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Ultrafast Laser Sources

Laser Technik Journal 3/2014 45

More Powerful Femtosecond LasersYb-doped laser crystals supporting the race Florian Kienle and Daniel Achenbach

There is a constant race for more power and higher pulse energies from femtosecond lasers to cater for de-manding applications in materials processing such as TFT-display mask repair, glass scribing, stent cutting or composite materials machining. This puts more and more stringent require-ments on the components used in the oscillators as well as amplifiers of such laser systems, particularly the laser gain medium. We would like to present our recent results with what we identi-fied to be appropriate gain media and which were used in regenerative am-plifier systems. The experiments were performed in our R&D labs as well as in the High Q Laser in-house applica-tion lab (cutting-edge pico- and fem-tosecond laser systems as well as laser scanning and microscopy tools).

Technology

Besides thin-disk or fiber technologies, bulk-crystals in a regenerative amplifier configuration are widely used due to their simple resonator design and abil-ity for very short femtosecond pulses. Such laser systems typically operate in the 1 µm range using Ytterbium-doped host materials. Highly efficient pump-ing with commercially available laser diodes in the 980 nm spectral region consequently leads to a low quantum defect, i.e. pump energy being con-verted to heat rather than to laser ra-diation. Promising candidates with good thermal properties and high gain bandwidth for sub-400fs pulses are Yb:KYW (as a well-known and widely used reference material), Yb:CaF2 and Yb:CALGO, which are the subject of our current R&D efforts. The striking feature of both Yb:CaF2 and Yb:CALGO is the very high figure of merit “emis-

sion bandwidth × thermal conductiv-ity”, mainly due to the broad and flat emission characteristic unusual for Ytterbium-doped crystals. Some prop-erties of these three crystals are summa-rized in Table 1.

A well-established method to pro-duce high-power and/or high-energy ultrashort pulses is the so-called regen-erative amplification approach. Here, a master oscillator generates high-fidelity femtosecond pulses at high repetition rate. The pulse energy is typically in the pJ or low nJ range, low enough to avoid detrimental non-linear effects negatively affecting the temporal pulse shape as well as the optical spectrum. Some of these pulses are switched into an amplifier resonator by means of a fast polarization switch, mostly a Pock-els cell. When the Pockels cell “opens”, a pulse is coupled into the amplifier and gets trapped when the Pockels cell is “closed” (two orthogonal states of po-larization). The pulse experiences gain on every resonator round-trip. After a certain number of round-trips, the Pockels cell opens again and releases the amplified pulse from the amplifier. This switching typically occurs at rates of 1 kHz for very high energy pulses in the mJ range up to 1 MHz for pulses in the tens of µJ range. The inherent mate-rial dispersion of the gain crystal as well as the Pockels cell crystal helