monolithic cavity microchip laser platform. The use of periodically poled MgO-doped Lithium Niobate (PPMgOLN) as the nonlinear frequency doubler together with gain material Nd3+:YVO4 allows obtaining a significant increase in the overall efficiency of the green microchip laser in comparison with other compact green laser



source architectures with comparable output power. We discuss our progress in miniaturization and efficient operation across a wide range of temperatures and application-specific modulation conditions. In particular, we demonstrate 50mW-180mW average green output power (30% duty cycle) with optical-to-optical efficiency from 808nm pump to 532nm output up to 27% and wall-plug efficiency over 12%. Efficient laser operation with duty cycle ranging from 10% to 100% (cw) in a wide range of repetition rates is also demonstrated. The laser is designed to be a part of the miniature and efficient RGB light source for microdisplay-based (LCOS, DLP or similar) mobile projector devices. While these projection architectures typically require modulation rates from 60Hz to about 2000Hz depending on design, we extended modulation speed up to 30kHz and higher that can be of interest for other applications. A very efficient and small microchip and alignment-free design allows to package this laser source into the very small volume of only 0.23cm3 (bounding box). We present results of performance tests for this packaged laser.

7912-17, Session 5

A compact optically pumped semiconductor laser emitting at 593 nmW. R. Seelert, Coherent Lubeck GmbH (Germany)

No abstract available

7912-05, Session 6

Fiber-laser-pumped CW OPO for red, green, blue laser generationY. Lin, Y. Huang, National Tsing Hua Univ. (Taiwan)

We report a CW, watt-level, red, green, and blue (RGB) laser pumped by a low-cost multi-longitudinal-mode Yb-fiber laser at 1064 nm. A bow-tie singly resonant optical parametric oscillator at 1560 nm contains two intracavity sum-frequency generators for red and blue laser generations. The red laser photon at 633 nm is summed from the pump and the near-IR signal photons. The blue laser photon at 450 nm is summed from the red and the signal photons. An extracavity second harmonic generator converts the residual pump power into green laser radiation at 532 nm. The OPO and RGB wavelength converters are Mg-doped PPLN crystals with lengths of 5, 1, 1, 0.5 cm, respectively. At 25-W pump power, the laser generated 3.9, 0.5, and 0.5 W at 633, 532, and 450 nm, respectively. The pump to visible laser efficiency is 20%. The multimode pump laser offers large temperature bandwidths for the RGB laser radiations, because different spectral components of the pump are used in different wavelength converters. The measured temperature bandwidths for the red, blue lasers are 11 and 4 deg., respectively. The measured rms output power variation is about 6% over ~10 min time.

7912-18, Session 6

Harmonic generation with fiber MOPAs and solid state lasers: technical challenges, state-of-the-art comparison, and future developmentsA. N. Starodoumov, N. Hodgson, Coherent, Inc. (United States)

Fiber MOPAs in the infrared wavelength region offer the advantage of high single mode output powers, independent selection of pulse repetition rates and pulse durations, and access to high repetition rates. Despite these performance advantages, most industrial and scientific applications in the visible and the ultraviolet spectral range are still dominated by solid state lasers. We will give an overview of the technical challenges of harmonic generation in fiber lasers/amplifiers and discuss the state-of-the-art and future of Fiber MOPAs and bulk solid state lasers with harmonic generation.

7912-19, Session 6

Raman lasers for yellow-orange spectrum coverageN. Landru, J. Rouvillain, G. Lebail, T. Georges, Oxxius SA (France)

Diode lasers have been demonstrated to operate over a great part of the visible spectrum: InGaN diodes cover the violet-blue-green part (<530nm) and InGaAlP diodes cover the red part (>635nm). Some fluorophorous in biotechnology applications are excited by intermediate wavelengths, from 540 to 630nm. Optically pumped InGaAs lasers were demonstrated from 460nm up to 580nm. Standard frequency doubled diode pumped solid state (DPSS) lasers lack of suitable transition to cover the 565-650nm region. It is possible to modify the semiconductor composition to extend the frequency range or to frequency mix DPSS laser wavelengths, but it comes with a significant R&D effort or a complexity in the design.

Raman scattering can red-shift the strong transitions of Nd or Yb lasers so that many wavelengths lying in the 1080-1300nm range can be achieved. Recently several CW didoe pumped Raman lasers were demonstrated, some of them including intra-cavity frequency doubling or mixing. The problems with these Raman lasers are the high pump threshold and high noise. We have built several Raman lasers with a reduced loss presenting a low pump threshold (<1W) and high slope efficiency. Output powers in excess of 100mW were achieved at 588nm with a 2.5W 808nm pump. Laser emissions from 543nm up to 610nm were demonstrated. Noise of these lasers was analyzed and means to reach low noise operation will be discussed at the conference.

7912-20, Session 6

575 nm laser oscillation in Dy3+-doped waterproof fluoro-aluminate glass fiber pumped by violet GaN laser diodesY. Fujimoto, Osaka Univ. (Japan); O. Ishii, M. Yamazaki, Sumita Optical Glass, Inc. (Japan)

Visible lasers are widely applicable to medicine, biology, metrology, optical storage, and display technology, and especially a yellow laser has potential applications in biomedicine, ophthalmology, and medical treatment for acne melasma and facial telangiectasia. Yellow lasers have been developed with several techniques, such as copper bromide laser, and upconversion, or second harmonic generation in Nd-doped crystals, Yb-doped fiber, Bi-fiber laser, optically pumped semiconductor lasers.

We have successfully drawn a low-loss Dy-doped fluoro-aluminate glass fiber. Fluoro-aluminate glass including 10,000 ppm of Dy concentration was used as a fiber core. The core and clad diameters of the drawn fiber were 8 and 300 µm, respectively. A 4-cm-long Dy-doped fiber was inserted into a zirconia-ferrule, and then both sides of the fiber surfaces were polished. Yellow laser oscillation was demonstrated in the Dy3+-doped fluoride fiber pumped by a 398.8-nm GaN-LD at 575 nm with 17.1% of the slope efficiency. The maximum output power was 10.3 mW, the threshold power was 10.2 mW, and the slope efficiency was calculated to be 17.1%.

Because the violet GaN-LD is commonly used as a pick up for Blu-ray Disc devices and the low-cost multi-hundred milliwatt LD is already available, it is expected that low-cost and compact yellow laser devices can be produced. Since the fluoro-aluminate-glass system has a remarkable water resistance advantage compared to ZBLAN glass, this type of Dy-doped fluoride glass fiber can greatly contribute to a yellow fiber laser with high chemical durability without a frequency doubling technique.



7912-21, Session 6

Efficient frequency conversion of pulsed microchip and fiber laser radiation in PPSLTB. Jungbluth, S. Nyga, E. Pawlowski, T. Fink, Fraunhofer-Institut für Lasertechnik (Germany)

Extensive studies on frequency doubling with ppSLT crystals are presented. This includes a detailed discussion on design aspects and theoretical modeling predictions as well as experimental studies comparing the performance of ppSLT crystals from different providers with and without MgO doping. Experimental analyses of their acceptance parameters and crystal homogeneity are conducted with a pulsed microchip laser with low peak (6 kW) and low average power (50 mW) resulting in a maximum conversion efficiency of up to 80 % for high quality MgO doped crystals. Based on these results a compact converter module with fiber coupling is designed and tested with the radiation from the microchip laser and a fiber laser source in comparison. The fiber laser provides an average power of about 1 W. Even at this - still very moderate - power level a significant efficiency drop can be observed. Despite the advantage of higher pulse peak (25 kW) power from the fiber laser source, careful design adaptations of the converter are required even to preserve a conversion efficiency beyond 50%.

7912-22, Session 7

Yb-doped ultrafast thin disk lasersT. Südmeyer, C. Bär, ETH Zurich (Switzerland); C. Kränkel, ETH Zurich (Switzerland) and Univ. Hamburg (Germany); C. J. Saraceno, O. H. Heckl, M. C. Golling, ETH Zurich (Switzerland); R. Peters, K. Petermann, G. Huber, Univ. Hamburg (Germany); U. Keller, ETH Zurich (Switzerland)

Since its first demonstration in the year 2000, SESAM modelocked thin disk lasers achieved higher pulse energies and average power levels than any other modelocked oscillators. Stable and self-starting passive pulse formation is achieved with SESAMs and soliton modelocking . The key components of ultrafast thin disk lasers are used in reflection, which is an advantage for the generation of ultrashort pulses with excellent temporal, spectral and spatial properties, because the pulses are not affected by excessive nonlinearities inside the oscillator. In this presentation, we review the development of ultrafast thin disk lasers with high average power levels and report latest results. Particularly attractive for power-scaling is the gain material Yb:Lu2O3, because it has a better thermal conductivity and a broader emission bandwidth compared to the well established thin disk gain material Yb:YAG.. We report on successful scaling of a SESAM mode-locked Yb:Lu2O3 thin disk laser to an average power of 141 W setting a new record for mode-locked laser oscillators. The laser generates 738 fs pulses with an optical-to-optical efficiency of 40.1% in a diffraction limited beam (M2 < 1.2). The pulses have an energy of 2.35 µJ and a peak power of 2.8 MW. At an average power of 108 W we observed the highest optical-to-optical efficiency of 43.8% so far obtained from a passively mode-locked thin disk laser. No amplification stages are required, which make such lasers interesting for a growing number of industrial and scientific applications such as material processing or driving experiments in high field science.

7912-23, Session 7

Highly flexible ultrafast laser system with 220W average powerT. G. Mans, J. Dolkemeyer, AMPHOS GmbH (Germany); P. Russbueldt, Fraunhofer-Institut für Lasertechnik (Germany); C. Schnitzler, AMPHOS GmbH (Germany)

With the Innoslab amplifier concept sub-picosecond pulsed radiation with more than 600W from a single and more than 1.1kW average power

from two coupled amplifier stages has been demonstrated. We present a compact, integrated InnoSlab system with output power of more than 200W and flexible pulse width and repetition rate. The amplifier does not require any changes to support the varied time structure of the seed pulses.

The system consists of a commercially available fiber-based cpa laser on the watt level as seed laser and a downscaled InnoSlab amplifier stage. The output pulse duration of the system is varied by an order of magnitude from 530fs to 5,4ps by changing seed pulse duration. The bandwidth of these pulses is close to the transform limit of the shortest possible pulses (<3nm) and arises from the amplification bandwidth of the employed Yb:YAG crystal. The small bandwidth ensures the usability of conventional mirrors and lenses for applications in materials processing and the spectrum does not significantly alter with pulse duration or output power.

Pulse repetition rate can be varied from a maximum of 26.6MHz down to 1MHz and an AOM after the oscillator can be used to gate flexible pulse trains at these repetition rates.

Pulse energy exceeding 20µJ does not show signs of self phase modulation and slope efficiency of the amplifier is observed to be 58%. Beam quality in the two axes are with Mx2=1.1 and My2=1.3 close to the diffraction limit and astigmatism is removed by using a telescope system.

7912-24, Session 7

Picosecond laser with 400W average power and 1mJ pulse energyK. Du, D. Li, EdgeWave GmbH (Germany)

To scale the power of picosecond laser an oscillator and amplifier system has been developed. The amplifier consists of preamplifier and end amplifier. Both amplifiers are based on INNOSLAB amplifier. With the oscillator/amplifier system higher than 400W average power and max pulse energy of 1mJ was obtained. In this paper the design and the results will be presented and discussed.

7912-25, Session 7

250W single stage Nd:YVO4 picosecond INNOSLAB MOPAM. Höfer, C. Prause, H. Sipma, Fraunhofer-Institut für Lasertechnik (Germany); S. Naumov, R. Knappe, LUMERA LASER GmbH (Germany); H. Hoffmann, Fraunhofer-Institut für Lasertechnik (Germany)

High-end micromachining with picosecond lasers became an established process during the recent years. Power scaling led to industrial lasers, generating average power levels well above 50 W. Such lasers are routinely used in industrial applications and offer a new quality in laser micro-machining, nearly without any thermal side-effects like burrs and micro-cracks.

We present further power scaling, achieved by combining state-of-the-art industrial picosecond laser sources from LUMERA LASER with Fraunhofer ILT INNOSLAB amplifier technology. Pulses with 10 ps-duration and a wavelength of 1064 nm were generated by two different LUMERA lasers and further amplified in a compact single stage Nd:YVO4 INNOSLAB amplifier.

With an input power of 2.6 W from a RAPID MOPA system, the amplifier generated an output power of > 200 W at 1 MHz PRF, corresponding to a pulse energy of 200 µJ. At 500 kHz a pulse energy of 360 µJ was measured.

With an input power of 3.6 W from a long-cavity oscillator, which was mode-locked at a PRF of 10.7 MHz, the average output power of the amplifier was as high as 250 W, corresponding to a pulse energy of 23 µJ. The optical-to-optical (diode to output) efficiency of the amplifier was 33%. This system is well suited as compact high-power ps-Laser for



micro structuring of large surface areas.

Good beam quality with an M² well below 1.5 was achieved within the whole parameter range.

7912-26, Session 7

Flexible and modular picosecond lasers for industrial cold-ablation micromachiningK. J. Weingarten, Time-Bandwidth Products AG (Switzerland); F. Brunner, Time-Bandwidth Products, Inc. (United States); I. Klimov, A. Bardorf, M. Benetti, Time-Bandwidth Products AG (Switzerland)

Picosecond pulsed lasers with high pulse energy (>10 µJ) and high average power (>10 W) provide reliable cold-ablation microprocessing in many different material systems. These laser sources open new process window regimes which cannot be addressed by typical nanosecond Q-switched lasers for applications in precision hole drilling, micromachining, thin-film scribing, and more. However, achieving optimized performance requires a laser system with appropriate flexibility.

The paper describes recent developments in flexible laser system performance of industrial picosecond lasers and the choices required to achieve optimum performance for the invested equipment cost. Often the first key choice a user must make is the maximum average power of the system, which is driven by the desired process speed but also requires high-speed scanning, careful process development, and increases the cost of the laser system. Wavelength selection, typically either 1 µm, 532 nm, or 355 nm. depends on the process and materials under use. The process speed can then be optimized by determining the minimum pulse energy required for cold ablation, then maximizing the repetition rate of the laser to achieve maximum ablation and process speed. This requires systems with “true” repetition rate flexibility, e.g. the ability to operate with maximum average output power over its entire repetition rate range.

Flexible pulse bursts are an additional feature that can improve the process performance. We present a new approach which allows for the generation of bursts with arbitrary user-defined pulse energy distribution on a nanosecond time scale.

7912-27, Session 8

Yb-doped ultrafast solid state lasersF. Druon, P. Georges, Institut d’Optique Graduate School (France)


7912-28, Session 8

1100W Yb:YAG fs INNOSLAB amplifierP. Russbueldt, Fraunhofer-Institut für Lasertechnik (Germany); T. G. Mans, RWTH Aachen (Germany); H. Hoffmann, R. Poprawe, Fraunhofer-Institut für Lasertechnik (Germany)

Today, ultra-short laser pulses have found widespread applications in micromachining, metrology and physics. Reducing processing or measurement time requires increasing repetition rate and average power. To transfer femtosecond technology to industry, laser sources of high average power are essential.

Mainly due to the ongoing improvements of laser diodes new solid state laser designs like thin-disk, fiber and partially pumped slab lasers (Innoslab) established beside rod lasers. These technologies pave the way to ultrafast lasers of much higher average power than the familiar Ti:sapphire lasers. Innoslab and fiber lasers already demonstrated 1100W resp. 830W average power at 700fs pulse duration. In terms of scalability of average power, beam quality and efficiency, fiber, thin-disk and Innoslab are comparable. But for short pulse MOPA systems

all technologies have distinct strong and weak points. The gain of fiber amplifiers is orders of magnitudes higher compared to thin-disks, but the opposite holds for damage and nonlinearity. At high gain, intermediate power and high repetition rate, fiber amplifiers and at very high average and peak power but low gain thin-disk amplifiers cannot be surpassed. Innoslab amplifiers rank between these two extremes.

The specific properties of Innoslab MOPAs are compared with fibers and thin-disks. The scaling in terms of average and peak power as well as repetition rate, pulse duration and spectral bandwidth is discussed. A simple and compact single stage MOPA with 620W and a dual stage MOPA with 1.1kW average power at 650fs pulse duration 20MHz repetition rate and almost diffraction limited beam quality is presented.

7912-29, Session 8

Short pulse and high repetition rate diode-pumped Yb:CaF2 regenerative amplifierS. Ricaud, Amplitude Systemes (France) and Lab. Charles Fabry (France); F. Druon, Institut d’Optique Graduate School (France); D. N. Papadopoulos, Lab. Charles Fabry (France); P. Camy, J. Doualan, R. Moncorgé, ENSICAEN (France); M. Delaigue, A. Courjaud, Y. Zaouter, Amplitude Systemes (France); P. Georges, Institut d’Optique Graduate School (France); E. Mottay, Amplitude Systemes (France)

Many industrial and scientific applications need ultra-short and energetic pulses. Diode-pumped systems based on ytterbium-doped crystals have a huge interest thanks to their good thermal and spectroscopic properties. Among them, Yb:CaF2, shows very promising results for short pulse generation, and its long fluorescence lifetime, 2.4 ms, indicates a high energy storage capacity.

We present a diode-pumped regenerative amplifier based on an Yb:CaF2 crystal optimized to produce short pulses for various repetition rates ranging from 100 Hz to 10 kHz. The experiment is performed with a 2.6-% Yb doped 5-mm-long CaF2 crystal grown by using the Bridgman technique and used at Brewster angle. To optimize the injection pulse spectrum in terms of bandwidth and maximum gain, the seed pulses are generated by a broadband Yb:CALGO oscillator centered at 1043 nm with a FWHM bandwidth of 15 nm at a repetition rate of 27 MHz. The pulses are then stretched to 260 ps with a transmission grating. The shortest pulse duration generated is 178-fs, and the corresponding energy is 1.4 mJ before compression (620 µJ after), at a repetition rate of 500 Hz for 16 W of pump power. The bandwidth is 10 nm centered at 1040 nm. At 10 kHz repetition rate, 1.4 W of average power before compression is obtained, corresponding to an optical-optical efficiency of 10%. We also noticed that the pulse duration tends to increase above 1 kHz, reaching 400 fs at 10 kHz

7912-30, Session 8

Steady state mode-locking of the Nd:YVO4 laser operating on the 1 34 µm transition using intracavity SHG in BIBO or PPMgSLTH. L. Iliev, I. Buchvarov, V. Alexandrov, Sofia Univ. (Bulgaria); S. Kurimura, National Institute for Materials Science (Japan); V. P. Petrov, Max-Born-Institut für Nichtlineare Optik und Kurzzeitspektroskopie (Germany)

Passive mode-locking of Nd-lasers operating on the 4F3/2 4I13/2 transition is problematic for semiconductor saturable absorber mirrors (SESAMs) not only because of difficulties in their fabrication process but also in relation to the achievable parameters and damage resistivity. We investigate an alternative approach based on second-order nonlinearity inside the laser cavity which utilizes negative (2)-lens formation in a SHG crystal assisted by nonlinear reflection of the so-called “frequency-doubling nonlinear-mirror” (FDNLM). This approach has been previously



employed only for mode-locking of Nd-lasers emitting at 1.06 µm. Here we demonstrate passive mode-locking of a diode-pumped Nd:YVO4 laser operating at 1342 nm based on negative (2)-lensing assisted by the FDNLM effect. Using a 7.5-mm-long BiB3O6 (BIBO) nonlinear crystal or 10-mm-long and 1-mm-thick periodically-poled Mg-doped stoichiometric lithium tantalate (PPMgSLT) crystal and output couplers highly-reflecting at the second-harmonic with optimized transmission at the fundamental, we achieve average output powers in the steady-state mode-locked regime of the order of 1 W at pulse durations in the 4-7 ps range. Such powers already exceed the best results achieved with SESAMs while the pulse duration is substantially shorter than in SESAM mode-locked Nd-lasers operating on this transition. Higher average powers have been obtained for this laser transition only by the complex additive mode-locking technique. In our case the average power limit is set by the maximum power achievable in the fundamental transversal mode in the continuous-wave (cw) regime. The shortest pulses (FWHM of 3.7 ps) can be very well fitted by sech2 temporal shape assumption.

7912-31, Session 9

A Joule-class, TEM00 spatial profile, narrow-linewidth laser systemA. Vaupel, N. Bodnar, M. Hemmer, M. C. Richardson, CREOL, The College of Optics and Photonics, Univ. of Central Florida (United States)

We report on a Joule-class, narrow-linewidth amplifier line. A flashlamp pumped Q-switched oscillator featuring a Nd:YAG gain medium (6x65 mm rod) provides 21 ns duration pulses with 2.1 mJ of energy. The spectral linewidth of the output is narrowed to 22 pm by an intra-cavity VBG with a narrow reflectivity bandwidth centered at 1064 nm.

The output pulses are amplified to 110 mJ in a Nd:YAG (6.35x105 mm rod) collinear double-pass amplifier. A Nd:YAG (10x105 mm rod) single-pass amplifier further amplifies the pulses to 326 mJ. The final amplifier consists of another single pass, Nd:YAG (10x105 mm rod) yielding 480 mJ energy at 1 Hz repetition rate. In this geometry, a TEM00 beam profile, a high signal-to-amplified spontaneous emission (ASE) ratio and a narrow linewidth are maintained. The maximum output energy was scaled to 0.99 J but at the expense of a 368 mJ ASE pre-pulse. Further energy-scaling is investigated to increase the pulse energy to 10 J at 0.1 Hz repetition rate with a 2.5 cm diameter rod.

This system will be used for several studies. The narrow linewidth at 1064 nm, its second (532 nm) and third harmonic (355 nm) is suitable for remote Raman spectroscopy and the high energy enables filamentation with ns pulses. The amplifier line will be also used as extension for the pump beam generation line of the HERACLES OPCPA facility to produce 200 mJ-level picosecond pulses to pump an OPA, potentially leading to few-cycle pulses with 20 mJ pulse energy.

7912-32, Session 9

UV power scaling of INNOSLAB lasersK. Du, D. Li, S. Fu, EdgeWave GmbH (Germany)

Because of their high absorptions UV lasers are widely used for precision machining in LED, and micro electronic production. To increase productivity high average power of UV lasers are required. Lasers with pulse energy and short pulse are favourable for UV sources through third and forth harmonic generation. Using an INNOSLAB laser more than 50W at 355nm and more than 30W at 266nm have been obtained. In this paper the experimental results will be presented and discussed.

7912-33, Session 9

All passive synchronization of a quasi-three-level Q-switched laser and a four-level Q-switched laserH. P. H. Cheng, P. Tidemand-Lichtenberg, O. B. Jensen, P. E. Andersen, P. M. Petersen, C. Pedersen, Technical Univ. of Denmark (Denmark)

Synchronization of Q-switched lasers has been an active field of research in recent years, including both actively and passively Q-switched systems [1-3]. Synchronization between quasi-three-level and four-level lasers is particularly interesting for sum-frequency generation into the blue and ultraviolet spectral region [2]. This is, however, complicated by reabsorption loss and a significantly reduced gain cross-section in the quasi-three-level laser line. We report, for the first time, a quasi-three-level Q-switched laser synchronized to a four-level Q-switched laser in an all passive approach.

We use two separately pumped Nd:YAG laser crystals to equalize the repetition rate at 946 nm and 1064 nm respectively, and a single Cr:YAG saturable absorber (SA), placed in a common section of the two cavities, to synchronize the two pulse trains. Synchronization is obtained by detailed control of the pump power for each laser, the mode overlap in the SA, and the length ratio of the two resonators [3].

While timing jitter of the 946 nm and 1064 nm lasers were 10 and 8 µs respectively for the free running lasers, the relative jitter was only 50 ns in the synchronized system, comparable to an actively synchronized system [2]. A delay of 240 ns between pulse peaks was observed, but can be reduced further by optimizing the cavity lengths and mode overlap inside the SA. The pulse widths of the lasers were 80 ns and 41 ns, respectively.

The results obtained show potential for non-linear frequency conversion, and could lead to high power pulsed blue and ultraviolet lasers.

[1] R. W. Farley and P. D. Dao, “Development of an intracavity-summed multiple-wavelength Nd:YAG laser for a rugged, solid-state sodium lidar system”, Appl. Optics, 34, 4269, 1995.

[2] Herault E, Lelek M, Balembois F and Georges P, “Pulsed blue laser at 491 nm by Nonlinear Cavity Dumping,” Opt. Express, 16, 19419, 2008.

[3] Tidemand-Lichtenberg P, Janousek J, Melich R, Mortensen JL, Buchhave P, “Synchronization of 1064 and 1342 nm pulses using passive saturable absorbers,” Opt. Commun., 241, 487, 2004.

7912-34, Session 9

Optically triggered Cr:YAG Q-switched Nd:YAG laserB. J. Cole, A. D. Hays, J. Lei, B. W. Schilling, L. Goldberg, U.S. Army RDECOM CERDEC NVESD (United States)

The passively Q-switched (PQS) Nd:YAG laser has many desirable qualities that include simplicity in laser design, compactness, low cost and weight. However, for certain applications the high pulse-to-pulse timing jitter associated with passive Q-switching makes the laser unusable. To address this problem, we have previously described a pulse timing jitter reduction technique (Photonics West 2010) via direct bleaching of the Cr:YAG saturable absorber. A 1cm wide single laser diode bar was used to bleach a thin sheet within the saturable absorber from a direction orthogonal to the lasing axis. More than an order of magnitude reduction in timing jitter was achieved, and a timing jitter of tens of nanoseconds was demonstrated.

In our current work, we have further refined this technique to enable its practical implementation. It was found that a fluence of 70 kW/cm2, required to enable the optical triggering effect, could be reached using a single 3 mm wide mini-laser diode bar mounted on a compact heatsink, with an attached cylindrical lens to collimate the emission. The diode bar width matched the length of the Cr:YAG Q-switch crystal, allowing simple close coupling of its emission into the crystal. In order to miniaturize the triggering setup, a compact 300 A pulse driver, with a <0.5 microsecond



rise-time and 3-5 microsecond duration was developed for pulsing the 3 mm diode bar. These components have been combined to demonstrate a compact brassboard implementation of the optically triggered passively Q-switched laser. This paper will highlight the recent progress we have made towards a packaged optically triggered PQS laser.

7912-35, Session 9

Q-switched fiber lasersS. Christensen, Nufern (United States)


7912-36, Session 9

High power 808 nm VCSEL arrays for pumping of compact pulsed high energy Nd:YAG lasers operating at 946 nm and 1064 nm for blue and UV light generationR. Van Leeuwen, Y. Xiong, L. S. Watkins, J. Seurin, G. Xu, Q. Wang, C. L. Ghosh, Princeton Optronics, Inc. (United States)

High power 808 nm VCSEL arrays were developed to pump compact pulsed Nd:YAG lasers. A QCW side-pumped passively q-switched Nd:YAG laser operating at 1064 nm produced linearly polarized 4 ns IR pulses with 4.7 mJ pulse energy. These pulses were externally frequency doubled and quadrupled resulting in 2.5 mJ pulse energy at 532 nm and 0.8 mJ at 266 nm respectively. A similar but actively q-switched side-pumped Nd:YAG laser operating at the weaker quasi three-level 946 nm transition produced 12 mJ IR in a 27 ns pulse. This output was frequency doubled to produce 473 nm blue laser light.

7912-30, Session 10

High-pulse energy operation of efficient and compactly packaged, ns-pulse Yb-doped photonic-crystal fiber-based lasers delivering high-spectral and spatial brightnessF. Di Teodoro, Northrop Grumman Aerospace Systems (United States)

Pulse fiber lasers (PFLs) are naturally amenable to field deployment owing to favorable size and weight, high efficiency, and simple thermal management. Large-core photonic crystal fibers (PCFs) have also enabled high pulse energy, peak power, beam quality (BQ), and spectral brightness (SB) suitable for air-/space-based nanosecond-pulse remote sensors. However, packaged PFLs typically exhibit lower power and/or SB compared to laboratory demonstrations.

Here, we describe two PCF-based, compactly packaged, nanosecond-pulse master-oscillator/power-amplifier (MOPA) architectures achieving high pulse power, SB, and BQ.

In the first, a pulse-programmable diode laser drives a multistage Yb-doped fiber amplifier terminated by a 100um-core, rod-type PCF. For compact packaging, the PCF is divided into four ~36cm-long segments laid out to ensure negligible efficiency penalty compared to a whole PCF. The MOPA outputs ~1064nm-wavelength, linearly polarized, ~1.5ns pulses of energy/peak power > 2mJ/1.5MW at 10kHz pulse repetition frequency (PRF), good BQ (M2 ~ 1.2), and high SB (>85% pulse energy into a 0.06nm window). The laser system is enclosed within a < 15-liter volume package designed to withstand field-level shock, vibration, and thermal excursions.

In the second architecture, four time-synchronizable MOPA systems of layout, BQ, and SB similar to that described above are packaged within a 48×35×18cm enclosure (~30-liter volume). Each MOPA generates < 2ns

pulses of ~50W average power at 50kHz PRF. Within the same enclosure, an integrated optical bench enables high-efficiency dense wavelength multiplexing (DWM) of the MOPA output beams. To our knowledge, this setup provides the smallest footprint for high-power fiber DWM reported to date.

7912-36, Session 10

All-fiber based amplification of 40 ps pulses from a gain-switched laser diodeS. Kanzelmeyer, H. Sayinc, T. Theeg, M. Frede, J. Neumann, D. Kracht, Laser Zentrum Hannover e.V. (Germany)

Amplification of a gain-switched laser diode is demonstrated in an all-fiber based setup. Due to the low output pulse energy of gain-switched laser diodes in the range of about 20 pJ, amplification with repetition rates below a few MHz requires the consideration of amplified spontaneous emission (ASE), generated in a fiber amplifier. A high amount of ASE in a pulsed laser system leads to an inefficient amplification and a power background noise, which can result in self lasing of the amplifier. Additionally, reliable information on the pulse energy can not be extracted from the average output power and the repetition rate, if the amount of ASE is unknown. As the contribution of ASE can not be distinguished from the pulse train in the spectral domain, the fraction of ASE was investigated in the temporal domain by using an acousto-optic modulator. A maximum pulse energy of 13 µJ at a repetition rate of 1 MHz and a pulse duration of 40 ps was extracted, corresponding to a peak power of 270 kW. To the best of our knowledge, this is the highest extracted pulse energy from a laser system seeded by a gain-switched laser diode. Temporal pulse deformation due to intrapulse Raman scattering was observed, which resulted in a temporal power shift to the leading edge of the pulses.

7912-37, Session 10

High-average power Nd:YVO4 regenerative amplifier seeded by a gain switched diode laserM. Lührmann, F. Harth, C. Theobald, T. Ulm, Photonik-Zentrum Kaiserslautern e.V. (Germany); R. Knappe, A. Nebel, LUMERA LASER GmbH (Germany); A. Klehr, G. Erbert, Ferdinand-Braun-Institut (Germany); J. A. L’huillier, Photonik-Zentrum Kaiserslautern e.V. (Germany)

We report on a Nd:YVO4 regenerative amplifier, end pumped by 888 nm diode lasers. The output power was about 46 W at repetition rates from 150 to 833 kHz with an M2-factor of 1.2. The amplifier was seeded by a gain switched diode laser, generating pulses with a duration of 65 ps and a pulse energy of ~ 5 pJ. The high gain of the regenerative amplifier of ~ 70 dB provides amplified pulse energies as high as 150 µJ. Bifurcations of the pulse energy could be avoided without a preamplifier despite the low seed energy. Pulse amplitude fluctuations of only 1.2 % for 10,000 succeeding pulses were measured. The long term output power stability of the laboratory setup was 0.3 %. Determined by the seed diode the pulse duration after amplification was 70 ps.

The laser combines the advantages of a small and efficient diode seed source with a reliable solid state regenerative amplifier, forming a compact, robust and powerful system. The gain switched seed diode delivers pulses on demand, rendering a pulse-picker unnecessary. It is also insensitive against feedback, reducing requirements on isolation between seed diode and amplifier.

Generation of ultra-short pulses with high repetition rate and high average power make this laser an ideal source for applications in non-linear optics and high-quality material processing.



7912-37, Session 10

Sub-10 picosecond pulses from a fiber-amplified and optically compressed passively Q-switched microchip laserA. Steinmetz, D. Nodop, A. Martin, J. Limpert, A. Tünnermann, Friedrich-Schiller-Univ. Jena (Germany)

We present an experimentally confirmed approach based on nonlinear optical compression of passively Q-switched pulses accessing sub-10 ps domain, which is so far dominated by mode-locked systems. The concept implements the SPM-induced spectral-broadening of Q-switched pulses in optical waveguides and a supplementary compression with bulk optics e.g. a pair of diffraction gratings or a chirped-bragg-grating. The used seed-source is a fiber-amplified, passively Q-switched microchip laser operating on a single longitudinal mode and consists of a monolithically bonded combination of Nd:YVO4-crystal and semiconductor saturable absorber mirror. The microchip laser provides pulses with a duration of 100-150 ps, a pulse energy of 210 nJ at 332 kHz and a line width of ~20 pm at wavelength of 1064.2 nm.

The seed pulses are amplified in an ytterbium-doped photonic crystal fiber to a pulse energy of 3.15 µJ and spectrally broadened to 50 pm. For an additional increase of the spectral bandwidth, a passive fiber with 10 µm core diameter and 3 m length is used leading to 0.53 nm bandwidth. It should be mentioned, that a clean SPM broadened spectrum is possible by applying pulses as short as a few 100 ps, longer, i.e. nanosecond pulses, typically lead to a Raman continuum. As a first proof-of-principle experiment the broadened signal is sent through a conventional 1740 l/mm diffraction-gratings based pulse compressor. Best compression has been found at a grating separation of 11 cm resulting in pulse duration of smaller than 6.5 ps assuming a numerically calculated de-convolution factor of 0.735.

7912-38, Session 10

High-power diode pumped crystal fiber amplifier for passively Q-switched Nd:YAG microlaserI. Martial, Lab. Charles Fabry (France) and FiberCryst (France); F. Balembois, Lab. Charles Fabry (France); J. Didierjean, FiberCryst (France); P. Georges, Lab. Charles Fabry (France)

The Master Oscillator Power Amplifier (MOPA) configuration is very useful to extend laser performance of passively Q-switched Nd:doped microlasers in view of material processing applications. Different configurations and gain media have already been used in the past few years: multipass bulk amplifiers in longitudinal pumping, fiber amplifiers and recently side-pumped crystals operating at grazing incidence.

In this work, we present the use of a Nd:YAG crystal fiber for the amplification. The geometry of this gain medium (diameter 1 mm length 50 mm) can provide the good thermal management and high gain like in fibers and may sustain high peak power pulses like in the bulk materials.

As a seed, we used a passively Q-switched Nd:YAG microchip laser (Teem Photonics, France), generating 80 µJ, 500 ps pulses with a repetition rate of 1 kHz. The amplifier was pumped in counter propagation by a fiber coupled laser diode emitting 60 W at 808 nm (core diameter 100 µm NA 0,2). In a single pass, the pulse energy was multiplied by 15, reaching 1.2 mJ. This corresponds to a peak power of 2.4 MW, clearly beyond the typical limit of fiber amplifiers. To our knowledge, this is the most simple amplifier (only one gain medium and only one pass) ever developed able to reach the MW level with a significant average power (1.2 W).

7912-39, Session 10

2nd and 3rd harmonic generation from a fiber-amplified 100-ps, high-repetition rate and single-frequency passively Q-switched microchip laserA. Steinmetz, D. Nodop, Friedrich-Schiller-Univ. Jena (Germany); G. Sommerer, A. Wissel, S. Spiekermann, I. Freitag, InnoLight GmbH (Germany); J. Limpert, A. Tünnermann, Friedrich-Schiller-Univ. Jena (Germany)

We demonstrate second and third harmonic generation at a high repetition rate of 1 MHz obtained from a single-frequency, fiber-amplified microchip laser. The passively Q-Switched microchip laser provides pulses with duration of 100 picoseconds at a central wavelength of 1064 nm and consists of a monolithically bonded combination of Nd:YVO4-crystal and semiconductor saturable absorber mirror. The passively Q-Switched pulses are amplified in an ytterbium-doped photonic crystal fiber to an average power of 40 watts. The initial spectral width of ~20 pm is SPM-broadened to ~0.6 nm after the amplification.

The second harmonic generation stage is based on a non-critically phase-matched, 20 mm long LBO with no AR-coatings. The phase matching is achieved with a compact crystal oven, specially developed by INNOLIGHT GmbH. With a correction for the Fresnel-reflections of fundamental and second harmonic wave at the crystal interfaces, the SHG-stage generates an average power of more than 23.7 watts at 532 nm resulting in a conversion efficiency of 62.6%. For the third harmonic generation, a subsequent sum frequency generator is placed in a small distance after the SHG-stage into collimated beam and is based on a 7 mm long, critically phase-matched LBO crystal with AR-coatings. An average power of 9.5 watts is achieved at 355 nm wavelength resulting in an IR-to-UV conversion efficiency of more than 23.8 %.

Finally, this pulsed laser source in combination with a high-repetition rate and sub-100 ps pulse durations in visible and ultraviolet region is an interesting tool for many scientific and industrial applications.

7912-67, Poster Session

Study on the 1123 nm continuous-wave ceramic Nd:YAG laserS. Zhang, Q. Wang, X. Zhang, Z. Liu, A. Long, Shandong Univ. (China)

A fiber coupled cw diode laser (Wavelength=808 nm, NA=0.22, dcore=600micro) was used as the pumping source. The pump beam was re-imaged into laser material and the waist diameter was about 600micro. The rear mirror was a 3000 mm radius-of-curvature concave mirror. Its entrance face was coated for anti-reflection at 808 nm (R<0.2%). The other face was coated for high-reflection at 1123 nm (R>99.8%) and high-transmission at 808 nm (T>95%). The output coupler was a flat mirror that was coated with a reflectivity of 98% at 1123 nm and high-transmission at 1064 nm and 1319 nm (T>95%). Two ceramic Nd:YAG rods with the same size of 4mm10mm were employed in our experiments. Rod 1 was with 1.0 at. % Nd-doping concentration and rod 2 with 0.6 at. % Nd-doping concentration. Both rods had AR coatings (R<0.8%) on each face at 808 nm and 1123 nm. During our experiments, the cNd:YAG rods were wrapped with indium foil and mounted in water-cooled copper blocks with the water temperature maintained at 18 Centidegree. Highly efficient 1123 nm continuous-wave lasers are realized with ceramic Nd:YAG materials. A fiber-coupled continuous-wave 808-nm diode laser is used as the pumping source. Two ceramic Nd:YAG rods with the same length of 10mm and different Nd-doping concentrations are employed during our experiments. With the same incident diode power of 26.1 W, a cw output power of up to 10.8 W is obtained with the 1.0 at. %-Nd-doped rod, while 8.2 W output power is generated from the 0.6 at. %-Nd-doped rod. The highest conversion efficiency from diode power to 1123-nm laser power is 43.8% and 31.4%, respectively, for the two Nd:YAG rods.




Precision Fresnel attenuator of the beam power of laser radiationJ. A. Owsik, Military Univ. of Technology (Poland); A. A. Kovalev, S. A. Moskalyuk, All-Russian Research Institute for Optical and Physical Measurement (Russian Federation); A. Z. Rembielinska, LOT Polish Airlines (Poland); E. B. Yankevich, All-Russian Research Institute for Optical and Physical Measurement (Russian Federation)

Fresnel attenuators would appear to play a leading role in the set of dividers and attenuators of laser radiation that function on the basis of various operating principles by virtue of the meticulous study which they have undergone and in view of their widespread use in different optical systems and optical devices. Fresnel radiation attenuators are highly practicable and simple to operate. The most elementary of such attenuators consists in a transparent plane-parallel glass plate. In all optical systems, it is initially used as a coupler of a portion of the laser radiation for purposes of measurement (radiation energy, pulse duration, etc.). However, these types of attenuators possess a major drawback: that is, the reflectance of laser radiation from the surface of such an attenuator depends on the polarization of the radiation and its slope to the interface between different media. In principle, optical circuits from which this drawback can be partially eliminated are known. This relates to the dependence of the attenuation coefficient on the polarization of the incident radiation. For this purpose, we may employ, for example, two successive reflections. The reflecting planes may be situated so that the resultant reflectance is not affected by the type of polarization of the incident radiation. This type of circuit will be investigated in detail. It is precisely the polarization independence of the reflectance that may serve as a basis for the creation of a calibrated attenuator of laser radiation power. Another important property of this type of attenuator is the fact that the efficiency of the conversion of the radiation power is independent of the transverse dimensions of the laser beam (at least to within 10-5).


747 nm Pr:YAP microchip-laser output characteristicsM. Fibrich, H. Jelinková, J. ?ulc, Czech Technical Univ. in Prague (Czech Republic); K. Nejezchleb, V. ?koda, Crytur Ltd. (Czech Republic)

Compact and reliable solid-state laser systems based on microchip geometry are promising radiation sources for many applications both in industry and medicine.

An attractive laser host for Pr3+ ions is the yttrium aluminium oxide YAlO3 (YAP) because of its good thermal and mechanical properties (similar to those of YAG) combined with nature birefringent character, so, the thermally-induced birefringence should not degrade the laser performance. In comparison with widely examined Pr:YLF crystal, as Pr-fluoride representative, Pr:YAP active medium exhibits positive value of the temperature coefficient dn/dT enabling to design a microchip laser system.

For the construction of the compact, efficient, longitudinally diode-pumped Pr:YAP laser system operating at 747 nm, microchip geometry was proposed. The microchip crystal was formed by directly coated dielectric films on the surfaces of the Pr:YAP gain medium to form the laser cavity. As a pumping source, 1-W GaN laser diode was used. In order to investigate the laser output properties at different temperatures, the active medium was mounted on a water cooled copper heat sink; temperature of the cooling medium was set and controlled by thermostat.

Laser output characteristics were examined at varying active medium temperature within the range of approx. 11-40°C. The maximum output power obtained from the Pr:YAP at 747 nm wavelength was 139 mW (at 11°C). The laser oscillation threshold and slope efficiency with respect to the incident pumping power were about 430 mW and 25 %, respectively.

The output radiation was confined in close to the fundamental Gaussian mode.


Design of micro-second pulsed laser mode for ophthalmological CW Self-Raman laserA. D. Mota, Opto Eletrônica S.A. (Brazil) and Univ. de São Paulo (Brazil); G. Rossi, T. Almeida Ortega, G. Zerbini Costal, F. M. Yasuoka, M. A. Stefani, Opto Eletrônica S.A. (Brazil); J. C. Caiado de Castro Neto, Opto Eletrônica S.A. (Brazil) and Univ. de São Paulo (Brazil); M. S. Paiva, Univ. de São Paulo (Brazil)

Micro-second pulsed laser exposure is a new procedure for retina disease treatments. This technique consists in applying laser power sequences of 200µs pulses (1000 times less than traditional treatments). Short laser exposure duration avoids photoreceptor lesions on the retina, providing a better final vision. The laser cavity in 586nm and 4W CW output power is based on Self-Raman conversion in Nd:GdVO4. Initially, it was developed to continuous mode, but now it is switched to 500Hz, with minimum pulse duration of up to 100µs. This operation mode makes the cavity electronic control system velocity vital and challenging. The energy delivery should be very fast for the pulse to reach the desired level before its termination, and the control feedback system has to correct any deviation of output power to keep it stable and constant. The strategy found was to build a duplicated current driver, one plugged to the laser head and the second plugged to a fake laser cavity. Each one works in synchronism to keep the drained current constant while the laser pulse in turned on and off during treatment procedure. Consequently, it eliminates the power supply time response dependency which can take around 1ms. The output power control is performed by a high priority software to minimize any delay in power reading. The laser head showed good performance, without any stability issues. The cavity pulse time response was of 30µs (worst case) to reach a stable mode. This work resulted in a commercial ophthalmic laser.


The research of the laser facula of laser ranger finder in the far distanceR. Fu, Nanjing Univ. of Science & Technology (China)

A beam of 1.06µm laser pulse light of a laser range finder shoots to the target about 2000 meters away. The diameter of the facula of the laser light beam is about 1 meter. The lateral intensity of the facula is not an ideal Gauss distribution. Influenced by the atmosphere and reflecting rate of the target and mainly by the laser ranger finder itself, the intensity distribution varies in different position in the facula.

A testing device is designed to measure the facula of the laser light in the far distance. The device has a near infrared CCD camera of high sensitivity on 1.06µm, a filter of 1.06µm is applied for the camera. Another white light CCD is applied to obtain the background. A lens is designed for both cameras, the focus length is 400mm. The testing device is attached to the laser ranger finder. The near infrared CCD is synchronous with laser ranger finder, so as to receive the laser signal reflected from the target. The target is made of wood and is covered with white paint, and is as large as 5m×5m in size.

The near infrared CCD receives a sequence of laser pulse faculae from the target and store them in the computer in the form of bitmap format. The image of the facula is analyzed and the intensity distribution of the facula is obtained.

The influence of the characteristic of the target and atmosphere to the facula is discussed, the precision of the device is given.




Compact intracavity frequency doubled diode laser at 465 nmK. Li, N. J. Copner, Univ. of Glamorgan (United Kingdom); C. B. E. Gawith, Covesion Ltd. (United Kingdom); I. Knight, Oclaro, Inc. (United Kingdom); H. Pfeiffer, Oclaro, Inc. (Switzerland); B. Musk, Gooch & Housego, Torquay (United Kingdom)

We report on continuous-wave blue light generation by direct intra-cavity frequency doubling (ICFD) of multi-emitters laser bar using a MgO-doped periodically poled lithium niobate (MgO: PPLN) bulk crystal, which has the potential to be scalable to high production volumes and low costs with immense implication for laser-based projection displays. An extended cavity 49-emitters edge-emitting laser bar was used as the fundamental pump source. Ultra-low reflectivity coating on the output facet of the laser bar gives less than 0.1% in the wavelength range of ±15nm, which eliminates the original diode laser cavity allowing the extended longer laser cavity to dominate. An external dichroic mirror coated for high reflectivity in the near-infrared range and transparent for blue light emission is used as output coupling mirror. An asylindrical lens is used to collimate the fast axis of the diode. The slow axis micro-lens array consisting of 49 lens is aligned with the individual emitters so as to create 49 beams with 24 um beam radius focused to the midpoint of a 10 mm-long 4.8 µm period MgO: PPLN crystal. A thin film narrow bandwidth IR filter is inserted in the cavity before the crystal to restrict the spectral laser bandwidth to <0.1 nm so that optimal frequency conversion can be obtained. A maximum of 1 W of second-harmonic light at 465 nm is generated at an operating injection current of 45 A with the optimal phase-matching MgO:PPLN temperature of 77 °C. To increase the efficiency further, careful design of the lens used on the fast and slow axis beam waists and use of lower-temperature MgO:PPLN planar-waveguide array can be considered.


Passively Q-switched quasi-continuously pumped 2 4% Nd:YAG laser in a bounce geometryM. Jelínek, V. Kubecek, M. Cech, P. Hir?l, Czech Technical Univ. in Prague (Czech Republic)

Pulsed, diode pumped solid state lasers are attractive sources of high power and high spatial quality pulses for a wide range of applications. Efficient configuration for quasi-continuous pumping is a slab active element in a grazing incidence geometry, where the laser beam experiences total internal reflection and amplification at the pumped face. This results in high gain for active materials with high absorption coefficient at pump wavelength as Nd:YVO4 or recently available highly doped Nd:YAG. Nd:YAG crystals have better thermal and mechanical properties than vanadates and longer upper state life-time resulting in better energy storage capacity which is important for pulsed Q-switched operation, which was demonstrated also in our previous results.

In this paper we report on quasi-continuously pumped laser based on highly 2.4at.% doped crystalline Nd:YAG in a bounce geometry passively Q-switched by a Cr:YAG and V:YAG saturable absorber respectively. In comparison with our recent results the optimization of laser parameters and successive pulse amplification in the second Nd:YAG crystal also in a bounce geometry led to more efficient operation. At 1.06 um the oscillator 5 ns long linearly polarized output pulse with energy of 1.5 mJ was further amplified to 4 mJ. At the wavelength of 1.3 um the 13 ns long output pulse with energy of 600 uJ was amplified to 800 uJ in single pass. The details of resonator and pumping optimization will be also discussed.


Influence of V:YAG saturable absorber orientation on linearly polarized laser Q-switchingJ. ?ulc, T. Koutn?, H. Jelinková, Czech Technical Univ. in Prague (Czech Republic); K. Nejezchleb, V. ?koda, Crytur Ltd. (Czech Republic)

The impact of the V:YAG saturable absorber nonlinear transmission polarization anisotropy on Q-switched laser system was investigated. Two various cuts ([111] and [100]) of V:YAG crystal with the same initial transmission 87 % @ 1.3 µm were used as a passive Q-switch for longitudinaly diode pumped linearly polarized Nd:YAP laser operating at 1342 nm. This laser consisted of 8.2 mm long Nd:YAP crystal placed in 100 mm long semi-hemisferical resonator. The flat mirror in vicinity of Nd:YAP crystal was transparent for pumping radiation at 805 nm and highly reflecting at operating wavelength 1342 nm. The curved output coupler (radius of curvature 146 mm) had reflectivity 92 % @ 1342 nm. The laser was tested under pulsed pumping for the duty-cycle 9 %and under CW pumping. The laser output parameters (mean output power, pulse width, repetition rate, energy, and peak power) were measured for absorber turning around longitudinal crystal axis. In this configuration for [111]-cut of V:YAG the pulse energy and pulse width (FWHM) varied from 30 up to 45 µJ and from 50 to 38 ns, respectively. This corresponds to peak power rise from 0.6 up to 1.2 kW. In case of [100]-cut the proper orientation of V:YAG in respect to oscillating radiation polarization allowed to increase pulse energy from 36 to 62 µJ and peak power from 1.3 to 3 kW. Simultaneously, the pulse width decreased from 28 to 20 ns. The results showed that proper choice of saturable absorber cut and orientation can significantly improve giant pulse parameters.


Noise performances of a high-power picosecond Nd:YVO4 oscillatorM. Nadeau, Univ. Bordeaux 1 (France) and Thales Optronique SA (France); S. Petit, Ctr. National de la Recherche Scientifique (France); S. Montant, Commissariat à l’Énergie Atomique (France); P. Balcou, Ctr. National de la Recherche Scientifique (France); C. Simon-Boisson, Thales Optronique S.A. (France)

We report on an experimental investigation of the noise properties of a free-running, high-power, picosecond Nd:YVO4 oscillator. The oscillator, diode-pumped at 888 nm with more than 100 W, is passively mode-locked by SESAM and it delivers 21 W of average output power with 15 ps pulse duration. In a first step, the amplitude noise has been measured with a photodiode and an electronic spectrum analyser. By the analysis of the noise spectrum, we show that this measurement is very sensitive to help for careful alignment of the oscillator, especially in terms of beam quality. Different kind of mode-locked (Ti:Sapphire, Yb:KGW, Yb:glass) have been also characterized and their noise spectra are compared to each other. Finally, we investigated the second harmonic amplitude noise spectrum and we found that frequency doubling does not affect the noise properties. The results are similar to noise spectra of DPSS CW intracavity frequency doubled Nd:YVO4. In a second step, we investigated the timing phase noise by making the measurements of noise of the Nd:YVO4 oscillator at higher harmonics of its repetition rate. We also recorded the long term evolution of the repetition rate and all these results will be used to properly design a stabilization scheme in order to lock this oscillator to a passive enhancement Fabry-Perot cavity.




Influence of UV illumination on the cold temperature performance of a LiNbO3 Q-switched Nd:YAG laserB. J. Cole, V. King, J. Leach, L. Goldberg, U.S. Army RDECOM CERDEC NVESD (United States)

A well known issue with military Nd:YAG lasers Q-switched by LiNbO3 is a loss of hold-off and subsequent pre-lasing that is associated with cold temperature operation and/or a rapid change in temperature. The phenomenon occurs due to a pyroelectric effect in LiNbO3, where a change in temperature results in an accumulation of charges on the z-faces of the crystal. The unscreened “pyrocharges” result in a large electric field across the sample and, via the electroptic effect, impart an uncontrolled birefringence on light propagating within the cavity. A common approach to circumvent this effect is the use of a radioactive alpha emitter, such as Americium, to ionize the air near the LiNbO3 faces. Although quite effective, radioactive Americium is subject to regulation that complicates its use.

This paper will describe a technique based on UV illumination as a means to compensate pyrocharges accumulated on the end face of a LiNbO3 Q-switch. This method uses commercially available low cost UV LEDs that flood illuminate the LiNbO3 from the side. The absorption of UV increases the conductivity for LiNbO3 via a photo-generation of carriers, where the strong pyroelectric fields enable sweeping of the carriers in the appropriate direction to compensate the surface charge. A LiNbO3 actively Q-switched Nd:YAG laser was evaluated over temperature and shown to have robust performance with rapid temperature cycling to cold temperatures (to negative 20C) in the presence of UV illumination, without evidence of pre-lasing that was otherwise observed with no UV illumination of the LiNbO3.


Comparison of LD-pumped soliton and passively mode-locked operations of Yb:(Gd1-xYx)2SiO5 (x=0 5) laserJ. He, X. Liang, J. Lee, L. Zheng, J. Xu, Z. Xu, Shanghai Institute of Optics and Fine Mechanics (China)

LD-pumped soliton and passively mode-locked Yb:(Gd1-xYx)2SiO5 (x=0.5)(Yb:GYSO) lasers have been demonstrated together for the first time to the author’s knowledge. With a pair of SF10 prisms as the negative dispersion elements, the pulses as short as 1.4ps were generated for the soliton mode-locked operation. The central wavelength was 1056nm and the repetition rate was 48MHz. For the passively mode locking, output power could achieve ~1.2W with pulse width ~ 20ps. After having compared the two mode-locked operations, we found that the critical pulse energy in the soliton-mode locked operation against the QML was much lower than the critical pulse energy in the non-soliton mode-locked operation. And the pulse width was very different for the two mode-locked operations. According to the two mode-locked operations of the Yb:GYSO laser, we could figure out that the GVD (group velocity dispersion) of Yb:GYSO crystal was between 40fs2/mm and 190fs2/mm. We also did a simulation of how nonlinear refractive index of gain medium and intracavity net GDD influence the critical pulse energy in soliton-mode-locked operation.


High power red laser oscillation in Pr3+-doped waterproof fluoro-aluminate glass fiber excited by a GaN laser diodeJ. Nakanishi, T. Yamada, NIDEK Co., Ltd. (Japan); Y. Fujimoto,

Osaka Univ. (Japan); O. Ishii, M. Yamazaki, Sumita Optical Glass, Inc. (Japan)

We have demonstrated a high power red fiber laser with a Pr3+-doped waterproof fluoro-aluminate glass fiber (Pr:WPFGF). Even though a Pr:WPFGF is a fluoride material, it possesses the low deliquescence and enough long-term stability. A blue/violet GaN laser diode (GaN-LD) was launched into the Pr:WPFGF (core diameter 8 µm, length 40 mm) whose both end surfaces were deposited with dielectric-multilayered-coatings to construct a resonator. The maximum output power of the obtained 638 nm laser beam was measured to be 311.4 mW at 800 mW pumping power which is the highest power in Pr3+- doped fiber lasers, such as Pr:ZBLAN fiber. The threshold power was 52.1 mW, and the slope efficiency was calculated to be 41.6%. Considering the resonator to be a Fabry-Perot resonator, we can calculate the output power to be 336 mW at 800 mW pump power and the slop efficiency to be 44.2%. These theoretical values show good agreement with experimental ones.

In future study, we will enhance the precision of our laser system design, such as, the wavelength matching at high power operation between pumping and absorption spectra, the optimization of coupling mirrors. This system easily enables oscillation of visible laser beam without using complex optical system such as wavelength conversion.


Solid state fs oscillators with direct laser-diode pumpingG. Kim, U. Kang, E. G. Sall, S. A. Chizhov, A. Koulik, Korea Electrotechnology Research Institute (Korea, Republic of); V. E. Yashin, S.I. Vavilov State Optical Institute (Russian Federation)

We desceibeds the development of solid-state fs oscillators which are based on Yb:KYW crystal with direct pumping by a semiconductor injection laser. Powerful laser-diode are pumping in the logitudinal direction. Femtosecond pulses are generated by using a semiconductor saturable absorber for the passive mode-locking and chirped mirrors for the dispersion compensation. Two types of oscillators are developed; One is compact with average power of about 1 W and pulse width of about 90 fs, the other is powerful with average power of 3.5 W and pulse width of about 200 fs. They can be used as a stand-alone source of femtosecond radiation pulses for material microprocessing and primary source for femtosecond laser amplification systems such as multi-pass or regenerative amplifiers.

We will report more details of experimental results for both oscillators.


Low threshold, stable soliton mode-locked Yb:SSO laser with pulse duration of 2 3 psJ. Lee, X. Liang, J. He, L. Zheng, Shanghai Institute of Optics and Fine Mechanics (China); J. Xu, Shanghai Institute of Ceramics (China)

Abstract: We report on a soliton mode-locked Yb:SSO laser. Average output power up to 1.87W and pulse durations down to 2.3ps are demonstrated.

We used a five mirror cavity for the continuous wave mode-locked operation. A 5×6×3mm3 5at. % Yb:SSO crystal was used as laser medium and was placed in the middle of the cavity. A SESAM with saturation fluence of 70µJ/cm2 and a maximum modulation depth of 1.2% was used in one end of the cavity to initiate and maintain the CW mode-locked pulse train. The cavity was designed to provide a mode size radii of 90µm inside the crystal and 50µm on the SESAM, respectively.

With no extra negative dispersion elements, we could achieve stable CW soliton mode-locked train with different OC transmissions (1%, 5%, 9% and 13%). The threshold output power for the various OC we used (0.096W for 1% OC, 0.30W for 5% OC, 0.580W for 9% OC and 0.498W



for 13% OC, respectively) was distinct lower than the counterpart of the passively mode-locked regime. And pulses as short as 2.3ps was obtained, which was relatively short in the medium-in-the-middle type of picosecond mode-locked lasers. A maximum output power of 1.87W was achieved at a pump power of 11.5W with a 13% OC, corresponding to a slope efficiency of 22.4%. The result shows that, Yb:SSO crystal is quite suitable and potential for stable high power ultra-short mode-locked lasers.


Effects of gamma-irradiation on optical, electrical, and laser characteristics of pure and transition metal doped II-VI semiconductorsT. Konak, M. Tekavec, V. V. Fedorov, S. B. Mirov, The Univ. of Alabama at Birmingham (United States)

We report a comprehensive study of gamma-irradiation on optical, electrical, and laser characteristics of pure and transition-metal doped single and polycrystalline ZnS and ZnSe. Polished pure, Cr-doped, and Ag, Au, Cu, Al, In, and Mn co-doped ZnS and ZnSe crystals after absorption and electro-conductivity characterization were gamma-irradiated at doses of 1.7x10^8 rad at +10C and/or 1.4x10^8 rad at -3C. Dynamic RT absorption studies, electro-conductivity measurements, and mid-IR lasing, performed for different exposition times of crystals at RT (from minutes to weeks), revealed the following results: 1) Immediately after gamma-irradiation both ZnS and ZnSe crystals exhibit color-center formation accompanied by Zn metal aggregates and appearance of a strong absorption covering the whole visible-mid-IR spectral range. 2) During ~ 15 min of crystals annealing at RT color-center absorption and Zn aggregates scattering reduces significantly. 3) Re-irradiation at -3C completely suppresses radiation induced absorption/scattering making the spectrum identical to that of non-irradiated crystal. 4) Absorption behavior of Cr:ZnS and Cr:ZnSe crystals co-doped with Ag, Cu, Al, In, and Mn was similar to that of an undoped crystals. 5) Cr:Au:ZnS crystal in addition to 650 nm band and Cr2+ 1700 nm absorption featured an additional absorption peak at 900 nm which we attribute either to Cr3+ or Au perturbed color-center. 6) gamma-irradiation did not change the conductivity of any of the crystals. 7) Cr:ZnSe and Cr:ZnS lasers based on identical gamma-irradiated and non-irradiated crystals after 30 min of RT annealing featured a very similar pump thresholds, slope efficiencies and output powers


A single frequency Nd:YAG Q-switched laser with timely controllable firing time by a gated Pockels cellF. F. Wu, A. I. Khizhnyak, V. B. Markov, MetroLaser, Inc. (United States)

A single frequency Q-switched Nd:YAG laser with time-controllable firing is discussed. A ring-cavity configuration is used to trap CW seeded radiation that equalizes a CW transformed injection pulse seeded in the slave laser. The process is completed at Pockels cell rising edge. Such seeding injection results in unidirectional Q-switched lasing, suppressing the bidirectional Q-switched oscillation. Our earlier design has utilized a two-Pockels cells configuration to open the slave cavity and dump the Q-switched laser pulse. Current design uses a single gated Pockels cell to realize the following two functions: (1) opening the slave cavity and at same time perform of seeding, and (2) dump the Q-switched pulse when the slave lasing is optimized. Since the Q-switch firing time is precisely controlled by the Pockels cell’s timing, thus laser firing can be precisely time controlled. The advantage of the realized regime is in stable laser operation with no need in adjustment of the cavity mode. We demonstrate that the frequency of the Q-switched laser matches well to

the injected laser. The output pulse energy can be above 80 mJ to close 100 mJ depending the operating condition and pulse width is about 10 ns.


A hybrid fiber/solid state regenerative amplifier with tunable pulse widths for satellite laser rangingD. Poulios, American Univ. (United States); D. B. Coyle, NASA Goddard Space Flight Ctr. (United States)

A fiber/solid-state hybrid seeded regenerative amplifier capable of achieving high output energy with tunable pulse widths has been developed for satellite laser ranging applications. The regenerative amplifier cavity utilizes a pair of Nd: YAG zigzag slabs oriented orthogonally to one another in order to symmetrize thermal lensing effects and simply optical correction schemes. The seed laser used is a fiber-coupled 1064 nm narrowband (<0.02 nm) diode laser which is intensity modulated by a fiber Mach-Zender electro-optic modulator, enabling continuously tunable seed pulse widths in the 0.2-2.0 ns range. The seed laser pulse energy entering the regenerative amplifier cavity is <10 pJ, and is amplified to ~1.7 mJ after 44 round trips, representing a gain of ~62 dB. When seeded with 200 ps pulses at a 2 kHz repetition rate, the regenerative amplifier produces >2 W of frequency-doubled output (>1.0 mJ/pulse at 532 nm).


Fe:ZnSe laser: comparison of active materials grown by two different methodsM. E. Doroshenko, A. M. Prokhorov General Physics Institute (Russian Federation); H. Jelinková, Czech Technical Univ. in Prague (Czech Republic); T. T. Basiev, A. M. Prokhorov General Physics Institute (Russian Federation); M. Jelínek, P. Koranda, M. Nemec, Czech Technical Univ. in Prague (Czech Republic); V. K. Komar, A. S. Gerasimenko, Institute for Single Crystals (Ukraine); V. V. Badikov, D. V. Badikov, Kuban State Technological Univ. (Russian Federation); D. Vyhlidal, J. Stoklasa, Czech Technical Univ. in Prague (Czech Republic)

For some application short pulses in the mid-infrared region (3-5 um) could be useful. The aim of the presented project is to compare two Fe:ZnSe lasers designed with Fe:ZnSe bulk active crystals grown by two different methods - Bridgman (crystal I) and floating zones method (crystal II). The dimension of Fe:ZnSe active materials were 20 x 15 x 2.86 mm3 (I) and 10 x 10 x 3.5 mm3 (II). For pumping the Q-switched Er:YAG laser generating 20 mJ giant pulses with duration of 200 - 300 ns was used. The laser output wavelength (2937 nm) corresponds to the maximum of the Fe:ZnSe active materials absorption spectrum. The initial transmittance of the crystals for this wavelength was 35% (I) and 17% (II). The pump radiation was directed into the Fe:ZnSe crystals which were placed into the resonator formed by plan mirror with T=86% for 2.94 um and R=100% for 4-5 um and concave mirror r=-500 mm, R=95% for 4-5 um. The resonator length was 12 mm. As a result it was seen that the highest output energy was ~1 mJ for both investigated crystals for the absorbed energy 3.5 mJ. The generated Fe:ZnSe output pulse duration was 150 - 200 ns. Output beam spatial profile was approximately Gaussian in both axes. Tuning properties using intracavity CaF2 prism were also investigated and tuning range 3.8 - 5 um was observed.




Yb doped lutetium sesquioxide laser ceramicsR. S. Gurjar, G. Baldoni, Y. Wang, W. H. Rhodes, K. S. Shah, Radiation Monitoring Devices, Inc. (United States)

Recently it has been demonstrated that intense ultrashort pulsed lasers can be used to provide large acceleration to electrons (GeV to TeV) that can act as source for coherent attosecond X-ray emissions for high-energy particle physics experiments. Existing femtosecond and picosecond Ti-Sapphire lasers can not be upgraded to obtain these energies and need to be replaced by new lasing media in the long wavelength range (~ 1m) with lower quantum defects and higher pumping efficiencies to obtain higher output energies. Certain sesquioxide and orthosilicate lasers have superior thermal properties suitable for high power laser operation, broadband emission appropriate for short pulse generation and with Yb doping can have overall energy efficiencies approaching ~ 60-70%.

In order to develop compact, high energy pulsed lasers for accelerators, we fabricated thin ceramic discs of lutetium sesquioxide doped with Yb as a potential lasing medium. Yb-doped lutetium oxide starting powder particles were prepared by the Liquid Phase Flame Spray Pyrolysis (L-FSP) and coprecipitation techniques. The L-FSP particles had smaller size suitable for higher transparency (average diameter 23 nm) and were consolidated using both hot pressing and Sinter HIPping processes. The hot pressed samples had lower transparency and larger grain size compared to the Sinter HIPped samples. The sintering temperatures were maintained low at 1450 C in order to minimize grain size and maximize transparency. The optical properties of the specimens were quantified in terms of transparency (>95%) and both a material and a fabrication process were identified for further development.


Efficient compensation of thermal birefringence of a flash-lamp pumped Nd:YAG laser by a simple but novel methodP. K. Datta, S. Mondal, S. P. Singh, S. Dutta, S. Bera, S. P. Dey, Indian Institute of Technology, Kharagpur (India)

Depolarization loss due to thermal stress induced birefringence is efficiently reduced by a tilted Glan-Taylor polarizer inside a stable cavity of a Xe flash-lamp pumped Nd:YAG laser. The cavity is made of a concave rear mirror of radius of curvature 500mm and a plane output coupler with 20% coupling. The flash-lamp and Nd:YAG rod occupies the two foci of elliptic pump chamber. The cavity length is varied from 300mm−490mm and the corresponding average Gaussian mode diameter within the Nd:YAG rod lies in the range 0.55mm−1.0mm. The Glan-Taylor polarizer with antireflection coated faces is placed near the output coupler within the cavity for polarized output. The flash lamp discharge voltage corresponding to the laser threshold is about 300V. The flash-lamp discharge voltage is varied from 300V to 1400V and the output power at 1064nm is recorded for different repetition rate. The electrical input power to laser output power slope efficiency corresponding to 10Hz repetition rate is about 0.09%. The output power is found to vary with both the rotation of the polarizer along the axis of the beam propagation and perpendicular to it. The maximum depolarization loss is measured to be 50%. However, when the polarizer angular position is optimized, the depolarization loss reduces to 13%. The rod axis of the Nd:YAG crystal is along the [111] direction. A depolarization function is derived for the Nd:YAG rod and the tilted polarizer. The corresponding depolarization loss accounts reasonably well with the measured value.


Intense red upconversion fluorescence emission in NIR-excited erbium-ytterbium doped laponite-derived phosphorA. da Silva, D. S. Moura, A. S. Gouveia-Neto, E. A. Arcanjo da Silva, Jr., L. Bueno, E. B. da Costa, E. Azevedo, Univ. Federal Rural de Pernambuco (Brazil)

Materials suitable for the development of solid-state visible and white light sources based upon near-infrared excited rare-earth doped frequency upconverters have drawn much scientific and technological interest lately, due to their potential applications in color displays technology, assay of biological compounds, remote sensors, optical data storage, optical printing, etc. Thus, it is of great interest to investigate frequency upconversion processes in alternative phosphor materials. Regarding optical applications, laponite represents an interesting host alternative due to its high transparency and small cystallite size. Laponite RD® is a synthetic polycrystalline swelling clay of a hectorite type. Because of these characteristics, this clay type material can be deposited as a film on a substrate and may find applications in the optical waveguide technology.

In this report the optical properties and energy-transfer frequency upconversion luminescence of Er3+/Yb3+-codoped laponite-derived powders under 980 nm infrared excitation was investigated. The 75%(laponite):25%(PbF2) samples, generated high intensity red emission around 655 nm and lower intensity green emission around 525, and 550 nm. The observed emission signals were examined as a function of the excitation power, and results indicate that energy-transfer is the major upconversion excitation mechanism for the erbium excited-state emitting levels.The precursor glass samples were also heat treated at annealing temperatures of 300 oC, 400 oC, 500 oC, and 600 oC, for a 2h period. The dependence of the visible upconversion luminescence emission upon the annealing temperature indicated the existence of an optimum temperature which leads to the generation of the most intense and spectrally pure red emission signal


Properties of rare earth doped thin film dielectric layers for laser waveguidesS. J. Pearce, G. J. Parker, M. D. B. Charlton, J. S. Wilkinson, Univ. of Southampton (United Kingdom)

Thin films of Yttrium Oxide, Y2O3 were deposited by reactive sputtering and reactive evaporation to determine their suitability as a host for a rare earth doped planar waveguide upconversion laser. The optical properties, structure and crystalline phase of the films were found to be dependent on the deposition method and process parameters. X-ray diffraction (XRD) analysis on the ‘as-deposited’ thin films revealed that the films vary from amorphous to highly crystalline with a small broad peak at 29° corresponding to the reflections of Y2O3. The samples with the polycrystalline structure had a stoichometry close to bulk cubic Y2O3. SEM imaging revealed a regular column structure confirming their crystalline nature. The thin film layers which allowed guiding in both the visible and infra-red region had lower refractive indices, higher oxygen content and had a more amorphous structure. Higher oxygen pressures during the deposition leads to a more amorphous layer.

7912-40, Session 11

Developing a more useful surface quality metric for laser opticsT. Turner, Q. Turchette, Research Electro-Optics, Inc. (United States)



Light scatter due to surface defects on laser resonator optics produces losses which lower system efficiency and output power. In the US, the traditional methodology for gauging surface quality is defined by military specification MIL-0-13830A. This involves visual comparison of a component to scratch and dig (SAD) standards under controlled lighting and viewing conditions. However, for laser manufacturers, this approach has two significant drawbacks. First, despite attempts to control the exact inspection conditions, the process is still subjective and operator dependent. Second, there is no clear correlation between these surface quality inspection results and the actual performance impact of the optic in a laser resonator. As a result, laser manufacturers often overspecify surface quality in order to ensure that optics will not degrade laser performance due to scatter. This can drive up cost and lengthen lead times unnecessarily because it lowers yields for the optics fabricator. Alternatively, an objective test system for measuring optical scatter from defects can be constructed with a microscope, calibrated lighting, a CCD detector and image processing software. This approach is quantitative (yielding a specific number for total scattered light) highly repeatable and totally operator independent. Furthermore, it is flexible, allowing the user to set threshold levels as to what will or will not constitute a defect. This paper details how this automated, quantitative type of surface quality measurement can be implemented, and shows how its results correlate against other measures of scatter loss, such as cavity ringdown, and total integrated scatter (TIS).

7912-41, Session 11

Novel concept for long-haul ultrashort pulse fiber delivery without pre-chirpingT. Le, G. Tempea, A. Stingl, FEMTOLASERS Produktions GmbH (Austria); K. G. Jespersen, OFS Fitel Denmark ApS (Denmark); K. Wiesauer, RECENDT GmbH (Austria)

Ultrashort pulse fiber delivery for Ti:Sapphire lasers is basically restricted to distances below several meters which is due to the application of dispersion compensating devices that are not capable managing third and higher order material dispersion. By the use of a novel fiber delivery concept ultrashort laser pulses in the 800 nm wavelength range can now be transmitted over tens of meters without the need for any pulse pre-compression. For the first time a long-haul fiber delivery module will be demonstrated revealing its potential for remote imaging or THz spectroscopy with femtosecond laser pulses.

Results for the delivery of 100 fs to 200 fs laser pulses over up to 50 meters will be presented without the need for gratings or prisms. The application of the fiber delivered pulses will be demonstrated by generating broadband THz radiation directly after the fiber.

7912-42, Session 11

Beam steering mirror for fine adjustment of visible laser single mode fiber coupling based on a birefringent crystalC. Kannengiesser, R. von Elm, W. Seelert, Coherent Lubeck GmbH (Germany)

High precision beam steering usually requires complex and sophisticated mechanics to keep the alignment tolerances stable over the long term. In this talk, we will present a simple temperature- controlled steering device that is based on a crystal with different temperature expansion coefficients along its axes. Results for single mode fiber coupling of visible lasers will be presented.

7912-43, Session 11

All-in-quartz optics for low focal shiftsM. Blomqvist, O. I. Blomster, S. Campbell, M. Pålsson,

Optoskand AB (Sweden)

High laser power levels in combination with increasing beam quality bring optics performance into focus, particularly with regard to systems with low focal shifts along the optical axis. In industrial applications, this often influences the overall performance of the process, especially if the focal shift is comparable to or in excess of the Rayleigh length. It is commonly accepted that the focal shifts are of thermal nature where lens material, lens coating, geometry and surface contamination all contribute to the direction and extent of the focal shifts. In this paper we will present a novel design of lens packages where an all-in-quartz concept is explored. By mounting quartz lenses in hermetically sealed quartz tubes and applying water cooling on the perimeter of the quartz tubes we will reduce or eliminate a number of contributing factors to focal shift problems. The hermetic sealing, carried out in a clean-room environment, will minimize lens surface contamination. Differences in thermal expansion between lens and housing are eliminated as the lens and housing will be of the same material. Absorption of scattered laser light will be efficient as the energy is removed quickly by cooling water and not absorbed by fixed surroundings. Finally, indirect heating from the housing transmitted by radiation and convection to the lenses is avoided. Values of the normalized System Focal Shift Factors (SFSF) for the all-in-quartz optics will be compared to standard lens assemblies at multi-kW laser power levels.

7912-44, Session 11

Novel all-hardware closed-loop adaptive optics system using massively-parallel neural processingM. Eichhorn, A. Pichler, P. Raymond, Institut Franco-Allemand de Recherches de Saint-Louis (France)

In most applications of laser technology and optics the beam quality, the ability to focus a laser beam and the achievement of a good optical resolution play an important role. Unfortunately, these properties are often limited by distortions of the wave front of the beam.

For the compensation of these distortions adaptive optics may be used. It allows compensating a phase distortion by use of a deformable mirror or a spatial light modulator. To do this it is necessary to measure the phase front of the incoming or emitted radiation, e.g. with a Shack-Hartmann sensor (SHS).

Classical adaptive-optics control schemes now link the shift of the measured spots on the SHS to the mirror actuator displacement by mathematical matrix operations. However, it is well known that matrix operations, especially when including complex tasks as pseudo-inverse calculations etc., show a non-linear computation time dependence with matrix size, i.e. with resolution of the SHS and the mirror actuator number. Thus it is difficult to achieve high control loop frequencies in highly resolved adaptive optics.

To avoid this, a novel closed-loop adaptive optics has been developed using a massively-parallel neural network in an all-hardware setup. The technology therein can be used for a fast real-time wave front sensor as well as for closed-loop operation. Experimental data and a comparison with the classical matrix algorithm are presented.

7912-45, Session 11

High-power pulse-pumped solid state laser gain modulesJ. Doster, R. Feeler, Northrop Grumman Cutting Edge Optronics (United States)

Northrop Grumman Cutting Edge Optronics (NGCEO) has developed a line of pulse-pumped solid-state laser gain modules for industrial and scientific applications. These modules produce output energies up to 4 J/pulse when used as oscillators. When used as amplifiers, these modules



produce small signal gains in excess of 600. Input apertures from 2mm to 10mm are available.

This paper focuses on the applications and reliability of these gain modules. Various case studies are presented and include: high-energy pulse generation, repetitive pulse amplification, and pulse-train amplification. Particular emphasis is paid to maintaining beam quality, minimizing depolarization, and maximizing pump uniformity. A comparison between fiber lasers and lasers built from these modules is presented in order to further define the application space.

The laser gain modules contain long-life laser diode arrays based on bars specifically designed for pulsed (or quasi-CW) operation and packaged with hard solder. Reliability data for the diode arrays is presented that shows product lifetimes exceeding 10,000 hours in a number of operating regimes, thereby proving the suitability of these modules for industrial applications.

7912-46, Session 12

Space variant optics based on nanophotonic structures for IR LasersE. G. Johnson, M. K. Poutous, Z. Roth, A. J. Pung, The Univ. of North Carolina at Charlotte (United States); R. K. Shori, Naval Air Warfare Ctr. Weapons Div. (United States)

Spatially variant resonant structures can be used to perform complex beam shaping functions for spatial, spectral, and polarization control. This approach exploits the dispersion of periodic structures that are fabricated with Integrated Circuit manufacturing techniques. The spatial features are encoded by introducing perturbations to the structure as a function of position, which can include polarization sensitivities. This paper will summarize recent advances in this approach and present examples of IR lasers that can exploit these features in advanced IR lasers.

7912-47, Session 12

Optical, mechanical, and thermal properties of undoped and Er-doped yittria (Er:Y2O3)A. Joshi, R. K. Shori, O. Stafsudd, Univ. of California, Los Angeles (United States); N. Haynes, D. E. Zelmon, Air Force Research Lab. (United States)

Transparent form of polycrystalline Yittria (Y2O3) is a promising new laser host material. The refractive index between 4xx nm - 3.39 micron, dn/dT, Sellmeier coefficients, and stress optic coefficients of undoped and varying level of Er doped Yittria will be presented.

7912-48, Session 12

LD pumped high efficiency, high power Tm:YLF laser with adhesive-free bond laser compositesX. Mu, H. E. Meissner, H. Lee, Onyx Optics Inc. (United States)

2-um Tm3+-doped YLF lasers have many important military and civilian applications. One of the advantages of Tm:YLF laser is that it can be pumped by commonly available high power diode lasers at wavelength of 792 nm. Due to the two-for-one cross-relaxation process, it is possible that the system is capable of over 100% quantum efficiency. However, the Thulium laser is a quasi-three level system, the laser emission suffers re-absorption loss and the output power is sensitive to the crystal temperature due to the thermal population at the laser terminal band. For high efficiency laser operation, efficient thermal management must be employed. In this work, adhesive-free bonded (AFB) un-doped end-capped 4% Tm:YLF and composites consisting of segments of un-doped

YLF bonded to 4% Tm:YLF have been fabricated and investigated for high efficiency, high power laser operations. Our simulations indicate that the AFB laser composites can efficiently increase the cooling efficiency and reducing the tensile stress of the laser crystals compared with the non-composite laser medium. With a fiber coupled 792-nm diode laser as pump source and an end-pump arrangement, 29 W cw output power has been achieved at a wavelength of 1908 nm for a pump power of 65 W. The measured optical-to-optical and slope efficiencies are 44.5% and 51.4%, respectively. The corresponding quantum efficiencies are 107% and 124%, respectively.

7912-49, Session 12

Energy scaling of room temperature Fe2+:ZnSe gain-switched 4 3 µm laserN. Myoung, D. V. Martyshkin, V. V. Fedorov, A. Martinez, S. B. Mirov, The Univ. of Alabama at Birmingham (United States)

Recent progress in iron doped ZnSe Mid-IR tunable lasers makes them promising for direct gain-switched lasing at 300K over 3.7-5µm spectral range. However, the maximum documented pulse energy in the gain-switched regime of operation with ns pulse durations has been only several hundreds of µJ for many years. The major problem with the output energy scaling-up was in the development of fabrication technology for large-aperture, good optical quality, high optical density Fe:ZnSe gain elements. We report an optimization of Fe:ZnSe crystals fabrication, as well as new records of the gain-switched Fe:ZnSe lasing pumped by the radiation of Q-Switched Cr:Er:YSGG (2.8µm) laser. A rectangular shape polycrystalline Fe:ZnSe gain element (8x8x3mm) with an iron concentration of 2x1019cm-3 prepared by post-growth thermal diffusion method was placed inside a sealed container on a cold finger cooled by a tandem of two TE elements. Lasing was studied over 236-300K temperature range. The flat-flat cavity was formed by a HR mirror and an output (8x8mm) facet of the Fe:ZnSe crystal. At 34mJ pump energy (energy density .35J/cm2, pulse duration 16ns, rep. rate 6.7Hz) the maximum output energy reached 4.7mJ @ 4.3µm and 3.6mJ @ 4.37 µm at 236K and 300K, respectively and was limited only by available pump energy. Threshold was about 8mJ and was practically unchanged over studied temperature range. The laser slope efficiencies decreased from 19% to 16% with an increase of temperature from 236 to 300K.

7912-50, Session 12

Miniaturized high power Er:YAG solid state laser pumped by a single laser diode barM. Krejci, Oclaro, Inc. (Switzerland); A. Heinrich, Pantec Biosolutions AG (Liechtenstein); J. Müller, Oclaro, Inc. (Switzerland); T. Bragagna, Pantec Biosolutions AG (Liechtenstein); N. Lichtenstein, Oclaro, Inc. (Switzerland)

Pulsed solid state lasers qcw pumped between 0.1 and 10 ms pulse duration and 1-20% duty cycle are attractive for medical applications. We present a portable laser system for transdermal drug delivery based on a miniaturized diode pumped Er:YAG laser pumped by a high power laser diode bar.

The 1mm diameter Er:YAG laser rod is side pumped by a single 10mm wide 9xx nm laser diode bar mounted on a (10x12x5)mm3 conductively cooled platform designed for two side cooling of the bar. The peak output power of 460W, measured at (200µs, 10%d.c, 25°C base plate temperature) is limited by the available maximum drive current. The capability of chip and cooling design is reflected in 320W output power and 65°C junction temperature measured for 300A drive current, 5ms pulse duration, 10%d.c and 25°C base plate temperature. Optimized cooling, E2 facet passivation and the hard solder assembly of the diode enable reliable qcw operation at power levels ≥300W. Life test data will be presented.

The high power of the laser diode and the small laser rod diameter enables side pumping of Er:YAG despites its low emission cross section.



The miniaturized diode pumped Er:YAG solid state laser system has a final overall dimension of only 30x25x17mm3 in a sealed, highly shock and vibration resistant package. The system generates more than 2W average power at 250Hz in 200µs pulses with a M2<4. The optical to optical efficiency is above 10% and the thermoelectric cooling can work with cooling water up to 40°C.

7912-51, Session 12

Energy scaling of nanosecond gain-switched Cr2+:ZnSe lasersV. V. Fedorov, The Univ. of Alabama at Birmingham (United States) and IPG Photonics - Mid-Infrared Lasers (United States); I. S. Moskalev, M. B. Mirov, IPG Photonics - Mid-Infrared Lasers (United States); S. B. Mirov, The Univ. of Alabama at Birmingham (United States) and IPG Photonics - Mid-Infrared Lasers (United States); T. J. Wagner, M. J. Bohn, Air Force Institute of Technology (United States); P. A. Berry, K. L. Schepler, Air Force Research Lab. (United States)

Recent progress in chromium doped ZnSe Mid-IR tunable lasers makes them the laser sources of choice when one needs a compact system with tunability over 2-3 µm. More than 13 W in the CW operation mode, and shorter than 100 fs oscillations have been demonstrated by several research groups. However, the maximum gain-switched pulse energy with ns pulse durations has been limited to several mJ for many years. The major problem with output energy scaling was in the development of fabrication technology for large-aperture, good optical quality, high optical density Cr:ZnSe gain elements. In this paper, we report on Cr:ZnSe crystals optimized for gain-switched pumping, as well as new peak power records for gain-switched Cr:ZnSe lasing pumped by Q-switched Cr:Tm:Ho:YAG (2.096 µm) and Raman shifted Nd:YAG lasers (1.906 µm). In these experiments we used Brewster cut Cr:ZnSe gain elements with chromium concentrations of 8x1018 cm-3. In the first demonstration, we used a Cr:Tm:Ho:YAG pump laser with 85 ns pulse duration. The Cr:ZnSe laser demonstrated 3.1 mJ of output energy, and 110 nm linewidth centered at a wavelength of 2.47 µm. In the next experiment we used a Raman shifted Q-switched Nd:YAG laser with a repetition rate of 10 Hz and a pulse duration of 7 ns. The 1.906 µm excitation was realized with an H2 Raman cell pumped in a backscattering geometry, and the maximum output energy of the Cr:ZnSe laser reached 10.1 mJ using a sapphire substrate as an output coupler.

7912-52, Session 13

Towards rare-earth-doped chalcogenide glass fibre lasers for the mid-infrared (IR)A. B. Seddon, Z. Tang, D. Furniss, S. Sujecki, T. M. Benson, The Univ. of Nottingham (United Kingdom)

The mid-IR, defined as the spectral region 3-25µm, covers important atmospheric windows and molecular fingerprints of numerous gases, liquids and solids diverse as: greenhouse gases; ground, water and air pollutants; pharmaceuticals; toxic agents; oil, oil products and plastics; biological tissue etc.. In this spectral range, molecular species exhibit fundamental vibrational absorption bands with large extinction coefficients, hence mid-IR spectroscopy potentially provides extremely sensitive chemical analysis.

Coherent sources both discrete and broadband in the mid-infrared (mid-IR) are of great interest for remote, real-time spectroscopy, with an impressively broad range of potential applications viz.: real-time control of manufacturing processes; ‘biophotonics’, -spectral tissue mapping for medical diagnosis, and remote sensing for security and environmental monitoring.

Moreover, mid-IR direct imaging, for instance for medical diagnosis, is to date, unexplored and a tantalising adjunct to current shorter (visible)

and longer wavelength (THz) imaging. Finally, new wavelengths for power delivery for medical surgery are of international interest.

Mid-IR fibre laser operation up to 3 µm has been demonstrated in fluoride glass fibre. For longer wavelength mid-IR operation, lower phonon energy glass hosts are required. Chalcogenide host glasses offer favourable properties with low phonon energies, down to 350 cm-1 for low non-radiative decay rates, good solubility of rare-earth (RE) ions and high refractive indices leading to high absorption and emission cross-sections. Additionally, Se glass fibres are mechanically robust and chemically durable both in water and in the open atmosphere.

We shall report on progress of developing RE-doped selenide glasses to deliver tuneable, monochromatic light beyond 3 µm.

7912-53, Session 13

High-power diode-pumped Q-switched Er3+:YAG single-crystal fiber laserI. Martial, Lab. Charles Fabry (France) and FiberCryst (France); J. Didierjean, N. Aubry, FiberCryst (France); F. Balembois, P. Georges, Lab. Charles Fabry (France)

Er3+:YAG is one of the most attractive materials for the development of compact eye safe laser sources around 1.6 µm addressing applications like remote sensing or free space communications. The optical transition between the 4I15/2-4I13/2 manifolds includes the possibility for in-band pumping (between 1.45 µm and 1.53 µm) and thus provides high laser quantum efficiency (more than 90%). Because of temperature dependence of the spectroscopic parameters and up-conversion effects, low doped samples are required to achieve efficient lasers. For this purpose, the use of single-crystal fibers (long and thin rods), where the pump is guided (like in fibers) and where the signal is unguided (like in bulk materials) represents one way to manage the poor beam quality of pump laser diodes and to achieve high pump intensities along the whole laser medium. In this work the experimental setup consists of an actively cooled 0.5% Er3+:YAG single-crystal fiber which has a diameter of 800 µm, a length of 60 mm in a bi-concave cavity. The pump light is provided by a high power fiber-coupled laser diode at 1532 nm. A CW laser operation occurs at 1645 nm for a maximum output power of 12.5 W for 45.8 W of absorbed pump power. In Q-switched operation 2 mJ pulses at 1 kHz with a duration lower than 40 ns were obtained under 58 W of absorbed pump power. To our knowledge, this is the highest power ever achieved with directly grown Er:YAG single-crystal fibers in both CW and Q-switched operation.

7912-54, Session 13

Cr:ZnSe planar waveguide mid-IR laserJ. E. Willimas, D. V. Martyshkin, V. V. Fedorov, The Univ. of Alabama at Birmingham (United States); I. S. Moskalev, IPG Photonics - Mid-Infrared Lasers (United States); R. P. Camata, S. B. Mirov, The Univ. of Alabama at Birmingham (United States)

Mid-IR Cr:ZnSe bulk lasers have attracted a lot of attention due to their unique combination of optical and laser properties facilitating a wide range of potential scientific, industrial, and medical applications. Utilization of thin film waveguide geometry enabling a good thermal management and control of beam quality is a viable pathway for compact chip-integrated optical laser design. Cr:ZnSe thin films can be also promising as saturable absorbers and mode-lockers of the cavities of solid state lasers operating over 1.3-2.1 µm. We report a development of pulse laser deposition (PLD) grown Cr:ZnSe planar waveguides and first successful demonstration of mid-IR Cr:ZnSe waveguides lasing as well as passive Q-switching of the cavity of Er:YAG laser. PLD grown Cr:ZnSe waveguide were fabricated on Sapphire and GaAs substrates with chromium concentration of 10^18-10^20 cm^(-3). Room temperature mid-IR photoluminescence of Cr:ZnSe/sapphire samples was studied as a function of Cr2+ concentration. The lasing of the Cr:ZnSe waveguide was achieved at 2.6 µm under short-pulse (7 ns) 1.56 µm excitation. The



laser threshold was equal to 100 mJ/cm^2. In addition, highly doped Cr:ZnSe/sapphire sample was used for passive Q-switching of the fiber-pumped Er:YAG laser operating at 1645 nm. Passively Q-switched Er:YAG laser operated at repetition rate of 20 kHz with an output power of 100 mW and the ratio of the Q-switched to free-running output power of 67%. Passive mode-locking of near-IR Er, Tm and Ho laser cavities is another promising application of Cr:ZnSe film structures that will be also discussed at the conference.

7912-55, Session 13

Mid-IR lasing of Cr:ZnSe/As2S3:As2Se3 composite materialsD. V. Martyshkin, V. V. Fedorov, The Univ. of Alabama at Birmingham (United States) and IPG Photonics Corp. (United States); J. T. Goldstein, Air Force Research Lab. (United States); S. B. Mirov, The Univ. of Alabama at Birmingham (United States) and IPG Photonics - Mid-Infrared Lasers (United States)

Recent progress in chromium doped II-VI semiconductor materials (ZnS, ZnSe, CdSe) makes them the laser sources of choice when one needs a compact system with tunability over 2-3.6 µm. Output powers exceeding 10W and efficiency up to 70% were demonstrated in several Cr doped semiconductors (ZnS, ZnSe). A further increase of the output power requires a thorough thermal management of the active element. Among different approaches to control beam quality and thermal lensing, fiber geometry of gain element is very promising for a variety of applications. The proposed transition metal doped ZnS:ZnSe/As2S3:As2Se3 composite materials with index matching of II-VI and V-VI components represent a new way for design of mid-infrared laser active fibers. Chalcogenide glasses are of interest for mid-IR fiber-laser applications due to their wide IR transparency range and capability to vary refractive index from n=2.1 to n=2.5 enabling a pathway to refractive index optimization and matching to ZnS (n=2.26) and ZnSe (n=2.44) crystals eliminating scattering losses. From the other hand, II-VI compounds provide a tetrahedral coordination of the chromium ions required for mid-IR lasing properties. We report the first mid-IR laser active Cr: ZnSe/As2S3:As2Se3 compound fabrication and room-temperature lasing at 2.4 µm. The Cr:ZnSe/As2S3:As2Se3 composites were prepared by annealing of the appropriate compounds under vacuum and by casting and drying of Cr:ZnSe microparticles suspension in As2S3:As2Se3 propylamine solution. All samples demonstrated mid-IR photoluminescence typical for Cr2+ ions in ZnSe host. High optical gain and low passive losses in Cr:ZnSe/As2S3:As2Se3 composite material were demonstrated in random lasing experiments.

7912-56, Session 13

Mode-locking of a Cr:ZnSe laser by use of a PPLN nonlinear mirrorJ. Dherbecourt, A. Godard, A. Denoeud, J. Melkonian, M. Raybaut, M. Lefebvre, E. Rosencher, ONERA (France)

Cr:ZnSe is now widely known as a promising mid-infrared laser material thanks to its interesting properties, such as its wide emission spectrum. Until now, wide tunability (2000 nm-3000 nm) has been obtained in the CW regime, and ultrashort-pulse generation has been demonstrated from SESAM or self-starting Kerr lens mode-locking. These techniques generally require high pump power (several watts) or specific devices (SESAM based on multiple InAs/GaSb quantum wells). As an alternative approach, we demonstrate here mode-locking of a Cr:ZnSe laser using an intracavity second-order nonlinear mirror. This technique, initially proposed by Stankov, was previously demonstrated on near-infrared (Nd-doped or Ti:Sapphire) laser sources. Here, taking advantage of the efficiency and reliability of periodically-poled nonlinear crystals, we demonstrate for the first time mode locking of a Cr:Znse laser by use of a PPLN nonlinear mirror.

We will present the main characteristics of our Cr:ZnSe laser, pumped by

a CW thulium-doped fiber laser, where the intracavity nonlinear mirror is based on a PPLN crystal coupled with a dispersive YAG wedged plate to enable the fine control of the fundamental and SH waves relative phase . We will then explore the effects of different parameters (phase mismatch, output coupling...) on the laser temporal dynamics, which can result in pure or Q-switched mode-locking. We will also investigate how mode-locking can be achieved at any central wavelength by tuning the PPLN nonlinear period, enabling us to benefit from the wide Cr:ZnSe gain bandwidth. Finally, we will discuss potential improvements, especially concerning the thermal lensing management.

7912-57, Session 14

Improved characterization of laser damage threshold and transmitted wavefront error on CADB epoxy-free bonded solid state laser materialsN. Traggis, N. Claussen, Precision Photonics Corp. (United States); A. J. Bayramian, K. I. Schaffers, Lawrence Livermore National Lab. (United States)

Current state-of-the-art and next generation laser systems-such as those used in the NIF and LIFE experiments at LLNL-depend on ever larger optical elements. The need for wide aperture optics that are tolerant of high power has placed many demands on material growers for such diverse materials as crystalline sapphire, quartz, and laser host materials. For such materials, it is either prohibitively expensive or even physically impossible to fabricate monolithic pieces with the required size. In these cases, it is preferable to optically bond two or more elements together with a technique such as Chemically Activated Direct Bonding (CADB©). CADB is an epoxy-free bonding method that produces bulk-strength bonded samples with negligible optical loss and excellent environmental robustness. The authors have demonstrated CADB for a variety of different laser glasses and crystals. For this project, we propose to CADB bond several sample material sets (sapphire, quartz, and doped laser glass) and determine the suitability of the resulting assemblies for large aperture high power laser optics. The assemblies will be evaluated in terms of their transmitted wavefront error, laser damage threshold, and other optical properties.

7912-58, Session 14

Lasing properties of new Yb-doped borate compounds with varying gadolinium and yttrium concentrationI. B. Manek-Hönninger, M. Chavoutier, V. Jubera, D. Descamps, P. Veber, M. Velazquez, A. Garcia, L. Canioni, Univ. Bordeaux 1 (France)

Ytterbium doped crystals have proven to be an attractive laser medium for continuous wave lasers as well as for short pulse generation and have achieved established application in industrial laser systems. This is mainly due to the relatively small quantum defect, which limits the heating effect during the laser process, and the absorption spectrum of this ion that matches perfectly the emission wavelength of high power InGaAs laser diodes which can be used for pumping those laser crystals. Owing to the relatively large gain bandwidth depending on the host matrices Yb-doped crystals are suitable for ultrafast lasers down to the femtosecond regime.

In this contribution we will show spectroscopic and lasing properties of new ytterbium-doped borate compounds with varying gadolinium and yttrium concentrations. The studied crystals dispose the structure Li6[Gd(1-x)Yx]0.75Yb0.25(BO3)3 with x = 0, 0.25, 0.5, 0.75 and 1, respectively. All compounds were grown by the Czochralski method. We studied the absorption and emission spectra and investigated the gain properties at 1030 nm. The cw laser performances were tested in a linear cavity where the crystals were pumped by a laser diode emitting around



975 nm. All crystals were prepared as samples of about 1 mm thickness and had the same doping concentration of about 22 %. The compounds show all lasing at wavelengths around 1040 to 1060 nm with a slope efficiency of 32 %. The maximum observed output power was 460 mW at an incident pump power of 1.6 W at 972 nm.

7912-59, Session 14

Tuning solid state organic lasersS. Klinkhammer, T. Woggon, N. Heussner, M. Stroisch, C. Vannahme, T. Mappes, U. Lemmer, Karlsruher Institut für Technologie (Germany)

In the past few years, solid-state organic lasers have attracted a lot of research attention. Key benefits of organic gain materials are low-cost fabrication methods while maintaining the possibility to tailor the emission wavelength on demand due to their spectrally broad gain spectrum. Lasers relying on distributed feedback are of special interest as they emit in only a few laser modes and are thus very efficient compared to lasers based on conventional cavities or more advanced resonators. Furthermore, DFB lasers can easily be integrated into photonic circuits as coherent light sources opening the way towards fully integrated optical systems. The emission wavelength of DFB lasers can be estimated with the Bragg formula lambda_las = 2 Lambda n_eff/m.

We report on the fabrication and characterization of organic laser devices based on the organic semiconductor tris(8-hydroxyquinoline) aluminum (Alq3) doped with the laser dye 4-dicyanomethylene-2-methyl-6-(p-dimethyl-amino-styril)-4H-pyrane and based on the conjugated polymer poly(9,9 ‘-dioctylfluorene-co-benzothiadiazole) (F8BT), with respect to their tunability. For that, we apply different fabrication and tuning methods where we try to influence the different parameters of the Bragg formula dynamically. We apply numerical calculations to support our findings. Some of these tuning methods open paths towards integrated tunable and optically pumped laser sources. Possible applications for these tunable laser devices are given.

7912-60, Session 14

Compositionally tuned Nd:YxLu3-xGa5O12-laser at 935 nm for H2O-dial systemsJ. Löhring, M. Schlösser, H. Hoffmann, Fraunhofer-Institut für Lasertechnik (Germany)

For future satellite based water vapour DIAL systems efficient and rugged sources preferably around 935 nm are required. Especially for the WALES system (Water Vapour Lidar Experiment in Space) four wavelengths between 935.561 nm and 935.906 nm have to be addressed. A promising candidate for the direct generation within this spectral range is a simple diode pumped setup with compositionally tuned Nd-doped mixed garnet crystals. Within the scope of this work, novel Nd:YxLu3-xGa5O12-crystals with different compositions x were investigated. Beside the characterization of some relevant crystals properties Q-switch experiments were performed. By the R2-Z5-transition wavelength between 935.3 nm and 936.6 nm (vac.) can be addressed with different compositions. At a repetition rate of 100 Hz more than 5 mJ were extracted in longitudinal multimode around 935.7 nm. The cavity was injection seeded and stabilized with the ramp-and-fire-method to obtain single frequency radiation. At 935.7 nm more than 4.7 mJ were generated. The laser could be tuned over a range of about ± 0.22 nm in single frequency mode.

7912-61, Session 14

Diode-pumped, cryogenically cooled Yb:CaF2 for high efficient and high power laserS. Ricaud, Amplitude Systemes (France) and Lab. Charles Fabry

(France); F. Druon, Institut d’Optique Graduate School (France); D. N. Papadopoulos, A. Pellegrina, Ecole Nationale Supérieure de Techniques Avancées (France); P. Georges, Institut d’Optique Graduate School (France); A. Courjaud, Amplitude Systemes (France); P. Camy, J. Doualan, R. Moncorgé, ENSICAEN (France)

Cryogenic cooling is a very interesting and promising apparatus for high power lasers, especially with Yb-doped materials. In fact, it is now well known that operating this type of laser materials at cryogenic temperatures such as 77K (liquid nitrogen temperature) positively affects their performance, especially at high power levels, because of increased thermal conductivities and absorption and emission cross sections. In this paper, we report, to our best knowledge, the first laser operation of a singly doped Yb:CaF2 at cryogenic temperature and high power level. The laser experiments were performed with a 5-mm-long, 2.2-%Yb-doped CaF2 crystal. The crystal was positioned in a cryostat on a copper mount directly cooled at 77 K by liquid nitrogen. The laser is a V-shape cavity optimized for high-power cw laser operation. The pump is a 400-µm fiber coupled diode emitting 245 W. The laser shows very interesting performance: a laser efficiency up to 70 % versus absorbed pump power and an output power of 97 W. A small signal gain exceeding 3 and a laser wavelength tunability compatible with femtosecond pulse amplification was also demonstrated showing then the potential of this crystal for femtosecond cryogenically cooled amplifiers. We also studied the influence of the temperature on the thermal conductivity and the thermo-optic coefficients of Yb:CaF2 crystal (doped and undoped) in order to quantitatively evaluate the interest of cryogenic cooling on the heat extraction, the stress and the thermal lens.

7912-62, Session 14

High power Raman diamond laserJ. Fève, M. J. Bohn, J. K. Brasseur, K. E. Shortoff, Directed Energy Solutions (United States)

Diamond has received recent attention as an attractive gain medium for a Raman laser. We report on recent experiments using low birefringence CVD grown diamond crystals. A cryogenically cooled Yb:YAG laser has been developed as the pump beam. It delivers up to 550W at 1030nm, with an optical efficiency of 80%. The beam was diffraction limited, with M2<1.05 up to 450W. For given power of the pump diodes, Q-switched operation at 50 kHz generated the same output power, with pulses of 85 ns FWHM. The Raman laser used an anti-reflection coated 8mm-long diamond crystal in a linear cavity. Measurement of spontaneous Raman gain showed extremely good homogeneity of the crystal, with no measurable depolarization losses. Total losses were estimated to <0.5%/cm which confirms the quality of the crystal.

A first set of experiments aimed at accurately measuring the Raman gain. The threshold of the Raman laser was measured with a set of mirrors with high reflectivity at the Stokes wavelength (R=99.2% at 1193nm). The best fit of the data was obtained with the Raman gain coefficient of 12 cm/GW. The total output power of the 2 Stokes beams was 4.4W with a slope efficiency of 75%, which are the highest values reported for diamond to the best of our knowledge.

A second set of experiments is underway and will be reported in January, with detailed study of the beam quality and polarization of the Stokes beam. Based on the above measurement of the gain coefficient we designed a Raman laser cavity that should optimize the output power of the Stokes beam. Using mirrors with lower reflectivity, we predict Stokes output powers of >100W. The corresponding experiment will be reported at the conference.

7912-63, Session 15

Overview of ceramic laser technologyJ. S. Sanghera, U.S. Naval Research Lab. (United States)

The field of transparent ceramic laser materials has seen significant



improvements since the first demonstration of lasing in Dy3+:CaF2 ceramic in 1964. Considerable effort has gone into powder synthesis and purification, along with better sintering processes. Consequently lasing has been demonstrated in halide, oxide and chalcogenide ceramics. The efficiency and laser output power has been steadily increasing, especially within the last decade, to the point where 100 KW output power has now been demonstrated using Nd:YAG. In addition, the ceramic technology enables fabrication of novel architectures, including undoped claddings and graded doping profiles, as well as high doping levels which are not very practical using traditional single crystal technology.

I will present an overview of the history and current status of ceramic laser technology, including recent developments from the author’s lab.

7912-64, Session 15

Last advances in Yb3+ doped CaF2 ceramics synthesisM. S. Mortier, A. Lyberis, A. Suganuma, A. Stevenson, P. Gredin, D. Vivien, Ecole Nationale Supérieure de Chimie de Paris (France); G. Patriarche, Ctr. National de la Recherche Scientifique (France)

Recent studies have proved the high efficiency, for broadband tunable laser emission of short pulse duration generation [1,2], of Yb3+ doped CaF2 single crystal.

We are studying this compound in the ceramic form and will describe the various steps of the preparation process from the synthesis of the nanopowders by soft chemistry route to the final sintering treatment [3] leading to a transparent material. The materials have been characterized for each preparation step, from the powder to the grain and grain boundary structure of the ceramic by Scanning Electron Microscopy and Transmission Electron Microscopy. The study of the residual light scattering sources [4] has allowed the reduction of the losses. The optical properties of the ceramics will be shown to reach a high quality equivalent to single crystal.

[1] M. Siebold, M. Hornung, S. Bock, J. Hein, M.C. Kaluza, J. Wemans, R. Uecker, Broad-band regenerative laser amplification in ytterbium-doped calcium fluoride (Yb:CaF2), Appl. Phys. B 89, 543-547 (2007)

[2] M. Siebold et al. , Femtosecond pulses reach terawatt power via Yb:CaF2, Opt. Lett. 33, 2770 (2008)

[3] P. Aubry, A. Bensalah, P. Gredin, G. Patriarche, D. Vivien and M. Mortier, Synthesis and optical characterizations of Yb-doped CaF2 ceramics, Opt. Mater. 31 (2009) 750-753

[4] A. Lyberis, G. Patriarche, P. Gredin, D. Vivien, M. Mortier, Origin of light scattering in ytterbium doped calcium fluoride transparent ceramic for high power lasers, to be published

This work is supported by DGA

7912-65, Session 15

Field assisted sintering of infrared sensor windowsA. Rape, J. Singh, Applied Research Lab. (United States); S. B. Trivedi, Brimrose Corp. of America (United States); V. Shukla, R. Sadangi, Plasmadynamics, LLC (United States); N. S. Prasad, NASA Langley Research Ctr. (United States)

Infrared (IR) transmitting materials are very important in many applications involving sensors and detectors. IR windows made from such materials need to be as transparent as possible in the desired spectrum range. Since the IR window materials are commonly used in applications such as protective domes for sensors where they are subject to the ambient external conditions, mechanical, optical, thermal properties as well as corrosion resistance need to be considered in their selection..

Several materials including MgAl2O4 (Spinel) and Lutetium Oxide (Lu2O3) are good candidate materials for such applications. Effort is underway to produce nano-grained polycrystalline transparent materials that will be fabricated by the emerging Field Assisted Sintering Technology (FAST). FAST allows sintering of ceramic materials close to 100% theoretical density. This is important for achieving high IR transparency in the windows. The high heating rate associated with FAST allows retention of nano-grained structure of the starting material with minimal grain growth in sintering. This produces strong mechanical properties. Several material systems will be sintered using the FAST technique. FAST parameters will be optimized to achieve fully dense samples of the above two materials. Optical and micro-structural properties of these IR windows materials will be characterized and described in this paper.

7912-66, Session 15

Processing and transparency of polycrystalline yttrium aluminum garnet (YAG) fiber for optical applicationsH. J. Kim, UES, Inc. (United States); G. E. Fair, Air Force Research Lab. (United States); H. Lee, K. A. Keller, T. A. Parthasarathy, UES, Inc. (United States); R. S. Hay, Air Force Research Lab. (United States)

YAG has attracted copious attention for its optical and mechanical properties, and there have been needs for fiber forms of polycrystalline YAG for both laser and structural applications. Various processing routes of YAG fibers for structural applications have been explored. However, the processes and properties of the polycrystalline YAG fiber for laser applications have not been investigated intensively even though it has significant potential to be used as a host material for high power laser. In this presentation, recent results of the study on YAG fiber for laser applications will be presented and detailed relationship between processes, microstructures, and optical properties of YAG fibers will be discussed. Specifically, details of the processes for green fiber preparation, sintering methods, and transparencies depending on the processes will be shown. Our recent advancement in fiber processing prior to sintering has improved the transparency of YAG fiber significantly. In addition, vacuum or air sintering followed by Hot Isostatic Press (HIP) also enhanced transparency of the fibers comparable to that of single crystal YAG fiber.

7912-92, Session 15

Chemical mechanical polishing for YAG single crystals and ceramicsM. S. Goorsky, S. Brightup, S. Sharma, Univ. of California, Los Angeles (United States)

The effectiveness of different chemical-mechanical polishing (CMP) processes is assessed for both single crystalline and polycrystalline (ceramic) YAG materials. As-supplied mechanically polished surfaces possessed sub-micron scratches and other evidence of subsurface mechanical damage. Such defects have significant impact on the quality of bonding of such materials and the damage threshold of dielectric coatings deposited on these materials. The CMP processes remove the residual surface scratches to yield optical components with a much smoother surface with reduced subsurface damage.



Conference 7913: Laser Resonators and Beam Control XIIISunday-Tuesday 23-25 January 2011Part of Proceedings of SPIE Vol. 7913 Laser Resonators and Beam Control XIII

7913-01, Session 1

Unstable ring resonator with bidirectional propagation through the gain medium: analysisA. H. Paxton, Air Force Research Lab. (United States)


7913-02, Session 1

Analysis of frequency dependent pump light absorptionM. Wohlmuth, J. M. Werner, C. Pflaum, Friedrich-Alexander-Univ. Erlangen-Nürnberg (Germany)

Advanced simulation techniques proved to be a useful tool to improve the design of diode pumped solid state laser resonators. Here, the prediction of thermal lensing is crucial for an accurate model of the resonator performance. The thermal lens is mainly determined by the generated heat load in the crystal. Thus, besides knowing the cooling configuration, a precise description of the absorbed pump light distribution is necessary. In this paper, we provide a detailed analysis of the frequency dependency of the pump light absorption in the laser material and how it influences thermal lensing and hence the laser performance. In general, both emission of the pump light in the pump diodes and its absorption in the laser crystal strongly depend on the frequency. To model that dependency, we sample the pump light over the emission band of the pump diodes and compute the pump light absorption for that frequency range. Then, the overall absorbed pump light distribution and the total heat load are calculated as the sum of all samples. In general, this leads to an overall absorbed pump light distribution which cannot be described by an effective absorption coefficient and an exponential absorption behavior according to the Beer-Lambert law. This effect is particularly significant at pump wavelengths coinciding with a peak of absorption, e.g., 808nm or 885nm in Nd:YAG. We analyze its influence on beam radius, beam quality and output power for end-pumped solid state lasers with different pump wavelengths and pump diode characteristics.

7913-03, Session 1

Constructing petal modes from a coherent superposition of Laguerre-Gaussian modesD. Naidoo, A. Forbes, Council for Scientific and Industrial Research (South Africa); K. Ait-Ameur, Ecole Nationale Supérieure d’Ingenieurs de Caen et Ctr. de Recherche (France); M. Brunel, Univ. de Rouen (France)

The electric field distribution of a multimode laser beam may be decomposed as an incoherent superposition of Laguerre-Gaussian and Hermite-Gaussian functions. A coherent superposition of two Laguerre-Gaussian modes with the same radial order but opposite azimuthal order is achieved experimentally by inducing radial symmetry within an end pumped plano-concave resonator cavity. Radial symmetry is obtained with the use of a thin opaque disk of diameter 200 µm which is positioned in the path of the beam on the plane mirror to completely obstruct the TEM00 mode at low input pump powers. This configuration allows for higher order modes to oscillate at higher input pump powers. The intensity distribution of the output beam reveals a “petal” structure where the number of lobes in the petal corresponds

to twice the azimuthal order. The intensity distributions of these “petal” structures are numerically simulated and compared to experimental data and additionally an analytical expression for the M2 is computed and compared to the experimental result.

7913-04, Session 1

Laser sustained plasma ball lensing effect controlled by means of coaxial gas flowV. P. Zimakov, A. Y. Kedrov, V. A. Kuznetsov, A. N. Shemyakin, N. G. Solovyov, M. Y. Yakimov, Institute for Problems in Mechanics (Russian Federation)

The results of the experimental investigation of laser beam refraction in a continuous optical discharge (COD), or laser sustained plasma, stabilized in a focused (f4.4) laser beam and coaxial gas flow are presented. Visible intensity patterns of the optical image of COD and the IR intensity profiles of sustaining laser beam (M2 = 6.6) transmitted through the plasma were detected simultaneously.

The plasma ball of COD forms a kind of defocusing plasma lens where lensing effect occurs due to distributed free electrons. The properties of the plasma lens depend on the electron density distribution and may be controlled through plasma control techniques, with a gas flow for instance.

In our experiments in argon flow under atmospheric pressure considerable refraction of the sustaining laser beam (up to 0.06 rad) was achieved at lower gas flow velocities when front bound of the plasma was moved in a beam cross section where beam and plasma ball diameters were close to each other. When gas flow velocity was increased up to 1.5 and then higher up to 10-15 m/s laser beam refraction was decreased to the order of milliradians and lower. It was shown that the nature of the curve of the refraction in COD on the gas flow velocity reveals the peculiarities of the lensing properties of the plasma ball. From the other hand, plasma lensing effect control by means of the gas flow was also demonstrated in the experiments.

7913-05, Session 2

Real time laser beam analysis system for high power lasersM. Scaggs, Neoteric Concepts, LLC (United States); G. J. Haas, Haas Laser Technologies, Inc. (United States)

A laser beam analysis system, with all passive optical components, has been developed that permits the real time measurement of a high power laser beam in the tens of kilowatts which can provide the laser’s spatial profile, circularity, centroid, astigmatism and M-squared values using all the optics of a process application, including the focus lens and cover glass. At the heart of the technique is a Fabry-Perot resonator used with a focusing lens that provide a means to both attenuate and provide a multiplicity of focused laser spots each representing a spatial slice of the focused beam waist of interest onto a single CCD or CMOS camera. This arrangement provides real time data on the laser system’s beam properties and is the basis upon which this work it done. The coatings of the Fabry-Perot resonator provide a high degree of attenuation of the input beam so that thermal lensing is not a factor in the measurement. By adjusting incident angle and spacing between the mirrors of the Fabry-Perot resonator, a large number of spatial cross sections can be seen on the detector. This permits then the possibility of evaluating any focusing objective whether long or short in focal length.


7913-06, Session 2

Enhanced high-speed coherent diffraction imagingJ. Potier, S. Fricker, PhaseView (France); M. Idir, Synchrotron SOLEIL (France)

The aim of this work is to overcome the diffraction imaging problem caused by the influence of the source’s quality and the limitations of the detector in order to be compatible to a practical tabletop CDI system.

Due to recent advances in X-ray microscopy, we are now able to image objects with nanometer resolution thanks to Synchrotron beamlines or Free Electron Lasers (FEL). The PCI (Phase Contrast Imaging) is a robust technique that can recover the wavefront from measurements of only two intensity pictures in the Fresnel field. With our fast straightforward calculus methods, we manage to provide the phase induced by a microscopic specimen in few seconds. We can therefore obtain high contrast from transmittive materials at very small scales. To reach atomic resolution imaging and thus make a transition from the near to the far field, the CDI (Coherent Diffraction Imaging) technique finds its roots in the analysis of diffraction patterns to obtain the phase of the altered complex wave. Theoretical results about existence and uniqueness of this retrieved piece of information by both iterative and direct algorithms have already been released . However, performances of algorithms remain limited by the coherence of the X-ray beam, presence of random noise and the saturation threshold of the detector.

We will therefore present reconstructions of samples using enhanced versions of the three different algorithms (HIO, RAAR & Difference-Map) improving the speed of convergence and its repeatability. As a first step toward a practical X-Ray CDI system, initial images for reconstructions are acquired with the laser-based CDI system working in the visible spectrum. Discussion will be provided about empirical strategies offering best compromises between time processing and efficiency of the reconstruction.

7913-07, Session 2

Broadband operation of a single broad area diode laser by spectral beam combiningA. M. Heuer, D. Skoczowsky, A. Saghati, Univ. Potsdam (Germany); A. Jechow, Griffith Univ. (Australia); R. Menzel, Univ. Potsdam (Germany)

The operation of a single broad area diode BAL in an external cavity with a on-axis spectral beam combining geometry is presented. The priority objective of this approach was to achieve a broad spectral width for low coherence applications combined with a good beam quality and high output power.

The BAL laser chip has a gain guided structure and a 400 µm broad facet.

A grating in Littrow geometry was used as diffractive element in the external cavity. The bandwidth of the laser radiation was varied by choosing gratings with different grating periods. With a grating period of 300 lines per millimeter the broadest bandwidth of 24 nm was achieved. In addition the beam quality was improved with this setup to value of M² < 1.6 at a corresponding output power of 400 mW

7913-08, Session 2

Shack-Hartmann wavefront sensor and its problemsA. V. Kudryashov, J. Sheldakova, A. G. Alexandrov, Moscow State Open Univ. (Russian Federation)


7913-09, Session 3

Beam shaping of fiber coupled lasers for plastics processing: concepts for m-shaped beam profiles for optimized scanning processesJ. Meinschien, P. Bruns, A. S. Mikhailov, Y. V. Miklyaev, T. Mitra, LIMO Lissotschenko Mikrooptik GmbH (Germany)

Herewith, concepts are presented for generating optimized spot geometries - namely a m-shaped radial intensity distribution - for improved plastics processing. Refractive micro lenses and micro lens arrays are key components of these approaches and leading to high efficient and rugged assemblies.

Fiber coupled lasers are a versatile tool for plastics processing. In most of these applications, the fiber tip is imaged onto the work piece. Especially for plastics welding, the circular spot of the fiber tip is laterally scanned across the work piece and generates a heat impact to create a welding seam. The width and shape of such a welding seam is large influenced by the intensity distribution of the laser spot.

The spot size is well defined in these configurations by the fiber core diameter and the magnification of the process head. However, the intensity distribution of the spot is typically not specifically arranged. In most cases, a top hat intensity profile is assumed. This easily leads to nonuniform welding seams due to a parabolic heat impact across the welding seam. This crucial issue can be solved by a specific - m-shaped - radial intensity distribution of the spot at the work piece.

7913-10, Session 3

Efficient diffractive optical elements from glass with continuous surface profilesY. V. Miklyaev, M. M. Ivanenko, A. S. Mikhailov, W. Imgrunt, L. Aschke, V. N. Lissotschenko, LIMO Lissotschenko Mikrooptik GmbH (Germany)

Diffractive optical elements (DOE) are of rising importance for many industrial laser applications, especially in order to shape special light field configurations or split the beams. Typically such applications require high damage threshold, and low background illumination (high efficiency). Usual DOE with binary phase (step-like) profiles are made microlithographically and suffer from substantial scattering on profile derivative discontinuities. That gives also tendency to lower damage threshold as compared to intrinsic material values. Here the LIMO approach is based contrarily on a direct material processing and is suitable for manufacturing of high-precision free programmable continuous surface profiles in optical glasses and crystals. We report on linear symmetric diffractive glass splitter 1:11 with efficiency > 95% and discuss also other DOE designs. The design data, simulations with the measured surface profiles and experimental intensity measurements will be compared.

With similar diffractive gratings we have also designed and tested a new type of optical attenuator. It is composed from two symmetric gratings placed very close to each other. The transmission of the “open” system can be as high as 98%. By a small lateral shift between the gratings the transmission drops down to 0.3%. We have studied functional features of this attenuator and requirements on the grating profile. Because the required lateral shift is very small (5 or 10 micron in the present design) the attenuation can be very fast. The system can also be used for dynamic power stabilization or like a fast laser switch.

Conference 7913: Laser Resonators and Beam Control XIII


7913-11, Session 3

Thermal lensing compensation optics for high power lasersM. Scaggs, Neoteric Concepts, LLC (United States); G. J. Haas, Haas Laser Technologies, Inc. (United States)

Athermalization of focusing objectives is a common technique for optimizing imaging systems in the infrared where thermal effects are a major concern. The athermalization is generally done within the spectrum of interest and not generally applied to a single wavelength. The predominate glass used with high power infrared lasers in the near infrared of one micron, such as Nd:YAG and fiber lasers, is fused silica which has excellent thermal properties. All glasses, however, have a temperature coefficient of index of refraction (dn/dT) where as the glass heats up its index of refraction changes. Most glasses, fused silica included, have a positive dn/dT. A positive dn/dT will cause the focal length of the lens to decrease with a temperature rise. Many of the fluoride glasses, like CaF2, BaF2, LiF2, etc. have a negative dn/dT. By applying athermalization techniques of glass selection and optical design, the thermal lensing in a laser objective of a high power laser system can be substantially mitigated. We describe a passive method for minimizing thermal lensing of high power laser optics.

7913-12, Session 3

Adaptive extracavity beam shaping for application in nanosecond laser micromachiningR. J. Beck, J. P. Parry, J. D. Shephard, D. P. Hand, Heriot-Watt Univ. (United Kingdom)

Nanosecond lasers enable high-precision machining of a wide range of materials with a comparatively high throughput. The typical Gaussian intensity distribution generated at focus is not always ideal for the application; instead other shapes such as flat-tops (circular or square), or doughnuts can in some cases give better results. Also, other more complex beam profiles might be beneficial for surface micro structuring. In order to realise, and rapidly change between such beam shapes, we are investigating an adaptive optics approach based on using an iterative simulated annealing algorithm to control the actuators of a deformable mirror. A 37-element piezoelectric deformable mirror and a 37-element bimorph mirror were applied in an extracavity arrangement and results for nanosecond laser ablation with these systems are presented in this paper. Both devices are capable of supporting the (relatively) high average powers in the nanosecond regime (30W) and enable rapid modification of the spatial beam profile with millisecond response time typically. The dynamic nature of these elements is used to improve the quality of laser machining as presented in this paper. The results enabled by the deformable mirrors are compared to previous results using a spatial light modulator (SLM) based on a liquid crystal microdisplay. The SLM has a much higher resolution and enables complex beam shapes to be generated, however is much slower in response (75Hz). Having an active beam shaping element incorporated in a laser machining workstation adds increased flexibility and improves process control.

7913-13, Session 3

Multistage phased electro-optical planar arrays for the manipulation of high power laser beamsM. M. Ivanenko, A. Krasnaberski, A. S. Mikhailov, Y. V. Miklyaev, L. Aschke, V. N. Lissotschenko, LIMO Lissotschenko Mikrooptik GmbH (Germany)

Electro-optical (EO) components in planar form are well known as

modulators and switches for optical networking. Microscopic single-mode waveguides of these devices are conceived however for very low light intensity. LIMO works on a principally new type of planar electro-optical devices for modulation, deflection and wave-front control of powerful laser beams. They employ ultra-thin large area multimode waveguides from EO active material. As an example the results with 0.032x10x26 mm3 MgO:LiNbO3 waveguides will be presented. The waveguides are manufactured by LIMO and covered with electrodes in different configurations. Depending on the shape and arrangement of the electrodes the waveguide can perform usual EO tasks, like modulation or deflection, or acts as a phased EO array. Low control voltage (half-wave voltage 5 V in the given example) is a huge advantage of such components.

We have considered a variety of possible beam-deflection configurations with the EO waveguides. Here it is important that at certain geometrical conditions an entrance light filed is exactly reproduced at the exit of the multi-mode waveguide (analog of classical Talbot effect, will be discussed). The experimental deflector [1] contains 1 planar waveguide with triangle electrodes and 2 phased array-waveguides combined with microoptical raster-lenses. The detailed optical characteristics, estimations for maximal laser power and minimal pulse duration will be reported, as well as considerations concerning different applications.

[1] Supported by BMBF Project 13N9370 “Technologies for ultra-compact and mobile laser projection systems”

7913-14, Session 3

Laser beam alignment and diagnostics using diagnostic fluorescent safety mirrorsT. E. Lizotte, Hitachi Via Mechanics (USA), Inc. (United States)

There are a wide range of laser beam delivery systems in use for various purposes; including industrial and medical applications. Virtually all such beam delivery systems for practical purposes employ optical systems comprised of mirrors and lenses to shape, focus and guide the laser beam down to the material being processed. The goal of the laser beam delivery is to set the optimum parameters and to “fold” the beam path to reduce the mechanical length of the optical system, thereby allowing a physically compact system. In many cases, even a compact system can incorporate upwards of six mirrors and a comparable number of lenses all needing alignment so they are collinear. One of the major requirements for use of such systems in industry is a method of safe alignment. The alignment process requires that the aligner determine where the beam strikes each element. The aligner should also preferably be able to determine the shape or pattern of the laser beam at that point and its relative power. These alignments are further compounded in that the laser beams generated are not visible to the unaided human eye. Such beams are also often of relatively high power levels, and are thereby a significant hazard to the eyes of the aligner. Obvious an invisible beam makes it nearly impossible to align laser system without some form of optical assistance. The predominant method of visually aligning the laser beam delivery is the use of thermal paper, paper cards or fluorescing card material. The use of paper products which have limited power handling capability or coated plastics can produce significant debris and contaminants within the beam line that ultimately damage the optics. The use of the cards can also create significant laser light scatter jeopardizing the safety of the person aligning the system. This paper covers a new safety mirror design for use with at various UV and Near IR wavelengths (193 nm to 1064 nm) within laser beam delivery systems and how its use can provide benefits covering eye safety, precise alignment and beam diagnostics.

7913-15, Session 4

Negative-branch unstable resonator in off-axis configuration for rectangular cross-sectionsC. Pargmann, T. Hall, Deutsches Zentrum für Luft- und Raumfahrt



e.V. (Germany)

An off-axis configuration of the negative-branch unstable resonator is examined numerically and experimentally. Due to less diffraction effects such a configuration yields lower beam divergences than the standard on-axis resonator. The emphasis is placed on the application to low gain media with cross-sections of rectangular symmetry. The output coupling and the adaptation to the geometry of the gain medium are attained by a scraper. Two different scraper profiles are examined. One profile resembles to a rectangular bracket “[“ and corresponds to a shift of the optical axis away from the center of symmetry towards one of the edges. The other profile resembles to the letter “L” and corresponds to a shift of the optical axis towards one of the corners. The experiments are performed with a 10 kW class chemical oxygen iodine laser (COIL). Both scraper profiles are applied to a resonator of the same magnification. Measurements of the phase of the near field and intensity distributions of the far fields are presented. The setup using the [-shaped scraper yields a higher output coupling and therefore a lower output power and a lower beam divergence. The setup using the L-shaped scraper has the advantage, that it makes use of the complete gain medium and that the scraper is reusable for different resonator magnifications. The numerical calculations coincide very well with the experimental results and are therefore the basis for the resonator design of a COIL of the 100 kW class.

7913-17, Session 4

Adaptive optics on petawatt lasers: current performance of the Texas Petawatt LaserM. D. Martinez, The Univ. of Texas at Austin (United States)


7913-18, Session 4

Laser beam formation by adaptive opticsJ. Sheldakova, V. V. Samarkin, A. V. Kudryashov, A. L. Rukosuev, Active Optics NightN Ltd. (Russian Federation)


7913-19, Session 4

Experimental investigation of intracavity fiber array beam combiningS. A. Dimakov, S. I. Klimentiev, V. V. Lyubimov, A. Y. Rodionov, D. I. Zhuk, S.I. Vavilov State Optical Institute (Russian Federation)

The report introduces the results of experimental studies of intracavity fiber array beam combining. For the first time it was experimentally demonstrated generation of Hermite- Gaussian anti-symmetric mode TEM30 in stable super resonator for coherent combination of beams of four fiber amplifiers. Maximum number of phasing fiber channels by means of suggested method has been estimated.

7913-20, Session 4

Applying of refractive beam shapers of circular symmetry to generate non-circular shapes of homogenized laser beamsA. V. Laskin, V. Laskin, Molecular Technology GmbH (Germany)

Creating of non-circular laser spots, for example of linear, elliptical or rectangle shape, with uniform intensity profile is important in various

laser techniques in industry, scientific and medical applications. This task can be successfully solved with applying of refractive beam shaping optics of field mapping type in combination with some additional optical components. Due to their unique features, such as: low output divergence, high transmittance and flatness of output beam profile as well as extended depth of field, the refractive field mappers provide a freedom in further manipulation with intensity profile and shape of a laser beam. Typically design of refractive field mapping beam shapers has circular symmetry; therefore creating of non-circular spot shapes requires applying anamorphotic optical components (cylinder lenses, prism pairs, etc.) ahead of or after a beam shaper. As result it becomes possible to provide various combinations of spot shape and intensity profiles, for example: roof-like spot with uniform intensity in one direction and Gaussian or triangle profile in another direction, linear spots with aspect ratio up to 1:1000, elliptical spots of uniform intensity, etc. Applications include flow cytometry instrumentation, particle image velocimetry, particle size analyzing, hardening, cladding, annealing, and others.

This paper will describe some design basics of refractive beam shapers of the field mapping type and optical layouts for creating laser spots of non-circular symmetry. Examples of real implementations will be presented as well.

7913-21, Session 5

Mode localization, Q-factor, and nonuniformities of a cylinder microresonator: theory and experimentM. Sumetsky, OFS (United States)

As opposed to the whispering gallery modes (WGMs) in an optical spherical/spheroidal microresonator, the WGMs in a long and uniform cylindrical microresonator are delocalized. Therefore, a circulating light beam, which is evanescently coupled into the cylinder, eventually radiates out along the cylinder axis. However, the self-interference of such a beam can produce a strongly localized WGM. It is shown that the mode characteristic width is (alfa*betta)^(-1/2), where alfa and betta are the attenuation and propagation constants of the cylinder material. The Q-factor of this mode can be as large as the Q-factor of WGMs in a spheroidal microresonator with the same divided by 2.542. In practice, the cylinder resonators are fabricated of uncoated optical fibers which are not ideally uniform. Slow variation of the fiber radius leads to formation of WGM microresonators, which are extremely elongated along the fiber length. These resonators are modeled by spheroidal microresonators with small equatorial and gigantic axial radii. Experimentally, the resonant transmission spectrum is measured by coupling to a microfiber positioned normally to an optical fiber and connected to the light source and detector. The theoretical results explain the structure of the experimentally observed high Q-factor transmission resonances for very uniform conventional optical fibers as well as for fibers with specially introduced radius perturbations. Measurement of the resonance Q-factor as a function of microfiber position along the fiber axis is demonstrated an extremely accurate method of testing the fiber surface quality, which is important for numerous applications.

7913-22, Session 5

Ultra-high Q whispering-gallery-mode bottle microresonators: properties and applicationsD. O’Shea, M. Pöllinger, C. Junge, S. Nickel, A. Rettenmaier, Johannes Gutenberg Univ. Mainz (Germany); A. Rauschenbeutel, Johannes Gutenberg Univ. Mainz (Germany) and Technische Univ. Wien (Austria)

Recently, highly prolate-shaped whispering-gallery-mode “bottle microresonators” have attracted considerable attention due to their advantageous properties. We fabricate such resonators from standard optical glass fibers using a heat-and-pull process and show that they exhibit ultra-high quality factors in excess of 100 million in conjunction



with microscopic mode volumes of around 1000 µm³. Moreover, they offer the possibility of near lossless coupling of light into and out of the resonator using ultra-thin optical fibers. While sharing these properties with other types of whispering-gallery-mode microresonators, the bottle microresonators additionally have a simple and customizable mode structure. This enables tuning over a full free spectral range using mechanical strain as well as simultaneous coupling of two ultra-thin coupling fibers in an add-drop configuration. We present two applications based on these characteristics: In a cavity quantum electrodynamics experiment we actively stabilize the frequency of the bottle microresonator to an atomic transition and operate it in an ultra-high vacuum environment in order to couple single laser-cooled atoms to the resonator mode. In a second experiment, we show that the bottle microresonator can be used as a low-loss narrow-band add-drop filter. Using the Kerr effect of the silica resonator material, we furthermore demonstrate that this device can be used for all-optical signal processing at ultra-low powers.

7913-23, Session 5

A hybrid quantum photonic interface for solid state qubitsD. R. Englund, L. Li, J. S. Hodges, Columbia Univ. (United States); B. Shields, Harvard Univ. (United States); K. Rivoire, F. Hatami, J. Vuckovic, Stanford Univ. (United States); H. Park, M. D. Lukin, Harvard Univ. (United States)

There has been growing interest in systems of coupled quantum emitters and nanocavities to explore cavity quantum electrodynamics (QED) in the solid state. The remarkable control over the light/matter interaction in such systems would provide an ultimate optical probe to not only detect, but also control the radiative properties of emitters. However, the development of an in-situ method for scanning the emitter-cavity distance has remained a challenge. In this talk, we will demonstrate a general technique, which we term a Scanning Cavity Microscope, to deterministically couple single solid state emitters to photonic crystal nanocavities. The cavity is fabricated in a high quality semiconductor membrane and can be coupled to a range of emitters that can be localized on a surface. We demonstrate the utility of the technique on nitrogen vacancy (NV) centers in diamond, an emitter system that provides optically accessible electron and nuclear spins that make it promising as a qubit for quantum information processing. In particular, we will demonstrate spectrally selective enhancement of the fluorescence of a single NV. Enhancement factors up to a factor of seven were observed and agree with theoretical predictions. We will also describe relative positioning between the NV emitter and the cavity. Scanning in small steps at the nanometer level, we are able to map out the spatial dependence of the spontaneous emission rate (Purcell) modification. The experimental results show excellent agreement with theory. This scanning technique may be viewed as a Purcell-enhanced near-field probe that provides more information that other optical near-field probe, such as Scanning Near-Field Optical Microscopy.

7913-24, Session 5

Thermal noise in optimized dielectric mirror coatingsM. L. Gorodetsky, N. M. Kondratiev, Lomonosov Moscow State Univ. (Russian Federation)

Optical multilayer coatings of high-reflective mirrors significantly determine the properties of Fabry-Perot resonators. Thermal (Brownian) noise in these coatings produce excess phase noise which can seriously degrade the sensitivity of high-pecision measurements with these cavities, in particular in Laser gravitational-wave antennas (project LIGO), where at the current stage it is one of the main limiting factors. We present a method to calculate this effect accurately and analyze different strategies to diminish it by optimizing the coating.

Traditionally this noise is calculated as if the beam is reflected from the surface of the mirror fluctuating due to the sums of the fluctuations of each layer. However the beam in fact penetrates a coating and Brownian expansion of the layers leads to dephasing of interference in the coating and consequently to additional change in reflected phase. Fluctuations in the thickness of a layer change the strain in the medium and hence due to photoacoustic effect change the refractive index of this layer. This additional effect should be also considered. It is possible to make the noise smaller preserving the reflectivity by changing the number of layers and thicknesses of high and low refractive components. We show how this optimized coating may be constructed analytically rather then numerically as before. We also check the possibility to use internal resonant layers and optimized cap layer to decrease the thermal noise.

7913-42, Session 5

Filters and electro-optic modulators on fiber end-facesS. Meister, D. Schweda, M. Dziedzina, R. Juhre, A. Al-Saadi, B. A. Franke, Technische Univ. Berlin (Germany); B. Grimm, S. K. Schrader, Technische Fachhochschule Wildau (Germany); S. J. Benight, D. H. Bale, I. Kosilkin, L. R. Dalton, Univ. of Washington (United States); H. Eichler, Technische Univ. Berlin (Germany)

The development of new integrated opto-electronic devices for sensor applications and data communication is hardly demanded by the industry especially regarding the request for energy efficient solutions. Examples are passive and tunable optical filters as well as optical modulators. Such devices directly fabricated on the end-faces of optical fibers can provide a fast and low cost production.

A hybrid layer system can be built up to a passive Fabry-Pérot micro-cavity, where alternating dielectric high and low refractive materials are used as mirrors and a highly transparent polymer as the spacer material. The mirror design and the spacer thickness define the center operation wavelength and the filter bandwidth. Bandwidths of less than 1nm (FWHM) at a wavelength of 1550 nm could be achieved for such micro-cavities on the end-faces of optical fibers.

Enhancing the hybrid layer system by transparent conductive electrodes and by adding electro-optically active chromophores to the polymeric spacer material, the filters become tunable. Materials used for the electrodes are thin gold films (simultaneously acting as mirrors) or transparent conductive oxides (TCOs), like indium tin oxide (ITO). The oxidic electrodes have to be merged with the dielectric mirrors and the polymeric spacer. Applying a voltage to the electro-optically active polymeric spacer utilizing such electrodes, the refractive index of the spacer can be changed and therefore the resonance criteria of the micro-cavity. A shift of the transmitted wavelength occurs.

7913-25, Session 6

Spectrum engineering in whispering gallery mode resonatorsA. B. Matsko, A. A. Savchenkov, V. S. Ilchenko, D. J. Seidel, L. Maleki, OEwaves, Inc. (United States)

We show that modification of the shape of the WGM resonator surface results in modification of its spectrum. Two extremes of such a modification are the white light and single mode WGM resonators. The morphology of the resonator structure is used to select the propagating modes in the same manner as the core and cladding in an optical fiber determine the propagation of the mode in an optical fiber. We discuss how to fabricate resonators with several spectral patterns and consider two viable approaches. One is based on the fabrication of the resonator from a preform being a part of an ideal sphere. The other approach is based on the building the resonator out of an ideal cylindrical preform.



7913-26, Session 6

Octave spanning frequency combs from microresonatorsT. Kippenberg, Max-Planck-Institut für Quantenoptik (Germany)


7913-27, Session 6

Phonon lasers in cavity optomechanicsK. J. Vahala, California Institute of Technology (United States)


7913-28, Session 6

Challenges in octave-spanning and short free-spectral-range optical frequency comb generation using monolithic whispering gallery mode resonatorsY. K. Chembo, Institut FEMTO-ST (France) and Jet Propulsion Lab. (United States); N. Yu, Jet Propulsion Lab. (United States)

Optical frequency combs find applications in various areas of science and technology, such as time-frequency metrology, aerospace engineering, and ultra-low phase noise microwave and terahertz generation. Even though they are usually generated with mode-locked ultra-stable femtosecond lasers, a new paradigm has recently been proposed for the generation of these combs. It is based on the excitation of the whispering gallery mode of an ultra-high Q monolithic resonator through the Kerr effect. This communication will review the main challenges that arise from this new perspective, in order to achieve the goal of octave spanning combs whose free spectral range is the shortest possible.

7913-29, Session 7

Ultra-high-Q microcavity-based visible microlasersA. M. Armani, H. Choi, X. Zhang, C. Shi, The Univ. of Southern California (United States)

High performance optical devices and circuits are of significant interest to the fields of medical diagnostics and environmental monitoring as well as telecommunications. These optical platforms must include not only optical routing structures, such as waveguides, but also optical storage and power sources. Optical microcavities, which have very low optical loss, are uniquely suited to forming the foundation of optical buffers and integrated microlasers. Specifically, by leveraging the high optical build-up intensities in ultra-high-Q microcavities, there are several different mechanisms which can be used to form a microlaser. While previous research focused on forming microlasers which operated in the near-IR based on Raman and various doping methods, more recent results have demonstrated the ability to extend the emission behavior into the visible, with the potential to lase in the UV. These microlasers have been demonstrated using either ultra-high-Q microtoroid microcavities or microsphere microcavities, verifying the flexibility of the method. This presentation will discuss these recent results, including an analysis of the lasing behavior and the optical device characterization. This work represents one of the first examples of using ultra-high-Q microcavities as visible lasers.

7913-30, Session 7

Multiple-port directional emission whispering-gallery mode microlasersY. Huang, Institute of Semiconductors (China)

Microcavity lasers with multiple ports are suitable for light sources and optical signal processing in photonic integrated circuits. Equilateral triangular and square InGaAsP/InP microlasers connected with an output waveguide were fabricated by planar technology processes, and so square microlsers with two and four ports connected with output waveguides were fabricated recently.

Mode coupling between two whispering-gallery modes (WGMs) was predicted by finite-difference time-domain (FDTD) simulation for circular microresonator connected with an output waveguide. For a 2D microcylinder resonator with refractive index of 3.2 and radius of 10 m connected with three 2-m-wide output waveguide, we find that the TE coupled mode at wavelength of 1542.3 nm can have the mode Q factor of 6.7104 and output coupling efficiency of 0.76. Room temperature continuous-wave operation with the threshold current of 4 mA is realized for an AlGaInAs/InP microcylinder laser with the radius of 10µm and a 2-µm-wide output waveguide.

7913-31, Session 7

Coupled microcavity resonators and lasersL. Xu, Fudan Univ. (China)

Whispering gallery mode resonators are high Q cavities that are promising and universal cavity structures for laser generation from UV to infrared. Coupled whispering gallery mode resonators on the other hand have drastically altered emission and spectroscopic properties, allowing for example uni-directional and single-frequency generation. In this talk, I will describe our works on coupled microcavity resonators and lasers.

7913-32, Session 7

On-chip programmable RF waveform generation with microring resonator arraysM. Qi, Purdue Univ. (United States)

The ability to generate high-frequency and ultrabroadband radio-frequency (RF) waveforms may lead to new opportunities and significant performance enhancement in applications ranging from high-bandwidth secure wireless communication to pulsed radar. Photonics means of RF waveform generation, e.g., by spectral shaping and frequency-time mapping, can overcome the bandwidth limit in electronic generation. However, the bulk optics setup does not offer integration advantage of electronics. In this talk, we first present radio frequency arbitrary waveform generation (RFAWG) with a programmable spectral shaper on a silicon chip, followed by frequency-time mapping. We then show that the functionality of frequency-time mapping can also be integrated onto the chip, by using thermally tunable all-pass microring filters as tunable delay lines.

7913-33, Session 7

Lasing in high-Q conical polymeric microcavitiesT. Grossmann, S. Schleede, M. Hauser, Karlsruher Institut für Technologie (Germany); M. B. Christiansen, Technical Univ. of Denmark (Denmark); C. Vannahme, C. Eschenbaum, S. Klinkhammer, T. Beck, J. Fuchs, G. U. Nienhaus, U. Lemmer, Karlsruher Institut für Technologie (Germany); A. Kristensen,



Technical Univ. of Denmark (Denmark); T. Mappes, H. Kalt, Karlsruher Institut für Technologie (Germany)

In recent years, whispering gallery mode (WGM) microcavities with high quality factors (Q factors) have emerged as highly versatile objects with interesting properties within the field of microoptics. Polymeric microcavities, for example, enable straightforward integration of additional gain media to the cavities, offering new possibilities for realisation of optically pumped, low-cost coherent light sources on a chip. Due to easy doping of polymers with various dye molecules, which exhibit high internal quantum efficiencies, lasing wavelengths can in principle be chosen throughout the whole visible spectrum.

We report on lasing in conical microcavities, which are made out of the low-loss polymer poly (methyl methacrylate) (PMMA) doped with the dye rhodamine 6G, and directly generated on silicon. Including a thermal reflow step during fabrication results in significantly reduced surface roughness, resulting in low scattering losses of the WGMs. The high cavity Q factors (above two million in passive cavities) in combination with the large oscillator strength gain material enable lasing threshold energies as low as 3 nJ, achieved by free-space excitation in the quasi-stationary pumping regime. Lasing wavelengths are detected in the visible wavelength region around 600 nm. Finite element simulations indicate that lasing occurs in fundamental TE/TM cavity modes, as these modes have - in comparison to higher order cavity modes - the smallest mode volume and the largest overlap with the gain material. In addition we investigate the effect of dye concentration on lasing wavelength and threshold and explain the observations using a modified standard dye laser model.

7913-34, Session 7

Experimental demonstration of light transmission property through a tapered fiber embedded in scattering nanoparticlesH. Fujiwara, T. Ikeda, K. Sasaki, Hokkaido Univ. (Japan)

Wavelength-scale-disordered structures (random structures) have recently attracted attention as low-cost and easily-fabricated resonators, because of their potential for photon localization due to the interference of multiply scattered light. However, since the randomness also makes difficult to induce intended modes within the structure, unlike the conventional (periodic) microcavity structures, technological developments are essential for their applications. In order to achieve mode control even in random structures, we have numerically proposed a simple structure for manipulating resonant conditions in a random structure, in which a “defect” region without scatterers is deliberately made. For the experimental verification of our proposed method, we examined light transmission properties of a waveguide defect embedded in a random structure. For this purpose, we prepared a tapered fiber as a waveguide defect, which waist was partly covered by ZnO nanoparticles as scatterers. From the experimental results, we found that the transmission band of a prepared sample was clearly observed and the frequency position of the transmission band was well corresponding to the transmission dip of a ZnO nanoparticle film. Since the behaviors of observed transmission property qualitatively matched to numerical results, we concluded that surrounding scattering nanoparticles confine photons at specific frequency band, which is determined by the size and refractive index of surrounding scattering nanoparticles, and the photons at the specific frequency band can transmit the sample. Thus, we considered that these experimental results suggested the possibility that mode control can be achieved by our proposed method.

7913-35, Session 7

Unstable resonators based on gain guiding in photonic bandgap waveguidesT. Her, L. Zhao, Y. Zhou, X. Ao, L. W. Casperson, The Univ. of North Carolina at Charlotte (United States)

Gain guiding in an index-antiguided waveguide can be regarded as a guided-wave unstable resonator due to its intrinsic leaky nature. By providing suitable gain, robust single transverse mode operation with large mode area in waveguides can be realized. Pure index-antiguiding, however, is not compatible with optical end pumping. We have proposed gain guiding in photonic bandgap waveguides to provide simultaneous confinement of laser and pump radiations, in which the signal is gain-guided a low-index core whereas the pump is guided by the photonic bandgap. We theoretically analyze several design configurations of gain-guided photonic crystal waveguides to be used for large-mode-area laser amplifiers with single-transverse-mode operation. As an example, we adopt the absentee-layer (half-wave) condition in thin film optics to the high-index layers in the Bragg cladding such that they have no effect on the propagation modes at the signal radiation. Under this condition, the low-order modes of the signal in the Bragg fiber and the corresponding IAG fiber have nearly identical propagation constants and mode fields, as well as modal gains when gain guided. The loss at the signal radiation can then be solely determined by the index contrast between the core and the low-index layers of the Bragg cladding, while the high-index layers are used to optimize the photonic bandgap at the pump radiation. This half-wave condition is expected to substantially simplify the design of gain-guided Bragg waveguides, which hold promise for high-power laser amplifiers and oscillators with robust single transverse mode and large mode area.

7913-36, Session 8

Optical resonators for nanomanipulation: from single molecule analysis to directed assemblyD. Erickson, Cornell Univ. (United States)

Recently, we have demonstrated how the electromagnetic fields in optical resonator devices are sufficiently strong that they can be used to physically manipulate biological (nucleic acids & proteins) and non-biological (nanoparticles) materials as small as a few nanometers in size. This represents as much as a 100 fold improvement over the state of the art and has the potential to open new area of materials science in which complex hybrid nanostructures with arbitrary properties and composition can be created at will and single molecule analytics in which small molecules can be held in place and observed for long periods of time. In this talk I will discuss our recent efforts on the design, fabrication and operation of 1D optical resonators as applied to these operations.

7913-37, Session 8

On-chip single nanoparticle detection and measurement using ultrahigh-Q whispering gallery microresonatorsL. Yang, J. Zhu, S. K. Ozdemir, L. He, W. Kim, Y. Xiao, L. Li, D. Chen, Washington Univ. in St. Louis (United States)

Ultra-sensitive and label-free chemical and biological sensing devices are of great importance to biomedical research, clinical diagnostics, environmental monitoring, and security and defense applications. Optical sensors based on ultra-high-quality Whispering-Gallery-Mode (WGM) micro-resonators, in which the light-matter interactions are significantly enhanced, have shown great promise in achieving compact sensors with high sensitivity and reliability. However, traditional sensing mechanisms based on monitoring the frequency shift of a single resonance faces challenges since the resonant frequency is sensitive not only to the sensing targets but also to instrumental noise and many types of disturbances in the environment, such as temperature variation, mechanical instability and irrelevant molecules. The resonator frequency is also affected by the positions of sensing targets on the resonator. Thus, it is difficult to avoid ‘false signals’ in the detection. We report a novel sensing mechanism based on mode splitting, a phenomenon resulting from the interaction of nanoparticles and an ultra-high-quality WGM. In



particular, we demonstrated that a single nanoparticle can ‘split’ a high-quality optical mode in a microtoroid resonator into two modes, which reside in the same resonator, experience the same noise and form a self-referencing scheme more immune to noise. The combined information from both modes allows us to develop a position-independent sensing scheme to accurately derive the size information of the nanoparticle. So far we have been able to achieve position-independent detection and accurate sizing of single nanoparticles down to 30 nm in radius with a single-shot measurement using an on-chip high-quality WGM resonator for the first time.

7913-38, Session 8

Microring and microdisk resonator-based devices for on-chip optical interconnects, particle manipulation, and biosensingA. W. Poon, S. Feng, H. Cai, T. Lei, H. Chen, X. Luo, Hong Kong Univ. of Science and Technology (Hong Kong, China)

In this paper, we will report our latest progress in experimentally demonstrating on silicon chips (i) data format conversion for clock recovery at GHz-speed using microring resonators, (ii) optical manipulation of microparticles using microring resonators, and (iii) fluidic refractive index sensing using coupled-microdisk resonator optical waveguides.

For on-chip optical interconnects, data format conversion and clock recovery at GHz-speed are of interest. We will discuss our recent experiments using a feedback-coupled microring resonator-based notch filter to convert non-return-to-zero (NRZ) data format to pseudo-return-to-zero (PRZ) data format, and using an off-chip electro-absorption modulator and an electrical phase-locked loop to proof the concept of GHz-speed clock recovery. We will discuss design tradeoffs of on-chip integrated microring resonator-based filters and modulators for clock recovery applications.

On the lab-on-a-chip application front, optical manipulation of microparticles on an optofluidic chip is gaining research interest. We will discuss our recent demonstration of sorting and buffering of 1-micron-sized polystyrene particles in water using a silicon nitride microring resonator-based add-drop filter operating in 1550-nm wavelengths.

Moreover, fluidic refractive index sensing is of the essence toward on-chip label-free biosensing. We will discuss our recent experiments using coupled-microdisk resonator optical waveguides as fluidic refractive index sensors. We will extract the refractive index change of the fluidic cladding by imaging the out-of-plane light scattering patterns of the coupled-microdisk array at a fixed wavelength onto a charge-coupled-device (CCD) camera. Prospects of extending the refractive index sensing technique for micro-particles sensing and sizing will be discussed.

7913-39, Session 8

Characterization of high index microsphere resonators in fiber-integrated microfluidic platformsO. Svitelskiy, D. Sun, A. Darafsheh, V. N. Astratov, The Univ. of North Carolina at Charlotte (United States)

Fiber-integrated microfludic systems are widely used for developing whispering-gallery mode (WGM) sensors and other optical devices. However, immersion of glass or plastic cavities in a liquid medium due to the low contrast of refractive indices causes a dramatic decrease of the WGM Q-factors, thus calling for usage of larger size microspheres, which is not desirable in many practical cases. In this work we show that one of the possible solutions to this problem can be found in using microspheres with high values of refractive indices. In particular, we studied coupling to WGMs for a large set of microspheres with sizes of 5-50 micron made of various materials with index varying in a broad

range from 1.5 to 2. In order to determine the best coupling conditions, the diameter of the fiber tapers varied in the range from one to ten micrometers. Measuring spectra of the light transmitted through the tapers, we observed asymmetric line shapes due to the Fano interference between the resonantly coupled WGMs in spheres and the coherent background. The strength of the coupling between the cavity and the microfiber taper is shown to depend on the contact position of the microsphere along the taper and on the refractive index contrast between the microsphere and the liquid environment. We demonstrate that barium titanate glass microspheres with index of 1.9 are highly promising for developing sensor applications and for studying novel resonant light pressure effects in such microfluidic systems.

7913-40, Session 8

Rotation sensing using ring resonatorsM. J. F. Digonnet, Stanford Univ. (United States)

In recent years, several configurations of coupled ring resonators have been proposed to detect rotation. Theoretical studies conducted by different research groups have concluded that these structures offer tremendous enhancement in rotation sensitivity over conventional optical gyroscopes. In this presentation, we provide physical and theoretical arguments showing that these conclusions are incorrect. Coupling resonators together does not improve the sensitivity to rotation over a single-ring resonator, such as the classic resonant fiber optic gyroscope (RFOG). This presentation includes a discussion of a relatively new structure, in which distributed instead of local coupling takes place between adjacent resonators.

7913-41, Session 9

Fabrication of horizontal-slot disk resonators and their applicationsS. Lee, J. H. Shin, KAIST (Korea, Republic of)

SiN microdisks with 40 nm thick, 2 um deep air slot around its circumference is fabricated. The slot is thin enough such that the two disks are tightly coupled, and supports only a single vertical mode, while it is deep enough such that the center spacer does not affect the whispering gallery mode. Due to the “slot” effect, the resonator concentrates as much as >20% of its optical mode in its slot region. Applying this structure for bio-sensing, a surface sensitivity exceeding 5nm/nm is achieved demonstrating its potential for bio-sensing applications.

7913-43, Session 9

Polarization-purity spectra of a tapered-fiber-coupled microsphere cavity system at cryogenic temperaturesM. Fujiwara, A. Tanaka, K. Toubaru, H. Zhao, H. Takashima, S. Takeuchi, Osaka Univ. (Japan) and Hokkaido Univ. (Japan)

An ultrahigh-Q optical micro-sphererical or micro-toroidal cavity coupled with a tapered fiber is an ideal system for the cavity quantum electrodynamics (cavity-QED) study. In particular, coupling these microcavities and tapered fiber at cryogenic temperatures is vitally important for realizing strong coupling between the atom-like system and the cavity, vibrational mode cooling, and photonic quantum gates. Such systems have been recently demonstrated at cryogenic temperature by a few groups including ours [1]. The tapered-fiber coupled micro-resonator system at cryogenic temperature suffers from mechanical vibrations due to cooling systems, or from distortions caused by large temperature change. These factors may cause the degradation in polarization of probe light field in the tapered-fiber-coupled microcavity system, which is crucial for cavity-QED experiments.



Here we report the analysis on the degree of polarization (DOP; classical analogue of polarization purity) in a tapered-fiber-coupled microsphere cavity at cryogenic temperature. By scanning the wavelength of the probe light at around 637 nm, which is near the zero-phonon line of the NV- centers in diamond, polarization purity spectrum was obtained for this microsphere system at temperatures ranging from 8 to 30 K. We have found that the DOP at off-resonance frequencies kept high and stable against the temperature change, while the DOP on-resonance drops and fluctuates under the temperature change.

[1] Takashima et. al., Opt. Exp. vol. 18, 15169 (2010).

7913-44, Session 9

FBG-nano-cavity on an optical nanofiberK. P. Nayak, The Univ. of Electro-Communications (Japan); K. Nakajima, National Institute for Materials Science (Japan); F. L. Kien, The Univ. of Electro-Communications (Japan); H. T. Miyazaki, Y. Sugimoto, National Institute for Materials Science (Japan); K. Hakuta, The Univ. of Electro-Communications (Japan)

We introduce the fabrication of periodic nano-grooves on a sub-wavelength diameter silica fiber, an optical nanofiber, using focused ion beam milling. Such structures induce rather strong modulation of refractive index (~ 1%) in contrast to the conventional fiber Bragg gratings (FBGs). And we can realize excellent single mode FBGs with more than 90% reflectivity for only small number of periods (~ 100 Periods). Using such FBG structures we have realized nanofiber cavity system. The typical finesse of such nanofiber cavity is F ~ 20 and the on resonance transmission is ~ 30% which may be mainly limited by propagation loss in the cavity. Moreover the structural symmetry of such FBGs results in polarization selective modes in the nanofiber cavity. Due to the strong transverse confinement of the field in the guided mode of the nanofiber even with such low finesse strong enhancement of the spontaneous emission of atoms around the nanofiber cavity can be realized. Such “atom + nanofiber cavity” system can become a promising workbench for cavity-QED and quantum non-linear optics and will find various applications in quantum information technology. Apart from atoms, solid-state quantum emitters like quantum dots or diamond nano-crystals can also be implemented.

7913-45, Session 9

Periodical focusing of light in chains of microspheres in the limit of geometrical opticsA. Darafsheh, V. N. Astratov, The Univ. of North Carolina at Charlotte (United States)

In our previous work we observed that the periodical focusing of light in chains of 3 µm polystyrene spheres leads to formation of progressively smaller sizes of the focused beams reaching the diffraction limited dimensions. Due to a possibility of integration with multimodal flexible waveguides and fibers such structures can find applications in developing novel focusing microprobes for laser-tissue surgery and nanoscale photo-patterning.

In this work, we studied this effect of periodical focusing of light using a ray tracing code ZEEMAX EE for a variety of microspherical chains with spheres’ indices from 1.35 up to 3.5. We show that the use of a spherical light source with the size matching the size of the spheres in chain leads to scaling of the electromagnetic intensity distributions as a function of the sphere sizes. It is shown that there is a narrow range of spheres’ indices around 1.78 where the effect of “tapering” of focused beams is well pronounced and the attenuation of the total transmitted intensity experiences a local minimum. We explain this effect by a topologically controlled resonance when the period of focusing modes, so called “nanojet-induced modes”, is equal to the size of two spheres. The results of numerical ray tracing were found to be in a very good agreement with

measurements of the focused beams in chains of macroscopic (diameter >>10 wavelengths) spheres. We also show that for mesoscopic (i.e. 4-6 wavelengths) spheres, the description of the chain-focusing problem requires a rigorous vector-Mie solution.

7913-46, Session 9

Perturbation of whispering modes of magnetorheological polydimethylsiloxane spheres by external magnetic fieldT. Ioppolo, M. V. Otugen, Southern Methodist Univ. (United States)

Magnetorheological Polydimethylsiloxane (MR-PDMS) spheres are investigated as possible micro-optical resonators. The effect of an external magnetic field on the morphology dependent resonance (MDR) of these optical resonators is studied. The magnetic pressure acting on the surface of the (MR-PDMS) sphere induces deformation, causing a shift in the MDR. Magnetically polarizable particles are mixed in with the PDMS when it is in liquid form and cured to solidify into a nearly spherical shape of several hundreds of micron diameter. The sphere is then coated with a thin layer of pure PDMS with thickness of several microns. Light from a tunable laser is tangentially coupled into this outer layer using a tapered single mode optical fiber in order to excite the optical modes of the sphere. An analysis is carried out to estimate the WGM shift induced by the applied magnetic field. Experiments are also carried out to demonstrate the magnetic-field induced WGM shift in a MR-PDMS resonator. The results show that the MR-PDMS resonator can be used as high Q-factor tunable optical cavities with potential applications in sensing.

7913-51, Session 9

Heterogeneously integrated microdisk lasers for optical interconnects and optical logicP. Mechet, L. Liu, R. Kumar, K. Huybrechts, T. Spuesens, G. Roelkens, Univ. Gent (Belgium); E. Geluk, T. de Vries, Technische Univ. Eindhoven (Netherlands); P. Regreny, Ecole Centrale de Lyon (France); D. Van Thourhout, R. G. Baets, G. Morthier, Univ. Gent (Belgium)

Optical interconnect and optical packet switching systems could take advantage of small footprint, low power lasers and optical logic elements. Microdisk lasers, with a diameter below 10 µm and fabricated in InP membranes with high index contrast, offer this possibility at the telecom wavelengths. The lasers are fabricated using heterogeneous integration of InP membranes on silicon-on-insulator (SOI) passive waveguide circuits, which allows to combine the active elements with compact, high-index contrast passive elements.

The lasing mode in such microdisk lasers is a whispering gallery mode, which can be either in the clockwise or counter clockwise direction, or in both. The coupling to the SOI wire waveguides is through evanescent coupling. Predefined, unidirectional operation can be achieved by terminating the SOI wires at one end with Bragg gratings.

For all-optical flip-flops, the laser operation must be switchable between clockwise (CW) and counter clockwise (CCW), using short optical pulses. Unidirectional operation in either direction is only possible if the coupling between CW and CCW direction is very small, requiring small sidewall surface roughness, and if the gain suppression is sufficiently large, requiring large internal power levels.

All-optical flip-flops based on microdisk lasers with diameter of 7.5µm have been demonstrated. They operate with a CW power consumption of a few mW and switch in 60ps with switching energies as low as 1.8fJ. Operation as all-optical gate has also been demonstrated. The surface roughness is limited through optimized etching of the disks and the large internal power is obtained through good heat sinking.




Hyper coherent linewidth control for a 30mW 405nm visible laser diode by delayed self-heterodyne beatM. Matsumura, T. Kuromori, W. Sasaki, Doshisha Univ. (Japan)

We have demonstrated a hyper coherent spectral linewidth evaluation for a frequency stabilized 30mW, 405nm GaN violet laser diode (LD) based on the delayed self-heterodyne beat. The laser light source was stabilized to a reference confocal Fabry-Perot (CFP) cavity by negative electrical feedback to the injection current of the LD under the Pound-Drever-Hall technique. In addition, by introducing optical feedback from another tilted CFP cavity, the spectral linewidth has been efficiently narrowed. In this scheme, we have achieved 1.65X10^-11 estimated with Allan variance for the feedback error signal from CFP cavity. We also measured the linewidth directly by a separate FP interferometer, which resulted in close coincidence with above Allan variance estimations. In this work, we have further measured our narrowed linewidth by the delayed self heterodyne technique which consists of optical fiber of about 30km length to have delay time comparable to the coherence time of our stabilized violet laser. As a result, the reduced linewidth under the double feedback conditions has been estimated to be narrower than at least several hundreds kHz, which seems to be the resolution limit of our measurement. In conclusion, we have achieved a practical and inexpensive linewidth control for a high poewr violet LD to attain hyper coherent conditions of the semiconductor lasers.


Laser beam delivery for industrial machinesC. I. Isarie, C. Oprean, V. D. Petrescu, F. T. Popescu, C. Bokor, S. Itu, A. Gligor, I. V. Isarie, Univ. Lucian Blaga din Sibiu (Romania)

Modern industrial equipment use rigid classic or modern flexible beam delivery systems for the light energy from source to the processing heads, which focus the light in order to process the metallic materials at the industrial level.

Classical beam delivery systems of optic energy are usually of Gaussian type, especially those who are low order. These classic models often apply to industrial machines, which works on infrared radiation.

The analysis will contain the beam divergence, spatial coherence, and radiation variations parameters. There will be some analytical solutions (approximations) because they can be easily transformed into numeric solutions and processed by modern informatics systems.

The used models are plane wave, spherical wave and group of waves (beam). Also the measuring systems are direct measuring methods, coherence measurement method and modal measurement method.

The modern flexible beam delivery systems use as delivery environment the optical fibers and superior orders, using doubling and tripling techniques, like SHG and THG, for the frequency of the light energy.

There will be a comparison between classical and modern delivery methods, establishing specific industrial applications for each beam delivery method along with its characteristics and parameters.


About the phenomenon produced by the successive jumps of the peripheral electrons, at the absorption of the intense photon beam by the metalC. I. Isarie, C. Oprean, I. Marginean, T. Nemes, I. V. Isarie, C. Bokor, S. Itu, Univ. Lucian Blaga din Sibiu (Romania)

When a photon beam is in impact with a metal, the peripheral electrons which belong to the bombarded material are made jumps, and in the same time, new photons are absorbed by electrons which had not time to come back to the fundamental levels.

At a high level concentration of the radiant energy, a peripheral electron, could sequentially absorb more photons and could realize energetic jumps in successive phase, equivalent with some photons of high energy which have wave-length smaller than the incidental photons.

After some successive photon absorption of the same electron, in the interval in which it is not activated by new photons, the electron comes back to the fundamental level and delivers the accumulated energy, in photons of higher energy, which have a lower energy than the incident beam.

Coming back to the fundamental level, the electrons disturb the electronic cloud of the atom or ion they belong. After a huge number of such phenomenon the electronic cloud which is successively disturbed, produces an oscillation which rises the temperature of the nucleus.

The authors have studied the conditions which generated the rise of temperature and multiple radiations at the place where the photons bombard the metal.


Intra-cavity decomposition of a dual-directional laser beamD. Naidoo, A. Forbes, Council for Scientific and Industrial Research (South Africa); K. Ait-Ameur, M. Fromager, Ecole Nationale Supérieure d’Ingenieurs de Caen et Ctr. de Recherche (France)

Higher order modes oscillating within a laser resonator cavity are dual-directional and the electric field distribution of these modes may be described by Hermite-Gaussian functions and Laguerre-Gaussian functions for rectangular symmetric and circular symmetric beams respectively. The dual-directional oscillation is viewed as a composition of a forward oscillating beam and a backward oscillating beam for any stable cavity. Fox and Li present a detailed theoretical analysis to determine the propagation of a mode in each of these directions and this theory is compared to the contrasting Laguerre-Gaussian decomposition present in literature. The separation of a laser beam into the respective directions is achieved experimentally by introducing a mirror of near perfect reflectance into the cavity. The resonator cavity is configured in an L shape, where the high reflecting mirror is positioned at 45˚ to the resonators mirrors on the apex of the L. The high reflecting mirror transmits about 0.2 % of any incident beam and this allows for convenient sampling outside the cavity to infer the propagation of the laser beam within the cavity. We present experimental data on the intra-cavity propagating field for each direction and discuss the implications on the methods used to model laser resonators.



Conference 7914: Fiber Lasers VIII: Technology, Systems, and ApplicationsMonday-Thursday 24-27 January 2011 • Part of Proceedings of SPIE Vol. 7914 Fiber Lasers VIII: Technology, Systems, and Applications

7914-01, Session 1

Thulium fiber laser lithotripsyN. M. Fried, The Univ. of North Carolina at Charlotte (United States)

Introduction: The Holmium:YAG (Ho:YAG) laser is the most popular laser lithotripter. However, the Thulium Fiber Laser (TFL) wavelength (Wavelength=1908 nm) more closely matches a water absorption peak in tissue than Ho:YAG (Wavelength=2120 nm), making TFL more efficient for vaporization of urinary stones. The excellent TFL spatial beam profile also allows coupling of higher laser power into smaller optical fibers for lithotripsy procedures that require extreme fiber bending, and opens up space through the working port of the flexible endoscope for improved irrigation, visibility, and safety. This study compares Ho:YAG laser and TFL for lithotripsy.

Methods: Stone ablation thresholds, vaporization rates, and retropulsion were measured. Human stones were used for ablation studies and Plaster-of-Paris phantoms for retropulsion studies. A 20-Watt, clinical Ho:YAG laser was used (Wavelength=2120 nm, Pulse energy=35-1000 mJ, Pulse Duration=350 microseconds, Pulse rate=10 Hz). A 100-Watt TFL was modulated to operate in pulsed mode (Wavelength=1908 nm, Pulse energy=5-70 mJ, Pulse duration=500-1000 microseconds, Pulse rate=10-400 Hz). Laser energy was coupled into 100-365 micrometer fibers.

Results: The stone ablation threshold for TFL was five times lower than for Ho:YAG laser, translating into TFL stone vaporization rates five times higher than for Ho:YAG laser at low pulse energies (35-70 mJ). The TFL produced efficient stone removal with minimal stone retropulsion at pulse rates <150 Hz, while Ho:YAG laser was limited to operation at 10 Hz and stone retropulsion increased linearly with pulse energy.

Conclusions: Development of affordable, high-pulse-energy, long-pulse, Thulium fiber lasers is warranted as an alternative to Holmium:YAG lasers for lithotripsy.

7914-02, Session 1

Application of supercontinuum sources in spectro-radiometric characterization, calibration, validation, and performance testingJ. P. Rice, S. W. Brown, J. T. Woodward, H. J. Patrick, C. J. Zarobila, K. R. Lykke, National Institute of Standards and Technology (United States)

Broadband fiber sources are replacing conventional lamps in test applications requiring either tunable monochromatic light or spectrally-profiled broadband light. The fundamental reason is that, for a given spectral resolution, the coupling of the source light through the monochromator is much more efficient for fibers than for lamps. Thus high brightness fiber sources enable achievement of spectral power density approaching a goal of 1 mW/nm at 5 nm resolution. We will review several spectrally-resolved testing applications at NIST where the use of such a supercontinuum source can provide a performance advantage over lamps. One area is spectral, spatial, and radiometric characterization of spectrally-resolved sensors, using either a spectrally tunable source or a hyperspectral image projector (HIP). Another area is spectrally-resolved measurements of the optical properties of materials. Common requirements for improved sources beyond the present state of the art are: 1) attainment of 1 mW/nm over a broader spectral range, especially below 450 nm and above 2200 nm, 2) higher stability, and 3) flatter spectra.

7914-03, Session 1

Material micromachining using bursts of high-repetition rate picosecond pulses from a fiber laser sourceA. Cournoyer, M. Drolet, L. Desbiens, D. Lemieux, M. Briand, Y. Taillon, INO (Canada)

In this paper, we demonstrate the benefits of using bursts of picosecond pulses for material micromachining and compare the results with those obtained when using a nanosecond source with similar pulse energy, pulse width and pulse shape. The picosecond laser source used for the experiments was delivering 60-ps pulses at a repetition rate of 1.8 GHz, grouped within arbitrarily-shaped bursts having a width that could be varied from 2.5 to 80 ns. For the experiments, the burst repetition rate was fixed at 100 kHz. The laser output central wavelength was at 1064 nm and the output beam M2 value was below 1.15. Micro-milling experiments were performed on silicon and fused silica. For silicon, we show that the maximum material removal efficiency can be increased by more than 10% when using bursts of picosecond pulses with respect to nanosecond pulses with similar energy per pulse. Although fused silica is transparent at 1064 nm, we also demonstrate the feasibility of laser micro-milling using bursts of picosecond pulses, taking advantage of the peak power that is significantly higher within a picosecond burst as compared to a nanosecond pulse of the same energy, shape and width. Effect of shaping the burst envelope of the picosecond laser on the maximum material removal efficiency is also presented.

7914-95, Session 1

Dynamic pulsing of a MOPA fiber laser for processing of thermally sensitive materialsS. T. Hendow, J. R. Salcedo, P. T. Guerreiro, J. M. Sousa, Multiwave Photonics (Portugal)

A MOPA pulsed fiber laser is configured to produce very fast bursts of nanosecond pulses, at any pulse repetition frequency from single shot to 500 kHz. Each pulse burst is composed of any sequence of stored pulses of arbitrary pulse widths and separation, each pulse width can range from 10 to 200ns, and pulse separations are typically few 10s of ns. Pulse bursts are shown with pulse sequences and separation that offer high peak power, as compared to an equivalent single pulse of similar pulse energy and overall pulse width.

A group of pulses, each is 10 ns wide and separated by 50 ns, is applied to thermally sensitive materials, such as silicon, steel, ceramics and rubber, and the results are compared to those by a single pulse of equivalent pulse energy. Benefits of Dynamic Pulsing include reduced HAZ and controlled ablation and oxidation of materials, as well as surface marking with reduced penetration.

The operating wavelength is 1064 nm, maximum average power is 20W and maximum pulse energy is 0.7 mJ per pulse burst. Individual pulses are 10 to 200 ns wide.

An alternate configuration to Dynamic Pulsing is also shown, where the MOPA laser is synchronously triggered on-the-fly at repetition frequency from single shot to 500 kHz. Emitted pulses are preselected from any number of 32 internally stored pulse shapes or widths. An example of such emission is shown applied to silicon where the pulse sequences alternate between two pulses of different pulse widths and peak powers.


7914-05, Session 2

High-power 2-micron single-frequency fibre laser sourcesM. Ibsen, Univ. of Southampton (United Kingdom); Z. Zhang, Heriot-Watt Univ. (United Kingdom); L. Pearson, J. W. Kim, J. Sahu, W. A. Clarkson, Univ. of Southampton (United Kingdom)

The past years almost relentless increase in the output power from fibre lasers has in part been facilitated by the availability of higher power pump diodes. Whereas the power from fibre lasers in general now is well into the kW regime and continuing to mount due to this availability of pump power and ever more ingenious ways of getting the pump power coupled to the gain medium, maintaining single-frequency and narrow linewidth operation at any significant power level pose challenges. Despite this, the power level for fibre lasers with truly single-frequency output is on the increase, and currently it is at the Watt level. With additional relatively simple post amplification steps this level has now broken the 100W barrier. One area that has attracted a lot of attention recently is the region around 2-micron where a number of exciting new opportunities are emerging for higher power single-frequency sources. We will in this presentation review the current status of high power single-frequency 2-micron fibre laser sources, and discuss design aspects and applications related to advances in their performance. We will also discuss routes for further power-scaling.

7914-06, Session 2

1kW cw Yb-fiber-amplifier with less than 0 5GHz linewidth and near-diffraction limited beam-quality, for coherent combining applicationD. Engin, W. Lu, M. Akbulut, B. McIntosh, H. R. Verdun, S. Gupta, Fibertek, Inc. (United States)

High power, single-frequency, single spatial mode operation of cw fiber amplifiers is limited by the onset of nonlinearities such as Stimulated Brillouin Scattering (SBS). SBS can be suppressed by increasing the fiber modal effective area and by phase-modulating the input seed source. Current kW-class systems with diffraction-limited output use fibers with core size less than 25um, and need phase modulation bandwidths of several GHz. This necessitates active path length matching of multiple amplifier chains, and also makes it difficult to achieve electronic beam-steering for such coherently-combined fiber-amplifier arrays.

In this paper, we present results on master-oscillator Yb-doped fiber amplifier, with SBS-free operation to 1 kW cw output power (at 1064nm), with a phase-modulated linewidth of only 450MHz, well below other reported results to date. The final amplifier stage uses very high Yb-doped 35-um core LMA fiber, using a new fiber process recipe that virtually eliminates photo-darkening. As a result, at the 1kW cw power level, near-diffraction limited beam-quality (M2<1.4) is obtained, with high (>83%) internal quantum efficiency.

To enable true single-frequency cw operation at kW level, we investigate further mode-area scaling, with gain-discrimination for the higher-order modes. Fibers with partially Yb-doped cores, having controlled dopant and index profiles are fabricated, having core diameters up to 80-um. SBS-free, single-frequency (few kHz) operation is demonstrated up to 0.9kW cw power. At lower cw powers (<200W) near-diffraction limited beam-quality is obtained, but is observed to deteriorate at higher power levels. We discuss potential causes, and compare the observed results to a detailed model of kW large-core fiber-amplifiers, that includes all guided modes, effect of fiber coiling, transverse spatial hole burning, gain-tailoring, mode-scattering, SBS nonlinearity, and various thermal effects.

7914-07, Session 2

On the Raman threshold for passive large mode area fibersC. Jauregui-Misas, J. Limpert, Friedrich-Schiller-Univ. Jena (Germany); A. Tünnermann, Fraunhofer-Institut für Angewandte Optik und Feinmechanik (Germany)

The output power of fiber optic laser systems has been exponentially increasing in the last years. However, non-linear effects, and in particular stimulated Raman scattering (SRS), are threatening to seriously limit the development pace in the near future.

SRS can take place anywhere along the laser system, however it is actually the passive delivery fiber at the end of the system, the section where SRS is most likely to occur. The common way to combat this problem is to use the so-called Large Mode Area (LMA) fibers. However, these fibers are expensive and have a multimode nature that will either reduce the beam quality of the laser output or require a careful excitation of the fundamental mode. Furthermore, the larger the core area, the more complicated it will be to sustain single-mode operation. Therefore, it is becoming increasingly important to be able to determine which is the minimum core area required in the delivery fiber to avoid SRS.

This calculation is usually carried out using the conventional formula for the Raman Threshold published by R.G. Smith in 1972: . In this work we demonstrate that this formula and the conclusions derived from it are inaccurate for short (several meters long) LMA fibers. For example, one widely spread belief (obtained from this expression) is that there is no dependence of the Raman intensity threshold (Ith=Pth/Aeff) on the mode area. However, our calculations show otherwise. Additionally, we have obtained a corrected Raman threshold formula valid for short LMA fibers.

7914-08, Session 2

Suppression of stimulated Brillouin scattering in gain optical fibers through phase-modulation: a time dependent modelC. Zeringue, I. A. Dajani, G. T. Moore, Air Force Research Lab. (United States)

We describe a time-dependent model that describes the spatial and temporal evolution of stimulated Brillouin scattering (SBS) in Yb-doped optical fibers under phase-modulated pump conditions. Suppression of the SBS process at modulation frequencies ranging from approximately the Brillouin gain bandwith (40 MHz) and up to frequencies approximately 6 times the Brillouin shift (100 GHz) is considered. In order to accurately model large frequency modulations, higher-order time derivatives are retained in the phonon equation. The full set of coupled system of photon and phonon fields along with the population inversion equation is solved for numerically using the method of characteristics. The SBS process is initiated from noise by using a Langevin source term. To validate our simulations, we initially consider sinusoidal single-frequency modulations as a function of both modulation amplitude and frequency and show excellent agreement with experimental results. We then investigate effective phase modulation techniques by simulating a broad range of modulation functional forms. We suggest optimal modulation schemes at the GHz level that allow for further power scaling of narrow linewidth Yb-doped amplifiers.

7914-09, Session 3

A novel method for increasing the efficiency of 1064nm two tone lasers through heating of the gain fiberL. J. Henry, T. M. Shay, Air Force Research Lab. (United States); D. Hult, TREX Enterprises Corp. (United States); K. Rowland,

Conference 7914: Fiber Lasers VIII: Technology, Systems, and Applications


Boeing LTS Inc. (United States)

Two tone 1064 nm lasers co-seeded with 1040 nm radiation at lengths of gain fiber (5 m) most optimum for high-power single-frequency have been plagued with lower efficiencies due to residual 1040 nm emission and incomplete power transfer. Previously, 80% of the gain fiber was spooled on a chilled spool with the remainder at room temperature. The length of fiber placed in each temperature zone was driven by the need to equalize the Brillouin gain in each fiber segment to obtain maximal suppression of Stimulated Brillouin Scattering (SBS). These experimental results show for the first time that a significant increase in 1064 nm efficiency could be obtained by heating the gain fiber, versus cooling it. This is due to an increase in the absorption cross-section of Yb at 1040 nm with temperature resulting in more efficient energy transfer from 1040 nm to 1064 nm. The amount of 1040 nm in the output of the laser could be reduced up to 93% depending on the seed ratio when the temperature of the gain fiber was increased from 20C to 80C. SBS suppression through usage of multiple heating stages could also be obtained with increased efficiency by utilization of temperature zones of 80C and 50C (in that order) while still maintaining the desired length of fiber in each temperature zone to enable maximal SBS suppression. Finally, through the utilization of hot heating stage(s) for the gain fiber, a 70% or greater efficiency two tone laser can be maintained at a short fiber length with a high SBS threshold.

7914-10, Session 3

Experimental studies of segmented acoustically tailored photonic crystal fiberC. A. Robin, I. A. Dajani, F. L. Chiragh, Air Force Research Lab. (United States)

We present experimental studies of a novel polarization-maintaining photonic crystal fiber (PCF) possessing a two-segment transverse acoustic profile and a mode field diameter (MFD) exceeding 30 µm. The concentrations of the dopants (fluorine, aluminum, germanium) in the two segments were selected such that the corresponding Brillouin shifts were sufficiently separated to allow for the introduction of a large thermal gradient for further SBS suppression. A pump-probe experiment was conducted in order to characterize the Brillouin gain spectrum (BGS) and to confirm the existence of two Brillouin gain peaks. The separation of the two peaks was approximately 220 MHz and the bandwidth of each was estimated to be 50 MHz. By performing a cutback study and fitting the results to a model that accounts for fiber loss due to laser interaction, we were able to estimate the effective Brillouin gain coefficient to be 8.6×10^-12 m/W. A similar study conducted on a reference fiber with similar core and cladding size but with without the benefit of acoustic segmentation, yielded a Brillouin gain coefficient of 1.6×10^-11 m/W. By applying an effective two-step temperature profile, the BGS displayed four distinct peaks; thus demonstrating further SBS suppression through a thermal gradient. As a preliminary test in an amplifier configuration wherein the entire fiber was placed on a cold plate (thus with little benefit of quantum defect heating for further SBS suppression), we obtained a single frequency near diffraction-limited output with a power level approaching 200 W.

7914-11, Session 3

Experimental investigation of suppressing stimulated Raman scattering in double clad fiber amplifiers employing long period gratingsD. Nodop, C. Jauregui-Misas, F. Jansen, J. Limpert, A. Tünnermann, Friedrich-Schiller-Univ. Jena (Germany)

We present the first experimental investigation of the suppression of stimulated Raman scattering (SRS) employing long period gratings in double clad fiber amplifiers. The long period gratings (LPGs), fabricated

with a CO2-Laser, achieve SRS suppression by coupling the Stokes wavelength from the active core into the cladding. With only three LPGs inserted into a fiber pulse amplifier, the extractable Raman free output power was nearly doubled. A useful approach for the determination of a Raman conversion threshold is developed to commonly discuss the power scaling of fiber amplifiers in term of SRS.

A numerical simulation of the fiber amplifier based on rate equations shows good agreement with the experimentally obtained results. The scaling potential of this approach is discussed and reveals that the suppression of SRS by means of LPGs is mainly limited by the insertion losses of the LPGs. We show that, with the appropriate fabrication technique, the insertion losses of LPGs can reach values close to zero, which constitutes LPGs as a promising concept for further power scaling of double cladd fiber amplifiers without the need of large mode area fiber concepts.

7914-12, Session 3

Improved phase modulation for SBS mitigationD. M. Brown, M. L. Dennis, W. E. Torruellas, The Johns Hopkins Univ. (United States)

We have achieved a factor of 2 improvement in the Stimulated-Brillouin-Scattering (SBS) threshold for a 1kW class ytterbium doped fiber amplifier (YDFA) . This improvement has been experimentally demonstrated using a commercial 1 kW fiber amplifier (Nufern), pump limited to 930W, employing an alternative filtering and phase modulation scheme to the single frequency master oscillator (MO) that seeds the amp. The MO has an instantaneous linewidth of less than 2MHz (resolution limited) and 30MHz long term optical bandwidth. By broadening the linewidth of our master oscillator, our technique has shown that it is possible to achieve ~1 kW output power with an optical linewidth of less than 5 GHz with excellent beam quality, M2<1.5. This is a significant improvement compared to currently available RF noise phase modulation schemes, which offer ~1 kW power levels at linewidths ranging from 10-30 GHz. To support our experimental work, we have implemented numerical models guiding our phase modulation approach and optimizing the SBS threshold for our 1 kW YDFA. Based on our analysis and experimental work we show that phase modulation resulting in optical spectral broadening can increase or decrease the SBS threshold and can be optimized to achieve an improvement in output power at reduced modulation and optical bandwidths.

7914-13, Session 4

A novel multiple port side pump couplers for high-power fiber lasersY. Sintov, A. Meir, Y. Glick, S. Pearl, Soreq Nuclear Research Ctr. (Israel)

A novel side pump coupling technique between multiple multimode high NA pump fibers and a single accepting double clad fiber is described. 0.46NA pump fibers are used, allowing for the use of high power low brightness diode sources as a pump source. The accepting Double clad fiber is of 0.46NA inner cladding, allowing for highly efficient pump power coupling for the implementation of both CW and pulsed high power fiber lasers that are being developed at Soreq.

7914-14, Session 4

High-power, fused assemblies enabled by advances in fiber-processing technologiesR. Wiley, 3SAE Technologies, Inc. (United States)

The power handling capabilities of fiber lasers are limited by the



technologies available to fabricate and assemble the key optical system components. Previous tools for the assembly, tapering, and fusion of fiber laser elements have had drawbacks with regard to temperature range, alignment capability, assembly flexibility and surface contamination. To provide expanded capabilities for fiber laser assembly, a wide-area electrical plasma heat source was used in conjunction with an optimized image analysis method and a flexible alignment system, integrated according to mechatronic principles. High-resolution imaging and vision-based measurement provided feedback to adjust assembly, fusion, and tapering process parameters. The system was used to perform assembly steps including dissimilar-fiber splicing, tapering, bundling, capillary bundling, and fusion of fibers to bulk optic devices up to 10mm diameter. A wide range of fiber types and diameters were tested, including extremely large diameters, non-silica fibers, and photonic crystal fibers. The assemblies were evaluated for conformation to optical and mechanical design criteria, such as taper geometry and splice loss. The completed assemblies met the performance targets and exhibited reduced surface contamination compared to assemblies prepared on previously existing equipment. The imaging system and image analysis algorithms provided in situ fiber geometry measurement data that agreed well with external measurement. The ability to adjust operating parameters dynamically based on imaging was shown to provide substantial performance benefits, particularly in the tapering of fibers and bundles. The integrated design approach was shown to provide sufficient flexibility to perform all required operations with a minimum of reconfiguration.

7914-15, Session 4

Single-polarization all-solid photonic bandgap fiber incorporating point-by-point Bragg gratingsR. Goto, S. D. Jackson, The Univ. of Sydney (Australia); R. J. Williams, N. Jovanovic, G. Marshall, M. J. Withford, Macquarie Univ. (Australia)

We demonstrate Point-by-Point (PbP) fiber Bragg grating inscription into the core of a single-polarization all-solid photonic bandgap (PBG) fiber using femtosecond laser pulses. The writing method does not require photosensitivity and offers a particular advantage over the conventional ultraviolet (UV) writing method for Bragg grating inscription into an all-solid PBG fiber. The cladding of an all-solid PBG fiber typically has highly germanium (Ge) doped regions which defines the PBG, thus the UV-writing method changes the refractive index of the Ge-doped regions by photosensitivity. The change in the refractive index leads to a shift in the PBG to a long wavelength and the shift can be up to several tens of nanometers. This can result in the target Bragg wavelength no longer being guided. On the other hand, the PbP grating writing process does not modify the cladding, thus Bragg grating inscription is possible without shifting the PBG. A 20-mm-long third-order PbP Bragg grating was written into the non photosensitive, pure silica core of the PBG fiber. The grating strength was 7.5 dB (>80% reflection) at 1064 nm. The 3-dB bandwidth of the reflection was less than 200 pm. A 90-mm-diameter single-turn coiling of the fiber produced a large polarization dependent loss at 1064 nm due to the single-polarization guidance in the fiber at this wavelength, realizing linearly-polarized reflection. This grating can be used as a wavelength and polarization selective element for the creation of a narrow-linewidth, linearly-polarized fiber laser.

7914-16, Session 4

Ultra rapid dispersion measurement of optical fibers and optical assemblies at 1310 nm and 1550 nmW. Wieser, B. R. Biedermann, T. Klein, C. M. Eigenwillig, R. A. Huber, Ludwig-Maximilians-Univ. München (Germany)

We present a novel method to measure the dispersion of km-long fibers

and through optical components. The technique is based on a rapidly swept Fourier domain mode locked (FDML) laser driven at 50kHz repetition rate. Amplitude modulation with 400MHz and phase analysis yield the time-of-flight values over a continuous wavelength tuning range. Setups covering both common telecom wavelengths ranges (130nm around 1550nm and 110nm around 1310nm) are demonstrated. The high acquisition speed of 10µs for individual wavelength-resolved traces enables real-time monitoring with update rates >100Hz even when averaging several hundred acquisitions for improved accuracy.

7914-17, Session 4

All-fiber single-mode PM thulium fiber lasers using femtosecond laser written fiber Bragg gratingsC. Willis, R. A. Sims, L. Shah, M. C. Richardson, CREOL, The College of Optics and Photonics, Univ. of Central Florida (United States); J. Thomas, R. Becker, C. Voigtländer, Friedrich-Schiller-Univ. Jena (Germany); A. Tünnermann, S. Nolte, Friedrich-Schiller-Univ. Jena (Germany) and Fraunhofer Institute for Applied Optics and Precision Engineering (Germany)

Two continuous wave Tm fiber lasers, at 1959 nm an 2055 nm, were implemented using single mode PM silica fiber and femtosecond written fiber Bragg gratings (FBG). 2 m of active fiber was pumped using a 35 W 790 nm diode spliced directly to a highly reflective (HR) FBG at either 1959 nm or 2055 nm. The laser output coupler was either the Fresnel reflection from the flat cleaved output fiber facet or a low reflectivity (LR) FBG. Slope efficiency and spectral output were measured for each of the four configurations. Spectral and power performance was measured for each system as the LR FBG was heated to change its spectral reflectivity. These systems represent the first monolithic fiber laser cavities formed from femtosecond written FBGs utilizing Tm-doped PM silica fiber.

The best performance was achieved with the 1959 nm system in the HR-LR FBG configuration, outputting up to 5 W with 47.5% slope efficiency and a 0.5 nm spectral width. The HR-LR FBG 2055 nm system also had a spectral width of 0.5 nm, but only yielded a slope efficiency of 21.6% with a maximum power of 1 W, limited primarily by the low emission cross-section at the longer wavelength and the relatively short length of the active fiber. The HR-LR FBG configurations had a narrower bandwidth of 0.2 - 0.5 nm in contrast to HR-Flat cleave configurations which were 0.7-1.0 nm wide.

7914-18, Session 5

Reliability and photodarkening in Tm-doped fibers and lasersB. N. Samson, A. L. G. Carter, K. Tankala, Nufern (United States)

Over the last five years significant progress has been made on improving the efficiency of 790nm pumped Tm-doped fibers. This has been achieved primarily through the optimization of the glass composition for efficient exploitation of a “2 for 1” cross-relaxation process that results in the generation of two signal photons for every pump photon [1]. Consequently increasing the slope efficiency for 2m fiber lasers/amplifiers to around 65%, more than double the efficiency for the more conventional cascaded (Er:Yb pumped Tm) pumping scheme.

In this paper we will review some of the recent Tm-doped fiber laser architectures that are maturing into commercial products. We will review the spectroscopy of 790nm pumping, the maturity and availability of the various key components, including high brightness pump diodes and fiber Bragg gratings, as well as the relevant high efficiency Tm-doped fibers. The role of photo-darkening and other effects on the reliability and life expectancy of these lasers will be reviewed and latest results presented.



7914-19, Session 5

Gamma-radiation-induced photodarkening in ytterbium-doped silica glassesT. Arai, K. Ichii, S. Tanigawa, M. Fujimaki, Fujikura Ltd. (Japan)

Photodarkening phenomenon is known as an increase in optical losses induced by light irradiation to glasses. When it happens in the rare-earth-doped fiber core of fiber lasers and amplifiers, the excess losses cause a continuous decrease of output power that limits the lifetime and reliability of the devices. Thus, it is important to understand the mechanism of photodarkening for the improvement of apparatuses such as fiber lasers from a practical viewpoint. Here we report on the experimental results of defect analysis in Al-Yb co-doped silica glasses and fibers, which have been irradiated by Gamma ray or pump light. These samples of Al-Yb co-doped silica glasses and fibers have been prepared by the conventional modified chemical vapor deposition (MCVD) process. To investigate the defects formed in the samples, we have used the techniques of electron spin resonance (ESR), X-ray absorption fine-structure (XAFS), and optical transmittance measurement in UV-VIS region. Our results show that both the gamma-ray irradiation and pump-light irradiation predominantly generate the defect of Al-oxygen hole center (Al-OHC). This fact suggests that we can readily investigate the characteristics of photodarkening using bulk samples in a given situation instead of using fiber samples. On the basis of experimental results, we conclude that the formation of Al-OHC is the prime cause of the photodarkening loss in Al-Yb co-doped silica glasses.

7914-20, Session 5

Distribution of photodarkening-induced loss in Yb-doped fiber amplifiersM. N. Zervas, Univ. of Southampton (United Kingdom)

It has now been realised that photodarkening can severely hamper the long-term stability and deployment of high power Yb3+-doped fiber lasers in the industrial applications sector. Most studies so far have been using core pumping and relatively short (<0.5m) Yb3+-doped fibers in order to achieve almost uniform inversion along the length. Various reports have shown inversion dependence to the power of 3-4 or even 7. It has also been observed that photodarkening in LMA fibres is non-uniformly distributed over the fiber cross-section.

In this work we report photodarkening-induced output power degradations and long-term stability in high power pulsed and CW fiber laser MOPA systems. The studied laser systems are based on aluminosilicate single-mode Yb-doped fibers and use the GTWave fiber technology for cladding pumping. The active fiber lengths are between 10-30m. We have tested Yb-doped fiber amplifiers operated under both pulsed and CW mode.

Using OTDR background-loss measurements we show, for the first time, that the photodarkening-induced loss is non-uniformly distributed along the length of the active fiber. By calculating the average inversion along the fiber length, we show that the induced loss follows closely an Yb-inversion dependence to the power of 2.

In addition we have studied the temperature dependence of the output power variation. It is shown that increasing (decreasing) operating temperature results in decrease (increase) of the laser output power, reaching the new equilibria over time scales of ~200hours.

7914-21, Session 6

Tm-doped silicate glass fibre lasers: the foundation technology for high-power mid-infrared light generationS. D. Jackson, The Univ. of Sydney (Australia)

The Tm-doped silicate glass fibre laser that operates in the 2 micron region of the spectrum is fast becoming a mature technology with output powers already exceeding 1 kW. In this paper, I will review a number of current and future experiments that involve lasers pumped with the output from Tm-doped silicate glass fibre lasers including linear systems e.g., the optical excitation of rare earth ions and nonlinear systems e.g., Raman fibre lasers.

7914-22, Session 6

Efficient deep-UV generation from sub-uJ 30 fs-pulses in Ar-filled hollow-core photonic crystal fibreN. Y. Joly, Max-Planck-Institut für die Physik des Lichts (Germany) and Friedrich-Alexander-Univ. Erlangen-Nürnberg (Germany); P. Hoelzer, Max-Planck-Institut für die Physik des Lichts (Germany); J. Nold, Friedrich-Alexander-Univ. Erlangen-Nürnberg (Germany); W. Chang, G. Wong, A. Nazarkin, P. J. Russell, Max-Planck-Institut für die Physik des Lichts (Germany)

Hollow-core photonic crystal fiber (PCF) offers an unprecedented diffractionless system, where the interactions between guided light and the material filling the core (gas or liquid) are greatly enhanced. In the talk, we report recent experimental advances in the interaction between fs-pulses and noble-gases in PCF, with particular focus on efficient generation of deep UV.

By contrast with conventional approach using large bore gas-filled capillaries, Ar-filled Kagomé-lattice PCF requires higher gas-pressure to achieve appropriate phase-matching condition. Tight confinement and good control of the dispersion properties of the waveguide act in concert to efficiently generate deep UV light. Two processes are presented here: phase-matching to higher-order modes, and growth of dispersive waves in the UV region. In the latter case, tunable deep UV (200-320nm) light is generated with efficiencies as high as 4%, using 35 fs sub-µJ pulses at 800 nm. Most significant is that the UV remains in the fundamental mode.

7914-23, Session 6

Simultaneous excitation of selective multiple Raman Stokes wavelengths (green-yellow-red) using shaped multi-step pulses from an all-fiber MOPA systemD. Lin, P. S. Teh, S. Alam, K. Chen, D. J. Richardson, Univ. of Southampton (United Kingdom)

At Photonics West 2010 we reported the selective excitation of Raman Stokes lines of up-to 9th order in a 1 km length of fiber pumped by rectangular shaped optical pulses at 530 nm (Paper 7582-17). Here we demonstrate an extension of this approach to the use of pump pulses incorporating multi-amplitude-levels which enables the generation of Raman-shifted pulses in which the frequency changes in discrete steps across the pulse form in accordance with the pump amplitude profile. The technique should allow the realization of a new generation of wavelength agile fiber based laser systems operating across broad and interesting wavelength regimes.

To illustrate the approach we demonstrate the generation of pulses of several hundred nanoseconds duration in which the wavelength changes sequentially from green (543nm, 1st Stokes), to yellow (585nm, 4th Stokes) to red (616nm, 6th Stokes) using 530nm pump pulses with a 3 step profile. The pump pulses were readily generated using a frequency-doubled fiber MOPA. Changing the ratio of the peak powers (step height) between the steps, and the order of the step-changes allows the generation of sequences of colored pulses according to any desired combination of Raman Stokes Shift, providing dynamic and agile frequency tuning between well-defined wavelengths. The energy/power distribution of the different Raman Stokes is adjustable by varying the step widths.



7914-24, Session 6

All-fiber mid-IR supercontinuum source from 1 5 to 5 µmL. B. Shaw, R. Gattass, J. S. Sanghera, I. Aggarwal, U.S. Naval Research Lab. (United States)

Mid-IR supercontinuum sources have been reported in a variety of optical fibers including fluoride, tellurite, sapphire, and chalcogenide. To date, the longest wavelengths in the mid-IR have been achieved in fluoride and tellurite fiber using Ti:Sapphire/OPO based sources with very high peak powers and short fiber lengths to overcome the intrinsic fiber loss at the multiphonon edge. Here, we report on an all-fiber supercontinuum source based on As2S3 fiber with a broadband supercontinuum extending from 1.5 to 5 µm. The pump source was a Raman shifted erbium doped mode-locked silica fiber laser. The As2S3 fiber was a 2 meter length of conventional step index 10 µm core, 140 µm clad single mode fiber. 140 mW broadband power was demonstrated with a spectral intensity variation of 10 dB from 1.9 to 4.4 µm and 20 dB from 1.65 to 4.78 µm.

7914-25, Session 6

High-power all fiber picosecond sources from IR to UVS. Pierrot, J. Saby, F. Salin, B. Cocquelin, EOLITE Systems (France)

Picosecond pulses are highly desirable for industrial applications. The main limitation for the expansion of the picosecond market is the cost of high power picosecond laser sources. This is due to the complexity of the architecture used for picosecond pulse amplification and the difficulty to keep an excellent beam quality at high average power. We have demonstrated several picosecond sources using single mode rod-type fibers doped with Yb. Rod type fibers offer a unique combination of very large mode area for very low non-linearities and very high gain. We used a passively modelocked fiber oscillator that produces after pre-amplification up to 5 W at 80 MHz and 1030 nm. The linewidth of this source is 0.08 nm for 30 ps pulses. The pulse train repetition rate can be adjusted using a pulse picker from 200 kHz up to 80 MHz. The laser is seeded into a 70 cm long rod type amplifier. With a single pass amplifier we obtained an average power of 70 W at 80 MHz. Using LBO crystals we produced 45 W at 515 nm and 20W at 343 nm. Using the amplifier in a double pass configuration led to over 90 W in the IR from 6 to 80 MHz while keeping the bandwidth below 0.5 nm. 64% to the green and 30% to the UV harmonic generation efficiencies were observed. At lower repetition rate, we also obtained energies up to 30 µJ without any damage. The simplicity and small size of this all fiber source will have a strong impact on the growth of the industrial picosecond laser market.

7914-26, Session 6

Resonantly cladding-pumped Yb-free Er-doped LMA fiber lasers: power scaling and efficiency improvementJ. Zhang, V. Fromzel, T. Sanamyan, M. Dubinskii, U.S. Army Research Lab. (United States)

Highly scalable, efficient, low quantum defect laser operation of the resonantly cladding-pumped Yb-free Er-doped laser based on the COTS LMA fiber has been demonstrated. Our experiments were the next step in power and efficiency scaling of this type of laser. Single transverse mode operation with over 88 W of output at 1590-nm was obtained with 1532-nm pumping. Maximum optical-to-optical efficiency obtained in these experiments is ~69%. This result presents, to the best of our knowledge, the highest power ever reported from Yb-free Er-doped LMA fiber laser, as well as the highest efficiency ever reported for any cladding-pumped Er-doped laser.

7914-27, Session 6

High-average power second harmonic generation of femtosecond fiber lasersS. Hädrich, J. Rothhardt, T. Eidam, T. Gottschall, J. Limpert, Friedrich-Schiller-Univ. Jena (Germany); A. Tünnermann, Friedrich-Schiller-Univ. Jena (Germany) and Fraunhofer Insitute for Applied Optics and Precision Engineering (Germany)

We present second harmonic generation of a high average power, high energy femtosecond Yb doped fiber chirped pulse amplifier. This system is operated at various repetition rates at a central wavelength of 1040 nm. After two pre-amplification stages a main amplifier is used to achieve the required pulse energy for efficient second harmonic generation. It is comprised of a 80 cm long rod-type photonic crystal fiber with an active core diameter of 100 µm. A dielectric grating based compressor is used for compression to a pulse duration of about 450 fs. Second harmonic generation is then achieved in a 500 µm thick BBO crystals. The conversion efficiency of the second harmonic generation remained almost constant at ~50 % for all repetition rates and average power levels. At 4.08 MHz the highest average power of 129 W at 520 nm was achieved.

7914-28, Session 7

Coherent phase locking of high-pulse energy fiber amplifiersS. Palese, E. Cheung, F. Di Teodoro, M. E. Weber, Northrop Grumman Aerospace Systems (United States)

Coherent beam combination architectures of continuous wave fiber lasers/amplifiers have proven a viable path to the power scaling of such systems. The extension of this approach to the high pulse energy regime in fibers (> 1 mJ/pulse @ 1ns FWHM) has proven more difficult due to the presence of non-linear interactions which cause significant temporal phase distortions. This talk will provide an overview of the fundamental sources of this non-linear chirp, active phase compensation schemes to correct for both interpulse and intrapulse distortions, factors that may limit amplifier number scalabilty along with the latest results for coherent locking of a multi-stage fiber amplifier system terminated with a large mode area 100 um photonic crystal fiber rod. The phase noise spectrum relative to an ultra-stable CW reference source and phasing fidelity measurements for pulse energies up to 2 mJ/pulse (25 KHz repetition rates @1 ns FWHM) will be presented along with demonstrations of intrapulse, sub-ns phase control.

7914-29, Session 7

100 kW peak power, single polarization, high-brightness nanosecond lasers based on 3C optical fiberM. L. Stock, C. Liu, A. Kuznetsov, G. E. Tudury, Arbor Photonics, Inc. (United States); A. Galvanauskas, Univ. of Michigan (United States); T. S. Sosnowski, Arbor Photonics, Inc. (United States)

3C (Chirally-Coupled Core, also known as CCC) optical fiber is a new, engineerable geometry that enables robustly single mode performance of large core diameter fibers and, consequently, establishes a technological platform for high beam quality, power scalable lasers. Based on this platform we have demonstrated short pulse lasers with peak powers > 100 kW and M2 < 1.1, and are now developing practical integrated fiber lasers based on passive and active 3C fibers along with 3C based components. An additional attribute of 3C fiber is its ability to transmit polarization with high fidelity. Here we report the demonstration of robust polarization preserving performance of 35 µm core 3C fiber based short pulse systems. A polarization extinction ratio (PER) of ~ 20 dB is obtained



from a Yb-doped double-clad 3C fiber amplifier and this PER is stably maintained with ambient temperatures varying over a 50° C range. We also demonstrate that this high-PER polarization output is insensitive to localized temperature gradients and mechanical perturbations in the 3C fiber amplifier. Based on this 3C Yb-doped fiber a polarized, nanosecond pulsed fiber laser with average powers exceeding 50 Watts and single mode output (M2 < 1.1) has been demonstrated. The ability to deliver high peak power pulses at high average powers while maintaining exceptional beam quality and a stable polarization state in an easily integrated format makes 3C fiber laser systems extremely attractive for harmonic generation to visible and UV wavelengths.

7914-31, Session 7

Coherent combining of low-peak-power pulsed fiber amplifiers with 80-ns pulse durationP. Bourdon, K. Cadoret, L. Lombard, A. Azarian, G. Canat, B. Bennaï, D. Goular, V. Jolivet, ONERA (France); Y. Jaouën, Telecom ParisTech (France); O. Vasseur, ONERA (France)

Coherent combining techniques involving active phase control of the laser emitters have demonstrated their potential to efficiently combine kilowatts of continuous-wave power from fiber amplifiers. Even if nanosecond pulsed fiber amplifiers have been successfully combined through incoherent techniques such as spectral combination, coherent combining with active phase control has never been applied in the case of pulsed amplifiers.

One major challenge, when trying to coherently combine pulsed amplifiers, is dealing with phase fluctuations during the pulses, due to nonlinear effects and signal-induced refractive index changes in rare-earth doped fiber sections. These very fast phase fluctuations cannot be actively controlled and compensated for.

We characterize pulsed fiber amplifiers with low peak-power in terms of phase fluctuations during the pulses. Two different interferometric measurement techniques are used. They both reveal that for peak-powers lower than 100 W, phase fluctuations remain below 1/30-th wave during the pulses. In such pulsed amplifiers, this means that noticeable phase fluctuations only occur between the pulses, due to slow variations of fiber length and temperature. So, they can be perfectly controlled using classical continuous-wave-efficient combining techniques.

We will present the results of such realization combining two low-peak-power pulsed fiber amplifiers, using classical frequency-tagging coherent combining techniques. Phase difference measurement has to be performed between pulses using a small signal leak from the common master oscillator. Combination efficiency and consequences on amplifier performances of the necessity of this signal leak between pulses will be discussed.

7914-38, Session 7

Complete measurement of nanosecond laser pulses in time made simpleR. P. Trebino, Georgia Institute of Technology (United States); P. R. Bowlan, Swamp Optics, LLC (United States)

It is relatively straightforward to completely measure both long (>10ns) and very short (<100ps) laser pulses in time. But intermediate pulse lengths-that of the most common laser pulses-remain nearly immeasurable and, not coincidentally, correspond to the least stable of all lasers. True, ultrahigh-bandwidth oscilloscopes[1] and streak cameras[2] can now resolve such pulses, but such exotic electronic devices are expensive (~$100,000) and fragile and only yield the temporal intensity and not the temporal phase. For pulses up to ~100ps long, several linear-optical and interferometric methods exist, and “temporal imaging” techniques stretch a pulse in time to more easily measured lengths. But all these methods require precise synchronization with an

expensive modulator or another, pre-measured reference pulse or both. Using a delay line and a high-resolution grating spectrometer, Fuchs, et al., developed a multi-shot frequency-resolved-optical-gating (FROG) device, which avoids these problems, and, using it, they have measured pulses up to 80ps long [3]. But a simple, practical single-shot device for completely measuring ns pulses remains elusive.

Here we describe a simple, elegant, accurate, complete, compact, all-optical, entirely passive, and single-shot FROG device that solves the problem. It simultaneously achieves a very large delay range of ~10ns and very high spectral resolution of <1pm. It accomplishes both feats using high-efficiency, high-finesse etalons, the first to tilt the pulse by 89.9˚, that is, by several meters over a centimeter beam, and another to generate massive angular dispersion for a high-resolution spectrometer.

We demonstrate this device for measuring pulses 100ps to several ns long from a fiber-amplified micro-disk laser.

1. T. S. Clement, P. D. Hale, D. F. Williams, C. M. Wang, A. Dienstfrey, and D. A. Keenan, “Calibration of sampling oscilloscopes with high-speed photodiodes,” IEEE Transactions on Microwave Theory and Techniques 54, 3173-3181 (2006).

2. D. J. Bradley, B. Liddy, and W. E. Sleat, “Direct linear measurement of ultrashort light pulses with a picosecond streak camera,” Optics Communications 2, 391-395 (1971).

3. H. Fuchs, D. Woll, T. Ulm, and J. A. l’Huillier, “High resolution FROG system for the characterization of ps laser pulses,” Appl. Phys. B 88, 393-396 (2007).

7914-32, Session 8

A monolithic thulium doped single mode fiber laser with 1 5ns pulsewidth and 8kW peak powerJ. Ding, B. Sampson, C. Wang, K. Tankala, A. L. G. Carter, Nufern (United States)

Here we report a compact monolithic 2000nm pulsed laser with a single spatial mode output, ~1.5ns pulsewidth, 8kW peak power and >200mW average power at 20 kHz repetition rate. The gain-switched 2000nm laser, consisting of a pair of fiber Bragg gratings and 0.5m of Thulium-doped single cladding fiber, was core pumped by a high peak power pulsed 1550nm laser. When the input pulse energy of the 20 kHz pump pulses was sufficient enough to saturate the Thulium fiber, a stable 20 kHz pulse train was observed with linewidth of 0.05nm which is resolution limit of the Optical Spectrum Analyzer. This compact pulsed 2um laser, to the authors’ limited knowledge, represents the first Thulium doped fiber laser with 8kW peak power and several ns pulsewidth which is narrower than the previously reported tens of ns pulsewidth from gained switched Thulium fiber laser.

7914-33, Session 8

Picosecond programmable laser sweeping over 50 mega-wavelengths per secondY. Kim, B. Buorgoyne, Genia Photonics Inc. (Canada); N. Godbout, Ecole Polytechnique de Montréal (Canada); A. Villeneuve, Genia Photonics Inc. (Canada)

Picosecond lasers are enhancing many diverse application areas; from techniques such as OCT, CARS, SRS, and STED for biomedical imaging to enhanced drilling, cutting, and etching for laser micromachining. However, the tuning ranage of most picosecond lasers today is not broad enough and conventional techniques such as active or passive mode locking and gain switching cannot provide the desired fast sweeping rate. An essential criteria of picosecond lasers in nonlinear biomedical imaging systems is fast sweeping capability for hyperspectral CARS, SRS microscopes and nonlinear OCT applications. Such fast sweeping would allow high-speed imaging with high sensitivity which is definitely desired



for clinical purposes.

We have already demonstrated in previous works that our programmable lasers (PL) exhibit picossecond pulses spanning over 80 nm. We report here the successful realization of more than 50 mega-wavelengths per second using an SOA based PL around 1565 nm at a 114 MHz repetition rate. The laser is simply composed of an SOA, a CFBG (10ps/nm) with a 100 nm bandwidth, an optical circulator, an EOM (intensity modulator), and an output coupler (20%). Pulse duration is around 45 ps and no noticeable pulse degradation is observed on a high speed oscilloscope while sweeping. Without sweeping, OSNR of the pulse is around 40 dB at 1565 nm. Long term stability of pulse was also measured with an OSA resulting in ~ 60 dB SNR within 200 kHz bandwidth.

7914-34, Session 8

High-power actively mode-locked ytterbium doped fiber laser delivering 15 ps at 40 MHzP. Deslandes, Univ. Bordeaux 1 (France) and EOLITE Systems (France); D. Sangla, E. Freysz, F. Salin, J. Saby, Univ. Bordeaux 1 (France)

For the generation of ultrashort pulses in the picosecond range with fiber oscillators, the actively mode locking technique so-called Frequency Shifted Feedback (FSF) method is an interesting alternative to passive mode-locking. In order to develop a reliable high power system, we studied this technique with a double clad photonic crystal fiber. The pulse generation mechanism relies on an interplay between self-phase modulation inside the fiber, the frequency shift imposed by an active modulator and the spectral filtering inside the cavity. For our experiments, a 170-cm long ytterbium doped polarization maintaining photonic crystal fiber with 15/135 um cores diameter was pumped by a 10-W laser diode at 976-nm coupled inside a 200-um diameter fiber. The linear cavity was aligned on the first order of an Acousto-Optic Modulator working at 80 MHz and a highly reflective Volume Bragg Grating mirror at 1030-nm with a spectral width of 0.3-nm. A 4% reflective output coupler was used to maximize the extracted energy from the oscillator. The length of the cavity was chosen to match the second harmonic of the frequency shift. We achieved an average output power of 2-W at 40-MHz with stable pulse trains of 15-ps duration. This corresponds to a energy of 50-nJ per pulse and 3.3 kW peak power. To our knowledge, these preliminary results represent the highest average power ever achieved from a FSF fiber laser. In the future, we will study the power scalability with the use of new types of photonic crystal fibers.

7914-35, Session 8

High-energy Yb-doped fiber MOPA in the ns-kHz regime for large-scale laser facilities front-endL. Lago, Commissariat à l’Énergie Atomique (France); A. Mussot, M. Douay, Univ. des Sciences et Technologies de Lille (France); E. Hugonnot, Commissariat à l’Energie Atomique (France)

The recent progresses of microstructured optical fibers correspond to an important breakthrough in powerful fiber amplifier development. In fact, they combine the advantage of enlarging the mode size to limit detrimental nonlinear effects while preserving the single mode operation. Consequently, milli-Joule pulse energy in the nanosecond and multi-kilo Hertz regime has been reported in fibered Master Oscillator Power Amplifier (MOPA) systems. Although these systems are very attractive, their performances do not meet the high level performances required by front-end amplifier systems seeding high-power laser facilities designed for laser matter interaction experiments. Indeed, typical general requirements are strictly single-mode beam, versatile temporal pulse shaping with more than 10 bits dynamics and sub-nanosecond resolution over few tens of nanoseconds, optical signal-to-noise ratio better than 40 dB, polarization extinction ratio better than 25 dB and long term energy

stability to achieve reliable operation. In this work, we report numerical and experimental results of a dual-stage amplifier that encompasses these whole characteristics. First of all, a combination of high power CW laser and electro-optical modulator allows the generation of 10 kHz nanosecond pulses with tailored temporal pulse profiles. This seed pulse is amplified in the first-stage amplifier to achieve a pulse of 20 µJ energy (33 dB gain) that is launched inside the second amplifier. We finally obtained pulses of 0.5 mJ energy. The choice of optimal parameters and the temporal pulse shaping have been performed thanks to numerical simulations (including forward and backward Amplified Spontaneous Emission) in excellent agreement with the results.

7914-30, Session 9

High-pulse energy operation of efficient and compactly packaged, ns-pulse Yb-doped photonic-crystal fiber-based lasers delivering high-spectral and spatial brightnessF. Di Teodoro, Northrop Grumman Aerospace Systems (United States)

Pulse fiber lasers (PFLs) are naturally amenable to field deployment owing to favorable size and weight, high efficiency, and simple thermal management. Large-core photonic crystal fibers (PCFs) have also enabled high pulse energy, peak power, beam quality (BQ), and spectral brightness (SB) suitable for air-/space-based nanosecond-pulse remote sensors. However, packaged PFLs typically exhibit lower power and/or SB compared to laboratory demonstrations.

Here, we describe two PCF-based, compactly packaged, nanosecond-pulse master-oscillator/power-amplifier (MOPA) architectures achieving high pulse power, SB, and BQ.

In the first, a pulse-programmable diode laser drives a multistage Yb-doped fiber amplifier terminated by a 100um-core, rod-type PCF. For compact packaging, the PCF is divided into four ~36cm-long segments laid out to ensure negligible efficiency penalty compared to a whole PCF. The MOPA outputs ~1064nm-wavelength, linearly polarized, ~1.5ns pulses of energy/peak power > 2mJ/1.5MW at 10kHz pulse repetition frequency (PRF), good BQ (M2 ~ 1.2), and high SB (>85% pulse energy into a 0.06nm window). The laser system is enclosed within a < 15-liter volume package designed to withstand field-level shock, vibration, and thermal excursions.

In the second architecture, four time-synchronizable MOPA systems of layout, BQ, and SB similar to that described above are packaged within a 48×35×18cm enclosure (~30-liter volume). Each MOPA generates < 2ns pulses of ~50W average power at 50kHz PRF. Within the same enclosure, an integrated optical bench enables high-efficiency dense wavelength multiplexing (DWM) of the MOPA output beams. To our knowledge, this setup provides the smallest footprint for high-power fiber DWM reported to date.

7914-36, Session 9

All-fiber based amplification of 40 ps pulses from a gain-switched laser diodeS. Kanzelmeyer, H. Sayinc, T. Theeg, M. Frede, J. Neumann, D. Kracht, Laser Zentrum Hannover e.V. (Germany)

Amplification of a gain-switched laser diode is demonstrated in an all-fiber based setup. Due to the low output pulse energy of gain-switched laser diodes in the range of about 20 pJ, amplification with repetition rates below a few MHz requires the consideration of amplified spontaneous emission (ASE), generated in a fiber amplifier. A high amount of ASE in a pulsed laser system leads to an inefficient amplification and a power background noise, which can result in self lasing of the amplifier. Additionally, reliable information on the pulse energy can not be extracted from the average output power and the repetition rate, if the amount of ASE is unknown. As the contribution of



ASE can not be distinguished from the pulse train in the spectral domain, the fraction of ASE was investigated in the temporal domain by using an acousto-optic modulator. A maximum pulse energy of 13 µJ at a repetition rate of 1 MHz and a pulse duration of 40 ps was extracted, corresponding to a peak power of 270 kW. To the best of our knowledge, this is the highest extracted pulse energy from a laser system seeded by a gain-switched laser diode. Temporal pulse deformation due to intrapulse Raman scattering was observed, which resulted in a temporal power shift to the leading edge of the pulses.

7914-37, Session 9

High-average power Nd:YVO4 regenerative amplifier seeded by a gain switched diode laserM. Lührmann, F. Harth, C. Theobald, T. Ulm, Photonik-Zentrum Kaiserslautern e.V. (Germany); R. Knappe, A. Nebel, LUMERA LASER GmbH (Germany); A. Klehr, G. Erbert, Ferdinand-Braun-Institut (Germany); J. A. L’huillier, Photonik-Zentrum Kaiserslautern e.V. (Germany)

We report on a Nd:YVO4 regenerative amplifier, end pumped by 888 nm diode lasers. The output power was about 46 W at repetition rates from 150 to 833 kHz with an M2-factor of 1.2. The amplifier was seeded by a gain switched diode laser, generating pulses with a duration of 65 ps and a pulse energy of ~ 5 pJ. The high gain of the regenerative amplifier of ~ 70 dB provides amplified pulse energies as high as 150 µJ. Bifurcations of the pulse energy could be avoided without a preamplifier despite the low seed energy. Pulse amplitude fluctuations of only 1.2 % for 10,000 succeeding pulses were measured. The long term output power stability of the laboratory setup was 0.3 %. Determined by the seed diode the pulse duration after amplification was 70 ps.

The laser combines the advantages of a small and efficient diode seed source with a reliable solid state regenerative amplifier, forming a compact, robust and powerful system. The gain switched seed diode delivers pulses on demand, rendering a pulse-picker unnecessary. It is also insensitive against feedback, reducing requirements on isolation between seed diode and amplifier.

Generation of ultra-short pulses with high repetition rate and high average power make this laser an ideal source for applications in non-linear optics and high-quality material processing.

7914-37, Session 9

Sub-10 picosecond pulses from a fiber-amplified and optically compressed passively Q-switched microchip laserA. Steinmetz, D. Nodop, A. Martin, J. Limpert, A. Tünnermann, Friedrich-Schiller-Univ. Jena (Germany)

We present an experimentally confirmed approach based on nonlinear optical compression of passively Q-switched pulses accessing sub-10 ps domain, which is so far dominated by mode-locked systems. The concept implements the SPM-induced spectral-broadening of Q-switched pulses in optical waveguides and a supplementary compression with bulk optics e.g. a pair of diffraction gratings or a chirped-bragg-grating. The used seed-source is a fiber-amplified, passively Q-switched microchip laser operating on a single longitudinal mode and consists of a monolithically bonded combination of Nd:YVO4-crystal and semiconductor saturable absorber mirror. The microchip laser provides pulses with a duration of 100-150 ps, a pulse energy of 210 nJ at 332 kHz and a line width of ~20 pm at wavelength of 1064.2 nm.

The seed pulses are amplified in an ytterbium-doped photonic crystal fiber to a pulse energy of 3.15 µJ and spectrally broadened to 50 pm. For an additional increase of the spectral bandwidth, a passive fiber with 10 µm core diameter and 3 m length is used leading to 0.53 nm

bandwidth. It should be mentioned, that a clean SPM broadened spectrum is possible by applying pulses as short as a few 100 ps, longer, i.e. nanosecond pulses, typically lead to a Raman continuum. As a first proof-of-principle experiment the broadened signal is sent through a conventional 1740 l/mm diffraction-gratings based pulse compressor. Best compression has been found at a grating separation of 11 cm resulting in pulse duration of smaller than 6.5 ps assuming a numerically calculated de-convolution factor of 0.735.

7914-38, Session 9

High-power diode pumped crystal fiber amplifier for passively Q-switched Nd:YAG microlaserI. Martial, Lab. Charles Fabry (France) and FiberCryst (France); F. Balembois, Lab. Charles Fabry (France); J. Didierjean, FiberCryst (France); P. Georges, Lab. Charles Fabry (France)

The Master Oscillator Power Amplifier (MOPA) configuration is very useful to extend laser performance of passively Q-switched Nd:doped microlasers in view of material processing applications. Different configurations and gain media have already been used in the past few years: multipass bulk amplifiers in longitudinal pumping, fiber amplifiers and recently side-pumped crystals operating at grazing incidence.

In this work, we present the use of a Nd:YAG crystal fiber for the amplification. The geometry of this gain medium (diameter 1 mm length 50 mm) can provide the good thermal management and high gain like in fibers and may sustain high peak power pulses like in the bulk materials.

As a seed, we used a passively Q-switched Nd:YAG microchip laser (Teem Photonics, France), generating 80 µJ, 500 ps pulses with a repetition rate of 1 kHz. The amplifier was pumped in counter propagation by a fiber coupled laser diode emitting 60 W at 808 nm (core diameter 100 µm NA 0,2). In a single pass, the pulse energy was multiplied by 15, reaching 1.2 mJ. This corresponds to a peak power of 2.4 MW, clearly beyond the typical limit of fiber amplifiers. To our knowledge, this is the most simple amplifier (only one gain medium and only one pass) ever developed able to reach the MW level with a significant average power (1.2 W).

7914-39, Session 9

2nd and 3rd harmonic generation from a fiber-amplified 100-ps, high-repetition rate and single-frequency passively Q-switched microchip laserA. Steinmetz, D. Nodop, Friedrich-Schiller-Univ. Jena (Germany); G. Sommerer, A. Wissel, S. Spiekermann, I. Freitag, InnoLight GmbH (Germany); J. Limpert, A. Tünnermann, Friedrich-Schiller-Univ. Jena (Germany)

We demonstrate second and third harmonic generation at a high repetition rate of 1 MHz obtained from a single-frequency, fiber-amplified microchip laser. The passively Q-Switched microchip laser provides pulses with duration of 100 picoseconds at a central wavelength of 1064 nm and consists of a monolithically bonded combination of Nd:YVO4-crystal and semiconductor saturable absorber mirror. The passively Q-Switched pulses are amplified in an ytterbium-doped photonic crystal fiber to an average power of 40 watts. The initial spectral width of ~20 pm is SPM-broadened to ~0.6 nm after the amplification.

The second harmonic generation stage is based on a non-critically phase-matched, 20 mm long LBO with no AR-coatings. The phase matching is achieved with a compact crystal oven, specially developed by INNOLIGHT GmbH. With a correction for the Fresnel-reflections of fundamental and second harmonic wave at the crystal interfaces, the SHG-stage generates an average power of more than 23.7 watts at 532 nm resulting in a conversion efficiency of 62.6%. For the third harmonic



generation, a subsequent sum frequency generator is placed in a small distance after the SHG-stage into collimated beam and is based on a 7 mm long, critically phase-matched LBO crystal with AR-coatings. An average power of 9.5 watts is achieved at 355 nm wavelength resulting in an IR-to-UV conversion efficiency of more than 23.8 %.

Finally, this pulsed laser source in combination with a high-repetition rate and sub-100 ps pulse durations in visible and ultraviolet region is an interesting tool for many scientific and industrial applications.


Refractive index changes due to gain/absorption in Yb-doped fibersD. N. Schimpf, E. Seise, C. Jauregui-Misas, D. Nodop, J. Limpert, A. Tünnermann, Friedrich-Schiller-Univ. Jena (Germany)

It is experimentally demonstrated that gain/absorption in Yb-doped fibers changes the refractive index. The corresponding phase-shifts are measured using crossed-beam spectral interferometry (XBSI). Employing this novel technique, we show for the first time to the best of our knowledge, on highly precise measurement of the phase change (with error < 0.05 rad) over a broad spectral window (~ 30 nm) in the vicinity of 1030 nm at ultra fine spectral resolution (~0.05 nm). XBSI gives the spectral phase-difference between a probe and reference arm . The probe arm contains a 52 cm-length of polarization maintaining (PM) Yb-doped fiber (PM YDFA with MFD of ~6µm), which is core pumped at a wavelength of 920 nm. The reference arm includes a passive PM single mode fiber (PM 980). The measured spectral phase profile is in excellent agreement with theoretical prediction using the small signal gain in the infra red. However, absorption in the UV also plays an important role. Using a small signal gain model, our measurement allows determining the relative phase-shift contribution due to UV and IR spectral features. For the present configuration it is shown that the contribution of IR to the overall phase change is about 7 %. The refractive index change due to both UV and IR gain/absorption is about 2E-5 for the present configuration.The result is of paramount importance for the design of next generation of high power fiber systems (pulse-shaping, mode-competition, coherent combination). Practical aspects will be highlighted.


Er-fiber laser based reference frequency standard for ultra dense WDM networksM. P. Nikodem, K. M. Abramski, Wroclaw Univ. of Technology (Poland)

Phase stabilized mode-locked lasers has already found application in laser metrology and spectroscopy. However, optical frequency combs (OFCs) stabilized using self-referencing stabilization scheme are too expensive to be widely used in the telecom industry. Also their stability is much higher then required for application in Ultra Dense WDM (UDWDM) networks for channel allocation and optical device calibration. On the other hand with the channel spacing of 6.25 GHz or lower laser sources and optical filters should be stabilized to some external frequency standard that is repeatable and only slightly depends on the environmental conditions. The perfect candidates for that are the absorption lines of hydrogen cyanide (HCN). Limited number of these lines is a disadvantage that can be overcome by merging them with optical comb. In proposed scheme frequency stabilized DFB diode is used to tie the OFC with the absorption line of the HCN. Beating between single-frequency diode and OFC is used to control the frequency of the OFC only by changing the pump power (cavity length is not controlled and its temperature is not stabilized). This low-cost method of absolute stabilization is realized using mostly telecom-based components. Optical comb is generated from all-fiber Er-laser with 169 MHz modes spacing. In our proof-of-principle experiment we have obtained the stability of the single OFC line below 1e-8 which was limited only by the stability of the DFB diode used. With presented method stability at the level of 1e-10

might be achieved which is sufficient for application in future UDWDM networks.


Actively Q-switched ytterbium-doped tapered fiber laserV. Filippov, J. Kerttula, Tampere Univ. of Technology (Finland); Y. K. Chamorovskii, K. Golant, Institute of Radio Engineering and Electronics (Russian Federation); O. G. Okhotnikov, Tampere Univ. of Technology (Finland)

Actively Q-switched fiber lasers are often the preferred choice for applications such as marking, machining, and range finding due to their high brightness, thermal robustness, and small footprint. Recently, we have proposed a tapered double-clad active fiber (T-DCF) as a favourable gain medium for high-power continuous-wave lasers and amplifiers. In this work, we have constructed an actively Q-switched T-DCF laser with an acousto-optic modulator (AOM). Actively Q-switched laser with T-DCF generated stable emission at 1065 nm over a wide repetition rate range of 0 Hz - 150 kHz, with the highest measured pulse energy of 2mJ limited by stimulated Brillouin scattering (SBS). We attribute the robust operation at low duty cycles to the intrinsically low amplified spontaneous emission (ASE), provided by the tapered fiber shape.

In addition to the advantage of the large mode area at the output end of the fiber, the T-DCF provides several other attractive features. First, the vignetting of co-propagating ASE results in ASE power loss in wide-to-narrow end propagation. Second, the low-mode nature of counter-propagating spontaneous emission, arising from mode selection in the narrow part of the fiber, leads to weaker amplification of the spontaneous emission. These two effects contribute to the low ASE background. Finally, fiber diameter modulation is a known method for SBS suppression, and another inherent property of the T-DCF. These characteristics allowed for generation of 2mJ, 64 ns pulses at very low duty cycles, up to single shot operation, illustrating the potential of the T-DCF for high-energy pulse generation and amplification.


Characterization of modal coupling of Bragg gratings in large-mode-area fibersA. T. Iho, A. Tervonen, Aalto Univ. School of Science and Technology (Finland); K. Ylä-Jarkko, Corelase Oy (Finland); S. Tammela, Beneq Oy (Finland); S. K. Honkanen, Aalto Univ. School of Science and Technology (Finland)

A new measurement technique for characterizing the effects of coupling of power from the fundamental mode to higher order core and cladding bounded modes occurring in a fiber Bragg grating (FBG) inscribed in a large-mode-area (LMA) fiber is demonstrated and studied. The method is based on inducing mode selective fiber bending losses on the modes propagating in the core and monitoring the power guided by the cladding of the LMA fiber. Besides transmitted, also reflected distributions of modes can be resolved in terms of the relative powers carried by them and thus the fraction of higher order modes (HOM) can be quantified. Additionally, the method can distinguish the mode content spectrally with high resolution. Sample FBGs having a chirped index profile are characterized using the method. The effect of direction reversal of light propagation on mode coupling in these gratings is also studied. Distinct wavelength regions corresponding to the fundamental mode coupling to the reflected fundamental mode, core HOM and cladding modes can be identified. The measurement method also reveals a wavelength dependent fine structure of the modal coupling, i.e. at certain wavelengths there can be coupling to all of the above mentioned modes simultaneously with varying amounts. These effects can be partly attributed to the phase matching between different modes being dependent on the spatial location in chirped gratings. It is shown that



our method can yield information that is useful for better design and optimization of fiber optic devices utilizing FBGs in LMA fibers, such as fiber lasers.


Arbitrarily-shaped bursts of picosecond pulses from a fiber laser source for high-throughput applicationsL. Desbiens, M. Drolet, V. Roy, M. M. Sisto, Y. Taillon, INO (Canada)

Picosecond laser sources are of great interest in industrial applications as they can be used to produce high quality features with greater throughput than femtosecond laser sources. Increasing the ablation efficiency of picosecond laser sources can be performed by bunching pulses in bursts and benefit from heat accumulation effects in the target. Pulsed fiber lasers are well suited for such a regime of operation, as the single pulse energy in a fiber is limited by the onset of nonlinear effects (SPM, SRS). Increasing the number of pulses to form a burst of pulses allows for average power scaling of picosecond fiber lasers. Furthermore, the ability to shape the burst envelope can be used to optimize the peak power along the entire burst duration (gain saturation compensation). It can also be used to control the heat deposition rate in the target and then optimize the ablation efficiency along the entire burst duration. We have developed a high-power fiber laser emitting arbitrarily-shaped bursts of picoseconds pulses at 20 W of average power. Burst duration can be varied from 2.5 ns to 80 ns. The burst repetition rate is externally triggered and can be varied from 100 kHz to 1 MHz. The single pulse duration is 60 ps and the repetition rate within a burst is 1.8 GHz. The output beam is linearly polarized (PER > 20 dB) and has a beam quality M2 < 1.15. The laser source has a tunable central wavelength at around 1064 nm and a spectral linewidth compatible with frequency conversion. A conversion efficiency higher than 60% has been obtained at 10 W of 1064-nm output power.


Yb-doped fiber amplifiers at 1014 8 nm and frequency quadruplingT. Hong, Shanghai Institute of Optics and Fine Mechanics (China) and Shanghai Advanced Research Institute (China)

We report our recent progress toward making a narrow-linewidth, 2-stage Yb-doped double-clad fiber amplifier at 1014.8nm and frequency quadrupling of this laser for 253nm. A narrow-linewidth continuous-wave semiconductor laser, tunable around 1014.8 nm, is used as a seed, feeding into the fiber amplifier, pumped by diode lasers at 980nm via fiber combiners. The gain fibers are cooled to liquid-nitrogen temperatures in order to suppress the absorption at the operating wavelength. The anticipate power could be multi-Watt. The emission will then be frequency quadrupled with optical cavities to achieve 253nm and used for laser cooling of mercury as a source for the new generation of optical frequency standards. This work is supported by Research Project of Shanghai Science and Technology Commission (Grant. No. 09DJ1400700) and National Natural Science Foundation of China (Grant No. 10974211).


Group-velocity dispersion in multimode photonic crystal fibers measured using time-domain white-light interferometryP. Böswetter, T. Baselt, F. Ebert, F. Basan, P. Hartmann, West Saxon Univ. of Applied Sciences Zwickau (Germany)

Optical fibers are used in various applications, e. g. optical communication, material processing, as a laser medium or to generate efficient supercontinua. For most of these applications the knowledge of the dispersion is an essential prerequisite. The dispersion and modal properties of photonic crystal fibers (PCF) strongly depend on the hole diameter and pitch. Since fabrication tolerances affect the structure of the photonic lattice the dispersion behavior as well as the number of guided transverse modes can differ from numerical calculations. Dispersion measurement of single-mode photonic crystal fibers is being well described in recent papers. However, the determination of dispersion in the presence of higher-order modes is much more difficult. To measure the dispersion of optical fibers with high accuracy, a time-domain white-light interferometer based on a Mach-Zehnder interferometer is presented. The experimental setup enables one to determine the wavelength-dependent differential group delay of light travelling through conventional fibers and PCFs within the wavelength range from 630 nm to 1550 nm. Interferences appear due to superposition of two laser beams, one propagating through the tested fiber and the other travelling through air. Measuring the different group delays of a step-index fiber shows the sufficient accuracy of the interferometer. This paper demonstrates a simple yet effective way to suppress higher-order modes to measure the chromatic dispersion of single-mode and multimode fibers as well.


Performance characterization of new erbium-doped fibers using MCVD nanoparticle doping processD. E. Boivin, A. Pastouret, E. Burov, C. Gonnet, O. Cavani, S. Lempereur, P. Sillard, Draka Comteq France (France)

In 2009, we introduced a new doping concept involving Al2O3/rare-earth nanoparticles (NP) in a MCVD-compatible process finding potential applications in Erbium-, Ytterbium- or Erbium-Ytterbium-doped fiber amplifiers and lasers [1].

This approach, motivated by the need for increased efficiencies and improved attributes, is characterized by the ability to control the rare-earth ion environment independently from the core composition. The NP matrix can therefore be viewed as an optimized sub-micronic amplifying medium for the embedded rare-earth ion.

The first experimental evidence to support this idea is reported in a comparative study with a standard process [2] where homogeneous up-conversion (HUC) and pair-induced quenching (PIQ) levels are extracted from Er3+ unsaturable absorption measurements. NP-based fibers are found to mitigate the effects of the Er3+ concentration increase seen in standard heavily-doped fibers.

This conclusion is particularly clear when focusing on the HUC coefficient evolution since, for a given type of NP, its level is independent from the Er3+ concentration in the doped zone.

In this paper, we address our most recent work completing these preliminary results. First, we investigate the quenching signatures of new NP designs and their behaviors when incorporated in different core matrices. Their interplay is further analysed by relating this set of measurements to practical EDFA performances. Gain and noise characteristics of typical WDM amplifiers operating points serve as key benchmarking indicators to identify the benefits of NP Erbium-doped fibers in the wide variety of EDFAs implementations.

[1] A. Pastouret et al., in Proc. SPIE 7195 (2009)

[2] D. Boivin et al., in Proc. SPIE 7580 (2010)


High-power double-clad Er-doped fiber laserM. Likhachev, L. Kotov, M. M. Bubnov, O. I. Medvedkov, A. M. Prokhorov General Physics Institute (Russian Federation); D.



Lipatov, Institute of Chemistry of High-Purity Substances of the Russian Academy of Sciences (Russian Federation); A. N. Guryanov, A. M. Prokhorov General Physics Institute (Russian Federation)

High-power single-mode fiber lasers operating at 1550nm are usually based on Er-Yb double clad (DC) fibers. The appearance of spurious lasing at 1064nm at the output power higher than 10W is a disadvantage of such lasers. Utilization of Er-doped fibers (EDF) could solve this problem, but due to a low absorption cross-section and high sensitivity to clustering, it is typically impossible to achieve high absorption from the cladding with a conventional pump scheme. The only exception is utilization of a 1530nm pump (absorption is ~ 3 time higher compared to the 980nm pump), but such pump sources are not commercially available at present.

Here we propose an all-fiber laser scheme based on DC EDF with a relatively low in-cavity fiber length (~10m) and a high slope efficiency ~ 30% compatible with commercially available pump sources at 980nm. A high clad absorption (>1 dB/m @ 980nm) was achieved by decreasing the outer diameter of the EDF down to 80um and by increasing the core diameter up to 20um. Pumping was performed with the help of a fiber taper, which delivers the pump from a conventional source (D=105um, NA=0.2) to the DC EDF (D=80um, NA=0.4). A high efficiency at low core aperture (NA~0.1) was achieved owing to the utilization of the novel P2O5-Al2O3-SiO2 (PAS) glass as the host for Er3+ ions. Optical power at 1560nm exceeded 6W at a pump level of 24W. The ways to increase the efficiency in the proposed laser scheme will be discussed.


Side-pumped, tapered fiber bundle for all-fiber counter-propagating pumped high-power fiber amplifiersT. Theeg, K. Hausmann, M. Frede, H. Sayinc, Laser Zentrum Hannover e.V. (Germany) and QUEST - Ctr. for Quantum-Engineering and Space-Time Research (Germany); J. Neumann, Laser Zentrum Hannover e.V. (Germany); D. Kracht, Laser Zentrum Hannover e.V. (Germany) and QUEST - Ctr. for Quantum-Engineering and Space-Time Research (Germany)

In this contribution we report an all-fiber side-pumped coupler for counter-propagating pumping of high power fiber amplifier. The fused fiber bundle consists of up to 6 pump units fused to a double-clad fiber with a pump cladding diameter of 250 µm (NA 0.46) and a signal core diameter of 25 µm. Each pump unit was made of a standard pump fiber with a core diameter of 105 µm (NA 0.15 or 0.22) and a cladding diameter of 125 µm fusion spliced to a short piece of coreless intermediate fiber with a cladding diameter of 125µm. Furthermore, the intermediate fibers were pre-tapered. The simulations showed that using the intermediate fiber increases the coupling efficiency and decreases the necessary taper length and taper ratio. Especially the reduced taper ratio and the consequential increased mechanical strength enabled the realisation of side-pumped tapered fiber bundles with 4 or 6 pump fibers. A coupling efficiency of typically > 90 % and a power handling of at least 25 W at a wavelength of 976 nm for each pump port was demonstrated experimentally. The maximum coupled power was limited by the available pump power. The reported technique is a promising approach towards side-pumped couplers for counter-propagating pumping that can handle hundreds of watts. Additionally, we demonstrated the implementation of the developed device to set up a high power continuous-wave single frequency fiber amplifier and a high peak power picosecond fiber amplifier operating at a wavelength of 1.06 µm.


A switchable and tunable dual-wavelength fiber laser based in a polarization-maintaining fiber Bragg grating and a Hi-Bi Sagnac fiber optical loop mirrorR. I. Alvarez Tamayo, Benemerita Univ. Autonoma de Puebla (Mexico); M. Durán Sánchez, Univ. Tecnológica de Puebla (Mexico); E. A. Kuzin, B. Ibarra-Escamilla, Instituto Nacional de Astrofísica, Óptica y Electrónica (Mexico); O. Pottiez, Ctr. de Investigaciones en Optica, A.C. (Mexico)

We demonstrate experimentally the operation of a linear cavity dual-wavelength fiber laser using a polarization-maintaining fiber Bragg grating (PM-FBG) as a reflector mirror generate the refection for the two laser emission lines. The PM-FBG is also used to tune the laser dual-wavelength total tuning range ~8 nm. The Hi-Bi Sagnac fiber optical loop mirror (FOLM) as a spectral filter is used to adjust the losses inside the cavity. The laser can be operating in wavelength-switch mode or in stable dual-wavelength mode by changes in temperature of the high birefringence fiber loop through the use of a Peltier device. We also study the laser dual-wavelength operation stability in a wider range of temperature changes through the variation of the amplitude transmission spectrum of the FOLM by twist application in the fiber loop. Results show that the proposed experimental setup can significantly increase the stability of dual-wavelength laser operation and from this point can be obtained a characterization to determine the temperature required for dual-wavelength laser emission with respect to the laser wavelength tuning, with a precision around the order of 10-1 °C.


Guiding and birefringent properties of a hybrid PDMS/silica photonic crystal fiberC. Markos, National Hellenic Research Foundation (Greece); K. G. Vlachos, Univ. of Patras (Greece); G. Kakarantzas, National Hellenic Research Foundation (Greece)

In this work, we demonstrate a Highly-Birefringent (Hi-Bi) Photonic Crystal Fiber (PCF) infiltrated with PDMS elastomer in order to enhance the sensitivity of the fiber to external temperature variations. Index guiding mechanism of the new PDMS/Silica structure and birefringent properties were investigated numerically using FDTD analysis. For the particular design of Hi-Bi PCF, the cut-off wavelength of the hybrid structure was calculated to be around 750 nm with acceptable loss of 0.0866 dB/cm at 633 nm wavelength. Due to the high thermo-optic coefficient of PDMS, the fiber exhibits high sensitivity to temperature changes. We experimentally demonstrate the sensitivity of the PDMS/Silica PCF that is ~ 0.37 rad.K-1 cm-1 for temperatures ranging from 20 to 80°C. The total length of the hybrid PCF examined was about 1.4 cm.


Influence of non-linear index on coherent passive beam combining of fiber lasersA. P. Napartovich, Troitsk Institute for Innovation and Fusion Research (Russian Federation) and Moscow Institute for Physics and Technology (Russian Federation); N. N. Elkin, D. V. Vysotsky, Troitsk Institute for Innovation and Fusion Research (Russian Federation)

Coherent laser beam combining is potentially attractive way to increase the combined beam brightness beyond the limits imposed on single-mode lasers by technological problems. The active control of individual laser beam characteristics is more flexible but essentially more



complicated in both, necessary equipment and service. Passive phase locking is a valuable alternative, since it does not need complex external management. A specific feature of fiber amplifiers and lasers is that they possess optical path differences of many wavelengths magnitude. Cold-cavity theory of coherent laser beam combining predicts in this case rather low efficiency of beam combining even for laser array of 8 lasers. Experiments, in contrast, demonstrated in such systems that high degree of phasing takes place for laser array of up to 20 lasers. Possible explanation of this discrepancy may be associated with a number of factors. These factors are: gain saturation, and intensity-dependent index, laser wavelength self-adjustment within the gain bandwidth. Besides, high degree of phase-locking can be established in self-sustained pulse periodic or spiky regime. Our approach takes injection controlled laser as a base unit of an ensemble. In an optically coupled laser array beams from the neighboring lasers are injected into the reference laser. Then a relationship between reference laser characteristics and whole wave field parameters can be found. As an example, fiber laser array with global coupling is numerically simulated by scanning laser wavelength within the gain bandwidth. It is found that non-linear index improves essentially passive phasing independent of sign of non-linearity.


Generation of 30 fs pulses at 670 nm using a frequency-doubled fiber laser system and a photonic-crystal fiber with two zero-dispersion wavelengthsR. Herda, T. Junggeburth, TOPTICA Photonics AG (Germany); K. P. Hansen, NKT Photonics A/S (Denmark); P. Leisching, TOPTICA Photonics AG (Germany)

The frequency-doubled radiation of an Erbium-doped fiber laser is used for supercontinuum generation in a small-core microstructured fiber with two zero-dispersion wavelengths (2ZDWs). The short-wavelength part of the generated supercontinuum is compressed. Furthermore we compare the spectra generated in fibers with one zero-dispersion wavelength and two zero dispersion wavelengths in experiments and simulations. The frequency-doubled radiation at 780 nm contains 102 mW of 100 fs pulses at a repetition rate of 80 MHz. Average powers up to 49 mW are launched into the 2ZDW photonic-crystal fiber. The photonic-crystal fiber is commercially available (NKT Photonics FemtoWHITE). The 2ZDW property of the fiber provides a different pulse shaping mechanism compared to supercontinuum generation with one zero dispersion wavelength. The broadening process is dominated by Four-Wave Mixing in contrast to the soliton-fission process in conventional fiber. While a conventional fiber gives a very structured spectrum, the 2ZDW fiber shows one broad distinct peak at wavelengths lower than the incident wavelength and one at wavelengths higher than the incident wavelength. Both peaks have a highly linear phase that indicates compressibility. In our experiment we observe the low wavelength peak around 670 nm and the long wavelength peak at 1100 nm. More than 35 mW is contained in the low wavelength peak. We use the anomalous dispersion of a SF10 prism compressor to compress the low wavelength peak of the spectrum. The compressed pulse has a central wavelength of 670 nm and a duration of 30 fs.


Photodarkening-induced refractive index change in ytterbium doped fibersC. Ye, J. J. Montiel i Ponsoda, A. Tervonen, S. K. Honkanen, Aalto Univ. School of Science and Technology (Finland)

Photodarkening is well known as an effect causing increase of optical loss in Yb-doped fibers under pumping. It has been recognized as a limiting factor of the long-term reliability of Yb-doped fiber lasers. In this paper, we show that photodarkening effect also causes refractive index

change in the Yb-doped fibers, and present quantitative measurement and analysis.

The refractive index change is measured by using a modal interference approach. The Yb-doped fiber sample is spliced with a core alignment offset, so that both LP01 and LP11 modes are excited. As the light propagates along the sample, a phase difference is generated between the two transverse modes. Modal interference occurs at the splice, resulting in periodic oscillation in the transmission spectrum. Consequently, the photodarkening-induced refractive index change is calculated from the phase shift of the interference fringe, with high accuracy.

The photodarkening-induced refractive index change is positive at the ytterbium lasing wavelengths near 1080nm, and it gradually approaches a saturated level. The index change is permanent at the room temperature. It is found that the value of refractive index change is linearly proportional to the excess loss at an arbitrary probe wavelength in the visible band. The maximum refractive index change in our experiment is close to 1×10-5, measured from different ytterbium fiber samples.

The potential influence of the photodarkening-induced refractive index change on fiber lasers is discussed, including mode distortion, phase retardation and degradation of fiber Bragg gratings.


Exact solutions for the pulse propagation in nonlinear optical mediumH. Lee, Korea Institute of Science and Technology (Korea, Republic of)

Priviously, the split-step Fourier method (SSFM) has been mainly used to solve pulse propagation in optical medium. However, SSFM, which is based on numerical analysis, cannot produce exact solution. In this paper, we get exact travelling wave solutions of the nonlinear partial differential equation (NPDE) based on the tanh, sech or tanh-sech methods. With these exact solutions, we consider the pulse-propagation problem and compare these with the results of the split-step Fourier method. In particular, we try to solve the NPDE including the higher-order nonlinear effects.


Second harmonic generation with continuous-wave fiber lasers in periodically-poled non linear crystalsM. Jacquemet, D. Harnois, A. Mugnier, D. Pureur, Quantel Group (France)

We present in this paper second-harmonic generation (SHG) results of multiwatts narrow-linewidth continuous-wave 1064 nm fiber lasers in periodically-poled crystals. We built two linearly polarized fiber lasers providing about 4.5W at 1064 nm with a PER greater than 15dB and a M² parameter <1.1. Both sources are based on an all-fiber master-oscillator power-amplifier scheme: this architecture allows separating linewidth management and power amplification. Combining the great beam quality and the narrow linewidth of both sources (<3MHz for single-frequency laser, 50 pm for the longitudinally multimode laser), efficient SHG of fiber lasers can be proceed in single-pass configuration with periodically-poled crystals. Among them, the most efficient for green generation at low power is certainly PPLN. We used a 20 mm long PPLN with a grating period of 6.9 µm and phase-matched temperature at 1064 nm around 70°C. We took care to perform SHG experiments in both cases with exactly the same focusing conditions, in order to compare only the influence of the spectral content of both sources. We generated more than 1W of green power (incident fundamental power of 4.5W) in the case of the spectrally multimode source. Frequency doubling of the single-frequency source yields to 0.47W at 532nm for 4.2W of 1064nm power. As previously theoretically predicted [1], we observe a factor of 2



between the efficiency of second-harmonic process between a spectrally highly multimode narrow-linewidth source and a single-frequency laser. Experimental results at higher power, as well as intensity noise measurements, will also be discussed.

[1] W-L Zhou, Y. Mori, T. Saski, and S; Nakai, “Theoretical analysis of multimode second harmonic generation”, Jpn. J. Appl. Phys., Vol. 34 (1995).

ACKNOWLEDGEMENT

This work has been partially funded by the Leadership in Fibre Laser Technology (LIFT) project under the 7th Framework Programme of the European Commission.


S2 imaging of LMA photonic crystal fiber amplifiersM. Laurila, Technical Univ. of Denmark (Denmark); T. T. Alkeskjold, J. Broeng, NKT Photonics A/S (Denmark); J. Laegsgaard, Technical Univ. of Denmark (Denmark)

Modal characterization of Photonic Crystal Fiber (PCF) amplifiers having Large-Mode-Area (LMA) is not trivial. The conventional cut-off wavelength of step-index fibers cannot usually be applied since the core NA is highly wavelength dependent. This forms a short-wavelength low-NA regime, where LMA PCFs can operate single-mode for wavelengths below the conventional cutoff wavelength, where step-index fibers typically operate Multi-Moded (MM).

Furthermore, conventional cut-off characterization methods, such as described in TIA-455-80C, are based on for example a bend reference technique, which is not applicable to rigid rod fibers.

Modal characterization using spatial and spectral resolved imaging (S2) has emerged as a novel method for characterizing modal content of LMA fibers without the need for bending. S2 imaging is a broadband interferometric method that requires a bandwidth covering multiple mode beating periods, which can be tens to hundred of nanometers with state-of-the-art LMA PCFs. Valid measurements require that the modal properties do not change significant over this bandwidth, but LMA PCFs typically have modal variations across this bandwidth and this sets some limitations of S2 imaging. In this paper, S2 imaging is applied to ytterbium-doped PCF and step-index LMA fiber amplifiers and the limitations of S2 imaging is addressed. The single-mode performance of LMA PCFs is further addressed by evaluating the higher-order mode content under different operating conditions such as offset launch, fiber coil diameter and orientation of the fiber cross-section within the coil.


Measurement of local polarization for multi-mode photonic crystal fibersO. A. Schmidt, C. Schulze, D. Flamm, M. R. Duparré, Friedrich-Schiller-Univ. Jena (Germany)

Radially and azimuthally polarized light are two examples of spatially inhomogeneous polarization. This so-called local polarization has several potential applications such as optical particle trapping, microscopy, and material processing. Fiber lasers generating radially and azimuthally polarized light use doughnut modes consisting of higher order modes that provide advantages for high power fiber lasers. Photonic crystal fibers (PCFs) allow the fabrication of large mode area fibers guiding a small and elaborately chosen number of modes. Polarization tailoring requires higher order modes, for example, radially and azimuthally polarized light is a superposition of LP modes. Therefore, we use a solid core PCF that can guide the fundamental mode as well as the next two higher order modes to demonstrate the measurement of the local polarization. This measurement technique is based on the combination of a rapid Stokes parameter measurement with the modal analysis using a computer-generated hologram. This combination enables us to

determine amplitude and phase delay of all guided modes. Thus, we know the complete field emerging from the PCF and we can specify the polarization state of every guided mode as well as the local polarization of the multi-mode beam. We use this method to obtain and visualize the modal polarization states depending on the input polarization and the spatially varying polarization ellipses. This measurement technique allows controlling and adjusting the local polarization at the fiber end facet as well as the design of new high power and polarization-optimized fiber lasers.


60 fs pulses from an all-fiber dissipative soliton erbium oscillatorN. B. Chichkov, K. Hausmann, D. Wandt, Laser Zentrum Hannover e.V. (Germany) and QUEST - Ctr. for Quantum Engineering and Space-Time Research (Germany); U. Morgner, Leibniz Univ. Hannover (Germany) and Laser Zentrum Hannover e.V. (Germany) and QUEST - Ctr. for Quantum Engineering and Space-Time Research (Germany); J. Neumann, Laser Zentrum Hannover e.V. (Germany) and QUEST - Ctr. for Quantum Engineering and Space-Time Research (Germany); D. Kracht, Laser Zentrum Hannover e.V. (Germany) and QUES - Ctr. for Quantum Engineering and Space-Time Research (Germany)

In this contribution we present our results of an all-fiber erbium oscillator with net-normal dispersion. The fiber oscillator consisted of a ring resonator with 40 cm SMF-28 fiber, 46 cm of Corning Metrocor fiber, and 100 cm of erbium-doped fiber with group velocity dispersions of -23 ps2/km, 10 ps2/km, and 28 ps2/km respectively, resulting in a total cavity dispersion of 0.023 ps2. The erbium doped fiber had a doping concentration of 0.23 mol-% and was core-pumped by a Raman laser at 1480 nm. A custom made 1480/1550 WDM consisting of the Metrocor fiber injected the pump light into the resonator and acted as a spectral filter with a bandwidth of 70 nm. Polarization controllers and a fiber pig-tailed polarizing beam splitter were used to initiate mode-locking by nonlinear polarization evolution and a fiber isolator ensured unidirectional operation. The laser operated at a repetition rate of 104 MHz and the output was taken from the rejection port of the polarizing beam splitter.

At a pump power of 2.5 W self-starting mode-locking was achieved with an output power of 164 mW corresponding to a pulse energy of 1.6 nJ. The output spectrum covered the spectral range from 1490 nm to 1600 nm at -10 dB and corresponded to a Fourier-limited pulse duration of 55 fs. The pulses are positively chirped throughout the resonator and had an output pulse duration of approximately 330 fs. A prism compressor was used to dechirp the output pulses to a duration of 60 fs.


Microstructured fiber with high-birefringence and low-noncircularity mode fieldA. N. Denisov, A. E. Levchenko, S. L. Semjonov, E. M. Dianov, Fiber Optics Research Ctr. (Russian Federation)

A new design of polarization-maintaining microstructured fiber will be presented. A solid core is surrounded by two or more concentric rings of holes of constant diameter. The distance between adjacent holes in the first ring is constant except for one or two pairs of holes, for which the spacing is increased.

The influence of the core shape and size on the phase and group birefringence as well as on the group velocity dispersion is studied. Birefringence and dispersion of fibers proposed are calculated by the finite element method.

It is found that the fiber of such a design can demonstrate high birefringence (up to 5x10-3) and simultaneously low circular asymmetry of the mode shape. Moreover, owing to introduction of small ellipticity



of the core (first ring of the holes) exact equality of the mode sizes in the two orthogonal directions was achieved.

A set of experimental samples was fabricated. The measured values of birefringence were in excellent agreement with the calculated ones. The results of the of h-parameter measurements of the samples demonstrate their capability to maintain polarization state of the guided light along the fiber length. Thus, the fibers developed hold promise for practical applications.


Modal analysis of beams emerging from a multi-core fiber using computer-generated hologramsC. Schulze, O. A. Schmidt, D. Flamm, M. R. Duparré, Friedrich-Schiller-Univ. Jena (Germany); S. Schröter, Institut für Photonische Technologien e.V. (Germany)

Multi-core fiber (MCF) lasers represent a promising opportunity in scaling the output of fiber lasers to yet increasing power levels. To ensure high brilliance and beam quality of MCF lasers, several approaches exist to favor the in-phase supermode like external Talbot or Fourier cavities or various types of fiber couplers.

Our method of using computer-generated holograms for modal analysis provides an opportunity to control the laser output modally resolved. The method is capable of online-monitoring the modal output of fiber lasers while tuning parameters such as pump power, temperature, bending radius etc., and could provide deeper insight into the dynamic processes going on in the laser.

Furthermore also the passive transport of highly brilliant laser light emerging from fiber lasers is an important feature. Our method can be used to control the beam delivery system as well.

Here we investigate the beam emerging from a 3m-long passive LMA-MCF at two different wavelengths. The wavelengths are chosen to demonstrate the modal analysis of differently complex mode mixtures provided by the differing number of excitable modes at these wavelengths. The MCF modes are calculated using a finite difference and a finite element approach respectively. At 1064 nm the MCF is measured to be single-mode whereas at 633 nm 10 modes can exist in the fiber. At 633 nm excitation of distinct mode mixtures is done by changing the input coupling situation enabled by a nano-positioning device. Furthermore it is shown how bending of the fiber alters the modal spectrum.


Impact of modal interference on high-power fiber laser systemsC. Jauregui-Misas, T. Eidam, J. Limpert, Friedrich-Schiller-Univ. Jena (Germany); A. Tünnermann, Fraunhofer-Institut für Angewandte Optik und Feinmechanik (United States)

In the last year there have been reports from various research groups around the globe about the onset of modal instabilities (or dramatic degradations of the beam quality) in high average power laser systems. Many of these reports describe how the usual output Gaussian beam of the fiber laser system becomes a higher order mode (most typically a LP11-like mode).This effect is commonly attributed to transversal hole burning. However, this theory alone cannot really explain reports of this effect being observed in fibers in which only the central part of the fiber core has been doped (such as the Rod-Type fibers).

As far as we know, up to date no theoretical work has been published in the subject to investigate the true physical origin of this effect. In this work we present such study and the conclusions obtained from it.

It has been found that conventional transversally-resolved rate equation models, able to take transversal hole burning into account, cannot

explain this effect when preferential gain designs are investigated. According to our investigations the inclusion of modal interference along the fiber is crucial to explain this effect in all type of fibers. Unfortunately current BPM models (able to account for modal interference along the fiber) are not able to take into account transversal hole burning. Thus, we have developed an advanced active fiber model that combines BPM and the transversally-resolved rate equations. This model reveals, for the first time, the important role played by modal interference along the fiber.


Defining coiling adiabaticityS. O’Reilly, B. Gauvreau, B. Sévigny, ITF Labs./Avensys Tech (Canada)

Coiling configurations are of major importance in fiber lasers performances and form factor. Therefore, we present a formalism to quantitatively express the adiabaticity of an optical fiber coil based on normalized coupling coefficient between modes. We present typical coiling configurations on few perspectives. The goal is to evaluate the capability of a coiled system to preserve the modal repartition of the optical intensity throughout the coil since it has a significant impact on losses and beam quality at output of the fiber.

The adabiatic condition over the normalized variation of bend radius can be written similarly as for adiabaticity in tapers. Basically, it is a quantitative comparison between coupling length of the first two modes and phase beatlength. We show that in order to preserve adiabaticity, a coil has to show smooth variations of its radius of curvature. Since smooth variations of radius of curvature represent a figure of interest, a free fiber coil fixed at both ends is really robust in terms of adiabaticity because the stress repartition tends to smooth variations in the radius of curvature.

A simple set-up for characterization is proposed. A large mode area (LMA) fiber is coiled between mode field adapters (MFA). Single mode fiber is put at both end so injection is made only in LP01. It results in an interferometer in which the spectrum fringes visibility scales with the coupling between the first modes. Therefore, it can be used as a characterization set-up to compare adiabaticity between various coil configurations.


Breaking the symmetry: enhanced transversal mode discrimination in large pitch photonic crystal fibersF. Stutzki, F. Jansen, C. Jauregui, Friedrich-Schiller-Univ. Jena (Germany); J. Limpert, Friedrich-Schiller-Univ. Jena (Germany) and Helmholtz-Institut Jena (Germany); A. Tünnermann, Fraunhofer-Institut für Angewandte Optik und Feinmechanik (Germany)

Photonic Crystal Fibers (PCFs) offer an excellent design flexibility, but, in contrast to step-index fibers, they are not single-mode in the strict sense. Single-transverse mode operation can be obtained by higher confinement losses for Higher Order Modes (HOMs), i.e. mode discrimination. Recent simulations and experiments with PCFs have shown that the hexagonal Large Pitch Fiber (LPFs) design offers high mode discrimination, thus allowing for mode field area scaling while maintaining a robust single mode operation. Nevertheless the mode discrimination is consistently reduced for large mode areas.

In this paper we discuss the influence of different geometries on the mode discrimination and propose non-hexagonal designs for a drastically increased mode discrimination. Some geometries discriminate several higher order modes intrinsically. For example, the LP11-like HOM in a 5-fold symmetry, i.e. a design with 5 holes in the inner ring, experiences much higher confinement losses compared to a typical hexagonal design (inner ring with 6 holes). For comparable fundamental mode properties



(1dB/m, 10000µm²), this simple 5 hole design increases the LP11-like HOM propagation losses from around 40dB/m (6-fold) to 65dB/m (5-fold).

Asymmetries can also be introduced by different hole sizes within the structure. This allows to break the symmetry in hexagonal structures, while still benefiting from the easy stack and draw production. Such a fiber design can boost the confinement losses of the LP11-like HOM to 70dB/m with a fundamental mode area of 10000µm². Thus, these asymmetric designs will enable effective single-mode fibers with mode field diameters well beyond 100µm.


All-fiber 7x1 signal combiner for incoherent laser beam combiningD. Noordegraaf, M. D. Maack, P. M. W. Skovgaard, J. Johansen, NKT Photonics A/S (Denmark); M. Blomqvist, Optoskand AB (Sweden); J. Laegsgaard, Technical Univ. of Denmark (Denmark)

The combiners are fabricated using filament based GPX glass processing stations from Vytran. These stations are used for fusing of bundles, tapering and splicing. The signal fibers are step-index single-clad fibers with a core diameter of 17 µm and an outer diameter of 125 µm. A bundle of 7 of these fibers is inserted into an F-doped glass capillary tube. This fiber filled capillary tube is fused and tapered down into a solid glass element. The tapered element will act as a MM waveguide with a core that consists of the fused SM fibers and a cladding formed by the low-index capillary tube. The tapered MM fiber tip is spliced to a MM fiber with a core diameter of 100 µm, an outer diameter of 660 µm and an NA of 0.22. The MM fiber is fitted with a high-power QBH connector from Optoskand. The connector includes a mode-stripping section and an anti-reflection coated end-cap.

The combiner is characterized at low power (~1 W of light from a 1060 nm fiber laser). The transmission loss for the signal in the combiner, as well as the far-field and M2 value out of the MM fiber are measured. Measurements of the far-field, show that the NA filling corresponds to 95% of the light having an NA lower than 0.06. A Spiricon laser beam profiler is used to measure the beam quality and the value for M2 was found to be 6.5. The average transmission loss for the signal is 0.21 dB.

The combiner is expected to enable signal beam combining with an average output power of several kW. A high-power test of the combiner has not yet been made. These tests are ongoing and will be included in the final conference paper.


LMA fibers modal decomposition using image factor analysisJ. Max, B. Gauvreau, B. Sévigny, M. Faucher, ITF Labs./Avensys Tech (Canada)

We propose a novel application of the factor analysis method commonly used in the field of spectroscopy. The aim is to quantitatively decompose light flux carried by a large mode area fiber on the individual contributions of its supported modes. Access to such cost effective and easy to implement diagnosis tool could largely help improving modal quality preservation in components and systems based on such fibers. Analysis is based on the near field imaging of the multimode fiber output facet and projection of the intensity pattern on refractive index profile based eigenvectors computation. Fictitious computer generated data sets with known modal power distributions are very precisely reconstructed with the method. Experimental measurement of LP01 conservation trough a mode field adapter revealed more than 95% of power carried by the fundamental mode of the large mode area fiber output. Both validation approaches confirmed the high precision and performance of the method.


Stimulated Raman scattering mitigation through amplified spontaneous emission simultaneous seeding on high-power double-clad fiber pulse amplifiersM. Melo, J. M. Sousa, M. O. Berendt, Multiwave Photonics (Portugal)

The evolution of stimulated Raman scattering (SRS) with simultaneous seeding of amplified spontaneous emission (ASE) and signal pulse on forward high power Ytterbium-doped double-clad fiber amplifier is studied. A cascaded amplifier chain configuration is used for the study. It is shown that the generated Raman-stokes pulses occur together with the signal pulse causing severe optical output signal pulse distortions. It is also experimentally demonstrated that increasing the ASE seeded power the energy contained in the first-order Stokes Raman pulse can be significantly reduced. The seeded ASE changes the gain dynamics of the high power amplifier stage, reducing SRS and having a strong impact on the enabled extracted energy within the signal pulse. The signal and Raman spectral components are discriminated and the temporal evolution of each component for different input ASE power levels is analyzed individually. Quantitative results of the energy transferred from the signal to the Raman pulse can in this way be accurately obtained.


Temporal compression of the wavelength swept output from a Fourier domain mode locked laserC. M. Eigenwillig, W. Wieser, B. R. Biedermann, T. Klein, R. A. Huber, Ludwig-Maximilians-Univ. München (Germany)

In a Fourier domain mode locked (FDML) laser a narrowband optical bandpass filter is driven resonantly to the roundtrip time of light in the resonator. FDML lasers are usually used as narrowband rapidly wavelength swept lasers for bioimaging and optical coherence tomography - however, the output is also equivalent to broadband (~100nm), long (~µs) and highly chirped (~10ns/nm) laser pulses. In this paper we investigate in how far these sweeps can be compressed, the observed factor of ~1000 strongly indicates at least partial coherent superposition of the different spectral components.


Radiation effects on fiber amplifiers: design of radiation tolerant Yb/Er-based devicesS. Girard, Commissariat à l’Énergie Atomique (France); A. Laurent, iXFiber SAS (France); M. Vivona, Lab. Hubert Curien (France); T. Robin, B. Cadier, iXFiber SAS (France); Y. Ouerdane, A. Boukenter, Lab. Hubert Curien (France)

Rare-earth (Ytterbium, Erbium of Yb/Er) doped optical fibers have been shown to be very sensitive to radiations. As a consequence and despite the small length used for space applications, the study of their vulnerability to the harsh environment associated with space missions remains crucial. Most of the previous studies have been devoted to their characterization in a passive configuration (with no amplification). These studies showed that for most of these fibers, the point defects at the origin of the fiber degradation, through the radiation-induced attenuation phenomenon, seem related to the glass matrix chosen for the active ion incorporation. In this paper, we present our recent results obtained in 2010 on a set of prototype RE-doped fibers, developed with specific designs to improve their radiation tolerance, exposed to gamma-rays at low dose rate (0.3 rad/s) and at doses up to 100 krad. The response of



the fibers in their passive and active configurations (pumped with high-power diodes and amplifying the infrared signal) will be presented with an analysis of the fiber amplifier performance changes in terms of power and spectral characteristics. Previous studies showing that applications at doses larger than 5 krad seem not possible due to the significant degradation of Er/Yb amplifier output power. However, we have observed a good tolerance of some of our specific fiber designs to radiations. We will show that with an optimized design, degradation of less than 20% is measured for the output power of the 1.0W Yb/Er fiber amplifier after a ~100 krad dose.


Synchronized dual wavelength programmable laser with 75nm wavelength difference tuningB. Buorgoyne, Y. Kim, A. Villeneuve, Genia Photonics Inc. (Canada)

Today, most tuneable fiber-based lasers generating dual wavelengths yield identical spacing between the wavelengths while tuning. Having variable spacing between the wavelengths would give way to a tuneable THz source, up to 10 THz, through difference frequency generation. We present here such a source which is a programmable laser based on an actively mode-locked dispersion-tuned picosecond fiber laser delivering two wavelengths in the C and L bands respectively. The difference between these wavelengths is rapidly adjustable over a 75 nm band while the laser is tuning. Laser pulses are as short as 30 ps.

The programmable laser is composed of a sigma cavity with a semiconductor optical amplifier (SOA), a circulator, a modulator and a 20% output coupler in the ring part. The branch part of the sigma cavity is made of C+L WDM with two identical chirped fiber Bragg gratings spanning 100 nm with 10 ps/nm dispersion on the C band and -10 ps/nm dispersion on the L band. The cavity is operated around 614 MHz which correspond to the 53rd and 54th harmonic of the signal in the C and L band respectively. Both pulses arrive synchronized at the laser output. Other noteworthy applications for this laser are differential absorption Lidar (DIAL) and CARS spectroscopy in the low end of the fingerprint area.


Coherent coupling of spectrally broadband fiber laser channelsA. I. Khizhnyak, V. B. Markov, J. M. Kilpatrick, I. Tomov, MetroLaser, Inc. (United States)

Fiber lasers turned into the most effective source of a high-power radiation with a good beam quality. However the output power of this type of laser though high, is limited by thermal and nonlinear effects. Coherent combining of multiple fiber channels should enable further increase of the output power.

This presentation discusses a novel approach in laser beam combining. The suggested beam-coupling technique is based on the primary feature of the stable-configuration laser cavity to automatically match the wave-front of the lasing modes to the output coupler (mirror) over the entire lasing spectral bandwidth. Another important feature of the proposed scheme is in its ability to match the outputs of the individual fiber channels to a corresponding high-order cavity mode. A simple phase corrector can then be applied to transform this high-order mode in the diffraction-limited output beam. Contrasting known schemes of coherent beam combining the proposed method enables to couple multiple laser channels with spectrally broadband oscillation and no need in an external control of each channel.

The presentation provides a detailed analysis of the proposed technique. A four-channel beam combining in the Hermit-Gaussian TEM11mode and its transformation into a single diffraction quality beam is used to illustrate the performance of the proposed method.


The high-power femtosecond pulses amplified by an all-fiber system based on the model of self-similar amplificationT. Yang, T. Wang, D. Yang, J. Wang, M. Sang, Tianjin Univ. (China)

First the parameters, such as the energy and the pulse width, of the input pulses into the amplifier are adjusted by a variable optical attenuator and by varying the pumping current of a femtosecond pulse fiber oscillator laser. It is found that the amplified pulses have the same pulse width of about 250 fs for the input pulses with different pulse widths from 125 fs to 230 fs when the input power and the output power to the amplifier are fixed at 1.1 mW and 500 mW, respectively. That means that the pulse width of the output pulses is not sensitive to the shape of the input pulses to the amplifier as the amplified pulse width did not change with the input pulse widths when their energy of input pulse and the gain of the amplifier are not changed. On the other hand, the pulse width of amplified pulses is increased from 239 fs to 265 fs when the average power of the input pulses increases from 0.4 mW to 1.2 mW along with a fixed input pulse width of 150 fs at a fixed gain in the amplifier. It is concluded that the pulse width of the amplified pulses is only related to the energy of the input pulses and has nothing to do with its pulse width. Combining the above results, we verified that the working mode of the amplifier is consistent with the typical self-similar amplification theory model. Finally, near transform-limited with the time-bandwidth product of 0.342, ultra-short pulses with high average power of 500 mW and pulse width of 113 fs at the central wavelength of 1552 nm are obtained by compressing the output pulses from the fiber amplifier by using a segment of DCFs with large negative group velocity dispersion.


High-gain resonance Er:glass amplifierP. Wan, J. Liu, PolarOnyx, Inc. (United States)

A novel and compact hybrid resonant amplifier has been demonstrated for further scaling energy/power level from 1550 nm fiber lasers by using Er3+/Yb3+ co-doped phosphate glass. The seed laser is a pulse shaping fiber laser at an eye safe wavelength of 1550 nm. The wavelength was temperature controllable and was stabilized at one of amplifier’s resonance wavelengths. Pulse shaping technology provides a vital solution in generating different input wave formats, for both CW and pulsed outputs. For Pulsed amplification, the pulse duration can be varied from microsecond to nanosecond and repetition rate from a few Hz to 250 kHz. Gain as high as 20 dB was obtained for nanosecond pulses at 10 Hz repetition rate, comparing with a single pass gain of only 0.64 dB. High OSNR, high extinction ratio and low background noise were also achieved at this low repetition rate by our new amplification method. In our CW input experiment, an optical conversion efficiency of up to 20% was obtained.

This new optical amplifier is very compact. The size of the amplifier is less than 5 cm. It has a great potential for broad applications.


Measurement techniques for the evaluation of photodarkening in fibers for high-power lasersG. Perrone, A. Braglia, M. Olivero, A. Neri, N. Boetti, J. Lousteau, D. Milanese, Politecnico di Torino (Italy)

The photodarkening is a performance degradation with time suffered by more or less all the ytterbium doped silicate fibers currently used in high power lasers. It is caused by chemical reactions triggered by exposure to high power densities for long periods, although the researchers are still debating whether it is due to the pump only or there



is a contribution from the signal power too. In any case, to the best of our knowledge, a procedure for comparing the behavior of two fibers with respect to photodarkening without having to build a laser and let it run for several hundred hours is still missing. This papers attempts to overcome this limitation analyzing the implications that can be inferred from the measurement of the UV-to-IR spectral emission in “fresh” fibers in different operating conditions. Both cladding and core pumping schemes are considered. The latter is particularly interesting since it allows obtaining a power density comparable to that achievable with several hundred W of pump in the cladding just by using a single mode diode with an output power of less than 1 W. The paper considers also the effect of launching signal pulses with high peak power (1kW - 10 kW) from solid state lasers to study the combined effect of pump and the signal powers, a situation that is particularly critical for fiber lasers delivering short and very intense pulses, like those used for example in solar cell scribing applications.


Fabrication of pump combiners for high-power fiber lasersG. Perrone, A. Braglia, M. Olivero, A. Neri, Politecnico di Torino (Italy)

Pump combiners are key components of high power fiber lasers since they allow working in very high power regimes by coupling several pump laser diodes with the active fiber. A typical all fiber pump combiner is made by collapsing into a single output a multimode fiber bundle; then in some cases a feedthrough is requested and this implies that a special fiber is placed in the center of the input bundle to allow the laser signal to pass though the combiner. This pump combiner is more complex to fabricate because it requires placing the fibers within the input bundle with tighter mechanical tolerances. Despite studies on the efficient combination of pump diodes are quite numerous, there are few manufactures of these devices, probably because their fabrication still requires a considerable amount of “hand-made” processes. Moreover, commercial devices are specific for certain types of output fibers only. In this paper we present a pump combiner fabrication procedure that we have devised to improve the manufacturing repeatability, at least for small production batches. A specific mechanical setup that simplifies placing the input fibers in their proper location prior to the tapering and fusion processes has been developed and tested in the 7-to-1 (i.e. seven high power inputs) and (6+1)-to-1 (i.e. six high power inputs and a feedthrough) cases. Preliminary results obtained fabricating pump combiners using different types of output double clad fibers have confirmed the validity of the proposed approach.


Thermal modeling of active fiber and splice points in high-power fiber laserZ. Huang, T. Y. Ng, C. P. Seah, DSO National Labs. (Singapore); H. T. Lim, Nanyang Technological Univ. (Singapore); R. Wu, DSO National Labs. (Singapore)

In the power scaling of all-fiber-based fiber laser, deleterious thermal effects exist in DCF, especially in the splice joint between active fiber and passive fiber after a TFB coupler. Splicing loss due to filament deposits is converted to a localized heat source that is hard to remove [1][2]. NA of LMA fiber is typically in the range of 0.06 and fluctuations in the temperature can affect this NA. With a refractive index change of 10-5/oC, it is found that a loss of as small as 0.004% of pumping power converted to heat across the splice joint can effect a temperature rise of 100 oC under 1-2kW of pumping and natural convection [3]. Application of a recoat can maintain the mechanical strength of fiber and NA of the inner cladding/outer cladding. However, a recoat or a thermal gel layer subsequently causes additional heating due to scattered power. Heating power is small and localized and often unmeasurable by optical means.

Unlike previous works, surface temperature monitoring of the fiber with pumping power up to 430W under various cooling configurations and then adjusting magnitude of the heat load in FEM software to match the temperatures has enabled an accurate characterization of the heating in splices, enabling an accurate scaling prediction of the splice point temperature at higher power. Excellent agreement between modelling and experimental data tested to 430W was achieved. Using rate equation modeling to estimate the heat load in the core, we have confidently estimated the heat load along critical sections of the fiber. Using suitable thermal interface materials, we can remove 100W/m of heat and also, the localized heat load at the splicing joint. More experimental results and details will be shown during the presentation.

References

[1] B. Zintzen, A. Emmerich, J. Geiger, D. Hoffman, and P. Loosen, “Effective Cooling for High-Power Fiber Lasers”, Proc. Fourth International WLT-Conference on Lasers in Manufacturing, Munich, (2007)

[2] Brown, D. C., Hoffman, H. J., Thermal, Stress, and Thermo-Optics Effects in High Average Power Double-Clad Silica Fiber Lasers, IEEE J. of Quant. Electronics, 37,2, (2001)

[3] Incropera, F., Dewitt, D., Fundamentals of Heat and mass Transfer, John Wiley & Sons, Hoboken, (2002)


Phase locking multiple fiber lasers using Talbot self-imaging effectR. Zhou, J. W. Haus, Univ. of Dayton (United States); B. Ibarra-Escamilla, Instituto Nacional de Astrofísica, Óptica y Electrónica (Mexico); Q. Zhan, P. E. Powers, Univ. of Dayton (United States)

Large mode area (LMA) fibers can be used as Talbot self-imaging device to design high power fiber lasers. In this paper, we launch beams from periodically arranged fiber amplifiers into a LMA fiber. The LMA fiber length is either full or half Talbot distance period, which is found numerically. In our numerical simulation, it is found that for 1D slab waveguide the self-images periodically appear; however, in 2D cylindrical waveguide it becomes non-periodic.

The Talbot distance we choose is the shortest distance at where the self-image most resembles the original image. We couple 4 periodically placed Gaussian beams which come from single mode fibers into a LMA fiber with a 200 micron core diameter, core and cladding index be 1.506 and 1.460, respectively, and the Talbot period distance is found to be about 11.28 mm. With up to 8 periodically placed Gaussian beams, the Talbot distance is the same. The LMA fiber length can be a full Talbot length with a structured reflectivity mirror that only reflects the most concentrated self-imaged beam spots. The mirror structure can be fabricated according to the original fiber amplifier placement. The LMA fiber length can also be half Talbot distance with a mirror that reflects the whole beams with a certain amount of reflection.

For the numerical simulations, we used half-Talbot-period length LMA fiber with a 50 percent reflector attached to its output end. The results show that the coupling efficiency between the LMA fiber and the single mode fibers is about 16 percent for 4 fiber amplifiers and 19 percent for 8 fiber amplifiers. We investigated how phase and amplitude variations among the fiber amplifiers could affect the coupling efficiency and the output far-field pattern.


Near-circular pump guidesJ. J. Morehead, M. H. Muendel, JDSU (United States)

In order to break the circular boundary’s conservation of angular momentum and scramble rays through the core, pump-guide shapes such as D’s and polygons such as octagons are often used. However, such fibers can be hard to cleave and lose power or brightness in splices with circular passive fibers. How nearly circular can a double-clad fiber’s



pump guide be and still have good absorption in the core?

In quantifying the core absorption for shapes which are nearly circular, ray tracing cannot be trusted. The perturbations can be smaller than a transverse wavelength, in which case even a large effect on rays has little effect on waves. Instead we find many eigenmodes of various pump-guide shapes and compare the statistics of their core-overlaps. Using a Fourier-Bessel-based algorithm, we find the lowest 10,000 modes of many pump-guide boundary shapes which are nearly circles. Actual absorption in the core is higher than any simple model, since uncontrolled refractive-index inhomogeneities (microbends) scatter pump light and couple the modes together. Thus we compare the mode overlap statistics for different shapes for a relative answer.

Some shapes with radial modulation less than 3% have better core overlaps than the standard octagonal shape, whose modulation is over 6%.


Experimental study of phase locking of fiber collimators using internal beam-tail interferenceL. A. Beresnev, U.S. Army Research Lab. (United States); M. A. Vorontsov, T. Weyrauch, Univ. of Dayton (United States); G. W. Carhart, J. Liu, U.S. Army Research Lab. (United States)

Experimental study of near-field phase locking of seven coherent fiber collimators array is described. As a metric of “internal” feedback loop the periphery areas (tails) of beams outgoing from three adjacent fiber tips are used before the beams are clipped by the lens apertures. The intercepted tails of beams were redirected back to collimator array forming the interference pattern in between adjacent collimators. One spot of the pattern was selected by pinhole with photodiode and used as a signal for SPGD controller to lock the phase of three adjacent collimators. For phase locking of seven beamlets three pinholes were used. Signals from three photodiodes placed behind of pinholes allowed us to lock the phase of all seven collimators without bulky beam splitters outside of collimator array.


High-average power optical demodulation of a fiber amplified phase modulated single-frequency signalS. Rhein, Friedrich-Schiller-Univ. Jena (Germany); O. Schmidt, Fraunhofer-Institut für Angewandte Optik und Feinmechanik (Germany); H. Zimer, JT Optical Engine GmbH + Co. KG (Germany); T. Schreiber, R. Eberhardt, A. Tünnermann, Fraunhofer-Institut für Angewandte Optik und Feinmechanik (Germany)

We report to the best of our knowledge on the first proof-of-principle experiment demonstrating high average power (10 W) optical demodulation of a fiber amplified phase-modulated single frequency diode laser signal with a sideband suppression of more than 30 dB. The single frequency signal with a bandwidth of <100 kHz is phase-modulated with an amplitude of 1.435 rad at an RF frequency of 80 MHz in a fiber integrated format with the goal to increase the SBS threshold. It is fiber amplified to 10 W average power in a standard double clad fiber (Liekki 125/10) of 2 m length pumped at 976 nm. Due to their low power handling capabilities, waveguide-type phase modulators cannot be applied for demodulation. Therefore, we send the collimated output through a free space lithium-niobate phase modulator. The RF signal for demodulation has been electronically clock synchronized to the modulation frequency with an additional phase shift of pi. This enables us to remove the modulation in a simple manner to recover a single-frequency signal with a long term (several minutes) sideband suppression

of more than 30 dB independent of the output power of the amplifier. Without fiber amplifier, the suppression is at least 36 dB (measurement limited) and indicates that the phase noise contribution from the fiber amplifier is negligible, which otherwise would degrade the demodulation quality. Additionally, no thermal lensing in the lithium-niobate crystal has been observed up to this power level, which suggests that the scheme allows for much higher power.


Fused fiber pump and signal combiners for a 4-kW ytterbium fiber laserD. Woll, J. Luu, C. Zhang, J. J. Morehead, J. Segall, K. Tai, B. M. Kharlamov, H. Yu, L. E. Myers, M. H. Muendel, JDSU (United States)

We report the development of fused-fiber pump and signal combiners. These combiners are critical enabling components of an Ytterbium fiber laser emitting 4 kW of 1080-nm radiation. The fiber laser system consists of seven fiber laser modules and a 7:1 signal combiner. The laser modules are end-pumped by 90 915-nm JDSU L4 diode-lasers, yielding a nominal pump power of 900 W. The diode laser radiation is coupled into the laser fiber through a 91:1 fused-fiber pump-combiner. The input fibers of this pump combiner are standard 105/125-um multimode fibers with an NA of 0.22. These fibers form a hexagonal fused-fiber bundle, which is tapered to match the cladding diameter of the laser fiber. 86% of the light exiting the pump-combiner is emitted within an NA of 0.32, and all measurable power is emitted within an NA of 0.45. The typical insertion loss of pump combiners is < 1%. The high-brightness radiation of seven laser modules is combined into a single output fiber using a 7:1 fused-fiber signal combiner providing a total power of 4.1 kW in the single output beam. The beam parameter product of the combined output is 2.5 mm-mrad. The low insertion loss of < 2% indicates that the signal combiner is suitable to handle even higher laser powers.


Comparative numerical study of efficiency of energy deposition in femtosecond microfabrication with fundamental and second harmonics of Yb-doped fiber laserA. Dostovalov, S. A. Babin, Institute of Automation and Electrometry (Russian Federation); V. K. Mezentsev, M. Dubov, M. Baregheh, Aston Univ. (United Kingdom)

Femtosecond (fs) laser microfabrication has become an important emerging technology in photonics. Several exciting applications have been recently reported such as low loss waveguides, gratings, couplers, microchip lasers etc. Although most of these demonstrations are produced using solid state fs lasers, fiber fs and subpicosecond fiber lasers are very attractive alternatives for fs microfabrication due to their turn key robustness and efficiency.

We present the results of numerical modelling of energy deposition for typical regimes of femtosecond inscription by fundamental harmonic of an Ytterbium-doped fiber laser at 1030 nm and its second harmonic at 515 nm. Laser wavelength is an important operation parameter since shorter wavelength usually allows for finer resolution of the inscription process. Numerical model comprises extended nonlinear Schroedinger equation including effects of diffraction, dispersion, Kerr nonlinearity, multi-photon absorption, plasma absorption and defocusing.

The results are presented in terms of dependence of the absorbed energy accumulated by the stationary cloud of electron-hole plasma versus initial pulse energy. All the modelling was performed for laser pulses with fixed FWHM pulsewidth of 300 fs focused with an objective of NA=0.4 routinely used in fs inscription experiments. We scanned inscribing pulse energy in the range between few nJ and few hundred nJ in both cases. Inscription with second harmonic is generally more efficient than with the



fundamental harmonic in terms of energy absorption. Absorbed energy is several times, up to an order of magnitude higher in case of second harmonic which should substantially decrease the inscription threshold.


Power-equalized multiwavelength Raman fiber laser based on random distributed feedbackS. A. Babin, Institute of Automation and Electrometry (Russian Federation); A. E. El-Taher, P. Harper, Aston Univ. (United Kingdom); D. V. Churkin, E. V. Podivilov, Institute of Automation and Electrometry (Russian Federation); J. D. Ania-Castanon, Consejo Superior de Investigaciones Científicas (Spain); S. K. Turitsyn, Aston Univ. (United Kingdom)

Raman fiber lasers (RFLs) having a broad and flat gain spectrum are treated as perspective sources of multiwavelength radiation with a great potential in various applications such as telecommunications and sensing. Different types of multiwavelength RFLs, mainly in ring cavity configurations, have been proposed and demonstrated utilizing multi-channel filters based on interferometers of different kind. Simpler linear schemes with fiber Bragg gratings (FBGs) as narrowband reflectors are of great interest, but not so many lines with relatively large spectral spacing has yet been obtained.

In this paper we compare output characteristics of the Raman fiber laser with linear cavity formed by an array of 22 FBGs with 0.8-nm spacing and broadband mirror of two principally different kinds: 4% Fresnel reflection at the fiber end facet and distributed feedback based on Rayleigh scattering (RS) in the whole ~22 km-long fiber. In spite of much lower reflection coefficient, RS-based random distributed feedback (RDFB) provides much better equalization between the generated lines at comparable and even slightly higher total output power. A qualitative model based on the analysis of axial power distributions and corresponding linear and nonlinear losses in two cases is proposed that explains the observed effects and principal advantage of RDFB scheme.

As a practical result, RFL with RS-based RDFB cavity generating 22 power-equalized stable lines with spacing of ~100 GHz (close to ITU grid) in 1552-1570 nm range at Watts power level with 40% efficiency is demonstrated.


On the theory of the modulation instability in optical fiber amplifiers and lasersA. M. Rubenchik, Lawrence Livermore National Lab. (United States); S. K. Turitsyn, Aston Univ. (United Kingdom); M. Fedoruk, Institute of Computational Technologies (Russian Federation)

Abstract: The modulation instability (MI) in optical fiber amplifiers and lasers with anomalous dispersion leads to CW radiation break-up and growth of multiple pulses. This can be both a detrimental effect limiting the performance of amplifiers, and also an underlying physical mechanism in the operation of MI-based devices. Here we revisit the analytical theory of MI in fiber optical amplifiers. The results of the exact theory are compared with the previously used adiabatic approximation model and the range of applicability of the later is determined. MI dependence on initial conditions is discussed.

7914-39, Session 10

Fibers and fiber-optic components for high-power fiber lasersH. Zimer, JT Optical Engine GmbH + Co. KG (Germany); M.

M. Kozak, JENOPTIK Optical Systems GmbH (Germany); A. Liem, JT Optical Engine GmbH + Co. KG (Germany); F. Flohrer, JENOPTIK Optical Systems GmbH (Germany); F. Dörfel, JENOPTIK Laser GmbH (Germany); P. Riedel, Guided Color Technologies GmbH (Germany); S. Linke, Friedrich-Schiller-Univ. Jena (Germany); R. J. Horley, F. Ghiringhelli, A. Harker, S. Desmoulins, M. N. Zervas, SPI Lasers (United Kingdom); J. Kirchhof, S. Unger, S. Jetschke, Institut für Photonische Technologien e.V. (Germany); T. Peschel, T. Schreiber, Fraunhofer-Institut für Angewandte Optik und Feinmechanik (Germany)

The major challenge in the development of monolithic kW class cw fiber lasers is the efficient conversion of pump photons into a high brightness laser beam under the constraints of heat management, long term stability and nonlinearities. This article reviews the interaction of some fiber related aspects as e.g. fiber core composition, photo darkening and modality, as well as their influence on system complexity and power scalability. Recent work on active fibers, pump couplers, mode field adaptors and other fiber-optic components will be presented.

7914-40, Session 10

Advanced specialty fiber designs for fiber lasersL. Dong, Clemson Univ. (United States)

Progress in advanced specialty fibers is the foundation to further breakthroughs in fiber lasers. Recently, we have helped to advance several areas of developments in specialty fibers and would like to review these efforts in this talk. One area is the development of wide band air-core fibers with an innovative square lattice cladding and the demonstration of a factor of two improvements in bandgap over conventional hexagonal lattice. These air-core fibers are critical for fiber delivery solution of both CW and pulsed fiber lasers in the future. Another area is a new development in simulation of SBS gains in optical fibers by incorporating leaky acoustic modes. These leaky acoustic modes are often omitted. It is essential that they are considered in SBS simulations in fibers, because they are normal solutions to the waveguide equations and have similar loss to guided modes where the mode loss is dominated by material loss. This leads to much improved resolution of SBS gain spectrum in fibers and to new insights on the limit of SBS suppression based on anti-guide acoustic waveguide designs.

7914-41, Session 10

Pulsed Tm3+-doped fiber amplifier with 300-W average output powerY. Tang, L. Xu, Y. Yang, Shanghai Institute of Optics and Fine Mechanics (China); J. Xu, Shanghai Jiaotong Univ. (China)

Combining a high-power Q-switched Tm:LuLiF crystal laser and two-stage large-fiber-core Tm3+ fiber amplifiers, 300-W average-power 2-µm pulsed laser output has been realized, which is believed to be the highest average-power ~2-µm pulsed fiber laser device. Pumped by 793-nm laser diode modules, the master oscillator fiber power amplifier system provided a slope efficiency of ~50% with respect to the launched pump. The high-power output was believed to originate from efficient suppression of ASE (amplified spontaneous emission) by end-fusing a short piece of 1mm-diamter passive silica fiber to the Tm3+ gain fiber, 10-degree angle cleavage of the fiber ends, and retro-feedback block of signal beam with optical isolators. At all amplification levels, no evident SBS (stimulated Brillouin scattering) or SRS (stimulated Raman scattering) signal was observed. With an acousto-optic modulator, the amplified pulse duration can be tuned between tens of ns and ~1 µs, and the pulse repetition rate can be changed from 500 Hz to 50 kHz. The maximum pulse energy was over 20 mJ, and the maximum pulse peak power was over 200 kW. By using the fiber-coiling-induced mode-filtering



effect, output beam quality of M2<1.2 was obtained. Further scaling of 2-µm average output power and pulse energy from Tm3+ fiber amplifiers were discussed.

7914-42, Session 10

Robust single-mode ytterbium-doped large pitch fiber emitting 294 WF. Jansen, F. Stutzki, T. Eidam, C. Jauregui, J. Limpert, A. Tünnermann, Friedrich-Schiller-Univ. Jena (Germany)

Photonic crystal fibers (PCF) such as large pitch fibers (LPF) and leakage channel fibers (LCF) provide large mode field areas by employing large hole-to-hole distances (pitch > 10 * wavelength). These designs enhance single-mode operation by a discontinuous core-cladding interface (“modal sieve”) resulting in selective high confinement losses for higher order modes. However, in contrast to LCFs, that usually require bending, the significantly higher mode discrimination of LPFs enables the design of straight rod-type fibers, which are essential for scaling the fundamental mode diameter beyond 50µm.

Here we present a rod-type LPF amplifier with nearly diffraction limited beam quality and a record average power of 294W with a simulated and measured mode field diameter of ~62µm. The seed in the experiments were broadband nanosecond pulses with an average power of 15 W. The LPF under study consists of two rings of air holes surrounding a one-hole-missing core in a hexagonal matrix. As predicted by simulations, the 1.2m long fiber exhibits a robust single mode behaviour even under x-y-misalignment of the seed coupling. Four fiber samples with four different hole diameters but constant pitch were experimentally studied, leading to M² values between <1.2 and <1.4. At the highest average output power, the M² was measured to be <1.4 corresponding to the simulated M² of the fundamental mode. Consequently, the higher M² is basically caused by the hexagonal geometry, resulting in an excellent single mode pointing stability.

7914-43, Session 10

4-kW fiber laser for metal cutting and weldingD. A. V. Kliner, K. Chong, J. Franke, T. E. Gordon, J. G. Gregg, V. M. Issier, B. M. Kharlamov, A. Kliner, M. Kobayashi, J. Lugo, J. Luu, J. J. Morehead, M. H. Muendel, L. E. Myers, K. Nguyen, JDSU (United States); H. Sako, Amada Co., Ltd. (Japan); K. R. Schneider, J. Segall, K. Shigeoka, JDSU (United States); D. B. Soh, Sandia National Labs., California (United States); R. Srinivasan, D. Tucker, D. Woll, D. L. Woods, H. Yu, C. Zhang, JDSU (United States)

We have developed a commercial 4-kW fiber laser suitable for metal cutting and welding. The laser consists of seven, 600-W modules whose outputs are combined with a fused-fiber 7:1 combiner. The system architecture has several advantages for practical applications:

1. The modules are pumped with single-emitter (L4) diodes, providing high reliability, high efficiency, and graceful degradation in case of diode failure.

2. The modules are end pumped using a 91:1 pump combiner, eliminating the need for complex pump/signal combiners that can cause loss and beam-quality degradation.

3. The system is monolithic (no free-space beams) for high reliability and environmental stability.

4. The average power can be varied by varying the diode drive current or by varying the duty cycle (pulse duration or repetition rate) in modulated operation.

5. The laser modules are field replaceable with a simple splicing procedure.

At 4 kW power, the measured beam-parameter product (BPP) is 2.5 mm-

mrad, the measured 8-hr power stability is 0.02% rms and 1.2% peak-to-peak, the central wavelength is 1081 nm, and the linewidth is 1.2 nm FWHM. A version of the laser system employing three modules and a 3:1 output combiner has also been developed to provide up to 1.8 kW power with higher beam quality (BPP = 1.8 mm-mrad). These lasers have been incorporated into Amada metal cutting machines; examples of aluminum, mild-steel, and stainless-steel sheet and plate cutting will be shown.

7914-44, Session 11

Challenges for fiber lasers in defense applicationsM. Niece, High Energy Laser Joint Technology Office (United States)


7914-45, Session 11

Fiber lasers in the materials processing marketD. A. Belforte, Photonics Spectra (United States)


7914-46, Session 11

Fiber lasers in PV manufacturingB. P. Piwczyk, IPG Photonics Corp. (United States)

Over the past decade lasers have played an increasingly important role in the manufacturing processes for solar cells, starting with wafer cutting and continuing with, fabrication of selective emitters, high speed hole drilling for back contacts, edge isolation, and edge deletion processes. The increasing capabilities of fiber lasers in terns of power, efficiency and reliability open up no possibilities to improve the efficiency of solar cells and the cost of manufacturing. This paper will describe recent work on laser fabrication of semiconductor junctions using high power fiber lasers and reduction of process steps for solar cell fabrication.

7914-47, Session 12

Laser beam projection with adaptive fiber array systemsM. A. Vorontsov, T. Weyrauch, Univ. of Dayton (United States); L. A. Beresnev, G. W. Carhart, U.S. Army Research Lab. (United States); S. L. Lachinova, L. Liu, Univ. of Maryland, College Park (United States); J. Liu, U.S. Army Research Lab. (United States)

We discuss recent progress in the development of a coherent fiber-array system composed of densely-packed fiber collimators with built-in capabilities for adaptive wavefront phase piston and tilt control at each fiber collimator. The paper presents the first experimental results of adaptive dynamic phase distortion compensation with an adaptive phase-locked array of fiber-collimator system in the target in the loop (TIL) arrangement. The control systems are based on the stochastic parallel gradient descent (SPGD) optimization technique. We also present a new near-field phase-locking techniques that don’t require either outgoing beam energy splitting or an unresolved target to close the feedback loop. We also discuss impact of photo-fly time delay on coherent beam combining over extended distances.



7914-48, Session 12

Coherent combining of ultrashort fiber-amplified laser pulsesE. Seise, A. Klenke, J. Limpert, A. Tünnermann, Friedrich-Schiller-Univ. Jena (Germany)

In this contribution we present a new peak-power scaling concept based on the coherent addition of ultrashort laser pulses. A related technique is the use of an enhancement cavity for power scaling. For continuous-wave and q-switched laser systems the concept of combining several laser beams to one final beam is well known. Coherent combining of ultrashort pulses results in new challenges for the realization. For instance, dispersion management and nonlinear effects in each channel have to be carefully considered. To analyze the requirements for the addition of two or more ultrashort laser pulses several simulations have been carried out. Based on these results we realized different setups to show the feasibility of the combination of ultrashort laser pulses. Both chirped pulse amplification (CPA) systems and non-CPA systems using active fibers have been studied. These setups consist of an interferometer (Michelson and Mach-Zehnder type) with an active stabilization. A polarization-dependent beam splitter was used to combine the two channels of the interferometer. The stabilization was done by measuring the polarization state after the combination with a Hänsch-Couillaud-detector and driving a piezo-stage to adjust the optical path length of one interferometer arm. With a laser system using a mode-locked laser source it was possible to coherently combine two fiber-amplified ultrashort laser pulses with a combining efficiency of more than 95% and a pulse width significantly below 1ps. Further work will include the adaptation of the shown concept to high energy chirped pulse amplification systems.

7914-49, Session 13

Coherent and spectral beam combining of fiber lasersS. J. Augst, MIT Lincoln Lab. (United States)


7914-51, Session 13

High-power spectral beam combining of fiber lasers by thermal tuning of volume Bragg gratings with ultra-high-spectral densityD. R. Drachenberg, I. B. Divliansky, CREOL, The College of Optics and Photonics, Univ. of Central Florida (United States); V. I. Smirnov, Optigrate Corp. (United States); G. B. Venus, CREOL, The College of Optics and Photonics, Univ. of Central Florida (United States); L. B. Glebov, CREOL, The College of Optics and Photonics, Univ. of Central Florida (United States) and OptiGrate Corp. (United States)

Lasers that produce 100 kW level diffraction limited power will require beam combining due to fundamental thermal and nonlinear limitations on the power of single emitter lasers. Towards this goal, we present high power, high spectral density beam combining by volume Bragg gratings of five 150 W beams with a spectral separation of 0.25 nm between beams, the narrowest to date for high power. Within 1 nm, 750 W total power is combined with greater than 90 % efficiency. Combined beam quality is discussed including the effect of unequal individual beam divergences on the combined beam quality. The individual input beams may have unique divergences as they enter the system, and the heated VBGs may introduce very slight changes in divergence to each beam. These small differences in beam divergence between the beams will not degrade the M2 of the individual beams, but the composite M2

after combination can be adversely affected if the beams do not have equivalent divergence at the output of the system. Tolerances on beam divergence variation are analyzed and discussed. High power beams transmitting through or diffracting from a VBG can experience different distortions resulting from thermal effects induced in the VBGs. Each beam also experiences a different aberration, as no two beams pass through the same number of identical VBGs. These effects are studied with experiment compared to modeling. Possible methods of beam quality improvement are discussed.

7914-52, Session 13

Global sensitivity analyses of coherent beam combining of fiber amplifier arrays by the use of numerical space filling designsA. Azarian, O. Vasseur, B. Bennaï, L. Lombard, G. Canat, ONERA (France); V. Jolivet, ONERA, (France); Y. Jaouën, Telecom ParisTech (France); P. Bourdon, ONERA (France)

Coherent beam combining of fiber amplifier arrays is a promising way to increase power of fiber lasers, and overcome the physical limitations to fiber laser power scaling. To increase power, a high number of fiber amplifiers are needed, and the relative output phase deviations need to be precisely controlled to preserve beam quality. Therefore, the study of the impact of phase errors at the output of each fiber on different combining efficiency criteria is crucial for the evaluation of the beam combining process stability. We perform sensitivity analyses on two different array configurations to study this impact: a square of 16 fibers and a hexagon of 19 fibers. Our method distinguishes from previous works by a global analysis: instead of considering the phases as independent Gaussian variables and propagating the uncertainties on the far field intensity, we modify simultaneously all the phase errors using numerical space filling designs. These designs are used to study the outputs of a system all over the domain of variation of the inputs, enabling metamodel constructions and high dimensional code explorations with limited numbers of code runs. This methodology makes it possible to investigate more precisely the role of each fiber and specially the role of interactions between fibers onto the combination with less runs than classical approaches. We present results exhibiting different interactions between fibers with respect to the array configurations and with respect to two different shaped wavefronts. Finally, we demonstrate that we can study arrays of more than 100 fibers.

7914-53, Session 14

Graphene mode locked ultrafast fiber lasersD. Tang, H. Zhang, Nanyang Technological Univ. (Singapore); Q. Bao, K. P. Loh, National Univ. of Singapore (Singapore)

Graphene is a single two-dimensional (2D) atomic layer of carbon atoms arranged in a honeycomb crystal lattice. It possesses not only unique electronic properties, e.g. zero energy band gap and linear energy-momentum dispersion relation, but also remarkable optical properties, e.g. despite being only one atomic thick, it absorbs a significant fraction of 2.3% of the incident white light. In this talk we show that light absorption of graphene can be readily saturated as a result of the Pauli blocking, and the saturable absorption of graphene has an ultrashort recovery time and small saturation intensity. Moreover, either the pristine graphene or graphene-polymer nanocomposite was used to mode lock the erbium-doped fiber lasers. It was found that stable mode locked pulses could always be obtained. In a dispersion-managed cavity laser we have achieved stable mode locked pulses with 7.3nJ pulse energy and 415fs pulse width, without damaging the graphene mode locker. A unique property of graphene saturable absorption is its wavelength independence. Taking advantage of the property, we have demonstrated wide range (~ 30nm) wavelength tunable soliton operation in erbium-doped fiber lasers, operating either in the normal or anomalous cavity dispersion regime. In addition, a graphene mode locked Yb-doped



fiber laser was also demonstrated. Our experiments clearly show that graphene is a true full-band saturable absorber that can be potentially used to mode lock multi-color lasers.

7914-54, Session 14

Power scaling and high-power applications of a femtosecond enhancement cavityI. Pupeza, Max-Planck-Institut für Quantenoptik (Germany) and Ludwig-Maximilians-Univ. München (Germany); T. Eidam, Friedrich-Schiller-Univ. Jena (Germany); J. Kaster, J. Rauschenberger, Max-Planck-Institut für Quantenoptik (Germany) and Ludwig-Maximilians-Univ. München (Germany); B. Bernhardt, A. Ozawa, E. E. Fill, Max-Planck-Institut für Quantenoptik (Germany); V. Pervak, Ludwig-Maximilians-Univ. München (Germany); A. A. Apolonski, Max-Planck-Institut für Quantenoptik (Germany) and Ludwig-Maximilians-Univ. München (Germany); T. Udem, Max-Planck-Institut für Quantenoptik (Germany); J. Limpert, Friedrich-Schiller-Univ. Jena (Germany); Z. A. Alahmed, A. M. Azzeer, King Saud Univ. (Saudi Arabia); A. Tünnermann, Friedrich-Schiller-Univ. Jena (Germany); T. W. Hänsch, F. Krausz, Max-Planck-Institut für Quantenoptik (Germany) and Ludwig-Maximilians-Univ. München (Germany)

By coherently overlapping the pulses of a high-repetition rate femtosecond laser in a high-finesse passive cavity, an intracavity power enhancement exceeding a factor of a thousand can be achieved. Enhancement cavities are employed to (i) increase the net efficiency of nonlinear interactions such as second-harmonic generation and (ii) to enable nonlinear processes requiring high peak intensities at multi-MHz repetition rates, such as high-harmonic generation. Other proposed applications, for which enhancement cavities constitute a very promising approach, include the generation of high-brilliance hard X-rays via inverse Compton scattering, THz generation and the detection of the birefringence of vacuum.

With the advent of high-power ultrafast laser systems, the investigation of the power and intensity scalability of enhancement cavities becomes crucial. In the present contribution we discuss the power scalability of a bow-tie cavity, seeded by a 78 MHz, Yb-based fiber CPA system producing 200 fs pulses and a maximum average power of 50 W. We demonstrate peak intensities exceeding 10^14 W/cm^2 at the 22 µm-radius cavity focus (1/e^2-intensity calculated at the resonator stability range center) for several enhancement regimes with circulating powers of up to 72 kW. We find that the enhancement is primarily limited by intensity-dependent processes occurring in the cavity mirrors, such as third-harmonic generation and nonlinear group delay dispersion and we propose solutions to overcome these limitations. Furthermore, we report on our group’s recent progress regarding the applications of this system.

7914-55, Session 14

High-energy fibre CPA system based on a single stage rod type fiber amplifier in double pass configurationY. Zaouter, F. Morin, C. Hönninger, E. Mottay, Amplitude Systemes (France)

In this contribution we report the generation of high energy, high temporal quality ultrashort pulses from a compact hybrid bulk-fiber non-linear chirped pulse amplifier setup. The main amplifier, made of a state-of-the-art Ytterbium doped rod-type fiber, uses a double-pass architecture and therefore benefits from a very high gain (up to 50 dB) together with high average power extraction and excellent beam quality (M2 < 1.3). Thanks to sufficient pulse stretching and good management of dispersion and non-linearities in the system, 250 fs pulses with

excellent temporal quality and energies of up to 200 µJ are extracted from the system. The achieved pulse peak power of ~750 MW is among the highest ever generated by a fiber CPA while the optical setup only requires one single power amplifier and fits in a 1.2 m x 0.45 m x 0.12 m housing. Furthermore, theoretical simulations are also carried out and revile that peak power well in excess of a GW can be generated with a slight modification of the system. A new setup is being built and very last results will be presented. The simplicity of the optical architecture is particularly important and attractive to highly demanding industrial and scientific applications. Excellent experimental results have already been obtained with this system on various topics such as thin film processing for photovoltaic applications, internal marking of transparent materials for traceability and anti-counterfeiting and optical parametric amplification of sub-10 fs pulses.

7914-56, Session 14

Passive mode-locking using multi-mode fiberE. Ding, J. N. Kutz, Univ. of Washington (United States); S. Lefrancois, F. W. Wise, Cornell Univ. (United States)

The performance of a multi-mode, mode-locked fiber laser cavity is investigated both experimentally and theoretically. The motivation for using multi-mode fiber is to circumvent the nonlinear penalties arising from the large peak powers in the mode-locked cavity. The nonlinear penalties induce multi-pulsing instabilities in the laser cavity, thus limiting the total energy delivered in a single pulse. It is this multi-pulsing instability that must be avoided in order to increase the performance of the laser cavity. Multi-mode fiber greatly reduces the peak power by providing a greatly enhanced effective cross-sectional core area.

Although the peak intensities are indeed reduced, the higher-order mode content that results from mode-coupling induces the multi-pulsing instability nonetheless at similar power levels, thus potentially negating the beneficial effects of the reduced peak powers. Regardless, by carefully engineering the cavity parameters, such as the coupling constant between modes and the linear loss in the higher-order modes, enhanced performance can be achieved over single-mode fiber lasers for the parameters considered. A detailed theoretical model, the first to characterize multi-mode mode-locking, is developed and is shown to be consistent with the experimental findings. It further provides a framework to optimize the performance of the multi-mode fiber laser as a function of the various cavity parameters such as the suppression of higher-order mode content and coupling between modes. Our findings suggest that, due to presence of higher order mode content, multi-mode fiber lasers must be carefully engineered in order for their performance to surpass that of standard single-mode fiber lasers.

7914-57, Session 14

Generation and amplification of 350 fs, 2 µm pulses in Tm:fiberR. A. Sims, P. Kadwani, L. Shah, M. C. Richardson, CREOL, The College of Optics and Photonics, Univ. of Central Florida (United States)

Amplified ultrashort pulses at 2 µm are of great interest for atmospheric sensing, medical, and materials processing applications. We describe the generation and amplification of femtosecond 2 µm pulses in thulium doped silica fiber. Mode-locked eye-safe laser pulses at ~ 2 µm were generated in a Tm:fiber oscillator using a single-walled carbon nanotube saturable absorber. A ring cavity was core pumped at 1.55 µm with an Er:Yb fiber laser. Stable mode-locking was achieved at a repetition rate of 87 MHz with soliton pulses reaching energies of ~ 40 pJ with a spectral bandwidth of ~12 nm. Autocorrelation pulsewidth measurements indicated bandwidth limited pulses of ~350 fs duration. This oscillator was used to seed a Tm:fiber amplifier with either free space or fiber coupling. Effects of dispersion compensation and pulse amplification are reported.



7914-58, Session 14

Non-adiabatic pulse compression using cascaded higher-order solitonsJ. N. Kutz, Univ. of Washington (United States); Q. Li, Hong Kong Polytechnic Univ. (United States); A. P. K. A. Wai, Hong Kong Polytechnic Univ. (Hong Kong, China)

A detailed investigation of a cascaded higher-order soliton compression scheme is presented. The results show clear and distinct advantages over the standard methods of adiabatic pulse compression, higher-order soliton compression and higher-order soliton compression in dispersion decreasing fiber. Specifically, the cascaded higher-order soliton compression can achieve a very large compression factor using two or three nonlinear fibers with different constant anomalous dispersion coefficients. Each fiber length is shorter than half of its soliton period. The 2-stage fifth-order soliton compression gives a compression factor of 284 and corresponding pedestal energy of 71%. The 3-stage second-order soliton compression gives a compression factor of 87 and corresponding pedestal energy of 27%. The 3-stage third-order soliton compression gives a compression factor of 600 and corresponding pedestal energy of 59%. These results are highly favorable when compared to the standard techniques previously used, thus suggesting that the cascaded higher-order soliton compression technique is a promising technology that is easy to implement with current technological components. In terms of compression factor and pedestal energy, the cascaded higher-order soliton compression clearly provides the best performance among the four compression techniques. Moreover, the fabrication of fiber with different constant dispersion segments is much easier compared to the fabrication of dispersion decreasing fiber. Specifically, photonic crystal fibers (PCFs) offer greatly enhanced design freedom, such as the precise control of the chromatic dispersion, compared to standard optical fibers. With fiber taping technologies and the use of PCF becoming commonplace, we anticipate the experimental realization of the cascaded N-soliton compression in the near future.

7914-59, Session 15

Measuring the spatial distribution of rare-earth dopants in high-power optical fibersA. D. Yablon, Interfiber Analysis (United States)

For the first time, a non-destructive technique for spatially resolving the location and relative concentration of rare-earth dopants in an optical fiber is demonstrated. This novel technique is based on computerized tomographic detection of spontaneous emission and achieves micron-scale spatial resolution with the aid of oil-immersion imaging. Spontaneous emission from a suitably pumped fiber specimen is shown to obey mathematical relations similar to those encountered in conventional computer-assisted tomography (CAT) medical X-ray scans. Recently, several groups have reported on performance benefits resulting from complex spatial distributions of rare-earth dopants in high-power optical fibers. For example, the relative gain experienced by a desirable signal mode can be enhanced by increasing its spatial overlap with the gain-producing dopant. In addition to elucidating interactions between the signal, pump, and dopant distributions, the measurement described here can reveal shortcomings in fiber manufacturing. Since the technique is non-destructive and can be scanned along the fiber length, it can map the full 3-dimensional distribution of complex rare-earth-doped fiber structures including gratings, physical tapers, fusion splices, and even couplers. Experimental data obtained from commercially available Yb-doped silica optical fibers is presented. In principle the technique can also be applied to Er-, Bi-, or Tm-doped silica or non-silica optical fibers.

7914-60, Session 15

High-power dissipative soliton laser using chirally-coupled core fiberS. Lefrancois, F. W. Wise, Cornell Univ. (United States); T. S. Sosnowski, Arbor Photonics, Inc. (United States); A. Galvanauskas, Univ. of Michigan (United States); C. Liu, Arbor Photonics, Inc. (United States)

We report an ultrafast modelocked laser based on large mode-area Chirally-Coupled Core (CCC) fiber and dissipative soliton pulses. This combines high-energy pulse shaping with effectively single-mode large core fiber technology. The fiber used has a core size of 33 um and a numerical aperture of 0.06. A secondary core wrapped around the central core ensures effectively single-mode operation through quasi-phase-matched resonances to higher-order modes. A 4 m long all-normal dispersion cavity delivers chirped pulse energies above 40 nJ that dechirp below 200 fs, with average powers above 2 W directly from a fiber oscillator. Peak power is measured in excess of 125 kW and was verified through non-linear broadening in single-mode fiber. Effectively single-mode operation is verified by an M2<1.15, as well as negligible amplitudes of secondary pulses induced by higher-order fiber modes. Numerical simulations show that both the chirped pulse duration and bandwidth breath by more than two times in the cavity. This confirms dissipative soliton pulse shaping based on spectral filtering of a chirped pulse. Using a shorter cavity with 2 m of fiber, pulses as short as 105 fs are obtained, owing to lower dispersion and larger spectral broadening. This demonstrates that Chirally-Coupled Core fiber delivers single-mode performance sufficient for systems as exacting as mode-locked oscillators. Due to its robust single-mode performance and all-glass structure, which enables fused component integration, we anticipate this fiber can lead to all-integrated, high peak power modelocked sources.

7914-61, Session 15

Hybrid large mode area photonic crystal fiber for distributed spectral filtering and single-mode operationF. Poli, E. Coscelli, Univ. degli Studi di Parma (Italy); T. T. Alkeskjold, NKT Photonics A/S (Denmark); D. Passaro, A. Cucinotta, S. Selleri, Univ. degli Studi di Parma (Italy); L. Leick, J. Broeng, NKT Photonics A/S (Denmark)

Efficient suppression of Amplified Spontaneous Emission (ASE) and Stimulated Brillouin Scattering (SBS) are two key challenges in the development of high-power fiber amplifiers and lasers. In-line filtering of both ASE and SBS can be obtained with hybrid Photonic Crystal Fibers (PCFs), where the cladding is formed by a combination of air-holes and high-index rods, thus providing guiding by both total internal reflection and photonic bandgap (PBG). The fiber transmission band can be tuned by adjusting the resonance wavelengths of the high-index inclusions.

In this paper, the properties of a large mode area Yb-doped double-cladding hybrid PCF have been analyzed. Two opposite rows of air-holes of the 10 µm-pitch photonic crystal cladding were replaced by high-index Ge-doped rods of different diameter, that is 7.1 µm on one side and 5.89 µm on the other, while 7 missing air-holes form the core, whose diameter is about 30 µm. Different air-hole size, ranging from 1 µm to 1.5 µm have been considered.

Experimental measurements and simulations, carried out through a finite-element based modal solver, have demonstrated the narrow spectral filtering capability of this PCF, both when straight and coiled, with a passband of about 80 nm in the Yb gain region. Moreover, high-order mode suppression can be achieved with a proper air-hole size or with the core down-doping. Finally, stress-induced birefringence due to the Ge-rods has been investigated, accounting for the polarization-maintaining behaviour of the manufactured fiber.

The Authors acknowledge the support of the EU funded FP7 ALPINE Project, n. 229231.



7914-62, Session 15

Design, fabrication, and characterization of acoustic antiguiding large core YDFAsW. E. Torruellas, M. C. Gross, M. L. Dennis, The Johns Hopkins Univ. (United States); M. Alam, K. F. Farley, V. Khitrov, K. Tankala, Nufern (United States)

We have achieved an improvement of a factor 6-7 of the Stimulated-Brillouin-Scattering (SBS) threshold large core YDFA with of 25m core diameter operating in single transverse mode and showing excellent beam quality, M2<1.5. Our design methodology shows the ability to readily increase this performance to values in excess of 10x compared to those achieved in fibers with step index profiles in both the optical and the acoustic domains. In order to achieve such an increase in SBS performance we accomplished the following: i) We identified design trade-offs based on a parametric designs of the acoustic and optical waveguides, ii) we characterized the acoustic response of fiber preforms appropriately doped and iii) characterized the SBS response in the associated fibers in both passive 1550nm and active 1070nm configurations. As a result, we have confirmed previously known acoustic-velocity dopant dependence and also, for the first time to our knowledge, we have been able to quantify in both preform and optical fiber geometries the acoustic velocity dependence of Ytterbium in silica.

7914-63, Session 15

M2-characterization of beams emerging from LMA fibers by means of modal decompositionD. Flamm, O. A. Schmidt, C. Schulze, M. R. Duparré, Friedrich-Schiller-Univ. Jena (Germany); S. Schröter, Institut für Photonische Technologien e.V. (Germany)

The beam propagation ratio M² is the measure of choice when characterizing the quality of a laser beam regarding the propagation properties of the beam width. As becoming an accepted and widespread standard, the beam propagation ratio specifies almost every novel laser system, including high-power fiber lasers. In this field of laser research and development, the yet increasing optical power necessitates the use of large-mode-area (LMA) fibers for active and passive applications. Here, the effective mode area is increased in comparison to single-mode fibers, enabling the excitation of higher-order transverse modes having a great influence on essential beam properties like focussability or divergence and, therefore, on the M²-parameter.

We report on a fast measurement procedure decomposing the investigated beam into its eigenmodes using computer-generated holograms. Measuring modal weights, meaning the relative power guided by a certain mode and intermodal phase delays, the full information about the optical field becomes available. This enables the numerical propagation of the beam through free space by convolution with the impulse response. The analysis of the simulated beam at several propagation lengths allows the realization of a synthetical caustic measurement. The calculation of the second order moments, in turn, yields the beam propagation ratio for stigmatic and even general astigmatic beams. The presented method is applied to conventional step-index fibers as well as to more sophisticated waveguides like photonic crystal fibers and multi-core fibers.

7914-64, Session 16

Low-NA single-mode LMA photonic crystal fiber amplifierT. T. Alkeskjold, NKT Photonics A/S (Denmark)

Enabling Single-Mode (SM) operation in Large-Mode-Area (LMA) fiber amplifiers is of high importance, since a SM output ensures highest beam quality and good pointing stability. SM operation can be achieved

in conventional Multi-Mode (MM) fiber amplifiers by either coiling the MM fiber to attenuate Higher-Order Modes (HOMs), launching mainly in the Fundamental Mode (FM) of the MM fiber or a combination of both. Having differential gain for the FM and HOM is also a possibility, but this comes with a penalty on reduced pump absorption and is mostly advantageous in high-gain amplifiers. Chiral side-cores can also be applied to filter out HOMs, but this requires very high production accuracy. The most straightforward way to achieve SM output is using a truly SM fiber i.e. using a fiber with the low NA required for true SM operation. But, with the constant demand for larger core dimensions, the ultra-low NA of such fibers becomes increasingly difficult to produce, even with the best achievable index accuracy of state-of-the-art fiber manufacturing.

In this paper, a new design approach for achieving ultra-low NA SM fibers is demonstrated. The design achieves SM operation even in short and straight fibers and allows relatively large preform tolerances to be compensated during draw. A low-NA straight SM fiber with mode field diameter > 55µm at 1064nm is demonstrated.

7914-65, Session 16

Effective absorption in cladding-pumped fibersM. N. Zervas, Univ. of Southampton (United Kingdom)

Power scaling in high power fiber lasers has been exclusively based on cladding pumping implemented in a number of different schemes. In addition, high brightness pumps (and pump combining arrangements) and high dopant concentrations are required in order to reduce fibre lengths and minimise the deleterious impact of background losses and nonlinear effects. Among the most commonly used cladding-pumping schemes are direct end-pumping (either free-space or through pump combiners) and coupled multimode fibers (GTWave).

It is known that pump absorption in cladding-pumped lasers and amplifiers departs considerably from pure exponential decay (Beer’s law) due to inefficient pump mixing. Such pump power absorption results in non-optimum inversion and can potentially affect severely the performance of the fiber laser.

In this talk we investigate for the first time experimentally and theoretically the wavelength dependence of the pump absorption along Yb3+-doped fibers, for cladding-pumped single as well as coupled multimode (GTWave) fibers. We show that significant spectral absorption distortions occur along the length with the 976nm absorption peak affected the most. We have developed a novel theoretical approach, based on coupled mode theory, to explain the observed effects. We have also investigated the mode mixing requirements in order to improve the absorption spectral distribution along the increase the overall absorption efficiency and discuss the implications on fiber laser performance.

7914-66, Session 16

New developments in high-power fiber lasers based on alternative materialsA. Langner, M. Such, G. Schötz, Heraeus Quarzglas GmbH & Co. KG (Germany); S. Grimm, F. Just, M. Leich, C. Mühlig, J. Kobelke, A. Schwuchow, IPHT Jena (Germany); O. Mehl, O. Strauch, B. Wedel, HIGHYAG Lasertechnologie GmbH (Germany); G. Rehmann, V. K. Krause, Laserline GmbH (Germany)

The sintering of Yb-doped fused silica granulates is a well established technique developed by the IPHT und Heraeus Quarzglas to produce very homogeneous rare earth doped bulk silica core rods for fiber laser applications. By the use of a new developed modified laser induced deflection (LID) technique we are able to pre-characterize directly the material absorption properties of the bulk material prior to the laser fiber production. The comparison of these bulk absorption results without



scattering effects and the total attenuation results measured in the fiber will be presented. The advantages and limits of the LID technique will be discussed in view of separation the different loss fraction.

Multimode double cladding laser fibers with an ultra large active core diameter (up to 100 µm) have been produced from the Yb-doped bulk silica rods by the use of two different techniques. One is a more or less classical jacketing technique, the other is a stacking technique of un-doped, Yb- and F-doped rods and F-doped tubes. This stacking and filament technique has the advantage that even very complex fiber structures can be set easily. We will present detailed studies of the refractive index and doping level homogeneity of the Yb-doped bulk materials and laser fibers to show the unique features of the Yb-doped bulk silica.

The different produced fiber types have been tested in high power fiber laser setups. We have achieved output powers above 1 kW with excellent long term laser stability. Detailed studies of the laser performance will be presented.

7914-67, Session 16

Single-mode regime of 19-cell Yb-doped double-cladding photonic crystal fibersE. Coscelli, F. Poli, Univ. degli Studi di Parma (Italy); T. T. Alkeskjold, NKT Photonics A/S (Denmark); D. Passaro, A. Cucinotta, S. Selleri, Univ. degli Studi di Parma (Italy); L. Leick, J. Broeng, NKT Photonics A/S (Denmark)

Yb-doped double-cladding Photonic Crystal Fibers (PCFs), characterized by large mode area and low numerical aperture, have been successfully exploited to realize high-power lasers and amplifiers. So far, the highest effective area values have been reached in 19-cell double-cladding PCFs, with core obtained by removing 19 air-holes from the inner cladding triangular lattice. However, the single-mode behaviour, which is a fundamental requirement for high-power applications, becomes a critical issue in fibers with so large core dimension.

In this paper the single-mode regime of 19-cell Yb-doped double-cladding PCFs has been studied, evaluating the cut-off wavelength c of the first Higher-Order Mode (HOM) and its overlap on the doped core at c. A full-vector modal solver based on the finite element method has been applied to evaluate the HOM dispersion curve and its magnetic field distribution, for the overlap calculation. Then, a spatial and spectral amplifier model has been exploited to analyze the role of the HOM near cut-off in the gain competition.

19-cell double-cladding PCFs with 5 rings of air-holes, with diameter d and spacing , and different d/ values, starting from d/ = 0.1, have been taken into account. The influence on the single-mode regime of the core diameter and of its refractive index, lower than the pure silica one, has been investigated.

Simulations have demonstrated the possibility to find generalized results for the HOM cut-off by scaling the whole 19-cell fiber cross-section, including the air-cladding.

The Authors acknowledge the support of the EU funded FP7 ALPINE Project, n. 229231.



Conference 7915: High Energy/Average Power Lasers and Intense Beam Applications VISunday-Tuesday 23-25 January 2011 • Part of Proceedings of SPIE Vol. 7915 High Energy/Average Power Lasers and Intense Beam Applications V

7915-01, Session 1

Recent electric oxygen-iodine laser experiments and modelingD. L. Carroll, G. F. Benavides, J. W. Zimmerman, A. D. Palla, CU Aerospace LLC (United States); M. Day, Univ. of Illinois at Urbana-Champaign (United States); J. T. Verdeyen, CU Aerospace LLC (United States); W. C. Solomon, Univ. of Illinois at Urbana-Champaign (United States)

Experiments and modeling have led to a continuing evolution of the Electric Oxygen-Iodine Laser (ElectricOIL) system. This continuous wave (cw) laser operating on the 1315 nm transition of atomic iodine is pumped by the production of O2(a) in a radio-frequency (RF) discharge in an O2/He/NO gas mixture. New discharge geometries have led to improvements in O2(a) production and efficiency. Further, size scaling is presently showing a super-linear growth in performance; a 95% enhancement in cw laser power was achieved via a 50% increase in gain length, flow rates, and discharge power. As understanding of the ElectricOIL system continues to improve, the design of the laser is systematically evolving. The gain has improved by more than 100-fold from the initial demonstration of 0.002% cm-1 to 0.26% cm-1, and similarly the outcoupled laser power has improved more than 500-fold from 0.16 W to 109 W.

7915-02, Session 1

Modeling of power scaling in EOILW. T. Rawlins, S. Lee, A. J. Hicks, I. M. Konen, D. B. Oakes, E. P. Plumb, S. J. Davis, Physical Sciences Inc. (United States)

We present modeling studies of the power scaling of EOIL.

7915-03, Session 1

Catalytic enhancement of singlet oxygen productionW. T. Rawlins, S. Lee, A. J. Hicks, I. M. Konen, E. P. Plumb, S. J. Davis, Physical Sciences Inc. (United States)

We will present recent results of power scaling of the electric oxygen iodine laser. We will include studies of the surface catalytic production of singlet oxygen.

7915-04, Session 1

A simplified kinetic model for the COIL active mediumM. C. Heaven, Emory Univ. (United States); V. N. Azyazov, S. Y. Pichugin, P.N. Lebedev Physical Institute (Russian Federation)

Kinetic data obtained in the last decade has considerably changed the concepts of some mechanisms of excitation and deactivation of excited states in the chemical oxygen-iodine laser (COIL) medium. The present review considers new kinetic data and an analysis of the mechanisms of pumping and quenching of electronically and vibrationally excited states in the oxygen-iodine media. An effective two-level model of I2 excitation and relaxation was developed. Calculated effective rate constants for deactivation of I2(X,11≤≤24) by O2, N2, He and CO2 are

presented. A simplified kinetic package of the COIL active medium, including 30-reaction set and 14 species is recommended. The results of calculations utilizing this simplified model are in good agreement with the experimental data.

7915-05, Session 2

Demonstration of a diode pumped continuous wave potassium laserB. V. Zhdanov, M. K. Shaffer, R. J. Knize, U.S. Air Force Academy (United States)

Diode Pumped Alkali Lasers (DPALs) attract increasing attention of researchers because of their potential for scaling to high powers while keeping a good spatial quality of the output beam. Efficient diode pumped Cs and Rb lasers have been demonstrated and are under extensive development now. Potassium (K) laser, which has been demonstrated only with surrogate pumping (CW Ti:Sapphire laser and pulsed Alexandrite laser) has several advantages compared to Cs and Rb lasers. In particular, K laser has higher quantum efficiency (99.6%) and can operate without hydrocarbon buffer gases. In this paper we present a first demonstration of a diode pumped Potassium laser. A narrowband laser diode array (LDA) with a linewidth about 10 GHz operating at 766.7 nm was used to pump Potassium vapor buffered by Helium gas at 600 torr. A stable laser cavity with longitudinal pumping and orthogonal polarization of the pump and laser beams was used in this experiment. The optimal laser cavity design and laser operation conditions were determined.

7915-06, Session 2

Cesium laser operating in the blue by direct optical excitation of the 7 2P3/2 stateK. C. Brown, G. P. Perram, Air Force Institute of Technology (United States)

The feasibility of an optically pumped alkali laser pumped on the blue 6 2S1/2 - 7 2P3/2 transition, followed by spin orbit relaxation to the 7 2P1/2 upper laser level and lasing to the ground state has been evaluated. Cesium atoms were heated in a sealed absorption cell at several temperatures up to 373 K and excited with a pulsed dye laser at 455.3 nm. The time dependent fluorescence of the 7 2P1/2 (459.1 nm) state was observed as a function of buffer gas density for various gases including helium, methane and ethane. The buffer gas pressures used are higher than previous spectroscopic work and similar to those used in DPAL experiments. The resulting fluorescence was fit to a two-level model and the temperature dependent cross section was calculated. We show that the fine-structure transfer rate of helium is rapid enough to support lasing on the 7 2P1/2 -> 6 S1/2 transition. The rate coefficients for spin-orbit relaxation for the 7 2P3/2, 1/2 levels are compared with the same rates for the 6 2P3/2, 1/2 levels corresponding to the normal near infrared Diode Pumped Alkali Laser system. Laser demonstration experiments will be described.

7915-07, Session 2

Small signal gain in DPAL systemsK. L. Galbally-Kinney, D. L. Maser, W. J. Kessler, W. T. Rawlins, S. J. Davis, Physical Sciences Inc. (United States)

We will present results of spectroscopic and kinetic studies including small signal gain for DPAL systems.


7915-08, Session 2

High-energy transversely pumped alkali vapor laserJ. Zweiback, A. M. Komashko, General Atomics Aeronautical Systems, Inc. (United States)

There is great interest in scaling diode pumped alkali lasers to high power. We believe that the most straightforward approach to further scaling is a transversely pumped architecture. This leads to automatic separation of pump and laser beams, a decrease in required diode brightness, reduced laser flux on optical windows, and reduced transport losses. In addition, transverse flow is enabled to remove heat from the gain volume.

General Atomics Aeronautical Systems has developed a transversely pumped alkali laser system. This is a low average power demonstration system which uses a pulsed alexandrite laser as a surrogate pump source. The system produces up to 40 mJ of output energy when pumped with 63 mJ, with a slope efficiency of 75 percent. We will discuss our theoretical and experimental studies.

7915-09, Session 2

Alkali atoms interacting with rare gas atoms and small hydrocarbonsM. C. Heaven, Emory Univ. (United States)

We will present results of studies of excited alkali atom collisions with rare gas species and hydrocarbons relevant to DPAL systems

7915-10, Session 2

XPAL modeling and theoryA. D. Palla, J. T. Verdeyen, D. L. Carroll, CU Aerospace LLC (United States)

The exciplex pumped alkali laser (XPAL) system was recently demonstrated in mixtures of Cs vapor, Ar, with and without ethane, by pumping Cs-Ar atomic collision pairs and subsequent dissociation of diatomic, electronically-excited CsAr molecules (exciplexes or excimers). The blue satellites of the alkali D2 lines provide an advantageous pathway for optically pumping atomic alkali lasers on the principal series (resonance) transitions with broad linewidth (>2 nm) semiconductor diode lasers. Because of the addition of atomic collision pairs and exciplex states, modeling of the XPAL system is more complicated than classic diode pumped alkali laser (DPAL) modeling. The development of a time-dependent finite-volume model including transport, thermal, and kinetic effects appropriate for the simulation of a cylindrical closed cell XPAL system is presented. The model equations and solution scheme are described in detail and a structured tessellation of the relevant physical domain is described as an appropriate grid for the required simulations. An updated kinetic set appropriate for modeling XPAL systems is presented. Two-dimensional, time-dependent simulations of an operating XPAL cell are presented and compared to data. Initial calculations of CW XPAL operation, a theoretical analysis of CW XPAL systems, along with advantages over the DPAL system are also presented.

7915-11, Session 3

Navy Laser Weapon System (LaWS) prototype development and testingB. J. Hankla, Naval Surface Warfare Ctr. Dahlgren Div. (United States)

The Navy’s Laser Weapon System (LaWS) is a NAVSEA weapon

development effort with the primary objective to add a high energy laser (HEL) weapon capability to the Navy Phalanx Close-in-Weapon System (CIWS). Over the past few years, system components and a full weapon prototype have been tested in increasingly challenging conditions. These tests have been designed to demonstrate the technical readiness of many of the key technologies, including fiber lasers, for use in Navy weapon applications. The most recent tests of the prototype occurred in May and June 2010 under very stressing, but typical, maritime conditions. This presentation will give an update of the status of the effort, show results of the recent live-fire testing, and discuss many of the system challenges that are currently being addressed.

7915-12, Session 3

High-power femtosecond hybrid Ti:sapphire: KrF laser facility and its applicationsA. A. Ionin, S. I. Kudryashov, A. O. Levchenko, L. V. Seleznev, D. V. Sinitsyn, N. N. Ustinovskii, V. D. Zvorykin, P.N. Lebedev Physical Institute (Russian Federation)

High-power hybrid femtosecond laser facility consisted of a front-end Ti:Sapphire laser system emitting the third harmonic radiation at the wavelength 248 nm and a set of excimer KrF laser amplifiers is now under development at the Lebedev Institute. Peak power comes up to a few TW right now. Recent experiments on applications of high-intensity femtosecond pulses with wavelengths 248nm and 744 nm for multiphoton and tunnel ionization of different gases, formation of mini-filaments in air under the tight focusing, and laser surface nanostructuring of various solid materials such as titanium, silicon, etc., are discussed.

7915-13, Session 3

Energy transfer kinetics of the np5(n+1)p excited states of Ne and KrM. C. Heaven, M. H. Kabir, Emory Univ. (United States)

We discuss the kinetics of excited rare gas species as gas phase laser candidates.

7915-14, Session 3

Mode-locked CO laser for isotope separation of uranium employing condensation repressionI. Y. Baranov, A. V. Koptev, Baltic State Technical Univ. (Russian Federation)

We have suggested a technical solution of a CO laser facility for industrial separation of uranium used in the production of fuel for nuclear power plants. There has been used a method of laser isotope separation of uranium, employing condensation repression in a free jet. The laser operation with nanosecond pulses irradiation can provide high efficiency in the separating block and the high effective coefficient of the laser with the wavelength of 5,3 µm. Receiving a uniform RF discharge under medium pressure and high Mach numbers in the gas stream solves the problem of an electron beam and cryogenic cooler of CO lasers and allows to simplify significantly the CO laser design. The laser active medium is cooled while being expanded in the nozzle; a low-current RF discharge is similar to a non-self-sustained discharge. We have received the lasing on the experimental small-scale facility with a RF discharge in the supersonic stream. In the present work we have developed a calculation model of optimization and have defined the parameters of a mode-locked CO laser with a continuous RF discharge in the supersonic stream. The conditions that provide high efficiency of the laser at the wavelength of 5,3 µm and high effective photon employing in the separating block have been determined. The CO laser average power of

Conference 7915: High Energy/Average Power Lasers and Intense Beam Applications VI


several kilowatts is sufficient for efficient industrial isotope separation of uranium at one facility. The uranium enrichment realized on this facility might be several times cheaper than by centrifuges.

7915-15, Session 3

Autocorrelation of femtosecond VUV pulses using multiphoton ionizationS. Kubodera, W. Nagaya, H. Zushi, M. Kaku, M. Katto, Univ. of Miyazaki (Japan)

We have developed femtosecond vacuum ultraviolet (VUV) laser system, which consists of femtosecond VUV seed pulses generated by wavelength conversion and an optical-field-induced ionization (OFI) argon excimer amplifier at 126 nm. Both the seed pulse generator and the OFI amplifier are driven by one high-intensity Ti:sapphire laser. To characterize the pulse width of the VUV femtosecond laser, autocorrelation technique has been utilized, where gaseous media have been chosen as nonlinear media. Gaseous media are transparent for the VUV pulses, and are ionized by multiphoton processes. The use of the gaseous media is thus inevitable to measure the VUV pulse widths. On the other hand, the same multiphoton technique can be utilized to characterize infrared (IR) laser pulse widths as well. We, therefore, have been developing a simple and versatile autocorrelator that characterizes femtosecond laser pulse widths in the wide spectral range between IR and VUV. For the femtosecond IR laser pulses at 882 nm, autocorrelated electron signals obtained in Xe were measured by an electron multiplier. Xe should have required nine photons to get ionized with the IR laser pulse. The electron signals, however, indicated that the four-photon process was dominated, indicating that the majority of electrons was produced in gaseous media other than Xe. By using the forth-order correlation function, the pulse width of 173 fs (FWHM, Gaussian assumed) was determined for the IR laser, which agreed with that of 168 fs (FWHM) measured by a standard two-photon autocorrelator using a nonlinear LBO crystal.

7915-16, Session 3

Thermodynamic equilibrium study of low temperature laser induced plasmas of air, argon, and cadmium by Thomson scattering and emission spectroscopyB. Bousquet, G. Travaillé, Univ. Bordeaux 1 (France); A. Mendys, K. Dzierzega, Jagiellonian Univ. in Krakow (Poland); S. Pellerin, Univ. d’Orléans (France); B. Pokrzywka, Cracow Pedagogical Univ. (Poland); L. Canioni, Univ. Bordeaux 1 (France)

Laser induced breakdown spectroscopy (LIBS) is a user friendly and, versatile technique based on the analysis of the optical emission spectra of laser induced plasmas (LIP) in various samples (aqueous, gaseous, solid, aerosols, etc.). Despite its significant development over the past 20 years, LIBS accuracy is still limited due to its high sensitivity to the matrix effect of the sample which results in enormous uncertainties in the prediction of the elemental. Several techniques were developed in order to by-pass this issue, among all one of the most promising is the Calibration Free LIBS procedure.

However, this method requires plasma in the state of Local Thermodynamic Equilibrium (LTE). Verification of LTE in LIBS plasma has been the subject of large number of papers, but where its assessment was made a posteriori using McWhirter’s criterion. Several critics aroused on this method, pointing out e.g. the dependence of the BP on plasma ionization dynamic. As a passive technique, this method depends only on the source emission. For complex (thick, recombining, etc.) plasmas, this may constitute a serious drawback.

We present results of our investigations of LTE on LIBS plasmas in air, argon (1atm) and cadmium sample. We used an active plasma diagnostic technique: the Thomson scattering (TS), to infer the physical parameters

of our plasmas (mostly ne, and Te). Other important parameters such as e.g. the ionization and excitation temperatures are also extracted so as to provide a direct diagnostic of the LTE state of the investigated plasmas.

7915-17, Session 3

Modeling of high-power spectral beam combining with thermally distorted volume Bragg gratingsS. Mokhov, D. R. Drachenberg, I. B. Divliansky, G. B. Venus, B. Y. Zeldovich, L. B. Glebov, CREOL, The College of Optics and Photonics, Univ. of Central Florida (United States)

High aperture volume Bragg gratings (VBG) recorded in photo-thermo-refractive (PTR) glass have been successfully implemented for spectral beam combining (SBC) of fiber lasers up to kilowatt power level recently and they are promising candidates for future SBC systems up to hundreds of kilowatts. In our combining scheme each narrow-band VBG reflects particular laser beam in the same angular direction with other beams of slightly different wavelengths for which this grating is almost transparent. Due to the complex chemical composition of the PTR glass each VBG demonstrates residual material absorption which becomes a source of its own heating. At 100kW laser power operation each grating is supposed to be actively cooled by gas flow in order to manage the heat load. As a result, the radial profile of the heating follows the profile of the beam intensity which is usually Gaussian. The thermal thickness expansion of the grating plate leads to corresponding Gaussian phase distortion that affects the beam quality. Such parasitic phase distortions due to heating from multiple transmitted beams are the main limitation of the SBC system performance. One approach to reduce such undesired phase distortions due to non-uniform heating is based on increase the material’s absorption with aperture radius in order to provide more uniform heating of the VBG. Another very promising approach is the use of super-Gaussian beams for SBC. Such beams have more uniform intensity profiles then ordinary Gaussian ones and though they are characterized by initial beam quality factor slightly higher than unity they have demonstrated much more stable behavior when propagating through heated gratings with corresponding phase profiles. In conclusion we would like to mention that our developed theoretical methods can be applied to other problems related to high-power beam propagation through absorbing optical elements.


Enhanced pulsed Nd:YAG laser modeling and simulation for the design of low and medium energy laser systemsS. S. A. Ghoniemy, A. Mohamed, Military Technical College (Egypt)

This paper presents a modified and enhanced version of our previously published passively Q-switched Nd:YAG laser model by taking into account the 4-levels energy model of the Cr4+:YAG saturable absorber and a combination of other parameters and by including the effect of thermal energy. The modified model is implemented using SIMULINK and used to simulate the behavior and predict the design parameters of practical passively Q-switched Nd:YAG laser systems. It is also used to predict and plot the radial temperature profile in Nd:YAG laser systems.

Our simulations showed that, thermal energy increases approximately linearly as the pump power increases, causing a reduction in the laser output energy by about 3-6% from its nominal value. We noticed that the surface heat transfer coefficient is monotonically increasing function with respect to the flow rate.

Our proposed model also enabled us to predict the time dependence of the temperature rise during the pump pulse and leaded to conclude that the rod temperature increases as the pump pulse width decreases. Using the proposed simulation tool, the induced birefringence can be obtained



and plotted which leaded to conclude that as the pump power increase the induced birefringence increases. The model capability is extended to predict the induced thermal lensing from which we noticed that the focal length is proportional to the rod cross-section and is inversely proportional to the dissipated power.

Finally, the proposed model is used to design three customized Nd:YAG systems and the simulation results showed a nearly real behaviour such as appearing the multiple pulse phenomena and in general they are in good agreement with the experimental measurements.



Conference 7916: High Power Lasers for Fusion ResearchTuesday-Thursday 25-27 January 2011 • Part of Proceedings of SPIE Vol. 7916 High Power Lasers for Fusion Research


Laser Mégajoule synchronization systemM. Luttmann, J. F. Pastor, V. Drouet, A. Adolf, M. Prat, J. Raimbourg, Commissariat à l’Énergie Atomique (France)

This paper describes the synchronization system under development on the Laser Mégajoule (LMJ) in order to synchronize the laser quads on the target to better than 40 ps rms. Our architecture is based on:

- An Integrated Timing System (ITS) validated on the LIL facility (LMJ prototype) which delivers trigger signals to the thousands of devices located in the laser and target bays with jitter down to 15ps rms.

- The ITS is coupled with an ultra precision timing system in order to trig the most demanding devices like the laser waveform generators. This system allows a programmable delay range of 100ns with a jitter 3 times better of 5ps rms.

- A chamber center sensor able to measure the arrival times of the 3w nano joule laser pulses generated by the 1J @1w front end shots.

- Optical delay lines to ensure the intra-quad beam synchronization.

The first part of this article presents the architecture, the major components and the expected performances of our system. The second part details the ITS and ultra precision timing system principle and features.


LIL laser performance statusX. B. Julien, H. Graillot, T. Chies, Commissariat à l’Énergie Atomique (France); J. G. Di Nicola, Lawrence Livermore National Lab. (United States); A. Adolf, G. Thiell, O. Henry, H. Vincent, P. Gendeau, C. Féral, R. Courchinoux, A. Martinez, E. Bar, E. Bordenave, V. Beau, A. Roques, L. Le Déroff, Commissariat à l’Énergie Atomique (France)

Initially, the Laser Integration Line (LIL) has been designed as a prototype to validate the concepts and the laser architecture of the Laser Méga Joule (LMJ). The LIL facility is a 4-beam laser representing a quad structure of LMJ. A careful set of test campaigns were conducted to safely ramp up laser performance. The main goal was to measure quad-specific features such as beam synchronization and focal spot (size, smoothing contrast ratio or irradiation nonuniformity) versus the LMJ requirements. Following qualification, LIL has become a major instrument dedicated to the achievement of plasma physics experiments for the Simulation Program but was also opened to the academic scientific community. One of the attributes of the LIL facility is to be very flexible to accommodate the requests of plasma physicists during campaigns. LIL is constantly evolving to best meet the needs of physicists. Changes made or planned are either to improve the quality of laser beams, or to increase LIL Energy-Power operating space. To optimize preparation and design of campaigns, the LIL performance status has been elaborated. It gives informations about the characteristics of the laser in terms of near field and far field, defines the steps to maintain performance, precises how the facility responds to the request, details settings (smoothing, shaping of the focal spot, energy, temporal pulse shaping, beam pointing) and gives the limits in energy and power.


Efficient spherical wavefront correction near the focus of the petawatt-level femtosecond CPA laser systemZ. Ren, X. Liang, L. Yu, X. Lu, R. Li, Z. Xu, Shanghai Institute of

Optics and Fine Mechanics (China)

By directly measuring the spherical wavefront near the focus of an f/4 off-axis parabola, we demonstrated a approach to efficiently correct convergent spherical wavefront of 150 mm output laser beam by installing a common small aperture deformable mirror (DM) with 50mm active aperture and 31 electrode actuators in the middle of 0.89PW/29.0fs Ti:sapphire CPA laser chain. In order to prove the correction effects of the approach, we also corrected the 50 mm beam wavefront in classical adaptive optical loop before the output laser beam is expanded to 150 mm. The experimental results show that spherical wavefront correction can effectively reduce the size of focal spot and greatly improved the intensity in target chamber of our CPA laser system. It is, to our knowledge, the first time attain the near perfect correction result in ultra-intensity laser system by correcting convergent spherical wavefront using a small aperture DM in adaptive optical loop. The size of the focal spot was improved to 1.52×1.61 times diffraction limitation (DL) at 10Hz, corresponding to a 1.63×1.78 DL focusing at single shot. Finally the maximum peak intensity of 2.36×10^21 W/cm2 was obtained at the output power of 0.89 PW. Comparing with the classical wavefront correction approach, the spherical wavefront correction approach is more simple, economic and validate. It is very fit for correcting the wavefront of large optical beam by a small aperture DM. Hence the correction approach is especially fit for PW-level laser system with large output optical beam size.


Carrier-envelope phase stabilized ~12fs, 0 54mJ, 1 8µm pulsesC. Li, L. Song, D. Wang, C. Xu, Y. Leng, R. Li, Z. Xu, Shanghai Institute of Optics and Fine Mechanics (China)

High-peak-power, carrier-envelope phase (CEP) stablized few-cycle laser pulses have intriguing application in attosecond pulses generation from high order harmonic generation (HHG). Especially the driven laser with central wavelength lies in the region of 1.5-3µm, as a balance between high enough hight-harmonic generation cutoff energy and acceptable conversion effciency, is regarded as a suitable driver for attoscond pulse generation[1-3].

In this paper, we present a three-stage collinear near IR optical parametric amplifier (OPA) with passively stabilized carrier-envelope phase. 1.4mJ/1.8µm/45fs/1kHz pulses are obtained pumped by a 8mJ/800nm/40fs/1kHz non-CEP-stabilized Ti:sapphire laser amplifier. The 1.8µm pulses are then spectrally broadened by nonlinear propagation in an argon-filled hollow-core fiber and subsequently compressed to ~12fs/0.54mJ by use of thin fused silica wedegs with 440 mrad rms CEP fluctuations, making this source suited as a driver for attoscond pulse generation.

Reference

1. V. S. Yakovlev, M. Ivanov, and F. Krausz, “Enhanced phase-matching for generation of soft X-ray harmonics and attosecond pulses in atomic gases,” Opt. Express 15, 15351 (2007).

2. C. Vozzi, C. Manzoni, F. Calegari, E. Benedetti, G. Sansone, G. Cerullo, M. Nisoli, S. De Silvestri, and S. Stagira, “Characterization of a high-energy self-phase-stabilized near-infrared parametric source,” J. Opt. Soc. Am. B 25, B112 (2008).

3. B. E. Schmidt, P. Béjot, M. Giguère, A. D. Shiner, C. Trallero-Herrero, E. Bisson, J. Kasparian, Jean-Pierre Wolf, D. M. Villeneuve, Jean-Claude Kieffer, P. B. Corkum, and F. Légaré, “Compression of 1.8µm laser pulses to sub two optical cycles with bulk material,” Appl. Phys. Lett. 96, 121109 (2010).



Spatiotemporal beam control of an OPCPA laser systemX. Zeng, Chinese Academy of Engineering Physics (China)

An arbitrary spatiotemporal beam shaping technique is proposed. An OPCPA laser system owning this ability is being constructed. The frond end seed of this OPCPA laser system is a millijoule-level 1053nm wavelength femtosecond clean pulse. This large energy clean seed pulse, which insures the high signal-noise-ratio of the whole laser system, is obtained through a femtosecond optical parametric amplification system based on Supercontinuum White Light injection. This clean pulse is then amplified to a joule level by a two-stage OPCPA. A novel arbitrary spatiotemporal control technique is also been proposed through the control of the pump pulse. An amplified OPCPA signal pulse owning a superGauss temporal distribution and pulse spectrum with a pit in the center is obtained, which shows the ability of arbitrary temporal shaping. As our OPCPA experiments show, temporal shape (spectrum) of the signal pulse can be valid controlled by the pump pulse shaping which is modulated by an Arbitrary Wave-shape Generator. Shaping the pump pulse nearfield distribution by a liquid crystal can also realize a well nearfield shaping of the amplified signal pulse.


The progress of beam smoothing on SG-II facilityS. Zhou, Shanghai Institute of Optics and Fine Mechanics (China)

The uniformity of focused laser beams is very important and necessary for high power laser facility, and a number of methods have been pursued to achieve much better uniformity on target plan on SG-II. The approach of SSD (smoothing by spectral dispersion) is used with other spatial smoothing technology, including lens array, distribution phase plate and continue phase plate. The characters of SSD have also been studied. It shows that uniformity of near field can be maintained when phase modulation is used with suitable bandwidth, and the uniformity of focal plane is more correlated with angular dispersion of modulated width than other parameters of SSD such as modulating frequency and the number of color cycle. However, the angular divergence is limited by laser system and the spatial smoothing technology. The intensity of speckles of diffraction element on focal plane is more uniform, the angular divergence of SSD is required less.


Hohlraum target pointing alignment from x-ray detector images using starburst design patternsR. R. Leach, Jr., Lawrence Livermore National Lab. (United States)

National Ignition Facility (NIF) is a high energy laser facility comprised of 192 laser beams focused on a hydrogen-filled spherical, cryogenic target with enough power and precision to initiate a fusion reaction. The target container, or hohlraum, must be accurately aligned to the in-coming laser beams, and the frozen fuel layer in the target must be carefully monitored. To achieve alignment, x-ray images are acquired through starburst shaped windows cut into opposite sides of the hohlraum. When the hohlraum is in alignment, the starburst pattern pairs match nearly exactly and allow a clear view of the ice layer formation on the edge of the target capsule. If the hohlraum is mis-aligned, the x-ray image analysis is applied to determine the direction and magnitude of adjustment required. The x-ray detector and source are moved in concert during the alignment process. Automated pointing alignment using image processing of the x-ray images is both accurate and efficient. In this paper, we describe the

control and associated image processing that enables automation of the starburst pointing alignment.


Defense in depth: laser safety and the National Ignition FacilityJ. King, Lawrence Livermore National Lab. (United States)

The National Ignition Facility (NIF) is the largest and most energetic laser in the world contained in a complex the size of a football stadium. From the initial laser pulse, provided by telecommunication style infrared nano Joule pulsed lasers, to the final 192 laser beams (1.8 Mega Joules total energy in the ultraviolet) converging on a target the size of a pencil eraser, laser safety is of paramount concern.

In addition to this, there are numerous high powered (Class 3B and 4) diagnostic lasers in use that can potentially send their laser radiation travelling throughout the facility. With individual beam paths of up to1500 meters and a workforce of more than one thousand, the potential for exposure is significant. Simple laser safety practices utilized in typical laser labs just don’t apply. To mitigate these hazards, NIF incorporates a multi layered approach to laser safety or “Defense in Depth.”

The purpose of this paper is to describe some of the “not so typical” laser hazards that exist in a large facility such as the NIF, and to explain how these unique complexities and hazards are mitigated.

7916-01, Session 1

Radiative properties in ICF plasmasD. Benredjem, Univ. Paris-Sud 11 (France)

We present new calculations on opacity and radiative power losses (RPL) of carbon and gold. Both ions are involved in inertial confinement fusion. The first element could also be utilized in the walls of future TOKAMAK reactors such as ITER while the second is present in holraums and its X-ray emission contributes to the heating in ICF. Because argon impurities may be used in the fusion core, in order to diagnose the electron temperature, we have calculated the intensities of the He- line and of the Li-like Ar satellite lines. In fact, the intensity ratio depends on electron temperature. The effect of the plasma electric field on the intensity ratio is discussed.

Our approach is based on a detailed line calculation where the atomic database is provided by the MCDF code [1]. Then a lineshape code [2] allowing for NLTE ionic populations was adapted to the calculation of opacity and RPL profiles. Because the calculation time is sometimes prohibitive a second approach, based on statistical distributions and involving the first four moments of the RPL, is investigated [3].

References

[1] P. Grant et al, Comput. Phys. Comm. 21, 207 (1980)

[2] B. Talin, A. Calisti, L. Godbert, R. Stamm, R. W. Lee and L. Klein, Phys. Rev. A 51 1918 (1995)

[3] J.-Ch. Pain, F. Gilleron, J. Bauche and C. Bauche-Arnoult, High Energy Density Physics 5, 294 (2009)

7916-02, Session 1

Opportunities for inertial fusion and high-energy-density physics research at the National Laser Users’ FacilityJ. M. Soures, Univ. of Rochester (United States)

State-of-the-art high-energy, high-power laser facilities, such as the Omega Laser Facility at the University of Rochester’s Laboratory for Laser Energetics (LLE), provide unique opportunities for conducting a

Conference 7916: High Power Lasers for Fusion Research


broad range of inertial fusion and high-energy-density physics studies. As part of the LLE National Laser Users’ Facility program, a significant portion of the shot time of the 60-beam UV 30-kJ, 30-TW OMEGA and the 4-beam, petawatt-class OMEGA EP Laser Systems is provided for external user experiments. These experiments include studies of matter compressed to super-high densities and pressures, inertial fusion, laboratory astrophysics, relativistic plasma physics, warm-dense-matter physics, and the development of advanced high-energy-density diagnostic systems. Some of the challenges, exciting results, and future opportunities for inertial fusion and high-energy-density physics research will be presented.

This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-08NA28302, the University of Rochester, and the New York State Energy Research and Development Authority. The support of DOE does not constitute an endorsement by DOE of the views expressed in this article.

7916-03, Session 1

Experimental study of high-Z gas buffers in gas-filled ICF enginesM. A. Rhodes, J. Kane, G. Loosmore, J. Demuth, J. F. Latkowski, Lawrence Livermore National Lab. (United States)

ICF power plants, such as the LIFE scheme under development at LLNL, may employ a high-Z, target-chamber gas-fill to moderate the first-wall heat-pulse due to x-rays and energetic ions released during target detonation. This gas-fill is heated and ionized by the target energy release. It must cool and recombine before the next shot (at nominally 70-ms intervals) to a temperature where the next target and laser pulse can propagate to chamber center with minimal degradation. While we expect rapid cooling to 2eV by radiation, our modeling of cooling below 2 eV has a high degree of uncertainty.

To reduce the uncertainties of cooling and beam/target propagation through such gas-filled chambers, we have developed an electrical discharge plasma source to produce warm plasma comprised of high-Z gases such as xenon. We are using this source to study the plasma cooling rates on both fast and slow timescales. We are also using this source to study laser beam propagation and eventually target propagation in the hot residual gas remaining in the chamber after each target shot. The source is a theta discharge configuration driven by a low-inductance, 5-kJ, 100-ns pulsed power system. This configuration delivers high peak power levels for effective plasma heating, has an electrode-less discharge for minimizing impurities, and has unobstructed axial access for diagnostics and beam (and/or target) propagation studies. Our diagnostics include Thompson scattering, time resolved spectroscopy, and plasma probes. We will report on the system design, operation, and initial results.

This work performed under the auspices of the USDOE by LLNL under Contract DE-AC52-07NA27344.

7916-04, Session 1

Laser safety at high profile projectsK. Barat, Lawrence Berkeley National Lab. (United States)

Laser safety has been an active concern of laser users since the invention of the laser. Laser safety in the typical research setting (government laboratory or university) is the greatest challenge to the laser user and laser safety officer. This is due to two factors. First, the very nature of research can put the user at risk, consider active manipulation of laser optics and beam paths, and user work with energized systems. Second, a laser safety culture that seems to accept laser injuries as part of the graduate student educational process. Major laser facilities have taken a more rigid and serious view of laser safety, its controls and procedures. Part of the rationale for this is that these facilities draw users from all around the world; presenting the facility with a work force of users coming from a wide mix of laser safety cultures. Another factor

is funding sources do not like bad publicity, which can come from laser accidents and a poor safety record. In addition to the fact, that injuries, equipment damage and lost staff time slow down progress. Hence high profile/large laser projects need to adapt a higher safety regimen both from an engineering and administrative point of view. The most successful high profile facilities take a dual track of control measures and a strong top down safety culture. This presentation will discuss all these points and present examples.

7916-05, Session 1

Modeling of the LIFE minichamber Xe theta pinch experimentJ. Kane, M. A. Rhodes, G. Loosmore, J. F. Latkowski, J. M. Koning, M. Patel, H. A. Scott, G. B. Zimmerman, J. Demuth, Lawrence Livermore National Lab. (United States)

In the LIFE fusion chamber, Xe buffer gas having ion density 1e16-4e16/cc will protect the chamber wall from 5-20 MJ in prompt X-rays, which will ionize the Xe and heat it to an electron temperature Te of 10 eV and higher. The Xe is expected to cool and recombine to below 1 eV and an ionization state Z* near zero. It is necessary to know what Te and Z* will be between shots (15 Hz repetition rate), because they affect propagation of drive beams and survival of the incoming cryogenic targets. At low Te and Z* there is a paucity of atomic data for Xe. Cooling by neutral collisions is important, and may be intractable to model. Xe radiates effectively, making it difficult to produce and study a hot homogeneous volume. In the LIFE minichamber experiment a theta pinch will inductively heat a few cc of Xe to a few eV. Te and Z* will be measured as the Xe cools using Thomson scattering. Ion temperature may be measured. Modeling of the experiments is being done using the magnetohydrodynamic code HYDRA with an external circuit model and inductive feedback from the plasma to the circuit. The scarcity of Xe opacity and conductivity data in the buffer gas regime presents a challenge to design of the experiment. Preliminary results suggest that the behavior of the theta pinch circuit and the plasma is complex, and that the Xe may be heated to Te of 2-3 eV before cooling begins.

7916-06, Session 2

MEGa-rays: the dawn of nuclear photonics with laser-based gamma-raysC. P. J. Barty, Lawrence Livermore National Lab. (United States)

The optimized interaction of short duration, intense laser pulses with high-charge, bunches of relativistic electrons can efficiently produce mono-energetic gamma-rays (MEGa-rays) of unprecedented peak brilliance. At 2 MeV, MEGa-ray sources currently under development at LLNL will exceed the peak brilliance of the world’s largest synchrotrons by more than 15 orders of magnitude. This revolutionary jump in brilliance is enabling to “nuclear photonics”, i.e. the isotope-specific interaction of the photon with the nucleus of the atom. In the highly penetrating spectral region between 1 MeV and 3 MeV, MEGa-ray beams can efficiently excite isotope-specific nuclear resonances and be used to detect, assay and image the isotopic content of objects without activating them. New solutions to an astonishingly wide variety of critical and near-term national nuclear problems such as high-confidence detection of clandestine nuclear weapons material and precision, non-invasive assay of spent nuclear fuel assemblies are possible with MEGa-ray beams. This presentation will review: the development of precision MEGa-ray technology, the construction of the world’s first 3rd generation MEGa-ray machine at LLNL and the development MEGa-ray enabled nuclear applications and science.



7916-07, Session 2

Low-cost diode arrays for the LIFE projectR. Feeler, E. Stephens, J. Junghans, Northrop Grumman Cutting Edge Optronics (United States)

One of the primary challenges of the Laser Inertial Fusion Engine (LIFE) project is the cost and availability of the laser diode arrays needed to pump the solid-state laser gain media in the system. Current projections indicate that the arrays need to be available for approximately one cent per Watt of output power, which is one to two orders of magnitude cheaper than currently available.

This work focuses on potential manufacturing approaches to meet the projected specifications of the LIFE project. Special attention will be paid to requirements related to power density (25 kW/cm2), bar pitch (200 - 400 microns), output wavelength (87x and 94x), and fast-axis divergence (+/- 4 degrees). A summary of the supply limitations and cost ramifications of each requirement is presented.

Also discussed are potential supply chain limitations that are anticipated as a result of the immense size of the LIFE project. Recommendations for future public and private investment are included.

7916-08, Session 2

Beam combination laser using phase controlled stimulated Brillouin scattering phase conjugation mirrors for laser fusion driverH. J. Kong, S. Park, S. Cha, KAIST (Korea, Republic of)

The output energy of 500kJ@10Hz@10ns pulsed laser is required for laser fusion energy realization. To get this 2.5kJ@10Hz@10ns laser module is requested. The current laser technologies allow only lasers of 100J@10Hz@10ns at most. By combining 25 of this 100J lasers by using phase controlled stimulated Brillouin scattering phase conjugation mirrors, 2.5kJ@10Hz@10ns laser module can be obtained.

7916-09, Session 2

HiLASE: high average power pulsed lasers for industry and researchT. Mocek, K. Jakubczak, M. Divok?, M. Sawicka, M. Chyla, P. Sikocinski, Institute of Physics of the ASCR, v.v.i. (Czech Republic)

Diode pumping of solid state laser (DPSSL) materials greatly improves the stability and efficiency in comparison to flashlamp pumped laser sources. The reduced heat deposition and maturity of laser diodes now allow to work on the development of pulsed lasers with average power of few kW. We will introduce the new Czech national R&D project HiLASE which specifically focuses on development of high-repetition DPSSL systems and associated technologies that may find use in industry and in the future European large-scale facilities such as HiPER and ELI. In HiLASE we explore two major concepts: thin-disk and cryogenically cooled multi-slab amplifiers. Using the thin-disk technology we plan to generate few ps to 20-30 ns laser pulses at repetition rate of up to 3 kHz with a total power level of 1 kW. The investigation of limits for maximum achievable output power will include the disk size scaling, cooling, doping and technological issues concerning the disk mounting. Second, by combining the concept of large aperture regenerative amplifier (LARA) with the technology of Yb:YAG thin-disks, high-pulse-energy DPSSL system for generation of 2-3 ps pulses with energy of 1-10 J at a repetition rate of 100 Hz will be developed. Third, we will conduct a programme of technology development to demonstrate the scalability of

DPSSL to the kJ level at repetition rates up to 10 Hz. The demonstration of scalability to high energy and high efficiency would make DPSSL the technology of choice for inertial fusion energy research and its commercialization.

7916-10, Session 3

Compact, efficient, low-cost diode power conditioning for laser inertial fusion energyA. J. Bayramian, R. J. Deri, E. S. Fulkerson, R. Lanning, S. Telford, Lawrence Livermore National Lab. (United States)

Several projects are underway worldwide which promise to demonstrate fusion ignition and gain. The first of these to be completed is the National Ignition Facility (NIF) at Lawrence Livermore National Laboratory. In 2010, the team of scientists and engineers operating the NIF will start a campaign to demonstrate fusion ignition, which could serve as the basis for a national program to develop Laser Inertial Fusion Energy (LIFE). A LIFE power generation system will have to meet all of the basic requirements of the NIF including laser requirements, target geometry, target illumination, radiation/hazardous materials handling and safety. In addition, the power generation system will have to address high average power operation of the laser system, average power effects on the target chamber, target injection and tracking, target mass production, blanket and tritium production, and the balance of plant to produce electricity. To meet efficiency requirements, not only the laser diodes but also the power conditioning for those diodes must be compact and efficient. A diode pulser has been designed to meet these specifications, operate efficiently at the laser design point, and utilize a minimal number of diagnostics and external controls. This architecture minimizes cost and size for the estimated 4.4 million pulsers needed for a power plant. A diode laser test station has been constructed at LLNL, where the first pulser designs have been assembled and will be tested on diode bar stacks. Operational characteristics, wallplug efficiency, cost basis, lessons learned, and next steps toward manufacture will be discussed.

7916-11, Session 3

Overview of Project OrionN. Hopps, AWE plc (United Kingdom)

Project Orion will provide a facility for performing high energy density plasma physics experiments at AWE. The laser consists of ten, nanosecond beamlines delivering a total of 5kJ with 0.1-5ns temporally shaped pulses and two short pulse beamlines, each producing 500J in 0.5ps .

The performance of the Orion laser is reported as the first phase of commissioning (one short and one long pulse beam) concludes. Target shots with all beamlines will begin in 2012.

The longer, nanosecond pulses are generated using a single fiber oscillator with modulators for pulse shaping. These are pre-amplified by modules containing a regenerative amplifier, 2D-SSD, and a four-passed rod amplifier. The output is injected into ten beamlines, which use four passes of four 200mm aperture disk amplifiers. The beams are frequency tripled and then focused onto target with f/4 lenses.

Pulses of 150fs duration are split into two Offner triplet stretchers and stretched to 4.8ns, then amplified by three-stage optical parametric amplifiers. The OPAs generate highly stable top-hat (temporal and spatial) pulses. The OPA outputs are amplified in dual glass, four-passed rod amplifiers, followed by disk amplifiers and Faraday isolators, increasing in aperture to 200mm. Finally, the beams are expanded to 600mm and compressed using single-pass compressors with gold diffraction



gratings. Vacuum propagation to target chamber is followed by focusing using f/3 paraboloidal mirrors. Deformable mirrors are used to achieve optimal focusing properties. One short pulse beam can be frequency-doubled at 300mm diameter for experiments demanding high contrast or a shorter laser wavelength..

7916-12, Session 3

Status of SG-III laser facilityW. Zheng, Chinese Academy of Engineering Physics (China)

In this paper, we review the present status of SG-III laser facility at China Academy of Engineering Physics. SG-III laser facility contains a 48 beams neodymium glass laser, the 48 beams arranged in bundles of 4×2 beams, six bundles to a cluster, that is designed to deliver 180kJ at 60 TW at 351 nm for Inertial confinement Fusion(ICF) application in a temporally shaped pulse of approximately 3.0 ns to 10.0ns, the average fluency is designed to operate at 5.8 J/cm2 in a 3.0 ns output pulse. The optical scheme of a four-pass main amplifier (Amp1) and a booster amplifier(Amp2) have been chosen. The main amplifier of 4×2 segmented array has been chosen as a module and the clear optical aperture is 40cm×40cm. It consist of multiple subsystems, including the front-end stage, pre-amplifier stage, injection section, main amplifier stage, beam transport stage, alignment system, switchyard, target system, diagnostic system and integrated control system. Two spatial filters are inserted in the system to remove high spatial frequencies from the beam, and CSF is the multi-pass spatial filter and TSF is the transport spatial filter. In order to correct the output wavefront for static and dynamic wavefront aberrations of disk amplifiers, a deformable mirror system is used in the main amplifier stage of SG-III. A small Pockels Cell is used in beam reverser to suppress parasitic oscillations and beam can be rotated by 90 degree in U-turn beam reverser located in the middle of the TSF. The main change is that the inject pulse of phase modulated at 9.2GHz to a total bandwidth of 0.3nm is used in SG-III insteads of inject pulse of chirped stacking pulse at 1.2nm, for temporal pulse shape modulation will be existing due to the pulse coherence for chirped stacking pulse.

7916-13, Session 3

Activation of LFEX laser and contamination problem of compression chamberT. Jitsuno, H. Murakami, T. Kawasaki, S. Motokoshi, Y. Nakata, N. Sarukura, H. Shiraga, N. Miyanaga, H. Azechi, Osaka Univ. (Japan)

In the activation of LFEX laser system (1 ps, 10kJ with 4 beam-lines), a heavy contamination of optical components was found in the vacuum compression chamber. After the first evacuation and recovery to the atmosphere, almost all mirrors are found in heavily contaminated condition. The contamination materials were mainly DBP (Di-n-butyl phthalate) and the paraffin oil. As the result of an intense investigation, it was found that the source of contamination was the machine oil from the wall of chamber with rough surface, and additional contamination of DBP came from the outside painting of the chamber. Several cleaning methods were tried, but no significant improvement was obtained. Because this oil contamination was seen only on the ion-assisted deposition mirror, we chose the silica gel as the absorbing material in the vacuum. We observed no decrease of damage threshold of mirror sample when it was evacuated with the silica gel.

7916-14, Session 3

HiPER laserB. Le Garrec, C. Edwards, J. Collier, Commissariat à l’Énergie Atomique (France)

Anticipating the success of the National Ignition Facility in the USA,

scientists from across Europe are developing the case for a next generation laser fusion facility to be constructed in Europe. The High Power Laser Energy Research Facility (HiPER) is being strategically driven by an ambitious goal for the production of clean energy via Inertial Fusion Energy (IFE) on the 2035 timescale.

HiPER is a European collaborative project between 26 institutions which seeks to establish proof-of-principle of commercial power production from laser driven inertial fusion, “Laser Energy”. The preparatory phase of the project, which commenced in April 2008, is funded jointly by European member states and the European Commission. The next phase, “Technology Development”, will run for approximately 8 years prior to facility construction. The case for funding is in preparation.

In March 2009, the HiPER Executive Board took the decision to down-select to a solution based on high repetition rate operation. This commits the project to a strategy of demonstrating ignition at 10Hz repetition rate.

HiPER is currently concentrating its efforts on some of the most significant issues associated with laser energy. In particular, optimised solutions for high efficiency, rep-rated operation in a number of key areas are under consideration including:

- A diode pumped solid state laser (DPSSL) architecture

- Advanced target design

- Mass production of targets, target injection and engagement

- Debris management and reactor design

This paper will highlight recent scientific and technical progress in the key areas above, set out the project’s delivery strategy, and explain how the project is moving forward in an international context.

Plans for the development of a prototype beam line operating in the 1 - 10kJ, 10Hz regime will be discussed.

7916-15, Session 3

Simulation tools to predict long-pulse system performance during shot operations on OMEGA EPM. J. Guardalben, L. J. Waxer, Univ. of Rochester (United States)

A requirement for the successful operation of an inertial confinement fusion facility is to maximize the energy transported to the fusion target, with the constraint that the peak intensity of the source beams should not exceed the damage threshold of the beam-transport optics. To optimize the long-pulse, on-target energy of the OMEGA EP Laser, we have developed simulation tools to use during shot operations that provide rapid prediction of laser-system performance. The OMEGA EP Laser is a petawatt-class, Nd doped phosphate glass laser system that can be operated in both 1053-nm, short-pulse (<1- to 100-ps) and 351-nm, long-pulse (1- to 10-ns) regimes. The beamline architecture consists of 40-cm, single-segment, disk amplifiers in a multipass configuration to provide the necessary gain and resulting IR energy. For long-pulse operation, type-I/type-II frequency-conversion crystals are used to convert the 1053-nm fundamental wavelength to its third harmonic. Daily shot operations begin with low-energy, preamplified shots to evaluate IR system energy and near-field beam quality prior to injection into the beamline amplifier chain. An important goal is to provide facility users with maximum UV energy on target, while maintaining UV peak fluence within an acceptable limit. Simulation tools that predict the UV near-field beam-fluence distribution and energy on target based on preamplified shot performance have been developed and are regularly used during shot operations. These tools have streamlined daily system qualification, making it possible for UV energy to be maximized within current system constraints. Their use and the methodology for daily system qualification will be described.

This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-08NA28302, the University of Rochester, and the New York State Energy Research and Development Authority. The support of DOE does not constitute an endorsement by DOE of the views expressed in this article.



7916-16, Session 4

Programmable beam spatial shaping system for the National Ignition FacilityJ. E. Heebner, M. Borden, P. Miller, S. Hunter, K. Christensen, M. Scanlan, C. A. Haynam, P. Wegner, Lawrence Livermore National Lab. (United States)

We report on the performance of a system of 48 spatial light modulators designed to mitigate laser-induced damage growth on the MegaJoule-class laser at the National Ignition Facility (NIF). The modulators are installed in a low-fluence relay plane upstream in the NIF amplifier chain, where they introduce “blocker” obscurations that shadow defects on downstream optics subjected to harsher fluences. In this two stage system, 1920x1080 bitmap images are first imprinted on incoherent, 470nm address beams via pixelated liquid crystal on silicon (LCoS) modulators. The address beam images are then projected onto custom fabricated optically-addressable light valves. Each valve consists of a large, single pixel liquid crystal cell in series with a layer of photoconductive Bismuth Silicon Oxide (BSO). The BSO layer enables bright and dark regions of the address image to locally control the voltage supplied to the liquid crystal layer which in turn modulates the amplitude of the coherent beams at 1053 nm. Using this technique, we are able to realize arbitrarily defined masking functions with smooth apodized shapes and no pixelization artifacts. Compared to previous reports, the valves possess a large aperture (22mmx36mm), low wavefront distortion (<0.5wvs) and incorporate coatings for high transmission (>90%) while avoiding etalon effects leading to spectral and temporal ripple. The fully automated system acquires and processes image data collected by the Final Optics Damage Inspection camera, determines which defects require blockers, writes the blocker images, and verifies that they have been correctly located to protect the optics before proceeding to fire system shots.

7916-17, Session 4

Recent advances in the front-end sources of the LMJ fusion laserJ. Gleyze, N. Beck, J. Dubertrand, A. Perrin, Commissariat à l’Énergie Atomique (France)

In lasers for the inertial confinement fusion, like LMJ, main amplifier is injected by a laser source in which the beam parameters must be controlled. For several years the CEA has developed front end fibre sources, based on the telecommunications fibre optics technologies.

The Fibre-Injection System in the LMJ facilities uses single-mode PM fibre based architecture. The laser pulse is phase modulated to suppress SBS and enhance spatial smoothing, amplified, split and then temporally shaped by an arbitrary waveform generator.

Residual amplitude modulation induced in the preamplifiers from the phase modulation is also precompensated. These sources meet the needs but as technology evolves, the solutions used could be more efficient at a lower cost and with a smaller footprint.

First, as the number of beams in LMJ is important the temporal shaping represents an important part of the cost. Moreover this kind of electronic is mastered by only a small number of manufacturers. A solution based on CAN was developed for the LMJ. This technology in addition to being less expensive to manufacture allows the realization of pulse duration limited only by the memory depth.

We’ll also approach control systems for spectral broadening. Indeed, recent works for the optimization of phase modulations result in a need for greater flexibility. A solution independent of the type of modulation that could permit more versatility in front end, has been studied for LMJ and will be discussed.

In brief, we’ll give an up-to-date description of some recent development issues of LMJ front end.

7916-18, Session 4

Homogenised pump laser illumination of high-energy-laser-active-materials used under tilted anglesF. Kubacki, T. Mitra, M. Jarczynski, L. Aschke, LIMO Lissotschenko Mikrooptik GmbH (Germany)

Large slabs are used in Brewster configuration to generate a polarized laser beam with minimized reflection losses. Recently, these materials are pumped by diode lasers or solid state lasers. We introduce a new method to pump laser (or large scale amplifier) materials by using a homogeneous illumination even if the pump light incidents under a certain angle. Tophat intensity profiles as well as concave or convex pump profiles are generated by micro-optics beam shaping, yielding in a new degree of freedom in the design of high-energy solid state lasers. Various illumination modes are feasible in order to improve the beam quality of the laser output. An unique homogeneity of <2% can be achieved. The homogeneous pump light distribution may assist to lower thermal lensing effects. Customized pump light distribution also allows for higher average laser power.

7916-19, Session 4

The progress of the front-end and preamplifier in PW and SG-IIX. Li, W. Fan, Shanghai Institute of Optics and Fine Mechanics (China)

This paper includes three main parts, which are the review of the function requirements and the design consideration of the front-end and PAM system for SGII and 9th PW, the discussion and proposal on the design scheme as well as the progress of the system.

In the design scheme, this front-end and PAM system include three function modules, which are the nanosecond pulse module, the wideband short pulse module and the synchronization module. The synchronization module can obtain low timing jitter through the method of optical short pulse trigger. This synchronization route has the advantage of lowering the requirements of high stable electronic precisions timing system. The nanosecond pulse module consists of SLM fiber laser, fiber amplifier, waveguide phase modulator, waveguide temporal shaping element, polarization control element and Nd:glass regen-amplifier. The pulse energy output by the nanosecond module is over 10 mJ at 1Hz. In the wideband short pulse module, the pulse generated by commercial mode-locked laser goes through single select element and dispersion extended element first. And then it is transferred into OPCPA PAM element and boosted to over ten mJ level .

The progress of front-end and preamplifier will be also reported in this paper. The timing jitter of less than 6 ps rms between short pulse and nanosecond pulse has been obtained in the demonstration .The side pump Nd:glass regen-PAM can reduce the spectral gain-narrow effects, which lead to the reduction of the FM-AM effects. The pump laser of OPCPA provided by front-end system with good timing accuracy is able to produce over 0.5 J energy in 527 nm. OPCPA has successfully outputs the energy of above 20 mJ. The relative present progress in this system will also be reported in this paper.

7916-20, Session 4

Wavefront sensing and adaptive optics implementation on the generation of petawatt class lasersJ. Ballesta, N. Lefaudeux, X. Levecq, Imagine Optic SA (France)

Like in astronomy, Adaptive Optics is now a quasi standard feature for the multi-Terawatt lasers. AO helps in reaching both maximum



peak energy and fluence in correcting both aberrations induced by the optical components and the thermal effects induced in the amplification media. The new generation of upcoming Petawatt class lasers created a technological gap and AO needs now a new breath to overcome larger aberrations, faster optics and higher risk of damaging optical components.

Imagine Optic is a pioneer company in the development of AO solutions dedicated to ultra-intense and high energy lasers. The French company has developed a standard range of products, featuring high performances-versatility-robustness-security, used in more than 20 ultra intense and high energy facilities, everyday. This year, Imagine Optic will present 2 new products dedicated to new generation of ultra intense and high energy lasers.

A new, affordable, technology of deformable mirror, mechanical actuation based,was designed to cope with large aberrations and high energy encountered in Petawatt lasers.

The “optical fuse” is a new laser safety system based on a patented concept using a DM.It is dedicated to protect sensitive components against possible unexpected local over-intensities that can be dramatic for compression gratings for instance.

7916-21, Session 4

Research of diode-pumped cryogenic Yb:YAG amplification at 10 Hz repetition rateJ. Wang, Y. Jiang, X. Li, X. Li, Shanghai Institute of Optics and Fine Mechanics (China)

A 1 Joule pulse energy laser system with 10 Hz repetition rate has been developing, which consists of three major components: an all fiber based master oscillator, a diode-pumped cryogenic Yb:YAG regenerative amplifier and a power amplifier with cryogenic Yb:YAG ceramics. A ~100 pJ optical pulse with 10 ns time duration and 10 Hz repetition rate at 1030 nm wavelength is produced by the master oscillator. Then, the optical pulse is injected into the regenerative amplifier and the power amplifier. The gain material of the regenerative amplifier is a 8% Yb:YAG ceramic with a 1.8 mm thickness, which is pumped by a fiber-coupled 110W laser diode at 935 nm wavelength. The Yb:YAG ceramic temperature was controlled about 188K and a single pass small signal gain of 1.71 was demonstrated. The optical pulse was amplified over 54 passes through the ceramic in the regenerative amplifier and ~10.5 mJ output energy at 10 Hz with a square-pulse distortion of ~1.5 was achieved. The gain spectrum dependence on Yb:YAG ceramic temperature was also investigated. The peak wavelength shifts to shorter wavelength by ~0.25 nm and the peak linewidth (FWHM) near 1030nm decreases from ~2.3 nm to ~0.7 nm, as the temperature changes from 203K to 103K.

7916-22, Session 5

Laser Mégajoule alignment to target chamber centerM. Luttmann, V. Denis, C. Lanternier, Commissariat à l’Énergie Atomique (France); M. Péalat, Sagem Defense Securite (France); E. Compain, Bertin Technologies (France)

This paper describes the alignment system developed for the Laser Mégajoule (LMJ) in order to focus the laser beams and to point the plasma diagnostics on the target. It is organized in 3 parts. First, we present the main laser components and the alignment architecture resulting from experience gained on LIL (i.e. LMJ prototype) and specificities of our focusing and frequency conversion system called SCF [1].

In a second part, we detail the major equipments dedicated to the alignment process [2] as the 6 tele-microscopes used to position the target, the SCF’s autocollimation plates, the plasma diagnostic green pointer and the common reference (CR). CR, cornerstone of chamber center alignment is an emitter/receptor system able to:

- Point various equipments at precise positions around the chamber

- Locate the 3w autocollimation spot reflected successively by each of the SCF,

- Define the target frame origin for the six tele-microscopes,

- Locate the two-green spots emitted by plasma diagnostic pointers.

The challenge for the CR is to perform these actions precisely (about 10 µm) whereas 3w back reflections from the SCF’s are typically 4 order of magnitude less intense than the beam emitted by the CR.

In the last part we present some results obtained on the tele microscope prototype and on a photometric prototype of the CR. The expected performances of our alignment system will also be discussed.

[1] A. Hugget, E. Journot, R. Ferbos, F. Macias, Ph. Fayollas, “New design of the Laser Megajoule final optics assembly” Proc. SPIE, Vol. 6665, 66650U (2007)

[2] M. Geitzholz, C. Lanternier, “Review of laser Megajoule target area: Design and processes” J. Phys. IV France 133 (2006) 631-636

7916-23, Session 5

Automatic alignment system for the National Ignition FacilityK. Wilhelmsen, Lawrence Livermore National Lab. (United States)

The automatic alignment system for the National Ignition Facility (NIF) is a largescale parallel system that directs all 192 laser beams along the 300-m optical path to a 50-micron focus at target chamber in less than 50 minutes. The system automatically commands 9,000 stepping motors to adjust mirrors and other optics based upon images acquired from high-resolution digital cameras viewing beams at various locations. Forty-one control loops per beamline request image processing services running on a LINUX cluster to analyze these images of the beam and references, and automatically steer the beams toward the target. This paper discusses the upgrades to the NIF automatic alignment system to handle new alignment needs and evolving requirements as related to various types of experiments performed. As NIF becomes a continuously operated system and more experiments are performed, performance monitoring is increasingly more important for maintenance and commissioning work. Data, collected during operations, is analyzed for tuning of the laser and targeting maintenance work. Handling evolving alignment and maintenance needs is expected for the planned 30 year operational life of NIF.

7916-24, Session 5

Adaptation of a cubic smoothing spline algorithm for multi-channel data stitching at the National Ignition FacilityC. G. Brown, Jr., A. B. Adcock, J. A. Liebman, E. J. Bond, Lawrence Livermore National Lab. (United States)

Some diagnostics at the National Ignition Facility (NIF), such as the Neutron Time of Flight (nTOF) and Gamma Ray History (GRH) diagnostics, require multiple data channels, with redundant time samples, to be combined or “stitched” into a single time series. Since we need a general, non-parametric method, we chose to apply the popular cubic smoothing spline technique to our stitching problem. The vast majority of cubic smoothing spline codes available to us were limited to non-redundant time samples, so we adapted one of the algorithms in the literature, by Hutchinson and deHoog, to our needs. The modified algorithm and the resulting code perform a cubic smoothing spline fit to multiple data channels with redundant time samples and missing data points. The data channels can have different, time-varying, zero-mean white noise characteristics. The method we employ automatically determines an optimal smoothing level by minimizing the Generalized Cross Validation (GCV) score. In order to automatically detect failure of the smoothing level selection, the Weighted Sum-Squared Residual



(WSSR) and zero-mean tests are performed on the residuals. Further, confidence intervals, both analytical and Monte Carlo, are also calculated. In this paper, we describe the derivation of our cubic smoothing spline algorithm. We outline the algorithm and test it with simulated and experimental data. Finally, we discuss integration of the code into and use in the automated NIF data processing system.

This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.

7916-25, Session 5

Image processing and control of a programmable spatial light modulator for optic damage protectionA. A. S. Awwal, R. R. Leach, Jr., G. Brunton, E. Tse, J. Matone, J. E. Heebner, Lawrence Livermore National Lab. (United States)

The heart of the National Ignition Facility is a megajoule-class laser system consisting of 192 beams used to drive inertial confinement fusion reactions. A recently installed system of programmable, liquid crystal based spatial light modulators adds the capability of arbitrarily shaping the spatial beam profiles in order to enhance operational flexibility. Its primary intended use is for introducing “blocker” obscurations shadowing isolated flaws on downstream optical elements that would otherwise be damaged by high fluence laser illumination. Because an improperly shaped blocker pattern can lead to equipment damage, both the position and shape of the obscurations must be carefully verified prior to high fluence operations. An automatic alignment algorithm is used to perform detection and estimation of the imposed blocker centroid positions compared to their intended locations. Furthermore, in order to minimize the spatially varying nonlinear response of the device, a calibration of the local magnification is performed at multiple sub-image locations. In this paper, we describe the control and associated image processing of this device that helps to enhance the safety and longevity of the overall system.

*This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.

7916-26, Session 6

1W unconverted light management for the National Ignition FacilityJ. G. Di Nicola, S. N. Dixit, C. A. Haynam, M. A. Johnson, D. H. Kalantar, J. J. Kroll, B. J. MacGowan, W. A. Molander, J. D. Moody, L. R. Siegel, L. J. Suter, T. L. Weiland, Lawrence Livermore National Lab. (United States)

The National Ignition Facility (NIF) at Lawrence Livermore National Laboratory contains a 192-beam 3.6 MJ neodymium glass laser that is frequency converted to 351nm light. It has been designed to support high energy density science (HEDS), including the demonstration of fusion ignition through Inertial Confinement.

To meet this goal, laser design criteria include the ability to generate pulses of up to 1.8-MJ total energy at 351nm, with peak power of 500 TW and precisely-controlled temporal pulse shapes with intensities that span two orders of magnitude. The focal spot fluence distribution of these pulses is conditioned, through a combination of special optics in the 1 (1053 nm) or 2 (526.5nm) portion of the laser (continuous phase plates), smoothing by spectral dispersion (SSD), and the overlapping of multiple beams with orthogonal polarization (polarization smoothing).

Frequency conversion efficiencies > 80% were demonstrated with flat-in-

time 5ns pulses [1]. However, for a high dynamic range shaped ignition pulse, integrated efficiency can drop to ~60%. The resulting unconverted light enters the target chamber, mostly in the form of 1 light with a lesser amount of 2 light. NIF has been designed to protect its equipment, both beamlines and target diagnostics, from the adverse effects of unconverted light. Features have recently been added to the target to enhance machine safety.

Our efforts have targeted the 1 light where residual energies of up to ~1MJ can be produced depending on the operating conditions of the laser. Some of the unconverted light management strategies will be presented along with supporting experimental results and design simulations, including a dimpled-shield used with the ignition-type targets to prevent specular reflection from entering conjugate beamlines.

[1] C. A. Haynam et al., “National Ignition Facility laser performance status,” Appl. Opt. 46, 3276-3303 (2007).

7916-27, Session 6

Thermo-mechanical response of silica based final optics under HiPER irradiation conditionsA. Rivera, J. Alvarez, D. Garoz, R. González-Arrabal, J. M. Perlado, Univ. Politécnica de Madrid (Spain)

The European laser fusion project, HiPER (High Power laser Energy Research facility) is now in the final year of the preparatory phase. After 7 years of technological phase a fusion reactor will be built. Initially, it will operate in bunches of 100 shots with up to 5 ignition shots and a maximum energy per bunch of about 100 MJ (HiPER 4a). Direct targets of up to 40 MJ will eventually be used. Finally, a reactor to operate in continuous mode with full capabilities and a power of 500 MWe will be built (HiPER 4b). The final optics choice is at the moment based on silica transmission lenses located 8 m away from the chamber center. This scheme might be adequate for a bunch mode operation (HiPER 4a), however, often lens replacement might be necessary depending on different irradiation scenarios (full/partial ion mitigation, different targets, lens operation temperature). In addition, the role of the unavoidable anti-reflecting layers that coat the lenses must be addressed. We carried out computational simulations of the thermo-mechanical behavior of the lenses under different scenarios and estimated the radiation induced damage in each case based on previous models of defect generation in silica. We will report realistic lens operation lifetimes under the different proposed scenarios. We will further discuss the much more exigent situation that must be faced to design the final optics under HiPER 4b conditions.

7916-28, Session 6

The experiment of the third harmonic generation with ‘I+II’ scheme in SGII NO 9L. Ji, Shanghai Institute of Optics and Fine Mechanics (China)

We have designed a frequency converter for the ShengGuangII upgrade laser with type I doubler and the type II tripler, it can convert 5000J fundamental laser to 3000J the third harmonics with 31cm×31cm aperture and 3ns pulse. The design scheme has been test at the SG-II NO.9, the result fits to our expection.

7916-29, Session 6

SG-II U final optics assembly: optical damage and clean gas controllingD. Zhao, L. Wang, S. Ping, Z. Lin, Shanghai Institute of Optics and Fine Mechanics (China)



The Shenguang-Upgrade (SG-U) is an under-constructed high-power laser driver, with a 8-beam, 24KJ, 3ns, ultraviolet laser output energy, in Shanghai Institute of Optics and Fine Mechanics, China. The function of Final Optics Assembly (FOA), one of the most important parts of SG-U facility, consists of :1) vacuum sealing to both the target chamber and the air, 2) frequency conversion from the 1 to the 3 light, 3) color separation, 4) focusing the whole beam to target surface, 5) beam sampling for 3 energy diagnostics, 6) target debris shielding, and 7) keeping dynamically FOA inner-environment clean and temperature stable. In order to avoid optical damage, simulation analysis have been done for on ghost images to the 4th order for 1, 2, 3, and for the antireflected light from the target surface. The optimization of the optics parameters has been investigated. The environment gas flow controlling process has also been designed to remove the aerosol contaminates out off the assembly.

In May, experimental research of the FOA prototype had completed on the Ninth Beam of SG- facility. 33 shots have been fired with the outpout 1 energy from 1000J to 4500J, and 3 energy from 460J to 2408J on target surface, 3ns square laser pulse width. In the process of the experiment, we tested many performances about physical, techniques and engineering. Emphasis was given on the process of the optical damage and the effects of the clean gas controlling. Now we will address some tested and analyzed result for the damaged final optical element. Conclusion may be drawn as, it is feasible that the aerosol deposition on the final optics is the first cause to the optics surface damage and to the increasing in turn for the damage level. This presentation addresses the optical configuration of the FOA, the simulation analysis of ghost and gas controlling and the result analysis of damage.

7916-30, Session 7

Thermal birefringence and depolarization compensation in glass-based high-average-power laser systemsA. L. Bullington, S. B. Sutton, A. J. Bayramian, J. A. Caird, R. J. Deri, A. C. Erlandson, M. A. Henesian, Lawrence Livermore National Lab. (United States)

Thermally induced birefringence degrades the beam quality in high-average-power laser systems with doped-glass substrates. In this work, we compare glass-laser slab amplifiers at either Brewster’s angle or normal incidence and discuss trade-offs between both designs. Numerical simulations show the impact of thermally induced depolarization in both amplifier systems. A non-uniform temperature profile and the resultant mechanical stress leads to depolarization that worsens as the beam propagates through the slab-amplifier chain. Reflective losses for depolarized light at Brewster’s angle cannot be compensated and severely degrade beam quality. This motivates the selection of normally incident slab amplifiers, allowing for birefringence compensation.

Tolerances for birefringence compensation of two matched normal-incidence glass-slab amplifiers balanced by a quartz rotator are also investigated. Imbalances in thermal load, relative amplifier position and beam magnification between amplifiers show the highest depolarization sensitivity and set limits for manufacturing tolerances and amplifier design.

This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.

7916-31, Session 7

KrF lasers for fusion energyJ. Sethian, M. C. Myers, M. F. Wolford, J. L. Giuliani, Jr., D. Kehne, S. P. Obenschain, U.S. Naval Research Lab. (United States); F. Hegeler, Commonwealth Technology, Inc. (United States); R. H. Lehmberg, U.S. Naval Research Lab. (United

States)

We are developing an approach to fusion energy using electron beam pumped krypton fluoride (KrF) lasers and direct drive targets. KrF has significant advantages in obtaining the high gain needed for an attractive power plant: Extremely uniform illumination allows the more efficient direct drive approach, ready ability to decrease the focal diameter during the implosion reduces losses, and short wavelength (248 nm) allows higher ablation pressures and suppresses laser-plasma instabilities. KrF fusion is evaluated with the Nike KrF laser and NRL’s radiation-hydrodynamic code (FASTRAD3D). Simulations predict a 1 MJ KrF laser can produce 200 MJ of fusion energy. Such high gain at modest laser energy substantially reduces the power plant cost.

The KrF technologies are being developed on the Electra facility. Electra produces 300 to 700 Joules laser energy at 2.5 to 5 Hz, and runs continuously for 10 hours (90,000 pulses.) The primary run limit is erosion of the spark gap based pulse power system. A new solid state module runs continually for 319 hours at 10 Hz, with components tested to over 300 million pulses. A system is planned for Electra. The overall laser efficiency (wall plug to energy on target) is projected to be over 7%, based on development of the individual components. The benchmarked KrF Orestes KrF code has been used to design a 16 kJ beam line for the proposed Fusion Test Facility. The amplifier would be a modest (25%) increase in dimensions over Nike, and use the durable, efficient, rep-rate technologies developed on Electra.

7916-32, Session 7

Progress of rep-rate plasma Pockels cell technology in LFRCX. Zhang, D. Wu, Chinese Academy of Engineering Physics (China)

Plasma Pockels cell (PPC), which can use a thin crystal to perform the uniform electro-optical effect, is ideal component as average-power optical switch with large aperture. In this paper, the key problems in PPC are analyzed for repetition-rate application, and thermo-optical effects are simulated by means of numerical modeling when average power laser is loaded on the electro-optical crystal. By reformative design and employing a capacity to share the gas discharge voltage, the PPC adopted DKDP crystal driven by one pulse is realized. As gas breakdown delay time is stable, and discharge plasma is uniformly filled the full aperture, it meets the demand of plasma electrode for the repetition-rate PPC with DKDP crystal. A rep-rate plasma Pockels cell (PPC) with 30mm aperture has been fabricated. It is optimized with the limited space of repeated frequency diode pumped laser. The specification of the PPC is: static transition of 97.2, switching efficiency of 99.8, the switch rising time of 8.6ns. In the LD pumped Yb:YAG plate laser system, the PPC can steadily work on 10Hz repetition rate performed as Q-switch. Furthermore, the principium design of rep-rate PPC with longitudinally conduction-cooled structure is described in this paper. It will efficiently abate the thermo-optical effects under repetition rate application.

7916-33, Session 7

Measurement and control of micro-errors on components-tiling and beams-combinationX. Wang, Y. Yang, F. Jing, F. Li, K. Zhou, Q. Zhu, Y. Zuo, L. Zhao, Z. Huang, X. Zeng, D. Jiang, X. Huang, X. Hao, X. Wei, Chinese Academy of Engineering Physics (China)

Fast ignition experiments need laser with energy of tens kJ, pulse width of tens ps, focused into about tens microns. To reach this need, TGC (tiled gratings compressor) and beam-combination technology have to been developed. TGC (tiled gratings compressor) is employed to make full use of the output of each single beam , and beam-combination technology will be used to meet the need of total energy and focused power. Both of these technologies are concerned with vibration



suppression and micro-errors control. In this article, development of TGC and beam-combination technology at CAEP is introduced.

On TGC technology, interferometric patern and far field distribution are used to initially eliminate the tiling error, and position sensor is used as feedback to maintain the state of the sub-gratings. About beam-combination technology, influences of a few factors on the combined focal spot are analyzed and simulated, and a method of feedback control in subsections is proposed and demonstrated in a test-bed.



Conference 7917: Nonlinear Frequency Generation and Conversion: Materials, Devices, and Applications XMonday-Thursday 24-27 January 2011 • Part of Proceedings of SPIE Vol. 7917 Nonlinear Frequency Generation and Conversion: Materials, Devices, and Applications X

7917-01, Session 1

Multi-watt orange light generation by intracavity frequency doubling in a dual-gain quantum dot semiconductor disk laserJ. Rautiainen, Tampere Univ. of Technology (Finland); I. L. Krestnikov, Innolume GmbH (Germany); J. Nikkinen, O. G. Okhotnikov, Tampere Univ. of Technology (Finland)

Various operation wavelengths can be achieved from semiconductor disk lasers based on quantum well material by band-gap engineering. The wavelength tailoring with this quantum-confined system, however, suffers from certain constrains related to critical growth conditions for specific spectral ranges. Quantum dot media allow expanding further the wavelength coverage of semiconductor disk lasers due to alleviated strain impact to the epitaxial structure. In this talk we present a frequency doubled dual-gain quantum-dot semiconductor disk laser producing 590 nm orange light. The multiple-gain concept is particularly relevant to quantum-dot systems whose single-pass gain is typically lower than the gain of quantum-well-based materials. This geometry can be applied to a number of gain elements and exhibit superior gain by consuming more pump power and sharing the thermal load among numerous active media, thus preventing excessive heating and rollover.

The structures grown by molecular beam epitaxy on a GaAs substrate comprise GaAs/AlAs distributed Bragg reflector and active region composed of 39 layers of InGaAs Stranski-Krastanov quantum dots designed for excited state emission at 1180 nm. The gain mirrors each assembled with intracavity diamond heat spreader produce individually 3 and 4 W of output power while the laser with both elements in a single cavity reveals 6 W at 1180 nm with beam quality factor M2<1.2. A BBO nonlinear crystal was employed for frequency doubling of the laser radiation. The loss induced by the nonlinear crystal is compensated by gain boosting in the dual-gain laser and 2.5 W of output power at 590 nm was achieved after frequency conversion.

7917-02, Session 1

1 W at 531 nm generated in a ppMgO:LN planar waveguide by means of frequency doubling of a DBR tapered diode laserD. Jedrzejczyk, R. Güther, K. Paschke, G. Erbert, Ferdinand-Braun-Institut (Germany)

Visible laser light sources with a moderate output power and spatially and spectrally single-mode operation are desired for many applications, such as spectroscopy or biotechnology. Frequency doubling of near-infrared (NIR) diode laser radiation in nonlinear crystals allows to realize such devices in a compact manner. One of the concepts to increase the conversion efficiency and the maximal output power at the same time is the application of a nonlinear planar waveguide.

In this work, we investigated experimentally second-harmonic generation (SHG) in a periodically poled 5 %mol MgO doped LiNbO3 (ppMgO:LN) planar waveguide. As a pump source a 6 mm long distributed Bragg reflector (DBR) tapered diode laser was applied. The DBR tapered laser emits nearly diffraction limited, spectrally single-mode CW radiation at 1063 nm and is therefore well suited for the SHG process. With the applied lens system in a bench-top experiment a coupling efficiency into the planar waveguide of 73 % was reached. A maximal SH power of 1.07 W was generated at an optical and electro-optical conversion efficiency of 26 % and 8.4 %, respectively. This is, to the best of our knowledge, the highest power level generated in a nonlinear waveguide by means of frequency doubling of diode laser radiation.

At the conference, we will present the experimental setup with the corresponding optimal focusing parameters. Furthermore, the SH power dependence on the NIR power and on the crystal temperature, as well as the spectral characteristics of the SH radiation will be shown.

7917-03, Session 1

High-power (1 1W) green (532nm) laser source based on single-pass second harmonic generation on a compact micro-optical benchP. Q. Liu, Princeton Univ. (United States) and Ferdinand-Braun-Institut (Germany); C. Fiebig, M. Uebernickel, G. Blume, D. Feise, A. Sahm, D. Jedrzejczyk, K. Paschke, G. Erbert, Ferdinand-Braun-Institut (Germany)

Compact, high-power lasers emitting green light with good beam quality are indispensable for many applications such as high-brilliance display technology [1] and certain spectroscopy techniques. Second harmonic generation (SHG) of green light using a high-performance edge-emitting diode laser and a nonlinear bulk crystal in a single-pass configuration is a straightforward and effective way to generate green light with >1 W output power and good beam quality, and offers the possibility of compact system integration. Several groups have already demonstrated such systems using a macroscopic setup with distributed Bragg reflector (DBR) tapered diode laser and periodically poled MgO:LiNbO3 crystal [2,3].

Here, we demonstrate results on a highly compact single-pass SHG system for green light generation. The whole system is integrated on a compact micro-optical bench (MIOB), which has a dimension of 50 mm x 10 mm x 5 mm. As pump source we used a DBR tapered diode laser with >8 W maximum optical output power. Using a 2.5 cm long periodically poled MgO:LiNbO3 crystal we achieved an optical output power of 1.1 W at ~532 nm with ~7.6 W pumping power. The maximum achieved optical conversion efficiency is nearly 15%, the corresponding wall-plug efficiency is 4.3%. The green laser beam shows a relatively good beam quality (measured at 0.9 W of green light power) of M²=1.8 in vertical and M²=4.9 in lateral direction, respectively. The details of the system design as well as the properties of the devices will be given at the conference.

[1] G. Hollemann, B. Braun, P. Heist, J. Symanowski, U. Krause, J. Kraenert and C. Deter, “High-power laser projection displays,” Proc. SPIE 4294, 36 (2001).

[2] O.B. Jensen, P.E. Andersen, B. Sumpf, K.H. Hasler, G. Erbert and P.M. Petersen, “1.5W green light generation by single-pass second harmonic generation of a single-frequency tapered diode laser,” Optics Express, Vol. 17, Issue 8, pp. 6532-6539 (2009).

[3] A. Jechow, R. Menzel, K. Paschke and G. Erbert, “Blue-green light generation using high brilliance edge emitting diode lasers,” Lasers and Photonics Reviews, published online Dec. 2009.

7917-04, Session 1

Modulation and efficiency characteristics of miniature microchip green laser sources based on PPMgOLN nonlinear materialJ. Khaydarov, A. V. Shchegrov, S. Essaian, S. Slavov, Spectralus Corp. (United States); H. Danielyan, G. Gabrielyan, A. Poghosyan, S. Soghomonyan, Spectralus CJSC (Armenia)


We developed a highly efficient diode-pumped solid-state (DPSS) green laser source, based on a monolithic cavity microchip laser platform. The use of periodically poled MgO-doped Lithium Niobate (PPMgOLN) as the nonlinear frequency doubler together with gain material Nd3+:YVO4 allows obtaining a significant increase in the overall efficiency of the green microchip laser in comparison with other compact green laser source architectures with comparable output power. We discuss our progress in miniaturization and efficient operation across a wide range of temperatures and application-specific modulation conditions. In particular, we demonstrate 50mW-180mW average green output power (30% duty cycle) with optical-to-optical efficiency from 808nm pump to 532nm output up to 27% and wall-plug efficiency over 12%. Efficient laser operation with duty cycle ranging from 10% to 100% (cw) in a wide range of repetition rates is also demonstrated. The laser is designed to be a part of the miniature and efficient RGB light source for microdisplay-based (LCOS, DLP or similar) mobile projector devices. While these projection architectures typically require modulation rates from 60Hz to about 2000Hz depending on design, we extended modulation speed up to 30kHz and higher that can be of interest for other applications. A very efficient and small microchip and alignment-free design allows to package this laser source into the very small volume of only 0.23cm3 (bounding box). We present results of performance tests for this packaged laser.

7917-17, Session 1

A compact optically pumped semiconductor laser emitting at 593 nmW. R. Seelert, Coherent Lubeck GmbH (Germany)


7917-05, Session 2

Fiber-laser-pumped CW OPO for red, green, blue laser generationY. Lin, Y. Huang, National Tsing Hua Univ. (Taiwan)

We report a CW, watt-level, red, green, and blue (RGB) laser pumped by a low-cost multi-longitudinal-mode Yb-fiber laser at 1064 nm. A bow-tie singly resonant optical parametric oscillator at 1560 nm contains two intracavity sum-frequency generators for red and blue laser generations. The red laser photon at 633 nm is summed from the pump and the near-IR signal photons. The blue laser photon at 450 nm is summed from the red and the signal photons. An extracavity second harmonic generator converts the residual pump power into green laser radiation at 532 nm. The OPO and RGB wavelength converters are Mg-doped PPLN crystals with lengths of 5, 1, 1, 0.5 cm, respectively. At 25-W pump power, the laser generated 3.9, 0.5, and 0.5 W at 633, 532, and 450 nm, respectively. The pump to visible laser efficiency is 20%. The multimode pump laser offers large temperature bandwidths for the RGB laser radiations, because different spectral components of the pump are used in different wavelength converters. The measured temperature bandwidths for the red, blue lasers are 11 and 4 deg., respectively. The measured rms output power variation is about 6% over ~10 min time.

7917-18, Session 2

Harmonic generation with fiber MOPAs and solid state lasers: technical challenges, state-of-the-art comparison, and future developmentsA. N. Starodoumov, N. Hodgson, Coherent, Inc. (United States)

Fiber MOPAs in the infrared wavelength region offer the advantage of high single mode output powers, independent selection of pulse

repetition rates and pulse durations, and access to high repetition rates. Despite these performance advantages, most industrial and scientific applications in the visible and the ultraviolet spectral range are still dominated by solid state lasers. We will give an overview of the technical challenges of harmonic generation in fiber lasers/amplifiers and discuss the state-of-the-art and future of Fiber MOPAs and bulk solid state lasers with harmonic generation.

7917-19, Session 2

Raman lasers for yellow-orange spectrum coverageN. Landru, J. Rouvillain, G. Lebail, T. Georges, Oxxius SA (France)

Diode lasers have been demonstrated to operate over a great part of the visible spectrum: InGaN diodes cover the violet-blue-green part (<530nm) and InGaAlP diodes cover the red part (>635nm). Some fluorophorous in biotechnology applications are excited by intermediate wavelengths, from 540 to 630nm. Optically pumped InGaAs lasers were demonstrated from 460nm up to 580nm. Standard frequency doubled diode pumped solid state (DPSS) lasers lack of suitable transition to cover the 565-650nm region. It is possible to modify the semiconductor composition to extend the frequency range or to frequency mix DPSS laser wavelengths, but it comes with a significant R&D effort or a complexity in the design.

Raman scattering can red-shift the strong transitions of Nd or Yb lasers so that many wavelengths lying in the 1080-1300nm range can be achieved. Recently several CW didoe pumped Raman lasers were demonstrated, some of them including intra-cavity frequency doubling or mixing. The problems with these Raman lasers are the high pump threshold and high noise. We have built several Raman lasers with a reduced loss presenting a low pump threshold (<1W) and high slope efficiency. Output powers in excess of 100mW were achieved at 588nm with a 2.5W 808nm pump. Laser emissions from 543nm up to 610nm were demonstrated. Noise of these lasers was analyzed and means to reach low noise operation will be discussed at the conference.

7917-20, Session 2

575 nm laser oscillation in Dy3+-doped waterproof fluoro-aluminate glass fiber pumped by violet GaN laser diodesY. Fujimoto, Osaka Univ. (Japan); O. Ishii, M. Yamazaki, Sumita Optical Glass, Inc. (Japan)

Visible lasers are widely applicable to medicine, biology, metrology, optical storage, and display technology, and especially a yellow laser has potential applications in biomedicine, ophthalmology, and medical treatment for acne melasma and facial telangiectasia. Yellow lasers have been developed with several techniques, such as copper bromide laser, and upconversion, or second harmonic generation in Nd-doped crystals, Yb-doped fiber, Bi-fiber laser, optically pumped semiconductor lasers.

We have successfully drawn a low-loss Dy-doped fluoro-aluminate glass fiber. Fluoro-aluminate glass including 10,000 ppm of Dy concentration was used as a fiber core. The core and clad diameters of the drawn fiber were 8 and 300 µm, respectively. A 4-cm-long Dy-doped fiber was inserted into a zirconia-ferrule, and then both sides of the fiber surfaces were polished. Yellow laser oscillation was demonstrated in the Dy3+-doped fluoride fiber pumped by a 398.8-nm GaN-LD at 575 nm with 17.1% of the slope efficiency. The maximum output power was 10.3 mW, the threshold power was 10.2 mW, and the slope efficiency was calculated to be 17.1%.

Because the violet GaN-LD is commonly used as a pick up for Blu-ray Disc devices and the low-cost multi-hundred milliwatt LD is already available, it is expected that low-cost and compact yellow laser devices can be produced. Since the fluoro-aluminate-glass system has a remarkable water resistance advantage compared to ZBLAN glass, this

Conference 7917: Nonlinear Frequency Generation and Conversion: Materials, Devices, and Applications X


type of Dy-doped fluoride glass fiber can greatly contribute to a yellow fiber laser with high chemical durability without a frequency doubling technique.

7917-21, Session 2

Efficient frequency conversion of pulsed microchip and fiber laser radiation in PPSLTB. Jungbluth, S. Nyga, E. Pawlowski, T. Fink, Fraunhofer-Institut für Lasertechnik (Germany)

Extensive studies on frequency doubling with ppSLT crystals are presented. This includes a detailed discussion on design aspects and theoretical modeling predictions as well as experimental studies comparing the performance of ppSLT crystals from different providers with and without MgO doping. Experimental analyses of their acceptance parameters and crystal homogeneity are conducted with a pulsed microchip laser with low peak (6 kW) and low average power (50 mW) resulting in a maximum conversion efficiency of up to 80 % for high quality MgO doped crystals. Based on these results a compact converter module with fiber coupling is designed and tested with the radiation from the microchip laser and a fiber laser source in comparison. The fiber laser provides an average power of about 1 W. Even at this - still very moderate - power level a significant efficiency drop can be observed. Despite the advantage of higher pulse peak (25 kW) power from the fiber laser source, careful design adaptations of the converter are required even to preserve a conversion efficiency beyond 50%.

7917-06, Session 3

Amplification of ps-pulses from freely triggerable gain-switched laser diodes at 1062 nm and second harmonic generation in periodically poled lithium niobateT. Schönau, S. M. Riecke, K. Lauritsen, R. Erdmann, PicoQuant GmbH (Germany)

Pulsed lasers based on gain-switched distributed feedback (DFB) laser diodes allow for arbitrary repetition rates from single shot up to a few GHz. The narrow spectrum (sub-200 pm FWHM) even under pulsed condition makes them well suited for optical amplification and nonlinear processes.

In this study we present a compact alignment-free setup, operating at 1062 nm that includes one Ytterbium-doped fiber amplifier stage. A freely triggerable seed diode laser delivers sub-100 ps pulses at any repetition rate up to 80 MHz with typical pulse energies in the Picojoule range. After amplification, the average power exceeds 300 mW at 40 MHz, which corresponds to 10 nJ pulses and about 100 W of peak power. The output beam is then focussed into periodically poled lithium niobate for second harmonic generation (SHG). The conversion efficiency into 531 nm is measured for different crystal lengths and input beam parameters. Measurements of the frequency conversion are compared to results of theoretical models. We also evaluate spectral properties as well as pulse shape and output stability of the amplifier and the second-harmonic signal. Additional focus is set on low timing jitter between pulses and long term power and pointing stability.

Applications for pulsed visible laser light can be found in material processing, display technology and time-resolved fluorescence spectroscopy. Depending on deployed fluorophores, there is a high demand for cost effective pulsed excitation sources covering all parts of the visible spectrum, especially in the yellow-green range.

7917-07, Session 3

Thermal optimization of the second harmonic generation with tapered diode lasersA. Sahm, M. Uebernickel, K. Paschke, G. Erbert, G. Tränkle, Ferdinand-Braun-Institut (Germany)

Recently, hybrid integrated compact laser sources with high optical output power in the visible range around 488 nm were demonstrated using tapered diode lasers and a single pass second harmonic generation (SHG) with a periodically poled LiNbO3 crystal of 30 mm length. The highest achievable conversion efficiency depends on the light source but is also a function of the temperature distribution along the crystal length.

The maximum conversion efficiency of a given beam is theoretically achieved by a homogenous temperature distribution. Experiments have shown that for high power SHG different absorption mechanisms are causing a temperature gradient in the crystal. This gradient leads to a inhomogeneous poling period which diminishes the effective crystal length and leads to a smaller conversion efficiency.

In this paper we present a method for the optimization of the temperature management during the SHG. This is done by a multizone heater package that can be integrated into compact laser sources. This package can be used to generate arbitrary temperature distributions and is especially able to compensate an arising temperature gradient.

For a given pump power of 4 W we were able to increase the SHG output power of more than 20%. At the same time the FWHM of the temperature tuning curve was decreased from 1 K to a nearly optimal value of 0.8 K. This shows the potential of this concept especially at higher pump powers where the temperature effects are more challenging.

7917-08, Session 3

Deep ultraviolet light generation at 193 nm by quasi-phase-matched quartzM. Harada, National Institute for Materials Science (Japan) and Nikon Corp. (Japan); M. Adachi, National Institute for Materials Science (Japan) and Nidek Co., Ltd. (Japan); K. Muramatsu, M. Ueda, Nikon Corp. (Japan); T. Yamada, NIDEK Co., Ltd. (Japan); S. Kurimura, National Institute for Materials Science (Japan)

Coherent all-solid-state light source of a wavelength below 200nm is attracting much attention for industrial applications such as semiconductor processing and eye surgery. Multi-stage wavelength conversion from a high power infrared solid-state laser is a promising solution. Conventional system is quite complex because sum-frequency generation (SFG) is used at the final stage of conversion. To simplify the optical system, practical second harmonic generation (SHG) materials have been researched to enable an in-line cascaded structure and enhance the versatility of all-solid-state light source.

We have developed quasi-phase-matching (QPM) in crystal quartz that utilizes twinning. Periodic twin structures are fabricated by spatially-modulated mechanical stress at elevated temperature. The tolerance of the device fabrication parameters such as temperature and stress becomes narrow because merging and back-switching easily occur at a short QPM period as well as the domain merging and back-switching in periodically poled ferroelectrics. Back-switching of twins are more likely to happen at a period shorter than 10 m, especially when releasing the stress at room temperature. We demonstrated the finest periodic twins with a period of 9.6 um by suppressing back-switching with the novel stress-maintaining module. The period was designed for 5th-order QPM-SHG at 193.4 nm wavelength around room temperature to match ArF excimer laser’s wavelength. We measured input/output characteristics of SHG in QPM quartz. Vacuum ultraviolet 193.4 nm light of 17 uW was generated from 180 mW fundamental light in QPM-SHG experiment. To the best of our knowledge, this is the shortest emission wavelength ever obtained in QPM technology.



7917-09, Session 3

70% frequency-doubling efficiency of 0 8-W mode-locked picosecond Ti:sapphire laser with external cavityT. Ohira, S. Maeda, Y. Takida, H. Kumagai, Osaka City Univ. (Japan)

Recently, high-performance light sources in the UV and VUV region is getting more and more essential in application fields such as photochemistry, spectroscopy, lithography, and material science. Therefore, many researches regarding on the frequency doubling of near-infrared coherent lights have been done to obtain high-efficiency and high-power coherent UV lights. However, the pump light of the high average power is necessary for such high-efficiency wavelength conversions. We have researched high-efficient, convenient and simple generation of UV continuous and quasi-continuous waves by optimizing an external cavity and then using a BiB3O6 (BiBO) as a nonlinear crystal of relatively high nonlinear optical coefficient. We report here the generation of a high-efficient 389-nm coherent light based on the second harmonic generation of a mode-locked picosecond-pulsed Ti:sapphire laser with the BiBO as a nonlinear crystal. As a result, more than 500 mW of output at 389 nm was obtained with the maximum input of 800 mW and a maximum efficiency of 63%. Furthermore, considering the reflection loss at the output mirror of the 389-nm light, we could obtain 70% conversion efficiency. This value is one of the best results of the second harmonic generation of less than 1W of average pump power. We will show you the detail in our presentation.

7917-71, Session 3

Multiple-wavelength synthetic green laser source for speckle reductionD. V. Kuksenkov, R. V. Roussev, S. Li, W. A. Wood, C. M. Lynn, Corning Incorporated (United States)

While “native” (direct semiconductor) green lasers with sufficient output power and efficiency for pico-projector applications are not yet commercially available, “synthetic” lasers generating green output by frequency conversion of a 1060 nm diode laser pump present an attractive alternative. Unfortunately, the high spectral and spatial coherence of these lasers results in a high level of speckle in projected images.

We report on the experimental demonstration of a synthetic green laser simultaneously emitting three spectral lines with ~ 0.5 nm separation and less than 2 dB intensity difference, enabling a factor of [radical sign]3 reduction of speckle contrast.

Our device concept consists of two main parts. The first is a diode laser emitting two relatively narrow spectral lines with equal power. While ideally this would be an integrated design such as DBR laser with super-structure grating, in the experiments we use an external cavity configuration with dual-wavelength reflection provided by a volume Bragg grating. Gain-switching is used to achieve stable dual-line operation and maximize conversion efficiency.

The second part is a nonlinear crystal designed to simultaneously phase match three nonlinear processes - second harmonic generation of each pump line and sum frequency generation. In the experiments, we use reverse proton exchanged MgO-doped lithium niobate waveguides with quasi-periodic (spatially phase-modulated) poling. We demonstrate 58 mW output power in three spectral lines and 33% nonlinear conversion efficiency.

The presented design concept is extendable to a larger number of wavelengths, for example 3-wavelength pump and 5-wavelength output, for speckle contrast reduction by a factor of [radical sign]5.

7917-11, Session 4

Multi-THz fields exceeding 100 MV/cm: an ultrabroadband source for sub-cycle nonlinear opticsA. Sell, F. Junginger, O. Schubert, B. Mayer, Univ. Konstanz (Germany); T. Kampfrath, M. Wolf, Fritz-Haber-Institut der Max-Planck-Gesellschaft (Germany); D. Brida, M. Marangoni, G. Cerullo, Politecnico di Milano (Italy); A. Leitenstorfer, R. Huber, Univ. Konstanz (Germany)

Phase-locked few-cycle pulses in the intriguing multi-terahertz (THz) spectral range have evolved into a versatile probe of femtosecond low-energy dynamics in condensed matter [1]. Novel studies of ultrabroadband nonlinear THz optics and coherent control have triggered a growing demand for transients with ever higher center frequencies and amplitudes.

Here we present a table top facility which allows for generation and electro-optic detection of THz transients with unprecedented intensities and bandwidths. Our hybrid laser combines the stability of a femtosecond Er:fiber system with the high power of a Ti:sapphire amplifier. Seed and gate pulses as short as 4.3 fs are provided by the multi-branch fiber laser [2]. Difference frequency mixing of the signal waves from two parallel optical parametric amplifiers warrants inherently phase-locked THz waveforms tunable throughout the far and mid infrared, from 0.1 to 140 THz [3,4]. Peak electric fields of up to 108 MV/cm are recorded [4]. By careful dispersion management, we achieve strict single-cycle pulses with amplitudes exceeding 10 MV/cm [5].

This source paves the way to studies of extremely non-perturbative THz nonlinearities on the sub-cycle scale. Lead-off examples include coherent control of intra-excitonic transitions in Cu2O [6]. Since our record THz intensities also come with extremely high magnetic fields of up to 33 T, we succeed in controlling coherent spin waves in the prototypical anti¬ferro¬magnet NiO by all-magnetic coupling [7]. Finally, an exciting new class of non-collinear four-wave-mixing studies with complete field resolution in semiconductors, superconductors and molecular systems comes into reach.

[1] see e.g. R. Huber et al., Nature 414, 286 (2001); T. Kampfrath et al., Phys. Rev. Lett. 95, 187403 (2005); G. Günter et al., Nature 458, 178 (2009).

[2] G. Krauss et al., Nature Photonics 4, 33 (2010).

[3] A. Sell et al., Appl. Phys. Lett. 93, 251107 (2008).

[4] A. Sell et al., Opt. Lett. 33, 2767 (2008).

[5] F. Junginger et al., Opt. Lett., accepted for publication (2010).

[6] S. Leinß et al., Phys. Rev. Lett. 101, 246401 (2008).

[7] T. Kampfrath et al., submitted for publication.

7917-12, Session 4

Nonlinear holographic imaging of terahertz radiationJ. Delagnes, P. Mounaix, L. Canioni, Univ. Bordeaux 1 (France)

Due to their non-ionizing properties and relative transparency in most materials, terahertz radiations or T-rays are appealing for several imaging and inspection applications. Nevertheless the currently available emitters/detectors sets limit the range of application either because of their limited tunability or low sensitivity that require prohibitive acquisition time.

In order to circumvent these limitations, one possibility is to transfer the THz information into the visible range. Here, we present a theoretical investigation of an up-conversion method to perform imaging of THz radiation We demonstrate that, in the weak-field regime, the non-collinear sum frequency generation in an electrooptic crystal between visible and THz radiation results in the encoding of the spatial phase of the THz object field onto a visible wave. Since THz and visible wavevectors are



strongly different, phase matching is weakly violated and the visible beam is partly deviated. When suitably focused, the spatial phase of the incident visible beam is imprinted in the Fresnel propagator after the interaction and the crystal thus behaves as a converging lens. As a consequence, the transfer of phase and amplitude of the THz field onto the optical field allows to record THz scene with optical detector that are much more sensitive than THz sensors.

We show that a modified conjugation formula can be derived from our analysis; in comparison to the classical one this generalized lens formula (Nonlinear Snell-Descartes’ Lens Formula) exhibits an additional magnification factor proportional to the ratio between the optical and THz wavelength. Intermediate and final images are computed.

7917-13, Session 4

Investigation of metamaterials for terahertz frequency rangeR. Yahiaoui, Univ. Bordeaux 1 (France); H. Nemec, P. Ku?el, F. Kadlec, C. Kadlec, Institute of Physics of the ASCR, v.v.i. (Czech Republic); J. Schilling, Martin-Luther-Univ. Halle-Wittenberg (Germany); M. Bari, Science and Technology Research Partners Ltd. (Ireland); J. Delagnes, P. Mounaix, Univ. Bordeaux 1 (France)

Metamaterials are artificial materials with unusual properties that do not exist in nature and basically could consist of subwavelength metallic patterns printed on dielectric substrate. In this paper, we present a theoretical and experimental investigations of metamaterials designed for THz applications. First, fishnet metamaterials which are composed with a thin dielectric material sandwiched between two metallic layers. Two techniques were used in order to fabricate our prototypes: double layer optical lithography and laser micromachining. We performed simulations and experiments using commercial software simulator based on finite element method (HFSS) and terahertz time domain spectroscopy THz-TDS respectively. A good agreement was reported between simulations and experiments while pointed out the dramatic influences of dielectric losses in the effective response. The second devices rely on dielectric metamaterials which exhibit a tunable range of negative effective permeability in the terahertz spectral region (0.2 - 0.36 THz). Our structures consist of an array of intrinsically non-magnetic rods made of an incipient ferroelectric SrTiO3 which shows a high tunable permittivity. The magnetic response and its tuning are achieved by a temperature control of the permittivity of SrTiO3 which defines the resonant confinement of the electromagnetic field within the rods.

7917-14, Session 4

Parametric generation of terahertz wave pumped by picosecond Ti:sapphire laser with MgO-doped LiNbO3 installed in external enhancement cavityY. Takida, S. Maeda, T. Ohira, H. Kumagai, S. Nashima, Osaka City Univ. (Japan)

Terahertz waves have been expected to be applied in comprehensive application spheres. As for the generation of terahertz wave, there is a way to utilize optical parametric process or optical rectification, which is the second-order nonlinear optical effect. Using the method of optical parametric process, a Q-switched Nd:YAG laser often have been utilized as a pumping source. In many cases using the method of optical rectification, femtosecond pulses from a Ti:sapphire laser have been utilized as a pumping source. However, a picosecond-pulsed light source which provides pulses with the pulse duration between nanosecond and femtosecond, to our knowledge, have never been reported as a pumping source for direct generation of terahertz wave. We can obtain much higher repetition-rate terahertz wave than that by a Q-switched Nd:YAG laser. In addition, the value of enhancement factor using picosecond pulses is able to become larger than that using femtosecond pulses.

Thus, we focus on the generation of terahertz wave by optical parametric process in nonlinear optical crystal pumped by a 81.7-MHz picosecond Ti:sapphire laser. Therefore, we built an external enhancement cavity with MgO-doped LiNbO3 (MgO:LN) and enhanced the pump light in the cavity. Considering that the idler light generated in MgO:LN with the different angle of 1 degree to the pumping light, we designed the cavity in order to circulating the idler light as well as the pumping light simultaneously. Circulating idler light in the cavity, the circulated idler light might contribute the terahertz parametric generation induced by the next pumping pulse. As a result, when the idler light circulating the cavity, we demonstrated a clear enhancement of the idler intensity.

7917-15, Session 5

Recent progress of THz generation and detection in ambient air or gasesX. Lu, B. W. Clough, I. Ho, J. Liu, J. Dai, X. Zhang, Rensselaer Polytechnic Institute (United States)

Since early 90s, THz time domain spectroscopy has been largely applied on the measurement of semiconductor, electro-optic crystals, and selected chemical, biological and explosive materials. However, majority measurements are linear transmission or reflection measurement. Here we will highlight THz wave sensing and imaging science, technology and its applications, with an emphasis on spectroscopic and imaging capabilities. The most recent results of using air (and selected gases) as the emitter and sensor material for both generation and detection of broadband THz waves will also be reported. Air, especially ionized air (plasma), has been used to generate intense peak THz waves (THz field > 1.5 MV/cm) with a broadband spectrum (10% bandwidth from 0.1 THz to 46 THz).

A focused optical pulse (mJ) in air creates a plasma.

- It is commonly known that pulsed, laser-induced plasma emits UV and visible fluorescence, as well as sound waves (photo-acoustics);

- It is less known that the fluorescence brightness and acoustic amplitude increase when a THz pulse is applied on the plasma.

- It is generally unknown that a broadband THz signal can be coherently detected with the use of dual-color laser beam excitation.

We have developed THz radiation-enhanced-emission-of-fluorescence (REEF) and THz-enhanced acoustic (TEA) techniques. By “seeing” the fluorescence, or “hearing” the sound, coherent detection of THz waves at standoff distance is feasible. Remote generation at 30 meters and remote detection at 10 meters are demonstrated.

7917-16, Session 5

Coherent monolithic THz generation based on quasi-phase-matched GaP bonding structures pumped by pulsed fiber lasers at ~ 1 5 umW. Shi, NP Photonics, Inc. (United States); E. B. Petersen, NP Photonics, Inc. (United States) and The Univ. of Arizona (United States); A. Chavez-Pirson, NP Photonics, Inc. (United States); N. Peyghambarian, NP Photonics, Inc. (United States) and The Univ. of Arizona (United States)

We report the coherent monolithic THz generation by using GaP quasi-phase-matched (QPM) bonding structures with different periods, based on difference-frequency generation (DFG). QPM semiconductor structures provide a promising approach to enhance THz conversion efficiency via DFG compared to using naturally formed cubic bulk crystals. In order to avoid the multi-photon absorption for GaP materials, we used the pulsed fiber lasers in the C-band to pump GaP QPM bonding structures. In our monolithic pulsed fiber laser system, the peak power can reach ~100 kW when the repetition rate is ~10 kHz



and the pulse width ~2.5 ns. This pulsed fiber laser has a monolithic MOPA configuration and the fiber laser pulses have the transform-limited linewidth. An arbitrary waveform generator (AWG) is used to pre-shape the optical pulse shapes by driving an electro-optic modulator (EOM) to directly chop a CW single-frequency fiber laser - producing pulses shaped to avoid dynamic saturation in the cascade of fiber amplifiers. In the DFG THz generation by using GaP QPM bonding structure, the highest average power is 10.9 uW at 1.5 THz by using 6-period GaP QPM bonding structure pumped by the pulsed fiber lasers. Comparing with the single layer GaP crystal, 6-period GaP QPM bonding structure has about 26 times of enhancement in THz generation. The azimuthal dependence of the THz generation for the GaP QPM bonding structure has been measured when the polarization directions of two pump beams are orthogonal and parallel, respectively.

7917-17, Session 5

Length dependence of forward and backward THz DFG in a strongly absorptive materialY. Huang, Y. Lin, Y. Lin, National Tsing Hua Univ. (Taiwan)

One popular approach to generate coherent THz radiation is difference frequency generation (DFG) of two input laser fields in a nonlinear optical material. However, owing to the vast difference in wavelengths for the mixing waves, a nonlinear optical material is often transparent to optical waves but could be highly absorptive to the THz wave. One typical example is the THz DFG in lithium niobate. It has been widely believed that, in such a strongly absorptive nonlinear optical material, the useful DFG length is on the order of the THz absorption length. To determine the length dependence of THz DFG in lithium niobate, we fabricated an array of PPLN crystals with crystal lengths varying from 1 to 25 mm. With a grating period of 65 microns, the PPLN crystal is phase matched to the generation of 1.5 and 0.6 THz waves in the forward and backward directions, respectively, when pumped at 1540 nm. We show in DFG theory and experiment that the THz wave can grow monotonically over cm distance with a collinear phase matched configuration in a PPLN crystal, despite an absorption length of 50-300 microns between 1.5 and 0.6 THz in lithium niobate.

7917-18, Session 5

Terahertz generation with tilted-front laser pulses: dynamical theoryM. I. Bakunov, Univ. of Nizhny Novgorod (Russian Federation); S. B. Bodrov, Institute of Applied Physics (Russian Federation); E. Mashkovich, Univ. of Nizhny Novgorod (Russian Federation)

Optical rectification of femtosecond laser pulses with tilted intensity front in electro-optic crystals provides record optical-to-terahertz conversion efficiencies nowadays [see, for example, A.G. Stepanov et al., Opt. Lett. 33, 2497 (2008)]. Despite the experimental success of this technique, the theory of terahertz emission from tilted-front optical pulses has still not been developed sufficiently. The stationary theory of the emission in an infinite crystal [M.I. Bakunov et al., J. Appl. Phys. 104, 073105 (2008)] becomes inadequate for the experiments with thin or low absorbing (for example, cryogenically cooled lithium niobate or GaAs) crystals. A dynamical theory, which accounts for the transient processes at the entrance boundary of the crystal, is required.

Here we develop such a theory for a tilted-front laser pulse propagating through a prism-shaped electro-optic crystal. Crystals of such a shape are typically used in experiments. The theory is based on a rigorous solution of Maxwell’s equations and allows us to explore the dynamics of terahertz generation in the crystal. In particular, transverse walk-off length is introduced as an important parameter of the terahertz field formation process. Two typical experimental situations - lithium niobate excited with Ti:sapphire laser (0.8 um wavelength) at room and cryogenic temperatures - are considered, and new schemes, in which GaAs is excited at 1.8 and 3.5 um, are proposed and analyzed. The parameters of

the laser pulse (transverse size, tilt angle, and pulse duration) and crystal size maximizing the terahertz yield are calculated.

7917-19, Session 5

Generation of difference frequency radiation by mutually orthogonal polarized few cycle laser pulses propagating in GaAs crystalA. S. Martirosyan, Institute for Physical Research (Armenia); D. L. Hovhannisyan, Yerevan State Univ. (Armenia); V. O. Chaltikyan, Institute for Physical Research (Armenia); G. D. Hovhannisyan, Yerevan State Univ. (Armenia)

We present the results of theoretical studies of the generation process of difference frequency radiation arising via interaction of mutually orthogonal polarized few-cycle laser pulses propagating in an isotropic nonlinear medium. The system of modified Korteweg-de Vries equations, describing the nonlinear interaction of mutually orthogonal polarized few-cycle laser pulses in GaAs in frame of unidirectional approximation wave, are obtained. Numerical time-integration of the system of modified Korteweg-de Vries equations by the method of lines has been performed. We consider the interaction of mutually-orthogonal linearly polarized pulses, both having the central wavelength of 1.98 um, duration of 30 fs and the electrical field amplitude of 100 MV/m, propagating along the normal to the plane in the 134 um - thickness GaAs crystal. The dependences of spectral distribution red shift of pump pulses from medium length and initial amplitudes are obtained.

7917-20, Session 6

CdSiP2 picosecond optical parametric generatorO. Chalus, ICFO - Institut de Ciencies Fotoniques (Spain); P. G. Schunemann, K. T. Zawilski, BAE Systems (United States); J. Biegert, M. Ebrahim-Zadeh, ICFO - Institut de Ciencies Fotoniques (Spain)

The nonlinear crystal, CdSiP2 (CSP) is a recently developed optical material which offers unique linear and nonlinear properties for parametric down-conversion into the mid-IR. It is a negative uniaxial chalcopyrite with a transparency above ~6500 nm, which possesses a noncritical phase-matching (NCPM) capability with a maximum effective nonlinear coefficient as high as 84.5 pm/V. Importantly, CSP has a band-gap well below 1 micron, which permits pumping at 1064 nm, and under type I (eoo) parametric generation with NCPM can provide an idler wavelength near 6400 nm, a spectral range of great interest for medical applications. In recent studies, the potential of CSP for the generation of mid-IR radiation using direct pumping at 1064 nm was demonstrated using optical parametric oscillators in the nanosecond. sub-nanosecond, and synchronously-pumped picosecond time-scales. Here, we report efficient generation of picosecond pulses in near- and mid-IR in CSP at a repetition rate as high as 100 kHz using single-pass optical parametric generation (OPG) pumped by a mode-locked and amplified Nd:YVO4 laser at 1064 nm. We demonstrate an average signal power of 1.16 W at 1282 nm and idler power of 154 mW at 6204 nm, for for 6.1 W of pump, corresponding to photon conversion efficiencies of 25% and 15%, respectively. We also report spatial, temporal and spectral characterization of the signal and idler output, where we obtain near-Gaussian profiles for both beams, a signal pulse duration of 6.36 ps with a FWHM bandwidth of 8.5 nm, and an idler bandwidth of 122 nm.



7917-21, Session 6

Ho:YAG laser pumped walk-off compensated mid-infrared ZGP optical parametric oscillationX. Mu, H. E. Meissner, H. Lee, Onyx Optics Inc. (United States)

Optical parametric oscillations (OPOs) in ZGP crystals have been demonstrated as an efficient method for mid-infrared radiations. Walk-off compensation (WOC) is desired for most of the applications because it can lower the OPO threshold, increase the conversion efficiency and improve the output beam quality. Conventional WOC can be achieved in a crystal pair when the two crystals are aligned with opposite walk-off directions. In order to maintain the angular tunability of the OPO output wavelengths, the two single crystals need to be separately mounted on a precision setup that can synchronously tune the incident angles of the two crystals. In this work, we have demonstrated a novel folded linear resonator for WOC OPO in a single ZGP crystal. The OPO uses the doubled ZGP crystal length in a WOC configuration meanwhile has simplified requirements on crystal alignment. The pump laser used in our experiment is a Q-switched Ho:YAG laser with pulse width of 11 ns and repetition rate 10 KHz. The ZGP crystal is cut at q=55.5° for type-I phase-matched OPO with crystal length of 15 mm. A maximum output power of 1.72 W at wavelength of 4.67 um has been measured at a pump power of 11.7 W. The measured OPO quantum slope efficiency is above 50%.

7917-22, Session 6

Development of a mid-infrared tunable optical parametric oscillator pumped by a Q-switched Tm,Ho:YAG laserH. Hazama, Osaka Univ. (Japan); M. Yumoto, T. Ogawa, S. Wada, RIKEN (Japan); K. Awazu, Osaka Univ. (Japan)

Resonant absorption of light due to a variety of different molecular bond with each specific vibration frequency occurs in the mid-infrared (MIR) wavelength region of 5-10 µm. Many medical and biological applications using the selective absorption in the MIR wavelength region have been reported. For example, treatments of arteriosclerosis and gallstone, mass spectrometric analysis of proteins with laser ionization, and infrared spectroscopy have been demonstrated with high-energy and widely tunable lasers such as free electron lasers (FELs) and solid-state lasers using difference-frequency generation (DFG). However, practical applications of FELs and DFG based solid-state lasers have been difficult because of the size and the cost of the equipments. Therefore, we have developed a compact and high-energy tunable MIR laser using an optical parametric oscillator (OPO). In order to obtain a high conversion efficiency in the MIR wavelength region, a diode-pumped and Q-switched Tm,Ho:YAG laser with a wavelength of 2.1 µm was adopted for the pump source of the OPO. A silver gallium-indium selenide crystal with a size of 7 mm × 5 mm × 20 mm was used in the singly resonant OPO. Output from the OPO at a wavelength of 6.0 µm was obtained with a pumping energy of 10 mJ. The pulse width of the output of the OPO was about 70 ns while that of the pumping laser was 130 ns.

7917-23, Session 6

Generation of watt level mid-infrared wavelengths using intracavity ZnGeP2 OPO within a 2 1µm Ho:YAG laserL. Tan, Nanyang Technological Univ. (Singapore) and DSO National Labs. (Singapore); P. B. Phua, DSO National Labs. (Singapore) and Nanyang Technological Univ. (Singapore)

We would like to report on watt level mid-infrared (MIR) wavelength generation using intracavity ZnGeP_2 (ZGP) optical parametrical

oscillator (OPO) within a 2.1µm Ho:YAG Laser. A compact cavity of less than 50cm was designed for the intracavity OPO setup. With the same laser setup, watt level of both 2.1µm and MIR wavelengths were generated concurrently. An average output power of >10W of 2.1µm and >1W of MIR wavelength at 5KHz repetition rate were achieved from a 36W Tm Fiber pump laser. The Ho:YAG laser was resonantly pumped by a 1.9µm Tm Fiber laser and nanosecond pulses were generated using an acousto-optics q-switch modulator. With the use of a /4 waveplate and a thin film polarizer, a variable output coupler for the Ho:YAG laser was formed where we could optimize the output coupling to achieve 12W of 2.1µm wavelength. MIR wavelengths were generated using commercial ZGP crystals from Inrad. A HR mirror for the MIR wavelengths was insert into the Ho:YAG cavity to form the intracavity ZGP OPO. The rear mirror of the Ho:YAG cavity act as the output coupler with R=70% for the MIR wavelengths. Optimizing of the MIR generation was done by tuning the phase-matching angle of the ZGP and adjusting the cavity length of the OPO. A preliminary result of the intracavity ZGP OPO generates >1W of MIR wavelength.

7917-24, Session 6

Improvement of mid-infrared pulsed OPOs efficiency by thermal management and cascaded nonlinear conversionsA. Godard, M. Raybaut, T. Schmid, M. Lefebvre, ONERA (France); A. Michel, M. Péalat, Sagem Defense Securite (France)

Optical parametric oscillators (OPOs) are versatile devices to downconvert a pump light into two tunable frequencies, signal and idler, through a nonlinear three-wave mixing process. Very high conversion efficiency, approaching 100 %, can be demonstrated in the continuous-wave regime while, in the nanosecond regime, the efficiency is mainly limited by the temporal dynamics of the three interacting waves, i.e. finite build-up time followed by parametric gain saturation. Nevertheless, still high pump depletions from 40 % to 60 % are routinely obtained with conventional pulsed OPOs. Hence, for the conversion of a 1-µm laser emission to a 4-µm wavelength under pulsed operation, the efficiency is typically limited to 10-15 %. Moreover, for devices pumped with several watts or tens of watts, thermal issues often lead to a significant reduction of the efficiency. As a consequence, in the case of a PPLN OPO pumped at 1 µm and emitting at 4-µm, these effects usually lead to an actual conversion efficiency of 6-7 %.

In this context, we thoroughly investigated such thermal effects both theoretically and experimentally. Based on our investigations, we developed two-zone temperature controlled ovens enabling thermal management of the periodically poled nonlinear crystals. With proper adjustments of the applied temperature gradient, we have demonstrated that a significant improvement (more than 30 %) of the conversion efficiency can be obtained. We also implemented devices where the resonant signal wave is recycled to pump cascaded nonlinear processes, leading to a two-fold improvement of the conversion efficiency toward the mid-IR.

7917-25, Session 6

Classical-quantum analogies: SU(1,1) and Glauber photonic latticesH. M. Moya-Cessa, Instituto Nacional de Astrofísica, Óptica y Electrónica (Mexico)

The purpose of this presentation is two-fold: to study photonic lattices with certain interactions by using quantum mechanical methods and to model quantum features, such as non classical states of a quantum system by propagating light through the lattices. We study several configurations to show SU(1,1), Glauber-Fock and regular systems. We also hint on the analogies between propagation in GRIN systems and quantum mechanical applications. In this last case we show how quantum mechanical beam splitters may be modeled.




Calculation characteristics of spontaneous parametric emission based on BaAlBO3F2 crystalH. Wang, Xi’an Univ. of Arts and Science (China); K. Li, N. J. Copner, Univ. of Glamorgan (United Kingdom)

The nonlinear optical properties analysis of the new BaAlBO3F2 (BABF) crystal have attracted great interesting because it is chemically stable, is not hygroscopic and possesses a moderate birefringence suitable for UV light generation. In this paper, we report on the characteristics of spontaneous parametric fluorescence in BABF crystal based on the amplification transfer function in the different quasi-phase matched modes. From the numerical calculation, we studied the spatial properties of optical parametric fluorescence in BABF crystal. The analysis results are shown that when the homochromatic signal is injected into optical parametric amplifier, the fluorescence signal is first emitted in a cone and then in a ring always centered on the pump wave direction, in addition, for the amplification of the polychromatic signal pulse, the spectrum of the fluorescence is over a wide wavelength range in the special phase matching angle. To our knowledge, this is the first time to present theoretically optical parametric fluorescence lifetime results with the parametric gain during the optical parametric generation in BABF. The numerical simulation results show that the lifetime of optical parametric fluorescence is great different with the variation of the phase matching angle. The lifetimes are close to 1ns over a 200nm wavelength range in a particular phase matching angle. However, when the parametric fluorescence photos are generated in a greater phase matching angle, the average lifetimes are decreased to around 200ps. Note that all the results presented in parametric fluorescence lifetime aspect are in good agreement with the results from the parametric fluorescence spectrum. These results provide a theoretical reference for further development and application of the new BABF crystal.


Optical studies of a semiorganic nonlinear optical crystal: lithium p-nitrophenolate trihydrateP. Arockiasamy, D. Sivasubramanian, Bharathidasan Univ. (India); J. Phillip, Cochin Univ. of Science & Technology (India)

A semi-organic nonlinear optical material, lithium p - nitrophenolate trihydrate (LPNP) was synthesized and the molecular structure was confirmed by FT-IR, FT-NMR and FT-Raman spectral analyses. The cell parameters and morphology of the grown crystal were identified from XRD. The thermal transport properties, thermal effusivity (e = 308.02 Jm-2K-1s-1/2), thermal diffusivity ( = 90.02 x 10-7 m2s-1), thermal conductivity (K = 9.24 Wm-1K-1) and heat capacity (Cp = 790.2 JKg-1K-1) were measured by the photopyroelectric technique at room temperature using He - Cd laser (120 mW, 442 nm). The dielectric constant at 305 K is 30, which remains invariant at higher frequencies. From the optical investigations, the transmittance window (450-1380 nm) and the direct optical band gap (2.47 eV) are found. Laser induced surface damage threshold is 0.95 GW/cm2 using Nd: YAG laser (1064 nm) and the relative powder SHG efficiency is 10 times that of KDP. Third order nonlinear response was studied following Z-scan technique with a He-Ne laser (632.8 nm, 35 mW). The magnitude and sign of the nonlinear absorption and nonlinear refraction are derived from the transmittance curve. It is identified that the nonlinear absorption is due to the saturable absorption while the nonlinear refraction leads to self defocusing. Thus the LPNP crystal is a promising candidate to make efficient light emitting diodes working in the visible region.

References

[1] S. Dhanuskodi, A. Pricilla Jeyakumari, S. Manivannan, J. Philip, S. K. Tiwari, Spectrochimica Acta Part A 66 (2007) 318.

[2] Q. Mohamed Ali, P. K. Palanisamy, Optik 116 (2005) 515.


Output characteristics of 579nm Raman laser for medical applicationY. Kim, W. Jeon, Dankook Univ. (Korea, Republic of); E. Hahn, Univ. of Suwon (Korea, Republic of)

It has been reported that the yellow laser light has good effect on the treatment of dermatological disease such as vascular lesion and pigmented lesion, because the yellow laser light is strongly absorbed into the hemoglobin. All-solid-state intracavity Raman laser with an output wavelength of 579 nm has been fabricated for the purpose of medical application by using ceramic Nd:YAG laser, KGW Raman crystal, and LBO crystal. The Raman crystal converts fundamental wavelength of Nd:YAG laser into the First Stokes wavelength of 1159 nm, and the yellow laser light at the wavelength of 579 nm can be obtained by 2nd harmonic generation using an LBO crystal from the 1st Stokes wavelength of 1159nm. Output characteristics of the Raman laser has been investigated according to the variation of reflectivity of output coupler, input power, pulse width, and repetition rate. Average output power of 30 mW could be obtained at the pump power of 20 W. Recently we replaced the ceramic Nd:YAG with the Nd:YVO4 to accomplish higher output power. The output characteristics of this Raman laser system will be presented and discussed.


Low-threshold, quasi-cw terahertz parametric amplification in an external ring cavity with an MgO:LiNbO3 CrystalS. Maeda, T. Ohira, Y. Takida, H. Kumagai, S. Nashima, Osaka City Univ. (Japan)

Terahertz waves have been attracted recently because of their wide ranging applications in various fields. Since the appearance of a high power near-infrared light source, coherent terahertz waves have been generated successfully using PC antennas, Q-switched Nd: YAG laser, or femtosecond ultrashort pulsed laser. Actually, however, there is no tunable and high-repetition terahertz wave source with narrow linewidth. Thus, we have been focusing on a picosecond-pulsed laser, because it has narrower linewidth than that of a femtosecond-pulsed laser and moreover higher-reputation than that of a Q-switch Nd: YAG laser. In addition, the picosecond-pulsed peak power can be enhanced in a high-finesse compact external cavity to overcome the threshold of terahertz parametric generation and amplification, because the pulse is relatively small spatially, and the spectrum is relatively narrow. Therefore, we developed a terahertz parametric amplifier with an MgO-doped LiNbO3 (MgO: LN) nonlinear crystal in an external ring cavity with the enhancement of pump power. Moreover, the generated idler light was recycled by 2 additional flat mirrors so as to provide a contribution to parametric amplification. As a result, we obtained terahertz wave radiation at high-repetition of 80MHz. We additionally examined seed injection, which seed was provided a continuous-wave diode laser along the generated idler. In this presentation, we will show you the seed injection experimental results together with the effect of terahertz parametric amplification.


A study on fabrication of BaMgF4 thin film toward frequency-conversion device in UV/VUV regionH. Matsukawa, T. Shimono, N. Hirano, H. Kumagai, Osaka City Univ. (Japan)

BaMgF4 is a novel ferroelectric fluoride which is transparent in the



wavelength region from 125 nm to 1300 nm. Recently, the trial production of the frequency conversion device with the BaMgF4 single crystal was reported for the ultraviolet (UV) and vacuum ultraviolet (VUV) wavelength regions, but there has been few report on it. The BaMgF4 is very attractive ferroelectric crystal because it can be used as a quasi phase matching (QPM) device such as LiNbO3 or LiTaO3. Nonlinear crystals have very large nonlinear coefficients generally, but these coefficients limited wavelengths to use due to the birefringent phase matching. The QPM technique can provide the phase matching in the whole transparent wavelength region, in contrast to the conventional birefringent phase matching, which limited to the wavelengths from 573 nm to 5634 nm. Thus the QPM technique is attractive to fabricate frequency-conversion device in UV/VUV region.

In this study, we have purpose to fabricate BaMgF4 hetero-epitaxial thin films toward frequency-conversion devices. The optical-grade BaMgF4 single crystal has been fabricated by Bridgman method. Rather, we focus on fabrication of BaMgF4 thin films by the precise and careful method of ion beam sputtering. It will be possible to fabricate also its waveguide structure under the vacuum-consistent process.


Study on periodic twinning of quartz crystal under bending stressT. Shimono, H. Matsukawa, N. Hirano, H. Kumagai, Osaka City Univ. (Japan); N. Fukuda, T. Takiya, N. Inoue, K. Nakayama, Hitachi Zosen Corp. (Japan)

Periodic inversion of spontaneous polarization in a ferroelectric substance realized quasi phase matching(QPM) and then revolutionized nonlinear optics. In these years, quartz crystal has been attracted attention as a nonlinear optical material without spontaneous polarization. Moreover, making of an inverted structure by impressing perpendicular stress has been researched because a quartz crystal turns to be twinned under sufficient stress. Thus, we paid attention that twinning of quartz crystal was able to be rather achieved effectively by impressing bending stress, and then studied on making a periodic inverted structure of polarization to generate deep ultraviolet lights by QPM quartz crystal. We fabricated experimentally the inverted structure locally by impressing the stress to choose the specific location and direction for the AT-cut quartz substrate that was heated just below the phase transition temperature. Then, we observed the twin boundaries by X-ray topograph. To control inverted area, we introduced local stress by processing the surface of quartz crystal with reactive ion etching. Moreover, the stress distribution was calculated when the bending stress was impressed to the quartz crystal with the rectangle structure, and then the condition for the polarization inversion was also calculated.


Effect of post-growth annealing on the optical properties of LiGaS2 nonlinear crystalsA. P. Yelisseyev, Institute of Mineralogy and Petrography (Russian Federation); M. K. Starikova, Novosibirsk State Technical Univ. (Russian Federation); L. I. Isaenko, S. Lobanov, Institute of Mineralogy and Petrography (Russian Federation); V. P. Petrov, Max-Born-Institut für Nichtlineare Optik und Kurzzeitspektroskopie (Germany)

Crystals of the LiBC2 family with B=In, Ga and C=S, Se, Te are considered promising for nonlinear optics in the mid-IR. Among them LiGaS2 (LGS) exhibits the largest band-gap and lowest two-photon absorption and group velocity mismatch. As in the case of other multi-component chalcogenides, deviations from the stoichiometric composition occur in the growth process. To develop a way to correct the composition and improve the optical quality of LGS, we studied changes in transmission, photoluminescence (PL) and photo-induced absorption

(PIA) produced by annealing LGS in vacuum, in the presence of Li2S, Ga2S3, and S. The S-containing inclusions were found responsible for light scattering and absorption near 8 µm in the mid-IR. Anion vacancies of two structural types, VS1 and VS2, produce absorption in the 370 nm band and intense PL in pairs of bands at 426/510 and 450/532 nm. The VS concentration grows after vacuum annealing and drops after annealing in Li2S, Ga2S3, and S. The concentration of cation antisite defects (GaLi) is determined mainly by the growth conditions. These defects are responsible for intense yellow PL which is excited through band-to-band transitions and for the intense 330 nm band near the fundamental absorption edge. PIA in the ≤850 nm range, responsible for the grey track effect, was found reversible: It appears after UV/blue light illumination and can be removed by IR light. PIA is a result of the capture of free charge carriers by the traps and parameters of the latter were estimated using thermo-activation spectroscopy.


Parabolic pulse generation in the nonlinear nonuniform fibre cascadeI. Zolotovsky, M. Yavtushenko, Ulyanovsk State Univ. (Russian Federation); A. A. Sysoliatin, A. M. Prokhorov General Physics Institute (Russian Federation); D. I. Sementsov, I. Yavtushenko, Ulyanovsk State Univ. (Russian Federation); O. Okhotnikov, Tampere Univ. of Technology (Finland)

Fibre cascade, created in FORC RAS with the help of specialists from UlSU and ORC, was used in this work. We investigate the parabolic pulse generation in the fibre cascade with normal dispersion. The secant-hyperbolic or gaussian pulse transforms to parabolic form if it has concrete correlation between chirp, spectra and energy. The first part of cascade is necessary for management the pulse chirp. The second one changes the pulse average form to parabolic form. We matches theoretical and experimental results. As it shown in this work, we can get a parabolic pulse in very short fibre cascade, only 20 metres length.


Brillouin enhanced four-wave mixing with the liquid fluorocarbonF. F. Wu, A. I. Khizhnyak, V. Markov, MetroLaser, Inc. (United States)

SBS is a nonlinear optical effect that is broadly used for correcting the beam quality of the laser beams, their mode control, amplification and phase conjugation. Two factors are essential when it comes to selection of the nonlinear medium for SBS, i.e. its efficiency or gain coefficient and safety. For example, a low SBS gain coefficient in the fluorocarbon liquid C8F18 that is at least one order of the magnitude lower than in other nonlinear media, typical limits its application in the high-power laser systems. However, a highly purified C8F18 is a very safe and stable nonlinear medium, and in combination with a high optical breakdown threshold it is becoming attractive for practical applications.

This report discusses the phase conjugate mirror (PCM) using the SBS effect in C8F18. The PCM reflectivity better than 90% has been achieved at an optimized experimental geometry of the incoming beam. The output energy of the phase conjugated pulse linearly increased with an increase of the energy of the input pulse after reaching the threshold level at about 3.3 mJ. The estimated slope efficiency is about 95%. As applied to a weak signal amplification, we have realized higher than 10^5-fold amplification when we use the Brillouin enhanced four wave mixing (BEFWM) with an input signal at the level of several nJs. The reflected energy as high as 11 mJ has been achieved with the 400 uJ incoming input signal. Further lowering of the signal energy should result in a higher amplification.




Modified Sellmeier equation for ZnGeP2 in the 0 97-1640µm rangeN. Umemura, Chitose Institute of Science and Technology (Japan)

It has been reported by Shi et al. that the refractive indices of ZnGeP2 at 1.0642µm have been modified through the annealing process, and the variation of the index dispersions near 1.0642µm can significantly change the phase-matching conditions for DFG between the Nd:YAG laser and Nd:YAG laser-pumped OPO in the THz region. In addition, these authors speculated that the refractive indices in the THz region has a negligible effect on the determination of the phase-matching conditions, which contrasts sharply with the speculation of Kumhkar et al. that the abrupt change in the refractive indices in the THz region by annealing may change the phase-matching conditions.

In order to reconcile these controversies, we have modified our Sellmeier equations presented previously and checked the phase-matching conditions for THz generation in the non-annealed and annealed crystals, and found that there is no significant difference in the refractive indices of the non-annealed and annealed crystals.


Analysis of single frequency Raman amplifier for guide star applicationI. A. Dajani, C. L. Vergien, C. Zeringue, Air Force Research Lab. (United States)

We analyze the scalability of amplifying the output from a single frequency diode laser operating at 1178nm through the utilization of a Raman fiber amplifier. A detailed model that accounts for stimulated Raman scattering (SRS), stimulated Brillouin scattering (SBS) and amplifier noise in relation to the fiber mode field diameter (MFD), length and available pump power in both co-pumped and counter-pumped configurations is developed. The backward travelling Stokes light is initiated from both spontaneous Brillouin and spontaneous Raman processes. In order to mitigate the SBS process, we employ a multi-step temperature distribution spatially optimized along the length of the fiber. Our theoretical results compare well with the recent experimental results obtained by the Raman amplifier research team at the European Southern Observatory (ESO). That research group was successful in obtaining approximately 40 W of 1178 nm from a single amplifier which was subsequently frequency doubled to 589 nm in nonlinear cavity for utilization in a sodium guide star system. We also consider here the feasibility of generating the D2a and D2b lines in a sodium beacon from a single Raman amplifier by examining four-wave mixing (FWM) in the co-pumped and counter-pumped configurations. Finally, we provide preliminary results from our SBS studies on a 10/125 µm Raman amplifier pumped with a 50 W IPG laser operating at 1120 nm.


Statistical properties of partially coherent CW fiber lasersD. V. Churkin, Institute of Automation and Electrometry (Russian Federation); S. V. Smirnov, Novosibirsk State Univ. (Russian Federation); E. V. Podivilov, Institute of Automation and Electrometry (Russian Federation)

The output radiation of partially coherent (PC) continuous-wave (CW) fiber lasers consists of numerous (10^3-10^8) longitudinal modes interacting nonlinearly via four wave-mixing processes. Statistical properties of PC CW radiation are of great interest, but have been only studied for stimulated Brillouin scattering based lasers. At the same time,

contradictory phenomenological models are used, despite the results of modeling problems of practical interest (supercontinuum generation, PC field propagation) strongly depend on the statistics of the input initial PC field.

We use 4 coupled NLSE to describe in details performances (including statistics) of the PC quasi-CW fiber laser using the high-Q cavity RFL as a test bed for our methods. The good quantitative agreement of modelling with experimental data for generated power and spectra is obtained. Precise spectral profiles of FBGs forming the laser cavity influence sufficiently real laser performances. We prove that the generation is not sensitive to the XPM between the pump and generated waves. It is shown that the amplitudes of different modes strongly fluctuate having Gaussian probability density function (PDF). The amplitudes fluctuations should be taken into account into phenomenological models of PC CW radiation. The intensity exhibits strong fast fluctuations with non-exponential and power dependent PDF revealing that longitudinal modes are correlated.

The developed iterative approach of solving boundary problem can be used for modelling and optimization of Yb-, Er-doped or ultra-long RFLs as well as for modelling of recently developed random distributed feedback fiber laser.


Demonstration of a high power 1 5344 micrometer output Nd:YAG pumped OPOM. D. Wojcik, R. Foltynowicz, Utah State Univ. (United States)



Reverse-proton-exchanged waveguide frequency doublers for green light generationR. V. Roussev, V. A. Bhagavatula, J. Himmelreich, K. Becken, J. Tingley, Corning Incorporated (United States)

Compact lasers producing 20-150 mW of green or yellow light are needed for miniature image projectors and flow cytometry. Single-pass waveguide frequency-doubling of 1050-1200-nm semiconductor laser radiation is currently among the most compact and efficient ways to obtain visible light at these power levels.

Anneal-proton exchanged (APE) and reverse-proton exchanged (RPE) lithium niobate waveguides enable high-efficiency optical-frequency mixing (OFM). The relative simplicity of fabrication has made these waveguides the favorite platform for developing and testing device concepts utilizing waveguide OFM. For visible-light generation, MgO-doped lithium niobate (MgLN) is used to avoid photorefractive sensitivity. The diffusion of protons in MgLN is different from that in the non-doped crystal. We describe the differences and the resulting limitations on second-harmonic-generation (SHG) conversion efficiency of MgLN waveguides. We also present the semi-empirical modeling that we use for the design of APE and RPE waveguides for green-light SHG, including the effects of the “dead layer” generated during the initial proton exchange.

The experimental results on SHG in channel waveguides agree with the predictions of numerical simulations based on the semi-empirical index profile. We observe conversion efficiency exceeding 200%/W and over 100-mW output green power from a 1-cm long RPE waveguide. Due to lower absorption, RPE waveguides support far superior green output power compared to APE waveguides containing a higher-index layer. We also observed that high-energy pulses produced by spiking of the pump fiber laser can produce catastrophic damage in the interior of the waveguide rather than at the input facet where the intensity is highest.



7917-26, Session 7

Linear light bullets based on Airy-Bessel wave packetsF. W. Wise, A. Chong, W. Renninger, Cornell Univ. (United States)

For applications it would be desirable to have pulsed light beams that do not spread in time or space, despite the presence of dispersion and diffraction. These are sometimes referred to as “light bullets.” Over the years researchers have demonstrated a small number of clever techniques for generating such localized 3-dimensional wave packets. All of these have required sophisticated experimental arrangements designed to tailor the light field precisely to the properties of the propagation medium. We have recently demonstrated the generation of Airy-Bessel wave packets, which offer major advantages over prior approaches to this problem. They are separable in time (the Airy function) and space (the Bessel function), in contrast to the strong and essential space-time coupling in prior work. The Airy-Bessel wave packets can be generated without detailed knowledge of the propagation medium; the dispersion can be normal or anomalous, for example, and the effects of dispersion and diffraction do not have to be equalized. These properties will make the Airy-Bessel wave packets useful for a wide range of applications.

7917-27, Session 7

Performing the triple auto-correlation of picosecond optical pulse train with a photo electromotive forces detectorA. S. Shcherbakov, S. Mansurova, P. Moreno Zarate, Instituto Nacional de Astrofísica, Óptica y Electrónica (Mexico); J. Campos Acosta, Consejo Superior de Investigaciones Científicas (Spain); S. A. Nemov, St. Petersburg State Polytechnical Univ. (Russian Federation)

Recently, photo-detectors based on the photo electromotive force (EMF) effect have been developed and utilized for various modern applications. Here, we consider the possibility of involving the photo-EMF detectors in registration of the parameters peculiar to ultra-short optical pulses. In so doing, the technique of implementing the triple auto-correlations of picosecond optical pulse trains is the subject for our analysis. Knowledge of triple auto-correlation function, whose Fourier transformation shapes the corresponding bispectrum, makes possible recovering such train-average parameters as, for instance, the pulse width and frequency chirp as well as revealing asymmetry of ultra-short pulse envelope. The main advantage of applying the photo-EMF detectors lies in an opportunity to detect triple correlations directly, without any intermediate frequency conversion with optical nonlinear processes in additional crystals. For this purpose, the theory of three-beam correlations at photosensitive layer of the photo-EMF detector is developed, so that principal possibility of registering the triple correlations is demonstrated. It can be done within schematic arrangement including the degenerated four-beam Michelson interferometer, so that the obtained triple correlations have non-traditional form and need rather specific algorithm for their further processing. Together with this, the experimental characterizations are presented for two different types of potentially suitable materials, namely, gallium arsenide (GaAs) semiconductor and the poly-fluoren 6-co-triphenyldiamine (PF6-TPD) photo-conductor polymer, which both exhibit the photo-EMF effect. They both exhibit high-pass transfer functions that give us high vibration stability. This novel approach provides more reliable analyzing train-average parameters of picosecond pulses due to significantly higher level of the output optical signals under processing

7917-28, Session 7

Broadband third-harmonic generation on interfaces using femtosecond pulsesL. Misoguti, E. C. Barbano, J. d. P. Siqueira, C. R. Mendonça, S. C. Zilio, Univ. de São Paulo (Brazil)

In the field of nonlinear optics, there has been a growing interest in the ultrashort (fs) laser pulses applications due to their unique spectral and intensity properties. Among nonlinear effects, the third-harmonic generation (THG) is interesting for studying isotropic materials. Here, we report on the third harmonic (TH) spectrum generated by ultrashort laser pulses using Z-scan method and how it is affected by the interface. As we know, the THG in interfaces is important on applications such as in THG scanning microscopy, for instance. Basically, we observed that the femtosecond pulse has broadband spectrum and such property also affects the TH spectrum. As light source we used a commercial optical parametric amplifier (OPA) pumped by 150 fs laser pulses at 775 nm, with a 1 KHz repetition rate. The OPA was set to generate 120 fs-duration pulses at near IR. To detect the TH, we have used a high resolution portable UV-Vis spectrometer (HR 4000-Ocean Optics). By playing with the lens focal length we are able to follows the transition between the bulk and interface THG. There are interesting fringes on the spectrum related to the fundamental and harmonic beam propagation when the TH is generated in the bulk, but these fringes disappear as soon the generation is related to the interfaces. We studied the TH intensity and spectrum as function of the z-position and the Rayleigh parameter, z0. By using different z0, we could discriminate the TH generated at the sample’s input and output interfaces.

7917-29, Session 7

MHz-rate picosecond laser discretely tunable from the near-IR to deep UVV. V. Yakovlev, Univ. of Wisconsin-Milwaukee (United States)

Ultrasfast, bandwidth-limited laser pulses, which can be broadly tuned throughout the spectrum, are in a great demand for numerous spectroscopic applications. One of the particular applications is resonant Raman spectroscopy, which requires narrow-band (<10 cm^-1) pulses, which can be tuned in or out of resonance with some electronic transition of a molecule to achieve selective excitation and resonance signal enhancement. Short pulses are also desired, since they allow interrogation the fast dynamics of molecular species. Optical parametric amplifiers are typically used for frequency conversion. However, in many situations there is no need in continuous tunability, and it is more desirable to have a set of wavelengths, whose spectral position is fixed, and the spectral bandwidth is reproducible for every day operation. Spectroscopy also benefits from the high repetition rate, since it allows the use of high-frequency modulation techniques with the lock-in detection with the exceptional signal-to-noise ratio.

We propose and experimentally demonstrate a new class of broadly tunable laser systems, which is based on stimulated Raman amplification with a subsequent nonlinear optical conversion. Using a widely available YVO4 as a nonlinear Raman material, we demonstrate a very efficient stimulated Raman generation and amplification in this material using 1064 nm fundamental wavelength of the high-repetition rate (1 MHz), Nd:YVO4 oscillator/amplifier system, capable of generating up to 10 microjoules per pulse. Pulses as short as 4 ps are routinely generated with the efficiency exceeding 60%. The use of a powerful seed generated through the white-light continuum generation reduces the incident power requirement and avoids the beam break-up and instability at the highest energy extraction efficiencies.

Nonlinear optical conversion through harmonics generation and frequency mixing allows efficient energy conversion from the near-IR to the visible and the deep UV spectral range. Potential applications of the developed laser system for remote resonance Raman water analysis will be discussed.



7917-30, Session 7

Highly simplified device for measuring the intensity and phase of picosecond pulsesJ. Cohen, V. Chauhan, Georgia Institute of Technology (United States); D. Lee, Swamp Optics, LLC (United States); P. M. Vaughan, R. P. Trebino, Georgia Institute of Technology (United States)

Ultrashort laser pulses in the few-ps range are important for many applications, from telecommunications to pulse shaping. As a result, their measurement is also important. Interestingly, however, while measuring fs pulses has become almost routine, measuring longer, ps, pulses can be challenging due to the need for relatively high spectral resolution and relatively large temporal range. In this paper we demonstrate an extremely simple frequency-resolved-optical-gating (GRENOUILLE) device for measuring the intensity and phase of relatively long-ps-pulses. In order to achieve the necessary spectral resolution and large temporal range, we make two device modifications to the standard GRENOUILLE design. First, we use a several-cm-thick highly dispersive crystal (LiIO3) to provide the required improved spectral resolution. Secondly, we use a very large beam crossing angle-more than twice as large as previous versions. This latter condition sounds easy, but in fact proved the most challenging task in designing the device. To achieve the necessary temporal range we modified the shape of the SHG crystal from the universally used rectangular shape to a pentagonal shape. Along with providing the required delay range, the pentagonal crystal has the additional advantage of reducing the device’s total number of components to as few as three simple easily aligned optics, making it the simplest device ever developed for self-referenced complete pulse measurement. We report complete intensity-and-phase measurements of pulses up to 15ps long with a time-bandwidth product of 21.

7917-31, Session 8

Rediscovered dynamics of nonlinear fiber optics: from breathers to extreme localisationB. Kibler, J. Fatome, C. Finot, G. Millot, Univ. de Bourgogne (France); F. Dias, Ecole Normale Supérieure de Cachan (France); G. Genty, M. Erkintalo, Tampere Univ. of Technology (Finland); N. Akhmediev, The Australian National Univ. (Australia); J. M. Dudley, Univ. de Franche-Comté (France)

Recent investigations have shown a renewed interest in certain classes of analytical solutions of the Nonlinear Schrödinger equation which, although present in the mathematical literature for 25 years, have been largely overlooked in studies of nonlinear fiber propagation in optics. In this paper we review recent experiments that have shown the power of this analytic approach. We first consider how the modulation instability of a quasi-continuous wave input in an optical fiber can be described by a class of analytical solutions known as Akhmediev Breathers and this allows the characteristic triangular shape of the initial supercontinuum spectrum in experiments aroun 1064 nm to be explained analytically. Secondly, we extend the concept of Akhmediev Breathers from the continuous wave to the pulsed regime and show that the initial evolution of a weakly modulated picosecond pulse can be described in terms of “local” breather states whose growth-return cycle and analytic properties are determined by the local peak power at the corresponding point on the initial pulse envelope. These analytical dynamics were confirmed experimentally by injecting 4 ps pulses centered at 815 nm with 100 W peak power into a photonic crystal fiber with a zero-dispersion wavelength at 760 nm. Finally, we discuss extreme temporal localisation dynamics in a low frequency limit of induced modulation instability where particularly simple rational soliton solutions can be observed and again confirm analytical results by experiments around 1550 nm.

7917-32, Session 8

Enhanced supercontinuum generation by minute continuous wave seedK. Y. K. Cheung, Y. Zhou, K. K. Wong, K. K. Tsia, The Univ. of Hong Kong (Hong Kong, China)

It is well-known that the properties of the supercontinuum (SC) radiation depend critically on the initial generation conditions. For instance, statistically rare “rogue” soliton events can trigger sizable redshift and temporal intensity in SC. Interestingly, such rouge events can be actively controlled by adding an external weak pulse or by modulating the pump-pulse envelopes. In contrast, we here present, for the first time to our best of knowledge, that a simple seeding mechanism using an extremely weak continuous wave (CW) can also achieve such “rogue” enhancement when the CW seed wavelength is close to the spectral region where modulation instability occurs. We experimentally demonstrated that a CW seed (~1000 weaker than pump) can considerably broaden SC bandwidth compared to the unseeded SC (~100 nm wider). The average SC power was increased by 23 dB. CW seeding also significantly alters the SC amplitude statistics, i.e. from extreme-value statistics in the unseeded SC to almost normal distribution in the seeded SC. Interferometric measurements also revealed the improvement in the coherence of the seeded SC in contrast to the unseeded one. The enhanced SC by minute CW seeding only requires the seed-wavelength tuning for optimization and eliminates the need for high-precision (down to picoseconds) timing between the pump and the seed as in the pulse-seeding case. It thus offers a more convenient and practical approach to realize an enhanced and stable SC for many applications, especially in which real-time, ultrafast and single-shot spectroscopic measurements are essential.

7917-33, Session 8

The use of the nonlinear optical loop mirror for investigations of pulse breakup in optical fibersE. A. Kuzin, B. Ibarra-Escamilla, Instituto Nacional de Astrofísica, Óptica y Electrónica (Mexico); O. Pottiez, Ctr. de Investigaciones en Óptica, A.C. (Mexico)

The pulse breakup and the formation of the bunch of solitons are principal processes at the initial stage of the supercontinuum generation when long pulses are used for pumping. The most investigations use the measuring of the spectrum in the fiber end to characterize the development of the supercontinuum. The extraction of an individual soliton or the group of solitons with similar parameters from the bunch can reveal details that are usually hidden when only output spectrum is measured. To do this we have to use some nonlinear device. In this work we theoretically and experimentally demonstrate that the nonlinear optical loop mirror is viable device that allows the extraction of an individual soliton, a group of the solitons with similar parameters, and also for measuring of the distribution of powers of solitons in the process of the pulse breakup of the long pulses. The operation principle is based on the fact that the NOLM has a maximum transmission for the solitons with a specific durations while solitons with shorter and longer durations are strongly rejected. The duration with high transmission depends on the NOLM length and also can be moved by amplification of solitons before entering to the NOLM. By an appropriate choice of the NOLM parameters and the amplification of the bunch of solitons, the extraction of the solitons with selected parameters is possible. An analysis of the solitons at the NOLM output at different amplification allows retrieval the power distribution of the solitons at the NOLM input.



7917-34, Session 8

Forward stimulated Brillouin scattering observed in standard highly-nonlinear optical fiberY. Zhu, Duke Univ. (United States); J. Wang, Duke Univ. (United States) and Beijing Jiaotong Univ. (China); R. Zhang, D. J. Gauthier, Duke Univ. (United States)

We observe forward stimulated Brillouin scattering (FSBS) in a standard highly-nonlinear optical fiber. The FSBS process results from the scattering of a confined light beam from guided acoustic modes, in which the acoustic vibrations are in directions perpendicular to the optical propagation direction axis of the waveguide. The tight confinement of light field enhances the light-sound coupling and favors resonant coupling to high-frequency acoustic modes. In the experiment, we pump the 2-km-long small-core room-temperature optical fiber (HNLF, OFS, Inc.) by a 1550-nm-wavelength monochromatic beam of ~ 8 mW and generate a weak co-propagating tunable single sideband from the same laser as the probe beam. The output beams are mixed with a frequency shifted reference beam and the beat signals are resolved on a spectrum analyzer. Approximately 30 different FSBS resonances are observed from ~ 80 MHz up to our detection limit of 1.3 GHz. The resonant frequencies are found to be in close agreement with the prediction of theory. A maximum gain of 1.3 (defined as ratio between the output and the input probe power) is obtained for the resonance at 1.027 GHz, corresponding to coupling with the radial acoustic mode of the 22nd order. This mode has the maximum spatial overlap with the transverse profile of the optical field tightly confined in the fiber core. We also find the FSBS resonances are asymmetric, which is explained by taking into account forward four-wave mixing arising from the intensity-dependent refractive index of the fiber (the nonlinear optical Kerr effect).

7917-35, Session 8

Failure and limitations of linear Raman gain approximation for fiber supercontinuum generation modellingM. Erkintalo, G. Genty, Tampere Univ. of Technology (Finland); B. Wetzel, J. M. Dudley, Univ. de Franche-Comté (France)

The propagation of short pulses in optical fibers is commonly modeled by the generalized Nonlinear Schrödinger equation which includes the frequency-dependence of the dispersion and nonlinear response and the non-instantaneous part of the nonlinear response of silica. It is also common to model the delayed -or Raman- response of silica based on the assumption of a material response that varies linearly with frequency. Here, we examine in detail the accuracy and limitations of this widely used approach. Our major conclusion is that the linear Raman gain approximation performs very poorly in parameter regimes typical of many experimental studies, introducing significant errors and artifacts into the spectral and statistical properties of the supercontinuum spectrum. Specifically, numerical simulations of supercontinuum generation in silica fiber using the linear Raman gain approximation are compared with simulations using the full Raman response, and differences in the spectral, temporal and stability characteristics are considered quantitatively. Among various artifacts, we show that the linear approximation leads to gradual energy transfer from the blue-shifted dispersive waves and pump remains to the redshifted solitons and to greatly enhanced soliton self-frequency shift.

7917-36, Session 9

Ultra-sensitive, room-temperature THz detection based on parametric upconversion by using a pulsed 1550nm optical sourceM. J. Khan, J. C. Chen, Z. Liau, S. Kaushik, MIT Lincoln Lab. (United States)

We demonstrate ultra-sensitive optical detection of terahertz by using nonlinear parametric upconversion. Terahertz radiation is mixed with pump light at 1550 nm in a nonlinear crystal to generate an optical sideband or idler wave. The idler signal is separated from the optical pump, coupled into an optical fiber and detected using a Geiger-mode avalanche photo-diode. Our scheme to detect THz waves leverages mature technology at 1550 nm developed for telecommunications to enable ultra-sensitive detection at room-temperature. We have fabricated a diffusion-bonded, quasi phasematched GaAs crystal, a (2) nonlinear material, that is pumped with a readily obtainable erbium doped fiber amplifier to perform the parametric conversion. We demonstrate efficient upconversion of terahertz radiation using both a continuous-wave THz source operating at 0.82 THz and a pulsed sub-picosecond THz source with spectral coverage from 0.5 THz to 1.5 THz. The resulting THz detector has a noise equivalent power of 78 fW/Hz1/2 with a timing resolution of 1 ns. (2) nonlinear interactions are intrinsically very fast; our temporal bandwidth is limited by the optical detector. Additionally, the THz detector demonstrates a broadband response with a phase-matching bandwidth approaching 1 THz. This noise equivalent power of 78 fW/Hz1/2 and the corresponding power conversion efficiency of 1.2 10-3 are the best reported, to our knowledge. This paper presents both theoretical and experimental results.

7917-37, Session 9

Amplification and wavelength conversion of a complex two-dimensional image by optical parametric amplificationP. M. Vaughan, R. P. Trebino, Georgia Institute of Technology (United States)

Nonlinear optics allows for a wide range of applications to imaging. Optical Parametric Amplification (OPA) can amplify and convert a mid-IR image into the visible or near-IR[1], allowing for standard CCD detection of IR images. The pulsed pumping of OPA’s also yields time-gating[2], which can isolate unscattered photons arriving through scattered media, rendering the medium transparent. And the interaction between the two beams may take place in the Fourier plane of one of the beams, allowing for real-time image processing[3].

Previously, the most complex image recorded using OPA contained 8lines/mm[4]. It involved picosecond-duration pulses. Use of a shorter pump pulse would increase the pump pulse intensity, allowing shorter-length nonlinear media and increasing nonlinear-optical efficiency. Even better, it would reduce geometrical smearing and image distortion. Thus a fs pump pulse would yield significantly higher spatial resolution[5]. Here we take advantage of recent improvements in fs pulse amplification to achieve much better detail and OPA images in two dimensions.

Mixing the Fourier transform of an image with a large, spatially filtered near- TEM00, fs-duration pump beam, we have achieved amplification by more than two orders of magnitude on features of 28lines/mm and more than an order of magnitude on features with up to 57lines/mm. We also wavelength-converted these images from the IR (930nm) to the visible (700nm). Resolvable features spanned the range from 0.5 to 22.5lines/mm.

We believe OPA imaging using fs pulses is a promising approach for imaging objects at a distance through scattering media.

[1.] Midwinter, J.E., IEEE J. Quantum Electron. QE-4(8)716-720. (1968).

[2.] Doule, C. et al. Opt. Lett, 25(5)353-355. (2000).

[3.] Faris, G.W. and M. Banks, Opt. Lett. 19(22)1813-1815. (1994).



[4.] Devaux, F. and E. Lantz, J Opt Soc Am B. 12(11)2245-2253. (1995).

[5.] Andrews, R.A., IEEE J. Quantum Electron. QE-6(1)68-81. (1970).

7917-38, Session 9

High resolution 2-D image up-conversion of incoherent lightC. Pedersen, J. S. Dam, P. Tidemand-Lichtenberg, Technical Univ. of Denmark (Denmark)

An optimized method for continuous wave 2-dimensional (2-D) up-conversion of incoherent or thermal light is demonstrated and quantified. Using a standard resolution target an image of 200x1000 pixels is obtained. To the authors’ knowledge this is the highest resolution ever reported using up-conversion. The suggested method is viewed in scope of modern near infrared (NIR) CCD cameras. Up-conversion based on Sum Frequency Generation (SFG) was already considered in the 1960’ties [1-2] as a possible means for light detection at wavelength ranges inaccessible for direct detection due to lack of efficient detector materials. Such wavelength ranges include the Mid infrared (MIR) and far infrared. However the quantum efficiency in the up-conversion process was low [3] and therefore the method was later abandoned, particular for the more challenging 2-D up-conversion case. Recently we have demonstrated approximately 25% quantum efficiency up-conversion of 2-D images using coherent illumination of the object [4].

In this work we extend the method demonstrated for coherent illumination to include incoherently illuminated objects, i.e. objects emitting thermal light. Particular we have optimized each step of the up-conversion process using modern technological advances. This include Quasi-Phase-Matching leading to high effective nonlinearity and elimination of walk-off, an intracavity design enhancing the amplitude of the interacting fields, and implemented modern NIR CCD detectors. These improvements have increased the up-conversion efficiency by 103 compared to prior designs, e.g. [3]. A particular advantage compared to state-of-the-art thermal cameras is the possibility to tailor the spectral response leading to functional spectral imaging.

1. Midwinter, J. E. Infrared up conversion in lithium-niobate with large bandwidth and solid acceptance angle. Appl. Phys. Lett. 14, 29-32 (1969). http://apl.aip.org/applab/v14/i1/p29_s1

2. Falk, J. & Tiffany, W. B. Theory of parametric upconversion of thermal images. J. Appl. Phys. 43, 3762-3769 (1972). http://jap.aip.org/japiau/v43/i9/p3762_s1

3. Lucy, R. F. Infrared to Visible Parametric Upconversion. Appl. Opt. 11, 1329-1336 (1972). http://www.opticsinfobase.org/abstract.cfm?URI=ao-11-6-1329

4. Pedersen, C., Karamehmedovi, E., Dam, J. S. & Tidemand-Lichtenberg, P. Enhanced 2D-image upconversion using solid-state lasers. Opt. Express 17, 20885-20890 (2009). http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-17-23-20885

7917-39, Session 9

A simple model for 2-D image up-conversion of incoherent lightJ. S. Dam, C. Pedersen, P. Tidemand-Lichtenberg, Technical Univ. of Denmark (Denmark)

We present a simple theoretical model and experimental data for 2 dimensional (2-D) image up-conversion of incoherent light. The model predicts the feasibility of a mid-infrared camera based on conventional silicon CCD technology combined with a wavelength converter module. While image up-conversion has been known for more than 40 years, the technology has been hindered by very low conversion quantum efficiency (~10-7). We show that our implementation compared to previous work can result in a feasible system: Using intracavity up-conversion and Quasi Phase Matching (QPM) nonlinear materials provide increased conversion efficiency. Using a QPM crystal and choosing

the wavelengths so the first order term in the phasematch wavelength acceptance vanishes, results in very large wavelength acceptance. Together these improvements allow for conversion quantum efficiencies theoretically reaching the 10-2 level, while allowing for a reasonably large image and spectral range to be up-converted. The thermal, temporal and spatial resolution will be discussed in detail. For a given implementation, some level of compromise between the parameters must be made. Furthermore, the system can be modified to provide functional imaging, i.e. images with spectral information. This will be discussed based on the suggested model. We believe that the presented technology opens up new possibilities for sub microsecond thermal imaging of hot events, as well as potentially low cost image acquisition devices in the mid- and far-infrared spectral range.

7917-40, Session 9

Characterizing the performances of an advanced acousto-optical filter exploiting the collinear calcium molybdate crystalline cellA. S. Shcherbakov, National Institute for Astrophysics, Optics, and Electronics (Mexico); J. Maximov, Molecular Technology GmbH (Germany); A. Luna Castellanos, D. Sanchez Lucero, K. J. Sanchez Perez, National Institute for Astrophysics, Optics, and Electronics (Mexico)

In 1970 - 80s, novel optical spectral devices, electronically tunable acousto-optical filters (AOFs) had been proposed and developed. During the years gone AOFs have been remarkably progressed, and now they are widely exploited, for instance, in astrophysical observations. Schematically, AOFs can be separated on collinear and non-collinear filters, depending on the relative directions of passing the waves through crystalline cell, as well as on sequential and parallel ones, depending on the algorithm of spectrum analysis. Their features are characterized by the amplitude and spectral parameters. Here, we consider the performances of radically advanced collinear AOF based on calcium molybdate single-crystal and exploiting essentially the acousto-optic nonlinearity. This new AOF with a 15-microsecond time-aperture operates over all the visible range exhibiting 60%-efficiency at the electric power 1.0 W. Its bandwidth includes two contributions. The first one does not depend on the divergence angles of optical and acoustic beams, and it prevails when these angles are under a critical angle. The second contribution dominates with really wide-angle divergence of these beams or when AOF crystal is sufficiently long; so that the bandwidth shows a quadratic growth as the divergence angles increase. Estimating that critical angle shows that it doesn’t exceed 10 degrees when the crystal is < 6 cm in length and the acoustic frequency is < 100 MHz. Within filtering optical images, the minimal size of resolvable element causes the divergence angle. A direct square-law dependence for crystal’s length and an inverse square-law dependence for its bandwidth on this minimal size make possible optimizing this advanced collinear AOF.

7917-41, Session 9

Compact all-solid-state integrated mid-IR SourceI. V. Melnikov, E.L.S. Co. (Russian Federation) and High Q Labs,, Inc. (Canada); A. V. Shestakov, E.L.S. Co. (Russian Federation); A. N. Knigavko, High Q Labs., Inc. (Canada)

Diode-pumped microchip lasers which integrate gain crystals with a layer of saturable absorber have been proven to be a compact robust source of short nanosecond/sub-nanosecond pulses with peak power sufficient for effective parametric wavelength-tuning applications. Although these capabilities for the mid-IR conversion have already been demonstrated, the demand for highly precise and compact remote spectroscopy tools prompts towards their further miniaturization along with bandwidth narrowing while gaining peak power and repetition rate. In this Report,



we describe a small-footprint near- to mid-IR source that is based on optical parametric generation in periodically-poled lithium niobate (PPLN) driven by a Nd:YAG/Cr4+:YAG microchip laser, which in turn, is boosted by a diode-pumped fiber amplifier.

The measured overall energy conversion of the system reached 30%, whereas the maximum efficiency of generation in the near IR range achieved 25 % (or approximately 0.15 mJ) indicating existence of noticeable loses for the idler. Further increase of the pump energy was found to be not favorable, as it might saturate the parametric conversion process via non-phase matched second- and fourth-harmonic generation for the pump and signal waves. The energy threshold of the seedless optical parametric generation amounted to few micro-Joules but can be lowered down significantly by switching on of the spectrally adjusted DFB laser. It is found that the mid-IR wavelength to be tuned as much as 250 nm in the temperature range given. This range can be further extended by increasing temperature up to 350 oC and by tunable seed within the parametric superluminescence spectrum. The generated bandwidth can be narrowed up to 0.1 nm by narrow-band seeding even at the highest level of pumping. The fine structure observed in the spectrum is attributed to generation of the two longitudinal modes by the microchip pump laser.

The laser system delivered nearly diffraction-limited beams both at the near- and mid-IR tunable wavelengths. The pulse-to-pulse energy instability does not exceed 5 %. Optical elements of the scheme can be easily incorporated into a housing with the dimensions of 50x15x15 cm3, which, along with the parameters described above, suggests the presented source to be ideal for many applications as high-precision spectroscopy, photo-medicine, environmental control, scanning of remote objects in the eye safe spectral domain, IR-countermeasures, telecommunications, and education, where the device can be used to demonstrate basics of lasers physics and non-linear optics.

7917-42, Session 10

Dual-comb molecular spectroscopy in the NIR and MIRN. R. Newbury, E. Baumann, I. Coddington, F. R. Giorgetta, W. C. Swann, A. Zolot, National Institute of Standards and Technology (United States)

Frequency combs can provide reasonably broadband, coherent light, which has obvious applications to spectroscopy. One method of exploiting combs for high resolution spectroscopy is through a coherent dual comb spectrometer, which uses two mutually coherent frequency combs. One comb acts as a source and is transmitted through a gas sample while the second combs acts as a local oscillator to read out the resulting attenuation and phase shifts of the transmitted source comb. This approach can be viewed either as a massively parallel laser heterodyne spectrometer or as an infrared time-domain spectrometer. The combination of an rf heterodyne signal and a high data update rate allows for very efficient suppression of 1/f noise in the spectrometer. In addition, we implement coherent signal averaging to dramatically improve the signal-to-noise ratio. Using the dual comb spectrometer around 1.5 microns, we are able to measure complex gas spectra in the near infrared with high signal-to-noise ratio, frequency resolution and accuracy. Recently, we have also explored extending the operation of the coherent dual comb spectrometer into the mid infrared (MIR) through difference frequency generation. The mutual coherence of the combs is preserved in the DFG process and we can measure the complex spectra of methane at 3.4 microns. We will discuss the basic performance of the coherent dual comb spectrometer at 1.5 microns and in the MIR. We will also discuss the various tradeoffs involved in extending its operation to the MIR.

7917-43, Session 10

Mid-IR spectral comb with broad instantaneous bandwidth using subharmonic OPON. C. Leindecker, A. Marandi, R. L. Byer, K. L. Vodopyanov, Stanford Univ. (United States)

We implement a new approach for generating broadband mid-infrared frequency combs via degenerate optical parametric oscillation. This technique efficiently transfers the desirable properties of shorter wavelength mode-locked sources to the mid-IR. Our OPO resonator is a 3m ring cavity composed of one pair of concave mirrors with R=50mm and four flat mirrors, all but one of which are gold coated with > 99% reflection. A single dielectric mirror is used to introduce the 1560nm pump (Menlo Systems C-fiber, 100 MHz, 70 fs, 350 mW). The dielectric mirror is transmissive for the pump and reflective in the 2.5- 4 micron range. Broadband parametric gain around 3.1-micron subharmonic is provided by short (0.2-0.5mm) periodically poled lithium niobate (MgO:PPLN) at Brewster angle. Crystals were cut from Crystal Technology Inc. material having QPM period of 34.8 microns for type 0 (e=e+e) phase matching at t=32 deg. C. The enormous acceptance bandwidth at degeneracy, typical for OPOs with type 0 (or type I) phase-matching, gives broad bandwidth and makes temperature tuning insignificant. Broadband oscillation is achieved when signal/idler are brought into degenerate resonance by fine-tuning the cavity length with a mirror on a piezo stage. Using an 8% reflective pellicle, we outcouple a frequency comb of more than 1000nm bandwidth, centered around 3.1 microns. A 1mm or 2.5mm thick ZnSe plate at Brewster angle provides 2nd-order group velocity dispersion compensation, improving the OPO bandwidth. The OPO threshold was measured to be < 35mW. When locked, the OPO outputs 60 mW of average power centered at 3.1 microns. With proper intracavity dispersion management including chirped mirrors, we expect to extend the spectral width to an octave or more.

7917-44, Session 10

Yb-fiber lasers for frequency comb spectroscopy: from XUV to mid-IRI. Hartl, A. Ruehl, M. E. Fermann, IMRA America, Inc. (United States); A. Cingoz, JILA-AMO (United States) and National Institute of Standards and Technology (United States); D. C. Yost, JILA-AMO (United States); F. Adler, J. Ye, JILA-AMO (United States) and National Institute of Standards and Technology (United States)

Our 80-W, 154-MHz, 120-fs high power Yb-fiber frequency comb is based on a modelocked Yb-fiber oscillator centered at 1050 nm with >100 mW average power. The pulses are stretched to >600 ps and amplified in three 15-dB Yb-fiber amplifier stages to 108 W average power. After compression we achieve nearly pedestal-free pulses at 80 W average power, a spectral bandwidth of 20 nm and 120 fs FWHM, measured by frequency-resolved optical gating. We use nearly linear amplification with no significant non-linear phase distortions with a B-integral <0.2. Only a small fraction of the pulse energy (1.5 nJ) is required for octave-wide spectral broadening and self-referenced -to-2 detection and stabilization of CEO.

For conversion to the XUV, 15W of the laser output is coupled to a passive cavity with an enhancement factor of 350. High harmonic radiation is generated in Xe gas jet at a 960 µm2 area intracavity focus. Even with only 10% output coupling using an intracavity diffraction grating, we determine 3.7, 3.8, 1.3, 1.8, and 0.7 µW of average power for the 9th through 17th harmonics, respectively [4].

10W of the Yb-frequency comb has also been used to pump an optical parametric oscillator (OPO) frequency comb optimized for mid-infrared applications [5]. The center frequency of the idler-OPO comb is tunable from 2.8 µm to 4.8 µm, and a power per comb line on the order of 1-10



µW (>1 W of maximum average power). The simultaneous spectral coverage ranges from 50 to 400 cm-1.

1. T. R. Schibli, I. Hartl, D. C. Yost, M. J. Martin, M. E. Fermann, and J. Ye, “Optical frequency comb with submillihertz linewidth and more than 10 W average power,” Nature Photonics 2, 355 (2008).

2. I. Hartl, H. A. McKay, R. Thapa, B. K. Thomas, A. Ruehl, L. Dong, and M. E. Fermann, “Fully Stabilized GHz Yb-Fiber Laser Frequency Comb,” in Advanced Solid-State Photonics, OSA Technical Digest Series (CD) (Optical Society of America, 2009), MF9.

3. A. Ruehl, A. Marcinkevicius, M. E. Fermann, and I. Hartl, “75 W Yb-Fiber Laser Frequency Comb,” in Conference on Lasers and Electro-Optics, OSA Technical Digest (CD) (Optical Society of America, CMN1.

4. A. Cingöz, D. C. Yost, T. K. Allison, J. Ye, A. Ruehl, M. E. Fermann, and I. Hartl, “Power Scaling of High-Repetition-Rate HHG,” at 17th International Conference on Ultrafast Phenomena, Snowmass Village, CO, 2010,

5. F. Adler, K. C. Cossel, M. J. Thorpe, I. Hartl, M. E. Fermann, and J. Ye, “Phase-stabilized, 1.5 W frequency comb at 2.8-4.8µm,” Opt. Lett. 34, 1330-1332 (2009).

7917-45, Session 10

Mid-infrared femtosecond frequency combs for sensing and optical clocksI. T. Sorokina, Norwegian Univ. of Science and Technology (Norway); E. Sorokin, Vienna Univ. of Technology (Austria)

We review recent advances in frequency comb sources based on Cr2+ chalcogenide - lasers, operating between 1.9 and 3.6 micron, and their applications to “Fourier-transform frequency comb spectroscopy” and other gas sensing techniques.

This year was marked by a leap forward in these lasers, exhibiting the broadest among existing lasers continuous tuning (1.9 to 3.3 microns for Cr:ZnSe and up to 3.6 microns for Cr:CdSe), as well as truly Kerr-Lens mode-locked 80 fs pulses. We demonstrated these lasers to operate in a compact hybrid fiber-bulk laser form as well as directly diode-pumped. The femtosecond frequency combs also serve as a perfect starting point for synchroniously pumped OPOs, opening the way towards frequency combs at even longer wavelengths.

The talk highlights the radical efficiency and sensitivity increase when such sources are applied to high-resolution molecular spectroscopy and gas sensing, and opens up bright perspectives for simple and cost effective sources for applications ranging from breath analysis to optical clocks, and their applications in space science and technology. Other potential applications include video-rate chemical imaging and transient spectroscopy of e.g. gas plumes, flames and plasmas, and non-repetitive phenomena such as those found, e.g. in protein folding dynamics.

7917-46, Session 11

Mid-infrared photonics in siliconA. Spott, Y. Liu, T. W. Baehr-Jones, R. Ilic, M. Hochberg, Univ. of Washington (United States)

Silicon waveguides have, to date, largely been designed to operate near the telecommunication bands in the near infrared. The mid-infrared (MIR) wavelengths, which range from two to twenty microns, are critical for a number of application areas, including chemical bond spectroscopy and thermal imaging. We show results, using commercially available silicon-on-sapphire wafers, for low-loss (~4 dB/cm) waveguides and what we believe to be the first working microresonators operating at wavelengths around 5.5 um in silicon guides. This talk will discuss the applications for mid-infrared integrated photonics in the silicon system, particularly for sensing and nonlinear optics.

7917-47, Session 11

Third harmonic generation in periodically poled crystalsJ. Delagnes, L. Canioni, Univ. Bordeaux 1 (France)

Third harmonic generation (THG) has already been demonstrated in various materials such as gasses, crystals or glasses. Third order-based methods usually exhibit moderate efficiency (up to few percent) limited by phase matching or necessitate high power to operate. Drastic improvement of the THG yield can be obtained in structured materials and/or with cascaded second order processes whose magnitude is inherently larger. Direct THG with cascaded second order processes has been demonstrated in periodically poled LiNO3 (ppLN) either as a side product of the quasi-phase matched (QPM) SHG or using complex ppLN domain pattern and architecture in order to optimize the SHG + SFG scheme.

Here we present results demonstrating the direct THG in poled materials. The method we propose relies on standard ppLN architecture. Neither the SHG nor the SFG optimization or their coupling is mandatory for cascaded THG. Complicated Fibonacci or quasi-periodic structures are thus unnecessary. Indeed, in our approach, the SHG signal remains weak and only serves as “virtual state” to seed the THG that satisfies the overall QPM-THG condition. Numerical simulations reveal that under QPM-THG condition, SHG intensity has rapid spatial variations and its intensity remains weak along the propagation. Adiabatic elimination of the second harmonic virtual state allows for an efficient THG, which is observed experimentally. Using this approximation, non-linear coupled wave equations reduce to THG process.

Experimentally, a well-collimated third harmonic signal is observed when a suitable ppLN is illuminated with a 1.5 µm OPA. The broad nature of the observed acceptance spectrum (~20 nm) finds its origin in the non-stepwise THG we have developed.

7917-48, Session 11

Flattop wideband wavelength converters based on cascaded sum- and difference-frequency generation using step-chirped gratingsA. Tehranchi, R. Kashyap, Ecole Polytechnique de Montréal (Canada)

There has been increasing interest in the flattop wide waveband wavelength converters based on quasi-phase-matched cascaded (2) in nonlinear-optic waveguides as it has several applications in optical networks including channel selective and multiple channel wavelength conversion. Whilst the pumps can be out of the conversion bandwidth in the cascaded sum and difference frequency generation (SFG + DFG), by increasing the difference between pump wavelengths, the bandwidth is enhanced along with a huge variation in efficiency response and thus only a limited flat bandwidth can be achieved using uniform gratings. Further, the pump bandwidth of the cascaded SFG + DFG is limited for uniform gratings. One approach for removing the non-uniformity and flattening the efficiency response is the manipulation of the periodic structure. The research on how to engineer the chirped gratings is therefore of great importance, to maintain the flattop response of conversion efficiency for a large pump wavelength difference and to enhance further the conversion efficiency stability by increasing the pump bandwidth.

Here, we investigate the role of step-chirped gratings (SCG) for flattening of conversion efficiency response and enhancing the pump bandwidth in the cascaded SFG + DFG with a 75-nm-large pump wavelength difference. To obtain a flattop response using the same waveguide length, the appropriate critical period shifts are presented for the reasonable number of sections and chirp steps feasible for fabrication. Furthermore, it is shown that adding the section numbers for the SCG structure increases the pump bandwidth.



7917-49, Session 11

Laser heated pedestal growth of potassium lithium niobate for UV generationG. Maxwell, D. Dalton, Shasta Crystals (United States); A. B. Petersen, Newport Spectra-Physics (United States)

Potassium lithium niobate (KLN), a non-linear optical material with high nonlinearity and other desirable properties, has the potential to improve the performance and reduce the cost of blue and UV lasers. However, KLN crystals have not entered the commercial mainstream because it is impossible to grow them reproducibly, and with the required performance and cost, by conventional techniques such as the Czochralski method. We have developed a proprietary process based on the laser heated pedestal growth (LHPG) technique that will eliminate the technical barriers to manufacturing of KLN crystals.

In principle, the KLN material system can be compositionally tuned for noncritical phase matching in the IR down to 760 nm, with the second harmonic at >380 nm. To date, we have grown crystals generating harmonics below 400 nm, near the short wavelength limit.

In this paper, we will describe the LHPG method, and its application to KLN crystal growth including improvements in crystal uniformity and transparency. We will also present our latest harmonic generation results in the UV.

7917-50, Session 12

Future directions in quasi-phasematched semiconductors for mid-infrared lasersP. G. Schunemann, S. D. Setzler, BAE Systems (United States)

As solid-state laser technology shifts to more affordable fiber-based devices that are rugged, compact, conformable, and ultimately continuous wave (cw) or quasi-cw, improved NLO crystals will be needed which can be non-critically phase-matched (NCPM) in order to achieve the tight focusing, long interaction lengths, and alignment insensitivity required for efficient conversion in a fiber-coupled package. NCPM is best achieved in a quasi-phase-matched (QPM) material, which also offers engineerable functionality such as parallel or serial grating structures for tunability or cascaded nonlinear processes respectively. QPM oxides like periodically poled lithium niobate (PPLN) or tantalate (PPLT) offer excellent performance in the visible and near-infrared, but exhibit excessive absorption losses for efficient operation beyond 4m. Orientation patterned semiconductors, however, are transparent far deeper into the mid-infrared than their oxide counterparts and offer many other outstanding properties including much higher nonlinear coefficients, higher thermal conductivity, higher purity levels, and very low losses when grown from the vapor phase. More importantly, this technology leverages a huge commercial investment and established infrastructure for the growth and processing of compound semiconductors.

Orientation-patterned GaAs (OPGaAs) was the first material to demonstrate the potential of all-epitaxial-grown QPM semiconductors, and it is already achieving record output powers and efficiencies in the mid-IR. In this paper we analyze and compare the properties, processing, and predicted performance of several potential orientation-patterned semiconductor compounds in order to identify the most promising candidates for further development, including preliminary results on a new material: orientation-patterned gallium phosphide (OPGaP).

7917-51, Session 12

Some properties of the mixed GaS0 4Se0 6 nonlinear crystal in comparison to GaSeG. Marchev, A. Tyazhev, V. L. Panyutin, V. P. Petrov, F. Noack, K. Miyata, Max-Born-Institut für Nichtlineare Optik und Kurzzeitspektroskopie (Germany); M. Griepentrog, Bundesanstalt

für Materialforschung und -prüfung (Germany)

Two essential advantages can be expected from adding S to the well known nonlinear crystal GaSe: increase of the band-gap value or the short wave cut-off limit and improved hardness. Recently, we confirmed that the non-centrosymmetric structure of GaSe is preserved up to a GaS content of 40 mol. % while the nonlinear coefficient d22 is reduced by only 24%. The increased band-gap results also in higher surface damage threshold. The first Sellmeier equations for GaS0.4Se0.6 were based on refractive index measurements. These equations were subsequently refined by fitting second-harmonic and difference-frequency generation phase-matching angle data in the mid-infrared as well as birefringence data in the visible and near-infrared obtained with thin phase retardation plates. The two-photon absorption effect was studied for GaS0.4Se0.6 and GaSe using amplified 1064 nm picosecond pulses at 10 Hz. The comparison indicated roughly an order of magnitude weaker two-photon absorption effect in GaS0.4Se0.6 in comparison to GaSe in the GW/cm2 intensity range. This means that GaS0.4Se0.6 could be safely used in Nd:YAG laser pumped nanosecond optical parametric oscillators or picosecond optical parametric amplifiers without nonlinear absorption. In the reported dynamic indentation measurements of GaS0.4Se0.6 and GaSe, the primary loading was controlled such that the strain rate was held constant at 0.05 s-1. The maximum indentation depth was 2 µm. Frequency and amplitude of the superimposed sinusoidal oscillation were 45 Hz and 2 nm, respectively. The Berkovich indenter used was a so-called AccuTipTM (Agilent), very sharp tips with a nominal tip radius of about 50 nm.

7917-52, Session 12

Frequency doubling of a CO2 laser using orientation patterned GaAsL. P. Gonzalez, D. Upchurch, Air Force Research Lab. (United States); P. G. Schunemann, K. T. Zawilski, BAE Systems (United States); S. Guha, Air Force Research Lab. (United States)

High efficiency second harmonic generation of a TEA CO2 laser operating at 9.569 um (pulse duration 200 ns) was demonstrated in a quasi-phase-matched GaAs structure, 1.48 mm thick, 39.7 mm long and 8.3 mm wide, and with 219.6 um grating period. The structure was grown by hydride vapor phase epitaxy and was dual-band anti-reflection coated on both entrance and exit surfaces. Energy of 1.2 mJ was obtained at 4.78 um for single pass conversion with an incident energy of 2.56 mJ (0.9 J/cm2).

7917-53, Session 12

Photoacousitc Z-scan for the measurement of third-order nonlinear absorption coefficientC. S. Yelleswarapu, Univ. of Massachusetts Boston (United States); S. Kothapalli, Stanford Univ. (United States); D. V. G. L. N. Rao, Univ. of Massachusetts Boston (United States)

Third order nonlinear optical parameters of materials are responsible for variation of refractive and absorptive properties. The measurement of these parameters is important for many practical applications. Among several methods, conventional Z-scan is a simple technique and most commonly used approach [1]. It is performed in transmission geometry and offers high sensitivity. Since its invention, several varieties of Z-scan have been demonstrated in order to improve the sensitivity and/or to study variety of materials [2-4]. In spite of these advances, it is still not convenient to use in certain situations. Here we present a novel Photoacoustic Z-scan (PZ-scan) technique to measure the nonlinear absorption coefficients of a wide variety of materials. It combines the advantages offered by the conventional optical Z-scan technique and the highly sensitive photoacoustic detection. In PZ-scan, instead of measuring the transmitted optical signal as in the case of traditional Z-scan, we record the generated photoacoustic signal while the sample



is translated along the focused laser beam. Since the signal strength is directly proportional to the optical absorption, PZ-scan displays nonlinear behavior depicting the nonlinear optical absorption of the material. As the generated acoustic waves are propagated outwards in all directions, including both the forward and backward directions, PZ-scan is performed in the reflection geometry to study the nonlinear absorption behaviors of saturable and reverse saturable absorbers. Our results demonstrate that PZ-scan can be used to study nonlinear optical properties of non-transparent materials. As photoacoustic signals are time resolved, it is possible to study the nonlinear optical characteristics of thick samples.

References:

1. M. Sheik Bahae, A. A. Said, T. H. Wei, D. J. Hagan, E. W. Vanstryland, IEEE J. Quant. Electron. 26, 760 (1990).

2. T. Xia, D. J. Hagan, M. Sheik-Bahae, and E. W. Van Stryland, Opt. Commun. 134, 529 (1994).

3. S. M. Mian, B. Taheri, and J. P. Wicksted, J. Opt. Soc. Am. B 13, 856 (1996).

4. P. Chen, D. A. Oulianov, I. V. Tomov, and P. M. Rentzepis, J. Appl. Phys. 85, 7043 (1999).

7917-54, Session 12

Thermally managed Z-scan measurements of titanium dioxide thin filmsC. C. Evans, J. D. B. Bradley, F. Parsy, K. C. Phillips, R. Senaratne, E. A. Marti-Panameño, E. D. Mazur, Harvard Univ. (United States)

Integrated photonic circuits are a sought after goal in the telecommunication industry. Thin films possessing both high linear and nonlinear indices are of particular interest as they may enable all-optical processing at nano-scale dimensions. One promising material for nonlinear optics is titanium dioxide (TiO2), which has a large linear and nonlinear index of refraction with low linear losses. Using sputtering, we deposit thin films of amorphous and polycrystalline TiO2 with planar waveguiding losses of less than 1 dB/cm for visible and near infrared wavelengths. To our knowledge, the nonlinear properties of similar films have not been reported at 800 nm. This work is devoted to presenting measurements of the nonlinear optical properties of these films using the z-scan technique.

Z-scan is a widely used method for determining the magnitude and sign of the complex nonlinear response of a material. However, cumulative thermal effects can produce signatures that may be misinterpreted as electronic in nature. We use the thermally managed z-scan technique to collect data which can be used to account for thermal effects and extrapolated to single pulse measurements. Using a Ti:Sapphire laser with 100-fs pulses at 800 nm, we observe ultrafast electronic effects near TiO2’s half band-gap.

We will present recent measurements of the nonlinear contribution to the refractive index as well as multiphoton absorption properties in several of these thin films. We explore the relation between material processing parameters and observed nonlinearity. Lastly, we will discuss the consequences for applications such as all-optical switching.

7917-55, Session 12

Excited state nonlinear optics: limitations and improvements upon the two-level approximation for moleculesD. L. Andrews, D. S. Bradshaw, M. M. Coles, Univ. of East Anglia Norwich (United Kingdom)

When parametric nonlinear processes are employed in the cause of efficient optical frequency conversion, the media involved are generally subjected to substantially off-resonant input radiation. As such, it is usually only electronic ground states of the conversion material that are significantly populated; higher levels are engaged only in the capacity of virtual states, and it is frequently assumed that just one such state dominates in determining the response. Calculating the nonlinear optical susceptibilities of molecules on this basis, excluding all but the ground and one excited state in a sum-over-states formulation, signifies the adoption of a two-level model, a technique that is widely deployed in the calculation and analysis of nonlinear optical properties. The two-level model offers tractable and physically simple representations of molecular response, including wavelength dependence; it is also the origin of the widely applied ‘push-pull’ approach to designing optically nonlinear chromophores. By contrast, direct ab initio calculations of optical susceptibility are commonly frustrated by a complete failure to determine such dispersion features. However, caution is required; the two-level model can deliver potentially misleading results if it is applied without regard to the criteria for its validity, especially when molecular excited states are significantly populated. On the basis of a precise, quantum electrodynamical basis for the theory, this paper explores the applicability and detailed conditions for the two-level model for electronically excited molecules, identifying problematic results and providing tractable methods for improving the accuracy of calculations on real molecule-photon interactions.

7917-56, Session 12

New mixed LiGa0 5In0 5Se2 nonlinear crystal for the mid-IRV. Vedenyapin, L. I. Isaenko, A. P. Yelisseyev, S. Lobanov, Institute of Mineralogy and Petrography (Russian Federation); A. Tyazhev, G. Marchev, V. P. Petrov, Max-Born-Institut für Nichtlineare Optik und Kurzzeitspektroskopie (Germany)

LiGaSe2 and LiInSe2 are promising nonlinear crystals for conversion of laser radiation to the mid-IR spectral range which are transparent down to the visible and UV. Their band-gaps are 3.57 and 2.86 eV, respectively. This difference results in rather different two-photon absorption coefficients at short pump wavelengths, e.g. 820 nm, where beta<0.07 cm/GW for LiGaSe2 and beta=0.6 cm/GW for LiInSe2. We synthesized a new mixed crystal as solid solution with composition LiGa0.5In0.5Se2 which has the same orthorhombic structure (mm2) as the parent compounds (LiGaSe2 and LiInSe2). The unit cell parameters of LiGa0.5In0.5Se2 are a=7.0376(2) A, b=8.3401(3) A, c=6.6855(2) A, and unit cell volume 392.4 A3. The new crystal is more technological with regard to the growth process in comparison with LiGaSe2 since its homogeneity range is broader in the phase diagram. We established that about 10% of the Li ions are found in octahedral position with coordination number of 3. This feature produces additional distortion in the crystal lattice and may result in enhanced nonlinearity and suppression of photo-induced effects. The band-gap of LiGa0.5In0.5Se2 is estimated to be ~3.2 eV. The dispersion characteristics were measured and Sellmeier equations will be presented. The fundamental wavelength range for the SHG process extends from 1.75 to 11.8 µm. The nonlinear coefficients of LiGa0.5In0.5Se2 are compared with those of LiGaSe2 and LiInSe2.



Conference 7918: High-Power Diode Laser Technology and Applications IXSunday-Tuesday 23-25 January 2011 • Part of Proceedings of SPIE Vol. 7918 High-Power Diode Laser Technology and Applications IX

7918-01, Session 1

Reliability of high power diode laser systems based on single emittersP. O. Leisher, L. Bao, J. Wang, M. Grimshaw, M. A. Devito, A. Brown, K. W. Kennedy, S. R. Karlsen, J. A. Small, R. J. Martinsen, J. Haden, nLIGHT Corp. (United States)

Diode laser modules based on arrays of single emitters offer a number of advantages over bar-based solutions including enhanced reliability, higher brightness, and lower cost per bright watt. This approach has enabled a rapid proliferation of commercially available high-brightness fiber-coupled diode laser modules. Incorporating ever-greater numbers of emitters within a single module offers a direct path for power scaling while simultaneously maintaining high brightness and minimizing overall cost. While reports of long lifetimes for single emitter diode laser technology are widespread, the complex relationship between the standalone chip reliability and package-induced failure modes, as well as the impact of built-in redundancy offered by multiple emitters, are not often discussed. In this work we present a system-level reliability methodology which separates failure modes which interact with the constituent emitters from those which do not. Accelerated lifetesting of each separable failure mode is used to build a comprehensive system-level reliability model. Aggressive reliability testing at the module level is then used to place a maximum upper bound on the consequences of emitter-package interaction effects on the reliability of the system. Results for various system configurations and limitations of the methodology are reported.

7918-02, Session 1

Catastrophic optical bulk damage (COBD) in high power multi-mode InGaAs-AlGaAs strained quantum well lasersY. Sin, N. Ives, S. LaLumondiere, S. C. Moss, The Aerospace Corp. (United States)

State-of-the-art broad-area InGaAs-AlGaAs strained quantum well (QW) lasers show an optical output power of over 20 W and a power conversion efficiency of over 70% under CW operation. Unlike broad-area (Al)GaAs QW lasers, broad-area InGaAs strained QW lasers show two failure types including facet catastrophic optical damage (COD) and bulk failure. Optimization of facet passivation processes has led to significant reduction in occurrence of facet COD (or COMD), but bulk failure (or COBD) has received little attention although it is crucial to understand degradation processes responsible for COBD and then develop COBD-free lasers for high reliability applications including potential satellite systems. Our group recently proposed a model for the COBD process and this paper further investigates the root causes of COBD in the broad-area lasers. We performed accelerated life-tests of MOCVD-grown broad-area strained InGaAs-AlGaAs single QW lasers at ~975 nm, which predominantly yielded catastrophic bulk failures. We employed various non-destructive techniques to study pre- and post-stressed lasers. Time resolved electroluminescence (TR-EL) was employed to observe formation and progression of dark spots and dark lines through windowed n-contacts during entire life-tests that eventually led to COBD. Deep level transient spectroscopy (DLTS) was employed to investigate trap characteristics in degraded devices at different stages of degradation to study the role that non-radiative recombination centers (NRCs) play in COBD processes. Time resolved photoluminescence (TR-PL) was employed to measure carrier lifetimes from both undamaged and damaged active areas to find correlation between dark line defects in degraded lasers and non-radiative recombination processes.

7918-03, Session 1

Temperature and current accelerated lifetime conditions and testing of laser diodes for ESA BepiColombo space missionG. Klumel, Y. Karni, S. Cohen, SCD Semiconductor Devices (Israel); M. Rech, K. Weidlich, Carl Zeiss Optronics GmbH (Germany)

System designers and end users of diode pumped solid state lasers often require knowledge of the operability limits of QCW laser diode pump sources and their predicted reliability performance as a function of operating conditions. Accelerated ageing at elevated temperatures, duty cycles and/or currents allows extended lifetime testing of diode stacks to be executed on compressed timescales with high confidence.

We present a novel, time-efficient technique for the determination of accelerated lifetest conditions using degradation rate data, rather than the traditionally used failures against time data.

To assess the effect of thermally accelerated ageing, 4 groups of 4 stacks each were operated for 60 Mshots at different temperature stress levels by varying the pulse repetition rate from 100Hz to 250Hz. The measured power degradation rates were fitted to an Arrhenius type model with an activation energy of 0.46eV - 0.60eV.

Similarly, for current accelerated ageing, another 4 groups of 4 stacks were tested at operation currents from 120A to 150A. The optical power degradation rates due to current stressing followed a power law behavior with a current acceleration factor of 1.6-1.7.

The acceleration parameters obtained allowed a considerable reduction of the lifetest duration, which would otherwise take an unacceptably long time under nominal operating conditions.

The results of the accelerated lifetest confirm the high reliability of SCD’s stacks that have undergone a full qualification as pump sources for a laser altimeter in ESA’s Bepi-Colombo space mission.

The presented reliability analysis allows lifetest qualification programs to be accelerated for generic QCW stacks and their lifetime to be predicted in various operating environments.

7918-04, Session 1

Reliable high-power long-pulse 8XX-nm diode laser bars and arrays operating at high temperatureL. Fan, C. Cao, G. Thaler, D. Nonnemacher, F. Lapinski, I. Ai, B. Caliva, S. Das, L. Zeng, R. Walker, M. McElhinney, P. Thiagarajan, Lasertel, Inc. (United States)

We report on the high-power high-temperature long-pulse performance of the 8XX-nm diode laser bars and arrays, which were recently developed at Lasertel Inc. for diode laser pumping within high-temperature (over 130 ºC) environment without any cooling. Since certain energy in each pulse is required, the diode laser bars have to provide both high peak power and a nice pulse shape. Optimizing the epi-structure of the diode laser, the laser cavity and the distribution of waste heat, we demonstrated over 40-millisecond long-pulse operation of the 8XX -nm CS bars at 130 ºC and 100 A. Pumping the bar with 5-Hz frequency 10-millisecond rectangular current pulses, we generate over 60 W peak power at 100 A and 130 ºC. During the pulse duration, the pulse shape of the laser output power is well-maintained and the power almost linearly decays with a rate of 2.0% peak power per millisecond at 130 ºC and 100 A. Regardless of the pulse shape, this laser bar can lase at very high temperature: 170 ºC/180 ºC for 10 ms/1ms pulse operation


(both with 5 Hz frequency), respectively. To the best of our knowledge, this is the highest operating temperature for a long-pulse 8XX -nm laser bar. Under the condition of 130 ºC and 100 A, the laser bars do not show any degradation after 310,000 10-millisecond current pulse shots. The kilowatt stack arrays are also demonstrated at 130 ºC and 100 A. The development of reliable high-temperature diode laser bar paves the way for diode laser long-pulse pumping within a high-temperature environment without any cooling.

7918-05, Session 1

High reliability and high performance of 9xx-nm single emitter laser diodesL. Bao, J. Wang, M. A. Devito, P. O. Leisher, D. Xu, M. Grimshaw, W. Dong, X. Guan, S. Zhang, C. Bai, J. G. Bai, D. Wise, R. J. Martinsen, nLIGHT Corp. (United States)

Improved performance and reliability of 9xx-nm single emitter laser diodes will be presented in this paper.

The ongoing multi-cell life-test has accumulated 12,000 hours duration, with drive currents from 14A to 18A and junction temperatures ranging from 60°C to 110°C. Out of 208 devices, only 12 devices have failed up to this point. The failure rate is so low that meaningful estimation of acceleration factors cannot be extrapolated. If nominal acceleration factors are used, multi-cell life-test data supports ~400 FIT, with 90% confidence, at 10W, 50°C junction temperatures. Even with no acceleration factors applied, the 9xx-nm diode lasers demonstrate highly reliable operation, under very high powers and temperatures. In addition to this 9xx-nm work, we have also extended the high performance and reliability to 880-nm, with efficiencies exceeding 68%.

In order to further scale up power and brightness, long cavity experiments have been conducted with epitaxial structure designs optimized for low internal loss. Initial results demonstrated 67% peak efficiency on lasers with 5mm cavity length and 63% peak efficiency on lasers with 7.6mm cavity length. Brightness is improved with reduced far-field and enhanced power and efficiency. Life-test of these devices has been running at greater than 20A and 50C junction temperature.

7918-06, Session 1

Optimized biasing of pump laser diodes in a highly reliable metrology source for long-duration space missionsI. Poberezhskiy, D. H. Chang, H. Erlig, Jet Propulsion Lab. (United States)

Nd:YAG-based Non-Planar Ring Oscillator (NPRO) lasers have been selected as the optical metrology source for several space missions, including NASA SIM Lite space interferometer. However, meeting stringent reliability requirements for prolonged missions is challenging due to the wear-out and random failure characteristics of pump laser diodes. For example, SIM Lite metrology source pump module must produce >2W of 808nm light with >99.7% reliability over 5.5 years. We developed a laser pump module architecture that meets these requirements by combining the outputs of multiple single-mode pump diodes operating at derated conditions in a high port count fiber coupler.

As pump lasers begin to fail in the course of a mission, the remaining ones must increase their output to continue supplying constant pump power. In this paper we consider the efficiency of several approaches to biasing the laser diode “farm,” including equal load sharing, as well as keeping spare diodes “dark” to switch on when needed. To compare the competing approaches, we introduce a new optimization metric, instantaneous laser farm aging, that should be minimized during the mission. We find that different approaches become preferable depending on the values of pump laser parameters, such as random failure rate, wear-out median lifetime, and power exponents in the Arrhenius relations. Using values typical for commercially available 808-nm diodes, we select

an optimal biasing approach for our laser pump module architecture. Although this work was motivated by the stringent reliability and optical power requirements of SIM Lite metrology source, the developed approach is broadly applicable to load-sharing optimization among multiple laser diodes.

7918-07, Session 1

QCW laser diode array reliability at 80x and 88x nmR. Feeler, E. Stephens, J. L. Levy, J. Junghans, D. Schnurbusch, Northrop Grumman Cutting Edge Optronics (United States)

Northrop Grumman Cutting Edge Optronics (NGCEO) has recently developed high-power laser diode arrays specifically for long-life operation in quasi-CW applications. These arrays feature a new epitaxial wafer design that utilizes a large optical cavity and are packaged using AuSn solder and CTE-matched heat sinks.

This work focuses on life test matrix of multiple epitaxial structures, multiple wavelengths, and multiple drive currents. Particular emphasis is given to the 80x and 88x wavelength bands running at 100-400 Watts per bar. Statistical analyses of the life tests are presented and reliability statistics are calculated in order to provide predictive data to laser system designers. Reliable operating points are identified for various applications including range finding (product lifetimes less than 1 billion shots) and industrial machining (product lifetimes greater than 20 billion shots).

In addition to life test data, a summary of performance data for each epitaxial structure and each bar design is also presented. Power, voltage, and wavelength are presented as a function of drive parameters (current, pulse width, repetition rate). Data is also presented as a function of device temperature ranging from -40 to +70 degrees Celsius.

7918-08, Session 2

Fiber coupled diode laser beam parameter product calculation and rule for optimized designZ. Wang, A. Segref, T. P. Koenning, R. Pandey, DILAS Diode Laser, Inc. (United States)

The BPP of a passive, lossless system is a constant and cannot be improved upon but the beams may be reshaped for enhance coupling performance. The function of the optical designer of fiber coupled diode lasers is to preserve the brightness of the diode sources while maximizing the coupling efficiency into the fiber. In coupling diode laser power into fiber output, the symmetrical geometry of the fiber core makes it highly desirable to have symmetrical Beam Parameter Product (BPPs) at the fiber input surface, but this is not always practical. It is therefore desirable to be able to know the ‘diagonal’ (fiber)BPP, where the BPPs of the fast and slow axes are known, before detailed design and simulation processes. A commonly used expression derived in an earlier work has been found to consistently under-predict the fiber BPP (i.e. better beam quality is predicted than is actually achievable in practice). In this paper, using a simplified model, we provide the proof of the proper calculation of the diagonal component of the fiber BPP using BPPs of the fast and slow axes as input. Measured data is shown to provide satisfactory correlation between the BPPs of the diode and the fiber BPP.

In designing the optics that launches an asymmetric beam into the fiber, a trade off exists between conjugate variables of beam sizes and the numerical apertures along the two axes. The design process often requires more detailed design, simulation and experimental work than would be desirable. Using the simplified model we offer proof that the fiber BPP can be shown to have a minimum (optimal) value for the given diode BPPs that is easily calculated. Because the calculation only depends on the BPPs from the diode, the optimized condition can be obtained before any detailed design and simulation are carried out.

Conference 7918: High-Power Diode Laser Technology and Applications IX


Although this optimization is based on a simple model and should not replace detailed design in practice, it should offer valuable insights for the detailed design.

7918-09, Session 2

Simulation and analysis of volume holographic gratings integrated in collimation optics for wavelength stabilizationS. Hengesbach, U. Witte, M. Traub, H. Hoffmann, Fraunhofer-Institut für Lasertechnik (Germany)

Integrating volume holographic gratings into micro-optical components such as cylindrical fast-axis collimation lenses (VHG-FAC) for diode lasers constitutes a promising concept for wavelength stabilization by forming an external cavity laser. Compared to standard wavelength stabilization configurations the integrated element reduces the alignment complexity and is furthermore insensitive to the smile-error of diode laser bars. In order to configure and optimize these components the diffraction of the divergent field distribution of a broad area semiconductor laser must be calculated.

In this paper the extension of the coupled-mode theory is presented in order to calculate the spectral distribution of the diffracted field and the coupling efficiency within the external cavity. The model was extended to three-dimensional space and supplemented to include surface effects, polarization dependency and wave-optical propagation.

The asymmetric spectral distribution emitted by an external cavity laser with VHG-FAC is tracked back to the feedback of highly divergent radiation diffracted at the holographic grating. Power losses due to the coupling efficiency within the cavity are also calculated for various field distributions and compared with experimental data.

In summary the mathematical model allows to estimate the minimum spectral width and the losses using a VHG-FAC in an external cavity. Thus the injection locking concept using the VHG-FAC can be compared to the spectral characteristics and estimated power losses of standard wavelength stabilization configurations, e.g. the alignment of the grating in the collimated beam.

7918-10, Session 2

High efficiency frequency stabilized tapered amplifiers with improved brightnessP. Friedmann, J. Gilly, S. Moritz, M. T. Kelemen, m2k-laser GmbH (Germany)

Semiconductor laser diodes with a tapered gain region provide a beam quality near to the diffraction limit combined with high output power. They can be configured as laser with a high-reflectivity coating on the rear facet as well as amplifier with an antireflection coating on both facets. In amplifier configuration they can be used in external cavity or MOPA - configuration with the advantage of a precisely tuneable wavelength.

Today amplifiers are commercially established with an optical output-power of 1-2W in a wide range of applications such as non linear spectroscopy, frequency doubling for blue-green outputs. Especially for the pumping of rare-earth-doped fiber amplifiers and materials processing higher power is requested.

By extension of the resonator length from 2000µm up to 5000µm the output power of the amplifiers has been increased up to 6W at 976nm with beam quality near diffraction limit. A wall-plug efficiency of more than 55% was reached in continuous wave operation. Improvements of the epitaxy structure and the mounting technology are essential for realizing longer resonators. Therefore an improved InGaAs/AlGaAs single quantum well vertical structure with low internal losses was grown by molecular beam epitaxy. The lateral design of the investigated devices consists of a ridge-waveguide section and a taper section with 4° taper angle. Due to the 4° taper angle the devices provide a small lateral far-field angle < 12° (95%).

7918-11, Session 2

Wigner distribution function of DBR tapered diode laserM. Uebernickel, B. Eppich, C. Fiebig, K. Paschke, G. Erbert, Ferdinand-Braun-Institut (Germany)

High power distributed-bragg reflector tapered diode lasers (DBR-TPL) are well suited for several applications which require a high focus ability e.g. single mode fiber coupling or second-harmonic generation. The improved beam quality enables high coupling efficiency and high conversion efficiency with such laser sources.

However, the precise prediction of coupling or conversion efficiencies as well as the design of optimized optical beam coupling systems is a non-trivial task. The beam propagation ratio M2 according to ISO 11146 is not sufficient to estimate coupling or conversion efficiencies since in case of DBR-TPLs it is dominated by small side lobes in the beam profiles. Although containing only a small fraction of the total power these side lobes dramatically increases the beam propagation factor and hence leads to a significant underestimation of the true beam quality.

Here instead we utilize the concept of the Wigner distribution function (WDF) to describe the laser beam properties in more detail. From the WDF of a laser beam, the phase and intensity distribution can be easily derived as well as the spatial coherence distribution. For a given single mode fiber the theoretical optimum of the coupling efficiency can be obtained by means of a simple overlap integral. Simultaneously, this approach delivers the corresponding beam forming requirements to meet the optimum coupling condition.

We will present experimental data of the WDF of DBR-TPL up to a power of 10 W and will experimentally and theoretically compare the coupling efficiency into a single mode fiber.

7918-12, Session 2

Wavelength stabilized diode laser based devices free of power or efficiency penaltiesK. W. Kennedy, nLIGHT Corp. (United States)

Utilization of external volume gratings to improve TEM00 power scaling and overall conversion efficiency is rapidly expanding, particularly in pumping the narrow upper laser level of Nd:YAG DPSS lasers at 885 nm and the 1532 nm absorption band of Er:YAG DPSS lasers. It is often believed that the use of such external gratings to wavelength lock diode lasers leads to unavoidable losses in power and efficiency. nLIGHT’s vertical integration and unique design methodology has eliminated these problem in our grating-locked diode laser products and will be demonstrated in several wavelength ranges targeting both DPSS as well as fiber laser systems.

7918-13, Session 3

Macro-channel cooled, high power, fiber coupled diode lasers exceeding 1 2kW of output powerT. P. Koenning, K. R. Alegria, Z. Wang, A. Segref, D. Spapleton, DILAS Diode Laser, Inc. (United States); W. Fassbender, M. Flament, K. Rotter, A. Noeske, J. Biesenbach, DILAS Diodenlaser GmbH (Germany)

We report on a new series of fiber coupled diode laser modules exceeding 1.2kW of single wavelength optical power from a 400um / 0.2NA fiber. The units are constructed from passively cooled laser bars as opposed to other comparably powered commercially available modules that use micro-channel heat-sinks. Micro-channel heat sinks require cooling water meeting demanding specifications and are therefore prone to failures due to contamination and increase the overall cost to operate



and maintain the laser. Dilas’ new series of high power fiber coupled diode lasers are designed to eliminate micro channel coolers and their associated failure mechanisms and cost.

Low-smile soldering processes were developed to maximize the brightness available from each diode laser bar. The diode laser brightness is optimally conserved using Dilas’ recently developed propriety laser bar stacking geometry and optics. A total of 24 bars are coupled into a single fiber core using a polarization multiplexing scheme. The modular design permits further power scaling through wavelength multiplexing. Other customer critical features such as industrial grade fibers, pilot beams, fiber interlocks and power monitoring are standard features on these modules.

The optical design and the beam parameter calculations will be presented to explain the inherit design trade offs. Results for single and dual wavelength modules will be presented.

7918-14, Session 3

70% efficient, near 1kW, single 1-cm laser-diode bar at 20°CM. T. Knapczyk, J. H. Jacob, H. Eppich, Science Research Lab., Inc. (United States); A. K. Chin, Somerville Laser Technology, LLC. (United States); K. D. F. Lang, J. T. Vignati, R. H. Chin, Science Research Lab., Inc. (United States)

The brightness of properly-designed laser-diodes is limited by the operating temperature and the power at which the device fails by catastrophic optical-damage (COD). In the past decade, there has been a remarkable order-of-magnitude increase in the brightness of laser diodes due to the development of super-large optical-cavity (SLOC) structures and facet-passivation methods. However, further improvements in brightness are possible by reducing the operating-temperature using improved heat-sinks. In this study, we attach a long-cavity (5mm), laser-diode bar to a novel EPIC (Enhanced Performance Impingement Cooler) heat-sink which has a heat-removal capacity of 3kW/cm2. The remarkable performance of the EPIC cooler is a result of water impinging perpendicular to the heat-removal surface whereas the water flows parallel in standard micro-channel coolers.

The previously-reported highest CW-power for a single bar[1] was 950W but the operating current was 1120A even at a set-point coolant-temperature of 7°C. However, in this study, a record, 960W, CW output-power has been achieved for a single, 1cm-wide, 5mm cavity-length, 77% fill-factor, 940nm, laser-diode bar operated at only 900A even at a 20°C coolant temperature. The output power was limited by COD failure of some of the 65 emitters. The electrical-to-optical efficiency at peak value and at 960W was 70% and 59%, respectively. Stable operation at 600W, 20°C over a period of 100hrs has also been demonstrated. These record results are in large part due to the EPIC’s low thermal-resistance of 6x10-2 K/W, about a third that of micro-channel coolers.

1. H. Li, T. Towe, I. Chyr, D. Brown, T. Nguyen, F. Reinhardt, X. Jin, R. Srinivasan, M. Berube, T. Truchan, R. Bullock, and J. Harrison, “ Near 1kW of continuous-wave power from a single high-efficiency diode-laser bar”, IEEE Phot. Tech. Lett. 19, 960-962 (2007)

7918-15, Session 3

Testing of active heat sink for advanced high-power laser diodesJ. Vetrovec, Aqwest, LLC (United States); R. Feeler, Northrop Grumman Cutting Edge Optronics (United States)

We have previously introduced an innovative active heat sink (AHS) for high-power laser diodes offering unparalleled capacity in high-heat flux handling and temperature control [1]. The AHS employs convective heat transfer by a liquid coolant flowing at high speed inside a miniature sealed flow loop. The coolant receives waste heat at a high flux, transports it by forced convection, and rejects it at a much lower heat

flux. Coolant flow in the loop is maintained electromagnetically without any moving parts. AHS can handle a heat load of several hundred watts at a heat flux over 1,000 W/cm2 with a thermal resistance as low as 0.1 °C/W. AHS thermal conductance can be electronically adjusted, allowing for precise control of diode temperature and the diode light wavelength. In particular, when pumping solid-state or alkaline vapor lasers, diode wavelength can be precisely tuned to the absorption features of the laser gain medium.

This paper presents the active heat sink theory, scaling laws, model predictions, and data from testing at high heat flux and high heat loads. This work was in-part funded by the National Science Foundation and by the US Air Force.

1. J. Vetrovec, “Progress in the Development of Active Heat Sink for High-Power Laser Diodes,” SPIE vol. 7583-19

7918-16, Session 3

Heat sink types for high-power diode laser barsM. Leers, E. Liermann, Fraunhofer-Institut für Lasertechnik (Germany); M. Goetz, Curamik Electronics GmbH (Germany); D. Schröder, JENOPTIK Laser GmbH (Germany)

The fields of application for high power diode lasers and their heat sinks are becoming increasingly diverse. In recent years, passive heat sinks have been establishing themselves more and more. Their advantage lies in the fact that no cooling liquid is required for passive heat sinks. Furthermore, this type of heat sinks can be inexpensively manufactured and it is efficient when used to cool diode laser bars with resonator lengths of over 2.5 mm. Nevertheless, actively cooled heat sinks are still being applied. Micro-channel heat sinks are mostly made of copper. Depending on the application, the laser bar is mounted on an expansion matched submount or it is directly soldered on the heat sink.

Recently there is an increased need for active heat sinks that do not use copper in the water circuit, but instead use an electrical insulator. In the case of the IlasCo heat sink AlN ceramic is used as the insulator material. Concurrently, it provides for the CTE adaptation to the laser bar.

Another important aspect is the smile of the laser bars after they have been mounted. For an acceptable smile, the machinability of the heat sinks, the influence of the laser bar, the different soldering systems and the heat sink materials are equally important.

The fields of application and the properties of the different types of heat sinks are demonstrated in this paper. Particularly the thermal performance, the smile and the induced stresses are compared. Furthermore, the results of long-term testing of different heat sinks are demonstrated.

7918-17, Session 3

Automated assembly processes for high power single emitter diode lasersJ. Pierer, M. Lützelschwab, S. Grossmann, C. A. Bosshard, Ctr. Suisse d’Electronique et de Microtechnique SA (Switzerland); B. Valk, R. Brunner, R. K. Bättig, N. Lichtenstein, Oclaro, Inc. (Switzerland)

The market for high power diode laser modules is steadily growing. Increased voltage/low current operation of single emitter pump sources are of particular interest to customers. However, the production and assembly of single emitter modules become more challenging.

In the framework of a development project with OCLARO, CSEM has realized solutions to a) satisfy the need for higher power from a fiber at low material costs and b) automate the complex assembly processes.

Utilizing OCLAROs diode laser portfolio a fiber coupled module was designed with the objective of coupling 100 W optical power into a 105 µm / NA 0.15 fiber. Two submodules accommodating 6 vertically



stacked single emitters each are combined by polarization multiplexing into a fiber. Additionally, a robust and low cost fiber pigtail was designed withstanding high optical powers not coupled to the fiber core or decoupled off the cladding.

CSEM developed a semiautomatic process enabling a skilled operator to assemble 40 optical elements way faster than possible in a purely manual procedure. Further improvements are possible with fully automated processes. Thereby a considerable part of the assembly time is consumed for curing of adhesives. The developed process is based on combined near/ far field monitoring and specific developed alignment algorithms.

The repeatable accuracy after component fixing was found to be better than 500 nm and 20 µrad, resulting in high yield rates. The latter in combination with the module performance, the robust fiber pigtail and low material costs indicate an excellent potential for the realization of cost efficient laser modules.

7918-18, Session 4

High brightness diode laser module for pump and direct applicationD. Schröder, A. Franke, E. A. Werner, S. Wagner, E. Deichsel, JENOPTIK Laser GmbH (Germany)

The industry of laser marking, direct application and solid state laser pumping requires highly reliable and high efficiency laser diodes.

The every market has a need for high brightness or power fiber coupled modules.

JENOPTIK will show the new 105µm fiber coupled module with an output power of more than 75W and a NA of less than 0.15. The form factor of the module has been decreased without losing stability. The already introduced building block principle has been enhanced to reduce the cost per watt. We will present the new features, power and there wide range of application.

7918-19, Session 4

Technologies for high brilliance and high efficiency beam shaping of high power diode laser: trade-offs between technology and cost driven approachesU. Fornahl, J. Meinschien, L. Aschke, LIMO Lissotschenko Mikrooptik GmbH (Germany)

Current status of high power laser technology is reviewed with special respect on beam shaping issues. Approaches with single emitters are evaluated as well as configurations with bars or arrays. Beam shaping and beam combination is the key for competitive solutions in the market of industrial applications with high power diode lasers.

Special emphasis is given on the issue that for most applications the trade offs between cost and desired technical parameter must by thoroughly assessed. Therefore, a holistic strategy along the chain of processes and components is pursued.

Latest results of specific laser diode modules are presented for several parameter ranges: for fibre coupled modules in the power range of 50-200 W with a single wavelength, for fibre coupled modules exceed 1 kW output power with multiple wavelength as well as for free beam lasers of up to 20 kW output power.

7918-20, Session 4

Record CW-brightness from a single, 20% fill-factor, 1-cm laser-diode bar at 20°CA. K. Chin, Somerville Laser Technology, LLC. (United States); M. T. Knapczyk, J. H. Jacob, H. Eppich, K. D. F. Lang, R. H. Chin, Science Research Lab., Inc. (United States)

Recently, fiber-coupled laser-diode modules for small-core fibers mainly incorporate single-emitters as the light sources. Due to their individual nature, single emitters can be mechanically-positioned independently and can be selected for high-performance and high-reliability. Due to the inter-related issues of thermal-management, mechanical-precision and mechanical-stability, designers have avoided the use of laser-diode bars (LD-bars) in fiber-coupled modules. These issues include thermal roll-over of the output power, increases in the slow-axis divergence, and beam-pointing variations as the power increases. However, there are significant cost and size advantages for laser systems if LD-bars, containing many single-emitters in a compact form, are used as the light sources. In this study, we have achieved record brightness from a LD-bar by significantly improving the thermal management of the LD-bar.

A record, 240W, CW output-power has been achieved for a single, 1cm-wide, 3.5mm cavity-length, 20% fill-factor, 976nm, laser-diode bar operated at 20°C. The remarkable laser-bar performance was in part the result of a novel EPIC (Enhanced Performance Impingement Cooler) heat-sink with a thermal resistance of 1.6x10-2K/W. The superb thermal-management resulted in record brightness for a LD-bar, i.e. a slow-axis divergence of 10° (95% power containment angle) and a collimated fast-axis beam-pointing of less than 1mrad was achieved at 200W output-power, resulting in an expected coupling efficiency of more than 85% into a 200µm core, 0.22NA fiber as modeled by ZEMAX. Several devices have been aged at 180W, 20°C. Device reliability will be discussed as well as results of actual fiber coupling.

7918-21, Session 4

Single emitter based diode lasers with high brightness and narrow linewidthS. W. Heinemann, B. Regaard, T. Schmidt, B. Lewis, Fraunhofer USA, Inc. (United States)

High brightness, narrow linewidth fiber coupled diode lasers based on multiple single emitters rely on four core technologies, a precision reflow process, a customized automated alignment station for high yield production of single emitter based high brightness diode lasers, wavelength stabilization with external grating and efficient optical stacking achieving 85% optical fill factors. Results on ultra high brightness fiber coupled diode laser are presented with 700W out of a 200um/0.22NA fiber and 2nm linewidth over the whole output power range.

Multiple Single Emitter (MSE) modules allow highest power and highest brightness fiber coupled diode lasers based on standard broad area diodes. 12 single emitters, each rated at 11W, can be stacked in fast axis and yield more than 100W in a fully collimated beam with a beam quality of 5mm*mrad in both axes. Coupling efficiencies of >90% are achieved resulting in compact fiber coupled modules. Volume Bragg Gratings (VBG) stabilize the wavelength over a tuning range of >15nm and narrow the linewidth of individual diodes to less than 1.8nm. The brightness is scaled by polarization multiplexing and optical stacking is deployed for larger fibers resulting in 700W delivered from a 200m fiber, 0.2 NA. Wall plug efficiencies of >35% are achieved. The 700W/200m/0.2 laser comprises 120 single emitters. Pointing tolerances and collimation errors of all emitters cannot exceed 10% of the spot size to realize the benefits of highest brightness from single emitters compared to bars. Respective manufacturing practices will be discussed.



7918-22, Session 4

Brightness and power scaling of diode-based light sourcesE. Deichsel, P. Heist, L. Wagner, E. A. Werner, JENOPTIK Laser GmbH (Germany)

Power and brightness of diode-based light-sources attract more and more attention. High-power and high-brightness diode laser systems address a wide range of applications in the field of direct material processing and pump sources for solid state and fiber lasers. Jenoptik’s single-emitter based high-brightness light source offers powers up to 75W with beam-parameter products in the range of 7,5mm*mrad. Based on such modules as base units in a modular building block system, we highlight strategies for power and brightness scaling. Different approaches such as fiber and free-space combining are investigated in detail. Powers in the several 100W to kW class range are demonstrated with remarkable brightness.

7918-23, Session 4

High-power, high-brightness, and low-weight fiber coupled diode laser deviceP. Wolf, B. Köhler, K. Rotter, S. Hertsch, H. Kissel, J. Biesenbach, DILAS Diodenlaser GmbH (Germany)

New solid-state laser devices, especially fiber laser systems, require increasingly higher optical output power provided by fiber-coupled diode laser pump modules. In particular for defense technology, robust but lightweight high-power diode laser sources with high brightness are needed.

A novel diode laser device has been developed combining high power, high brightness, wavelength stabilization and low weight, which becomes more and more important for a multitude of applications. Heart of the device is a specially tailored diode laser bar, which epitaxial and lateral structure is designed such that only standard fast- and slow-axis collimator lenses are required to couple the beam into a 200 µm fiber.

In this paper we present a detailed characterization of the new diode laser device with 675 W of optical power coupled into a 200 µm, NA 0.22 fiber with a lightweight design due to a special housing optimized for low weight. Spectral quality is ensured over a wide range of temperature and current by means of volume holographic gratings for wavelength stabilization. Nevertheless, an overall efficiency of more than 40 % has been achieved.

In addition we present a compact diode laser source with 200 W of optical power coupled into a 200 µm, NA 0.22 fiber. This diode laser device is optimized with regard to highest efficiency and yields an overall electro-optical efficiency of more than 50 %.

7918-24, Session 4

Suppression of lateral modes in wide aperture laser diodes by digital planar hologramsV. Svetikov, I. Ivonin, A. Koshelev, Nanooptika, LLC (Russian Federation); L. V. Velikov, Y. Vorobiev, Nano-Optic Devices, LLC (United States); A. Goltsov, Nanooptika, LLC (Russian Federation); V. V. Yankov, Nano-Optic Devices, LLC (United States)

High power (HP) laser diodes with apertures around 100um pump solid state and fiber lasers, used for material processing. The necessity for the second stage lasers originates from the well-known limitation of brightness of laser diodes with the aperture increase due to appearance of multiple lateral modes. For the first time we report suppression of lateral modes of 100um wide laser diodes by digital planar holograms. Digital planar hologram narrows spectrum of laser diodes, similar to simple gratings, used in DFB and DBR lasers.

7918-25, Session 4

Bright laser source with high power single mode emitting diode laser stacked array assembly and fiber couplingM. Forrer, FISBA OPTIK AG (Switzerland); C. A. Bosshard, Ctr. Suisse d’Electronique et de Microtechnique SA (Switzerland); N. Lichtenstein, Oclaro, Inc. (Switzerland)

Within this project a technological platform to assemble laser modules (stacks) for ultrabright laser sources was realized. Specific goals included the reduction of production costs and the increase of the laser stack coupling efficiency into a glass fiber. This new laser system is based upon a revolutionary concept of 2D diode laser arrays (stacks) from OCLARO(SEAL: Single-Mode Emitter Array Laser) and dedicated micro-optics (MO) from FISBA for beam collimation and fiber coupling. CSEM tasks included the study and the development of the MO supports and the corresponding high-precision assembly process to the laser stack, exploiting a combination of glass soldering and adhesive fixing. Applications of the new laser system are in the direct use of collimated laser diodes for materials processing.

A compact module based on a laser diode stack consisting of 4 laser diode arrays and micro-optical lens arrays was developed for coupling up to 100 W of laser power into a multimode fiber. To achieve this goal, the design of the laser stacks and the MO had to be combined with the design of a suitable assembly process for the MO. The alignment and fixing development of the micro-optical lens arrays was based on a novel combination of glass soldering, laser glass soldering, and adhesive fixing. To fabricate the prototype, first, a fast axis collimation optics was soldered into a specially designed glass support using vision-assisted alignment. Then, the glass support was actively aligned and fixed to the laser stack with submicrometer precision. Permanent attachment was achieved using a UV curable adhesive. Finally, a slow axis collimation lens was attached to the glass support using a newly developed laser soldering process for glass again requiring a submicrometer alignment precision.

7918-26, Session 5

High-brightness distributed-Bragg-reflector tapered diode lasers: pushing your application to the next levelC. Fiebig, Ferdinand-Braun-Institut (Germany); S. Pekarek, ETH Zurich (Switzerland); M. Uebernickel, Ferdinand-Braun-Institut (Germany); T. Südmeyer, U. Keller, ETH Zurich (Switzerland); K. Paschke, G. Erbert, Ferdinand-Braun-Institut (Germany)

High brightness diode lasers with a nearly diffraction limited beam are increasingly important in diode laser industry and research. Especially, tapered diode lasers are well suited for applications requiring a high optical output power in combination with a good beam quality, e.g. laser display technology, direct material processing and pumping of fiber lasers and fiber amplifiers. The combination of such devices with monolithic internal gratings led to the development of Distributed-Feedback (DFB) or Distributed Bragg-Reflector (DBR) Master Oscillator Power Amplifiers (MOPA) as well as DBR tapered diode lasers (DBR-TPL). The latter became the first single chip diode laser providing a brightness close to 1 GW cm-2 sr-1. Due to their high brightness and compactness these monolithic diode lasers are paving the way for new applications and allow the miniaturization of existing diode laser systems, respectively.

At the conference we will show results concerning the development of such DBR-TPL at different wavelengths up to the 10 W levels. Based on that, we will present two examples benefiting from the excellent brightness of these lasers.

1. Using these diode lasers and nonlinear bulk crystals for second harmonic generation (SHG) in a single-pass configuration bench-top



experiment we produced visible light up to 1.8 W. The compactness of these lasers allows for miniaturizing this setup to a compact 2.5 cm³ micro-optical bench (MIOB) showing output powers up to 1 W.

2. The brightness of the DBR-TPL enabled the development of a femtosecond gigahertz SESAM-modelocked Yb:KGW with a record high peak power of 3.9 kW. At the repetition rate of 1 GHz and the wavelength of 1041 nm pulses with a duration of 281 fs and an average output power of 1.1 W were generated. This Yb:KGW laser has a high potential for stable frequency comb generation.

7918-27, Session 5

New developments of high-power single emitters and laser bars at JENOPTIKM. Zorn, R. Huelsewede, H. Schulze, J. Sebastian, JENOPTIK Diode Lab GmbH (Germany); D. Schröder, P. Hennig, JENOPTIK Laser GmbH (Germany)

High-power single emitters and laser bars find several growing industrial applications such as materials processing. A steady increase in efficiency and output power is needed to conquer these new markets. Furthermore laser properties have to be designed according to new applications. These needs make high demands on the development and fabrication of laser diodes either as single emitters or as laser bars.

Single emitter laser diodes are optimized for fiber coupling with a minimal loss of light. Optimizing the laser design and the underlying epitaxy structure and process, we developed single emitters with high output powers at a small far field angle. Laser diodes at 938 nm show output powers of more than 12 W having a far field angle of 25°.

The high-quality laser bars developed at JENOPTIK within the last years have been extended to new wavelength ranges. At the lower end, laser bars have been developed around 780 nm. At the higher end, design parameters have been optimized for 1060 nm.

For the whole wavelength range latest high-performance device data will be presented.

7918-28, Session 5

Next generation 8xxnm laser bars and single emittersU. Strauss, M. Müller, T. Swietlik, R. Fehse, C. Lauer, G. Grönninger, H. Koenig, OSRAM Opto Semiconductors GmbH (Germany); M. Stoiber, I. Scholl, J. Biesenbach, DILAS Diodenlaser GmbH (Germany); T. Fillardet, A. Kohl, Quantel Group (France)

High power laser bars at 9xx nm are typically operated at 100-250W cw for 1cm width. However, power levels at shorter wavelengths are significantly lower: 808nm laser bars for long term continuous wave operation emit typically 100W for a 1cm wide bar at room temperature. The output power is limited by self heating and catastrophic optical mirror damage at the laser facet (COMD). The high power conversion efficiency of 60% enables long term operation at these power levels. The COMD limit of a 50% fill factor bars is approx. 250W, equivalent to 10W per 200µm emitter (conditions: 200µs, 5Hz).

Now we present data on bars fabricated with our new facet technology, which enables us to double the maximum facet load of the 200µm wide emitters to 24W at 200µs pulse tests. The new process pushes cw operation at 808nm to 150-200W/bar. The increase of the power level is also observed in q-cw operation using 200µm pulses. The q-cw laser bar with 80% fill factor achieves output powers above 350W in long term operation at 200µs and 100Hz. The COMD limit of the bar is as high as 700W. Mini stack with three bars of 5mm width are tested up to 190A resulting in 64% power conversion efficiency at 600W output power.

The next limitation of 8xxnm lasers is bulk degradation at high heat sink temperatures. Values of 60°C and more are common for consumer

applications of single emitters. Therefore Osram developed a new epitaxial design. The corresponding single emitters operate in TO can at case temperatures of 80°C and current densities as high as 1.4kA/cm² at 850nm emission wavelength.

7918-29, Session 5

New approach for high-power diode laser modules with homogenized intensity distributionB. Köhler, F. Ahnepohl, K. Rotter, J. Biesenbach, DILAS Diodenlaser GmbH (Germany)

In the last few years high-power diode laser modules with homogenized intensity distribution have found a growing number of applications, like annealing, hardening and surface illumination. The standard beam shaping concepts in such modules are using an optical waveguide or microoptical lens arrays for homogenization. For the generation of long lines with high aspect ratio these concepts have some significant drawbacks, especially if the line is composed of several sub-modules with shorter line segments. The homogeneity in the transition zone of these segments is always difficult to handle.

In this paper we report on a new approach for homogenization of high-power diode laser modules by using linearly arranged fiber bundles to generate homogeneous lines. The main advantage of this concept is that scaling of line length is easily achieved by increasing the number of linearly arranged fibers. We present a detailed characterization of such a modular diode laser system with 3 kW output power and homogenization by means of a fiber bundle. The dimension of the homogenized line is 200 mm x 2 mm.

In addition we present a number of different diode laser modules with homogenization by means of classical approaches, like microoptical cylindrical lens arrays. The output power of these modules ranges from 50 W to 11 kW with line dimensions from 3 mm x 50 µm up to two dimensional homogenized areas of 55 mm x 20 mm.

7918-30, Session 5

100W high-brightness multi emitter laser pumpR. Duesterberg, L. Xu, J. A. Skidmore, J. Guo, J. Cheng, J. Du, B. Johnson, D. L. Vecht, N. Guerin, B. Huang, P. Cheng, R. Raju, K. W. Lee, J. Cai, V. V. Rossin, E. P. Zucker, JDSU (United States)

We report results of a spatially-multiplexed broad area laser diode platform designed for efficient pumping of fiber lasers or direct-diode systems. Optical output power in excess of 100W from a 105um core, 0.15NA fiber is demonstrated with high coupling efficiency. The compact form factor and low thermal resistance enable tight packing densities needed for kW-class fiber laser systems. Broad area lasers have been optimized for maximum coupling / wall plug efficiencies for this design. In addition to performance characteristics, an update on long term reliability testing of 9XXnm broad area laser diodes will also be provided.

7918-31, Session 5

Scaleable multi-format QCW pump stacks based on 200W laser diode bars and mini bars at 808nm and 940nmY. Berk, Y. Karni, G. Klumel, Y. Openhaim, S. Cohen, D. A. Yanson, SCD Semiconductor Devices (Israel)

Advanced solid state laser architectures place increasingly demanding requirements on high-brightness, low-cost QCW laser diode pump



sources, with high power concentration desired within a custom aperture both for side and end pumping configurations. To meet this need, we have developed a new series of scaleable pump sources at 808nm and 940nm. The stacks, available in multiple output formats, allow for custom aperture filling by varying both the length and quantity of stacked laser bars. For these products, we developed next-generation laser bars based on improved epitaxial wafer designs delivering >55% efficiency at power densities of 20W/mm of emission aperture. With >200W of peak QCW power available from a full-length 1cm bar, we have demonstrated power scaling to over 1.8 kW in 9-bar stacks with 55% wall plug efficiency. We will also present the design and performance of several stack configurations using full-length and reduced-length (mini) bars that demonstrate the versatility of both the bar and packaging designs. We will illustrate how the RobustHead packaging technology developed at SCD is capable of accommodating variable bar length, pitch and quantity for custom pumping geometries. The excellent all-around performance of the stacks is supported by reliability data in line with the previously reported 20 Gshot space-grade qualification results of SCD’s stacks.

7918-32, Session 5

High-power, high-brightness QCW laser diodes based on cm-bars and mini-barsH. Huang, J. Wang, M. A. Devito, L. Bao, D. Xu, S. Zhang, D. Wise, W. Dong, M. Grimshaw, C. Bai, G. Fanning, A. L. Hodges, D. Balsley, C. E. Hamilton, R. Ferina, J. Bell, nLIGHT Corp. (United States)

Progresses on high power, high brightness QCW laser diodes are presented for lasing wavelengths of 808, 880 and 940 nm. These devices demonstrate the effectiveness of nLight’s proprietary nXLT facet passivation technology to ensure reliable operation at hundreds of watts output.

With improvements in the epitaxial structures, at 808 nm, reliable 150W operation for a 3.0 mm wide mini-bar and reliable 200 W operation for a 5.0 mm wide mini-bar have been demonstrated with peak efficiencies approaching 65%. At 880 nm, reliable 400 W operation for a 10.0 mm wide bar with a peak efficiency near 65% has been demonstrated. At 940 nm, reliable 200 W operation for a 5.0 mm wide mini bar with peak efficiency near 65% has been demonstrated as well.

These high-efficiency QCW sources are well-suited for portable eye-safe laser range finding and other applications requiring a compact, high-brightness source. Reliability studies and failure mode analyses are shown to demonstrate long-term stability under severe military and industrial environments.

7918-33, Session 5

Concept and experimental implementation of a scalable high power and highly homogeneous laser line generator for industrial applicationsA. Bayer, M. Brodner, J. Meinschien, A. S. Mikhailov, T. Mitra, LIMO Lissotschenko Mikrooptik GmbH (Germany)

High power diode laser line generators are nowadays industrial standard for applications like plastic processing, vision inspection and drying. With increased beam quality, especially peak intensity and homogeneity, they also enable new applications like hardening, annealing or cutting of various materials. All of these applications have in common that simultaneous processing is limited by the scalability of the generated line length without changing process relevant parameters of the line like working distance, peak intensity, homogeneity and depth of focus. Therefore, a patent pending beam shaping concept is presented that enables the interconnection of an arbitrary number of nearly free selectable laser sources to generate scalable laser lines with outstanding beam parameters. System design, experimental setup and results

of a laser line generator is shown. It is based on a stitching concept consisting of ten fiber coupled high power diode lasers, which generates a 200mm long and 2mm wide laser line with a homogeneity level of 97% p-v over a depth of focus of +/- 5 mm with an overall output power of up to 4.2 kW. The concept is discussed regarding industrial 24/7 applications and the options for even higher beam quality, especially the capability of generating lines with increased power densities up to several kW/cm² and a line length of several meters.

7918-34, Session 5

Facet engineering of high power single emittersM. Shamay, D. A. Yanson, M. Levi, R. Tessler, Y. Karni, Y. Don, N. Rapaport, I. Schnitzer, SCD Semiconductor Devices (Israel)

The ever increasing demand for high-power, high-reliability operation of single emitters at 9xx nm wavelength requires the development of laser diodes with improved facet regions immune to both catastrophic and wear-out failure modes. In our study, we have evaluated several laser facet definition technologies in application to 90 micron aperture single emitters in asymmetric design (In)GaAs/AlGaAs based material emitting at 915nm and 980nm.

We will present the performance of non-absorbing mirror designs based on epitaxial regrowth and quantum-well intermixing. It is demonstrated that neither of these techniques delivers high-reliability operation without a carefully developed facet passivation process. Such a process should include a facet pre-clean cycle, encapsulation by a passivation layer and an AR/HR coating deposition, and the effect and importance of each cycle will be discussed. We will describe the use of an ion assisted deposition system to perform the passivation and coating processes and a range of analysis techniques (XPS, TEM and FIB, EDAX and ellipsometry) for process design and optimization. It is shown that process parameters that deliver the highest start-of-life catastrophic optical damage (COD) levels are not necessarily the ones that guarantee high-reliability operation.

The optimized facet design has delivered COD powers in excess of 12 MW/sq.cm and product-grade 980nm single emitters with a slope efficiency of >1 W/A and a peak efficiency of >60%. The devices have accumulated over 1,200 hours of CW operation at 11W with a power degradation <0.1%. Up-to-date reliability data will be presented at the Conference.

7918-41, Session 5

Ultra-high brightness, wavelength-stabilized, kW-class fiber coupled diode laserR. K. Huang, B. Chann, J. D. Glenn, TeraDiode, Inc. (United States)

Abstracts should contain enough detail to clearly convey the approach and the results of the research. Accepted abstracts may be published and made available at the meeting. Please submit only 250-word abstracts for review.

High-power and high-brightness fiber-coupled direct diode lasers offer superior efficiency, price, reliability, and performance for materials processing applications such as keyhole welding and for pumping fiber and solid state lasers. TeraDiode, Inc. has developed extremely high brightness fiber-coupled diode lasers using a novel beam combining and shaping technology pioneered at MIT Lincoln Laboratory. TeraDiode’s ALPHA-19 laser is a fiber-coupled direct diode laser with a power level of 1,040 W from a 200 µm core diameter, 0.18 numerical aperture (NA) output fiber (90% power content). The kW-class ALPHA-19 laser, with a Beam Parameter Product (BPP) of 18 mm-mrad, has demonstrated substantially higher brightness than other commercially available fiber coupled, direct diode lasers. Furthermore, the center wavelength of this laser is wavelength stabilized. ALPHA-19’s center wavelength is 966 nm. The central wavelength remains approximately 966 nm over



the entire power range of the laser up to full power. This wavelength stabilized property is important for fiber and solid state laser pumping applications. We used the ALPHA-19 laser in a demonstration of keyhole welding of stainless steel sheet metal. Extension of this direct-diode laser technology to higher performance will also be discussed.


Short wavelength limitation in high power AlGaInP laser diodesT. Nishida, N. Shimada, T. Ogawa, M. Miyashita, T. Yagi, Mitsubishi Electric Corp. (Japan)

Laser displays have been gathering much attention because of their many advantages. In the wavelength region of red color, luminosity factor rapidly increases as wavelength shortens. Therefore, red LD with wavelength as short as possible is required. Experimental results for short wavelength limitation in AlGaInP LDs were shown and discussed in this paper. 625, 630, and 638 nm broad area LDs with output power exceeding 200 mW CW were successfully fabricated. But P-I characteristics and its temperature dependence of 625 nm LD are extremely inferior to 630 and 638 nm ones. The main reason might be carrier leakage, and the results indicate that an additional countermeasure to carrier leakage should be adopted to realize a 625 nm LD with good temperature characteristics. Conversion efficiency from electrical power input to luminous flux output with the LDs was also studied. 625 nm LD has extremely low efficiency, though brightness of 625 nm light is 1.7 times of 638nm one. And 630 nm LD shows better conversion efficiency at high luminous flux region than 638 nm one, though the P-I characteristics of 630 nm is worse than that of 638 nm one. The tendency is inverted at low flux region, indicating that the lasing wavelength of red LD for laser display should be chosen carefully.


Variation of refractive index step of 635nm ridge waveguide lasers for optimized index guidingD. Feise, G. Blume, C. Kaspari, K. Paschke, G. Erbert, Ferdinand-Braun-Institut (Germany)

Ridge waveguide lasers at 635 nm can be used for a variety of applications which require excellent beam quality. Such applications are for instance: medical imaging, distance measurement equipment, and display applications.

High-brightness ridge waveguide lasers provide an excellent beam quality because the radiation is guided by a refractive index step achieved by etching a ridge of a certain width and height. This refractive index step determines the distribution of the optical mode in the lateral dimension and, therefore, the characteristics of the diode laser.

We will present results from our investigations regarding the impact of the variation of the refractive index step on the beam quality and the optical output power of ridge waveguide lasers at a wavelength of 638 nm. The beam profiles of these lasers were investigated to obtain the beam propagation factors and to observe potential beam steering.

The diode lasers under study are 2 mm long and made from the same epitaxial structure. I-line lithography was used to define the widths of the ridge of 3 and 5 µm. Using reactive ion etching the ridge waveguide was formed with four variations of the effective refractive index step from 0.4 to 5 x 10^-3. The facets were passivated and coated to give a reflectivity of 1% at the front and 96% at the rear. These lasers are capable to provide a maximum optical output power of > 200 mW and have an excellent beam quality of M^2 (1/e^2) = 1.3 and M^2 (2nd order moments) = 2 at 100 mW.


Optical elements for optimal brightness of single emitter devicesO. Homburg, M. Jarczynski, U. Fornahl, T. Mitra, LIMO Lissotschenko Mikrooptik GmbH (Germany)

Due to their increasing power and brightness multi-mode single emitter devices are becoming increasingly widely used for the assembly and packaging of high power diode lasers. Typical emitter widths are 50, 90 (100) and 200 µm with power levels available > 15 W. Also larger stripes are available - up to 1000 µm - with power levels > 25W. For highest power laser devices not only the power of the emitter is important - but of equal importance is the subsequent optics to collect all the emitted power while maintaining the brightness of the source. High NA acylindrical micro-lenses very well account for the strong asymmetric emitter characteristics of the fast and slow axis and thus, result in best collimation and coupling efficiencies in contrast to spherical lenses. LIMO’s cost-effective micro-optics wafer technology is most suited for such acylindrical optics. It allows the manufacture of different materials to cover wavelengths ranges from the UV to the NIR, e.g. 380 - 2000 nm. Since both sides of a wafer can be structured with crossed cylindrical lenses one single monolithic optical element simultaneously shapes the fast and slow axis of the emitted light. Additionally, mechanical reference planes can be integrated in such monolithic optics for precise and simple integration. Apart from rather straight-forward collimation and focussing optics also more complex application-specific concepts are exemplarily demonstrated.


Spectral narrowing of a 980nm tapered diode laser barD. Vijayakumar, O. B. Jensen, P. M. Petersen, Technical Univ. of Denmark (Denmark); G. Lucas-Leclin, Institut d’Optique Graduate School (France); B. Thestrup, Technical Univ. of Denmark (Denmark)

High power diode laser bars are interesting in many applications such as solid state laser pumping, material processing, laser trapping, laser cooling and second harmonic generation. Often, the free running laser arrays emit a broad spectrum of the order of several nanometers which limit their scope in wavelength specific applications and hence, it is vital to stabilize the emission spectrum of these devices. In our experiment, we describe the wavelength narrowing of a 12 element 980 nm tapered diode laser bar using a simple Littman configuration. The tapered laser bar which suffered from a big smile has been “smile corrected” using individual phase masks for each emitter. The external cavity consists of the laser bar, both fast and slow axis micro collimators, smile correcting phase mask, 5X beam expanding lens combination, a 1200 lines/mm reflecting grating with 85% efficiency in the first order, a slow axis focusing cylindrical lens of 40 mm focal length and an output coupler which is 10% reflective. In the free running mode, the laser emission spectrum was 6 nm wide at an operating current of 30A. The output power was measured to be in excess of 12W. Under the external cavity operation, the wavelength spread of the laser could be limited less than 0.1 nm with an output power in excess of 8 W at an operating current of 30A. The spectrum was found to be tunable in the range of 20 nm.




Coherent Polarization Combining of Multiple Spatial Modes Beams in a Diode BarP. Purnawirman, Nanyang Technological Univ. (Singapore); P. B. Phua, Nanyang Technological Univ. (Singapore) and DSO National Labs. (Singapore)

We present the coherent polarization locking of multiple spatial modes emitters in a single diode bar. Active area of individual emitter in a diode bar consists of many spatial modes, which often results in irregular multi peaked pattern in each beam. In spite of the randomly different profiles, our experiment shows that the each emitter can passively adjust its amplitude and phase to achieve the lowest optical loss for the combined beam. The coherent locking method consists of a series of birefringent waveplates and walk-off crystals to spatially combine the emitter beams. Polarizer beam splitter is used to passively filter and ensure the coherent phase of the combined beams. We obtained 6.4 W of coherently locked laser power from four diode emitters under 58 A injection current. The coherently locked beam preserves the spatial profile similar to the single emitter, with M2 values of 1.5 x 12. In comparison with the standard commercial laser of the same material, the brightness is improved over 10 times. Our experiment shows that the multimode coherent polarization combining allows more power extraction from several emitters in diode bar while preserving the beam quality of single emitter.



Conference 7919: Vertical External Cavity Surface Emitting Lasers (VECSELs)Monday-Tuesday 24-25 January 2011 • Part of Proceedings of SPIE Vol. 7919 Vertical External Cavity Surface Emitting Lasers (VECSELs)

7919-01, Session 1

Advances in directly visible VECSELsJ. E. Hastie, Univ. of Strathclyde (United Kingdom); A. B. Krysa, The Univ. of Sheffield (United Kingdom); S. Calvez, M. D. Dawson, Univ. of Strathclyde (United Kingdom)

Vertical External-Cavity Surface-Emitting Lasers (VECSELs), also now commonly known as Semiconductor Disk Lasers (SDLs), deliver high output power with excellent beam quality across a broad spectral range utilising a variety of semiconductor materials. In particular, InGaAs/GaAs devices have been shown to reach output powers of 10s of Watts and may target a wide array of visible applications in the blue to yellow spectral range via efficient intracavity second harmonic generation. The use of dilute nitride material, GaInNAs, can expand the frequency-doubled wavelength coverage further into the orange/red, still with multi-Watt output power. We have, however, focussed on the delivery of direct visible emission from AlGaInP-based VECSELs, utilising either quantum wells or more recently InP quantum dots, to cover the wavelength range 640 - 750 nm. Direct visible emission is attractive for broader spectral coverage, broad tuneability from a single device, frequency doubling to the ultraviolet, and, with the advent of high power GaN laser diodes suitable as pump lasers, the prospect for improved efficiency. We review the design, performance, challenges and future prospects of directly visible VECSELs.

7919-02, Session 1

High power optically pumped VECSELs emitting in 1310 nm and 1550 nm bandsA. Sirbu, A. Mereuta, A. Caliman, E. E. Kapon, Ecole Polytechnique Fédérale de Lausanne (Switzerland); J. Rautiainen, J. Lyytikainen, O. Okhotnikov, Tampere Univ. of Technology (Finland); J. Walczak, T. Czyszanowski, Technical Univ. of Lodz (Poland)

We present InAlGaAs/InP-AlGaAs/GaAs optically pumped vertical external cavity surface emitting lasers (VECSELs) operating in 1310 nm and 1550 nm bands. The gain mirror structures are formed by wafer fusion of InAlGaAs/InP active cavities on AlGaAs/GaAs DBRs. The InAlGaAs/InP active cavities comprise 5, or 3 groups of AlGaInAs QWs positioned at the antinodes of the standing wave of the optical electric field.

The active region and DBR wafers are bonded together at 600°C using the wafer fusion technique. Special care was taken for decreasing the defect formation as a result of different thermal expansion coefficients of GaAs-based and InP- based compounds during cooling down from 600°C to room temperature. Two types of heat dissipation schemes with AlGaAs/GaAs DBR soldered on heat spreader (Type 1) and with additional intra-cavity diamond heat-spreader (Type 2) have been tested in this work. The gain mirrors are optically pumped by fibre coupled 980 nm pump lasers. External mirrors with output coupling of 2-2.5 % and highly reflective curved mirrors complete VECSEL cavities of z-type.

Experimental results and modelling show thermal impedance of Type 1 and Type 2 heat dissipation schemes of 20 K/W and 2 K/W respectively. Maximum CW output of devices with 5 groups of quantum wells and diamond intra-cavity heat spreaders reached the level of 2.5 W in both 1310 nm and 1550 nm bands. Devices with 3 groups of quantum wells show increased CW output as high as 4.8 W.

7919-03, Session 1

High-power 1 25 µm InAs QD VECSEL based on resonant periodic gain structureA. R. Albrecht, T. J. Rotter, C. P. Hains, A. Stintz, The Univ. of New Mexico (United States); T. Wang, The Univ. of Arizona (United States); Y. Kaneda, J. V. Moloney, College of Optical Sciences, The Univ. of Arizona (United States); K. J. Malloy, G. Balakrishnan, The Univ. of New Mexico (United States)

Edge-emitting semiconductor lasers using MBE-grown, self-assembled InAs quantum dots (QD) have resulted in excellent device performance, including low temperature sensitivity and thresholds. Achieving sufficient gain for use in surface-emitting lasers requires stacking of QD layers, deteriorating material quality and device performance due to strain.

We compare a traditional design consisting of 4 groups of 3 closely spaced QD layers placed at adjacent antinodes of the standing wave inside the VECSEL cavity with a design employing a resonant periodic gain (RPG) structure. In this design, each of the 12 QD layers is placed at a separate electrical field antinode, increasing the spacing between the QDs.

Devices were mounted to a temperature controlled heat sink using thermal grease. A fiber-coupled 808 nm diode laser, focused to a 300 µm diameter spot was used as pump source. A 1% transmission output coupler with 25 cm radius of curvature completed the external cavity. CW operation with thermally limited output powers of 400 mW was achieved for the RPG structure, compared to 100 mW for the conventional design. Making use of growth non-uniformity across the 3” GaAs substrate, the emission wavelength could be tuned from 1220 nm to 1280 nm.

For thermal management, the substrate of the RPG sample was thinned to 100 µm and indium bonded to a thermal grade CVD diamond, mounted to a water cooled heat sink. This greatly increased the usable pump power before thermal rollover, resulting in over 3 W cw output power using 1% or 2% transmission output couplers.

7919-04, Session 1

11 W single gain element dilute nitride disk laser emitting at 1180 nmT. Leinonen, V. Korpijärvi, J. Puustinen, Tampere Univ. of Technology (Finland); R. Epstein, Areté Associates (United States); M. Guina, Tampere Univ. of Technology (Finland)

Future earth-based telescopes are expected to surpass the resolution of space telescopes in the visible range with just a fraction of the cost by making use of adaptive optics systems. One of the key elements of such a system is a 589 nm laser that creates a so-called guide star. Currently, the guide stars are generated by using dye lasers, which are hard to maintain, costly, and exhibit poor reliability. Frequency doubled semiconductor disk lasers (SDLs) have the potential to overcome these problems and to provide new features such as pulsed operation. Amber SDLs are expected to have an impact also on medical applications, such as flow cytometry, dermatology, and eye surgery. Owing to recent technological advances, dilute nitride GaInNAs/GaAs SDLs offer a practical solution to access the 1180 nm-1250 nm wavelength range and to produce efficient visible lasers via frequency-doubling.

Here we report the latest advances in the development of high power 1180 nm disk lasers exploiting dilute nitride gain media. A systematic power scaling study was performed as a function of output coupler transmission and of the laser mode size on a single gain chip. With a pump spot diameter of 390 µm and an output coupler of 1.5% we have achieved slightly more than 11 W output power with a slope efficiency


of 25.5%. The maximum slope efficiency was 28%, and was achieved with a pump spot of 320 µm and a 2.5% output coupler. Preliminary experiments led to generation of about 6.2 W of frequency doubled laser radiation.

7919-05, Session 1

Gain coupling VECSELsR. G. Bedford, Air Force Research Lab. (United States); C. Hessenius, College of Optical Sciences, The Univ. of Arizona (United States); N. Terry, Air Force Research Lab. (United States); M. Fallahi, J. V. Moloney, College of Optical Sciences, The Univ. of Arizona (United States)

Vertical external cavity surface emitting lasers (VECSELs) provide a flexible platform in order to explore curious laser designs and systems as their high-power, high-brightness make them attractive for many applications, and their flexibility eases this exploration. In considering the methods of coupling VECSELs as well as their potential uses, we begin by reporting on the development of a gain coupled VECSEL for use in optical switching. In particular, two VECSEL cavities share a common gain region; the competition for a common set of carriers dictate how these cavities interact. The easiest manifestation to realize gain coupling is to utilize a linear cavity as well as a v-cavity, built around a single half-VCSEL chip. The cavity gain/loss of each cavity can be controlled independently through use of birefringent filters, allowing us to explore the design space, which can be divided up into coarse behavior, easy to analyze through comparing the two uncoupled lasers, and a fine behavior, where one cavity will affect the other and each cavity can laser simultaneously, sometimes at dramatically different wavelengths. These two regions may be explained with simple rate equations, and it will be shown that if prepared properly, spontaneous emission plays a large role in balancing out the two laser cavities within the fine regime, while may be completely neglected in the coarse regime.

7919-06, Session 2

GaSb-based semiconductor disk lasers: versatile lasers for the 2-3 µm wavelength rangeM. Rattunde, B. Roesener, S. Kaspar, C. Manz, K. Koehler, J. Wagner, Fraunhofer-Institut für Angewandte Festkörperphysik (Germany)

Optically-pumped semiconductor disk lasers (OPSDLs) have attracted increasing interest due to their capability of delivering simultaneously high output-power and excellent beam quality [1]. Recently, the wavelength coverage of OPSDL has been extended to above 2µm by exploring OPSDL structures based on the (AlGaIn)(AsSb) material system [2]. In this contribution we report on the development of GaSb-based OPSDLs emitting in the 1.9-2.8µm wavelength range.

These long-wavelength OPSDL deliver multiple-Watt CW output power near room temperature (e.g. 3W at 20°C and over 4W at 0°C heatsink temperature at 2.0µm emission wavelength), using commercial 980nm laser diodes for barrier pumping. The optical quantum efficiency reaches very high values of 44 % at 20°C and 49 % at 0°C, values that are already close to that of state of the art GaAs-based disk lasers emitting at around 1µm wavelength [3,4]. Further power scaling can be achieved by operating multiple gain-elements in a single cavity [5].

Another advantage of OPSDL is the flexibility provided by the external cavity concept. Using a birefringent filter inside a V-shaped cavity, a tunable OPSDL was demonstrated operating at a single cavity mode with a linewidth below 2.3MHz (limited by the resolution of the measurement setup) and an output power exceeding 200mW. Actively stabilizing the laser cavity using locking techniques, a linewidth below 400kHz was achieved, with room for further improvement. This demonstrates the great potential of GaSb-based OPSDL to serve as compact and powerful

narrow-linewidth seed lasers for solid state lasers or amplifiers operating in the 2-3µm range.

References

[1] M. Kuznetsov et al., IEEE J. Sel. Topics In Quantum Electron. 5, pp. 561-573 (1999).

[2] N. Schulz et al. Laser & Photon. Rev., DOI 10.1002/lpor.200710037 (2008)

[3] J. Chilla et al., Proc. SPIE 5332, 143 (2004)

[4] B. Rudin et al., Opt. Lett. 33, 2719 (2008)

[5] B. Rösener et al., IEEE Photon. Technol. Lett. 21, 848 (2009)

7919-07, Session 2

Recent progress on high power optically pumped semiconductor lasersJ. L. Chilla, Coherent, Inc. (United States)

Optically pumped semiconductor lasers offer significant advantages with respect to all traditional diode-pumped solid state lasers (including fiber lasers) in regards to wavelength flexibility, broad pump tolerance, efficient spectral and spatial brightness conversion and high power scaling. For a commercial laser maker like Coherent, these technical advantages translate into the possibility of creating products that are used in diverse applications ranging from entertainment to medicine. Recent work has concentrated on multi-Watt visible lasers for scientific applications with very demanding output requirements. We have developed a family of lasers operating at different wavelengths (532 nm, 488 nm, 460 nm, etc) and power levels (up to 7 W in green). These are TEM00 lasers with stable beam parameters at all power levels. Besides wavelength and power, the single most important requirement for scientific application is low noise. In particular, 532 nm pump lasers for ultrafast Ti:Sapphire lasers with Carrier Envelope Phase Stabilization (CEP) are one of the most challenging applications, a benchmark to judge our laser’s performance. In these ultra short pulses, made up of a small number of cycles, the relative phase of the electric field oscillating at the optical frequency is locked to the envelope of the pulse. Fluctuation on the pump power as small as 0.01 % RMS translates into phase noise that cannot be removed by the feedback electronics. We have demonstrated successful CEP lock using our new OPS pump lasers on several different set ups. A level of performance on par to previous DPSS lasers from Coherent that -to our knowledge- is not achieved by lasers from our competitors.

7919-08, Session 2

Lateral lasing and ASE reduction in VECSELsC. Hessenius, M. Fallahi, J. V. Moloney, College of Optical Sciences, The Univ. of Arizona (United States); R. G. Bedford, Air Force Research Lab. (United States)

Vertical external cavity surface emitting lasers (VECSELs) are attractive for many applications due to their high-power, high-brightness outputs. In order to power scale the devices, the pump spot size should be increased. However, the large pump area greatly amplifies the guided spontaneous emission in the epitaxial plane. In order to efficiently power scale the devices, amplified spontaneous emission (ASE) and lateral lasing must be reduced. We first begin by reporting on the temperature dependence of lateral lasing. Lateral lasing and ASE are greatly dependent on the operating temperature and the pump power as the quantum well gain and bandgap are functions of temperature. In order to reduce the ASE and lateral lasing we introduce a loss mechanism outside of the pumped section by depositing a low bandgap material such as Ge on the facets of the chip. High index of refraction of the film helps reduce Fresnel reflection and the absorption in the emission range. We observed significantly reduced lateral lasing and ASE degradation. Higher slope efficiency and higher output power is achieved for various spots and operating temperatures. Detailed results will be presented.

Conference 7919: Vertical External Cavity Surface Emitting Lasers (VECSELs)


7919-09, Session 2

2 W cw OPO in mid-IR pumped by OPSL laser intra-cavity radiationA. Caprara, Coherent, Inc. (United States)

Optically pumped semiconductor lasers (OPSL) are particularly suitable for intra-cavity non-linear frequency generation. Multi-Watt second harmonic, and sub-Watt third harmonic OPSL sources are commercially available in the visible and ultra-violet spectrum. We present work extending the OPSL operation to the mid infra-red by means of an intra-cavity pumped optical parametric oscillator. An output power of 2 Watts at 3.47 microns is reported.

7919-10, Session 3

Wavelength tunable red AlGaInP-VECSEL emitting at around 660 nmT. Schwarzbäck, H. Kahle, M. Eichfelder, W. Schulz, R. K. Rossbach, M. Jetter, P. Michler, Univ. Stuttgart (Germany)

Vertical external cavity surface-emitting lasers (VECSELs) have emerged recently as an important category of power-scalable semiconductor lasers in a wide range of applications as in biophotonics, television or projectors, spectroscopy and lithography. With usage of external cavities and optical excitation, VECSELs arise with high continuous-wave output power and near-diffraction-limited beam quality with a TEM00 Gaussian beam profile.

We present a non-resonantly pumped red-emitting VECSEL system based on a multi-quantum-well structure with 20 compressively-strained GaInP quantum wells (QWs) for an operation wavelength of around 660 nm. Five QW packages are placed in (Al0.55Ga0.45)0.51In0.49P cladding layers in a resonant periodic gain design. Each package consists of four QWs embedded in (Al0.33Ga0.67)0.51In0.49P barriers, respectively. The 3 cavity is fabricated on a 55 /4 pairs Al0.50Ga0.50As/AlAs distributed Bragg reflector.

By bonding an intra-cavity diamond heatspreader to the chip, continuous-wave operation exceeding 700 mW output power at a wavelength of 662 nm with a low threshold power of 0.8 W was achieved. Using a birefringent filter in a folded cavity, a tuning range of around 19 nm could be shown. With this method wavelengths down to 650 nm were observed. Utilizing a non-linear crystal for intra-cavity frequency doubling, coherent light at 325 nm could be produced matching the HeCd laser line. Additionally, to expand the emission range, InP/AlGaInP quantum dots are currently under investigation for usage as active material.

7919-11, Session 3

Blue light source based on spectrally stabilized external dual grating reflector coupled surface emitter arrayY. O. Yilmaz, O. V. Smolski, V. O. Smolski, E. G. Johnson, The Univ. of North Carolina at Charlotte (United States)

Second harmonic generation (SHG) from Near-IR diode lasers is an attractive solution for blue-light sources with high power and narrow linewidth. IR sources based on broad stripe high power devices with narrow linewidths makes it possible to achieve a wide range of wavelengths throughout the blue region. This paper summarizes recent results utilizing a configuration of external dual grating reflector coupled surface emitting laser array for blue light generation. The emitters are wavelength locked by means of a diffraction grating array in Littrow arrangement. The diffraction gratings are etched into a single Silicon substrate in an array configuration. The gratings are coated with Cr-Au for increased reflectivity. The reflectivity of the gratings as a function of

grating depth is numerically calculated using FDFD and compared with the experimental values. The output from each emitter is focused into a multi-channel Mg:O-PPLN crystal. Each emitter in the array is locked to a different wavelength, which are matched to the individual channels of the nonlinear crystal. SHG conversion efficiencies on the order of 10-3 W-1 are measured from each channel.

7919-13, Session 3

Recent advances in VECSELs for laser projection applicationsH. H. Lindberg, S. Illek, I. Pietzonka, M. Furitsch, A. Plössl, S. Haupt, M. Kuehnelt, R. Schulz, T. Hoefer, U. Strauss, OSRAM Opto Semiconductors GmbH (Germany)

The vertical external cavity surface emitting laser (VECSEL) is a relatively new member of the semiconductor laser family. One major advantage of the technology is the relatively high output power combined with an excellent beam quality. This, in combination with the external cavity concept, enables efficient intracavity frequency doubling from infrared into visible wavelengths. Building on the VECSEL technology OSRAM Opto Semiconductors has in the last years developed and commercialized efficient green laser sources for use in laser projectors on which the latest results will be reported.

A key characteristic of the VECSEL is also the output power scalability. This relies on the thermal properties and geometry of the VECSEL chip where there exist two main concepts: The thinfilm variant where the epitaxial layers are transferred to a heat sink and the monolithic chip where the epitaxial layers remain on their growth substrate. Whereas the former offers significant advantages in terms of maximum achievable output power, it is usually fabricated in a single chip manner and thus not suitable for a large scale production. The monolithic variant, on the other hand, offers wafer level processing but has thermal limitations due to the poor thermal properties of common growth substrates. Building on the thinfilm LED technology we will demonstrate a wafer level approach of the thinfilm VECSEL concept, which offers a large scale production of thinfilm VECSEL chips. This technology, in which a 1060nm InGaAs based quantum well structure and an AlGaAs Bragg mirror is transferred to silicon heat sinks, and its challenges, will be presented alongside experimental results.

7919-14, Session 4

Properties of (GaIn)As-based near-infrared VECSEL grown by low-temperature metal organic vapour phase epitaxy processW. Stolz, Philipps-Univ. Marburg (Germany)

The application of a specific metal organic vapour phase epitaxy (MOVPE) process for (GaIn)As-based VECSEL using less toxic and thermally more efficiently decomposing MO-group-V-sources like tertiary butyl arsine (TBAs) and tertiary butyl phosphine (TBP) results in an extended emission wavelength range and facilitates the necessary strain compensation of the highly compressive-strained (GaIn)As-quantum well layers by specific tensile-strained Ga(PAs) barrier layers. Applying a closed-loop-design concept of detailed microscopic modelling and experimental realization as well as laser characterization allows for an efficient optimization of these complex laser devices. Specific care is devoted to efficient heat spreading and heat sinking of these high-power VECSEL structures. The achieved status of the laser properties, i.e. reaching continuous wave output powers in excess of 60 W at 1040nm wavelength, will be presented and discussed.



7919-15, Session 4

MBE growth challenges of quantum dot saturable absorbers integrated into a MIXSELM. C. Golling, Y. Barbarin, T. Südmeyer, U. Keller, ETH Zurich (Switzerland)

More recently we have achieved wafer integration of gain and saturable absorber with the modelocked integrated external-cavity surface emitting laser (MIXSEL). Here we discuss in more details the MBE growth challenges we had to solve.

From a cavity design point of view, one important degree of freedom is lost when integrating gain and absorber within one wafer. The MIXSEL semiconductor structure is only about 10 µm thick which is well within the confocal parameter length of a gigahertz laser cavity. Thus, we have the same laser beam diameter in the gain and the absorber, but we also need to have a faster absorber than gain saturation for stable modelocking. Initially we resolved this challenge with a higher laser field enhancement in the absorber layer with an additional intermediate distributed Bragg reflector (DBR). The result was a serious trade-off in growth tolerances. Recently we have resolved this issue with new quantum dot saturable absorbers with lower saturation fluence. This made an antiresonant MIXSEL possible with much better MBE growth tolerances and lower dispersion. Every growth run then resulted in working MIXSELs with chips from all over the wafer.

Another challenge has been the growth temperature for the different sections within the MIXSEL. The MBE growth temperature for the DBRs is ≈600°C, for the active quantum well region ≈520°C and for the optimized quantum dot saturable absorber ≈400°C. When subsequent layers are grown, the absorber becomes annealed and a bandgap blue-shift occurs. This had to be taken into account as well.

7919-16, Session 4

Lattice mismatched growth for mid-IR VECSELsG. Balakrishnan, The Univ. of New Mexico (United States)

We present epitaxial techniques for the growth of optically pumped vertical-external-cavity surface-emitting lasers (VECSEL) with high output powers at 2 - 3.5 µm emission wavelength through the integration of III-Sb active regions with AlGaAs Distributed Bragg Reflectors (DBRs). The novelty in this approach is the combination of a GaSb based active region with a GaAs based DBR, both of which are monolithically grown on a GaAs substrate. The strain due to the lattice mismatch between the GaSb-based active region (lattice constant close to 6.1 Å) and the GaAs-based DBR (lattice constant close to 5.65 Å) is relieved by the formation of an interfacial misfit (IMF) array at the GaSb/GaAs interface.

We shall discuss the technical challenges related to the MBE growth of such structures. First, the IMF based growth of III-Sb on GaAs leads to differences in strain and relaxation of the epilayer as compared to growth on GaSb substrates. Second, the transfer of the IMF growth technology from growth on GaAs substrates to growth on the MOCVD grown AlGaAs/GaAs DBR is nontrivial. The challenges include the IMF formation on the DBR surface, which is slightly rougher than a GaAs substrate’s surface, and the growth temperature control, since the DBR’s heat conductivity differs from that of a GaAs substrate. We shall also discuss the development of longer wavelength VECSELs in the MWIR (2.5 to 3.5 µm). This discussion will include active region development, digital-alloy based quintinary barrier development and DBR performance at these wavelengths.

Collaborators include T.J. Rotter, A. Albrecht, C.P. Hains, P. Ahirwar, L.R. Dawson, J.V. Moloney, J. M. Yarborough, Yi-Ying Lai, Yushi Kaneda, Jörg Hader, S. W. Koch and R. Bedford.

7919-17, Session 4

Recent advances in electrically pumped VECSELs for modelockingY. Barbarin, ETH Zurich (Switzerland)

Vertical external cavity surface emitting lasers (VECSELs) are excellent high power semiconductor lasers with diffraction limited circular output beam and outstanding modelocking performance even at tens of GHz repetition rate. The output power can be scaled up by simply increasing the mode area on the gain region. It makes them very attractive for numerous applications such as RGB displays, biomedical imaging or optical clocking of multi-core processors. Passively modelocked optically pumped VECSELs, using a semiconductor saturable absorber mirror (SESAM), have generated shorter pulses and higher average powers than any other modelocked semiconductor laser (135-fs pulses at 35-mW average power and 2.1-W in 4.7-ps pulses).

Electrical pumping (EP) of modelocked VECSELs is the obvious next step towards compact high-power ultrafast laser sources. In 2003, Novalux Corporation reported a continuous-wave (cw) output power of nearly one Watt from their proprietary EP-VECSEL (NECSEL). The modelocking of a NECSEL has been demonstrated with 40 mW of average power in 57-ps pulses. Since then, very few EP-VECSEL results have been reported. Recently, we started to develop EP-VECSELs designed for modelocking, which require an optimized balance between electrical resistance, optical losses, dispersion and beam quality. We discuss our design approach and present initial EP-VECSEL devices generating >100-mW cw power. Homogeneous current injection is achieved even for large devices, showing very good agreement with our numerical simulations. Sufficient power in a diffraction-limited beam and a carefully designed SESAM are required to modelock an EP-VECSEL.

7919-18, Session 4

Beam quality optimization of electrically pumped VECSELs for passive modelockingW. P. Pallmann, M. Hoffmann, ETH Zurich (Switzerland); M. Miller, Philips Technologie GmbH U-L-M Photonics (Germany); J. Baier, Philips GmbH (Germany); H. Moench, Philips Research (Germany); I. Dahhan, B. Witzigmann, Univ. Kassel (Germany); M. C. Golling, Y. Barbarin, T. Südmeyer, U. Keller, ETH Zurich (Switzerland)

Vertical external cavity surface emitting lasers (VECSELs) offer diffraction-limited circular output beams and high power levels. Modelocked VECSELs using semiconductor saturable absorber mirrors (SESAMs) achieve short pulses and high repetition rates. To date modelocked VECSELs have been mostly optically pumped, however, electrical pumping is advantageous for reduced packaging requirements.

A major challenge for ultrafast electrically pumped VECSELs (EP-VECSELs) is the balance between the competing requirements of low electrical resistance, low optical losses and controlled dispersion while maintaining fundamental mode operation for stable modelocking. To compensate for the optical losses in the doped structure, it is necessary to enhance the field in the quantum wells for more gain using an intermediate DBR. A higher reflectivity of this DBR yields higher gain, but also reduces the mode control of the external curved output coupler and therefore leads to deteriorated beam quality.

We present an experimental study on the influence of the intermediate DBR reflectivity on the beam quality and output power of EP-VECSELs. Three different designs with internal reflectivities of 90%, 82% and 71% were characterized in a straight cavity using different output couplers (OCs). The best combinations found for the devices with intermediate DBR reflectivities of 90%, 82% and 71% were 10% OC, M²=2.6, 34 mW; 5% OC, M²=1.1, 15.1 mW and 2.5% OC, M²=1.0, 5 mW, respectively. We can demonstrate that a correctly balanced field enhancement and output coupling is necessary to achieve both good beam quality and high output power, and a trade-off in power has to be accepted.



7919-19, Session 4

High power Bessel beams from EP-VECSELsG. S. Sokolovskii, Ioffe Physico-Technical Institute (Russian Federation) and Univ. of Dundee (United Kingdom); S. A. Zolotovskaya, Univ. of Dundee (United Kingdom); S. N. Losev, V. V. Dudelev, A. G. Deryagin, V. I. Kuchinskii, Ioffe Physico-Technical Institute (Russian Federation); W. Sibbett, Univ. of St. Andrews (United Kingdom); E. U. Rafailov, Univ. of Dundee (United Kingdom)

Non-diffracting light fields such as Bessel beams are of interest for a number of applications such as optical trapping and tweezing or manipulation of micromachines. To date, Bessel beams have generally been produced by reconfiguring the output beams from vibronic lasers, but many practical applications require sources to be compact and efficient. For long time, it was believed that the main condition for generation of non-diffracting light fields is the coherence of light but recently we have shown that non-diffracting beams can be formed from low-coherent light sources, including high-power laser diodes and light-emitting diodes and spatial coherence rather than the temporal one plays the leading role in generation of Bessel beams. In our experiments, it was shown that in the case of poor laser beam quality the propagation length of the Bessel beam can be limited by the divergence of the constituting quasi-Gaussian beam rather than the axicon aperture and apex angle. In this sense, generation of Bessel beams from the broad-area electrically-pumped vertical external cavity surface-emitting lasers (EP-VECSELs), offering watt power levels with excellent beam quality could be the best opportunity for replacement of gas and solid-state lasers for more power-demanding applications in optical manipulation. In our experiments, we have demonstrated output powers of sub-watt level and central lobe diameters of 10-100 um with propagation lengths up to few tens of centimeters. To our knowledge, this is the best result for Bessel beams generated from semiconductor light sources and is comparable to that achievable from vibronic lasers.

7919-20, Session 4

Design and characterization of electrically pumped vertical external cavity surface emitting lasersJ. R. Orchard, D. M. Williams, D. T. D. Childs, L. C. Lin, B. J. Stevens, J. S. Roberts, R. A. Hogg, The Univ. of Sheffield (United Kingdom)

Whilst optically pumped vertical external cavity surface emitting lasers (VECSELs) offer the ultimate in freedom in laser design, electrically pumped versions offer the ultimate in compactness and cost effectiveness. This is at the expense of sacrifices made due to the need for current injection schemes. By contrast to conventional edge emitting laser characterization, where the derivation of gain-current density characteristics is trivial, the vertical cavity and dual cavity nature of EP-VECSELs presents new challenges. Furthermore, for mode-locking applications it is desirable to have no intra-cavity mirror in the semiconductor chip which places stringent requirements on device design.

In this paper we report on the design, epitaxial growth, device fabrication and characterization of continuous wave, room temperature operating EP-VECSELs. Our design for 980nm emission incorporates substrate emission for current spreading, and the use of 6 partially strain balanced QWs (GaAsP barriers, InGaAs wells). Strain balancing allows for future scalability in gain to allow the elimination of intra-cavity mirrors. The effect of the efficiency of current spreading on power scaling is discussed.

In addition to discussing device design trade-offs in our EP-VECSELS, we describe the characterization of the material via both the analysis of edge emitting lasers of different length and EP-VECSELs as a function of output coupler mirror. This allows insight into the fundamental parameters and characteristics necessary for further device optimisation.

7919-21, Session 4

Power and brightness scaling in large aperture semiconductor lasersJ. McInerney, Univ. College Cork (Ireland)

Invited paper - to be provided later

7919-22, Session 5

Design and optimisation of VECSELs for the IR and mid-IRJ. Hader, Nonlinear Control Strategies Inc. (United States) and College of Optical Sciences, The Univ. of Arizona (United States)

Vertical-external-cavity surface-emitting lasers (VECSELs) pose strong challenges to design, analysis and optimisation. The intrinsic temperatures and carrier densities vary strongly over the operating range leading to significant changes in the underlying material properties. A thorough understanding of these properties, like, optical gain and carrier losses, is required for successful design and optimisation. Simple, non-destructive analysis using surface-luminescence and reflectivity measurement is hampered by cavity effects.

We will show how modern modelling tools based on fully microscopic many-body theory can be used to solve these challenges for the example of VECSELs for the IR and mid-IR and discuss optimisation strategies.

Of particular importance at longer wavelengths is the possible suppression of the dominating carrier losses due to Auger processes. These losses not only reduce the carrier density, but also introduce heat which, in turn, leads to reduced gain and requires even higher densities - eventually leading to thermal roll-over. We will show how modifications in the well design and variations of the substrate can reduce the Auger losses while remaining good optical gain and leading to significantly improved device performance.

Optimized performance also requires highly efficient pump conversion. Pump light energy that is not converted into lasing light leads to heating which strongly reduces the efficiency as well as the maximum achievable power. We show how modifications of the heterostructure design as well as the use of metallization layers and optical coatings could be used to improve the pump efficiency.

Work done in collaboration with Jerome V. Moloney and Stepahn W. Koch.

7919-23, Session 5

Quantum design and nonequilibrium effects in VECSELsS. W. Koch, Philipps-Univ. Marburg (Germany)

Based on the semiconductor Bloch and the semiconductor luminescence equations, we compute absorption/gain, spontaneous emission, as well as the radiative and non-radiative (Auger) losses using bandstructures obtained from a multi-band kp theory. Only structural layout and material parameters are required as input.

In this talk, it is shown how the theory can be used for the quantum design of VECSELs. The importance of nonequilibrium effects is emphasized and it is discussed how kinetic hole burning allows for the cw operation of more than a single mode.

This work reviews the collaborative efforts of the groups at the College of Optical Sciences, University of Arizona, Tucson/AZ, NLCSTR, Tucson/AZ, and the Physics Department, Phlipps Universität Marburg/Germany. The contributors include Ada Bäumner, Christina Bückers, Mahmoud Fallahi, Jörg Hader, Yushi Kaneda, Bernardette Kunert, Jerome V. Moloney, Wolfgang Stolz (in alphabetical order).



7919-24, Session 5

Design and simulation of electrically pumped mode-locked VECSELsB. Witzigmann, Univ. Kassel (Germany)

We report on the physics-based design of an electrically pumped vertical external cavity surface emitting laser suitable for passive mode-locking. Using a fully coupled, two-dimensional electro-opto-thermal model, a design is presented that balances optical losses, current injection efficiency, self-heating and mode-locking capability for optimum operation. While optically pumped VECSELs show excellent power-scaling by increasing the device radius, electrical pumping introduces additional constraints. We discuss those and propose designs for maximum single mode output power. Comparison to experimental results will be given.

In order to assess the dynamic mode-locking properties of VECSELs, a novel frequency-domain based method is proposed and compared in detail to time-domain based methods.

7919-25, Session 5

Scaling high-power ultrafast VECSELs into the femtosecond regimeO. D. Sieber, M. Hoffmann, V. J. Wittwer, W. P. Pallmann, Y. Barbarin, M. C. Golling, T. Südmeyer, U. Keller, ETH Zurich (Switzerland)

Passively modelocked Vertical External Cavity Surface Emitting Lasers (VECSELs) or Modelocked Integrated eXternal-cavity Surface-Emitting Lasers (MIXSELs) using semiconductor saturable absorbers have achieved multi-Watt average output power in the picosecond regime. Femtosecond pulse generation, however, has been power limited well below 1 W. Continuum generation applications such as a simple white light sources for illumination in laparoscopy surgery or gigahertz frequency combs would benefit from a higher peak power at low cost.

To explore the limitations in the femtosecond domain, a quantitative understanding of the pulse formation processes is required. Our simulations are based on numerical iterations of a circulating pulse inside a VECSEL cavity. Simulations showed a good qualitative agreement with experimental results in the picosecond regime. By minimizing intracavity group delay dispersion (GDD) and improving gain bandwidth and SESAM parameters, our model predicts pulses as short as 750 fs.

In practice, the intracavity GDD is minimized with a top coating on the VECSEL and the SESAM. The design of the top coating consists of 6 AlAs/AlGaAs pairs and a fused silica layer on top. The thicknesses are optimized using a Monte Carlo algorithm. This provides a flat GDD with values between ±10 fs2 over a range of 30-nm around the design wavelength. Compared to an anti-resonant design these new structures show significantly improved GDD flatness with an even higher field enhancement factor. We expect femtosecond pulses from such ultrafast VECSELs.

7919-26, Session 5

Numerical modelling of optical Stark effect saturable absorbers in mode-locked femtosecond VECSELsA. H. Quarterman, G. J. Daniell, S. Carswell, K. G. Wilcox, Z. Mihoubi, A. L. Chung, V. Apostolopoulos, A. C. Tropper, Univ. of Southampton (United Kingdom)

Quasi-soliton modelocking has been identified as the mechanism responsible for the formation of picosecond pulses in passively mode-locked VECSELs, but neither this mechanism nor Kerr lens modelocking can account for the formation of sub-picosecond pulses from these

lasers. It has been suggested that these pulses are shaped by the optical Stark effect. Numerical simulations have shown that the optical Stark effect is capable of shortening pulses in the absence of bleaching, but to date no studies have been performed under realistic operating conditions.

We model the interaction of an optical pulse with an absorbing quantum well using a semi-classical two level atom approximation. As the bandwidth of a VECSEL pulse is small compared to the spread of energies within a semiconductor band the population of two level atoms is divided into “live” atoms which interact with the optical field, and “dead” atoms which do not. Live and dead states are coupled by carrier-carrier scattering. Populations are calculated numerically, allowing changes in the pulse duration and energy to be evaluated.

Results from this model show an increase in pulse shortening above that due to saturable absorber bleaching alone. This increase occurs at pulse durations below one picosecond, implying that this effect is responsible for the formation of femtosecond pulses. At these pulse durations the model predicts that the absorbing resonance broadens and decreases in amplitude. This is recognisable as a result of the optical Stark effect, indicating that this mechanism is responsible for the formation of femtosecond pulses in VECSELs.

7919-27, Session 6

Power scaling of the MIXSEL: an integrated picosecond semiconductor laser with >6 W average powerT. Südmeyer, ETH Zurich (Switzerland)

High power picosecond lasers are important for numerous applications. However, so far, multi-watt power levels required ion-doped dielectric laser materials in combination with additional intra-cavity components for pulse formation, resulting in high complexity and costs. Modelocked semiconductor lasers have the potential for cost-efficient production, enabling mass applications such as biomedical imaging or optical clocking. Vertical external cavity surface emitting lasers (VECSELs) are well suited for high power levels, because the pulses propagate mostly in an external cavity and experience only low dispersion and nonlinearities from the vertical propagation through epitaxial semiconductor layers of a few µm thickness. Passive modelocking with a semiconductor saturable absorber mirror (SESAM) enabled sub-100 fs pulses and picosecond pulses with up to 2.1 W average power. In 2007, we demonstrated the integration of both elements into a single structure, the MIXSEL (modelocked integrated external-cavity surface emitting laser). However, insufficient heat management limited the first MIXSEL to 200 mW of average output power even at -50°C heat sink temperature. Furthermore, its design relied on a strong field enhancement in the absorber section for achieving stable modelocking, which required extremely high growth accuracy (better 1%). Here, we discuss an improved MIXSEL design based on a low-saturation fluence quantum dot (QD) layer in an anti-resonant structure, which substantially improves growth tolerances. The 8-µm structure is directly mounted onto a diamond heat sink. The simple straight cavity with only two intracavity components generates 28-ps pulses with 6.4 W average power, which is higher than for any other modelocked semiconductor laser.

7919-28, Session 6

Power scaling of cw and pulsed IR and mid-IR VECSELsJ. V. Moloney, College of Optical Sciences, The Univ. of Arizona (United States)

High-power operation of VECSELs/OPSLs relies on proper wafer design, precise wafer growth, and efficient system heat removal. We will discuss the development of high power VECSEL sources around 1040 nm and 2 micron based on III-As and III-Sb material growth. The 2 micron mid-IR VECSEL uses a novel lattice mismatched growth where the antimonide



RPG structure consisting of InGaSb QWs and AlGaSb barriers is grown on an AlAs/GaAs DBR and GaAs substrate.

An uncoated OPSL device which has a significant micro-cavity effect that enhances the effective gain was pumped by an 808 nm diode laser beam with 810 µm diameter pump spot. Good agreement is demonstrated between the present experimental results and theory, supporting precise growth of the quantum wells and the entire resonant periodic gain (RPG) active structure. We observe up to 64 W of continuous wave output from a single OPSL device and a pulsed peak power of 275W when pumped with an Alexandrite 755nm with pulsewidth of 800ns.

The 2 micron VECSEL was pulsed pumped with a 1064nm YAG oscillator and amplifier chain system. Optimal performance was achieved with pulsed pump durations on the order of 100 ns and we observed peak powers of up to 340W. We have designed and grown lower barrier InGaSb/AlGaSb RPG structures that can be pumped at longer wavelength (1320nm YAG and 1470nm diode bar) to reduce the quantum defect for barrier pumping and additionally, suppress Auger losses. We will report on a comparative study of high-barrier and low-barrier pumped 2 micron VECSELs under both CW and pulsed conditions.

This work was performed as part of a JTO MRI project in collaboration with T.-L. Wang, Y. Kaneda, J. M. Yarborough, Jörg Hader, S.W Koch, B. Kunert, W. Stolz, R. Pandey, J. Alexander, Yi-Ying Lai, T. J Rotter, G. Balakrishnan, C. Hains and R. Bedford

7919-29, Session 6

QD-based saturable absorbers for ultrafast lasersE. U. Rafailov, S. A. Zolotovskaya, M. Butkus, Univ. of Dundee (United Kingdom)

Novel materials, notably quantum-dot (QD) semiconductor structures offer the unique possibility of combining exploitable spectral broadening of both gain and absorption with ultrafast carrier dynamic properties. Thanks to these characteristics QD-based devices have enhanced the properties of ultrashort pulse lasers and opened up new possibilities in ultrafast science and technology. In this paper we review the recent progress on the development of novel quantum-dot SESAM structures for different lasers. We also demonstrate that QD structures can be designed to provide compact and efficient ultrashort pulse laser sources with high and low repetition rates.

7919-30, Session 7

Tailoring the wavelength of semiconductor disk lasersO. G. Okhotnikov, Tampere Univ. of Technology (Finland)

A wide flexibility in operation wavelengths can be achieved with semiconductor disk lasers based on quantum well material by band-gap engineering. The wavelength tailoring with this quantum-confined system, however, still suffers from certain constrains. The operation of disk lasers at specific wavelengths is hard to achieve due to the shortage of suitable semiconductor compounds and obstacles associated with monolithic growth of high-reflection distributed Bragg reflector and gain region. The increased lattice mismatch results in critical growth of high-quality materials due to high strain.

This talk presents two approaches used to tailor operation wavelength of semiconductor disk lasers. One utilizes the quantum dot media which allow expanding further the wavelength coverage of semiconductor disk lasers due to alleviated strain impact to the epitaxial structure. The broad spectral tuning range and improved temperature stability of quantum dots suggest the promising potential of these media for wavelength scaling of disk lasers.

Another technique appeals to wafer fusion technology used for combining disparate semiconductor materials, e.g. GaAs and InP, which cannot be grown monolithically. The presentation describes recent

developments in high-power continuous-wave and mode-locked disk lasers using these concepts, with a particular focus on the scaling to long-wavelengths.

7919-31, Session 7

VECSEL technology for modelocking at 1 55 µmS. Bouchoule, A. Khadour, Z. Zhao, Ctr. National de la Recherche Scientifique (France); J. Decobert, Alcatel-Thales III-V Lab. (France); J. Harmand, J. Oudar, Ctr. National de la Recherche Scientifique (France)

Low-timing jitter, short pulse sources at 1.55 µm are required in optical clock recovery or sampling systems, to act as clock or gate. Mode-locked vertical-extended-cavity-surface emitting lasers (ML-VECSEL) are promising candidates for the generation of stable short pulses, owing to the small thickness of the active semiconductor medium and the high finesse of the cavity. Unfortunately, a high intra-cavity power is generally required to achieve an efficient saturation of the fast SESAM, and the poor thermal behavior of quaternary InP-based semiconductor compounds often limits the performance of ML-VECSELs operating at 1.55 µm. Very efficient heat dissipation has been demonstrated for optically pumped VECSELs with a top-mounted intra-cavity diamond heat spreader. In another approach optimizing downward heat sinking, we have developed a ½-VCSEL chip with a low thermal resistance using a hybrid metal-metamorphic GaAs/AlAs mirror and bonded onto a highly thermally conductive host substrate. A quasi 1D vertical heat dissipation regime suitable for power scaling is achieved using CVD diamond. The thermally-optimized ½-VCSEL chip assembled in a 4-mirror cavity with a 1.55µm fast InGaAsN/GaAsN SESAM generates nearly Fourrier transform-limited mode-locked pulses at an operation temperature of 25 °C. The RF linewidth of the free running laser (repetition frequency of ~ 2 GHz) is measured to be less than 1000 Hz, indicating that the 1.55 µm ML-VECSEL has the potential of producing pulse train with an intrinsic low timing-jitter. A practical advantage of the downward heat sinking approach is to be compatible with surface post-processing of the ½-VCSEL structure and therefore compatible with optical and electrical pumping.

7919-32, Session 7

Timing jitter characterization of a quantum dot SESAM modelocked VECSELV. J. Wittwer, W. P. Pallmann, A. E. H. Oehler, B. Rudin, M. C. Golling, Y. Barbarin, T. Südmeyer, U. Keller, ETH Zurich (Switzerland)

We present timing jitter measurements of a free-running SESAM modelocked VECSEL. For VECSELs we would expect much better noise performance compared to edge-emitting semiconductor lasers because the interaction length with the quantum well gain is very short and the output coupling is very low, which results in a high-Q cavity, and therefore small saturated gain resulting in low noise. We have indeed observed very good performance comparable to ion-doped solid-state lasers which typically show excellent stability.

Our laser is an InGaAs quantum well VECSEL operating at 955 nm passively modelocked with a quantum dot SESAM generating 7-ps pulses with 1.88-GHz repetition rate and 80-mW average output power. The VECSEL is optically pumped perpendicular to the surface using an 808-nm fiber coupled multimode diode laser. The gain structure is located in the middle of the Z-shaped cavity to reduce multiple pulsing instabilities due to the limited gain lifetime at these low pulse repetition rates. For good mechanical stability all optical elements including the pump diode are mounted inside a metallic housing to shield against air currents and vibrations. This laser is free-running without any active stabilization. We measured the two-sided noise power spectral density at the 10th harmonic of the laser output with the von der Linde method. The



rms timing jitter integrated over an offset frequency range from 100 Hz to 100 kHz gives a free-running timing jitter of ≈400 fs which is an upper limit because the measurement was already system noise limited above 10 kHz.

7919-33, Session 7

All quantum dot based femtosecond VECSELM. Hoffmann, O. D. Sieber, W. P. Pallmann, V. J. Wittwer, Y. Barbarin, T. Südmeyer, U. Keller, ETH Zurich (Switzerland); I. L. Krestnikov, S. S. Mikhrin, D. A. Livshits, Innolume GmbH (Germany); G. Malcolm, C. Hamilton, M Squared Lasers Ltd. (United Kingdom)

VECSELs (vertical external cavity surface emitting lasers) combine the advantages of semiconductor and diode pumped solid state lasers. Bandgap engineering provides a large wavelength range, and the vertical emission geometry allows for power scaling with an excellent transverse beam profile suitable for modelocking. Using quantum well gain material, output powers of 2.1 W, repetitions rate of 50 GHz and ultra-short pulses of 60 fs could be demonstrated using a SESAM (semiconductor saturable absorber mirror) for modelocking.

Quantum dot (QD) gain materials offer a larger inhomogeneously broadened spectral bandwidth potentially supporting shorter pulse durations. Modelocked QD-VECSELs have recently been reported at 1060 nm with 18-ps pulses at 27.4 mW. Here, we demonstrate a femtosecond QD-based VECSEL using a QD-SESAM for passive modelocking. For better thermal management we removed the substrate and mounted the VECSEL gain structure directly onto a copper heat sink. We obtained fundamental modelocking with 63 mW at 3.2 GHz with 780-fs pulses. In the same cavity configuration we could also achieve 108 mW with somewhat longer pulses of 820 fs. In both cases the emission wavelength was around 960 nm. In cw operation we obtained output powers of more than 5.2 W using an intra-cavity diamond heat spreader, which has been the highest output power from a QD-VECSEL so far. At this point the intra-cavity diamond heat spreader introduces etalon effects which are detrimental for modelocking. However, we expect that with a diamond substrate we can scale up the average power in modelocked operation as well.

7919-12, Session 8

Design and properties of high-power highly-coherent single-frequency VECSEL emitting in the near- to mid-IRA. Garnache, Univ. Montpellier 2 (France)

We will present the results we obtained in Vertical-External-Cavity Surface-Emitting Laser geometry, together with a full physical study of the emitted highly coherent wave. We will discuss the targeted photonics applications. We demonstrate high power, low noise single frequency operation of a tunable compact VECSEL exhibiting low divergence high beam quality. The lasers are operating in CW at RT at 1µm (GaAs) and 2.3µm (Sb) technologies, with thermal management. Both diode pumping and electrical pumping are developed. The design and physical properties (intensity/frequency noise, beam quality) will be presented. The key parameters limiting the laser will be discussed.

7919-34, Session 8

Femtosecond surface emitting lasersA. C. Tropper, Univ. of Southampton (United Kingdom)

Diode-pumped solid state lasers, long dominant in many high power applications, now face a challenge from optically-pumped semiconductor lasers, which add spectral versatility to the good beam quality and

potential for power-scaling that are characteristic of optically-pumped disc lasers. The vertical-external-cavity surface-emitting semiconductor laser, or VECSEL, readily exhibits passive mode-locking with the inclusion of a semiconductor saturable absorber mirror (SESAM) in the external cavity [1]. These devices most often emit picosecond pulses, recruiting only a small fraction of the available gain bandwidth of the quantum wells. If the dispersive and filtering effects of the multilayer gain and saturable absorber structures are well-controlled, however, it is possible to observe clean sub-picosecond pulses of duration down to 100fs and below [2].

The presentation will describe SESAM-mode-locked VECSELs based on compressively strained InGaAs/GaAs quantum wells that generate trains of near-transform-limited femtosecond pulses at wavelengths around 1 micron with average power of 30 - 300 mW. The nonlinear optical response of the quantum well SESAM in this regime is investigated using a numerical model in which the resonantly excited carriers are coupled by scattering to states outside the laser bandwidth.

1. U. Keller, and A. C. Tropper, “Passively modelocked surface-emitting semiconductor lasers,” Physics Reports-Review Section of Physics Letters 429, 67-120 (2006).

2. A. H. Quarterman, K. G. Wilcox, V. Apostolopoulos, Z. Mihoubi, S. P. Elsmere, I. Farrer, D. A. Ritchie, and A. Tropper, “A passively mode-locked external-cavity semiconductor laser emitting 60-fs pulses,” Nat Photon 3, 729-731 (2009).

7919-35, Session 8

Pulse generation in the 100-fs range from a mode-locked semiconductor disk laserU. Griebner, P. Klopp, Max-Born-Institut für Nichtlineare Optik und Kurzzeitspektroskopie (Germany); M. Zorn, M. Weyers, Ferdinand-Braun-Institut (Germany)

Femtosecond mode-locked semiconductor disk lasers (SDLs) with their typically high repetition rates of ≥1 GHz are attractive, e.g., for THz time-domain spectroscopy with asynchronous optical sampling. Here we report about practically chirp-free pulses with a duration close to 100 fs obtained in the single-pulse regime. The SDL cavity consisted of only three elements, an InGaAs/AlGaAs gain chip, a fast semiconductor saturable absorber mirror and an output coupler. The pulse in our mode-locked SDL was shaped mainly by the spectro-temporal behavior of saturable absorption and gain and the associated self-phase modulation; group delay dispersion is small.

Pulses as short as 107 fs were generated with a spectral width of 10.2 nm (FWHM), centered at 1030 nm. This results in a time-bandwidth product of ≈0.31, which is close to the transform-limit. The output power amounted to 3 mW for 2.8 W of pump power at 808 nm. The pulse repetition rate was ≈5 GHz, corresponding to fundamental mode-locking. Harmonically mode-locked SDLs are capable of operating at much higher pulse repetition rates. A maximum pulse repetition rate of 92 GHz was achieved while preserving a pulse duration shorter than 200 fs.

In principle, pulses from SDLs even shorter than 107 fs are possible. Quarterman et. al. (Nature Photon. 3, 729 (2009)) observed pulse bunches, which had an envelope of a few ps and revealed a substructure of ≈60 fs peaks. For single-pulse operation, to the best of our knowledge, the sub 110 fs pulses reported here are the shortest pulses generated from any semiconductor oscillator.

7919-36, Session 8

169 GHz repetition rate passively harmonically mode-locked VECSEL emitting 265 fs pulsesA. H. Quarterman, K. G. Wilcox, A. Perevedentsev, V. Apostolopoulos, Z. Mihoubi, A. L. Chung, Univ. of Southampton (United Kingdom); H. E. Beere, I. Farrer, D. A. Ritchie, Univ.



of Cambridge (United Kingdom); A. C. Tropper, Univ. of Southampton (United Kingdom)

High repetition rate passively mode-locked sources are of significant interest due to their potential for applications including optical clocking, optical sampling, communications and others. Due to their short excited state lifetimes mode-locked VECSELs are ideally suited to high repetition rate operation, however fundamentally mode-locked quantum well-based VECSELs have not achieved repetition rates above 10 GHz due to the limitations placed on the cavity geometry by the requirement that the saturable absorber saturates more quickly than the gain. This issue has been overcome by the use of quantum dot-based saturable absorbers with lower saturation fluences leading to repetition rates up to 50 GHz, but sub-picosecond pulses have not been achieved at these repetition rates.

We present a passively harmonically mode-locked VECSEL emitting pulses of 265 fs duration at a repetition rate of 169 GHz with an output power of 20 mW. The laser is based around an antiresonant 6 quantum well gain sample and is mode-locked using an optical Stark effect saturable absorber. Harmonic modelocking is achieved by bonding a sapphire etalon to the gain sample. The sapphire then acts as a coupled cavity, setting the repetition rate of the laser while still allowing a tight focus on the saturable absorber. RF spectra of the laser output show no peaks at harmonics of the fundamental repetition rate up to 26 GHz, indicating stable harmonic modelocking. Autocorrelations reveal groups of pulses circulating in the cavity as a result of an increased tendency towards Q-switched modelocking due to the low pulse energies.

7919-37, Session 8

FROG measurements of a femtosecond modelocked VECSELS. P. Elsmere, K. G. Wilcox, A. H. Quarterman, Z. Mihoubi, A. C. Tropper, Univ. of Southampton (United Kingdom)

Femtosecond VECSELs can produce pulses as short as 60fs [1] and have peak powers up to 315W [2]. Whilst many reported femtosecond VECSELs produce near transform limited sech2 pulses, femtosecond VECSELs can operate in regimes where pulses up to twice the transform limit are obtained.

Here we report on measurements performed using second harmonic frequency resolved optical gating (SH-FROG) to investigate the phase structure of chirped pulses with durations between 500 and 750fs, at repetition rates from 1 to 6GHz. For non-transform limited pulses we observe a nonlinear chirp arising from a cubic temporal phase. We postulate that gain and SESAM saturation effects causing nonlinear refractive index changes are the source of this.

We used an own built SH-FROG system with a standard setup using a slow delay line such as in [3]. We used a 300µm BBO crystal, which was chosen to provide the highest SHG signal for the required phase matching bandwidth. The second harmonic signal was spectrally resolved using a 1m spectrometer with a silicon CCD sensor which had a resolution of 0.006nm.

We also investigated the pulse quality when multiple pulses are circulating intracavity and find that no significant pulse degradation occurs, highlighting the potential for producing high quality pulse trains at high repetition rates using harmonically modelocked VECSELs.

[1] Nature Photonics 3, 729-731 (2009)

[2] Photonics Technology Letters 22, 1021-1023 (2010)

[3] JOSA B (Optical Physics), 11, 2206-15 (1994)

7919-38, Session 8

High peak power femtosecond pulse VECSELs for terahertz time domain spectroscopyK. G. Wilcox, A. L. Chung, A. H. Quarterman, Z. Mihoubi, Univ. of Southampton (United Kingdom); I. Farrer, H. E. Beere, D. A. Ritchie, Univ. of Cambridge (United Kingdom); V. Apostolopoulos, A. C. Tropper, Univ. of Southampton (United Kingdom)

Recently we reported an all semiconductor THz-TDS system using a 480fs pulse VECSEL with a peak power of 52W [1]. However, the bandwidth and power of the THz signal generated was limited by the low peak power and long pulses. Here, we report on a femtosecond modelocked VECSEL with six times the peak power of that used in [1], and its application as in an all semiconductor terahertz time domain spectrometer (THz-TDS).

The VECSEL produced near-transform-limited 335 fs sech2 pulses at a fundamental repetition rate of 1 GHz, a centre wavelength of 999 nm and an average output power of 120 mW. This factor of two increase in peak power over previously reported modelocked VECSELs was achieved by optimising our sample design and MBE growth to produce a pump power limited CW output power of 550 mW from an unprocessed sample. This antiresonant gain sample, combined with a high modulation depth surface recombination SESAM utilising the optical Stark effect ensured ~300fs pulse operation, whilst optimised cavity design allowed the high power characteristics of the gain sample to be exploited, whilst maintaining stable single pulse operation.

We report on the effect that this high peak power femtosecond VECSEL has on our generated THz signal, where an increase in the THz power and bandwidth is observed. The increased signal and bandwidth improve the characteristics of a compact VECSEL THz-TDS system to a level where it can be utilised in applications such as quality control, imaging and spectroscopy.

[1] Opt. Lett. 33, 2125-2127 (2008)

7919-39, Session 8

Applications of high power OPS lasers in the visible and UV spectral rangeB. Morioka, Coherent, Inc. (United States)

In the past few years, optically pumped semiconductor lasers (OPSL) have developed from low cw powers (10s-100s of milliwatts) to high power (multi-Watt) in the visible wavelength region. In addition, multiple 100s of mW of cw power are available in the UV spectral range to address new applications in life sciences. This presentation will discuss current applications of multi-Watt visible OPSLs for scientific and medical markets, entertainment and forensics, as well as UV applications in bio-technology. Future wavelength and power requirements of emerging and existing applications will be presented as well.



Conference 7920: Laser Applications in Microelectronic and Optoelectronic Manufacturing (LAMOM) XVIMonday-Thursday 24-27 January 2011 • Part of Proceedings of SPIE Vol. 7920 Laser Applications in Microelectronic and Optoelectronic Manufacturing (LAMOM) XVI

7920-01, Session 1

Fundamental studies of the role of photo-induced defect production on laser modification and etching of surfaces: single crystal ZnOE. H. Khan, J. Unverferth, S. C. Langford, J. T. Dickinson, Washington State Univ. (United States)

UV-Laser interactions with wide bandgap insulators and semiconductors has generated a number of examples of point defect production, surface and bulk modification, etching and re-deposition processes, as well as numerous PLD related applications involving the emitted particles. In metal containing compounds such as oxides and halides, aggregation of metals into nanoparticles has been observed. In this talk we examine such modifications in oriented single crystals of the transparent semiconductor ZnO with a band-gap of ~3.4 eV. We first discuss results on interactions of strongly absorbing 248 nm (5 eV), 193 nm (6.4 eV), and 157 nm (7.8 eV) excimer laser light as well as sub bandgap irradiation at 532 nm (2.33 eV; doubled Nd:YAG) with high purity ZnO (1010) surfaces in UHV. Using various time resolved spectroscopies, we examine in detail the particle emissions and related light emissions induced by exposure of single crystals to these lasers and the surprising surface modification that results. The latter includes 10 -20 nm nanoparticle growth on the exposed surface and extremely clean etching (e.g., holes through mm thick samples). The laser fluence dependence of these changes are correlated with the emission of neutral Zn, O, and O2 providing evidence of laser generated point defects in ZnO accompanying electron-hole pair production and their role they play. The extremely efficient etching phenomena observed in our experiments allows a very brittle high technology material to be machined with ease.

7920-02, Session 1

Absorption of femtosecond laser pulse in fused silica: experiments and modellingN. Varkentina, O. P. Uteza, B. Chimier, N. Sanner, Lasers, Plasmas et Procédés Photoniques (France); T. E. Itina, Lab. Hubert Curien (France); M. L. Sentis, Lasers, Plasmas et Procédés Photoniques (France)

Laser-matter interaction with femtosecond lasers includes the transfer of the electromagnetic energy to electronic excitation of the matter followed by electron-lattice relaxation and conversion of the laser energy into heat.We present experimental results of laser-dielectric interaction (450 fs pulse @ 1025nm; SiO2-suprasil sample, surface experiments) for the single-shot laser regime.The evolution of the reflection, transmission and absorption signals is studied as a function of fluence.The experimental curves are accompanied by a modelling which takes into account the photoionization and avalanche ionization depicting absorption of the laser energy by the material.We outline the distinction between the ablation and the damage thresholds for dielectric materials. The incident pulse propagation into the material, the temporal evolution of the electron density, reflection and transmission illustrate the beginning and the duration of the laser pulse absorption. The magnitude of the absorption process is energy sensitive and, with the increase of the deposited energy, the onset of absorption is moved temporally to the beginning of the pulse. We show the influence of the electron-phonon collision frequency on the calculated values of reflection, transmission and absorption. The frequency of electron-phonon collision is a time dependant function of electronic density and electron temperature. From the model we deduce the crather depth corresponding to the applied fluence. We further compare it to experimental data measured by atomic force microscopy technique.

7920-03, Session 1

Goos-Hänchen effect enhanced by surface plasmon resonance in Kretschmann-Raether configurationT. Duan, Xi’an Institute of Optics and Precision Mechanics (China)

We investigate theoretically the large positive and negative Goos-Hänchen displacements enhanced by surface plasmon resonance in a Kretschmann-Raether attenuated total reflection configuration where a thin silver film is coated onto an optical prism. The phenomenon has been observed in experiments and applied in optical sensors and optical switches. The purpose of this paper is to explained theoretically negative as well as positive displacements of TM-polarized light beams by Artmann on the basis of stationary-phase argument. There exists critical thickness to separate the thickness of the metallic film into two regions due to the absorption of the metallic film surface plasmon resonance. The result of the calculations show that the metallic film is thinner or thicker than the critical thickness corresponds to positive resonant displacements and negative ones. The prediction of Artmann’s formula agrees qualitatively with the experimental result, but the predicted peaks are much higher. The results of numerical simulations of the Gaussion -shape beam show better agreement with the observed data than those of Artmann’s formula.

7920-04, Session 1

Effects of the source, surface, and sensor couplings and colorimetric of laser speckle pattern on the performance of optical imaging systemM. Darwiesh, A. F. El-Sherif, Military Technical College (Egypt); H. El-Ghandour, Ain Shams Univ. (Egypt); H. Aly, Military Technical College (Egypt)

Optical imaging systems are widely used in different applications include tracking for portable scanners; input pointing devices for laptop computers, cell phones, and cameras, fingerprint-identification scanners, optical navigation for target tracking, and in optical computer mouse.

We presented an experimental work to measure and analyze the laser speckle pattern (LSP) produced from different optical sources (i.e. various color LEDs, 3 mW diode laser, and 10mW He-Ne laser) with different produced operating surfaces (Gabor hologram diffusers), and how they affects the performance of the optical imaging systems; speckle size and signal-to-noise ratio (signal is represented by the patches of the speckles that contain or carry information, and noise is represented by the whole remaining part of the selected image).

The theoretical and experimental studies of the colorimetry are used to get color correction for the color images captured by the optical imaging system with different optical sources to produce realistic gray images which contains most of the information in the image by selecting suitable gray scale which contains most of the informative data in the image, this is done by calculating the accurate Red-Green-Blue (RGB) color components making use of the measured spectrum for light sources, and color matching functions of International Telecommunication Organization (ITU-R709) for CRT phosphorus, Tirinton-SONY Model to present the relations between the signal-to-noise ratios with different diffusers for each light source.

The source/ surface coupling has been discussed and concludes that the signal- to -noise ratio (which give an indication about the performance of the optical imaging system) varies from high to low according to the combination between the used diffusers (five Gabor hologram diffusers)


and the used optical sources (He-Ne laser, diode laser, red LED, blue LED, green LED, yellow LED, and orange LED). So, we can select perfect combination which gives high performance of the optical imaging system.

The sensor/surface coupling has been discussed and concludes that the speckle size is ranged from 4.59 to 4.62 m, which are slightly different or approximately the same for all produced diffusers (which satisfies the fact that the speckle size is independent on the illuminating surface). But, the calculated value of signal-to-noise ratio takes different values ranged from 0.71 to 0.92 for different diffuser. This means that the surface texture affects the performance of the optical sensor because, all images captured for all diffusers under the same conditions.

7920-05, Session 2

Glass welding technology using ultra short laser pulsesS. Roth, K. Cvecek, I. Miyamoto, M. H. M. Schmidt, BLZ Bayerisches Laserzentrum GmbH (Germany)


7920-06, Session 2

Reliable laser micro-welding of copperC. Rüttimann, U. Duerr, LASAG AG (Switzerland); A. Moalem, Laser Zentrum Hannover e.V. (Germany)

The reliability of copper welds is still a problem today concerning the high demands of spot or contact welding for the electronic or medical industry. Two aspects characterising this reliability are tackled: the reproducibility of the weld geometry and the reduction of thermally induced side effects, e.g., sputtering.

One problem in copper welding is the low absorption of a clean polished surface (approx. 5%) at room temperature and 1 micron emission of standard industrial lasers. This low absorption combined with the unique thermal properties of copper lead to the fact that even small surface contaminations lead to drastic variations in weld quality. The frequency doubled Nd:YAG at 532 nm is much better absorbed by the copper surface at room temperature.

Combining these two wavelengths and using the drastic increase in absorption even at 1 micron at higher temperatures leads to an efficient spot welding solution by using pulse forming with the thermal pulses of a Nd:YAG laser. At the beginning of the pulse the high intensity 1 micron radiation is partly converted to 532 nm radiation which preheats the surface to a level where the absorption for 1 micron radiation is high enough. From this point on the pulse power is reduced and welding is finished by the 1 micron radiation. The total pulse length is typically less than 5 ms. A significant increase in weld reliability will be demonstrated.

7920-07, Session 2

Advanced micro-machining applications for low nanosecond high brightness fiber lasersA. P. Hoult, IPG Photonics Corp. (United States)

Master Oscillator Power Fiber Amplifier lasers (MOPFA) lasers have been available for a few years but very short nanosecond pulses (1-10ns) along with low brightness (<1.3) and high repetition rates up to 600kHz has only been achieved recently. These improvements have come about due to the availability of a wide range of standard fiber laser components such as new pump diodes and new fiber oscillators and amplifiers. The work presented here shows that these lasers are capable of the full range of laser micro-machining processes. In addition, new polarisation maintaining pump lasers and a novel second harmonic frequency generation module has led to a low nanosecond green (532nm) laser, and some micro-machining applications are also presented. These new fiber

laser designs are capable of far higher pulse repetition rates than those currently available from most conventional lasers. A number of laser micro-machining applications are presented in detail and a simplified experimental design technique is used to show how these processes were optimised. It is shown that in some cases this leads to far higher processing speeds.

7920-08, Session 2

Processing of metals with ps-laserpulses in the range between 10ps and 100psB. Neuenschwander, M. Schmid, V. Romano, B. Jaeggi, U. W. Hunziker, Berner Fachhochschule Technik und Informatik (Switzerland)

Currently, ultra short laser pulses below 10ps, are used, when high requirements concerning accuracy, surface roughness, heat affected zone etc. are demanded. To generate sub 10ps pulses, today complicated and expensive turnkey systems that are set up in a MOPA arrangement with rod or disk amplifier are used.

For many applications, especially when machining metals, pulselengths in the range of 20ps-100ps might lead to results that fulfill the necessary precision requirements. In this longer pulselength range laser systems can be conceived that are less complex, have lower costs and display higher compactness, e.g. MOPA based fiber laser systems. Such systems would probably find an increased acceptance in industrial production.

However data of the ablation process with laser pulse durations above the thermalization time of about 10ps, where the ablation process can’t longer be considered as “cold”, are hardly available.

With this paper we will report on thresholds and ablation rates for pulse durations ranging from 10ps to 100ps for selected metals and processes. Additionally, the ablation quality for pulse durations in the range of 10ps- 100ps is presented as well.

7920-09, Session 2

Novel micromachining process using optical breakdown of a microdropletD. Ahn, D. Jang, D. Kim, Pohang Univ. of Science and Technology (Korea, Republic of)

In the conventional laser micromachining process, the ablation efficiency is limited by plasma shielding because the plasma absorbs a part of the incident laser energy. Also, thermal side effects lowers the micromachining performance in various aspects. In this work, we propose a novel micromachining technique which has a significantly increased ablation efficiency and reduced thermal effect, compared with the conventional laser processing. When a laser pulse (Q-switched Nd:YAG laser, FWHM: 6 ns, wavelength: 1064 nm) is focused on a small liquid droplet (100 ~ 500 µm), optical breakdown of the droplet is generated with high-speed (~1000 m/s) microjet ejection from the liquid droplet. As the microjet carries a transmitted portion of the laser/plasma beam in addition to the hydrodynamic momentum, it can be employed in micromachining with significantly reduced thermal effect and increased ablation rates. It is shown that the direction, size, divergence angle and velocity of the microjet can be controlled by adjusting the process parameters, such as laser energy, focal position, droplet size, etc. Experiments using various materials (SS 304, Ti, Cu and Ni) exhibit substantial enhancement of the ablation rate, as large as ~10 times compared with the conventional processing. We believe that the novel process has strong potential in numerous applications including micromachining of hard-to-process materials and high-aspect ratio drilling.

Conference 7920: Laser Applications in Microelectronic and Optoelectronic Manufacturing (LAMOM) XVI


7920-10, Session 3

Generation of superfine structure smaller than 10 nm by interfering femtosecond laser processingY. Nakata, K. Momoo, T. Hiromoto, N. Miyanaga, Osaka Univ. (Japan)

Interfering femtosecond laser can induce periodic induction of energy. When an opaque thin film deposited on a substrate is irradiated by an interfering femtosecond laser, periodic thermal process is induced. This results in liquid motion of the solute film. The process has an analogy with a motion of liquid water taken by a high-speed camera. Each spot melts, inflates, flows and shrinks according to the nature of the solute thin film, then it freeze due to temperature fall by thermal radiation and conduction. The temperature distribution, which governs the natures such as viscosity, surface tension, changes in time and space, but the resultant structures are very simple and unique. The shapes of the structures are bump, spike, waterdrop, and can be controlled by laser fluence, thin film thickness, substrate material, etc.. The size of some structures is smaller than 10 nm in curvature radius, and the aspect ratio is over 20. In addition, an interference pattern changes according to the phase shift and power ratio between the interfering beams, and duplicated structure of two shapes, or complicated structures can be generated in a single shot of laser irradiation. In the case of duplicated structure, the density of nano-structures is doubled, and two different nano-structures appear alternately. These structures will be useful in nanotechnology, metamaterial technology, etc..

7920-11, Session 3

Surface morphology of SiO2 coated InP/InGaAs/InGaAsP microstructures following irradiation with the ArF and KrF excimer lasersN. Liu, K. Moumanis, J. J. Dubowski, Univ. de Sherbrooke (Canada)

It has been known that excimer laser irradiated surfaces of III-V quantum well (QW) semiconductor microstructures could provide conditions attractive for selective area bandgap engineering. Depending on the laser used, the investigated microstructure and the irradiation environment both the enhanced quantum well intermixing (QWI) leading to blue shifting of the bandgap energy, as well as the suppressed QWI processes have been reported. Excimer lasers are attractive for bandgap engineering as they can be used to pattern large size wafers, without the need for photolithography masks that normally are required to achieve selective area processing.

Successful fabrication of devices from the QWI material requires efficient control of its surface morphology. To address this problem, we have investigated surface morphology of InP/InGaAs/InGaAsP QW microstructure coated with dSiO2 = 50, 150, 234 and 244 nm thick SiO2 films. Both ArF (193 nm) and KrF (248 nm) excimers have been used to irradiate a series of samples with up to 400 pulses of fluence 76 to 185 mJ/cm2. We have observed that following the laser irradiation, the surface roughness of the InP cap increased from RMS = 1.44 nm to 10.31 nm. Generally, the amplitude of the surface roughness increased with the laser fluence and pulse number, but it became saturated at a large pulse number. Following the RTA step, a smoother surface morphology was observed for all irradiated samples. The sample coated with 244 nm thick SiO2 (dSiO2≈KrF), following the 25-pulse-radiation with the KrF laser at 122mJ/cm2 and RTA, showed a smooth surface (RMS = 2.02 nm) and maximum blueshift of 82 nm. To date, this is one of the best results we have observed for this material system, but the optimization of the process is the subject of ongoing research.

7920-12, Session 3

Generating sub-micron features on rough surfaces using optical trap assisted nanopatterningY. Tsai, R. Fardel, C. B. Arnold, Princeton Univ. (United States)

Probe-based, nanoscale direct-write techniques are somewhat limited in their ability to accommodate non-standard or structured surfaces containing existing micron-scale roughness or significant features. Here we apply optical trap assisted nanopatterning (OTAN) to this challenge and demonstrate that this approach can overcome many of these issues facing other probe-based methods. In this technique, a micron-scale dielectric sphere is positioned in close proximity to a substrate using an optical trap and is subsequently used as a near-field focusing objective to create nano-scale features on a surface. By using a Bessel beam optical trap, the microsphere is able to self-position itself above the surface due to the force balance between the surface interaction forces and the optical scattering force. Once positioned, the spheres, can be translated along the surface over existing features without direct contact or sticking to the substrate. We experimentally show the use of OTAN for creating continuous and isolated nanoscale features across pre-structured polyimide substrates with 1.7 um surface steps. The mean diameter and standard deviation of feature positions above and below the step are measured and we use this to characterize the patterning accuracy on these surfaces. Moreover, we explore the effects of a step height, elevation angle, and trapping power on the ability of spheres transverse steps.

7920-13, Session 3

Novel surface cleaning method using high-speed micro jet generated by laser-induced breakdown of a microdropletD. Jang, D. Ahn, D. Kim, Pohang Univ. of Science and Technology (Korea, Republic of)

This work presents a novel surface cleaning technique using laser-induced breakdown (LIB) of a microdroplet in the ambient air. In the process, a laser pulse (Q-switched Nd:YAG laser, FWHM: 6 ns, wavelength: 1064 nm) induces successive optical breakdown of a microscale droplet of several hundred microns in diameter. If the laser irradiance focused on the liquid overcomes the optical breakdown threshold, laser-induced plasma (LIP) is produced, leading to explosion of liquid droplet. Because the LIP has high pressure, it puts the liquid to the forward direction (shadow direction of the incident beam) and makes high-speed micro water jet (~ 1000 m/s). In this work, we analyzed and optimized the hydrodynamics of laser-induced liquid jet using laser flash shadowgraphy. It was shown that the jet direction, divergence angle and velocity could be controlled by adjusting the process parameters, such as laser energy, position of focal spot, droplet size, etc. The high-speed micro liquid jet was impinged onto the contaminated surface at an oblique angle, demonstrating removal of nanoscale contaminants from the surface. Cleaning efficiency better than 95 % is demonstrated using a microjet with a speed over 1000 m/s for 30 nm polystyrene particles on a Si surface.

7920-14, Session 4

Controlled growth of carbon nanotubes: from locations to structuresY. Lu, Y. Zhou, M. Mahjouri Samani, W. Xiong, M. Mitchell, Univ. of Nebraska-Lincoln (United States)




7920-15, Session 4

High rep-rate UV laser precipitation of silicon nanocrystalsW. Mustafeez, Stanford Univ. (United States); D. Lee, C. P. Grigoropoulos, Univ. of California, Berkeley (United States); A. Salleo, Stanford Univ. (United States)

We explore a new laser regime for preparation of Si nanocrystals. The study was done on PECVD deposited silicon rich oxide films that were 300-450nm thick with varying excess Si content. Loosely focused 355nm, 12ps pulses at 80MHz rep rate with energies in the 4.7nJ to 6.2nJ range with translation at 2mm/sec were used to precipitate Si nanocrystals in the oxide matrix. Localized formation of silicon nanocrystals is observed through a photoluminescence and Raman study. PL shows enhanced red shifted emission intensities at 750nm in regions along the edge of laser tracks. Raman peaks are observed at 486-510 cm-1 which correspond to different sized nanocrystals as well as amorphous clusters. The topography is characterized through AFM which shows a bulge formation centered at the laser affected region in contrast to usually observed bulge at the rim.

7920-16, Session 4

Femtosecond laser doping of TiO2 for photocatalysisK. C. Phillips, M. Sher, A. Co, E. C. Landis, C. M. Friend, E. D. Mazur, Harvard Univ. (United States)

We present a novel method for femtosecond-laser doping of titanium dioxide (TiO2) for enhanced absorptance in the visible electromagnetic spectrum.

With a bandgap of 3.2 eV for the anatase crystalline phase, TiO2 most strongly absorbs in the UV range ( < 387 nm). However, doping with metals and nitrogen has been shown to create intermediate states in the bandgap, generating a new material for visible-light photocatalysis that has the potential for watersplitting. Using femtosecond laser doping techniques on bulk TiO2 in a gaseous environment, we produce laser-induced periodic surface structures. We compare how the surface morphology and optical properties vary with gas composition and laser parameters. In addition, we present compositional data from x-ray photoelectron and Raman spectroscopy and structural data from scanning electron microscopy. We also show the creation of oxygen deficiencies after raster scanning and investigate their role in altering absorptance. Our research presents an innovative approach using laser scanning techniques to alter the structure of the TiO2 crystal for solar harvesting.

7920-17, Session 4

Sintering of solution-based nano-particles by a UV laser pulse trainJ. Zhang, M. Li, Panasonic Corp. of North America (United States); K. Morimoto, Panasonic Corp. (Japan)

Due to low melting point of Nanoparticles (NPs), sintering can be carried out at relatively low temperatures. In addition, a laser beam makes it possible to confine the heat to a localized area. Taking advantage of these two effects, one can perform laser sintering of NPs on existing devices without causing significant collateral damages. This technology can be useful in a wide range of applications, such as semiconductor, and biotechnology industries, etc.

Motivated by this idea, sintering of palladium (Pd) and silicon (Si) nano-particles (NPs) by a 266nm laser pulse train on ink-printed films was investigated. Organic Pd-ink (~3.0 nm diameter Pd-NPs, Tanaka Kikinzoku Group), and organic Si-ink, (~10s nm-~100s nm sized Si-NPs,

Primet Precision Material Inc.,) were used as precursors. A high repetition rate DPSS laser (up to 300 kHz, 25ns, 266nm, Coherent AVIA series), which produces a ns pulse train with 3.3 µs -33.3 µs interval of pulse-to-pulse, was used as the heating source. The pulse train heating combines the advantages of single laser pulse and continuous wave laser heating.

Highly electrically conductive Pd (Resistivity=~150µΩcm) thin film on PET substrate and semi-conductive Si (Resistivity=~22.6kΩcm) thin film on glass substrate were successfully obtained with this laser pulse train sintering process. In this paper, in addition to the sintering procedure and recipe, characterizations of sintered films with transmission electric microscopy, atomic force microscopy, scanning electric microscopy as well as Raman spectroscopy will be reported. The mechanism of laser pulse train induced sintering was also discussed.

7920-18, Session 5

Industrial production with ultra fast laser workstationsE. Audouard, Univ. Jean Monnet Saint-Etienne (France); H. Soder, Impulsion SAS (France)

Femtosecond lasers have proved to be great tools for precise, accurate and high quality micro- but also nano-machining. Numerous works have been carried out to study laser-matter interaction mechanisms on the one hand, and laser machining processes on the other hand.

Very naïve first industrial approaches, such as “ultra fast machining is interesting because it’s not a thermal process”, are only used by manufacturers who do not really want to sell workstations for industrial use. Technical development and good knowledge of the process have to be simultaneously settled to allow a real practical application.

In this work, we will underline some practical applications and the link between a better knowledge of physical mechanisms and the development of industrial processes. Of course, we have to pay attention to typical parameters of industrial development, such as the processing time. With the knowledge of an “efficient” ablation rate, the physical time can be easily calculated, using the total length of the machining, speed and laser parameters. This calculated physical process time can be compared to the effective time needed to machine the sample. This kind of information can evidence the possible improvement to be done on the mechanical and computer environment to reach this minimum process time.

Femtosecond technology has now proven its capacity to lead innovative production, even if it’s still in some well defined fields, but its contribution is non negligible in a context of economical crisis.

7920-19, Session 5

An investigation of piezoelectric cutting by femtosecond laserY. Di Maio, E. Audouard, J. Colombier, Univ. Jean Monnet Saint-Etienne (France); P. Cazottes, J. Beitia, Sagem Defense Securite (France)

Among several piezoelectric actuators, the PZT ceramics (Pb(ZrxTi1-x)O3) are one of the preferred candidates for industrial applications requiring very high precision, speed and good controllability, as micrometric transducers. This paper investigates an innovative method to cut a PZT wafer with an extreme precision.

The most used method to cut PZT wafers today is a mechanical cut with an automatic saw. However, when a better precision is required, other methods such as femtosecond lasers should be used. Most specific tests performed with these lasers have revealed high efficiency thanks to modulable parameters and ablation improvements. Although not an issue for more common materials, some undesired features require a better understanding of the laser-matter interaction, a characterization of the unexpected phenomena and a determination of the optimal parameters for cutting.



Describing the propagation of a laser beam through optical devices provides more insights on the ceramic behaviour. Furthermore, key parameters such as the ablation rate and the ablation threshold must be determined to precisely characterise this material. In the meantime, laser induced phenomena are observed, from rippled-like nanostructurations to cracking, leading sometimes to a spallation of the edges of the material after cutting. The research of the optimal parameters to reach uniform processes is a straight forward way to avoid or control these effects which could be negative for the purposes of the machined pieces.

Associated with adapted laser settings and eventually with beam shaping, femtosecond processes should improve PZT machining in terms of precision, speed, cleanliness, scalability and after-processed treatments comparatively to usual cutting methods.

7920-32, Session 5

Fundamentals and industrial applications of ultrashort pulsed lasers at BoschJ. Koenig, T. Bauer, Robert Bosch GmbH (Germany)

Well-known trends in automotive industry are for example miniaturization, higher precision, diversification of materials, variety of variants and smaller lots. Always on scope has to be cost-effectiveness and outstanding quality.

About ten years ago ultrashort pulsed laser material processing showed the potential to satisfy a lot of the named requirements. BOSCH coordinated public funded projects together with universities and industrial partners to develop the ultrashort pulse laser technology from an academic level to a cost-effective production technology.

First step was to build up knowledge of the fundamental process, in detail the interaction of ultrashort laser pulse and material. This gives the possibility to define the specifications of applicable laser systems, for example pulse duration, pulse energy and repetition rate.

Out of this, the laser systems and further system technology is optimised in an iterative way with respect of robust and cost-effective processes.

Since 2007 are ultrashort laser pulses used at the BOSCH plant in Bamberg for production of exhaust gas sensors, shown in figure 1 left. They are made of a special ceramic layer system and can measure the exhaust gas properties faster and more precisely. This enables further reduction of emissions by optimised combustion control.

Since 2009 BOSCH ultrashort pulsed lasers are micro structuring the injector of common rail diesel systems. A drainage groove allows a tight system even at increased pressures of up to 2000 bar. Diesel injection systems become even more reliable, powerful and environment-friendly.

7920-33, Session 5

Directly induced ablation of metal thin films by ultra short laser pulsesG. Heise, C. Hellwig, J. Konrad, S. Sarrach, H. P. Huber, Hochschule München für Angewandte Wissenschaften (Germany)

Molybdenum (Mo) films of 0.5 µm thickness on a glass substrate are used as a back contact for CIS thin film solar cells. These thin films can be ablated from the glass side by picosecond laser pulses at low fluences < 1 J/cm2, without any visible thermal effects and damage; in contrast to nanosecond pulses. In our previous work we showed that the perfect ablation of the Mo films is based on a directly induced laser ablation occurring at fluences below the limit of complete thermodynamical heating and evaporation. Similar processes are often referred to as “laser lift-off”. To gain more insight in the underlying ablation mechanisms of directly induced laser ablation, we performed experiments with different film thicknesses and film materials at various fluence levels and laser spot diameters: Thin films of Molybdenum, chromium, titanium and platinum with thicknesses between 200 nm and 1 µm, produced by cathode sputtering, were examined. In the case of molybdenum and

chromium, the influence of an intermediate buffer layer of silicon nitride on the ablation behavior was analyzed. It is shown that ablation form the glass side clearly suggests a higher grade of ablation efficiency and a better structural quality in contrast to metal side patterning. A model will be presented, in which the ablation characteristics are connected with the mechanical ductility of the different metals.

7920-34, Session 5

Colorizing of the stainless steel surface by single-beam direct femtosecond laser writingM. S. Ahsan, KAIST (Korea, Republic of) and Khulna Univ. (Bangladesh); Y. G. Kim, M. S. Lee, KAIST (Korea, Republic of)

This paper reports on the colorizing of the stainless steel surface by controlling the irradiation conditions of a single-beam femtosecond laser. We change the color of the stainless steel surface by femtosecond laser induced periodic microholes or microgratings on the sample surface. Colorizing of metal surface by periodic microholes, produced by femtosecond laser, is achieved for the first time without any kind of coating. The laser modified stainless steel surfaces show different colors under different incident or azimuthal angles of the incident light, which changes in color indicate the dependence of the metal color on the angles (incident and azimuthal) of the incident light. We report, for the first time, the changes of metal color due to the change of the azimuthal angles of the incident light. Furthermore, the changes in the color of the laser modified metal surfaces are mainly due to the excitation of surface plasmons (SPs) on the metal surface. The resonant angle of SPs is different for different wavelength of light. As a result, under different incident or azimuthal angles different wavelength of light is trapped on the surface depending on the resonance for that particular wavelength; light of other wavelengths react naturally and contributes for the color change of the stainless steel surface. Finally, we discovered that the nanostructures produced on the top of the microgratings and micro holes play important roles for the propagation of the SPs in parallel with the mcirogratings and mcroholes, which nanostructures are responsible for a complex SPs on the sample surface.

7920-20, Session 6

Laser ablation of AgInSe2: a clean and effective approach for ternary semiconductorsD. Pathak, Guru Nanak Dev Univ. (India)

Laser ablation has attracted special interest for the formation of thin films Compared with other formation technique . A distinctive feature of laser ablation is that it allow high quality and stochiometry of films of even very complex element material. In this presentation laser ablation of AgInSe2 chalcopyrite semiconductor will be discussed in which it is difficult to maintain stochiometry by conventional method. High Quality AgInSe2 (AIS) films were grown on Glass substrates by the ultra-high-vacuum pulsed laser deposition technique from the AIS target synthesized from high-purity materials. The X-ray diffraction studies of the films show that films are textured in (112) direction..The substrate temperature appears to influence the properties of films. Increase in substrate temperature results in more order structure .Compositional analysis has been carried out by EDAX. It is observed that compositional stochiometry is maintained to the more extent by PLD technique than other traditional methods like thermal evaporation. The optical studies of the films show that the optical band gap is about 1.20 eV.



7920-21, Session 6

Investigation on solid state Nd3+:YAG line beam laser annealing and texturing of amorphous silicon thin filmsN. J. Vasa, A. I. Palani, M. Singaperumal, Indian Institute of Technology Madras (India)

Crystalline silicon plays a major role in photovoltaic application, however the cost is high and overall size is limited. As an alternative approach, Nd3+:YAG laser annealing technique can be used to convert amorphous silicon (a-Si) film into polycrystalline silicon. In addition to laser annealing, lasers can also be used to produce a textured surface on the silicon surfaces by the methodology of spot overlap.

In the present work, an influence of the laser fluence and laser beam overlap on a-Si film is studied. An attempt is made to crystallize and simultaneously form a nanotextured surface using a pulsed solid-state Nd3+: YAG laser at a wavelength 355 nm with a Gaussian profile. Nanotexturing is studied by 50% and 90% overlapping of the diameter of the laser spot. At the laser fluence values between 350 mJ/cm2 and 500 mJ/cm2 and with the beam overlap of 90%, the broadband Raman spectrum around 480 cm-1 was quenched and a narrow-band Raman shift around 520 cm-1 was observed. Further, electrical resistance of 10 k was observed. With 90% overlap, surface roughness was increased and films also show a higher absorbance than that of treated with 50% overlap. Theoretical studies were performed to understand the annealing and the nanotexturing of a-Si films. Currently, experimental studies are performed in which the Gaussian laser beam with the circular spot is transformed into a flat-top beam profile with a line beam. The line beam with a flat-top beam profile is expected to allow a wide area scanning.

7920-36, Session 6

The effect of ambient conditions on thin wafers processed with fs-laser machiningS. C. Jeoung, Korea Research Institute of Standards and Science (Korea, Republic of)

Micromachining based on laser has become important tools in the fields of mass production for modern -devices including multi-layered microelectronics, LED, solar voltaic cells, displays, multilayered PCB and so on. While conventional mechanical processes have come to their limitations in the course of size miniaturization, ultrafast laser techniques have been known to have a potential application in high precision processing because of their minimized heat affected zone. The other potential application of fs-laser -processing is to modify the surface to form functional micro- and nanostructures.

We will present the effects of ambient processing conditions on the surface topological changes as well as the mechanical properties of processed materials. For example, we observe that ambient gases and laser fluence used in processing play an important role in encapsulation of Ge nanostructures with oxidized layer as well as their size distribution. Further, the substrate temperature of silicon wafers also affects the dependence of surface roughness on the laser fluence. This observation can be understood in terms of the changes in ablation mechanism underlying fs-laser material ablation process between optical penetration and thermal diffusion processes. We also propose an empirical relation between maximum yield stress and cumulative temperature increment of dielectric materials based on the measurement of the stress of small die formed with varying the repetition rate of fs-laser pulse and the ambient gas. The current work should be helpful to understand the effect of ambient conditions on fs-laser-material interaction for practical usage.

7920-37, Session 6

On the damage behaviour of Al2O3 insulating layers in thin film systems for the fabrication of sputtered strain gaugesO. Suttmann, U. Klug, R. Kling, Laser Zentrum Hannover e.V. (Germany)

A new approach for strain measurement is the integration of laser patterned sputter thin film strain gauges. After deposition of the film system, consisting of sensing and insulating films, the sensing film is patterned by laser ablation. To ensure proper functionality of the strain sensors, the sensing film has to be removed completely. Damage of the insulating film bears the risk of shortcutting the sensor with the substrate. Hence, damage in the insulating film between metal work piece and sensing films has to be avoided when fabricating strain gauges.

We report on ablation experiments of thin film systems of NiCr and Al2O3. Ablation is performed with a Nd:YVO4 laser with a pulse duration of 15 ps, a wavelength of 532 nm and a repetition rate of 100 kHz. The effect of fluence, number of irradiated pulses and Al2O3 film thickness are investigated. This paper concentrates on the damage thresholds and damage mechanisms of the Al2O3 film. Two kinds of damage mechanisms appear: ablation based material removal and stress induced cracking. The damage thresholds increase with the film thickness. Increasing the number of irradiated pulses leads to decreasing damage thresholds until reaching a limit. Fluences below the limit allow for damage free ablation of the sensing layer. This behaviour is in agreement with an incubation model for bulk dielectrica. The role of defects in the Al2O3 films on the damage behaviour is discussed.

The knowledge gained enables patterning of thin film strain gauges without damaging the insulating layer.

7920-38, Session 7

In-situ coherent imaging to monitor and control laser micro machining processesJ. M. Fraser, P. J. Webster, J. X. Yu, B. Y. Leung, L. G. Wright, K. D. Mortimer, Queen’s Univ. (Canada)

In applications ranging from noncontact microsurgery to semiconductor blind hole drilling, precise depth control of laser processing is essential. Even a priori characterization and design of the machining process cannot compensate for material heterogeneity and stochasticity inherent to the ablation process. We take a different approach: by imaging sample morphology along the machining beam axis at high speeds (up to 300 kHz), we can guide the machining process in real time, even in high aspect ratio holes. The in situ metrology is based on coherent imaging (similar to the medical imaging modality optical coherence tomography) and is practical for a wide-range of light sources and machining processes (e.g., thermal cutting using a quasi-continuous wave ytterbium fiber laser, or nonlinear ablation achieved with ultrafast pulses from a diode-pumped regenerative amplifier). Coherent imaging has high dynamic range (> 60 dB) and strongly rejects incoherent signals allowing weak features to be observed in the presence of high power machining light and bright plasmas. Broadband imaging light is required to achieve high axial resolution (~5 micron) but center wavelength can be chosen appropriate to the application. Infrared (wavelength: 1320±35 nm) allow simultaneous monitoring of both surface and subsurface interfaces in nonabsorbing materials like tissue and semiconductors (e.g., silicon). Silicon based detector technology can be used with near infrared imaging light (804 ± 30 nm) enabling high speed acquisition or a low cost implementation (total imaging system <10k$). Machining with an appropriate ultrafast laser (broadband) allows machining and imaging to be done with the same light source.



7920-39, Session 7

In-situ observation of the hole formation during deep drilling with ultrashort laser pulsesS. Döring, S. Richter, Friedrich-Schiller-Univ. Jena (Germany); S. Nolte, A. Tünnermann, Friedrich-Schiller-Univ. Jena (Germany) and Fraunhofer Institute Applied Optics and Precision Engineering (Germany)

We report on the in-situ observation of the laser drilling process using ultrashort laser pulses. Our technique is based on transmission imaging of a silicon sample at 1060 nm. For drilling, we used a laser system that provides pulses with a duration of 8 ps at 1030 nm. This wavelength is below the band edge and silicon shows linear absorption. The beam is focused on the sample surface perpendicular to the transillumination. Therefore the temporal evolution of the longitudinal silhouette of the hole can be visualized during the drilling progress. Our observations show that the dynamics of the drilling process change in the depth of the material. For deep drilling, effects like the decrease in ablation rate, the formation of bulges, deviations in drilling direction and finally a branching of the hole end occur due to the influence of the previously excavated capillary. That causes a perturbation of the beam by irregular internal reflections and the interaction with ablation products as well as an additional abrasive effect by the ablated particles and plasma. The dependence of hole depth and shape on the process parameters, especially fluence and pulse energy, is studied. The depth of the hole shows a stepwise increase, while the ablated volume increases continuously, corresponding to intermediate periods with predominantly transverse expansion. The maximum achievable hole depth in deep drilling is chiefly determined by the pulse energy but largely independent from the fluence.

7920-24, Session 8

Laser-induced breakdown spectroscopy with tailored femtosecond pulses for 3-dimensional chemical imaging with high spatial resolutionJ. Mildner, C. Sarpe-Tudoran, L. Englert, D. Otto, N. Goette, M. Wollenhaupt, W. Wessel, A. Brueckner-Foit, T. Baumert, Univ. Kassel (Germany)


7920-25, Session 8

Uniform near-field nanopatterning due to the field distribution control by oblique femtosecond laser irradiation to nanoparticlesT. Miyanishi, M. Terakawa, M. Obara, Keio Univ. (Japan)

We present near-field optical properties around silica, silicon and gold nanoparticles aligned on a silicon substrate excited by oblique incidence femtosecond laser for nanohole processing. Using an enhanced localized near field, a nanohole can be fabricated even with near-infrared laser excitation. Near-field nanofabrication will open up smart applications for new optical devices with high-throughput processing. We have been investigating near-field processing with an oblique irradiation to gold nanoparticles placed on silicon substrate. Theoretical study revealed that the incident laser energy is concentrated into the contact point between the particle and the substrate due to the image charge inside the substrate at any incident angles. Here, we investigate the near-field distribution around silica and silicon nanoparticles excited by oblique incidence femtosecond laser. The near field around silica and silicon

particles is explained by Mie scattering theory, while the near field around gold nanoparticles is explained by plasmon polaritons inside nanoparticles. By using particles with a dielectric constant as low as silica, the position of the peak intensity point is controllable by changing the incident angle of the laser. With particles with a dielectric constant as high as silicon, the polarized charge shows a similar effect to the plasmon charge. Therefore the distribution of the concentrated energy provided with silicon nanoparticles is similar to that of gold nanoparticle.

This work is supported in part by a Grant-in-Aid for Japan Society for the Promotion of Science (JSPS) from the Ministry of Education, Culture, Sport, Science and Technology, Japan.

7920-26, Session 8

Nanostructure formation on silicon surfaces by high repetition-rate sub-15fs near-infrared laser pulsesM. H. Straub, K. König, Univ. des Saarlandes (Germany)

Laser-induced micro- and nanostructures on surfaces of crystalline silicon have been investigated intensely during the past decades. In particular, two types of ripple formation have been identified as self-organization phenomena. Ripples at a period of approx. 1 µm were explained by interference of the incident light field and a surface wave generated by periodic structural surface changes. In addition, tiny ripples at much shorter period of less than 100 nm were observed. So far, these structures were produced by lasers operating at pulse lengths in the order of 100 fs to 10 ns. Here, we report on the formation of characteristic surface features of low-indexed silicon surfaces such as Si(100), which were generated by sub-15 fs Ti:Sapphire laser pulses (centre wavelength 800 nm, bandwidth 120 nm, repetition rate 85 MHz). The high peak intensity in the tight focus of a high-numerical aperture objective allowed for profound structural and compositional changes at sub-nJ pulse energies. For example, lines written onto a Si(100) surface in water revealed changes in morphology at a period of 1.0 µm due to oxide particle formation at higher focal intensities or sub-10 nm hole arrangements surrounded by elevated areas at lower intensities. In contrast, a similar experiment with the Si(100) surface in oil mainly resulted in nano-scale deposition of carbon compounds in the exposed area. Our contribution includes analysis and discussion of a large variety of phenomena as well as a comparison with observations made using laser light of longer pulse width.

7920-28, Session 8

Optimization methods of hologram for holographic femtosecond laser processingY. Hayasaki, S. Hasegawa, Utsunomiya Univ. (Japan)

Parallel femtosecond laser processing with high throughput is indispensable to perform a wide volume fabrication composed of a huge number of processing points. Computer-generated holograms (CGHs) give features of high throughput and high light-use efficiency to the femtosecond laser processing. The CGH variably generates a desired arbitrary beam, such as a spatially-shaped beam, a split beam, and a wave-front corrected beam with low loss of light by use of a liquid-crystal spatial light modulator (LCSLM). A key requirement in an optimization of the CGH is a precise control of the diffraction peak intensity. We developed some optimization methods of the CGH. The diffraction peaks generated with multiplexed phase Fresnel lenses (MPFL) were made uniform by changing the center phase and size of each PFL, while taking account of the intensity distribution of the irradiated laser pulse and the spatial frequency response of the SLM. The uniformity of the diffraction peaks U was 88% at 10 parallel beams and the fabrication uniformity Ud was 80% at the irradiation energy of E = 2.4µJ. To obtain an MPFL with higher uniformity in the optical system, the MPFL was optimized using the diffraction peak intensities that were optically measured. The optimization obtained U=97% at 10 parallel beams and Ud=84% at E=2.4µJ. Recently we developed the second harmonic optimization



method. The method obtained U=97% at 18 parallel beams and the fabrication uniformity was fairly improved. In our presentation, our recent progresses of the CGH optimizations for holographic femtosecond laser processing are demonstrated.

7920-40, Session 8

Ultrafast microsphere near-field nanostructuringK. Leitz, U. Quentin, Lehrstuhl für Photonische Technologien (Germany) and Erlangen Graduate School of Advanced Optical Technologie (Germany); B. Hornung, Consultant (Germany); A. Otto, I. Alexeev, M. Schmidt, Lehrstuhl für Photonische Technologien (Germany) and Erlangen Graduate School of Advanced Optical Technologie (Germany)

Due to the steadily advancing miniaturisation in all fields of technology nanostructuring becomes increasingly important. Whereas the classical lithographic nanostructuring suffers from both high costs and low flexibility, for many applications in biomedicine and technology laser based nanostructuring approaches, where near-field effects allow a sub-diffraction limited laser focusing, are on the rise. In combination with ultrashort laser sources, that allow the utilisation of non-linear multi-photon absorption effects, a flexible, low-cost laser based nanostructuring with sub-wavelength resolution becomes possible. Among various near-field nanostructuring approaches the microsphere based techniques, which use small microbead particles of the size of the wavelength for a sub-diffraction limited focusing of pulsed laser radiation, are the most promising. Compared to the tip or aperture based techniques this approach is very robust and can be applied both for a large-scale production of periodic arrays of nanostructures and in combination with optical trapping also for a direct-write. Size and shape of the features produced by microsphere near-field nanostructuring strongly depend on the respective processing parameters.

In this contribution a basic study of the influence of processing parameters on the microsphere near-field nanostructuring with ultrashort pico- and femtosecond laser pulses will be presented. The experimental and numerical results with dielectric and metal nanoparticles on dielectric, semiconductor and metal substrates show the influence of particle size and material, substrate material, pulse duration, number of contributing laser pulses and polarisation on the structuring process.

7920-45, Session 8

Towards all-in-glass micro-actuators fabricated with femtosecond lasersY. Bellouard, Technische Univ. Eindhoven (Netherlands); A. A. Said, M. A. Dugan, P. Bado, Translume, Inc. (United States)

Femtosecond lasers exposure combined with a chemical etching step have been recently demonstrated as an efficient and versatile process for manufacturing complex three-dimensional structures in glass materials. This new fabrication method is particularly interesting for MEMS and in particular for monolithic structures that embed multiple functions, like waveguides, channels and mechanical elements.

Recently, we demonstrated that, and although it may sound counterintuitive, fused silica has excellent mechanical properties. Furthermore, we showed that flexures manufactured using this process can withstand very high stress and large deflection. As a proof-of-concept, an all-in glass monolithic micro-displacement sensor combining waveguides and a double compound linear guidance was made.

Here, we present a step towards fully integrated MEMS devices. In addition to the mechanical guidance and the sensing, we report on a scheme to introduce actuating functions to the device in which a set of electrodes embedded in the structure are used to drive the mechanism. In this paper, we describe the fabrication process and the performances of a first prototype of a femtosecond laser-manufactured actuator.


Generation of complicated or duplicated structure by interfering femtosecond laser processing of metallic thin filmT. Hiromoto, K. Momoo, Y. Nakata, N. Miyanaga, Osaka Univ. (Japan)

Interfering ultra-short pulse laser processing can generate periodic nano-structures in a single shot. We have processed thin metallic films, and generated nano-structures such as nano-waterdrop, nanocrown, nanobump etc.. They are generated by thermally process such as melting and inflation like bubbling, and the structures change according to the character of target and interference pattern on a target surface. We use a demagnification system attached to a transmission beam splitter as a beam correlator. An interference pattern of four 1st beams, diffracted by a transmission grating, is like a matrix. The distribution of nano-structures is periodic, according to the interference pattern. The period of the interference pattern can be changed by parameters of wavelength, correlation angle, intensity and phase shift distributions between the beams. In this experiment, we generated arranged periodic structures different from the past experiments, by changing the configuration of four 1st beams. With same intensity distribution between the beams, the peaks in an interference pattern is uniform. On the other hand, by changing the intensity distribution and phase shift between the beams at a same time, duplicated or complicated structures can be generated. In the former case, the density of nano-structures is doubled, and two different nano-structures appear alternately. In the latter case, each spot has not-circular distribution with some interference conditions. In addition, we changed a target structure to control the unit structure. All the structures generated in our experiments were observed by scanning electron microscope (SEM).


F2 laser surface modification of UV transparent polymer for selective cell culture and its mechanism studyY. Hanada, K. Sugioka, K. Midorikawa, RIKEN (Japan)

In this paper, we demonstrate surface modification of UV transparent polymer by F2 laser for selective cell culture. Most cell-based biological assays yield data averaged across large group of cells, yet it is well known that individual cells even those identical in appearance, differ in numerous characteristics. For this reason, much attention has been paid to selective cell culturing on transparent polymers for individual cell measurements. Recently, Asahi glass Co. Ltd. has developed an amorphous fluoric-polymer called CYTOP. CYTOP has refractive index of 1.34 which is comparable to that of water and has wide transmission range from 200 nm to 2 µm. Using this unique features, we demonstrated F2 laser surface modification of CYTOP to hydrophilize the CYTOP for the selective cell culturing. After the laser modification, wettability of the CYTOP was improved from high hydrophobicity. Therefore, selective cell patterning of an adhesion cell and that of 3-D microscopic observation can be performed. In the meanwhile, we investigated the mechanism of the F2 laser surface modification of CYTOP using X-ray photoelectron spectroscopy (XPS) and attempted to improve the wettabilitiy of the laser modified CYTOP. By immersing CYTOP in an organic solvent after the laser surface modification, wettability of the CYTOP was improved with water contact angle decrease to 58°at the minimum. Detailed mechanism of the laser modified CYTOP will be discussed at the presentation.




Multiphoton lithography and ITO structuring by high repetition-rate sub-15 femtosecond laser pulsesM. Afshar, S. Saremi, H. Völlm, D. Feili, H. Seidel, M. H. Straub, H. Zhang, K. König, Univ. des Saarlandes (Germany)

Due to its enormous application potential in recent years femtosecond fabrication of micro- and nanostructures has become the subject of intense research. Here, we report on experiments using a near-infrared Ti:Sapphire laser system based on a 85 MHz, sub-15 fs resonator. In combination with a precompensation unit of chirped mirrors the laser beam was focused tightly into the material using a microscope with a high-numerical aperture objective.

In the negative photoresist SU-8 multiphoton polymerization of 3D structures via combined beam and stage scanning resulted in a minimum line width of approximately 80 nm at aspect ratios in excess of 100. For comparison, structures were also produced at different pulse width of up to 2.7 ps using a glass rod as pulse stretcher. Moreover, the influence of scan parameters on the 3D structures was studied.

The second part of our contribution deals with sub-wavelength nanostructuring and laser-annealing of thin indium-tin-oxide (ITO) films sputtered on glass substrates. The ablation experiments allowed for the generation of cuts of 80 nm in width. The dependence of the minimum size of cuts on scan parameters as well as on film properties such as oxygen content was examined.

In conclusion, both our experiments on multiphoton lithography of polymers and laser structuring of ITO demonstrate that sub-100 nm structural features can readily be achieved using sub-15 fs pulsed light.


Non-destructive analysis on femtosecond laser doped siliconY. Lin, Harvard Univ. (United States); M. J. Smith, S. Gradecak, Massachusetts Institute of Technology (United States); E. D. Mazur, Harvard Univ. (United States)

Irradiating silicon substrates using femtosecond laser pulses in the presence of dopant precursors can result in the incorporation of high dopant concentrations beyond equilibrium values. This drastically alters the optical properties of the resulting material, extending absorption throughout the visible spectrum into the mid-infrared. In addition, this material exhibits metallic-like carrier transport. These properties make this material a unique candidate for optoelectronic devices. To further understand this material, the effects of laser irradiation and dopant precursors need to be correlated to structural and optical properties. In this work, we utilize non-destructive optical tools including Raman spectroscopy and Fourier transform infrared spectroscopy to characterize silicon irradiated by single femtosecond pulses. Using these techniques, we study laser-induced changes in crystallinity, dopant profiles, and optical properties as a function of processing parameters. Our findings contribute to a better understanding of the structural and optical characteristics, completing a picture of the silicon surface condition after femtosecond laser irradiation.


Manufacture of stacked-layers pattern by femto-second laser-induced forward transferC. H. Lu, B. H. Chen, C. M. Chang, M. L. Tseng, National Taiwan Univ. (Taiwan); C. H. Chu, H. Chiang, National Taiwan Ocean Univ. (Taiwan); D. P. Tsai, National Taiwan Univ. (Taiwan)

Laser-induced forward transfer (LIFT) is a simple, fast and only one-step process technology, which utilizes laser pulse to remove the thin film material (donor) from a transparent substrate and transfer the irradiated material forward to a receiver substrate. In this paper, we utilize LIFT to print the pattern of multi-layer thin film consisted of stacked gold and dielectric material on glass substrate. A transfer mechanism based on bursting of the chalcogenide material dynamic release layer has been demonstrated. The thin films are deposited on transparent glass substrates in a 5 x 10-1 pa argon chamber by sputter. After deposition, the donor and receiver substrates are contacted with each other. The combined sample is mounted on a x-y-z moving stage with a displacement resolution of 0.4 nm, and subsequently irradiated by the Ti:sapphire laser (wavelength of 800 nm) with pulse duration of 140 fs and repetition rate of 80 MHz. The characteristic of sample is studied by scanning electron microscopy (SEM). The dots patterns with different volume and size are fabricated by changing the laser power. The evaporation of sacrificial layer causes the explosive pressure as a driving force to transfer the materials in solid phase. The manufacture of three layers pattern by LIFT technique has been demonstrated. The proposed method has potential for manufacturing of three-dimension structure in plasmonic application.


Evaluation of a refractive index profiles for a modification induced by focused femtosecond laser irradiation in the optical glassesT. Hashimoto, S. Tanaka, New Glass Forum (Japan)

We report on a refractive index modification (Δn) induced by femtosecond irradiation and evaluation of the profile for created lines inside the different types of optical glasses, i.e., silicate or borate glass with/out containing the metal oxides such as BaO, TiO 2, or La 2O 3. The lines are fabricated by scanning a stage and focusing the femtosecond laser pulses, 800nm wavelength, a 250 kHz repetition rate and 300 fs pulse duration, from the Ti:sapphire regenerative amplifier system. The Δn profiles of modification were obtained with Qualitative Phase Microscopy technique and presented systematically for a different input power and a variety of glasses. The Δn profile changed with focusing condition using 10× (N.A.=0.3) or 40× (N.A.=0.85), and input power in a single glass. However, the Δn and a trend of the sign was different depending on glass types. For example, silicate glass containing TiO 2, exhibited negative Δn trend. Furthermore, the glass showed relatively large negative Δn, < -0.01 in the investigated power range. These results could be useful for a design or selection of glasses for micro optics produced by femtosecond laser fabrication.


Laser cutting of carbon fiber reinforced plastics (CFRP) by UV pulsed laser ablationH. Niino, R. Kurosaki, National Institute of Advanced Industrial Science and Technology (Japan)

We used DPSS UV laser (355 nm, P=4.5 W, rep.rate= 30 kHz, 20 ns). The laser beam on the sample surface (scan speed: 50 mm s-1) was scanned by multiple-scan-pass irradiation in the air.

The beam was focused with an f-theta lens (f=150 mm). 2 mm- and 0.5 mm-thickness CFRPs were employed for the cutting.




F2 laser formation of SiO2 protective layer onto polycarbonate for lightweight vehicle windowS. Sonobe, National Defense Academy (Japan); Y. Nojima, National Defense Academy (Japan) and Renias Co., Ltd. (Japan); M. Okoshi, National Defense Academy (Japan); H. Nojiri, Renias Co., Ltd. (Japan); N. Inoue, National Defense Academy (Japan)

Transparent, light polycarbonate (PC) is one of the leading candidates for lightweight window material, in addition to the excellent shock-resistance. However, PC shows a poor scratch-resistance; conventionally a protective coat is formed on PC before practical uses. The protective coat is generally made of silicone ([SiO(CH3)2]n) resin, but the scratch-resistance is still not sufficient as an alternative material of vehicle windowpane. In this paper, we formed a transparent SiO2 protective layer on the protective coat made of silicone on PC by the irradiation of a 157 nm F2 laser.

The present work is based on our previous finding; silicone rubber surface was photochemically modified into carbon-free SiO2, accompanied by swelling of the F2 laser-irradiated area. Experimentally, a conventional silicone-protective coat was formed on a 3-mm-thick PC plate through a 4-micron-thick acrylic primer. The thickness of the silicone-protective coat was changed from 3.7 to 10 micron. A laser beam generated from the F2 laser irradiated the sample surface. The single pulse fluence of the F2 laser was 14 mJ/cm2.

In the Taber abrasion test, the SiO2 layer remarkably reduced the number of the scratches, showing a low haze value. As a result, the Hz was successfully reduced to 3.6 %, compared with the cases in the nonirradiated sample and a bare polycarbonate of approximately 11.3 and 41.3 %, respectively, which is comparable to the case in a bare silica glass of approximately 1.6 %. We also discuss on the better conditions to obtain the higher scratch-resistance equal to silica glass.


Patterning of aluminum thin films by 157nm F2 laserK. Iwai, M. Okoshi, National Defense Academy (Japan); H. Nojiri, Renias Co., Ltd. (Japan); N. Inoue, National Defense Academy (Japan)

The fabricated patterns of metal thin films are used as an electrode or antenna in the devices. The use of laser is effective for the precise and fine pattering of thin films on substrate. Unlike the lithography process, the pattering of metal thin films based on laser ablation has been widely studied in laser processing; direct removal and laser-induced forward transfer are useful methods, for instance. In another approach, we used a 157 nm F2 laser to induce strong oxidation reaction only on the irradiated surface of metal thin films for selective chemical etching. The methodology is not based on laser ablation; only tens mJ/cm2 single pulse fluence of F2 laser is required. The use of F2 laser is essential for the photochemical reactions; active oxygen (O(1D)) atoms can be effectively obtained through the photodecomposition of O2 molecules in air. In this study, we applied the F2 laser to aluminum (Al) thin films on silica glass substrate to induce the photochemical surface modification for selective metallization. The Al thin film surface was strongly oxidized to show high resistance to potassium hydroxide (KOH) aqueous solution to etch the nonirradiated Al thin films for patterning. Moreover, high adhesion between Al and silica glass was found by the F2 laser irradiation. The mechanism of the high adhesion, that is the photochmiecal interface modification, was discussed.


Microwelding of glass substrates by double pulse irradiation of femtosecond laserM. Iida, RIKEN (Japan) and Tokyo Denki Univ. (Japan); Y. Hanada, K. Sugioka, RIKEN (Japan); H. Takai, Tokyo Denki Univ. (Japan); K. Midorikawa, RIKEN (Japan)

In this paper, we propose to use the double-pulse irradiation method with femtosecond laser for glass welding to improve both welding quality and efficiency. In the experiment, we irradiate double-pulse of femtosecond laser to the interface of stacked two glass substrates with a repetition rate of 200 kHz. The delay time of the two pulses is changed from 100 fs to around 2 ps. Efficient welding of glass can be achieved by double-pulse irradiation with the delay time of sub ps, since the avalanche ionization should occur within a couple of ps after the last pulse irradiation.


Laser micro welding of copper using a 532nm Nd:YAG laserG. J. Shannon, Miyachi Unitek Corp. (United States)

The problem of micro welding conductive parts is defined not only from a laser perspective but from a welding perspective. The reflectivity of copper to 1064nm wavelength is shown experimentally as a clear issue to welding success. Previous alternatives and process augmentations were reviewed and also attempted, and are shown either theoretically of experimentally to be flawed. The absorption of 532nm in copper is around 20 times that of 1064nm wavelength making it the ideal choice for copper micro welding, however until this point such a welding laser did not exist. The development of a patented stable intra cavity frequency doubling resonator enables millisecond pulses, 1.5kW peak power and pulse energy’s of 4J. This capability provides the necessary conditions for stable welding not only for copper but for other conductive materials and coatings such as silver. Experimental data of the laser is provided along with specific industrial examples that cannot be welded reliably by any other means.

7920-29, Session 9

Designer pulses for optimal ablationR. I. Stoian, J. Colombier, M. Guillermin, F. Garrelie, E. Audouard, Lab. Hubert Curien, CNRS, Univ. de Lyon, Univ. Jean Monnet Saint-Etienne (France)

Determining thermodynamic trajectories is an essential factor for controlling the nature and the energetic characteristics of the ablation products following laser irradiation of materials on ultrafast scales. In this respect, designing the energy delivery rate using pulse shaping methods in the temporal domain is a powerful way for controlling the excitation and thermodynamic relaxation of the material and its hydrodynamic advance. Using experimental and theoretical adaptive loops based on hydrodynamic codes we indicate the shapes of optimal pulses on ultrashort and short scales required to reach extreme thermodynamic states at limited energy input. These affect the excitation level and the energetic content of the ablation products, as well as the balance between thermal and mechanical energy, and usually imply light coupling into the incipient material hydrodynamic motion. Consequences are visible in the formation of atomic and cluster species, their kinetics and spectral emissivities, and in the ejection of nanoscale liquid droplets. A discussion on the nature of these resulting exotic thermodynamic states, mostly implying supercritical paths, will be given. The results are interesting for remote spectroscopy applications, e.g. LIBS, resulting ablation quality, and for generation of nanoparticles.



7920-30, Session 9

Micromachining display glass with picosecond lasersD. Mueller, H. Haloui, B. H. Klimt, LUMERA LASER GmbH (Germany)

As display glass substrates become thinner and thinner, mechanical cutting techniques are pushed to their limits and as a consequence yield is worsening significantly. At the same time, laser cutting techniques utilizing ns or even CW laser sources show little promise in mastering the increasing quality requirements of ever thinner substrates in the display glass business.

Future generations of cover glasses in the display market will as thin as 50µm. These thicknesses play to the advantages of cutting with picosecond lasers as the volume that needs to be removed is commensurate with ps-laser ablation rates and generation of micro-cracks can be minimized.

We will compare various cutting techniques utilizing picosecond lasers and present cutting data on widely used display glass substrates (such as Gorilla and Jade). More recently tests on glass as thin as 50µm have demonstrated cutting speeds beyond 100mm/s. These cutting tests have shown that excellent quality with minimal micro-cracks can be achieved and that cutting rates are quickly approaching an economic throughput.

Hybrid techniques that combine laser machining and mechanical force can achieve even higher cutting speeds while maintaining excellent edge quality. We will also show that picosecond lasers allow cutting complicated shapes such as cutouts in 600µm-thick display glass for mobile devices within about 1 minute.

7920-31, Session 9

Novel applications of sub-surface laser machiningB. R. Campbell, D. M. Bernot, L. A. Forster, The Pennsylvania State Univ. Electro-Optics Ctr. (United States)

Lasers can uniquely be used to create physical changes inside a bulk material. Traditional manufacturing processes are limited to surface modifications, but a laser can be focused at any location inside a material transparent to that wavelength. Using sub surface machining methods with ultrashort pulse lasers, two practical applications are demonstrated. First, a laser is used to sever wires causing unwanted connections embedded deep inside a thick piece of glass, effectively repairing a defective wire network. Second, subsurface bar-coding was shown to produce readable markings. Surface laser markings were shown to weaken the glass, but subsurface marking had virtually no effect on strength.

7920-32, Session 9

CO2 laser scribe of chemically strengthened glass with high surface compressive stressX. Li, B. R. Vaddi, Corning Inc. (United States)

Chemically strengthened glass is finding increasing use in handheld, IT and TV cover applications. Chemically strengthened glass, in particular those with high compressive stress (>600 MPa) and deep depth of layer, enables glass to retain higher strength than non-strengthened glass when its surface is abraded. CO2 laser scribe and break enables plurality of devices being made from a sheet of glass. Furthermore, CO2 laser scribe and break enables debris-free separation of glass with high edge strength. We investigated laser scribe of chemically strengthened glass with surface compressive stress greater than 600 MPa. We present the underlying laser scribing mechanisms. We further developed a method of mechanical initiation which enables consistent laser scribing process.

Using the initiation method and suitable beam parameters cross-scribe was repetitively carried out on GEN 2 size chemically strengthened glass substrates. Edge strength specimens were prepared using the laser scribe and break technique for glass with different thicknesses and different chemical strengthening. The specimens were tested using the 4-point bend method. Resulting strength distribution is presented.

7920-33, Session 9

Laser machining and plasma analysis using a novel trepanning systemG. Illing, D. Ashkenasi, T. Kaszemeikat, N. Müller, Laser- und Medizin-Technologie GmbH, Berlin (Germany); D. Diego-Vallejo, Instituto Politécnico Nacional (Mexico); H. Eichler, Technische Univ. Berlin (Germany)

Laser-induced micro ablation using diode-pumped short-pulsed solid state laser systems offer the possibility to machine and structure a whole range of different materials at adequate speed and high precision. To avoid negative thermal and mechanical effects at given laser parameters that may obstruct precision, LMTB has designed and implemented a novel optical concept for a versatile trepanning system enabling the adjustment of beam displacement and inclination during the fast circular rotation of the optics. The presented trepanning systems are designed for laser machining through-holes with a diameter in a range from 50 to 1000 µm. The through-holes can be processed with a taper of +3° to -3°, including the special case of cylindrical drilling. Starting from an early stage of implementation, the novel trepanning system has been customized for different applications with different industrial partners. The presentation outlines the development steps and advanced performance, accenting laser micro machining results utilizing the novel LMTB trepanning system in operation at varying laser parameters, such a pulse width and wavelength. In addition, laser induced breakdown spectroscopy (LIBS) has been carried out with the trepanning system to identify correlations between material processing effects and plasma spectra. The presence of characteristic patterns in the spectral data can be used to implement a control strategy to guarantee an efficient material removal rate.

7920-34, Session 10

Laser surface structuring with long depth of focusA. Michalowski, C. Freitag, R. Weber, T. Graf, Univ. Stuttgart (Germany)


7920-35, Session 10

A study on 8 beam laser diode to adopt laser scanning unitD. Song, QSI Co., Ltd. (Korea, Republic of)

In this paper, next generation 780nm monolithic individually addressable 8-beam diode laser with 10mW optical power for laser scanning unit were developed. Beam to beam spacing is 30µm and air bridge interconnection process was developed for individual operations.

Measured average values of threshold current(Ith), operating current(Iop), operating voltage(Vop), slope efficiency(SE), horizontal beam divergence(FFH), vertical beam divergence(FFV), and peak wavelength from 5 specimens are 14.91mA, 28.79mA, 1.91V, 0.72mW/mA, 8.28, 31.89, and 785.67nm respectively. All major electro-optic parameters from 8 emitters are within 2.5% variation for each device. Also we measured power droop that had a strong influence on printing image at 600Hz with duty 10% and 90% and we can obtained droop



rate of the within 1% in each channel at room temperature and 10mW power . Reliability life test at 70°C is on going and the test results will be discussed.

From the experimental measurement results, we can assure that the developed 8-beam diode laser is suitable optical source for high speed laser scanning unit in multi-function printing system and laser beam printers.

7920-36, Session 10

Advantages offered by high average power picosecond lasersC. J. Moorhouse, Coherent Scotland Ltd. (United Kingdom)

As microelectronic devices shrink in size, thinner material thicknesses are required to reduce material costs, device size and weight. A single step laser direct write process is an attractive alternative to conventional, multiple step photolithography processes by eliminating process steps and the cost of chemicals. The fragile nature of these thin materials makes them difficult to handle or machine either mechanically or by conventional Diode Pumped Solids State (DPSS) lasers. Picosecond pulses can cut materials with reduced damage regions and selectively remove thin films due to the reduced thermal effects of these short pulsewidths. Also, the high repetition rate allows high speed processing for industrial applications. Results of dicing of thin silicon wafers (100µm thick) are discussed, along with thin film removal processes using conventional Gaussian and flattop beam profiles are compared. For many picosecond processes, throughput can be increased by the use of a novel technique to using multiple beams from a single laser source is outlined.

7920-37, Session 10

Novel 1 2kW UV Laser system for micro fabrication and annealingL. Herbst, R. Paetzel, K. Schmidt, Coherent GmbH (Germany)

The growing demand for laser micro fabrication drives further requirements on higher production speed per part and lower manufacturing costs. A newly developed 1.2 kW 308 nm excimer laser addresses both micro-manufacturing and high production throughput.

Solid state UV laser sources usually cannot emit UV laser radiation directly. The inherently required frequency conversion modules limit the total output power to several 10 Watts below 350 nm. Furthermore these UV modules limit the long term reliability of high power UV solid state lasers significantly because of the wear of the conversion crystals.

Excimer lasers, however, overcome these issues by direct emission at 308, 248, or 193 nm. By now up to 540 Watts at 308 nm are established in production. With the new laser we have more than doubled the available output power to 1.2 kW.

The combination of short wavelength and highest available UV laser power makes it ideal for processing of small features or to modify thin surfaces. Furthermore, pulsed UV laser radiation is very suitable for removing delicate electronic devices from manufacturing substrates.

High-power UV laser systems are capable of processing large areas with resolution down to several microns in one single laser ablation step without using multiple lithography and wet chemical processes. For instance, laser Lift-Off and large area annealing have proven to be very efficient manufacturing techniques for volume production. In this paper, high-power excimer laser micro-machining, laser Lift-Off and annealing relevant applications will be presented and discussed.

7920-38, Session 10

Three-dimensional structuring inside transparent materials by a phase modulated fs laser beam with a LCOS-SLMM. Sakakura, K. Miura, Kyoto Univ. (Japan); T. Sawano, New Glass Forum (Japan); Y. Shimotsuma, Kyoto Univ. (Japan); K. Hirao, New Glass Forum (Japan)

A femtosecond laser processing system with a spatial light modulator (SLM) and its application are presented. Three-dimensional refractive index structures can be fabricated inside glasses by foscuing femtosecond laser pulses. To induce large refractive index increase without crack formation, irradiation time is needed. Therefore, fabrication efficiency is a critical problem. Our laser processing system with an SLM can improve the fabrication efficiency, because multiple light spots can be generated by modulating the spatial phase distribution of laser beam with an SLM. We will talk about the principle of the laser machining system as well as the applications for parallel writing of 3D optical waveguides, diffractive gratings, and optical data strage.

7920-26, Session 11

Recent status and prospects of the EU-funded ALPINE projectS. Selleri, Univ. degli Studi di Parma (Italy)

The EU funded ALPINE (Advanced Lasers for Photovoltaic INdustrial processing Enhancement) project aims to push forward the research on laser systems for the scribing of photovoltaic modules, by developing low cost, high energy, short pulse, all-in-fiber lasers for IR, visible and UV micromachining. A necessary requirement for these innovative systems is having fibers capable of amplifying the optical power without distorting pulse parameters. This requires very large mode area single-mode fibers and interfacing technologies enabling these to be integrated into the laser system. ALPINE is addressing these challenges, for both MOPA and Q-switched lasers, through the so called photonic crystal fiber technology, which has prior shown unsurpassed potential for very large mode area fibers. First steps are investigating and optimizing the fiber design in order to improve the polarization properties and laser stability, developing splicing processes for these fibers, achieving an all-in-fiber pump/signal combiner, and optimizing the efficiency of frequency doubling and tripling process.

ALPINE is expected to work on new materials other than silicon and to provide CdTe/CdS and CuInGaSe2/CdS thin film solar cells both on rigid and flexible substrates like glass, polymide or metallic foils. These different configurations pose challenges to the patterning processes P1, P2, P3, all of them targeted with fully laser-based scribing. This includes beam handling, new concepts for beam splitting and automation control, when integrating the laser into industrial machines. The quality process for each scribing pattern and for the whole cell has to be validated, in terms of increased efficiency and reliability.

7920-28, Session 11

The change of electrical properties of CIGS thin-film solar cells after structuring with ultrashort laser pulsesA. Wehrmann, H. Schulte-Huxe, M. Ehrhardt, K. Zimmer, Leibniz-Institut für Oberflächenmodifizierung e.V. (Germany); A. Braun, S. Ragnow, Solarion AG (Germany)

The laser patterning of Cu(In,Ga)Se2-based thin-film solar cells has recently received increasing attention. For a low-cost manufacturing it is essential to transfer single solar cells into a module by electrical interconnections in series. Whereat, the front contact of the previous



cell is connected with the back contact of the following cell. Therefore, specific scribing procedures are required to selectively separate layers from each other. The electrical contacts require optimisation with respect to the ohmic losses and the utilized active cell area. Laser processing techniques are promising for rapid scribing and numerous investigations were carried out. However, current studies show that due to the laser processing the material system is modified and the cell efficiency decreases as a result of edge degradation and local shunt formation.

The influence of laser processing parameters on the electrical behaviour of the solar cell was investigated. Hence, thin-film CIGS solar cells were scribed with ultrashort laser pulses and the changes of the electrical properties were measured directly after the structuring process. The material modifications of the processed areas were analyzed by scanning electron microscopy (SEM) in combination with electron dispersive X-ray spectroscopy (EDX) and focused ion beam (FIB). Additional investigations of the scribing area with electrical and optical methods are realised. The results are discussed and a model is introduced to address the understanding of the physical mechanisms and to enable further optimizations.

7920-29, Session 11

Laser processing of organic photovoltaic cells with a roll-to-roll manufacturing processT. Petsch, J. Hänel, B. Keiper, M. Clair, C. Scholz, 3D-Micromac AG (Germany)

Flexible large area organic photovoltaics are currently one of the fastest developing areas of organic electronics. New light absorbing polymer blends combined with new transparent conductive materials provide higher power conversion efficiencies while new and improved production methods are developed to achieve higher throughput at reduced cost. 3D-Micromac AG, a leading supplier of custom-made laser micro machining tools, develops new and innovative laser processes for OPV manufacturing. Various transparent conductive materials, used as front contacts, can be machined using ultra-short pulsed lasers such as pico- and femtosecond lasers. These lasers in combination with high performance galvanometer scanning systems achieve superior scribing quality without damaging the flexible substrate. With scribing speeds of 10 m/s and up it is possible to integrate this technology into a roll-to-roll manufacturing tool. In addition the laser can also be used for other necessary machining steps such as the patterning of the active layer and the back contact electrode. Besides ablation process steps 3D-Micromac AG is also developing a laser welding process for the improvement of the encapsulation characteristics of organic solar cells. As penetrating water and oxygen lead to a fast degradation of the active layer the encapsulation is vital for the OPVs performance and lifetime. The combination of large area printing technologies, high speed laser processes and roll-to-roll equipment enables high volume production of organic photovoltaics at reasonable costs. Therefore 3D-Micromac is currently designing a fully integrated production tool which incorporates all the aforementioned technologies into a single production line.

7920-30, Session 11

Multi-spot laser processing of crystalline solar cellsO. Haupt, V. Schütz, U. Stute, Laser Zentrum Hannover e.V. (Germany)

We report on fast and flexible laser processing technology for crystalline solar cells by using ultra-short laser pulses and a combination of Diffractive Optical Elements (DOE´s) for beam splitting with conventional scanner technology. The focus is laid on damage reduction, decreasing processing times, and efficient processing strategies. We demonstrate the process conversion from single-spot to multi-spot ablation of thin-films and bulk material, eg. nitride ablation and edge isolation. We will point out an increase in ablation efficiency by a factor of 3 and an

additional increase in processing speed by a factor of > 50 for surface ablation processes. The DOE in combination with scanner technology provides a fast and flexible system where only an industrial proven DOE has to be implemented in front of the scanner. Due to this modification the technology can be easily adapted. Using multi-spot technology for processing of crystalline solar cells, heat accumulation has to be analyzed. Limitations in spot distance and geometrical arrangements are discussed and described mathematically. Results and process windows will be shown for a thin-film ablation (surface) and a laser edge isolation (bulk) process on crystalline solar cells. An estimation of cycle times and area throughput will show the potential for using DOE´s especially combined with ultra-short pulse lasers.

7920-39, Session 11

Innovative laser based solar cell scribingB. Frei, S. Schneeberger, R. Witte, Solneva SA (Switzerland)

The solar photovoltaic market is continuously growing with boths crystalline silicon (c-Si) as well as in the thin film technology. This continuous growth is directly dependant on the manufacturing costs for solar cells. One factor for cost reduction are innovative ideas for an optimization of precision and throughput. Lasers are excellent tools to provide highly efficient processes with impressive accuracy. They need to be used in combination with fast and precise motion systems for a maximum gain in the manufacturing process, yielding best cost of ownership.

In this article such an innovative solution is presented for laser scribing in thin film Si modules. A combination of a new glass holding technique on a machine with a fast and precise motion system is the base for a cost effective scribing machine. In addition, the advantages of fiber lasers in beam delivery and beam quality guarantee not only shorter setup and down times but also high resolution and reproducibility for the scribing processes P1, P2 and P3. The precision of the whole system allows to reduce the dead zone to a minimum and therefore to improve the efficiency of the modules.

7920-40, Session 11

Femtosecond laser doped silicon for photovoltaic applicationsM. Sher, M. T. Winkler, Y. Lin, E. D. Mazur, Harvard Univ. (United States)

Silicon is doped to concentrations above the metal-insulator transition threshold via femtosecond laser doping techniques creating a novel material that has potential for photovoltaic applications. By focusing femtosecond laser pulses on the surface of silicon in a sulfur hexafluoride (SF6) environment, silicon is doped with 1% atomic sulfur. This material exhibits near-unity, broadband absorption from visible to lower than 0.5 eV near infrared light, deep below the silicon bandgap. This material also exhibits metallic-like conduction. These unusual optical and electronic properties suggest the formation of an intermediate band. We report on the femtosecond laser doping techniques and material properties. By changing the laser parameters and ambient environment we can control the dopant profiles, crystallinity and surface morphology. In addition, we use temperature-dependent Hall measurements to investigate electron transport as well as identify the energy states of the sulfur donors and hence the location of the intermediate band. We will also discuss potential applications for intermediate band photovoltaics.



Conference 7921: Laser-based Micro- and Nanopackaging and Assembly VTuesday-Thursday 25-27 January 2011 • Part of Proceedings of SPIE Vol. 7921 Laser-based Micro- and Nanopackaging and Assembly V

7921-01, Session 1

Direct laser writing for nanophotonicsG. von Freymann, M. Thiel, J. Fischer, I. Staude, M. Wegener, Karlsruhe Institute of Technology (Germany)

Direct Laser Writing (DLW) recently evolved into an established and commercially available technique for rapid prototyping of three-dimensional nano- and micro-architectures. Based on two-photon polymerization a broad variety of photoresist materials can directly be structured. Usually, ultrafast lasers operating at near-infrared wavelengths are employed as light sources. With 780 nm laser wavelength 100 nm lateral feature sizes are routinely achieved. However, the index contrast achievable with photoresist materials is most of the time not sufficient to open complete photonic bandgaps in photonic crystals fabricated via DLW. A conversion into high-index-of-refraction materials, e.g., silicon, overcomes this problem [1,2]. Here, we report on our recent progress on the fabrication of photonic crystals with a complete bandgap containing functional defects. The challenge is the further reduction of the feature sizes to comfortably reach telecommunication wavelengths or wavelengths even below. Two approaches will be presented: (i) Utilizing a continuous-wave laser operating in the green (532 nm) [3] and (ii) sDLW (Stimulated-emission depletion inspired DLW). While (i) increases the robustness of the system and decreases the lateral feature size due to the reduced diffraction limit, (ii) completely overcomes the diffraction limit and allows for smaller feature sizes in the lateral and axial directions [4].

[1] N. Tetréault et al., Adv. Mater. 18, 457 (2006)

[2] I. Staude et al., Opt. Lett. 35, 1094 (2010)

[3] M. Thiel et al., in preparation (2010)

[4] J. Fischer et al., Adv. Mater., DOI 10.1002/adma.201000892, in press (2010)

7921-02, Session 1

Laser forward transfer for digital microfabricationA. Piqué, H. Kim, R. A. Auyeung, A. Birnbaum, N. A. Charipar, K. Metkus, S. Mathews, U.S. Naval Research Lab. (United States)

Digital microfabrication processes are non-lithographic techniques ideally capable of directly generating patterns and structures of functional materials for the rapid prototyping of electronic, optical and sensor devices. Laser Direct-Write is an example of digital microfabrication that offers unique advantages and capabilities. A key advantage of laser direct-write techniques is their compatibility with a wide range of materials, surface chemistries and surface morphologies. These processes have been demonstrated in the fabrication of a wide variety of microelectronic elements such as interconnects, passives, antennas, sensors, power sources and embedded circuits. Recently, a novel laser direct-write technique able to digitally microfabricate thin film-like structures has been developed at the Naval Research Laboratory. This technique, known as Laser Decal Transfer is capable of generating patterns with excellent lateral resolution and thickness uniformity using high viscosity metallic nano-inks. The high degree of control in size and shape achievable has been applied to the digital microfabrication of 3-dimensional stacked assemblies, MEMS-like structures and free-standing interconnects. Overall, laser forward transfer is perhaps the most flexible digital microfabrication process available in terms of materials versatility, substrate compatibility and range of speed, scale and resolution. This talk will describe the unique advantages and capabilities of laser forward transfer, discuss its applications and explore its role in the future of digital microfabrication.

This work was sponsored by the Office of Naval Research.

7921-03, Session 1

Formation of Si and Ge films and micropatterns by wet process using laser direct writing methodA. Watanabe, Tohoku Univ. (Japan)

The challenges toward solution processing of Si and Ge semiconductor materials have been made recently. The development of Si and Ge semiconducting films by wet process provides the possibilities to realize the low cost manufacturing of electronic devices. In this presentation, the attempts toward the formation of Si and Ge films and micropatterns by wet process using laser direct writing method are reported. First one is the laser direct writing of polycrystalline Ge micropatterns using Ge ink. In the study, an organogermanium nanocluster (OrGe) which has a germanium nanocluster of a few nanometers and organic groups modifying the nanocluster was used as a Ge ink. Such an organosoluble germanium nanocluster was prepared from tetrachlorogermane in a solution using Mg, and then the surface was modified by an organic group. The OrGe is a stable Ge source which can be handled even in the air. The structural changes of the Ge skeleton were investigated by micro-Raman spectroscopy. The second one is the formation of the Si and Ge micropatterns at liquid/substrate interface by laser direct writing. The absorption of the laser beam at the surface of a substrate causes the formation of Si and Ge films at the liquid/substrate interface and the crystallization. The third one is the formation of Si film by laser irradiation to a Si-particle dispersed film. Some additives induce the melting of Si particle by laser irradiation effectively.

7921-05, Session 2

Extending the process limits of laser polymer welding with high-brilliance beam sources (recent status and prospects of POLYBRIGHT)A. M. Olowinsky, Fraunhofer-Institut für Lasertechnik (Germany)

Plastics play an important role in almost every facet of our lives and constitute a wide variety of products, from everyday products such as food and beverage packaging, over furniture and building materials to high tech products in the automotive, electronics, aerospace, white goods, medical and other sectors.

The objective of PolyBright, the European Research project on laser polymer welding, is to provide high speed and flexible laser manufacturing technology and expand the limits of current plastic part assembly. New laser polymer joining processes for optimized thermal management in combination with adapted wavelengths will provide higher quality, high processing speed up to 1 m/s and robust manufacturing processes at lower costs. Key innovations of the PolyBright project are fibre lasers with high powers up to 500 W, high speed scanning and flexible beam manipulation systems for simultaneous welding and high-resolution welding, such as dynamic masks and multi kHz scanning heads.

With this initial step, PolyBright will break new paths in processing of advanced plastic products overcoming the quality and speed limitations of conventional plastic part assembly. Completely new concepts for high speed processing, flexibility and quality need to be established in combination with high brilliance lasers and related equipment. PolyBright will thus open new markets for laser systems with a short term potential of over several 100 laser installations per year and a future much larger market share in the still growing plastic market. PolyBright will hence establish a comprehensive and sustainable development activity on new high brilliance lasers that will strengthen the laser system industry.


7921-06, Session 2

Effect of different laser intensity profiles on laser brazed ceramic-steel jointsI. J. Suedmeyer, M. Rohde, H. Besser, B. Liesching, M. Grein, J. Schneider, Karlsruhe Institute of Technology (Germany)

Oxide and non oxide ceramics (Al2O3, SiC) were brazed to commercial carbon steel with active filler alloys using a CO2-laser (l = 10.64 µm). Two different laser intensity profiles were used for heating up the compound: A laser output beam presenting a Gauss profile and a homogenized, nearly top head profile were applied for joining the compounds in an Argon stream.

The temperature distribution with and without the homogenizing optic was measured by an infrared camera during the process and compared to the results of a finite element model simulating the brazing process with the different laser intensity profiles. Polished microsections were prepared for characterization of the different joints by scanning electron micrographs and EDX-analysis. In order to evaluate the effects of the different laser intensity profiles on the compound, the shear strengths of the braze-joints were determined. Additionally residual stresses which were caused by the gradient of thermal expansion between ceramic and metal were determined by finite element modeling.

The microsections did not exhibit differences between the joints, which were brazed with different laser profiles. However the shear tests proved, that an explicit increase of compound strength up to 34 MPa of the ceramic-metal joints can be achieved with the top head profile, whereas the joints brazed with the Gauss profile achieved only shear strength values of 24 MPa. Finally tribological pin-on-disc tests proved the capability of the laser brazed joints with regard to the application conditions.

7921-07, Session 2

Micro joining of thin metallic films on flexible substrates with nanosecond laser pulsesM. Ehrhardt, K. Zimmer, A. Wehrmann, Leibniz-Institut für Oberflächenmodifizierung e.V. (Germany)

The joining of thin-film components for mechanical or electrical connection has become progressively more important for different applications in micro technology, thin-film technique and photovoltaic. However, the application of well-established techniques like wire bonding and soldering requires either specific metal layers such as gold or a sufficient film thickness.

Classical laser processes for material joining have been developed mainly for steel and heavy industrial as well as for precision engineering. However, due to the much lower thickness of the used films compared to classic approaches and the usage of thermal sensitive substrates, e.g. polymers or semiconductor materials, new approaches have to be developed.

In this contribution a novel laser process for joining thin films will be presented which enables the mechanical and electrical connection of two thin metal films or foils of similar or different metallic materials like Ag/Ag, Ag/Ti, Ag/Al, Al/Cu, Ag/Cu, Al/Cu which have a thickness down to 500 nm.

The connection is achieved without an additional auxiliary material and a minimal damage of the thermal sensitive substrate. In dependence on the selected material system and processing parameter the connection is created by a combination of geometric tooting or thermal intermixing of the metals.

Results of the joined metal films are characterized in terms of electrical properties like current carrying capacity and mechanical stability like the tensile strength.

7921-31, Session 2

Laser welding assembling of an implantable bio-medical device: investigation of temperature fieldY. Lin, G. Jiang, J. Calderon, Alfred E. Mann Foundation for Scientific Research (United States)

We report a process of hermetically sealing the Functional Electrical Battery Powered Microstimulator (FEBPM) by laser welding technology for medical implants application. A pulsed 1064nm Nd:YAG laser system with a peak power capability of 6 Kw is employed in the study. An Eyelet Subassembly and a Case Subassembly are laser welded together, making a strong, hermetic joint. The materials are Ti-6Al-4V. In order to protect the subcomponents such as the battery and the electronic modules inside the case, and to maintain the mechanical integrity of the subassemblies, it is decided that the temperature distribution along the laser weld joint to be determined. A temperature indicating liquid (paint) is applied on the surfaces near the laser weld joint and allowed to dry before the laser welding is initialized. During laser welding, the color of the paint at the surfaces changes, indicating the temperature at the surfaces reached the melting point of the paint. A numerical simulation by means of finite element analysis (FEA) is performed to obtain the information of the temperature distribution. The numerical model simulates the laser beam welding process. The numerical model is established and refined based on experimental investigation. The effect of laser input power and the laser beam size on the temperature distribution is investigated subsequently and the predicated results are in accordance with experimental investigation. The established numerical model can be used not only in estimating the temperature distribution, but also in the predication of the mechanical responses including the residual stresses and the displacements in the assembly, due to the laser welding process.

7921-09, Session 3

Laser based manufacturing of shunt lines for OLED lightingM. Ruske, H. Schwab, Philips Technologie GmbH (Germany)

OLED lighting is a new player in the arena of high efficiency, long lived light sources. In contrast to inorganic LED, OLED is a technology to realise large area light sources. The base technology is already in wide use for displays like mobile phones or PDA’s. One of the main attention points in this technology is the need to realise large are light sources by at the same time maintaining low cost as we have to compete with classical light sources. The main cost driver in OLED lighting is the substrate compromising a glass plate, a transparent conductive oxide and a metal grid to support current distribution in the area. These substrates are manufactured using passive matrix display technology. The glass plate is coated with a layer of the conductive oxide and a second layer of the matal which is then sequentially structured again using photo litography. In collaboration with the Fraunhofer ILT group we have developed a new technology to realise metal shunt lines on a transparent electrode. This process makes use of a pulsed laser system used to melt and partially vaporise metal from a carrier foil. This metal is transferred onto the OLED substrate. In order to assure good line definition the carrier foil is structured in such a way that the structure acts as a shadow mask defining the deposition area. Using this technology it is possible to relaise shunt lines of various materials with a conductivity close to bulk conductivity at high transfer rates.

Conference 7921: Laser-based Micro- and Nanopackaging and Assembly V


7921-10, Session 3

Strategy of fabrication of complex shape parts based on the stability of single laser melted trackI. Yadroitsev, M. Doubenskaia, I. Smurov, Ecole Nationale d’Ingénieurs de Saint-Etienne (France)

Selective laser melting (SLM) is known as a unique technology to produce objects from metal powders with complex geometry and mechanical properties comparable to those of bulk materials. The essential operation at SLM is the laser beam scanning over the surface of a thin powder layer previously deposited on previously remelted layer. Line-by-line, a laser beam melts material along a row of powder particles, thereby forming a molten pool. Under the effect of surface tension, the molten pool takes the shape of a circular or segmental cylinder. Fragmentation of the remelted tracks is a well-known drawback of SLM referred to as the “balling” effect. The features of the tracks’ instability depend of laser power, scanning speed, powder layer thickness, substrate material, physical properties and granulomorphometry of the powder used.

Analysis of the formation of single tracks showed that the process has a threshold character: there are “stability zones” where the track is continuous, and “instability zones”. Instabilities appear at low scanning speed in the form of distortions and irregularities, and, on the contrary, excessively high speed gives rise to the balling effect. The range of the optimal scanning speed is larger for higher laser power, and it narrows for material with high thermal conductivity. The influence of processing parameters and manufacturing strategies on the mechanical properties of the fabricated objects was analyzed. Near full-density samples with excellent mechanical strength were manufactured from SS grade 316L, CoCr alloy and Inconel 625 powders by two-zone cross-hatching technique.

7921-11, Session 3

Optical monitoring of heat processes in selective laser meltingM. Doubenskaia, M. Pavlov, Y. Chivel, Ecole Nationale d’Ingénieurs de Saint-Etienne (France)

An original optical diagnostic system was developed and integrated with commercial PHENIX PM-100 Selective Laser Melting machine.

Variations of the pyrometer signal with operational parameters were analyzed for the following cases:

- Variation of the hatch distance for 50 µm thickness powder layer (good metallurgical contact with substrate): Three different zones of signal variation are found: (a) constant signal for low hatch distances when the consecutive track is so close to the previous one that they form direct metallurgical contact; (b) constant signal for large hatch distances when interaction between the tracks is practically absent; (c) transition zone where the pyrometer signal increases with hatch distance.

- Variation of the hatch distance for 1 mm thickness powder layer (no metallurgical contact with substrate): For the hatch distance less than 300 µm, a significant increase of the pyrometer signal at the beginning of the scanning is caused by heat accumulation in the previously fabricated tracks which are in direct metallurgical contact, and by low heat losses into the surrounding powder layer.

- Variation of the powder layer thickness: The increase of the pyrometer signal with powder layer thickness is the result of the energy balance between powder melting and heat losses into the substrate. Starting from a certain critical value, the pyrometer signal reaches the maximum and stable value because the contact between melted powder layer and substrate is lost, and, thus, all the laser energy is absorbed by powder, and no heat losses into the substrate takes place.

7921-12, Session 3

Laser-assisted synthesis of diamond crystals in open air through vibrational excitation of precursor moleculesZ. Xie, Y. Zhou, X. He, Y. Gao, J. B. Park, T. Guillemet, Y. Lu, Univ. of Nebraska-Lincoln (United States)

Fast growth of diamond crystals in open air was achieved by combustion synthesis with resonant laser energy coupling. A wavelength-tunable CO2 laser (spectrum range from 9.2 to 10.9 µm) was used for the coupling of laser energy in synthesis of diamond crystals. A pre-mixed C2H4/C2H2/O2 gas mixture was used as precursors. Through resonant excitation of the CH2-wagging mode of ethylene (C2H4) molecules using the CO2 laser tuned at 10.532 µm, high-quality diamond crystals were grown on silicon substrates with a high growth rate at ~139 µm/hr. Diamond crystals with a length up to 5 mm and a diameter of 1 mm were grown in 36 hours. Sharp Raman peaks at 1332 cm-1 with full width at half maximum (FWHM) values around 4.5 cm-1 and distinct X-ray diffraction spectra demonstrated the high quality of the diamond crystals. The effects of the resonant excitation of precursor molecules by the CO2 laser were investigated using optical emission spectroscopy.

7921-13, Session 3

Diameter modulation of carbon nanotubes by rapid temperature modulation in laser-assisted chemical vapor depositionM. Mahjouri Samani, Y. Zhou, W. Xiong, Y. Gao, M. Mitchell, Y. Lu, Univ. of Nebraska-Lincoln (United States)

Periodic diameter modulation of carbon nanotubes by quick temperature variation was successfully achieved in laser-assisted chemical vapor deposition process. Tapered and alternating-diameter CNTs were grown by periodic modulation of the temperature due to inverse relationship between the temperature and the diameter of the CNTs. The modulated-diameter single-walled carbon nanotubes (SWNTs) were integrated into field-effect transistors (FETs) structure to investigate their electronic transport properties. The tapered SWNTs showed electronic properties similar to Schottky diodes indicating clear evidence of different bandgap structures at two ends of the tubes. However, the electronic transport of the alternating-diameter SWNTs showed a huge resistance which is attributed to the large number of defects and the electron confinement in the periodic quantum well arrays. Transmission electron microscopy and Raman spectroscopy were also studied to investigate the structural and electronic properties of the structures.

7921-14, Session 4

Ultrafast laser fabrication of 3D photonic structures in rare-earth doped glasses and nonlinear optical materialsK. P. Chen, Univ. of Pittsburgh (United States)

Femtosecond ultrafast lasers have been shown to be effective for inducing refractive index changes in transparent dielectric materials. Embedded waveguides have been fabricated by ultrafast lasers in wide array of transparent materials including silica glasses, nonlinear crystals, and laser ceramic materials.

In this paper, we present research results on ultrafast laser fabrication of complex multi-layer optical structures in active glass and nonlinear optical materials. Multi-layer 4×4 lightwave circuits were fabricated in silica glass for inter-chip optical interconnect; ring oscillators were fabricated in Nd:YAG ceramic materials for on-chip waveguide laser; and o-ring resonators were laser written in LiNbO3 nonlinear crystals for all-



optical optical switching.

This paper demonstrates that an ultrafast laser can be a unique and versatile fabrication tool to produce complex and useful 3D lightwave and optical structures. Although waveguide loss is higher than 1 dB/cm in some materials, the capability of producing multi-layer structures via one-step direct writing make the ultrafast laser a useful fabrication tool for some highly specialized applications, which are not easily attainable by other means.

7921-15, Session 4

3D adaptive spatio-temporal control of laser-induced refractive index changes in optical glassesR. I. Stoian, Lab. Hubert Curien (France)

The nonlinear optical absorption character has underlined the potential of ultrafast laser pulses for 3D processing of transparent materials, particularly for adding optical functions by refractive index engineering involving thermo-mechanical and structural rearrangements of the dielectric matrix. Nevertheless, challenges are related to the time-effectiveness of irradiation, correct beam delivery and the influence of material properties on the exposure results. Particularly for light-guiding applications, it is for example suitable to master positive refractive index changes in a time-efficient manner, where the result depends on deposited energy and its relaxation paths.

To address these challenges several irradiation concepts based on adaptive optics in spatial and temporal domains were developed. We review here some of the applications from different perspectives. A physical aspect is related to time-synchronized energy delivery tuned on material transient reactions, enabling thus a synergetic interaction between light and matter and, therefore, optimal results. Examples will be given concerning refractive index flip in thermally expansive glasses by thermo-mechanical regulation, energy confinement by nonlinear control, accompanied by time-resolved investigations of the refractive index change. A second engineering aspect is related to processing efficiency. We give insights into 3D parallel complex photoinscription techniques and beam-delivery corrections utilizing dynamic wavefront engineering.

Additionally, in energetic regimes ultrafast laser radiation can generate an intriguing nanoscale spontaneous arrangement, leading to form birefringence and modulated index patterns. Using the birefringence properties and the deriving anisotropic optical properties, polarization sensitive devices were designed and fabricated. The polarization sensitivity allows particular light propagation and confinement properties in 3D structures.

7921-17, Session 4

Laser-chemical finishing of micro forming toolsA. Stephen, C. Gerhard, F. Vollertsen, Bremer Institut für angewandte Strahltechnik GmbH (Germany)

In this contribution, we report on a laser-chemical removal method for precise machining of micro forming tools. Here, a focused machining laser beam is guided coaxially to an etchant jet stream. Since the material removal is caused by laser-induced chemical reactions using this method, machining is achieved at low laser powers. Hence, material stressing involving micro cracks and further parasitic effects can be avoided. Due to those advantages, this method offers a suitable technique for the finishing of precision micro tools. Several experiments have been performed at rotary swaging jaws made of Stellite 21 in order to chamfer the edged transition section between operating sphere and tool flank. The influence of both different laser powers and work piece traverse speeds has been investigated. For this purpose, several parallel laser paths were applied along the edged transition section when varying the process parameters.

Here, the incident laser beam is subjected to different angles of incidence. Due to reflection effects, the process parameters have to be matched with respect to the particular angle of incidence during the machining. In this vein, the edged transition section of rotary swaging jaws was chamfered at radii in the range of 120 microns.

7921-27, Session 4

Laser selective patterning of ITO on flexible PET for organic photovoltaicsS. Xiao, A. Ostendorf, Ruhr-Univ. Bochum (Germany)

Flexible organic photovoltaics have gained increasing interests during the last decade. Toward increasing the efficiency and decreasing the cost per Watt, they are on their way to the market. The approach of laser patterning technology has been expected to motivate the industrialization of organic photovoltaics. In this paper high repetition picosecond laser radiation fabricated trenches of ITO on flexible PET(Polyethylene terephthalate) substrate are presented. In order to obtain clean removal ITO layer without damaging PET substrate, various laser wavelengths, frequencies, fluencies with different scanning speeds and scanning strategies are applied and optimized. The results analyzed by SEM and white-light interference microscopy demonstrate that picosecond laser selective etching ITO on PET substrate is feasible.

7921-18, Session 5

High aspect ratio taper-free micro and nano-channel fabrication in glass with ultrafast nondiffracting Bessel beamsM. K. Bhuyan, F. Courvoisier, M. Jacquot, P. Lacourt, R. Salut, L. Furfaro, J. M. Dudley, Univ. de Franche-Comté (France)

Micro and nanochannels are essential structures for micro/nano fluidic and nanophotonics devices. Femtosecond machining is particularly attractive for the fabrication of such devices because of its low cost and its ability to rapidly machine complex micron and nanometer scale structures in two and three dimensions. In this presentation, we will review our recent work using for the first time nondiffracting femtosecond Bessel beams to overcome many of the difficulties of Gaussian beam femtosecond laser micro and nano-processing. Our results show that Bessel beams can be used to generate taper-free channels of ~2 µm diameter and ~80 µm length in glass in a straightforward setup without the need for any sample translation. We identify a working window for the practical use of Bessel beams for glass micromachining. We also demonstrate the generation with a single laser pulse of terminated and through nano-channels in glass with diameters in the range 200-800 nm with aspect ratio that can exceed 100. We interpret our results in terms of a specific regime of stable stationary propagation regime of Bessel beams even at ablation-level intensities. These results represent a significant advance of femtosecond laser ablation technology into the nanometric regime.

7921-19, Session 5

Nanoprocessing of glass and PMMA by means of near infrared sub-15 femtosecond laser pulsesH. Zhang, K. König, M. Afshar, D. Feili, H. Seidel, Univ. des Saarlandes (Germany)

Laser material processing through multiphoton absorption has attracted high attentions in the past years. It allows non-contact, sub-wavelength and three-dimensional structuring. Especially, extreme short laser pulses provide high light intensity with low average power and permit



generation of nanostructures with high precision comparing to other laser sources. In this work, a near infrared sub-15 femtosecond laser scanning microscope was employed for structuring of colored glass and PMMA. The 400 mW Ti-Sapphire laser works at 85 MHz with an M-shaped emission spectrum with maxima at 770 nm and 827 nm. By using a high NA objective, light intensity of about 10 TW/cm² at the focal plane can be reached. The absorption edge of the glass and the plexiglass lies at 435 nm and 450 nm, respectively. A mean power of less than 17 mW, which corresponds to the pulse energy of 0.2 nJ, was sufficient for drilling holes and ablating materials. Two-photon fluorescence measurements, which can be performed with the same microscope, reveal the creation of filaments within the specimens, likely caused by self focusing effects. Holes with a diameter of less than 170 nm, were produced without chemical etching. The results are compared with those obtained by using 300 fs and 3 ps laser pulses. This work was supported by the German Research Association within the priority program 1327.

7921-20, Session 5

Realization of high-performance optical element by optical near-field etchingK. Hirata, Sigma Koki Co., Ltd. (Japan)

Recent year, they require the high performances of laser as a light source in variety application area. For instance, those are a shorter wavelength, a shorter pulse width.In order to serve those needs, an improvement of the laser damage threshold value of optical element used in the laser applications is required. And they reported that a surface-roughness of glass substrate as used coated optical element exert also influence that. Currently, the chemical mechanical polishing method (CMP method) is general used as the polishing method of optical element. This method is a friction method. Therefore, the reduction of the surface-roughness is prevented by generation of scratches and digs that keep happening by contamination in slurry. In order to solve this problem, we propose the optical near-field etching method (ONE). The ONE is operated by irradiation of a SHG light (532nm) of Nd:YAG laser on glass substrate in chlorine gas atmosphere that have a optical absorption band edge of 400nm. The radical formation of the chlorine molecular is created by non-adiabatic photochemical reaction due to optical near-field occurred in glass surface. And the etching is progressed in the projection of glass surface. With this processing, we can achieve the reduction of Ra value of surface-roughness from 0.2nm to 0.13nm. In addition, we gave the mirror coating to the glass substrate to which the surface-roughness was improved by ONE and measured the laser damage threshold value, and so we obtained 14.0J/cm2 as the laser damage threshold value. The laser damage threshold value of the glass substrate without ONE is 8.2J/cm2. It is shown that the laser damage threshold increase by 1.7 times by ONE.

7921-21, Session 5

Highly efficient diffractive beam splitters surface-structured on submicron scale using deep-UV laser interference lithographyJ. Amako, D. Sawaki, E. Fujii, Seiko Epson Corp. (Japan)

Diffractive beam splitters are an effective means of attaining a high throughput in laser-based processes. To make use of laser power, the splitter’s light-use efficiency needs to be maximized by reducing Fresnel reflections. Usually anti-reflective coating is employed and yet thin-film coats may be damaged under prolonged intense laser irradiation and no appropriate films are available at some laser wavelengths. We will report highly efficient diffractive beam splitters surface-structured on submicron scale. Submicron relief structures formed on the surfaces of a splitter function as an anti-reflective layer to improve the efficiency. Surface structuring is conducted using deep-UV, liquid-immersion interference lithography in combination with dry etching. The resist layer, as immersed in an index-matching liquid, is exposed to the interference fringes set intersecting the peaks and troughs on diffractive surfaces. Rigorously

designed structures having a period of 140 nm and a depth of 55 nm are lithographed onto fused-silica splitters. Splitting efficiencies at 266 nm are increased by 8.0% to compare favorably with a theoretical value, while the reflections are substantially reduced. The splitting uniformity obtained after structuring is over 0.90, which is practically sufficient. These performances are sensitive to the profile and depth of the sub-wavelength structures, and the dimensions of the structures depend on a wavelength. Potential of the surface structuring will be elucidated through discussions over fabrication errors and design wavelengths. The surface-structured beam splitters presented here are of great use in industrial machining applications using high-power pulsed lasers, such as drilling, scribing and joining.

7921-22, Session 6

Effects of incident laser energy on device performance in printed microbatteries and supercapacitorsC. B. Arnold, Princeton Univ. (United States)

Laser induced forward transfer printing has been shown to be a successful method of producing thick-film patterns of complex materials for integrated energy storage devices. Battery and supercapactor electrodes, separators, and electrolyte have been deposited to produce small scale prototypes. In this presentation, we focus on the effects of the incident laser on the properties and performance of these materials. We demonstrate that irradiation of electrochemically active materials during the printing process can have a significant effect on the energy storage and power delivery performance by modifying the surface structures and chemistry. The influence of incident laser energy during deposition is studied and exploited in order to modify and optimize electrochemical properties. At low transfer energy densities, the material maintains its structural, morphological, and electrochemical properties in comparison to control samples. However, higher energy density produces noticeable changes to the morphology, structure, and electrochemistry leading to modifications in the transport properties. Implications of this process for cell fabrication on low temperature substrates and integration with existing devices will be discussed.

7921-23, Session 6

Laser processing of electrode materials for manufacturing of 3D micro-batteriesR. Kohler, J. Pröll, C. Ziebert, M. Bruns, M. Rinke, S. Heissler, C. Adelhelm, Karlsruhe Institute of Technology (Germany); M. Przybylski, ATL Lasertechnik GmbH (Germany); W. Pfleging, Karlsruhe Institute of Technology (Germany)

The material development for advanced lithium ion batteries plays an important role in future mobile applications and energy storage systems. It is assumed that electrode materials made of nano-composited materials will improve battery lifetime and will lead to an enhancement of lithium diffusion and thus improve battery capacity and cyclability. Lithium cobalt oxide (LiCoO2) is commonly used as a cathode material. Thin films of this electrode material were synthesised by r.f. magnetron sputtering of LiCoO2 targets in pure argon plasma on silicon and stainless steel substrates. A major problem concerning thin film electrodes is, that increasing film thickness leads to an increase in lithium diffusion path lengths and thereby a decrease in power density. To overcome this problem, the investigation of a 3D-battery system with an increased surface area is necessary. UV-laser micromachining is applied to create defined line or grating structures via mask imaging. Within the patterning process conical surface structures with high aspect ratios and structure sizes in the range of 0.5 µm - 2 µm were formed. Because of cone growing processes only little material loss was observed.

For the formation of the high temperature phase of LiCoO2 (HT-LiCoO2), which exhibits good electrochemical performance with a specific capacity of 140 Ah/kg and high capacity retention, a subsequent



annealing treatment is necessary.

For this purpose laser annealing of thin film LiCoO2 using a high power diode laser system (wavelength =940 nm) was investigated in detail. Different annealing temperatures (between 200°C and 700°C) and process gases (argon, oxygen, helium, nitrogen, ambient air) were applied. The influence of the annealing treatment on the surface micro-structures was discussed.

The as-prepared and the laser treated films were studied with Raman spectroscopy, X-ray photoelectron spectroscopy and X-ray diffraction to determine their stoichiometry and crystallinity. The electrochemical properties of the manufactured films were investigated via electrochemical cycling against a lithium anode.

7921-24, Session 6

Laser modification and characterization of Li-Mn-O thin film cathodes for lithium-ion batteriesJ. Pröll, R. Kohler, C. Adelhelm, M. Bruns, M. Rinke, S. Heissler, Karlsruhe Institute of Technology (Germany); M. Przybylski, ATL Lasertechnik GmbH (Germany); C. Ziebert, W. Pfleging, Karlsruhe Institute of Technology (Germany)

The development of future battery systems, e.g. for electric vehicles, is mainly focused on powerful rechargeable lithium ion batteries (LIB). To satisfy this demand, current studies are focused on cathodes based on nano-composite materials which lead to an increase in power density of the LIB primarily due to large electrochemically active surface areas. Electrode materials made of Li-Mn-O (e.g. LiMn2O4) are assumed to replace the commonly used cathode material LiCoO2 due to less toxicity, lower costs and higher specific capacities.

Thin films in the Li-Mn-O system were synthesized by non-reactive r.f. magnetron sputtering of LiMn2O4 target on silicon and stainless steel substrates. Heat treatment processes for Li-Mn-O thin films were investigated in order to achieve an appropriate crystalline phase which shows excellent electrochemical cycling performance. Laser annealing processes of unstructured and structured thin films were used as well as conventional furnace annealing for an increase in energy density and control of grain size. Laser annealing was realized via a high power diode laser system (wavelength of 940 nm) and the temperature was controlled during the process by a pyrometer. In order to enhance power density and cycle stability of the cathode material, direct laser structuring methods were investigated using laser systems operating at wavelengths of 193 nm and 248 nm. High aspect ratio micro- and sub-micron structures were created on thin films.

The effects of laser treatment of thin films were studied with Raman spectroscopy, X-ray diffraction and scanning electron microscopy. The formation of electrochemically active and inactive phases such as spinel LiMn2O4 or Li2MnO3 was discussed. Stoichiometry and surface chemistry were investigated via X-ray photoelectron spectroscopy, inductive coupled plasma atom emission spectroscopy (ICP-AES) and carrier gas hot extraction analysis. Interaction between UV-laser radiation and the thin film material was analyzed with laser ablation ICP-MS (inductive coupled plasma mass spectroscopy) as well as through ablation experiments. Finally, to investigate the electrochemical properties, the manufactured thin film cathodes were cycled against a lithium anode using liquid electrolyte EC/DMC 1:1 with 1 M LiPF6.

7921-25, Session 6

Laser selective thin film structuring with different wavelengthsK. Du, P. Shi, EdgeWave GmbH (Germany)

Selective thin film structuring with laser beams were already adapted in such as display industry and photovoltaic. Developments in tailored performances of laser beams, such as spatial profile, temporal behaviour

and wavelength will improve the resource efficiency and reduce the production cost and this in turn will make more applications accessible. For optimizing selective thin film structuring different wavelengths were used. In this paper the results will be presented and discussed.

7921-26, Session 7

Recent status and prospects of the EU-funded ALPINE projectS. Selleri, Univ. degli Studi di Parma (Italy)

The EU funded ALPINE (Advanced Lasers for Photovoltaic INdustrial processing Enhancement) project aims to push forward the research on laser systems for the scribing of photovoltaic modules, by developing low cost, high energy, short pulse, all-in-fiber lasers for IR, visible and UV micromachining. A necessary requirement for these innovative systems is having fibers capable of amplifying the optical power without distorting pulse parameters. This requires very large mode area single-mode fibers and interfacing technologies enabling these to be integrated into the laser system. ALPINE is addressing these challenges, for both MOPA and Q-switched lasers, through the so called photonic crystal fiber technology, which has prior shown unsurpassed potential for very large mode area fibers. First steps are investigating and optimizing the fiber design in order to improve the polarization properties and laser stability, developing splicing processes for these fibers, achieving an all-in-fiber pump/signal combiner, and optimizing the efficiency of frequency doubling and tripling process.

ALPINE is expected to work on new materials other than silicon and to provide CdTe/CdS and CuInGaSe2/CdS thin film solar cells both on rigid and flexible substrates like glass, polymide or metallic foils. These different configurations pose challenges to the patterning processes P1, P2, P3, all of them targeted with fully laser-based scribing. This includes beam handling, new concepts for beam splitting and automation control, when integrating the laser into industrial machines. The quality process for each scribing pattern and for the whole cell has to be validated, in terms of increased efficiency and reliability.

7921-28, Session 7

The change of electrical properties of CIGS thin-film solar cells after structuring with ultrashort laser pulsesA. Wehrmann, H. Schulte-Huxe, M. Ehrhardt, K. Zimmer, Leibniz-Institut für Oberflächenmodifizierung e.V. (Germany); A. Braun, S. Ragnow, Solarion AG (Germany)

The laser patterning of Cu(In,Ga)Se2-based thin-film solar cells has recently received increasing attention. For a low-cost manufacturing it is essential to transfer single solar cells into a module by electrical interconnections in series. Whereat, the front contact of the previous cell is connected with the back contact of the following cell. Therefore, specific scribing procedures are required to selectively separate layers from each other. The electrical contacts require optimisation with respect to the ohmic losses and the utilized active cell area. Laser processing techniques are promising for rapid scribing and numerous investigations were carried out. However, current studies show that due to the laser processing the material system is modified and the cell efficiency decreases as a result of edge degradation and local shunt formation.

The influence of laser processing parameters on the electrical behaviour of the solar cell was investigated. Hence, thin-film CIGS solar cells were scribed with ultrashort laser pulses and the changes of the electrical properties were measured directly after the structuring process. The material modifications of the processed areas were analyzed by scanning electron microscopy (SEM) in combination with electron dispersive X-ray spectroscopy (EDX) and focused ion beam (FIB). Additional investigations of the scribing area with electrical and optical methods are realised. The results are discussed and a model is introduced to address



the understanding of the physical mechanisms and to enable further optimizations.

7921-29, Session 7

Laser processing of organic photovoltaic cells with a roll-to-roll manufacturing processT. Petsch, J. Hänel, B. Keiper, M. Clair, C. Scholz, 3D-Micromac AG (Germany)

Flexible large area organic photovoltaics are currently one of the fastest developing areas of organic electronics. New light absorbing polymer blends combined with new transparent conductive materials provide higher power conversion efficiencies while new and improved production methods are developed to achieve higher throughput at reduced cost. 3D-Micromac AG, a leading supplier of custom-made laser micro machining tools, develops new and innovative laser processes for OPV manufacturing. Various transparent conductive materials, used as front contacts, can be machined using ultra-short pulsed lasers such as pico- and femtosecond lasers. These lasers in combination with high performance galvanometer scanning systems achieve superior scribing quality without damaging the flexible substrate. With scribing speeds of 10 m/s and up it is possible to integrate this technology into a roll-to-roll manufacturing tool. In addition the laser can also be used for other necessary machining steps such as the patterning of the active layer and the back contact electrode. Besides ablation process steps 3D-Micromac AG is also developing a laser welding process for the improvement of the encapsulation characteristics of organic solar cells. As penetrating water and oxygen lead to a fast degradation of the active layer the encapsulation is vital for the OPVs performance and lifetime. The combination of large area printing technologies, high speed laser processes and roll-to-roll equipment enables high volume production of organic photovoltaics at reasonable costs. Therefore 3D-Micromac is currently designing a fully integrated production tool which incorporates all the aforementioned technologies into a single production line.

7921-30, Session 7

Multi-spot laser processing of crystalline solar cellsO. Haupt, V. Schütz, U. Stute, Laser Zentrum Hannover e.V. (Germany)

We report on fast and flexible laser processing technology for crystalline solar cells by using ultra-short laser pulses and a combination of Diffractive Optical Elements (DOE´s) for beam splitting with conventional scanner technology. The focus is laid on damage reduction, decreasing processing times, and efficient processing strategies. We demonstrate the process conversion from single-spot to multi-spot ablation of thin-films and bulk material, eg. nitride ablation and edge isolation. We will point out an increase in ablation efficiency by a factor of 3 and an additional increase in processing speed by a factor of > 50 for surface ablation processes. The DOE in combination with scanner technology provides a fast and flexible system where only an industrial proven DOE has to be implemented in front of the scanner. Due to this modification the technology can be easily adapted. Using multi-spot technology for processing of crystalline solar cells, heat accumulation has to be analyzed. Limitations in spot distance and geometrical arrangements are discussed and described mathematically. Results and process windows will be shown for a thin-film ablation (surface) and a laser edge isolation (bulk) process on crystalline solar cells. An estimation of cycle times and area throughput will show the potential for using DOE´s especially combined with ultra-short pulse lasers.

7921-39, Session 7

Innovative laser based solar cell scribingB. Frei, S. Schneeberger, R. Witte, Solneva SA (Switzerland)

The solar photovoltaic market is continuously growing with boths crystalline silicon (c-Si) as well as in the thin film technology. This continuous growth is directly dependant on the manufacturing costs for solar cells. One factor for cost reduction are innovative ideas for an optimization of precision and throughput. Lasers are excellent tools to provide highly efficient processes with impressive accuracy. They need to be used in combination with fast and precise motion systems for a maximum gain in the manufacturing process, yielding best cost of ownership.

In this article such an innovative solution is presented for laser scribing in thin film Si modules. A combination of a new glass holding technique on a machine with a fast and precise motion system is the base for a cost effective scribing machine. In addition, the advantages of fiber lasers in beam delivery and beam quality guarantee not only shorter setup and down times but also high resolution and reproducibility for the scribing processes P1, P2 and P3. The precision of the whole system allows to reduce the dead zone to a minimum and therefore to improve the efficiency of the modules.

7921-40, Session 7

Femtosecond laser doped silicon for photovoltaic applicationsM. Sher, M. T. Winkler, Y. Lin, E. D. Mazur, Harvard Univ. (United States)

Silicon is doped to concentrations above the metal-insulator transition threshold via femtosecond laser doping techniques creating a novel material that has potential for photovoltaic applications. By focusing femtosecond laser pulses on the surface of silicon in a sulfur hexafluoride (SF6) environment, silicon is doped with 1% atomic sulfur. This material exhibits near-unity, broadband absorption from visible to lower than 0.5 eV near infrared light, deep below the silicon bandgap. This material also exhibits metallic-like conduction. These unusual optical and electronic properties suggest the formation of an intermediate band. We report on the femtosecond laser doping techniques and material properties. By changing the laser parameters and ambient environment we can control the dopant profiles, crystallinity and surface morphology. In addition, we use temperature-dependent Hall measurements to investigate electron transport as well as identify the energy states of the sulfur donors and hence the location of the intermediate band. We will also discuss potential applications for intermediate band photovoltaics.



Conference 7922: Synthesis and Photonics of Nanoscale Materials VIIIMonday-Tuesday 24-25 January 2011 • Part of Proceedings of SPIE Vol. 7922 Synthesis and Photonics of Nanoscale Materials VIII

7922-01, Session 1

Selective annealing by picosecond laser pulses for conductive patterns on the transparent zinc oxide thin filmD. Lee, H. Pan, Univ. of California, Berkeley (United States); S. H. Ko, KAIST (Korea, Republic of); C. P. Grigoropoulos, Univ. of California, Berkeley (United States); H. K. Park, AppliFlex LLC. (United States)

Spin-coatable high concentration (up to 20wt% ZnO) transparent ZnO nanoparticle solution was made in a unique process. All procedures were done below 60°C without any vacuum process. This solution can be used for a host of applications, including inkjet printing, nano imprinting, etc. Highly transparent ZnO thin film was made on the quartz substrate by spin-coating at room temperature without vacuum process. Before annealing the film was not conductive. Laser annealing imparted large conductivity increase while annealing via a hot plate at 500°C did not achieve good conductivity. Laser radiation enables selective annealing directly on the thin film. Conductive patterns were constructed without lithography and high temperature process. Highly conductive patterns were achieved with undoped ZnO thin film as well as variation of conductivity can be obtained by varying the laser power or scanning speed or ambient gas environment. Parametric studies were conducted on laser power, scanning speed and ambient gas environment versus the sheet resistance and optical transmittance of annealed ZnO thin film. The lowest resistivity of the undoped ZnO film by laser annealing was about 0.0675[Ω.cm].

7922-02, Session 1

Fabrication of layer structured ZnO nanowire by nanoparticle-assisted pulsed laser deposition for optoelectronic applicationD. Nakamura, A. Kumeda, K. Toya, K. Okazaki, K. Kubo, K. Tsuta, M. Higashihata, T. Okada, Kyushu Univ. (Japan)

ZnO nanowires have attracted a great attention as building blocks for the optoelectronic devices. For the practical optoelectronic applications based on the ZnO nanowires, a synthesis technique for layered structure has significant advantage to fabricate a pn junction, a core/shell structure, and a multiple quantum well structure. We have been succeeded in growing nanowires on the pre-deposited ZnO film and core/shell structure by a newly developed nanoparticle-assisted pulsed-laser deposition (NAPLD) using multi-target changer. In this presentation, recent progresses of synthesis of layer-structured ZnO nanowires by the NAPLD will be discussed.

7922-03, Session 1

In situ spectroscopic diagnostics of SnO2 nanowire growth by laser-CVD at high temperaturesA. Puretzky, J. Shin, C. Rouleau, J. D. Readle, N. Thonnard, A. Goyal, D. B. Geohegan, Oak Ridge National Lab. (United States)

SnO2 nanowires exhibit n-type semiconductor properties with the band gap of 3.6 eV and are very attractive nanomaterial for many important applications, including flexible electrical and optoelectronic devices, photovoltaic cells, and gas sensors. Therefore it is important to develop

different approaches to control the growth of these nanowires and understand their growth mechanism. Here we demonstrate a laser ablation approach to grow tin oxide nanowires on different substrates (Si, SrTiO3, etc.) covered with a thin layer of gold (1-10 nm). Au nanoparticles formed by roughening of the film are typically used as catalysts for the VLS growth of oxide nanowires. In the laser-CVD approach described here, material ablated from a SnO2 target at 700°C by a KrF-laser (248 nm, 4J/cm2, 5Hz) inside a quartz tube reactor supplied reactants to grow the SnO2 nanowires at relatively high pressures (100-200 Torr) of flowing Ar gas (100 sccm). Under these conditions, relatively long (~5 µm) and thin (~ 20 nm) tin oxide nanowires are formed with preferential vertical orientations on the substrate. The nature of the reactants were investigated using fast gated ICCD imaging and spectroscopy to clarify if the nanowires grow from tin oxide nanoparticles formed during laser ablation into the high pressure background gas or from tin atoms generated by laser ablation of gas suspended nanoparticles on the subsequent laser shots. The possible growth mechanisms will be discussed.

Research sponsored by the Materials Science and Engineering Division, U.S. Department of Energy. A portion of this research was conducted at the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, U.S. Department of Energy.

7922-04, Session 1

Parametric study of laser assisted silicon nanowire growth mechanismS. Ryu, D. J. Hwang, C. P. Grigoropoulos, Univ. of California, Berkeley (United States)

We experimentally explored the laser assited growth mechanism of silicon nanowires (SiNWs) on silicon film by employing a focused laser. With continuous wave (CW) Ar-ion laser absorbed by optically thick silicon film sputtered on fused silica substrate, SiNWs are grown in spatially confined diffusion area by vapor-liquid-solid (VLS) mechanism with gold nanoparticles on the film. Taking advantage of fast local heating capability of a laser, multi-parametric study of temperature, growth time, and laser illumination direction is conveniently and systematically carried out in a single platform. While direct heating of gold nanoparticles allows one-dimensional nanowire growth at very early stage with localized heating of catalysts through plasmonic resonance, the subsequent growth is dominated by a secondary silicon deposition on the pre-grown portion of nanowires. On the contrary, it is kinetically confirmed that indirect heating of gold nanoparticles through the local hot film enables the growth to follow Arrhenius law appearing in traditional growth trend of SiNWs under the furnace. The growth rate is orders of magnitude greater than that by conventional CVD technique, since the chemical reaction is confined by focused laser spot with spatial resolution of O (1um) favorably allowing three dimensional diffusion path of gas. Moreover, With fast heating of O(10us), the growth rate close to the maximum at given gas condition by short time growth enables more precisely exploration of the growth mechanism than before.

7922-05, Session 1

The single walled carbon nanotube: a model system for excitons in 1DJ. Lefebvre, National Research Council Canada (Canada)

No abstract available.


7922-06, Session 2

Nanoscale optoelectronic probes for carbon nanotube based nanobiohybridsY. Xu, Vanderbilt Univ. (United States)

Carbon nanotube (CNT) based nanobiohybrids have emerged as one of the most promising materials for biological and biomedical applications such as drug delivery. We have successfully delivered CNT based small interfering RNA hybrids into human neuroblastoma cells to block tumor growth and metastasis. Moreover, we have developed an optoelectronic probing system combining scanning photocurrent microscopy, fluorescence microscopy and optical trapping techniques with CNT transistors to investigate the physical properties and complicated interactions of CNT based RNA/DNA hybrids at the single-molecule level. These fundamental studies provide key information to the delivery mechanism in drug delivery.

7922-07, Session 2

Photo-thermal mediated transformation of carbon nanoparticlesV. Varadrajan, N. D. Ingle, A. R. Koymen, S. K. Mohanty, The Univ. of Texas at Arlington (United States)

Photothermal therapy offers a solution for the destruction of cancer cells without significant collateral damage to otherwise healthy cells. Functionalized-nanoparticles interact readily with biomolecules specific to cancer cell membrane. Several attempts are underway in using carbon nanoparticles (CNPs) and nanotubes due to their excellent absorption properties in the near-infrared spectrum of biological window. However, minimizing the required number of injected nanoparticles, to ensure least risk of cytotoxicity, is a major challenge. We report on the introduction of magnetic carbon nanoparticles (MCNPs) onto cancer cells, localizing them in desired region by applying external magnetic field and irradiating them with near-infrared laser beam. The MCNPs were prepared using an electric plasma discharge in benzene generated in an ultrasonic cavitation. The CNPs were made magnetic by use of Fe-electrodes to dope, as confirmed by SQUID magnetometry. Transmission electron microscopy measurements showed the size distribution of these MCNPs in the range of 5-10 nm. For photothermal irradiation, a tunable (690-1040 nm) CW Ti: Sapphire laser beam was weakly focused on to the cell monolayer under an inverted fluorescence microscope. The response of different cell types (HeLa, HEK and Prostate cancer cells) to photothermal irradiation was investigated. Cell death in the presence of both MCNPs and laser beam was confirmed by morphological changes and propidium iodide fluorescence in nucleus. The results of our study suggest that MCNP based photothermal therapy is a viable approach to remotely guide photothermal therapy offering significantly reduced cytotoxicity.

7922-08, Session 2

Femtosecond laser-induced formation of AuAg nanoalloys from aqueous mixture of metallic ionsY. Herbani, T. Nakamura, S. Sato, Tohoku Univ. (Japan)

One step synthesis of AuAg alloy nanoparticles of various compositions has been performed for the first time by femtosecond laser irradiation of the mixture solution of metallic ions prepared in water. The mixture of Au and Ag metallic ions was simply introduced into a 3-ml glass vial and exposed for several minutes to femtosecond laser pulses with the excitation wavelength at 800 nm and the pulse width of 100 fs. A tightly focusing condition using an aspheric lens with high numerical aperture has been set to assure that a strong optical breakdown occurred in the solution and promote the formation of a large number of hydrogen radicals and the solvated electrons, which act as a reducing agent for

metallic ions. Using aqueous mixtures of silver-ammonia complex and chloroaurate ions, a large number of 5 - 7 nm AuAg has been produced and the spectra of colloidal nanoalloys have plasmon peak positions that were linearly shifted from 400 to 520 nm with the increase of the gold molar fraction in the solution. Furthermore, the absence of the intrinsic peaks to the pure Ag and Au nanoparticles in the spectra of the AuxAg(1-x) nanoparticles suggested that the produced structure is rather homogeneous and did not consist of segregated metal forming a core-shell structure. These results suggested that the atomic gold fraction in the nanoparticles were the same as the gold molar fractions in the solution. No oxidation of Ag nanoparticles caused by reactive oxygen species produced has been observed.

7922-09, Session 3

Plasmonic nanoparticle optical antennas and cavities for manipulation of radiative emission rate and directivityH. A. Atwater, Jr., California Institute of Technology (United States)

Metallic nanoparticle structures can serve simultaneously as optical frequency antennas and resonant cavities that can dramatically enhance the rate of radiative emission as well as control the direction of emission. Coupling of nanoscale semiconductor emitters to highly localized surface plasmon modes of nanoscale metallic antennas and cavities can enable enhancement of the radiative emission rate by more than 1000x compared to free dipole emission, and can reorient emission from in-plane to surface-normal with respect to the underlying substrate. Examples of nanoparticle antennas and cavities employing polymeric semiconductors and III-V compound semiconductors will be discussed.

7922-10, Session 3

Surface plasmon lasersH. Grebel, New Jersey Institute of Technology (United States)

We have demonstrated for the first time, surface plasmons laser (SPL) by imbedding gain material within a nano-scale periodic structure on top of a metal substrate. Threshold, gain, spectral line narrowing and feedback were demonstrated in the visible range. The surface metallic waveguides were consisted of a nano-scale hole-array in a 50 nm thick layer of aluminum oxide on top of aluminum substrate (anodized aluminum oxide or, AAO). Two-layer graphene was added on top of the perforated oxide layer in order to maintain the gain media (dye chromophores or quantum dots) just above the structure ‘hot spots’. The sub-wavelength array of holes enabled coupling to and from the waveguides as well as, providing feedback to the surface modes. Gain of a few ten-thousands per cm was demonstrated for this small (less than 5 micron square) optically pumped laser. Threshold and spectral line narrowing, by more than a factor of 4, signified the laser operation. Incorporating such concept into an optical surface circuit or bio-sensors is an on-going effort.

7922-11, Session 3

Tailoring the interaction between matter and polarized light with plasmonic optical antennasP. Biagioni, Politecnico di Milano (Italy)

Plasmonic antennas represent a powerful tool to engineer localization and enhancement of optical fields. The analysis and control of polarized fields on the nanoscale is a crucial issue in plasmonics, since the scaling down of widely used optical techniques relies upon polarized near fields. The possibility to shape the polarization properties of local fields would open the road towards controlled interaction between polarized light and

Conference 7922: Synthesis and Photonics of Nanoscale Materials VIII


matter at the nanoscale.

We present extended simulations for a plasmonic cross antenna structure, constituted by two perpendicular dipole antennas with a common gap, and show how this novel configuration allows for a complete control of light with an arbitrary polarization state in the antenna plane [1]. We also show that the complex phase response of plasmonic antennas represents a further degree of freedom for nanoscale polarization engineering and demonstrate an antenna-based nanowaveplate [2].

Finally, we present strategies for the structuring of gold cross antennas by focused ion-beam milling, based on our previous experience with linear antennas [3]. We discuss recent developments obtained by structuring cross antennas starting from single-crystal Au microplates grown by electrochemical methods and introduce possible strategies to experimentally characterize the degree of circular polarization in the feed-gap of a cross antenna by means of near-field circular dichroism [4].

[1] P. Biagioni et al., Phys. Rev. Lett. 102, 256801 (2009).

[2] P. Biagioni et al.,Phys. Rev. B 80, 153409 (2009).

[3] J. S. Huang et al., Nano Lett. 10, 2105 (2010).

[4] M. Savoini et al., Opt. Lett. 34, 3571 (2009).

7922-12, Session 3

Damping parameters of localized surface plasmon polariton resonances of gold nanoparticlesF. Hubenthal, F. Träger, Univ. Kassel (Germany)

The optical properties of noble metal nanoparticles are dominated by localized surface plasmon polariton resonances, i.e., by collective excitations of the conduction band electrons. Recently, the quantitative determination of the damping mechanisms of plasmons has gained increasing interest. In particular, the damping parameter A was in the focus of a variety of studies, since its knowledge may deliver a fundamental understanding of the plasmon.

In this contribution, we present systematic measurements of the damping parameter A of gold nanoparticles as a function of photon energy and chemical surrounding. The two most essential among the numerous results are: first, different damping parameters between A = 0.20 nm/fs and A = 0.40 nm/fs have been extracted for gold nanoparticles with plasmon resonances located between 1.40 eV and 2.15 eV. Second, a threshold energy for chemical interface damping (CID) has been observed. In particular, the latter result cannot be explained within the classical picture due to stepwise dephasing of the collectively oscillating electrons. Since CID is based on tunnelling of electrons into adsorbate states, the plasmon energy has no significant impact on the average energy of an electron. As a consequence, the tunnelling probability should dependent only weakly on the plasmon energy, which in contrast to our results. The measured threshold energy for CID demonstrates, that the plasmon decays in a single process due to tunnelling of an electron, which carries the entire plasmon energy. As a consequence, the plasmon must be described as a two level system. The impact on the understanding of plasmons and their damping, as well as consequences on applications of noble metal nanoparticles will be discussed.

7922-13, Session 4

Resonant optical transmission through hole arrays in metal films: physics and applicationsR. Gordon, Univ. of Victoria (Canada)

Since the discovery of extraordinary optical transmission through an array of subwavelength holes in a metal film in 1998, there has been abundant research activity aimed at understanding the physics of this phenomenon. There has also been development in the many applications

of subwavelength hole arrays. This talk will overview the rich physics associated with hole arrays, as well as their many applications including polarization control, filtering, switching, nonlinear optics, surface plasmon resonance sensing, surface-enhanced fluorescence, surface-enhanced Raman scattering, absorption spectroscopy, and quantum interactions. The various approaches and developments in hole-array fabrication and integration of hole-arrays into devices will described.

7922-14, Session 4

Design parameters of surface-enhanced Raman spectroscopy templates for obtaining strong power enhancement factor with high area density at 532 nm excitation wavelengthA. Zenidaka, Y. Tanaka, T. Miyanishi, M. Terakawa, M. Obara, Keio Univ. (Japan)

We present the optical power enhancement on the surface of the 2D periodic arrays of convex and concave gold nanostructures to compare and explain the advantage of the nanostructures for surface enhanced Raman spectroscopy (SERS) templates. The power enhancement due to the surface plasmon polaritons is simulated by 3D Finite-Difference Time-Domain (FDTD) method at conventional 532 nm pump light. 2D periodic arrays of closely-packed gold particles are defined as convex nanostructures, while 2D periodic arrays of hemispherical voids on gold substrate are defined as the concave nanostructures. The peak power enhancement factor, the average power enhancement factor and the activity rate for both structures are investigated and compared. The convex nanostructures show a stronger enhancement factor in localized hotspots than the concave system, while the concave nanostructures show a higher spatially-averaged power enhancement factor and higher activity rate than the convex system, due to the dense distribution of high electric field on the substrate. We also reveal the dependence of the void diameter on the inter-void distance to maximize the power enhancement in the concave system. This provides a guideline for the fabrication of the SERS template giving a strong power enhancement factor with a high area density. Finally, we will present the experimental results to validate the above theoretical results.

7922-15, Session 4

Nanostructure formation by self-assembled monolayers: influence of the isomerization state of azobenzene ligands on monolayer formationF. Vogel, F. Bretthauer, U. Siemeling, F. Träger, Univ. Kassel (Germany)

With the objective of preparing novel nanoscale interfaces, two azobenzene-derivatized 1,2-dithiolanes were synthesized and used for the preparation of self-assembled monolayers (SAM) on gold substrates. Azobenzene derivatives can be switched between their “cis” and “trans” isomers by irradiation with photons. They represent the most widely studied system for photoresponsive SAMs. In our studies, the kinetics of film formation of the two azobenzene-derivatized molecules have been investigated by in situ optical second harmonic generation (SHG), a powerful nonlinear optical swiss army knife for the investigation of interfacial processes. For this purpose, SAMs have been prepared of both isomers of the molecules. We find that the isomerization state of the molecules significantly affects the kinetics of film formation. The kinetics are best described by a second order Langmuir dependence for the investigated concentrations between 25 and 200 µmol/l for both kind of molecules.



7922-16, Session 4

Hyperspectral imaging of surface plasmon resonance effects induced by uncollimated semiconductor radiationD. Lepage, J. J. Dubowski, Univ. de Sherbrooke (Canada)

We have recently proposed an innovative SPR microstructure comprising a metal coated dielectric layer deposited on top of a photoluminescence (PL) emitting quantum well (QW) wafer [1-2]. The entire device, thanks to the built-in light source and the application of a SPR imaging technique, has the potential to become a highly compact SPR biosensor for simultaneous detection of numerous biomolecules.

The device takes advantage of uncollimated and incoherent emission from the QW microstructures, which result in a spectro-angular far-field SPR response. Our results indicated that the injected in-plane wavevectors could increase the SPs coupling efficiency up to 100 times in comparison to indirect SPs injection [2]. To adequately monitor the emitted spectro-angular far-field, we have presented the general idea of an experimental setup required for the collection the 3D measurements of SPR dispersion relations w(kx,ky), enabling a much richer picture of surficial biochemical events. Preliminary results indicated that the proposed methodology would produce simultaneously the equivalent of 10^5 to 10^8 conventional SPR scans achievable with commercial systems [2-3].

In this communication, we present what we believe to be a novel surficial imaging technique, based on the hyperspectral photoluminescence mapping of SPR events (HI-PLM-SPR). A comparison between experimental and analytical dispersion relation maps w(kx,ky) are presented, along with initial trials with biochemical agents.

References:

[1] Lepage D., Dubowski J. J., App. Phys. Lett., vol.91[163106] (2007) 1-3

[2] Lepage D., Dubowski J. J., Opt. Express 17[12] (2009) 10411-10418

[3] Lepage D., Dubowski J. J., SPIE-LA12 (Proceedings Paper), vol. 7586[758607] (2010) 1-8.

7922-17, Session 4

Optical limiting behavior of Au-Ag nanoparticles under CW laser illuminationP. K. Palanisamy, R. Kasu, K. Elangovan, Anna Univ. Chennai (India)

Metallic nanoparticles such as Ag, Cu and Au, show characteristic optical absorption in the visible wavelength region due to the excitation of plasmons (collective oscillation of electrons). The peak position and the bandwidth of the plasmon transition depend on the size and the form of the nanoparticle. Au-Ag nanoparticles have been synthesized in aqueous solution by chemical reduction method. The structure of Au-Ag could be either a core-shell or an alloy phase depending on the preparation conditions. The TEM image of the Au-Ag nanoparticles sample confirms the core-shell structure. In the core-shell structure, the core belongs to gold .It appears to be highly dense because it has high electron density than silver. The Au-Ag colloidal nanoparticle samples are characterized by UV-VIS absorbance and Z-scan technique. Under CW 532 nm excitation, Z-scan measurements show that the Au-Ag nanoparticles exhibit intensity dependent refractive index n2. Due to the large nonlinear refractive index, it can be used for variety of photonic applications. Optical limiting behavior of Au-Ag colloidal nanoparticles is observed. Optical limiting behavior suggests the saturable absorption nature of the Au-Ag colloidal nanoparticles.


Synthesis of silicon fibrous nanoparticles aggregate structures using femtosecond laser radiationM. Sivakumar, Ryerson Univ. (Canada) and Amrita Vishwa Vidyapeetham (India); K. Venkatakrishnan, B. Tan, Ryerson Univ. (Canada)

Irradiation of silicon samples with femtosecond laser pulses at megahertz pulse repetition rate under ambient condition lead to the generation weblike fibrous nanostructure. Electron microscopy analysis revealed that the fibrous nanostructure is formed due to aggregation of nanoparticles of size varying between 2 to 40 nm. The amorphous nature of silicon nanoparticles present in the nanoparticles aggretae is confirmed by microraman analysis. The nanoparticles aggregate starts forming at a pulse repetition rate of 2 MHz and interaction time 0.1ms. The nanoparticles formation is explained by nucleation and condensation of vapour in the plasma plume generated during the irradiation process. The laser dwell time is approximately equal to the critical time for nucleation process. As a result the nanoparticles generated from the previous laser pulse are at a high temperature and agglomerates with particles formed by the subsequent pulses. This study provides evidence that femtosecond laser irradiation can be an ambient condition physical method for silicon fibrous nanoparticles aggregate structures generation.


Fabrication of gold-platinum alloy nanoparticles by high-intensity laser irradiation of aqueous solutionT. Nakamura, Y. Herbani, S. Sato, Tohoku Univ. (Japan)

Gold-platinum alloy nanoparticles were fabricated by high-intensity femtosecond laser irradiation of mixed aqueous solutions of auric and platinum ions with different mixing ratio. The resulting particles were characterized by UV-visible spectroscopy, TEM and XRD. After irradiation, the color of the solution represented gradual change from red in auric solution to brown in platinum solution. Absorption peak arise from surface plasmon resonance of gold nanoparticles were observed in the spectrum of auric solution, and the peak position shifted to shorter wavelength range and the absorption peak in the spectrum decreased with decreasing the composition of auric ion in the solution. The position of XRD peaks of the particles shifted from that of pure gold to platinum, and XRD peaks of the particles prepared in the solutions with a certain mixing ratio of auric and platinum ions were observed between the peak positions of gold and platinum. This finding demonstrates bimetallic nanoparticles of gold and platinum are successfully fabricated only by high-intensity laser irradiation of mixed aqueous solution of auric and platinum ions.


Atomic layer deposition of amorphous TiO2/ZnO multilayers for soft x-ray coherent opticsY. Sanjo, Y. Tanaka, M. Murata, H. Kumagai, Osaka City Univ. (Japan); T. Shinagawa, Osaka Municipal Technical Research Institute (Japan)

The development of high-reflection mirrors with amorphous metal-oxide multilayers in the “water-window” (=2.332-4.368nm) wavelengths is desirable for various water-window applications, because the combination of two different metal-oxides may make big difference in refractive index between two different layers at the wavelengths and moreover decrease roughness on the surface and interface.

One of the authors have already studied and fabricated amorphous



Al2O3/TiO2 multilayers for the “water-window” wavelengths by controlled growth with atomic layer deposition (ALD), and then acquired reflectance of 33.4 % at 2.73nm and at the incidence angle of 18.2° from the normal incidence. In order to fabricate amorphous multilayers with higher reflectance, we propose amorphous TiO2/ZnO multilayers , because utilizing combination of TiO2/ZnO is able to make refractive index difference for TiO2 larger than that of Al2O3/TiO2 at “water-window” wavelengths. In this study, amorphous TiO2/ZnO multilayers are fabricated by ALD and then reflectance characteristics are investigated using monochromatized synchrotron radiation (SR) located Ultraviolet Synchrotron Radiation Facility (UVSOR), Institute for Molecular Science, Okazaki, Japan. In the presentation, ALD of amorphous TiO2/ZnO multilayer mirrors will be discussed in detail, comparing with reflectance characteristics of amorphous Al2O3 / TiO2 multilayers.


Horizontal alignment of zinc oxide nanowires for UV optoelectronic applicationsK. Okazaki, Kyushu Univ. (Japan)

The ZnO with the wide direct band-gap of 3.37 eV and the large exciton binding energy of 60 meV at room temperature is one of the most promising materials for ultraviolet optoelectronic devices due to its abundant natural resources and low cost. We have succeeded in growing the ZnO nanowires, which have high crystalline and excellent optical property, by nanoparticle-assisted pulsed-laser ablation deposition (NAPLD). In this study, horizontal alignment of synthesized ZnO nanowires was achieved by simple friction technique. The horizontally aligned ZnO nanowires are expected to be applied to optoelectronic devices such as UV laser and sensing device.


Laser interactions with graphene: in situ and ex situ studiesC. Rouleau, M. Regmi, A. Puretzky, G. Eres, J. D. Readle, D. B. Geohegan, Oak Ridge National Lab. (United States)

Single layer graphene (SLG) is currently synthesized on Cu foils at 900-1000 C by chemical vapor deposition from hydrocarbon precursors. However, the kinetics and mechanisms of the growth process are poorly understood. Here we report the use of time-resolved laser reflectivity to monitor the growth of graphene in situ, and the use of scanning electron microscopy and Raman spectroscopy to characterize the nature of the deposited graphene layers ex situ. The results not only indicate that the quality of the copper surface is important for graphene growth, but growth can be rapid. Experiments with single crystal Cu substrates, as well as polished and as-received substrates will be described. It was also found that the graphene layers exhibit high thresholds for laser ablation, and studies with ps- and fs-laser irradiation will be described to understand the mechanisms and thresholds for damage for SLG on Cu.

Research sponsored by the Materials Science and Engineering Division, U.S. Department of Energy. A portion of this research was conducted at the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, U.S. Department of Energy.


In situ density measurements of carbon nanotube arrays by laser reflectometry: flux dependent density variations induced by pulsed CVDD. B. Geohegan, J. J. Jackson, A. A. Puretzky, G. Eres, C. Rouleau, K. More, Oak Ridge National Lab. (United States)

The density of vertically-aligned carbon nanotube arrays have been measured in real-time during continuous growth by chemical vapor deposition and found to decline up to a factor of 1.6 during the first 10 microns of height. Derivation of the evolution of nanotube array density during growth is described from in situ optical measurements of the array height and extinction coefficient. Ex situ Z-contrast scanning transmission electron microscope density analysis of sections of the arrays confirm this natural decline in array density with length and time. Using this real-time optical diagnostic technique, the response of array density to rapidly varying feedstock flux was explored using pulsed-CVD in fast flows at low pressures. Acetylene pulses of ~0.2 s duration were found to induce the growth of distinct ~1-2 micron-long layers within which densities varied up to ~ 200%. This highly responsive flux-dependence to the density of the arrays, along with the described optical density measurement technique, is shown to provide a pathway not only to monitor the density of VANTAs as they grow but to synthesize interesting new multilayered array architectures with regions of controllable length and density variations.

Research sponsored by the Materials Sciences and Engineering Division, Office of Basic Energy Sciences, U.S. Department of Energy. The Z-STEM characterization in this research was conducted at the Center for Nanophase Materials Sciences, and HRTEM was performed at the Shared Research Equipment Collaborative Research Center, which are sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, U.S. Department of Energy.


Laser-based synthesis in liquid environment of silicon nanoparticles and their related structural and optical propertiesR. Intartaglia, K. Bagga, F. Brandi, G. Das, A. Genovese, E. Di Fabrizio, A. Diaspro, Istituto Italiano di Tecnologia (Italy)

Silicon nanoparticles (Si-NPs) are of great interest due to their unique physical properties which make them suitable for a wide range of applications specially in the production of nanostructured materials with tailor made properties to be used in the field of medicine, energy technology and in particular in bio-medicine as efficient photosensitizers of oxygen molecule [1]. Recently, it was demonstrated that Si-NPs can generate singlet oxygen state upon UV-irradiation [2]. So far, a variety chemical approach such as electrochemical etching, wet chemistry route was developed to produce Si-NPs [3,4]. In the last decade, the production of nanomaterials by laser based-synthesis, started to represent an alternative to the chemical ones which could be inconvenient for certain applications owing to the contamination of the resulted nanomaterials [5]. The use of toxic compound during the chemical synthesis procedure complicates their applications in, in vivo biosensing and imaging. Laser ablation of a solid target in liquid environment allows the production of colloid nanocrystals with unique surface chemistry (free from any contaminant) since the synthesis is carried out in a contamination-free environment. Many irradiations parameters have to be taking in account for controlling the size and shape of nanoparticles. Some of these parameters include laser wavelength, pulse energy, pulse duration, repetition rate, and liquid environments [6-9]. In particular, the laser pulse duration is found to affect directly the ablation, nucleation, growth, and aggregation mechanisms. Long laser pulses (nanosecond) release energy on a time-scale comparable with the thermal relaxation processes of the



target, while femtosecond laser pulses release energy to electrons in the target on a time-scale much faster than electron-phonon thermalization processes. Thus local heating on the target is reduced in the case of femtosecond pulse. Moreover temporal overlap between laser pulses and the ablated material induces thermodynamic instability of the plasma during his expansion. Consequently, some differences are observed in the generated nanoparticles with pulses of different duration . Only few works on silicon nanoparticles generation in liquid environment has been reported [10,11]. Nanosecond laser ablation of silicon show generation of nanoparticles which stabilize into clusters due to agglomeration effect. ie Silicon nanoparticles are held together by an irregular network.

Here, isolated silicon nanoparticles were prepared by femtosecond laser ablation of a silicon target in deionized water. The mean size is found to vary from 60 nm to 2.5 nm in the absence of any reducing chemical reagents, decreasing the laser fluence of one order of magnitude (Fig.1). High resolution transmission electronic microscopy together with Raman spectroscopy confirms the crystalline structure of the generated silicon nanoparticles. The energy confinement of carriers which is evaluated from optical experiment varies from 90 meV to 550 meV when the mean nanoparticles size decrease from 60 nm to 2.5 nm. In particular, the evaluated nanoparticles size from optical analysis and LCAO theoretical model are found in agreement with TEM and Raman measurements for the silicon nanoparticles with a size less than 6 nm. Moreover, agglomeration effect is observed for smallest nanoparticles produced at lower fluence.

[1] E. Gross, Physical Review B, 68, 115405, (2003).

[2] M. Meunier, Journal of Biomedical Optics, 14, 021010 (2009)

[3] Milena Rosso-Vasic,a Evan Spruijt, Zoran Popovi, Karin Overgaag,c Barend van Lagen, Bruno Grandidier, Daniel Vanmaekelbergh, David Domınguez-Guti_errez, Luisa De Cola and Han Zuilhof , J. Mater. Chem., 2009, 19.

[4] Xiaoming Zhang, Doinita Neiner, ShizhongWang, Angelique Y Louie and Susan M Kauzlarich Nanotechnology 18 (2007) 095601.

[5] A.V Kabashin and M. Meunier, Laser Ablation-Based Synthesis of Nanomaterials,in Recent Advances in Laser Processing of Materials, ed,E. Fogarassy,(Elsevier, Oxford, 2006).

[6] Takeshi Tsuji, Kenzo Iryo, Norihisa Watanabe, Masaharu Tsuji, Applied Surface Science 202 80 (2002).

[7] Fumitaka Mafune, Jun-ya Kohno, Yoshihiro Takeda, and Tamotsu Kondow, Hisahiro Sawabe J. Phys. Chem. B, 104, 9111 (2000)

[8] G.A. Shafeev, E. Freysz, F. bozon-verduraz, Appl. Phys. A 78, 307 (2004)

[9] Takeshi Tsuji, Toshihiko kakita, Masaharu Tsuji, Applied Surface Science 206 314 (2003)

[10] Vladimir vrek Davide Mariotti and Michio Kondo 19 January 2009 / Vol. 17, No. 2 / Optics express 520

[11] I Umezu, H Minami, H Senoo and A Sugimura, Journal of Physics: Conference Series 59 (2007) 392-395.



Conference 7923: Free-Space Laser Communication Technologies XXIIIWednesday-Thursday 26-27 January 2011 • Part of Proceedings of SPIE Vol. 7923 Free-Space Laser Communication Technologies XXIII

7923-01, Session 1

Overview of the lunar laser communications demonstrationB. S. Robinson, D. M. Boroson, D. A. Burianek, D. V. Murphy, MIT Lincoln Lab. (United States)


7923-02, Session 1

Coherent inter-satellite and satellite-ground linksR. Lange, Tesat-Spacecom GmbH & Co. KG (Germany)

Hih data rate optical communication systems are operational in-orbit for three years. Based on homodyne BPSK, data is transmitted at a rate of 5.625 Gbps in LEO-LEO, LEO-to-ground and ground-to-LEO links. With a bit error rate for inter-satellite links much better than 10-9 the performance of 1.8 Gbps LEO-to-GEO links is verified. The respective terminals are under qualification and will be applied first in EDRS, the European data relay system. Today the laser communication terminals are applied in programs o investigate coherent beam propagation from a LEO satellite through the atmosphere to the ground and vice versa.

7923-03, Session 1

5 625 Gbps bidirectional laser communications and ranging measurements between the NFIRE Satellite and an optical ground stationR. A. Fields, The Aerospace Corp. (United States); M. Gregory, B. Wandernoth, Tesat-Spacecom GmbH & Co. KG (Germany); D. Kozlowski, H. T. Yura, The Aerospace Corp. (United States); F. F. Heine, Tesat-Spacecom GmbH & Co. KG (Germany); R. L. Wong, C. T. Lunde, The Aerospace Corp. (United States); R. Meyer, Deutsches Zentrum für Luft- und Raumfahrt e.V. (Germany)


7923-04, Session 1

Tactical line-of-sight optical communications networksL. Wasiczko Thomas, W. Rabinovich, R. Burris, U.S. Naval Research Lab. (United States)


7923-05, Session 1

Orbital angular momentum (OAM) based LDPC-coded deep-space optical communicationI. B. Djordjevic, The Univ. of Arizona (United States)

Power-efficient modulation schemes such as PPM are widely adopted in current deep-space optical communications. The very large bandwidth of these links (compared to RF links) has made the low spectral efficiency of PPM less of a concern. However, in order to achieve multi-gigabit transmission (projected for 2020) for the use in interplanetary communications, the usage of large number of time slots in PPM is needed, which imposes stringent requirements on system design and implementation. In order to satisfy high-bandwidth demands of future interplanetary communications, while keeping the system cost and power consumption reasonably low, we propose the use of multidimensional coded modulation schemes. Namely, it is well known, that photons can carry both spin angular momentum (SAM), associated with polarization; and orbital angular momentum (OAM), associated with azimuthal phase of the complex electric field. Because OAM eigenstates are orthogonal, in principle, arbitrary number of bits per single photon can be transmitted. The ability to generate/analyze states with different OAMs, by using interferometric or holographic methods allows the realization of deep-space optical communication systems with ultra-high photon efficiencies expressed in terms of number of bits per photon. The OAM modulation and multiplexing can, therefore, be used, in combination with other degrees of freedom, to solve the high-bandwidth requirements of future deep-space optical communications. By increasing the number of OAM states we can increase the aggregate data rate of the system, while enabling reliable transmission at these higher speeds by using LDPC codes at each level. Dramatic increase in photon efficiency (over 10 bits/photon) through OAM based LDPC-coded PPM will provide revolutionary capabilities for future deep-space optical communications. The main challenge for OAM deep-space communication represents the link between a spacecraft probe and the Earth station because in the presence of atmospheric turbulence the orthogonality between OAM states is not longer preserved. We will show that proposed LDPC-coded OAM based PPM can operate under strong turbulence regime when used in combination with receiver spatial diversity.

7923-06, Session 1

Architecture, design, and numerical simulation of a code/pulse-position-swapping (CPPS) direct translating receiverA. J. Mendez, Mendez R&D Associates (United States); V. J. Hernandez, C. V. Bennett, Lawrence Livermore National Lab. (United States); R. M. Gagliardi, The Univ. of Southern California (United States)

Pulse position modulation (PPM) is recognized as a power efficient means of communication and thus is favored for communication links that are average power limited (such as space links). The advantages of PPM can be combined with multiple access communications by translating the pulse positions to optical-code-division-multiple-access (O-CDMA) codes, which we define as code/pulse-position-swapping (CPPS). CPPS retains the multiple bits per symbol modulation format of PPM as well as the asynchronous multiple access of O-CDMA. Additionally, CPPS allows variable data rates or bits per symbol for each user (granular communications); common electrical bandwidth for all users at all data (symbol) rates; compatibility with free-space or guided (fiber and waveguide) communication links; and compatibility


with intensity modulation/direct detection, depending on the selection of the O-CDMA codes. In this paper we describe the architecture and design of a CPPS receiver for a known set of 32 wavelength/time codes. The architecture is based on map-coding, resulting in a composite correlator (CRi) that improves upon a previously proposed CPPS receiver architecture that required a bank of correlators and multi-user detection. The CRi design is based on planar lightwave circuits and consists of tapped delay lines of different weights together with differential receivers to affect signal polarity. Numerical simulations characterize the receiver in terms of symbol and bit error rates.

7923-07, Session 2

Design of a high-speed space modem for the LLCDL. E. Elgin, MIT Lincoln Lab. (United States)


7923-08, Session 2

Characterization of chaos generated by bistable optical systems with a nonlinearity of the form sin2(xn)A. K. Ghosh, P. Verma, The Univ. of Oklahoma - Tulsa (United States)

Generation of chaos from bistable optical systems has been studied for several years [1]. Such chaotic signals have widespread applications in providing secure encryption in optical communication systems. Lyapunov exponent is an important parameter for measuring the entropy of a chaotic signal [2]. In this paper we calculate the Lyapunov exponents of nonlinear or bistable optical systems in which the input-output relation is of the form

y_n = Csin^2(A+Bx_(n-m))

where A, B, and C are parameters of the system, y_n is the output optical signal at time t = nT and x_(n-m) indicates the input or a state-variable at time t = (n-m)T, time-delay m < n and n = 1, 2, 3, ... . Several single-input-single-output bistable optical systems such as the electro-optical and acousto-optical modulators with feedback, nonlinear Fabry-Perot etalons and optical loop mirrors exhibit characteristics shown by the equation given above. By calculating the Lypunov exponent we are able to understand the dependence of each of the systems on their system-parameters and compare the entropy of the chaotic signal produced by each of them. We can then select the optimum bistable optical system for the most secure optical communications.

References:

[1] V. Annovazzi-Lodi, et al., “Secure Chaotic Transmission on a Free-Space Optics Data Link,” IEEE Journal of Quantum Electronics, vol. 44, pp. 1089-1095, 2008.

[2] W. Kinsner, “Characterizing chaos through Lyapunov metrics,” IEEE Transactions on Systems, Man and Cybernetics, Part C (Applications and Reviews), vol. 36, pp. 141-151, 2006.

7923-09, Session 2

Modulated laser radar decoding by inter symbol interferenceX. Mao, D. Inoue, H. Matsubara, M. Kagami, Toyota Central Research and Development Labs., Inc. (Japan)

Pseudo Random Noise (PN) coded laser radar can improve the target detection ability without the demand on high power laser. However, the reflected echoes are generally so weak that they are buried in the

thermal noise of the receiver, which raises the problem of choosing an optimal threshold for correctly decoding them. Theoretically, it should be the half of average voltage level of code ‘1’. However, in laser radar applications, the power of echoes varies from time to time and it is not known in advance. Thus, threshold for decoding can not be a fixed value. In this work, a novel method basing on Inter Symbol Interference (ISI) is proposed for improving correct decoding ratio, which shifts the received signal level to make preset threshold optimal, while not the same as previous adaptive algorithms that shifts threshold according to received signal power.

ISI is defined as a form of distortion in which one symbol interferes with subsequent symbols, which is usually caused by multipath propagation, inherent nonlinear frequency response of a channel or the bandwidth limited channel. In simulation work, a bandpass filter which limits the bandwidth of the channel is used to create the ISI. The system structure is similar to a photo diode AC coupled to an amplifier, in which the main purpose is to increase the gain bandwidth. The cut-off frequency parameter range of the filter that improves decoding ratio two times larger than that without using ISI is pointed out. A theoretical analysis on the effective parameter range with relation to the PN code chip width will be discussed. To show the effectiveness of the method, a simple experiment is performed and compared with the simulation work.

7923-10, Session 3

Study of optical inter-orbit communication technology for next generation space data-relay satelliteT. Hanada, S. Yamakawa, H. Kohata, Japan Aerospace Exploration Agency (Japan)

JAXA has made efforts to build the next generation space data relay network. The inter-orbit optical links are essential segments for such a network in order to fulfill requirements of high resolution earth observation satellite applications (such as Advanced Land Observing Satellite (ALOS) follow-on missions by JAXA) and manned space flight missions. JAXA’s R&D activities for advanced optical communication terminals are introduced. The target of the terminals is to establish the optical data relay link between the LEO user satellite and the GEO data relay satellite up to 2.5 Gbps of data-rate. JAXA has started the development of a Bread Board Model (BBM) of the terminal in order to evaluate the feasibility of the terminal.

The terminal is aimed to be small and light-weighted, which is helpful for an onboard capability of the LEO satellite. Furthermore, the modulation of carrier and the acquisition and tracking sequence are selected in order to achieve the interoperability of optical space communication systems.

In the inter-orbit link, we adapt the homodyne BPSK modulation whose carrier wave length is 1.06 micrometer. The beaconless spiral scan are adapted as the acquisition and tracking sequence.

We recently study the feasibility of the acquisition and tracking sensor, the waveguide high power amplifier for a transmitter and the homodyne coherent receiver etc. in the development of BBM.

7923-11, Session 3

Deep-space optical terminals concept conceptual designH. Hemmati, W. H. Farr, A. Biswas, K. M. Birnbaum, W. T. Roberts, Jet Propulsion Lab. (United States)


Conference 7923: Free-Space Laser Communication Technologies XXIII


7923-12, Session 4

Fiber-MOPA based, multi-aperture, multi-kW, uplink laser beacon for deep-space laser communication linksD. Engin, W. Lu, F. Kimpel, Y. Chen, M. Akbulut, S. Gupta, Fibertek, Inc. (United States)

An uplink laser beacon with multi-kW average power is needed to serve as an absolute reference for accurate pointing/tracking of spacecraft during deep-space lasercomm downlink. However, this beam goes through atmospheric layers, and hence suffers from both scintillation and beam wander in the far-field, that is detrimental to maintaining a narrow beam divergence. Use of a multi-aperture laser beacon with up to 8 beams can alleviate this scintillation, significantly improve the fade characteristics of the link, and is a good compromise against increased complexity of system, while providing for system redundancy and reliability.

Our proposed solution for the uplink multi-kW laser beacon is based on an 8-channel Yb-fiber-MOPA architecture, fully controlled by high-speed FPGA, both for pulse-formation and pulse-shaping, as well as for the overall laser system control. A seed laser-diode is split into 1xN channels (N=8), and routed through an intensity-modulator (and an optional phase-modulator), before being fed into a specially designed Yb-fiber-amplifier chain, to amplify the signal to multi-kW level. The modulators are either discrete or custom-integrated LiNbO3-based devices. The seed laser source (at 1064nm) is programmable for any desired modulation pattern, and for the current demonstration is set to 100nsec pulsewidth, at 1 MHz repetition rate (10% duty-cycle), representative of 8-ary/16-ary PPM pulse format. Peak power >5kW (avg. power >500W) for a channel is demonstrated, with >70% optical conversion efficiency, near diffraction-limited beam (M2<1.5), and OSNR>35dB. Time-domain pulse shaping due to gain-saturation is mitigated using FPGA-based digital control. To facilitate use of Si-APD as uplink photoreceivers, shorter wavelength laser-beacon is desired. For 1030nm, only minor compromise (~5%) to power efficiency is estimated, while for wavelengths <1020nm, a larger core/clad ratio Yb-fiber is needed to maintain acceptable power conversion efficiency.

7923-13, Session 4

Compact laser transmitter providing precision aligned visible and infrared beamsH. Lee, Y. Yoon, C. Park, H. Kim, Kwangwoon Univ. (Korea, Republic of); S. Lim, Korea Telecom (Korea, Republic of); S. Lee, Kwangwoon Univ. (Korea, Republic of)

Recently the signal transmission based on the free space optics has received enormous attention in view of its variosu applications such as the visible light communications, the last-mile free-space connection, and especially the multiple integrated laser engagement systems (MILES). A laser transmitter module for the MILES involves an infrared light at ~900 nm wavelength. It is widely believed that the initial alignment of the transmitter module between a laser beam and an actual bullet mimicked by the infrared light could be substantially facillitated by taking advantage of an auxiliary visible laser beam.

In this paper, we have proposed and built a laser transmitter providing precision aligned visible and infrared beams, incorporating an optical sub-assembly module. Both TO-can type visible (~650 nm) and infrared (~904 nm) laser diodes were combined by a thin-film type wavelength division multiplexing (WDM) filter and coupled to an identical optical fiber embedded in a ceramic stub. The light coming out of the fiber was simultaneously collimated to give rise to collinear visible and infrared beams. In this way, the 650 nm and 904 nm beams were almost perfectl aligned each other, furthermore they were subject to no external environmental influence such as the vibration, temperature change, mechanical shock, etc. As for the achieved performance, the alignment angle made by the visible and infrared beams was less than 0.01o;

the beam profiles of the visible and infrared beams were thoroughly investigated in terms of the spot size, the beam shape, and the divergence angle. Finally the total length of the implemented transmitter was less then ~4 cm, fitting well to a conventional transmitter module.

7923-14, Session 4

The optical intensity distribution in the far fieldJ. Vitásek, J. Látal, P. Koudelka, F. Hanácek, P. ?i?ka, J. Skapa, V. Va?inek, Technical Univ. of Ostrava (Czech Republic)

This article describes an intensity distribution of optical beam at the far field. In free space communication there are used laser transmitters, optical lenses and other components. When an optical beam impacts optical lens, the diffraction can appear at the edge of lens. A laser transmitted optical beam whose intensity distribution was Gaussian. This beam was aimed at the circular slots of different diameters. These circular slots represented transmitting lens. We could use circular slots instead of lenses, because we obtained the same effects of diffraction. The optical beam passed by slots and optical intensity distribution was observed at the shade, which was placed at the far field.

7923-15, Session 4

Absolute time position of picosecond laser pulseJ. Blazej, I. Prochazka, J. Kodet, Czech Technical Univ. in Prague (Czech Republic)

We are presenting some new aspects of detection of absolute time position of picosecond laser pulses. The ground-space optical links for precise and accurate time scale synchronization are running or preparing in frame of several space agencies. We are involved in several projects in position of single photon detector and timing designer. The typical optical link consists of ground segment - picosecond laser pulse transmitter, telescope, single photon avalanche detector, timing electronics, and time reference; and space segment - corner cube retroreflectors, single photon sensitive optical receiver with event timer board, and time reference to be synchronized. To ensure absolute time position we have to calibrate absolute internal delay of several semiconductor photodiodes in picosecond range detecting picosecond laser pulses on several wavelengths. The experimental results demonstrating unobvious dependence on detector type and entire experiment arrangement and its possible theoretical interpretation will be presented.

7923-16, Session 5

Design of a transportable ground telescope array for the LLCDD. Fitzgerald, MIT Lincoln Lab. (United States)


7923-17, Session 5

Design of a fiber-coupled superconducting nanowire detector array system for the LLCDM. E. Grein, MIT Lincoln Lab. (United States)




7923-18, Session 5

Coherent homodyne receiver with a compensator of Doppler shifts for inter orbit optical communicationT. Ando, E. Haraguchi, K. Tajima, Y. Hirano, Mitsubishi Electric Corp. (Japan); T. Hanada, S. Yamakawa, Japan Aerospace Exploration Agency (Japan)

We have been developing next generation optical inter-satellite communication terminals for space data relay of high resolution earth observation satellite data. A high sensitivity receiver is one of the important issues for high data ratefree space communication with lightweight optical communication terminals. Coherent homodyne BPSK (Binary Phase Shift Keying) receiver with wavelength of 1 micron is the most promising candidate because of its theoretical sensitivity. This paper presented Bread Board Model (BBM) of coherent homodyne receiver with an optical phase locked loop and a frequency compensator of Doppler shifts. The performance tests clearly demonstrated an error free demodulation from 2.5 Gbps BPSK modulated signals with large frequency offsets (+/-7 GHz) as well as optically phase-locking.

7923-19, Session 5

Deep space uplink receiver prototype for optical communicationsS. Sburlan, K. M. Birnbaum, W. H. Farr, Jet Propulsion Lab. (United States)

A hardware prototype of a flight transceiver for deep space optical communications has been developed. A single detector array is used for acquisition, tracking, and high speed data recovery. A counting algorithm accumulates pulses on every pixel in a photon-counting array and extracts signal levels relative to background from the uplink outer modulation. After acquisition, the beam spot is transferred to 4 pixels that receive data from the high bandwidth inner modulation.

7923-20, Session 6

Design of an inertially-stabilized telescope for the LLCDJ. W. Burnside, S. D. Conrad, C. E. DeVoe, A. D. Pillsbury, MIT Lincoln Lab. (United States)

DesThe Lunar Laser Communication Demonstration (LLCD) program will demonstrate the first high-bandwidth optical communication payload on a NASA space mission, and is a first step towards development of a deep-space optical communication system. The inertially stabilized telescope, which will fly on NASA’s LADEE spacecraft, is a key element of the program. The inertially stabilized 107 mm aperture telescope is fabricated nearly entirely of beryllium, providing high stiffness to weight and excellent thermal and optical properties. The telescope consists of a two-axis fine positioning stage using a combination of inertial sensors and coarse and fine optical tracking systems. The stabilized telescope is in turn mounted on a two-axis coarse positioning gimbal that provides a large field of regard. Inertial stabilization provides high local disturbance rejection while requiring modest optical uplink power to provide a low frequency absolute pointing reference. The telescope is a three-color design providing separate uplink acquisition and communication wavelengths, and a downlink communication wavelength. Acquisition and tracking of the uplink beacon is via a photodiode quadrant detector, while tracking on the fine communication beam is via nutation tracking and piezoelectric actuation of the receive fiber. Control of the point-ahead angle between the uplink and downlink beams is via piezoelectric actuation of the transmit fiber. The entire telescope is thermally stabilized to maintain optical quality during operation. The transmit and receive beams are fiber-coupled to a separate optoelectronic payload and the

telescope line-of-sight will be stabilized to better than 4 microradians during normal operations. Provision for self-test and boresighting during on-orbit operations is provided.

7923-22, Session 6

High-performance two-axis gimbal system for free space laser communications onboard unmanned aircraft systemsM. Locke, M. Czarnomski, A. Qadir, B. Adkins, J. LeBlanc, N. Baer, The Univ. of North Dakota (United States)

To improve the performance of our current free space laser communications payload for Unmanned Aircraft Systems (UAS), we have opted to design and manufacture a custom gimbal with a wide field-of-view and fast response time. This enhanced custom design will be a 24 volt system with integrated motor controllers and drivers which will offer a full 180o field-of-view in both azimuth and elevation; this will provide a more continuous tracking capability as well as increased velocities. The addition of active high-frequency vibration isolation, to complement the passive vibration isolation system in our current design, is also in development.

The ultimate goal of this research is to achieve affordable, reliable, and secure air-to-air laser communications between two separate remotely piloted aircraft. As a proof-of-concept, we have been working on the practical implementation of an air-to-ground, laser-based video communications payload system flown by a small Unmanned Aerial Vehicle (UAV).

While we have faced many challenges with this project, we have also had our share of successes. Our team has written and tested a numerical tracking algorithm used to aim the airborne laser transmitter at a stationary ground-based receiver with known GPS coordinates; however, further refinement of our tracking capabilities is dependent on an improved gimbal design for precision pointing of the airborne laser transmitter.

Our current gimbal pointing system is a two-axis, commercial-off-the-shelf component, which is limited in both range and velocity. The current design is capable of 360o of pan and 78o of tilt at a velocity of 60o per second. The control algorithm used for aiming the gimbal is executed on a PC-104 format embedded computer onboard the payload to accurately track a stationary ground-based receiver. This algorithm autonomously calculates a line-of-sight vector in real-time by using the UAV autopilot’s DGPS (latitude, longitude, and altitude) and IMU (roll, pitch, and yaw) data, along with the known GPS location of the ground-based photodiode array receiver.

7923-23, Session 6

Optical inter-satellite communication with dynamically reconfigurable optical device using Sn2P2S6 crystalK. Nishimaki, A. Okamoto, T. Fujita, Hokkaido Univ. (Japan); A. A. Grabar, Uzhgorod National Univ. (Ukraine); M. Takabayashi, Hokkaido Univ. (Japan); J. Uozumi, Hokkai-Gakuen Univ. (Japan); A. Tomita, Hokkaido Univ. (Japan); Y. Takayama, National Institute of Information and Communications Technology (Japan)

Since the optical inter-satellite communication has attractive advantages such as high-speed transmission with high confidence, almost no electronic magnetic interference, and low power consumption, it has been activity investigated. However, directivity control of the laser beams requires a bulky and complicated system in satellite mobile communications. A more flexible and high accurate system with small and simple mechanism has been desired. In this study, we propose a new method of optical inter-satellite communication with a dynamically reconfigurable optical directional device in which diffraction gratings are



automatically rewritten and reorganized in response of incident conditions by moving satellites. For realizing such a device, we have developed Sn2P2S6 crystals which have a high sensitive photorefractivity and dynamic reconfigurable property. Furthermore, this crystal has hundreds times faster response than conventional photorefractive materials such as BaTiO3. These features are extremely advantageous to construct a high-speed and flexible communication system with a large tolerance to displacement of moving satellites. To investigate the possibility of the dynamically reconfigurable optical inter-satellite communication system, we experimentally evaluate the temporal and spatial characteristics of Sn2P2S6 crystals for the variation of the beam incident angle. Moreover, the diffraction beam from the crystal has phase conjugate wavefronts of the beam entering from the counter direction. We try to utilize this behavior to suppress the beam spread and to reduce the background light such as sunlight with a spatial filtering technique that has sensitivity in wavefront differences of the signal and background light.

7923-24, Session 7

Operational condition of direct single-mode-fiber coupled FSO terminal under strong atmospheric turbulenceY. Arimoto, National Institute of Information and Communications Technology (Japan)

A new category of the free-space optical (FSO) communication terminals in which diffraction limited laser beams are directly transmitted from and coupled into a single mode fiber (SMF) has been developed at NICT to realize multi-Gbps class point-to-point wireless link. This paper discusses the operation condition for this FSO terminal under the various adverse weather conditions, such as strong atmospheric turbulences and rains. By using a dedicated propagation measurement system within the FSO terminal, a good correlation between the scintillation index of the intensities of beacon receiving power and the signal fading depth has been observed, which allows us to predict the signal link quality based on the beacon scintillation index provided by the classical scintillation theory. This prediction method concludes that the scintillation index for the beacon beam should be less than 0.1 for a stable and robust FSO terminal operation. However, under the extreme conditions, enough beacon power will not available due to the deep fading occurred by the strong atmospheric turbulence. The most recent FSO terminal developed by NICT solves this problem by using an adaptive controller which makes a switching between the error signal from the beacon tracker and the pointing prediction from the internal digital filter. This paper also reports the effect of performance enhancements provided by the new controller.

7923-25, Session 7

Experimental analysis of the effects of atmospheric turbulence on a 29-km free-space laser communication linkV. V. Nikulin, Binghamton Univ. (United States); J. Malowicki, Air Force Research Lab. (United States); V. Bedi, Binghamton Univ. (United States); D. Hughes, H. Bloss, Air Force Research Lab. (United States)

Laser beams propagating through the atmosphere are affected by optical turbulence, whose static and dynamic properties can be characterized by spatial and temporal fields of the refractive index. The resulting wave front distortions lead to performance degradation in the form of reduced signal power and increased bit-error-rates (BER), even in short links; however, it is impossible to obtain closed-form solutions for instantaneous realizations of these distortions and all the subsequent events. Instead, the statistical properties of the refractive index fluctuations can be studied using one of the well-known spectral models and extended further into the scintillation analysis and analysis of communication performance. From a practical stand point, it would

be very advantageous to relate the expected system performance to specific factors responsible for wave front distortions, which are typically linked to some weather variables, such as the air temperature, pressure, wind speed, etc. In this paper, we present the results of a detailed experimental study, where some of these relationships are mathematically justified based on the tests conducted over the period of several months. The measurement data was obtained using a 29-km free-space laser communication link established between two fixed-point terminals and operating at a wavelength of 1550 nm.

7923-26, Session 7

Performance analysis of atmospheric field conjugation adaptive arraysA. Belmonte, Univ. Politècnica de Catalunya (Spain); J. M. Kahn, Stanford Univ. (United States)

System configurations based on single monolithic-apertures that are immune to atmospheric fluctuations are being developed. Main goal is the improvement of the performance achievable in coherent, free-space optical communication systems using atmospheric compensation techniques such as adaptive optics. As an alternative to a single monolithic-aperture coherent receiver with a full-size collecting area, a large effective aperture can be achieved by combining the output signal from an array of smaller receivers. The advantage of a coherent array in terms of the coupling efficiency is that the number of turbulence speckles over each subaperture in the array is much smaller than it would be over a single large aperture.

We study the communication performance of field conjugation adaptive arrays applied in synchronous laser communication through the turbulent atmosphere. We assume that a single information-bearing signal is transmitted over the atmospheric fading channel, and that the adaptive array coherent receiver combines multiple dependent replicas to improve detection efficiency. We consider the effects of log-normal amplitude fluctuations and Gaussian phase fluctuations, in addition to local oscillator shot noise. We study the effect of various atmospheric parameters and the number of branches combined at the receiver. We consider both maximal-ratio combining (MRC) and equal-gain combining (EGC) coherent schemes.

7923-27, Session 7

Study on the implementation of spatial light modulator liquid crystal device atmospheric simulator for short wavelength infrared applicationsF. Santiago, C. O. Font, C. C. Wilcox, T. Martinez, J. A. Duperre III, J. R. Andrews, S. R. Restaino, C. Gilbreath, U.S. Naval Research Lab. (United States); S. Myers, Ctr. for High Technology Materials (United States); D. M. Payne, Narrascape, Inc. (United States)

The use of Spatial Light Modulators (SLM), Liquid Crystal Devices for atmospheric turbulence simulation in optical system has increased in the recent years. These devices allow the development of test-beds that can be used to simulate, analyze and improve optical components or systems in a controlled laboratory environment before further implementation on the field. Most research has been performed at visible wavelengths with the use of a vast array of atmospheric turbulence simulation algorithms. We present preliminary work on an atmospheric simulation test bed which uses an algorithm developed at NRL and a comparison between a reflective and transmissive high definition Liquid Crystal Device SLMs for applications in the short-wavelength infrared, with the main focus of interested at 1550nm.



7923-28, Session 8

Active laser ranging simultaneously with communications at planetary distancesY. Chen, K. M. Birnbaum, H. Hemmati, Jet Propulsion Lab. (United States)

We will report real time laboratory realization and field implementation of active laser ranging with two terminals for interplanetary distance. We have achieved sub-millimeter accuracy in the lab real-time active laser ranging.

Laser communications is a part of the integrated ranging system, as the ranging data collected from two terminals and clock synchronization are coordinated using laser communications. This will be discussed in the presentation as well.



Conference 7924: Atmospheric and Oceanic Propagation of Electromagnetic Waves VTuesday 25 January 2011 • Part of Proceedings of SPIE Vol. 7924 Atmospheric and Oceanic Propagation of Electromagnetic Waves V

7924-01, Session 1

Generation of various partially coherent beams and their propagation properties in turbulent atmosphereY. Cai, Soochow Univ. (China)

Partially coherent beams, such as Gaussian Schell-model beam, partially coherent dark hollow beam, partially coherent flat-topped beam and electromagnetic Gaussian Schell-model beam, have important applications in free space optical communications, optical imaging, optical trapping, inertial confinement fusion and nonlinear optics. In this paper, experimental generations of various partially coherent beams are introduced. Furthermore, with the help of matrix optics and tensor method, analytical formulae for such beams propagating in turbulent atmosphere are derived, and the propagation properties, such as average intensity, beam spreading, propagation factor, polarization and spectral shift, of such beams in turbulent atmosphere are reviewed.

7924-02, Session 1

Scintillation of Airy beam arrays in atmospheric turbulenceG. Gbur, Y. Gu, The Univ. of North Carolina at Charlotte (United States)

Scintillation is one of the fundamental limitations in the development of free-space optical communication systems. Some time ago, it was demonstrated that the scintillation of a partially coherent field can be lower than of its fully coherent counterpart. However, a partially coherent beam has a larger angular spread and forms a large spot at the receiver plane, which results in a smaller proportion of the transmitted energy being received by the on-axis detector. Beam arrays have been suggested another solution for scintillation reduction. Nevertheless, the problem of low received energy also exists because the constituent beamlets partially overlap at the receiver plane.

Airy beams were recently theoretically predicted and experimentally observed. The most exotic feature of Airy beams is their seeming ability to transversely accelerate, i.e. to propagate along parabolic trajectories in free space. It has also been shown that Airy beams can self-reconstruct themselves after propagating through obstacles and retain their intensity profiles under turbulent conditions. These intriguing properties suggest that Airy beams could be a choice for beam arrays in turbulence applications.

By numeric simulations, it is shown that on average, an Airy beam also follows a parabolic path when propagating in turbulence. Therefore, with appropriately chosen parameters, the constituent beamlets of an Airy beam array propagate through relatively independent regions of turbulence and fully overlap at the on-axis detector. The scintillation of the beam array is significantly reduced compared to a single Airy beam.

7924-03, Session 1

Scintillation of pseudo-Bessel correlated beams in atmospheric turbulenceY. Gu, G. Gbur, The Univ. of North Carolina at Charlotte (United States)

It is now well appreciated that the scintillation of a partially coherent beam can be lower than that of its fully coherent counterpart. So far, most studies on the propagation of partially coherent beams in

turbulence have focused on the beams whose spatial correlation function is Gaussian. It has been shown that the scintillation reduction by a Gaussian correlated beam is negligible in strong turbulence. However, a Bessel correlated beam of infinite size is propagation-invariant in free space. On noting the unusual properties of Bessel correlated beams in free-space propagation, we investigated their scintillation properties in turbulence propagation.

By using a discretized form of Bessel correlation function, a pseudo-Bessel correlated beam is synthesized by the incoherent superposition of a collection of beamlets whose wave vectors form a cone. Through the Rytov approximation and numeric simulations, it is demonstrated that, with an appropriately-chosen coherence parameter, pseudo-Bessel correlated beams have lower scintillation than comparable fully coherent beams in both weak and strong turbulence. It is also noted that the scintillation rapidly decreases to an asymptotic limit as the number of beamlets is increased. This suggests that, in general, the optimal scintillation reduction can always be achieved with a relatively small and finite number of such beamlets.

We have also studied the scintillation properties of pseudo-Bessel correlated beams combined with a central horizontal beamlet. The additional beamlet keeps the scintillation of the so-called modified pseudo-Bessel correlated beams at a low level for a relatively wide range of values of the correlation length.

7924-04, Session 1

Spectral, coherence, and polarization properties of beam-like optical fields propagating in non-Kolmogorov atmospheric turbulenceO. Korotkova, Univ. of Miami (United States); E. Shchepakina, Samara State Aerospace Univ. (Russian Federation)

The changes in all the second-order statistical properties of stochastic electromagnetic beams propagating in clear-air atmospheric turbulence described by the power spectrum with fractal constant (slope) , 3<<5 are examined. The detailed study is made for the spectral composition, the degree of coherence and the polarization properties. Propagation through the atmosphere is treated on the basis of the extended Huygens-Fresnel principle and the electromagnetic nature of the beam is accounted with the help of the unified theory of coherence and polarization. We demonstrate, in particular, that the atmosphere affects the beam statistics at most in the regime when takes a critical value that is slightly greater than 3 (3.11) and at least when it approaches 5 (free space-like propagation). We found that spectral and polarization properties of stochastic beams reconstruct at best for turbulent medium with the critical fractal constant. Our analysis will be useful for communications and sensing in the atmosphere at altitudes more than 2 km, where the non-Kolmogorov turbulent regime is believed to exist.

7924-05, Session 1

Method of evaluation of the mutual coherence function of laser beams and its application for symmetric dark hollow beamsV. A. Banakh, D. Marakasov, D. Rytchkov, V.E. Zuev Institute of Atmospheric Optics (Russian Federation); Y. K. Baykal, H. T. Eyyuboglu, Çankaya Univ. (Turkey)

In our report, a method of evaluation of mutual coherence function of optical wave propagating in turbulent atmosphere is proposed. The


method is based on inversion of Fresnel integral and 2-D Fast Fourier Transformation procedure that provide high accuracy in propagation conditions ranging from weak up to strong optical turbulence regime. This technique allows a significant reduction in the evaluation time of mutual coherence function. The proposed method is suitable for optical waves with arbitrary initial distribution of amplitude and phase. Results of investigations on the degradation of coherence of symmetric dark hollow beams (DHB) propagating in turbulent atmosphere are presented. Analysis of evolution of the mutual coherence function is carried out, and some characteristics of DHB, such as mean intensity distribution, and moments of Wiegner distribution are calculated for various kinds of profiles of structural characteristic of refractive index. Kolmogorov spectrum of correlation function of refractive index fluctuations is used in evaluation.

7924-06, Session 1

Polarization properties of stochastic electromagnetic pulsed beams in turbulent atmosphereC. Ding, Z. Zhao, L. Pan, Luoyang Normal Univ. (China); X. Yuan, Soochow Univ. (China)

Using the coherence theory of non-stationary fields and the method of two-time Fourier transform, the analytical expression for the spectral degree of polarization of stochastic spatially and spectrally partially coherent electromagnetic pulsed beams is derived, and used to study the polarization properties of stochastic electromagnetic pulsed beams. The influence of pulse frequency and refraction index structure constant on the spectral degree of polarization is emphasized. It is shown that, in comparison with free-space case, the turbulent atmosphere plays an important role on the distribution of the spectral degree of polarization of stochastic electromagnetic pulsed beams propagating in turbulent atmosphere. The illustrative numerical examples are given.

7924-07, Session 2

Long-range (149km) atmospheric propagation experiments with multi-wavelength laser beaconsM. A. Vorontsov, Univ. of Dayton (United States)

We report results of the experimental analysis of atmospheric effects on laser beam propagation over a long (149 km) propagation path between Mauna Loa (Island of Hawaii) and Haleakala (island of Maui) mountains. The experiments were performed in July and August 2009 using the Coherent Multi-Beam Atmospheric Transceiver (COMBAT) system that includes three co-aligned CW laser beacons operating at 530 nm, 1060 nm and 1550 nm.

7924-08, Session 2

Probability density function of fluctuating intensity of laser beam propagating in marine atmospheric turbulenceS. Avramov-Zamurovic, U.S. Naval Academy (United States); O. Korotkova, Univ. of Miami (United States); R. Malek-Madani, U.S. Naval Academy (United States)

Fluctuating intensity of a laser beam propagating through ground and marine atmospheric channels was measured under weak atmospheric conditions. The data was fitted to the Barakat’s model for the probability density function (PDF) which uses Gamma distribution for accounting for the first two statistical moments and generalized Laguerre polynomials for accounting for moments of orders higher than two. Comparison of the

above-ground and the above-water PDFs is made and the dependence of the shape of the PDF of the intensity on the radial position within the propagating beam is revealed.

7924-09, Session 2

Picosecond laser pulse propagation delay fluctuation through atmosphereI. Prochazka, L. Kral, J. Blazej, Czech Technical Univ. in Prague (Czech Republic)

The influence of Earth atmospheric turbulence on the propagation of a picosecond laser pulse has been investigated from point of view detection with high temporal resolution on single photon level. The results has been interpreted for optical time scale synchronization link allowing picosecond precision and accuracy in ground-orbit experimental scale. The details in laser beam position changes, phase wavefront deformation or beam profile changes were not studied like in adaptive optics as the goal of time transfer link is not the imaging but time tagging. The figure of merit of presented results is the time of propagation, its absolute delay and jitter. The correlation of the atmospheric turbulence with the propagation delay fluctuation was measured. The physical reason of the fluctuation of propagation time of laser pulse on picosecond level is the same, but the entirely different approach in comparison to adaptive optics was used to describe the effect.

7924-10, Session 2

Study of optical phase change measurement using the Hilbert transform and interferometric techniquesJ. A. Duperre III, C. O. Font, D. Bonanno, E. Tarpara, A. Golden, C. Gilbreath, U.S. Naval Research Lab. (United States)

Advances in the fields of optics and optical communications have created a demand for effectively measuring relative phase changes along an optical path or within an optical system. We present a method for obtaining these measurements using an interferometric setup with processing involving the Hilbert Transform. In this work, the Hilbert Transform algorithm is justified by accurately measuring the phase changes in both software generated signals and signals obtained via experimental setup. The NRL developed bench-top experimental setup is capable of extracting phase changes from signals obtained from various optical systems, including simulated freespace atmospheric propagation. This provides proof of the practicality of the application of our method to real-world applications.

7924-11, Session 2

Experimental study on high-order Bessel beams propagating in a turbulent atmosphereJ. Pu, Huaqiao Univ. (China)

The propagation of laser beams in a turbulent atmosphere is a subject of considerable importance in connection with a remote sensing, imaging, and optical communication. It is well known that a general laser beam will expand its beam width on its propagation in turbulent atmosphere. A non-diffracting beam, such as a Bessel beam, is demonstrated to keep its beam width on propagation. It is also shown that a vortex beam whose wave front is spiral is less influenced by the turbulent atmosphere. In this paper, the generation of a high-order Bessel beam and its propagation in turbulent atmosphere are experimentally investigated. We first of all convert a Gaussian beam into a vortex beam by a spiral phase plate, then a high-order Bessel is generated by passing the vortex beam through a axicon. It is demonstrated that the order of the generated high-order Bessel beam is the same as the topological charge of the

Conference 7924: Atmospheric and Oceanic Propagation of Electromagnetic Waves V


vortex beam, and the generated high order Bessel beam will keep the beam width unchanged during propagation in free space. We extend the study to the propagation of the beam in turbulent atmosphere, and the propagation properties of the high-order Bessel beam in turbulent atmosphere are investigated, including the scintillation index. Moreover the partially coherent high-order Bessel beams propagating in turbulent atmosphere are also studied.

7924-12, Session 3

Optical wireless communication through random mediaS. Arnon, Ben-Gurion Univ. of the Negev (Israel)


7924-13, Session 3

Adaptive mitigation of the laser wavefront perturbationsA. I. Khizhnyak, V. Markov, MetroLaser, Inc. (United States)

Optimal delivery of the laser energy to the remote target is the task that is difficult to achieve. Propagation of the laser beam through turbulent atmosphere results in an increased spot size, beam jitter and substantial reduction of its power density on a remote target or detector. Mitigation of these effects can be achieved by pre-distorting the wavefront of the output beam. To be effective, this technique necessitates for a localized beacon on the target. However, a conventional adaptive optics (AO) based wavefront correction doesn’t perform well in the operational scenario against a rough-surface target and turbulent atmospheric path. This is because the intricacy in decomposition of the speckle-pattern formed by atmospheric turbulence and by a target-scattered light at the pupil plane of the receiving telescope.

This presentation outlines a novel approach to the target-in-the-loop (TIL) regime for correcting turbulence-induced perturbations of the laser beam propagating against a rough-surface target. The approach is based on our prior studies of the beacon formation in the TIL regime with a phase conjugate mirror (PCM). The performed analysis of the TIL system shows that by using a spatial selector in its image plane enables to select turbulence-induced perturbations, thus making it is possible to recover the data on required pre-distortion of the outgoing laser beam.

In conclusion, this presentation discusses the results of the analysis and experimental modeling of the performance of the proposed TIL-PCM system with a rough-surface target and simulated turbulence.

7924-14, Session 3

Wavefront sensing and adaptive control in phased array of fiber collimatorsS. L. Lachinova, Univ. of Maryland, College Park (United States); M. A. Vorontsov, Univ. of Dayton (United States)

We introduce a new wavefront control concept for coherent combining of multiple laser beams that are generated by an array of fiber collimators (subapertures) and transmitted to a remote target plane through atmospheric turbulence. The target-plane beam combining requires compensation of optical path differences (piston phases) originated along the propagation path between the transmitter aperture and the target; in the present research, the phase shifts of the outgoing beams at the system pupil plane are considered compensated (locked). In the proposed and analyzed adaptive control technique, sensing of the piston phases at each fiber collimator is performed through local measurements of average wavefront tilts from the received (backscattered) wave, followed by reconstruction of the input wavefront over the entire aperture of the fiber array. Local piston phases, as well as higher

order aberrations, are subsequently calculated from the reconstructed wavefront. Results of analysis and numerical simulations are presented. Operation of adaptive array of fiber collimators is compared for various adaptive system configurations characterized by the number of fiber collimators, the adaptive compensation resolution, and atmospheric turbulence conditions. We show that the suggested concept can be effectively applied for long-range and near-horizontal beam propagation paths.

7924-15, Session 3

Scintillation reduction for combined Gaussian-vortex beam propagating through turbulent atmosphereG. P. Berman, Los Alamos National Lab. (United States); V. N. Gorshkov, Institute of Physics (Ukraine); S. V. Torous, National Technical Univ. (Ukraine)

We suggest a novel method to generate a partially coherent laser beam by combining two laser beams - Gaussian and vortex beams, with different frequencies (the difference between these two frequencies being significantly smaller than the frequencies themselves). In this case, the effective suppression of the scintillation index does not require high-frequency modulators. We present the results of numerical simulations for different dimensions of the detector, distances of propagation, and strengths of the atmospheric turbulence. The results are important for achieving gigabit data-rates in long-distance laser communication through turbulent atmospheres.

7924-16, Session 3

On fading probability density functions of fast-tracked and untracked free-space optical communication channelsZ. Zhao, R. Liao, Michigan Technological Univ. (United States)

Free-space optical (FSO) communication systems suffer from average power loss and instantaneous power fading due to the atmospheric turbulence.

The channel fading probability density function (pdf) is of critical importance for FSO communication system design and evaluation. The performance and reliability of FSO communication systems can be greatly enhanced if fast-tacking devices are employed at the transmitter in order to compensate laser beam wander at the receiver aperture. The fast-tracking method is especially effective when communication distance is long. This paper studies the fading probability density functions of both fast-tracked and untracked FSO communication channels. Large-scale wave-optics simulations are conducted for both tracked and untracked lasers. In the simulations, the Kolmogorov spectrum is adopted, and it is assumed that the outer scale is infinitely large and the inner scale is negligibly small. The fading pdfs of both fast-tracked and untracked FSO channels are obtained from the simulations. Results show that the fast-tracked channel fading can be accurately modeled as gamma-distributed if receiver aperture size is smaller than the coherence radius. An analytical method is given for calculating the untracked fading pdfs of both point-like and finite-size receiver apertures from the fast-tracked fading pdf. For point-like apertures, the analytical method gives pdfs very close to the well-known gamma-gamma pdfs if off-axis effects are omitted in the formulation.

When off-axis effects are taken into consideration, the untracked pdfs obtained using the analytical method fit the simulation pdfs better than gamma-gamma distributions for point-like apertures, and closely fit the simulation pdfs for finite-size apertures where gamma-gamma pdfs deviate from those of the simulations significantly.



7924-17, Session 3

RF-modulated pulsed fiber-optic lidar transmitter for improved underwater imaging and communicationsF. Kimpel, Y. Chen, B. McIntosh, J. Fouron, S. Gupta, Fibertek, Inc. (United States)

We report on the design, development and initial testing of an innovative fiber-optic pulsed lidar transmitter operating at 532nm, for underwater imaging and communication application in littoral waters. Unlike standard lidar transmitters that produce a single pulse at a fixed repetition rate, this lidar transmitter produces a coherent RF-modulated (0.5 - 1.56 GHz) pulse-packet at a programmable repetition rate (0.5 - 1.0 MHz). In addition to time-gating of the few nsec pulse-packet, the high frequency content of the transmitted pulse reduces the effect of volume backscatter in turbid waters, thereby enhancing the image contrast. The pulse-format, number of pulses and width of the pulse-packet can be programmed, thereby enabling enhanced detection via digital correlation methods. The laser transmitter architecture is based on an all-fiber multi-stage Yb-fiber-amplifier system operating at 1064nm, that is frequency-doubled in a single-pass configuration to produce up to 5W of average power at 532nm. Flexible pulsing capability is achieved via high-speed FPGA-based control of the seed laser drive, modulator drive and bias-control, as well as for other laser control loops.

The green laser output was projected through a 7-meter diameter water-tank to a white or black target. A PMT next to the laser recorded the double-pass return signal. Even under high attenuation in simulated turbid waters (by adding Maalox), the return signal was clearly distinguished from the backscatter. Future testing of this lidar transmitter with an I/Q RF-demodulator is expected to lead to even further improvement in image contrast. Preliminary testing as an underwater communication transmitter was also conducted, using much longer pulse patterns. Even at such reduced pulse energies, feasibility of high bit-rate communications in turbid waters was demonstrated.

7924-18, Session 4

USAF airborne laser: HEL-generated extinction effects and degradation along extended H2O vapor lines-of-sight (including oceans and atmosphere)C. A. Paiva, BSM Research Associates (United States)

This research addresses missile exhaust plume ionization as a function of altitude variable water vapor concentrations (including missile exhaust plume) increases, rocket plume expansions in H20 environments, reverse flows and HEL-generated plasmas, cumulatively as these processes affect USAF-AB (Airborne Laser) Advanced Tracking Illuminator (ATILL) and adaptive optics Beacon Illuminator Laser (BILL). Boost-phase missile exhaust plumes have been shown to generate a variety of very challenging exhaust-plasma and HEL electromagnetic extinction effects. This results in reducing returned energy to the sensor suite aboard USAF-ABL when considering missiles plume LOS penetration by ATILL and BILL. Specifically such exhaust plasma/HEL/plume interactions generate a reduction in coherence due to engagements with LOS missile exhaust reverse flows. This requires changes in automatic target detection, classification and identification (ATDCI) components and the primary HEL weapon system (USAF Airborne Laser). Missile expanded and reversed exhaust H2Ovapor plumes are shown to generate very severe propagation extinction fields within the Prandtl-Meyer reverse flows and HEL engagement regimes. This further results in inadequate automatic target recognition and pattern reference library efficiencies of USAF-ABL ATILL and BILL.

7924-19, Session 4

A tunable diode laser absorption system for long path atmospheric transmission and high energy laser applicationsG. P. Perram, C. A. Rice, Air Force Institute of Technology (United States)

An open-path Tunable Diode Laser Absorption (TDLAS) system composed of narrow band (~1 MHz) diodes fiber coupled to a 12” Ritchey-Chrétien transmit telescope has been developed to study atmospheric transmission of key High Energy Laser wavelengths. The ruggedized system has been field deployed and tested for propagation distances of greater than 1 km. Initial experiments were performed in the vicinity of the oxygen X3g - b1g+ electronic transition lines near 760 nm. The potassium version of the Diode Pumped Alkali Laser (DPAL) operates in between two of the sharp oxygen rotational features in the PP and the PQ branches. By scanning across many laser free spectral ranges and monitoring the laser frequency with a very precise wavemeter, the full structure of the oxygen molecular feature is observed. Rotational temperatures, oxygen concentrations, and total atmospheric pressure is accurately retrieved from Lorentzian profiles. Atmospheric scattering is also evaluated from transmission in the far wings and side scatter using a second 12” receiver. By comparing the high resolution TDLAS spectra with a non-imaging FTIR absorption spectra at 1 cm-1, validation of a method for monocular passive ranging is also achieved.

7924-20, Session 4

Atmospheric absorption spectroscopy using Tm:fiber ASE source around 2 micronP. Kadwani, CREOL, The College of Optics and Photonics, Univ. of Central Florida (United States); J. Chia, College of Optical Sciences, The Univ. of Arizona (United States); R. A. Sims, C. Willis, C. Jollivet Salvin, L. Shah, CREOL, The College of Optics and Photonics, Univ. of Central Florida (United States); D. Killinger, Univ. of South Florida (United States); M. C. Richardson, CREOL, The College of Optics and Photonics, Univ. of Central Florida (United States)

We investigate atmospheric absorption using a thulium doped silica fiber ASE source. This source is based on ~2-m long active fiber and generates a broadband spectrum from 1880 nm to 2040 nm. We measure the transmission through a 1-m long propagation cell for several gases from ~1x10^-3 torr to atmospheric pressure, and compare to High Resolution Transmission simulation (HITRAN and PNNL databases). This facility is ideal for controlled short-range experiments to calibrate long propagation conditions.

Several interesting atmospheric constituents have distinct signatures in this wavelength regime, primarily water vapor and carbon dioxide. These results are of crucial importance for a wide range of applications such as LIDAR, directed energy and free-space communications as variations in these components can significantly affect the atmospheric transmission windows.

In addition to reporting our experimental results, we will discuss how these results inform several current Tm:fiber laser development projects. These results will also be discussed in relation to long-range (1 km) propagation tests using a high power, tunable, narrow linewidth Tm:fiber laser at the Innovative Science & Technology Experimentation Facility (ISTEF) laser range on Merritt Island, FL.



7924-21, Session 4

Temporal-frequency spectra for laser propagation through non-Kolmogorov turbulence in a maritime environmentI. Toselli, Naval Postgraduate School (United States); S. R. Restaino, U.S. Naval Research Lab. (United States); B. Agrawal, Naval Postgraduate School (United States)

Experimental data show different atmospheric turbulence behavior in marine environments compared to terrestrial environments. Essentially the classical Kolmogorov refractive index power spectrum does not properly describe the behavior of the atmosphere in a maritime environment. Also, data show different refractivity measurements in a marine atmosphere, especially at wave numbers near the characteristic Hill’s bump. In this paper we analyze the Temporal-frequency spectra for laser beam propagation in a maritime environment by using a generalized power spectrum of the refractive index fluctuations. This spectrum is very flexible and can be used to fit experimental data by setting appropriate values of its coefficients.

7924-22, Session 4

Hybrid technique for propagation and scattering from random medium containing random distribution of particlesZ. Tong, O. Korotkova, Univ. of Miami (United States)

On the basis of the first Born approximation for weak scattering and Rytov approximation for propagation in weakly fluctuating medium we formulate a new technique for wave transmission in a random medium which contains random collection of scatterers. This new method unifies previously separately treated propagation in continuous medium and scattering from particulate medium via decomposition of field realizations into the angular spectra of plane waves. We apply this approach for deterministic and stochastic wave propagation in non-clear atmospheric turbulence, i.e. the one which might contain aerosol, water, dust, etc. particles. We also tackle the inverse problem of determining the structure function of random collection of scatterers from the knowledge of the incident wave,statistics of the clear-air turbulence and measurements of the transmitted wave. The new approach will be useful in laser communications and LIDARs operating in non-clear air atmospheric turbulence.



Conference 7925: Frontiers in Ultrafast Optics: Biomedical, Scientific, and Industrial Applications XISunday-Wednesday 23-26 January 2011 • Part of Proceedings of SPIE Vol. 7925 Frontiers in Ultrafast Optics: Biomedical, Scientific, and Industrial Applications XI

7925-01, Session 1

Microcavity biosensingF. Vollmer, Harvard Univ. (United States)


7925-02, Session 1

Ultrafast laser induced condensation of moleculesL. Gu, S. K. Mohanty, The Univ. of Texas at Arlington (United States)

We have found that ultrafast laser microbeam induced bubble can lead to condensation of fluorescent dye and antibodies in solution. The laser microbeam was generated by coupling a near infrared mode-locked femtosecond laser beam onto a Nikon inverted fluorescence microscope and focusing it with a 100X objective. Fluorescence imaging revealed that cavitation bubbles created high-concentration regions of dye or dye-labeled antibodies in the aqueous solution. Though, the nuclear staining propidium iodide (PI) gives order of magnitude higher fluorescence upon binding to DNA, the fluorescence of PI at the condensed spot outside cell was found to be higher than that in the nucleus. This implied concentration of PI to be increased by at least 20 times due to ultrafast laser assisted bubble formation. However, the fluorescence emission peak was found to be shifted towards the green region with even higher excitability under blue excitation. This is in contrast to the natural excitation (green) and emission (red) spectral characteristics of DNA-bound PI. The ultrafast laser microbeam could also be successfully employed to condense anti-mitochondrial antibodies stained with Alexa-488. High-concentration regions of the dyes and antibodies are found to be retained for a longer period of time and therefore provide an opportunity for collection of the condensed molecules using microcapilary and/or for further analysis. Kinetics of ultrafast laser assisted bubble formation and condensation of molecules will be presented.

7925-03, Session 1

Method for imaging quantum dot photoluminescence during exposure to radiationA. N. Immucci, J. J. L. Carson, Lawson Health Research Institute (Canada)

Quantum dots have been widely used in a variety of biomedical applications. A key advantage of these particles is that their optical properties depend predictably on size, which enables tuning of the emission wavelength [Alivasatos, et al. Science 1996, 271, 933-937.]. Recently it was found that CdSe/ZnS quantum dots lose their ability to photoluminescence after exposure to gamma radiation [Stodilka, et al. J. Phys. Chem.C 2009 Vol. 113, pp 2580-2585]. A method for readout of the loss of quantum dot photoluminescence during exposure to radiation could enable a multitude of real-time dosimetry applications. Here, we report on a method to image photoluminescence from quantum dots from a distance and under ambient lighting conditions. The approach was to construct and test a time-gated imaging system that incorporated pulsed illumination. The system was constructed from a pulsed green laser (Nd:YAG, 20 pulses/s, 5 ns pulse duration, ~5 mJ/pulse), a time-gated camera (LaVision Picostar, 2.5 - 5 ns gate width), and optical components to enable coaxial illumination and imaging. Using the system

to image samples of equivalent concentration to previous end-point work, quantum dot photoluminescence was measureable under ambient room lighting at a distance of 25 cm from the sample with a signal to background of 7.5:1. Continuous exposure of samples to pulsed laser produced no measureable loss of photoluminescence over a time period of an hour. With improvements to the light collection optics the range of the system is expected to increase to a distance exceeding 5 m, which will enable imaging of samples during exposure to a gamma radiation source.

7925-04, Session 1

Femto second single optical fiber tweezers enabled two-photon fluorescence excitation of trapped microscopic objectsY. N. Mishra, Cochin Univ. of Science & Technology (India); N. D. Ingle, S. K. Mohanty, The Univ. of Texas at Arlington (United States)

Analysis of trapped microscopic objects using fluorescence and Raman spectroscopy is gaining considerable interest. We report on the development of single fiber femto second optical tweezers and its use in two-photon fluorescence excitation (TPE) of trapped fluorescent polystyrene beads. The single fiber based optical tweezers was achieved by coupling a tunable Ti: Sapphire laser beam into an optical fiber with microfabricated conical tip. While TPE is known to cause less photobleaching, trapping of the floating objects led to stable fluorescence emission intensity over a long period of time. Owing to the propagation distance of the Bessel-like beam emerging from the axicon-fiber tip, a relatively longer streak of fluorescence was observed along the microsphere length. Large cone angle axicon provided better trapping stability and high axial confinement of TPE as compared to small cone angle fiber tips. The possibility of using the same fiber for collecting the two-photon fluorescence is also explored. We will present theoretical simulation of femtosecond fiber optical microbeam profiles emerging from the axicon tip and the experimentally observed TPE patterns. The combined use of TPE and optical trapping using single fiber optical tweezers will enable in-depth analysis of non-adherent samples. The microfabricated tip can also be used for two-photon imaging of cells adhering to substrate by scanning either the tip or the sample stage.

7925-05, Session 2

Femtosecond laser micro/nano patterning of biological materialsC. P. Grigoropoulos, H. Jeon, Univ. of California, Berkeley (United States); H. Hidai, Univ. of California, Berkeley (Japan); D. J. Hwang, Univ. of California, Berkeley (United States)

This talk aims at presenting recent work at the Laser Thermal Laboratory on the microscopic and nanoscale laser modification of biological materials using ultrafast laser pulses. We have devised a new method for fabricating high aspect ratio patterns of varying height by using two-photon polymerization process in order to study contact guidance and directed growth of biological cells. Studies using NIH-3T3 and MDCK cells indicate that cell morphology on fiber scaffolds is influenced by the pattern of actin microfilament bundles. Cells experienced different strength of contact guidance depending on the ridge height. Cell morphology and motility was investigated on micronscale anisotropic cross patterns and parallel line patterns having different aspect ratios. A significant effect on cell alignment and directionality of migration was observed. Cell morphology and motility were influenced by the


aspect ratio of the cross pattern, the grid size, and the ridge height. Cell contractility was examined microscopically in order to measure contractile forces generated by individual cells on self-standing fiber scaffolds. We have also introduced a method for generating user-defined nanopatterns of cell adhesion ligands by ablating an ultrathin protein adsorption resistant poly(ethylene glycol) brush layer using focused femtosecond laser pulses to expose an underlying adhesive substrate. The ablated regions were modified with peptides designed to engage with specific receptors. We were able to generate nanometer scale regions of cell adhesive peptides, while independently controlling feature size and spacing, hence allowing direct specification of the number and area of focal adhesion patterns. We have also conducted experiments to determine the effect of laser-induced nanoscale laser topography on cell adhesion.

7925-06, Session 2

Generation of UV and blue light by using off-axis pumping for fluorescence lifetime spectroscopyS. Taccheo, Swansea Univ. (United Kingdom); C. D’Andrea, A. Bassi, R. Cubeddu, Politecnico di Milano (Italy); K. Schuster, J. Kobelke, K. W. Morl, IPHT Jena (Germany); S. Soria Huguet, G. Nunzio Conti, G. C. Righini, Istituto di Fisica Applicata Nello Carrara (Italy)

We report on fluorescence lifetime spectroscopy in the visible range using light generated by high-order mode propagation in microstructured fiber. We extended the wavelength interval generated by supercontinnum generation in a microstructre fiber by coupling the light in the second order mode. Efficient generation of light in the visible range down to 420 nm is achieved by choosing the fiber dispersion to be close to zero for the second high-order mode at the 800 nm pump wavelength. Generation of UV light down to 350 nm was also observed with power level in the order of 1 mW.

We report the last performance of the source and a detailed investigation of the generated spectrum from UV/visible to mid-infrared.

This coupling in higher-order mode generated spectra by continuum generation with sharp peak in the visible range that can be tune from red to blue by changing the input power. A new regime for visible light generation seems to occurs in our case compared to pumping close to the zero dispersion wavelength for the fundamental mode. In addition this systems is far more stable and very easy to couple with respect other microstructured fiber solutions as we will discuss at the conference. Peaks apperas on the continuum spectrum and can be tuned by changing the coupling and input power. We were able to obtain a blue output beam concentrated at around 480 with several mW nm. We demonstrated the effectivness of this solution by performing a fluorescence lifetime spectroscopy measurement of a DCM dye dissolved in glycol sample. Recent results shows we can generate peaks in continuum spectrum in the UV region at around 350 nm. With this technique light can be efficiently generated in the whole visible range down to 350 nm.

7925-07, Session 2

Holographic spatiotemporal lens (HSTL)K. Kimura, S. Hasegawa, Y. Hayasaki, Utsunomiya Univ. (Japan)

Two-photon microscopy using infrared excitation realizes an imaging for living tissue in a high depth with small photobleaching, low absorption, and low scattering. To minimize the two-photon excitation spot, it is effective to increase a spatial photon density using a high numerical aperture lens and to increase a temporal photon density by shortening a pulse width. But the resolution of two-photon excitation spot decreases due to chromatic dispersion of an objective lens and other dispersive devices. We propose a holographic spatiotemporal lens (HSTL) to improve spatial resolution of two-photon excitation as a new focusing

technique of femtosecond laser pulse. The HSTL is composed of a diffraction grating or a prism-pair for a dispersion of femtosecond laser and chirped diffractive lens (CDL) displayed on a spatial light modulator (SLM). The CDL is designed for its corresponding wavelength, and hence the beam is spatiotemporally focused to its focal plane without chromatic aberration.Therefore, the area of high peak power is minimizaed, and high-resolution two-photon excitation spot is obtained. From experimental results, shortest pulse that is equivalent to pulse width of the laser system was obtained at only a focal plane of the HSTL, and the pulse width was stretched at other planes. And also, simulation results of propagation showthat the distribution of pulse duration along the optical axis is similar to the experimental results. Furthermore, we experimentally demonstrated parallel spatiotemporal focusing using a multiplexed HSTL that condense the beam to 9 points by taking advantage of straightforward reconfigurability of an SLM.

7925-08, Session 2

New femtosecond sources for laser surgery of the anterior segment of the eyeF. Deloison, C. Crotti, T. Marciano, D. A. Peyrot, L. Kowalczuk, Ecole Nationale Supérieure de Techniques Avancées (France); M. Savoldelli, J. Legeais, Hopital Hotel Dieu (France); K. Plamann, Ecole Nationale Supérieure de Techniques Avancées (France)

Femtosecond laser surgery is limited in depth for transfixing and lamellar cuts in oedematous cornea and even more so in sclera because of optical scattering. By increasing the wavelength while avoiding tissular absorption peaks, scattering phenomena are greatly reduced and the laser penetration depth is reduced. An optimal compromise may be obtained using the window of relative optical transparency centred at 1650 nm. Currently, no compact femtosecond laser emitting at this wavelength is available. We have developed a new compact laser source based on a diode-pumped solid-state laser emitting at 1030 nm using Ytterbium technology, with a spectral width of 5 nm, a pulse duration of 600 fs, a repetition rate of 10 kHz. It pumps a periodically poled crystal in the configuration of an Optical Parametric Generator (OPG). The system is single pass; it is simple and compact, stable and has a high conversion efficiency (up to 20 %) without needing the injection of a seed. The output wavelength can be tuned between 1450 nm to 2060 nm; the pulse energy available at 1650 nm is about 15 µJ. Several series of experiments on corneal tissue have been performed while varying the wavelength from 1450 nm to 1700 nm. The outcome has been compared to results obtained at 800 nm and 1000 nm. Using identical pulse energies, the penetration depth obtained at 1650 nm is improved by a factor of about three compared to 1030 nm while the excellent quality of the incisions is maintained.

7925-09, Session 3

Elucidating optimal photodisruption parameters in the femtosecond laser nanoablation of cellular membranesD. S. Eversole, S. Haering, A. Ben-Yakar, The Univ. of Texas at Austin (United States)

Excitation of surface plasmons on spherical nanoparticles by pulsed laser irradiation has provided a platform for the confinement of photoactivated processes. In conjunction with nanoparticle targeting methods, plasmonic laser phototherapy provides the highest level of therapeutic selectivity. Here we describe a novel plasmonic laser Nanosurgery (PLN) technique that relies on the large enhancement of femtosecond laser pulses in the vicinity of gold nanoparticles to photodisrupt nanoscale structures in live cells. To demonstrate the feasibility of PLN, we studied optoporation of MDA-MB-468 and NIH-3T3 cells in vitro. We labeled cells with functionalized 50 nm gold nanospheres and optoporated them with PLN using 250-fs, 760 nm laser pulses repeating at 80 MHz.

Conference 7925: Frontiers in Ultrafast Optics: Biomedical, Scientific, and Industrial Applications XI


To probe membrane integrity after photodisruption, we studied the influx of 10 kDa FITC-Dextran after irradiating of cells (~ 100) for 10 sec with a scanned laser beam. Membrane barrier dysfunction occurred at powers as low as 1 mW average power, in which 20% of irradiated cells incurred FITC-Dextran influx. The threshold for membrane dysfunction for labeled cells was approximately 50 times less than that for unlabeled cells. With increasing laser power, the number of cells incurring dye influx rapidly increased; an increase in laser power by 3-fold rendered nearly 100% of cells with membrane integrity dysfunction. We determined that cells irradiated with powers between 1 to 2.5 mW showed transient pore formation, while powers above that rendered cells unviable. To optimize optoporation efficiency, we studied the affects of particle labeling, irradiation duration, and laser polarization.

7925-10, Session 3

Towards high-throughput automated targeted femtosecond laser based transfection of adherent cellsM. K. Antkowiak, M. L. Y. Torres-Mapa, F. J. Gunn-Moore, K. Dholakia, Univ. of St. Andrews (United Kingdom)

Femtosecond laser induced cell membrane poration has proven to be an attractive alternative to the classical methods of drug and gene delivery. It is a selective, sterile, non-contact technique that offers a highly localized operation, low toxicity and consistent performance. However, its broader application still requires the development of robust, high-throughput and user-friendly systems. We present two systems capable of unassisted enhanced targeted optoinjection and phototransfection of adherent mammalian cells with a femtosecond laser. We demonstrate the advantages of a dynamic diffractive optical element, namely a spatial light modulator (SLM) for precise three dimensional positioning of the beam. It enables the implementation of a “point-and-shoot” system in which using the software interface a user simply points at the cell and a predefined sequence of precisely positioned doses can be applied. We show that irradiation in three axial positions alleviates the problem of exact beam positioning on the cell membrane and doubles the number of viably optoinjected cells when compared with a single dose. As an alternative we discuss a system that uses a fast piezo mirror in combination with a non-diffracting Bessel beam, which offers faster beam switching and larger accessible irradiation area at a cost of limited axial control and increased power requirements. Additionally, both presented systems enable untargeted raster scan irradiation which provides transfection of adherent cells at the throughput of 1 cell per second. Finally, we discuss possible imaging modalities, namely phase contrast, quantitative phase and epi-fluorescence that can be used for automated unassisted cell localization and targeting.

7925-11, Session 3

Near-field nanoablation thruster of gold solid nanoparticles by backside femtosecond laser irradiation for biomedical applicationG. Obara, T. Miyanishi, T. Honda, N. N. Nedyalkov, P. A. Atanasov, M. Terakawa, M. Obara, Keio Univ. (Japan)

We present a novel method of the gold particles transfer by backside irradiation of femtosecond laser. This method is to push the SOLID phase gold nanoparticles on the substrate, thrusting by nanoablation underneath the particle.

The laser-induced forward and backward transfer of materials has recently been reported for Al and Ni (Willis APL 2005), Cu (Yang APL 2006), and FeSi (Niino Appl. Phys. Express 2008). These materials are transferred in the MOLTEN phase in a form of small droplets. Kuznetsov, et al. (Optics Express 2009, JASA-B 2009, HPLA 2010) have demonstrated the femtosecond-laser-induced transfer of gold MOLTEN NANODROPLETS. They succeeded in the controllable fabrication of

spherical gold micro- and nanodroplets with sizes between 170 and 1500 nm.

We attempt to launch a SOLID gold nano spherical particle itself driven by the thrust of the enhanced near-field nanoablation underneath the gold nanoparticle. The gold nanoparticle is on the silicon substrate. An 800 nm, 100 fs laser is used to illuminate a gold sphere of 40 nm to 200 nm in diameter from the backside of the substrate. The angle of incidence is varied to control the projectile directionality.

Using 3D FDTD method, the near-field intensity distribution and the Poynting vector are calculated to explain the thrust and directionality of the lift-off of the gold particle. We also calculate the temperature rise of the gold nanoparticles using Two-Temperature Model. The absorption compared to the scattering of the gold particle is larger in the shorter spectral domain. The DDS dislikes the temperature rise.

7925-12, Session 3

Near-field enhancement in plasmonic laser nanoablation using gold nanorods on a silicon substrateR. K. Harrison, A. Ben-Yakar, The Univ. of Texas at Austin (United States)

Plasmonic laser nanoablation (PLN) using ultrafast laser pulses is of great interest for applications ranging from targeted nanosurgery to nanolithography, but the mechanisms behind this phenomenon are poorly understood. We study ultrafast PLN of silicon using gold nanorods with both experiments and numerical simulations to understand the relationship between observed nanorod ablation sites and plasmonic near-field enhancement. Using PLN with gold nanorods and ultrafast near-infrared laser pulses, we fabricated isolated features in a silicon surface as small as the dimensions of the nanorods used in the study - 25 nm x 75 nm. The fabricated nanorod ablation sites are studied in detail using high-resolution scanning electron microscopy. Nanorods on a silicon substrate are also simulated extensively using Discrete Dipole Approximation simulations. We compare the shape, depth and fluence observed at experimental nanorod ablation sites with the distribution, localization, and near-field enhancement magnitude observed from calculated electromagnetic fields. We use gold nanorods in our study as a plasmonic ‘lens’ rather than spheres to distinguish between proposed mechanisms of ablation. We conclude that the Poynting vector magnitude is a better predictor of ablation size, shape and threshold for ultrafast PLN than the square of the electric field, as had previously been supposed. Finally, we examine the impact of this research for potential applications.

7925-13, Session 3

Optoporation and transfection of cells with kHz and MHz repetition rate femtosecond lasersA. A. Davis, M. J. Farrar, N. Nishimura, M. Jin, C. B. Schaffer, Cornell Univ. (United States)

Femtosecond laser optoporation has emerged as a high efficiency technique for transient disruption of the membrane of targeted single cells to induce uptake of exogenous dyes, gold nanoparticles, or DNA plasmids, in vitro. However, an understanding of the mechanics of hole formation and resealing as well as how substances such as DNA plasmid enter the cell remain poorly understood. In addition, most previous work has utilized high repetition rate trains of femtosecond pulses from laser oscillators for optoporation, although it is not clear this regime is optimal and such laser systems may not allow easy translation of the method to in vivo experiments. Here, we examined hole formation and resealing dynamics by monitoring efflux of a small dye molecule, assessed cell viability post optoporation by measuring intracellular enzyme activity and membrane integrity, visualized DNA plasmid movement after



optoporation using fluorescently-labeled DNA, and quantified expression of exogenous DNA in optoporated cells. We compared two laser regimes for optoporation: a small number of high energy pulses from a low repetition rate regenerative amplifier (1 kHz) and a large number of low energy pulses from a high repetition rate oscillator (76 MHz). With simple modeling, we were able to determine the diameter and lifetime of the laser-produced hole from our data. We also examined how the hole mechanics varied with laser parameters and how they influenced cell viability as well as DNA uptake and expression. Our results indicated that acute resealing of laser-produced holes controlled long-term cell viability after optoporation in both laser regimes. Hole radii increased with laser energy for both low and high repetition rate optoporation. The diameter of the hole--independent of the laser regime used to produce it--predicted the probability of resealing and thus long-term cell viability. The optimal transfection efficiency was approximately 26% for the amplifier (12 nJ, 2 pulses) and 32% for the oscillator (1.0 nJ, 3.8 x 10^6 pulses). Our results were consistent with two potentially inclusive mechanisms for DNA entry: diffusive entry through the laser-produced hole (0-15 plasmids enter) and/or DNA that sticks to locally damaged cell membrane at the irradiation site and is later endocytosed.

7925-14, Session 4

Mechanisms of gold nanoparticle mediated ultrashort laser cell membrane perforationM. Schomaker, Laser Zentrum Hannover e.V. (Germany); J. Baumgart, Ecole Polytechnique de Montréal (Canada); D. Motekaitis, D. Heinemann, J. Krawinkel, Laser Zentrum Hannover e.V. (Germany); M. Pangalos, W. Bintig, Leibniz Univ. Hannover (Germany); É. Boulais, R. Lachaine, B. St.-Louis Lalonde, Ecole Polytechnique de Montréal (Canada); A. Ngezahayo, Leibniz Univ. Hannover (Germany); M. Meunier, Ecole Polytechnique de Montréal (Canada); A. Heisterkamp, Laser Zentrum Hannover e.V. (Germany)

The gold nanoparticle (GNP) mediated ultrashort laser cell membrane perforation has been proven as an efficient delivery method to bring membrane impermeable molecules into the cytoplasm. Nevertheless, the underlying mechanisms have not been fully determined yet. Different effects may occur when irradiating a GNP with ultrashort laser pulses and finally enable the molecule to transfer. Depending on the parameters (pulse length, laser fluence and wavelength, particle size and shape, etc.) light absorption or an enhanced near field scattering can lead to perforation of the cell membrane when the particle is in close vicinity. Here we present our theoretical and experimental results to clarify the perforation initiating mechanisms. The generation of cavitation and gas bubbles due to the laser induced effects were observed via time resolved imaging. Additionally, pump-probe experiments for bubble and pressure wave detection as well as modelling of the occurring effects was performed. Furthermore, in our patch clamp studies a depolarization of the membrane potential and the current through the membrane of GNP loaded cell during laser treatment was detected. This indicates an exchange of extra- and intra cellular ions trough the perforated cell membrane for some milliseconds. Additionally investigations by scanning electron microscope (SEM) and environmental scanning electron microscope (ESEM) imaging were applied to study the interaction of cells and GNP after co incubation. The images show an attachment of GNP at the cell membrane after several hours of incubation. Moreover, images of irradiated and GNP loaded cells were taken to visualize the laser induced effects.

7925-15, Session 4

Basic mechanisms of the femtosecond laser interaction with a plasmonic nanostructure in waterM. Meunier, É. Boulais, R. Lachaine, C. Caron, Ecole

Polytechnique de Montréal (Canada)

The irradiation of a plasmonic nanostructure embedded in a biological media by a femtosecond laser could result in a highly localized plasma and heat production yielding to the nanosurgery of cells. To understand the basic mechanisms underlying this process, a combination of simulation work and experimental measurement is used to study the formation of cavitation bubbles and the generation of strong pressure waves in the medium following the irradiation of plasmonic nanostructures in a water solution. Complete model allows the calculation of near-field distribution, plasma production as well as transient temperature and pressure distribution surrounding a gold nanostructure in water after an interaction with a 800nm 45 fs laser pulse. The electromagnetic field distribution is directly calculated from the Maxwell equations using a finite-element based method. Transient temperature distribution is determined through a two-temperature model and hydrodynamic phenomena are modeled through Navier-Stokes equations. Simulation results show plasmonic enhanced temperature and pressure variation in the surrounding medium due to the presence of the nanostructure. Transient optical absorption measurement and pump-probe experiments are then used to detect and measure the generation of cavitation bubbles in the water as well as the generation of strong pressure shock wave following the irradiation of different plasmonic nanostructures in water. Influence of laser parameters such as pulse duration, wavelength and fluence is discussed.

7925-16, Session 4

Nanoparticle mediated femtosecond laser photodisruption mechanismsS. Haering, R. K. Harrison, A. Ben-Yakar, The Univ. of Texas at Austin (United States)

Photodisruption using nanoparticles and pulsed lasers offers an extremely selective and precise tool for phototherapy. Femtosecond laser illumination of gold nanoparticles excites damage pathways not achievable at longer pulses using similar energy levels. However, confirmation of the actual damage mechanisms is difficult due to exceedingly small size and time scales associated with the processes. To this end, we have directly studied bubble formation around 50 nm plasmonic gold spheres irradiated with single femtosecond off-resonance near-infrared pulses. Utilizing a high sensitivity pump-probe scattering system, we measured the size of bubbles initiated in a circulating solution of nanoparticles with a single laser pulse. The results indicate a 30 times reduction in the formation threshold of 600 nm diameter bubbles as compared to the threshold in solution without nanoparticles. To provide an insight on the underlying damage mechanisms involved, we utilized several theoretical models, including: 1) Mie solution for nanoparticle-light interactions, 2) a two-temperature nanoparticle heating model, including particle interfacial conduction to the surrounding liquid medium, 3) electron photoemission based on the Fowler-DuBridge theory of thermionic and multiphoton assisted emission. These calculations indicate the observed single pulse enhancements may be due to a combination of particle ablation and explosive heating of liquid surrounding the particles. We conclude these threshold reductions are largely a result of the particle near-field enhancement acting through electrostatic particle ablation and cavitation bubble formation for single pulses.

7925-17, Session 4

Plasmonic enhanced femtosecond-laser optoporation and transfection of human melanoma cellsJ. Baumgart, Ecole Polytechnique de Montréal (Canada); L. Humbert, Royal Victoria Hospital (Canada); B. St.-Louis Lalonde, Ecole Polytechnique de Montréal (Canada); J. Lebrun, Royal Victoria Hospital (Canada); M. Meunier, Ecole Polytechnique de



Montréal (Canada)

Melanoma is a complex and aggressive cancer that it responsible for 75% of all skin cancer deaths and over the past 50 years, the incidence of melanoma in most developed countries has increased faster than any other cancer. We have investigated the use of a femtosecond (fs) laser to create localized small holes in the membranes of targeted cells to develop an alternative virus-free technique to allow transfer of genetic material for treating these cancer cells with high efficiency and minimal collateral damage. This technique is based on the highly localized process resulting from the plasmonic nanostructure field amplification of the fs laser irradiation, allowing to locally perforate the cell membrane. This plasmonic enhanced fs laser process is efficient and highly selective with high cell viability, up to 90%. An optimum perforation rate with efficient dye or DNA uptake was found for different types of gold nanostructures, spherical (100-200nm) and rod shaped (10x40nm). The cell selectivity is realized by using bioconjugated nanostructures, that couple specifically to a special cell type. Furthermore, spatial selectivity is accomplished due to the fact that only irradiated cells are perforated offering the possibility to directly compare the treated and untreated cells in the same dish. In cancer cells, the aggressiveness of the cells is shown in the migration and invasion velocity. These assays are performed by laser-scribing the dish and laser transfection, resulting in a fully non-contact and therefore sterile method with direct comparison of treated and control cells in the same dish.

7925-18, Session 4

Gold nanoparticle mediated cell manipulation using fs and ps laser pulses for cell perforation and transfectionD. Heinemann, M. Schomaker, D. Motekaitis, J. Krawinkel, Laser Zentrum Hannover e.V. (Germany); D. Killian, Univ. Rostock (Germany); H. Murua Escobar, Univ. of Veterinary Medicine Hannover (Germany); C. Junghanss, Univ. Rostock (Germany); A. Heisterkamp, Laser Zentrum Hannover e.V. (Germany)

Manipulation of cells requires the delivery of membrane-impermeable substances like genetic materials or proteins into the cytoplasm. Thus delivery of molecules across the cell membrane barrier is one of the key technologies in molecular biology. Many techniques concerning especially the delivery foreign DNA have been developed. Notwithstanding there still is a range of applications where these standard techniques fail to raise the desired results due to low efficiencies, high toxicity or other safety issues. Especially the transfection of sensitive cell types like primary and stem cells can be problematic. Here we present an alternative, laser based technique to perforate the cell membrane and thus allowing efficient delivery of extra cellular molecules: Gold nanoparticles (GNP) were incubated with the cells and the laser-GNP interaction leads to membrane perforation. This allows the utilisation of a weakly focused laser beam leading to fast scanning of the whole cell culture and thus to a high throughput. To investigate the GNP-laser interaction in more detail we have compared membrane perforation obtained by different laser pulse lengths. From our results we assume strong light absorption for ps laser pulses and relatively small particles as the initiating perforation mechanism, whereas an enhanced near field scattering occurs at 200 nm GNP when using fs laser pulses. SEM and ESEM imaging were applied to give a deeper insight into the GNP-cell interaction and the effects of laser radiation on the GNP. Additionally dextran-FITC derivatives of varying sizes were used to investigate the impact of molecule size on delivery efficiency.

7925-19, Session 5

Direct laser written waveguide coupler with an optically-tunable splitting ratioM. Ams, R. J. Williams, M. J. Withford, CUDOS @ Macquarie (Australia)

Significant attention has been directed to the use of femtosecond laser pulses for fabricating optical devices in transparent materials since it was demonstrated in 1996 that focussed femtosecond laser pulses can induce an increase in the refractive index of bulk transparent glasses at the point of focus. This discovery led to the realisation of a variety of laser-inscribed waveguide devices, including evanescent couplers/splitters, waveguide Bragg-gratings, waveguide amplifiers and monolithic distributed-feedback waveguide laser oscillators. The femtosecond laser direct-write technique also offers unique opportunities for the fabrication of three-dimensional photonic waveguide devices. Further advantages of the technique include rapid prototyping, compatibility with existing fibre systems, and freedom from lithographic masks and clean room fabrication environments.

Here we report on the application of the femtosecond laser direct-write technique to create a 2x2 single-mode waveguide coupler with an optically-tunable splitting ratio. The device was inscribed inside a ytterbium-doped (5 wt.%) phosphate glass sample. Tuning of the coupling ratio at 1550 nm is achieved through resonant optical excitation at 976 nm of the ytterbium ions, which shifts the refractive index through heating and the direct pumping of a saturable optical absorption (Kramers-Krönig causality). Initial experiments and modelling demonstrate that a reversible change in splitting ratio of more than 20% is possible (i.e. 50:50 to 30:70) using less than 500 mW of injected pump power. The implications for reconfigurable logic gates in quantum information systems will be discussed.

7925-20, Session 5

Direct laser writing of nonlinear properties in photosensitive glassG. Papon, Univ. Bordeaux 1 (France); J. Choi, CREOL, The College of Optics and Photonics, Univ. of Central Florida (United States); A. Royon, Univ. Bordeaux 1 (France); M. C. Richardson, L. B. Glebov, CREOL, The College of Optics and Photonics, Univ. of Central Florida (United States); L. Canioni, Univ. Bordeaux 1 (France)

Silver containing glasses have been reported to exhibit intense fluorescence properties. A 1030 nm pulsed femtosecond laser has been used to induce modifications in silver containing glass, namely femto-photo luminescent glass (FPL) and Photo-thermo refractive glass (PTR). The 5W output power of the laser at 10MHz is focused at a depth of 200 m in the glass using a 0,52 NA objective. The output polarization of the laser is TM. The tailoring of the number of pulses, pulse energy and repetition rate is achieved by acousto-optic filtering. The interaction resulted in the creation of stable pipe-shaped silver clusters forming below refraction-limit 3D structures. Those nano-structures exhibit non-linear properties such as SHG and THG as well as fluorescence. Due to multiphoton absorption, free electrons are created in the central part of the beam, enabling the reduction of Ag+ silver ions into Ag and subsequently into ionized Ag-clusters. The cumulated thermal effect of the pulses weakens the glass matrix allowing diffusion of the clusters. The ion concentration gradient creates an internal electric field enabling non-linear properties. Influences of polarization, dose and fluence on the non-linear properties are investigated. Our theoretic model taking into account all these parameters confirms the experimentally measured SHG and THG emission. Moreover, by varying the beam polarization direction we could prove the presence of an ion concentration gradient in the pipe structures. Therefore we have been able to characterize our structures and its formation mechanism.

7925-21, Session 5

3D patterning at the nanoscale of fluorescent emitters in glassM. Bellec, A. Royon, K. Bourhis, G. Papon, B. Bousquet, T. Cardinal, L. Canioni, Univ. Bordeaux 1 (France)



Recent interest has grown for generating local optical contrast in silver doped glasses using laser irradiation for many applications (waveguides, gratings, data storage...). Direct laser writing of nanostructures is challenging and relies on the precise control of the local phase separation and the diffusion processes. Up to now, structures with sizes down to a few hundreds of nanometers have been demonstrated.

Using a new approach, nanostructures have been written with sizes well below the diffraction limit (< 100 nm), taking advantage of the high repetition rate of a near infrared femtosecond laser. Fluorescent structures (80 nm-thick) composed of silver clusters were obtained in a silver containing glass. The laser/glass interaction offers the opportunity to have access to a paintbrush and an eraser working together at the nanoscale, allowing infinite structure designs.

Here we show three-dimensional fluorescent nanostructures fabrication by a near infrared high repetition rate femtosecond laser in a silver containing femto-photo-luminescent glass. By adjusting the laser dose (fluence, number of pulses and repetition rate), these stabilized intense fluorescent structures, composed of silver clusters, can be achieved with a perfect control of the luminescence intensity, the emission spectrum and the spatial distribution at the nanometer scale. The photo-induced structures are characterized by confocal fluorescence microscopy, high-resolution scanning electron microscopy, atomic force microscopy, absorption spectroscopy and fluorescence spectroscopy. This novel approach opens the way for the fabrication of stable fluorescent nanostructures in three dimensions in glass for applications in photonics and optical data storage.

7925-22, Session 5

New step towards the future perennial high capacity optical recording mediumA. Royon, K. Bourhis, M. Bellec, G. Papon, B. Bousquet, Y. Deshayes, T. Cardinal, L. Canioni, Univ. Bordeaux 1 (France)

Ancestral civilizations have succeeded in transmitting their cultural heritage, using stone then paper as storage media. Paradoxically, our numerical civilization which produces a lot more of information and knowledge, has not answered this challenge yet. Current magnetic and optical recording media have improved in terms of storage capacity and access speed over the past decades. Nevertheless, they suffer from a critical drawback: their lifetime, which is no longer than five to ten years. On the one hand, magnetic media mainly suffer from high sensitivity to electromagnetic radiations and can accidentally “crash”. On the other hand, current optical media have limited storage capacity and material durability. We propose a perennial optical recording medium which can satisfy both issues of high data capacity and very long lifetime.

Here we show three-dimensional optical recording by exploiting the fluorescence features of femtosecond-laser-induced silver clusters in glass. Enhancement of the storage capacity can be achieved by encoding the information onto many levels instead of two, 0 or 1. The fluorescence properties of these stable clusters are controlled and depend on the glass recording exposure conditions. The high dynamics of the fluorescence intensity permits the encoding of the information on tens of levels. The combination of a blue laser diode with a high numerical aperture focusing set-up, similar to the “Blu-ray” drive, enables the readout of the information inside the glass without cross-talk and photo-bleaching. This original recording medium can be immediately used to answer the societal problem of perennial high density data storage.

7925-23, Session 6

Femtosecond filamentation induced micro and nano-restructuring in the bulk of dielectrics and polymersS. Tzortzakis, D. Abdollahpour, D. G. Papazoglou, S. K. Georgiou, Foundation for Research and Technology-Hellas (Greece)

Ultrashort laser pulse filamentation is a well known phenomenon of laser beam self-organization in a narrow intense filamentary structure extending over very long distances, significantly longer than the characteristic Rayleigh length. Filaments appear for input powers above the critical threshold for self-focusing, while their dynamics and the stationary nature of some of their features can be qualitatively explained as due to a competition between linear and nonlinear effects including the Kerr self-focusing, ionization defocusing, nonlinear losses and dispersion effects. In the filaments’ core high intensities are attained and a plasma string is generated along their path.

Filaments have been successfully used to induce in the bulk of transparent glasses permanent structural modifications, like for instance waveguides, birefringent elements, and nanogratings. In this presentation we will discuss results of our recent experimental studies on the physics that lead to the formation of the permanent restructuring in fused silica as well as the physico-chemical reactions induced in corresponding experiments in polymers (PMMA). In these studies we employ a powerful time and spectrally resolved in-line holographic technique to monitor the temporal material evolution from the initial excitation through its successive relaxation stages and up to the final permanent amorphous lattice state.

7925-24, Session 6

Breaking stress of glass welded with femtosecond laser pulses at high repetition ratesS. Richter, S. Döring, Friedrich-Schiller-Univ. Jena (Germany); T. Peschel, R. Eberhardt, Fraunhofer-Institut für Angewandte Optik und Feinmechanik (Germany); S. Nolte, Friedrich-Schiller-Univ. Jena (Germany); A. Tünnermann, Friedrich-Schiller-Univ. Jena (Germany) and Fraunhofer-Institut für Angewandte Optik und Feinmechanik (Germany)

In recent years the modification of transparent materials with ultrashort pulses has attracted great interest. Due to nonlinear absorption processes the laser energy is deposited in the focal volume within the bulk of the transparent substrate. This leads to various structural modifications inside the material.

Here, we use ultrashort pulses to establish bonds between transparent glass substrates. Therefore, the laser is focused at the interface between the two substrates, where it is absorbed. The heat accumulation of successive pulses, which occurs at high repetition rates, leads to melting and subsequent resolidification. This way the laser acts as a local heat source directly at the interface. Therefore this is a local, flexible and thermal stress minimizing welding process, since only small regions of the material are heated. Moreover, tailored welding only in specific areas adapted to the application becomes easily possible.

In this presentation, the breaking stress achievable with this technique and the influence of laser parameters and writing geometries will be discussed. The behaviour of different glasses (fused silica and a Titan-oxide glass) will be presented. Maximal breaking stress of up to 70% of the bulk material has been obtained so far. We will explain the limits of this technique and possibilities to improve.

7925-25, Session 6

The influence of glass structure on femtosecond laser micro machining of waveguide amplifiers inside bulk Er-Yb doped polyphosphate glassL. B. Fletcher, N. Troy, J. J. Witcher, D. M. Krol, Univ. of California, Davis (United States); R. K. Brow, Missouri Univ. of Science and Technology (United States)



Permanent modification to the glass structure induced by femtosecond laser pulses can be used inside a variety of active glasses to fabricate waveguide lasers and amplifiers, with applications in three-dimensional photonic circuits. Zinc phosphate glasses near the metaphosphate regime, with a ratio of [O]/[P]=3, are excellent glass systems for achieving high rare-earth oxide concentrations with low luminescence quenching effects, and are ideal for fabricating compact high gain waveguide lasers that operate in the C-band. Previous research with zinc polyphosphate glasses has demonstrated important relationships between the initial composition of simple phosphate glasses and the structural changes that result from fs-laser modification. In particular, waveguides fabricated in glasses with the [O]/[P] ratio near 3.25 demonstrate local densification of the glass that occurs inside the irradiated area, which can be used to fabricate single mode waveguides. In this study we have investigated the fabrication of sub-surface waveguide amplifiers in Er-Yb zinc polyphosphate glasses by utilizing this relationship between the glass composition and the resulting changes to the network structure after modification by fs-laser pulses. Waveguides inside Er-Yb doped zinc polyphosphate glass have been fabricated using a regenerative amplified Ti:sapphire 1 kHz, 180 fs-laser system. Fs-laser writing parameters such as the laser pulse energy, the scan speed, the beam focusing, and the writing geometry have been studied. Near field guiding profiles and white light images as well as insertion losses and gain characteristics were measured after waveguides were written. Laser-induced structural changes in the glass were characterized using confocal fluorescence and Raman microscopy.

7925-26, Session 6

Time-resolved imaging of bulk a-SiO2 upon various ultrashort excitation sequencesA. Mermillod-Blondin, Max-Born-Institut für Nichtlineare Optik und Kurzzeitspektroskopie (Germany); C. Mauclair, Lab. Hubert Curien (France); A. Rosenfeld, Max-Born-Institut für Nichtlineare Optik und Kurzzeitspektroskopie (Germany); J. Bonse, Bundesanstalt für Materialforschung und -prüfung (Germany); R. I. Stoian, E. Audouard, Lab. Hubert Curien (France)

Ultrashort pulses lasers are tools of choice for functionalizing the bulk of transparent materials. In particular, direct photoinscription of simple photonic functions have been demonstrated. Those elementary functions rely on the local refractive index change induced when focusing an ultrashort pulse in the volume of a transparent material.

The range of possibilities offered by direct photoinscription is still under investigation. To help understanding, optimizing and assessing the full potential of this method, we developed a time-resolved phase contrast microscopy setup. The imaginary part (absorption) and the real part of the laser-induced complex refractive index can be visualized in the irradiated region. The setup is based on a commercially available phase contrast microscope extended into a pump-probe scheme. The originality of our approach is that the illumination is performed by using a pulsed laser source (i.e. a probe beam). Speckle-related issues are solved by employing adequate sets of diffusers. This laser-microscopy technique has a spatial resolution of 650 nm, and the impulse response is about 300 fs. The laser-induced refractive index changes may be tracked up to milliseconds after the energy deposition. The excitation beam (the pump) is focussed with a microscope objective (numerical aperture of 0.45) into the bulk of an a-SiO2 sample. The pump beam can be temporally shaped with a SLM-based pulse shaping unit. This additional degree of flexibility allows for observing different interaction regimes. For instance, bulk material processing with picosecond duration pulses, double pulse sequences or any temporal envelope of interest may be studied.

7925-27, Session 7

CPA-free picosecond fiber amplifier with >10µJ pulse energy and >300kW peak powerY. Zaouter, M. Lebugle, F. Morin, E. Mottay, C. Hönninger, Amplitude Systemes (France)

We demonstrate a compact and robust picosecond fiber amplifier that produces >10µJ pulse energy and average powers exceeding 10W while maintaining a narrow spectral bandwidth. This fiber amplifier system is well suited for CARS spectroscopy, micro-machining applications or nonlinear frequency conversion.

The amplifier architecture is extremely simple and optimised for high stability. It consists only of a picosecond fiber oscillator, a fiber-coupled AOM pulse picker, a pre-amplifier, and a booster amplifier. The fiber oscillator operates at 20MHz and emits almost transform-limited pulses of about 40ps pulse duration. The oscillator pulse train can be down-counted to the desired amplifier repetition rate by means of a fiber-coupled acousto-optic pulse picker. The following core-pumped pre-amplifier assures sufficient average power to saturate the booster amplifier stage that is formed by a single mode, large mode area photonic crystal fiber. The oscillator pulse duration is maintained during the entire amplification process. The spectral bandwidth increases due to self phase modulation encountered by the pulses during amplification. It varies as a function of the extracted pulse energy from 0.1nm at low pulse energy to about 1nm for pulses with 14µJ pulse energy. The simplicity of the fiber amplifier concept without the need of chirped-pulse amplification opens the door for industrial applications requiring high precision and high system robustness.

Even higher pulse energies and peak powers will be achieveable by using a rod-type photonic crystal fiber as booster amplifier stage. As an alternative approach for narrow-bandwidth picosecond amplification an architecture exploiting nonlinear spectral compression will be studied for comparison.

7925-28, Session 7

Technical advantages of disk laser technology in short and ultra-short pulse processesP. Graham, J. Stollhof, TRUMPF Inc. (United States)

This paper demonstrates that disk-laser technology introduces advantages that increase efficiency and allows for high productivity in micro-processing in both the nanosecond (ns) and picosecond (ps) regimes. Some technical advantages of disk technology include not requiring good pump beam quality or special wavelengths for pumping of the disk, high optical efficiencies, no thermal lensing effects and a possible scaling of output power without an increase of beam quality. With cavity-dumping, the pulse duration of the disk laser can be specified between 30 and hundreds of nanoseconds, but is independent of frequency, thus maintaining process stability. TRUMPF uses this technology in the 750 watts average power laser TruMicro 7050. High intensity, along with fluency, is important for high ablation rates in thin-film removal. Thus, these ns lasers show high removal rates above 60 cm2/s in thin-film solar cell production. In addition, recent results in paint-stripping of aerospace material prove the green credentials and high processing rates inherent with this technology as it can potentially replace toxic chemical processes. The ps disk technology meanwhile, is used in, for example, scribing of solar cells, wafer dicing and drilling injector nozzles, as the pulse duration is short enough to minimize heat input in the laser-matter interaction. In the TruMicro Series 5000, the multi-pass regenerative amplifier stage combines high optical-optical efficiencies together with excellent output beam quality for pulse durations of only 6 ps and high pulse energies of up to 0.25 mJ.



7925-30, Session 7

Optimal control of the population dynamics of the ground vibrational state of a polyatomic moleculeL. E. De Clercq, Council for Scientific and Industrial Research (South Africa) and Stellenbosch Univ. (South Africa); L. R. Botha, H. Uys, Council for Scientific and Industrial Research (South Africa); A. Du Plessis, Council for Scientific and Industrial Research (South Africa) and Stellenbosch Univ. (South Africa); E. G. Rohwer, Stellenbosch Univ. (South Africa)

The selective excitation of an arbitrary vibrational level within a polyatomic molecule, without passage through an intermediate electronic excited state was demonstrated. This was achieved by simulating the interaction of a shaped femtosecond pulse with one vibrational mode of the molecule. The carrier frequency of the pulse was chosen near resonant to the ground-to-first-excited vibrational transition of the mode and the pulse shape was optimized via closed-loop feedback and an optimization algorithm. The simulation concentrated on the first few vibrationally excited states since the density of states is still low, thus ensuring that the inter-vibrational decoherence time is long compared to the pulse length. Published spectroscopic data of a hexafluoride molecule was used, this included anharmonic splitting, as well as forbidden transitions. The effect of rotational levels was not included. A density matrix approach was followed, because this allows for the introduction of dephasing of the coherent excitation via thermalizing collisions with the reservoir, as well as inter-vibrational relaxation. A transform limited 150 femtosecond input pulse was used. Pulse shaping via a Spatial Light Modulator (SLM) was simulated. The effect that the shaped pulse had on the molecules was investigated by solving the Von Neumann equations. Various aspects of the optimized pulse shape were investigated these include, uniqueness and the stability of the solution.

7925-31, Session 7

Ultrasensitive resonant-cavity-enhanced spatial-spectral interferometryX. Gu, I. Pupeza, T. Udem, E. E. Fill, J. Kaster, Max-Planck-Institut für Quantenoptik (Germany); T. Eidam, J. Limpert, A. Tünnermann, Friedrich-Schiller-Univ. Jena (Germany); F. Krausz, Max-Planck-Institut für Quantenoptik (Germany)

We achieve ultrahigh phase measurement sensitivity by combining two techniques - spectral interferometry and resonant enhancement in a high-finesse passive cavity. As linear interference techniques, spectral interferometry, and its variant, spatial-spectral interferometry (SSI), are known to have superior sensitivity and resolution. In a high-finesse passive cavity, any intracavity dispersion accumulates upon the many round trips, which strongly affects the steady-state pulse shape inside the cavity. Moreover, the power enhancement leads to high intracavity intensities, which might give rise to nonlinear processes altering the dispersion characteristics.

In our setup, replicas of the pulse seeding an enhancement cavity and of the circulating intracavity pulse are combined and interfere in a SSI interferometer, whereby the phase difference of the two pulses is measured along with their individual spectra. Using this information and a theoretical cavity model, the cavity round-trip group delay dispersion can be measured with ultrahigh resolution.

As a demonstration, we applied this method to measure the GVD of air (16 fs^2/m @ 1 bar) in a 4-m-long enhancement cavity at 1040 nm, with results in very good agreement with the published Sellmeier equations. Moreover, this measurement method enables the nonintrusive and ultrasensitive investigation of nonlinear intracavity processes. For the first time, we were able to measure intensity-related GVD in the cavity optics.

7925-46, Session 7

Versatile approach for frequency resolved wavefront characterizationE. Frumker, NRC-Univ. of Ottawa Joint Attosecond Science Lab. (Canada) and Texas A&M Univ. (United States); G. G. Paulus, Friedrich-Schiller-Univ. Jena (Germany) and Texas A&M Univ. (United States); D. M. Villeneuve, National Research Council Canada (Canada); P. B. Corkum, NRC-Univ. of Ottawa Joint Attosecond Science Lab. (Canada)

Spatial characterization of high harmonics (HH) and XUV coherent radiation is of paramount importance, along with its temporal characterization. The high harmonics carry the combined signature of underlying quantum physical processes at the atomic level and of the cooperative phase matching . Accurate reconstruction of the high harmonic spatial wavefront, along with its temporal profile, gives us a complete range of tools to apply to the fundamental quantum properties and dynamics associated with high harmonic generation . For many applications it will be necessary to accurately measure the beam properties, just as it is important to know the beam characteristics for many laser experiments. For example, high harmonics and attosecond pulses are being proposed as a front-end for the next generation X-ray free electron Lasers. This oscillator-amplifier-like arrangement will require well characterized high harmonic sources.

We present a new concept of frequency resolved wavefront characterization that is particularly suitable for characterizing XUV radiation . In keeping with tradition in the area we give it an acronym - SWORD (Spectral Wavefront Optical Reconstruction by Diffraction). Our approach is based on an analysis of the diffraction pattern of a slit situated in front of a flat-field spectrometer. As the slit is scanned, the spectrally resolved diffraction pattern is recorded. Analyzing the measured diffractogram, we can reconstruct the wavefront. The technique can be easily extended beyond the XUV spectral region. When combined with temporal characterization techniques all information about the beam can be measured.

7925-47, Session 8

Femtosecond laser processing of fused silica substrates for molding of polymeric materialsM. Matteucci, F. Madani-Grasset, Y. Bellouard, Technische Univ. Eindhoven (Netherlands)

In recent years patterning of glass substrates by means of combined femtosecond laser exposure under the ablation threshold and wet etching has gained considerable interest. Glass materials such as fused silica combine very interesting chemical, mechanical and optical properties for the fabrication of microstructures.

To produce large numbers of low-cost micro-parts, plastic molding is considered the most promising micro-fabrication techniques. Micro molding can be used for the fabrication of elements such as active or passive components in MEMS devices, hydrophobic surfaces, cell-growth scaffolds or arrays of microlenses. We here propose to combine the nonlinear energy deposition provided by femtosecond lasers and wet etching to fabricate molds for the replica of polymer microstructures. To illustrate the use of femtosecond micromachined molds, we here demonstrate the fabrication of an array of large aspect ratio PDMS pillars. A method to enhance the de-molding of structures made in Polydimethylsiloxane (PDMS) and based on the deposition on the mold of a release coating will also be presented.



7925-48, Session 8

Fs-induced apodised Bragg waveguides in fused silicaC. Voigtländer, Friedrich-Schiller-Univ. Jena (Germany); P. Zeil, Royal Institute of Technology (Sweden); J. Thomas, A. Tünnermann, S. Nolte, Friedrich-Schiller-Univ. Jena (Germany)

Femtosecond laser pulsed inscription techniques allow non-photosensitive and active glasses to serve as platforms for integrated devices. Bragg waveguides represent essential building blocks providing both narrow and broad-band integrated mirrors. However, femtosecond pulse written broad-band Bragg waveguides have yet suffered from poor apodisation resulting in unwanted side-lobes. Here we present apodised Bragg waveguides inscribed in fused silica using a high repetition rate laser system with a pulse picker. By varying its modulation, the mean refractive index over the grating length could be kept constant, while the grating strength is varied. Thus, Bragg waveguides with Gaussian modulation profiles could be demonstrated.

7925-49, Session 8

On the role of stress in the chemical etching of fused silica exposed to low-energy femtosecond laser pulsesA. Champion, Y. Bellouard, Technische Univ. Eindhoven (Netherlands)

Recently, it was demonstrated that femtosecond lasers pulses with energies below the ablation threshold locally enhance the etching rate of fused silica: regions that are exposed to the laser beam are etched faster. This remarkable property has been exploited for fabricating a variety of micro-structures like fluidic channels, tunnels or more complex devices, like mechanical flexures.

The physical effect causing the etching-rate local enhancement is still debated and various hypotheses have been proposed among which localized densification models seem to prevail.

In that context, we recently demonstrated that the amount of deposited energy plays a very important role. It was found that for laser repetition rates where no cumulative effects are observed, there exists an optimal amount of energy deposited to achieve the fastest etching rate. These observations suggest that the stress introduced during laser exposure plays also an important role in the process. Our hypothesis is that not only structural changes such as the appearance of lower-order ring structures enhances the etching rate but also the stress surrounding laser-exposed regions.

To further understand the role of the stress, here we report on experiments where we accurately control laser writing parameters of lines and patterns not only with respect to their spatial localizations in the specimen but also in term of deposited energy. Etching profiles observations are then compared with finite-element predictions of the stress distribution surrounding laser-patterns to estimate the magnitude of the stress-induced etching.

7925-18, Session 9

Industrial production with ultra fast laser workstationsE. Audouard, Univ. Jean Monnet Saint-Etienne (France); H. Soder, Impulsion SAS (France)

Femtosecond lasers have proved to be great tools for precise, accurate and high quality micro- but also nano-machining. Numerous works have been carried out to study laser-matter interaction mechanisms on the one hand, and laser machining processes on the other hand.

Very naïve first industrial approaches, such as “ultra fast machining is interesting because it’s not a thermal process”, are only used by manufacturers who do not really want to sell workstations for industrial use. Technical development and good knowledge of the process have to be simultaneously settled to allow a real practical application.

In this work, we will underline some practical applications and the link between a better knowledge of physical mechanisms and the development of industrial processes. Of course, we have to pay attention to typical parameters of industrial development, such as the processing time. With the knowledge of an “efficient” ablation rate, the physical time can be easily calculated, using the total length of the machining, speed and laser parameters. This calculated physical process time can be compared to the effective time needed to machine the sample. This kind of information can evidence the possible improvement to be done on the mechanical and computer environment to reach this minimum process time.

Femtosecond technology has now proven its capacity to lead innovative production, even if it’s still in some well defined fields, but its contribution is non negligible in a context of economical crisis.

7925-19, Session 9

An investigation of piezoelectric cutting by femtosecond laserY. Di Maio, E. Audouard, J. Colombier, Univ. Jean Monnet Saint-Etienne (France); P. Cazottes, J. Beitia, Sagem Defense Securite (France)

Among several piezoelectric actuators, the PZT ceramics (Pb(ZrxTi1-x)O3) are one of the preferred candidates for industrial applications requiring very high precision, speed and good controllability, as micrometric transducers. This paper investigates an innovative method to cut a PZT wafer with an extreme precision.

The most used method to cut PZT wafers today is a mechanical cut with an automatic saw. However, when a better precision is required, other methods such as femtosecond lasers should be used. Most specific tests performed with these lasers have revealed high efficiency thanks to modulable parameters and ablation improvements. Although not an issue for more common materials, some undesired features require a better understanding of the laser-matter interaction, a characterization of the unexpected phenomena and a determination of the optimal parameters for cutting.

Describing the propagation of a laser beam through optical devices provides more insights on the ceramic behaviour. Furthermore, key parameters such as the ablation rate and the ablation threshold must be determined to precisely characterise this material. In the meantime, laser induced phenomena are observed, from rippled-like nanostructurations to cracking, leading sometimes to a spallation of the edges of the material after cutting. The research of the optimal parameters to reach uniform processes is a straight forward way to avoid or control these effects which could be negative for the purposes of the machined pieces.

Associated with adapted laser settings and eventually with beam shaping, femtosecond processes should improve PZT machining in terms of precision, speed, cleanliness, scalability and after-processed treatments comparatively to usual cutting methods.

7925-32, Session 9

Fundamentals and industrial applications of ultrashort pulsed lasers at BoschJ. Koenig, T. Bauer, Robert Bosch GmbH (Germany)

Well-known trends in automotive industry are for example miniaturization, higher precision, diversification of materials, variety of variants and smaller lots. Always on scope has to be cost-effectiveness and outstanding quality.

About ten years ago ultrashort pulsed laser material processing showed



the potential to satisfy a lot of the named requirements. BOSCH coordinated public funded projects together with universities and industrial partners to develop the ultrashort pulse laser technology from an academic level to a cost-effective production technology.

First step was to build up knowledge of the fundamental process, in detail the interaction of ultrashort laser pulse and material. This gives the possibility to define the specifications of applicable laser systems, for example pulse duration, pulse energy and repetition rate.

Out of this, the laser systems and further system technology is optimised in an iterative way with respect of robust and cost-effective processes.

Since 2007 are ultrashort laser pulses used at the BOSCH plant in Bamberg for production of exhaust gas sensors, shown in figure 1 left. They are made of a special ceramic layer system and can measure the exhaust gas properties faster and more precisely. This enables further reduction of emissions by optimised combustion control.

Since 2009 BOSCH ultrashort pulsed lasers are micro structuring the injector of common rail diesel systems. A drainage groove allows a tight system even at increased pressures of up to 2000 bar. Diesel injection systems become even more reliable, powerful and environment-friendly.

7925-33, Session 9

Directly induced ablation of metal thin films by ultra short laser pulsesG. Heise, C. Hellwig, J. Konrad, S. Sarrach, H. P. Huber, Hochschule München für Angewandte Wissenschaften (Germany)

Molybdenum (Mo) films of 0.5 µm thickness on a glass substrate are used as a back contact for CIS thin film solar cells. These thin films can be ablated from the glass side by picosecond laser pulses at low fluences < 1 J/cm2, without any visible thermal effects and damage; in contrast to nanosecond pulses. In our previous work we showed that the perfect ablation of the Mo films is based on a directly induced laser ablation occurring at fluences below the limit of complete thermodynamical heating and evaporation. Similar processes are often referred to as “laser lift-off”. To gain more insight in the underlying ablation mechanisms of directly induced laser ablation, we performed experiments with different film thicknesses and film materials at various fluence levels and laser spot diameters: Thin films of Molybdenum, chromium, titanium and platinum with thicknesses between 200 nm and 1 µm, produced by cathode sputtering, were examined. In the case of molybdenum and chromium, the influence of an intermediate buffer layer of silicon nitride on the ablation behavior was analyzed. It is shown that ablation form the glass side clearly suggests a higher grade of ablation efficiency and a better structural quality in contrast to metal side patterning. A model will be presented, in which the ablation characteristics are connected with the mechanical ductility of the different metals.

7925-34, Session 9

Colorizing of the stainless steel surface by single-beam direct femtosecond laser writingM. S. Ahsan, KAIST (Korea, Republic of) and Khulna Univ. (Bangladesh); Y. G. Kim, M. S. Lee, KAIST (Korea, Republic of)

This paper reports on the colorizing of the stainless steel surface by controlling the irradiation conditions of a single-beam femtosecond laser. We change the color of the stainless steel surface by femtosecond laser induced periodic microholes or microgratings on the sample surface. Colorizing of metal surface by periodic microholes, produced by femtosecond laser, is achieved for the first time without any kind of coating. The laser modified stainless steel surfaces show different colors under different incident or azimuthal angles of the incident light, which changes in color indicate the dependence of the metal color on the angles (incident and azimuthal) of the incident light. We report, for the first time, the changes of metal color due to the change of the azimuthal

angles of the incident light. Furthermore, the changes in the color of the laser modified metal surfaces are mainly due to the excitation of surface plasmons (SPs) on the metal surface. The resonant angle of SPs is different for different wavelength of light. As a result, under different incident or azimuthal angles different wavelength of light is trapped on the surface depending on the resonance for that particular wavelength; light of other wavelengths react naturally and contributes for the color change of the stainless steel surface. Finally, we discovered that the nanostructures produced on the top of the microgratings and micro holes play important roles for the propagation of the SPs in parallel with the mcirogratings and mcroholes, which nanostructures are responsible for a complex SPs on the sample surface.

7925-35, Session 9

Core techniques for precise and productive microdrilling in steel with ultrashort pulsed laser radiationM. Kraus, A. Michalowski, J. Fruechtenicht, R. Weber, T. Graf, Univ. Stuttgart (Germany)

The use of ultrashort laser pulses enables the fabrication of high-precision microstructures in metals without post processing, due to the minimization of thermal effects such as melt generation and recast formation. Since industry standard picosecond laser sources with high average power and high repetition rate have been launched onto the market in 2008, surface processing and cutting applications have found broad use in industrial production because in many cases, the available laser power can be converted into productivity by simply increasing the feed rate. In high-aspect-ratio microdrilling however, the use of high pulse energies and high repetition rates causes a strong ionization of the gas atmosphere inside the capillary, the formation of melt due to heat accumulation and, consequently, a deterioration of the borehole quality. These phenomena affect the drilling process particularly during the initial phase.

In this work we have studied the influence of time-controlled drilling strategies and different process gas types on ablation velocity, borehole morphology and machining quality at picosecond microdrilling in CrNi steel. Cylindrical and negative conical microholes with a diameter in the range of 100 µm have been fabricated in 1 mm sheets, employing a helical drilling process together with circularly polarized infrared radiation at 1030 nm. In order to observe the ionization behavior of the atmosphere inside and above the capillary, the optical plasma emissions were recorded using a fast Si photodiode.

It is shown that by systematically controlling the governing process parameters, the borehole quality can be significantly improved, whereas the machining time can be reduced to 10 s.

7925-20, Session 10

Laser ablation of AgInSe2: a clean and effective approach for ternary semiconductorsD. Pathak, Guru Nanak Dev Univ. (India)

Laser ablation has attracted special interest for the formation of thin films Compared with other formation technique . A distinctive feature of laser ablation is that it allow high quality and stochiometry of films of even very complex element material. In this presentation laser ablation of AgInSe2 chalcopyrite semiconductor will be discussed in which it is difficult to maintain stochiometry by conventional method. High Quality AgInSe2 (AIS) films were grown on Glass substrates by the ultra-high-vacuum pulsed laser deposition technique from the AIS target synthesized from high-purity materials. The X-ray diffraction studies of the films show that films are textured in (112) direction..The substrate temperature appears to influence the properties of films. Increase in substrate temperature results in more order structure .Compositional analysis has been carried out by



EDAX. It is observed that compositional stochiometry is maintained to the more extent by PLD technique than other traditional methods like thermal evaporation. The optical studies of the films show that the optical band gap is about 1.20 eV.

7925-21, Session 10

Investigation on solid state Nd3+:YAG line beam laser annealing and texturing of amorphous silicon thin filmsN. J. Vasa, A. I. Palani, M. Singaperumal, Indian Institute of Technology Madras (India)

Crystalline silicon plays a major role in photovoltaic application, however the cost is high and overall size is limited. As an alternative approach, Nd3+:YAG laser annealing technique can be used to convert amorphous silicon (a-Si) film into polycrystalline silicon. In addition to laser annealing, lasers can also be used to produce a textured surface on the silicon surfaces by the methodology of spot overlap.

In the present work, an influence of the laser fluence and laser beam overlap on a-Si film is studied. An attempt is made to crystallize and simultaneously form a nanotextured surface using a pulsed solid-state Nd3+: YAG laser at a wavelength 355 nm with a Gaussian profile. Nanotexturing is studied by 50% and 90% overlapping of the diameter of the laser spot. At the laser fluence values between 350 mJ/cm2 and 500 mJ/cm2 and with the beam overlap of 90%, the broadband Raman spectrum around 480 cm-1 was quenched and a narrow-band Raman shift around 520 cm-1 was observed. Further, electrical resistance of 10 k was observed. With 90% overlap, surface roughness was increased and films also show a higher absorbance than that of treated with 50% overlap. Theoretical studies were performed to understand the annealing and the nanotexturing of a-Si films. Currently, experimental studies are performed in which the Gaussian laser beam with the circular spot is transformed into a flat-top beam profile with a line beam. The line beam with a flat-top beam profile is expected to allow a wide area scanning.

7925-36, Session 10

The effect of ambient conditions on thin wafers processed with fs-laser machiningS. C. Jeoung, Korea Research Institute of Standards and Science (Korea, Republic of)

Micromachining based on laser has become important tools in the fields of mass production for modern -devices including multi-layered microelectronics, LED, solar voltaic cells, displays, multilayered PCB and so on. While conventional mechanical processes have come to their limitations in the course of size miniaturization, ultrafast laser techniques have been known to have a potential application in high precision processing because of their minimized heat affected zone. The other potential application of fs-laser -processing is to modify the surface to form functional micro- and nanostructures.

We will present the effects of ambient processing conditions on the surface topological changes as well as the mechanical properties of processed materials. For example, we observe that ambient gases and laser fluence used in processing play an important role in encapsulation of Ge nanostructures with oxidized layer as well as their size distribution. Further, the substrate temperature of silicon wafers also affects the dependence of surface roughness on the laser fluence. This observation can be understood in terms of the changes in ablation mechanism underlying fs-laser material ablation process between optical penetration and thermal diffusion processes. We also propose an empirical relation between maximum yield stress and cumulative temperature increment of dielectric materials based on the measurement of the stress of small die formed with varying the repetition rate of fs-laser pulse and the ambient gas. The current work should be helpful to understand the effect of ambient conditions on fs-laser-material interaction for practical usage.

7925-37, Session 10

On the damage behaviour of Al2O3 insulating layers in thin film systems for the fabrication of sputtered strain gaugesO. Suttmann, U. Klug, R. Kling, Laser Zentrum Hannover e.V. (Germany)

A new approach for strain measurement is the integration of laser patterned sputter thin film strain gauges. After deposition of the film system, consisting of sensing and insulating films, the sensing film is patterned by laser ablation. To ensure proper functionality of the strain sensors, the sensing film has to be removed completely. Damage of the insulating film bears the risk of shortcutting the sensor with the substrate. Hence, damage in the insulating film between metal work piece and sensing films has to be avoided when fabricating strain gauges.

We report on ablation experiments of thin film systems of NiCr and Al2O3. Ablation is performed with a Nd:YVO4 laser with a pulse duration of 15 ps, a wavelength of 532 nm and a repetition rate of 100 kHz. The effect of fluence, number of irradiated pulses and Al2O3 film thickness are investigated. This paper concentrates on the damage thresholds and damage mechanisms of the Al2O3 film. Two kinds of damage mechanisms appear: ablation based material removal and stress induced cracking. The damage thresholds increase with the film thickness. Increasing the number of irradiated pulses leads to decreasing damage thresholds until reaching a limit. Fluences below the limit allow for damage free ablation of the sensing layer. This behaviour is in agreement with an incubation model for bulk dielectrica. The role of defects in the Al2O3 films on the damage behaviour is discussed.

The knowledge gained enables patterning of thin film strain gauges without damaging the insulating layer.

7925-38, Session 11

In-situ coherent imaging to monitor and control laser micro machining processesJ. M. Fraser, P. J. Webster, J. X. Yu, B. Y. Leung, L. G. Wright, K. D. Mortimer, Queen’s Univ. (Canada)

In applications ranging from noncontact microsurgery to semiconductor blind hole drilling, precise depth control of laser processing is essential. Even a priori characterization and design of the machining process cannot compensate for material heterogeneity and stochasticity inherent to the ablation process. We take a different approach: by imaging sample morphology along the machining beam axis at high speeds (up to 300 kHz), we can guide the machining process in real time, even in high aspect ratio holes. The in situ metrology is based on coherent imaging (similar to the medical imaging modality optical coherence tomography) and is practical for a wide-range of light sources and machining processes (e.g., thermal cutting using a quasi-continuous wave ytterbium fiber laser, or nonlinear ablation achieved with ultrafast pulses from a diode-pumped regenerative amplifier). Coherent imaging has high dynamic range (> 60 dB) and strongly rejects incoherent signals allowing weak features to be observed in the presence of high power machining light and bright plasmas. Broadband imaging light is required to achieve high axial resolution (~5 micron) but center wavelength can be chosen appropriate to the application. Infrared (wavelength: 1320±35 nm) allow simultaneous monitoring of both surface and subsurface interfaces in nonabsorbing materials like tissue and semiconductors (e.g., silicon). Silicon based detector technology can be used with near infrared imaging light (804 ± 30 nm) enabling high speed acquisition or a low cost implementation (total imaging system <10k$). Machining with an appropriate ultrafast laser (broadband) allows machining and imaging to be done with the same light source.



7925-39, Session 11

In-situ observation of the hole formation during deep drilling with ultrashort laser pulsesS. Döring, S. Richter, Friedrich-Schiller-Univ. Jena (Germany); S. Nolte, A. Tünnermann, Friedrich-Schiller-Univ. Jena (Germany) and Fraunhofer Institute Applied Optics and Precision Engineering (Germany)

We report on the in-situ observation of the laser drilling process using ultrashort laser pulses. Our technique is based on transmission imaging of a silicon sample at 1060 nm. For drilling, we used a laser system that provides pulses with a duration of 8 ps at 1030 nm. This wavelength is below the band edge and silicon shows linear absorption. The beam is focused on the sample surface perpendicular to the transillumination. Therefore the temporal evolution of the longitudinal silhouette of the hole can be visualized during the drilling progress. Our observations show that the dynamics of the drilling process change in the depth of the material. For deep drilling, effects like the decrease in ablation rate, the formation of bulges, deviations in drilling direction and finally a branching of the hole end occur due to the influence of the previously excavated capillary. That causes a perturbation of the beam by irregular internal reflections and the interaction with ablation products as well as an additional abrasive effect by the ablated particles and plasma. The dependence of hole depth and shape on the process parameters, especially fluence and pulse energy, is studied. The depth of the hole shows a stepwise increase, while the ablated volume increases continuously, corresponding to intermediate periods with predominantly transverse expansion. The maximum achievable hole depth in deep drilling is chiefly determined by the pulse energy but largely independent from the fluence.

7925-24, Session 12

Laser-induced breakdown spectroscopy with tailored femtosecond pulses for 3-dimensional chemical imaging with high spatial resolutionJ. Mildner, C. Sarpe-Tudoran, L. Englert, D. Otto, N. Goette, M. Wollenhaupt, W. Wessel, A. Brueckner-Foit, T. Baumert, Univ. Kassel (Germany)


7925-25, Session 12

Uniform near-field nanopatterning due to the field distribution control by oblique femtosecond laser irradiation to nanoparticlesT. Miyanishi, M. Terakawa, M. Obara, Keio Univ. (Japan)

We present near-field optical properties around silica, silicon and gold nanoparticles aligned on a silicon substrate excited by oblique incidence femtosecond laser for nanohole processing. Using an enhanced localized near field, a nanohole can be fabricated even with near-infrared laser excitation. Near-field nanofabrication will open up smart applications for new optical devices with high-throughput processing. We have been investigating near-field processing with an oblique irradiation to gold nanoparticles placed on silicon substrate. Theoretical study revealed that the incident laser energy is concentrated into the contact point between the particle and the substrate due to the image charge inside the substrate at any incident angles. Here, we investigate the near-field distribution around silica and silicon nanoparticles excited by oblique incidence femtosecond laser. The near field around silica and silicon

particles is explained by Mie scattering theory, while the near field around gold nanoparticles is explained by plasmon polaritons inside nanoparticles. By using particles with a dielectric constant as low as silica, the position of the peak intensity point is controllable by changing the incident angle of the laser. With particles with a dielectric constant as high as silicon, the polarized charge shows a similar effect to the plasmon charge. Therefore the distribution of the concentrated energy provided with silicon nanoparticles is similar to that of gold nanoparticle.

This work is supported in part by a Grant-in-Aid for Japan Society for the Promotion of Science (JSPS) from the Ministry of Education, Culture, Sport, Science and Technology, Japan.

7925-26, Session 12

Nanostructure formation on silicon surfaces by high repetition-rate sub-15fs near-infrared laser pulsesM. H. Straub, K. König, Univ. des Saarlandes (Germany)

Laser-induced micro- and nanostructures on surfaces of crystalline silicon have been investigated intensely during the past decades. In particular, two types of ripple formation have been identified as self-organization phenomena. Ripples at a period of approx. 1 µm were explained by interference of the incident light field and a surface wave generated by periodic structural surface changes. In addition, tiny ripples at much shorter period of less than 100 nm were observed. So far, these structures were produced by lasers operating at pulse lengths in the order of 100 fs to 10 ns. Here, we report on the formation of characteristic surface features of low-indexed silicon surfaces such as Si(100), which were generated by sub-15 fs Ti:Sapphire laser pulses (centre wavelength 800 nm, bandwidth 120 nm, repetition rate 85 MHz). The high peak intensity in the tight focus of a high-numerical aperture objective allowed for profound structural and compositional changes at sub-nJ pulse energies. For example, lines written onto a Si(100) surface in water revealed changes in morphology at a period of 1.0 µm due to oxide particle formation at higher focal intensities or sub-10 nm hole arrangements surrounded by elevated areas at lower intensities. In contrast, a similar experiment with the Si(100) surface in oil mainly resulted in nano-scale deposition of carbon compounds in the exposed area. Our contribution includes analysis and discussion of a large variety of phenomena as well as a comparison with observations made using laser light of longer pulse width.

7925-28, Session 12

Optimization methods of hologram for holographic femtosecond laser processingY. Hayasaki, S. Hasegawa, Utsunomiya Univ. (Japan)

Parallel femtosecond laser processing with high throughput is indispensable to perform a wide volume fabrication composed of a huge number of processing points. Computer-generated holograms (CGHs) give features of high throughput and high light-use efficiency to the femtosecond laser processing. The CGH variably generates a desired arbitrary beam, such as a spatially-shaped beam, a split beam, and a wave-front corrected beam with low loss of light by use of a liquid-crystal spatial light modulator (LCSLM). A key requirement in an optimization of the CGH is a precise control of the diffraction peak intensity. We developed some optimization methods of the CGH. The diffraction peaks generated with multiplexed phase Fresnel lenses (MPFL) were made uniform by changing the center phase and size of each PFL, while taking account of the intensity distribution of the irradiated laser pulse and the spatial frequency response of the SLM. The uniformity of the diffraction peaks U was 88% at 10 parallel beams and the fabrication uniformity Ud was 80% at the irradiation energy of E = 2.4µJ. To obtain an MPFL with higher uniformity in the optical system, the MPFL was optimized using the diffraction peak intensities that were optically measured. The optimization obtained U=97% at 10 parallel beams and Ud=84% at E=2.4µJ. Recently we developed the second harmonic optimization



method. The method obtained U=97% at 18 parallel beams and the fabrication uniformity was fairly improved. In our presentation, our recent progresses of the CGH optimizations for holographic femtosecond laser processing are demonstrated.

7925-40, Session 12

Ultrafast microsphere near-field nanostructuringK. Leitz, U. Quentin, Lehrstuhl für Photonische Technologien (Germany) and Erlangen Graduate School of Advanced Optical Technologie (Germany); B. Hornung, Consultant (Germany); A. Otto, I. Alexeev, M. Schmidt, Lehrstuhl für Photonische Technologien (Germany) and Erlangen Graduate School of Advanced Optical Technologie (Germany)

Due to the steadily advancing miniaturisation in all fields of technology nanostructuring becomes increasingly important. Whereas the classical lithographic nanostructuring suffers from both high costs and low flexibility, for many applications in biomedicine and technology laser based nanostructuring approaches, where near-field effects allow a sub-diffraction limited laser focusing, are on the rise. In combination with ultrashort laser sources, that allow the utilisation of non-linear multi-photon absorption effects, a flexible, low-cost laser based nanostructuring with sub-wavelength resolution becomes possible. Among various near-field nanostructuring approaches the microsphere based techniques, which use small microbead particles of the size of the wavelength for a sub-diffraction limited focusing of pulsed laser radiation, are the most promising. Compared to the tip or aperture based techniques this approach is very robust and can be applied both for a large-scale production of periodic arrays of nanostructures and in combination with optical trapping also for a direct-write. Size and shape of the features produced by microsphere near-field nanostructuring strongly depend on the respective processing parameters.

In this contribution a basic study of the influence of processing parameters on the microsphere near-field nanostructuring with ultrashort pico- and femtosecond laser pulses will be presented. The experimental and numerical results with dielectric and metal nanoparticles on dielectric, semiconductor and metal substrates show the influence of particle size and material, substrate material, pulse duration, number of contributing laser pulses and polarisation on the structuring process.

7925-45, Session 12

Towards all-in-glass micro-actuators fabricated with femtosecond lasersY. Bellouard, Technische Univ. Eindhoven (Netherlands); A. A. Said, M. A. Dugan, P. Bado, Translume, Inc. (United States)

Femtosecond lasers exposure combined with a chemical etching step have been recently demonstrated as an efficient and versatile process for manufacturing complex three-dimensional structures in glass materials. This new fabrication method is particularly interesting for MEMS and in particular for monolithic structures that embed multiple functions, like waveguides, channels and mechanical elements.

Recently, we demonstrated that, and although it may sound counterintuitive, fused silica has excellent mechanical properties. Furthermore, we showed that flexures manufactured using this process can withstand very high stress and large deflection. As a proof-of-concept, an all-in glass monolithic micro-displacement sensor combining waveguides and a double compound linear guidance was made.

Here, we present a step towards fully integrated MEMS devices. In addition to the mechanical guidance and the sensing, we report on a scheme to introduce actuating functions to the device in which a set of electrodes embedded in the structure are used to drive the mechanism. In this paper, we describe the fabrication process and the performances of a first prototype of a femtosecond laser-manufactured actuator.


Simulation the temperature increase in porcine cadaver iris during direct illumination by femtosecond laser pulsesH. Sun, R. M. Kurtz, T. Juhasz, Univ. of California, Irvine (United States)

PURPOSE: Femtosecond lasers are approved by the US Food and Drug Administration (FDA) for ophthalmic surgery, including creation of corneal flaps in LASIK surgery. During the surgery, about 50-60% of laser energy may pass beyond the cornea, with potential effects on the iris. As a model for laser exposure of the iris during femtosectond corneal surgery, we simulated the temperature rise in porcine cadaver iris during direct illumination by the laser.

METHODS: The temperature increase induced by a 60 kHz iFS Advanced Femtosecond Laser (AMO Inc. Santa Ana, CA) in porcine cadaver iris was simulated using COMSOL (Comsol Inc. MA) finite element software. To model the effect of the laser radiation on the iris during the surgery, a cross section of the excised porcine cadaver iris was calculate for the temperature distribution during direct illumination by femtosecond laser pulses.

RESULTS: Temperature increases up to 2.45 Celsius degree (corresponding to 2 microJ and 24 second illumination) were observed in the porcine cadaver iris from simulation with little variation in temperature profiles compared with specimens for the same laser energy illumination in experiment.

CONCLUSIONS: The commercial iFS Advanced Femtosecond Laser operating with pulse energies at approximately the lower limit of the range evaluated in this study would be expected to result in a 1.23 °C temperature increase and, therefore, does not present a safety hazard to the iris.


Scattering-controlled femtosecond-laser induced nanostructuring of TiO2 thin filmsS. K. Das, A. Rosenfeld, M. Bock, Max-Born-Institut für Nichtlineare Optik und Kurzzeitspektroskopie (Germany); A. Pfuch, Innovent e.V. (Germany); W. Seeber, Friedrich-Schiller-Univ. Jena (Germany); R. Grunwald, Max-Born-Institut für Nichtlineare Optik und Kurzzeitspektroskopie (Germany)

Laser induced periodic surface structuring (LIPSS) is a straightforward and cost-effective approach to generate high-spatial-frequency periodic or random gratings and can be applied to many kinds of materials like metals, semiconductors and dielectrics. The periods and orientation of the nanostructures, their depth profile and spatial contrast can be controlled by varying peak power density, wavelength and polarization of the laser beam and its spatial distribution and uniformity. Recently it was demonstrated by several groups that extremely small features even with sub-wavelength dimensions can be obtained in dielectric materials by exploiting multiphoton excitation with femtosecond laser pulses. Here we report on first investigations of femtosecond-laser induced nanostructuring of sputtered and pyrolytically deposited titanium dioxide (TiO2) layers in comparison to bulk material. TiO2 surfaces of defined corrugation are of essential interest for applications like superwetting, photocatalysis, antifogging, photovoltaics, or gas sensors. At a pulse duration of 150 fs and a center wavelength of 800 nm, the formation of nanoripples with periods of 150 nm was observed in our experiments even at low pulse numbers in the range of N= 5 if the process was favoured by a sufficiently high density of initial scattering centers. It is indicated that the mechanism is strongly influenced by the surface morphology and the intrinsic optical quality of the layer. We conclude that a more subtle control of scattering should well enable for an improved performance of nanostructuring processes, in particular on extended areas.




Invisible two-dimensional barcode fabrication inside a synthetic silica glass by femtosecond laser processing using a computer-generated hologramH. Kawashima, M. Yamaji, J. Suzuki, S. Tanaka, New Glass Forum (Japan)

We have developed a three-dimensional (3D) femtosecond laser processing method using a Computer-Generated Hologram (CGH) that generates a 3D structure inside transparent materials such as silica glass and PMMA. As an application of our method, we report an invisible two-dimensional (2D) barcode embedded into a synthetic silica glass by femtosecond laser processing using a CGH. When we illuminate it with a 254 nm ultraviolet light, a red photoluminescence (PL) is observed, and we can read it. For an invisible 2D barcode fabrication, we design a CGH with long-focal-depth to efficiently extend optical damage in the depth direction resulted by femtosecond laser irradiation. We fabricate a 2D barcode pattern inside bulk synthetic silica glass with a spatially extended femtosecond pulse beam generated by the CGH. In the irradiated 2D barcode area, a PL band peaking at 650 nm (1.9 eV) is appeared with the excitation wavelength, 260 nm (4.8 eV). The PL band and PL excitation spectra are measured by means of a fluorescence spectrometer. We extract the 2D barcode information from the observed PL data by using a CMOS camera and image processing technology. This work provides a novel barcode fabrication method by femtosecond laser processing using a CGH and a barcode reading method by a red PL.


Optical device fabrication using femtosecond laser processing with glass-hologramJ. Suzuki, Y. Arima, S. Tanaka, New Glass Forum (Japan)

Femtosecond laser processing with glass-hologram is an efficient processing method that an arbitrary three-dimensional intensity distribution produced by the glass-hologram is directly patterned into transparent materials at one laser shot. It has higher productivity of devices with higher accuracy, comparing to the conventional femtosecond laser processing that translates tightly focused femtosecond laser pulses. We have demonstrated the fabrication of three-dimensional spiral structure [1], straight-waveguides of 5 mm long [2], curved-waveguides of 3 mm radius [3], and so on, using this laser processing method.

A glass-hologram is one of the key elements in this femtosecond laser processing and is obtained as follows. At first, we calculate a phase distribution that produces a targeting output beam from an input femtosecond laser beam, using computer-generated holography. Then the phase distribution is transferred on a glass surface by means of electron beam lithography and reactive ion etching. The performance of the fabricated glass-hologram depends on hologram parameters, such as phase level, pixel size, and so on. In consequence, the glass-hologram parameter will affect characteristic of the optical devices.

In this work, we report on optical devices fabricated by femtosecond laser processing with glass-hologram. We also investigate relation between characteristic of the optical device and the glass-hologram parameters or performance.

[1] M. Yamaji, H. Kawashima, J. Suzuki and S. Tanaka, App. Phys. Lett. 93, 041116 (2008)

[2] J. Suzuki, M. Yamaji, and S. Tanaka, Proc. SPIE, Vol. 7201, 72011C (2009)

[3] J. Suzuki, Y. Arima, M. Yamaji, H. Kawashima, and S. Tanaka, Proc. SPIE, Vol. 7589, 75890T (2010)


technical summariesspie.org/documents/conferencesexhibitions/pw11l...1-um region. this technology...

Documents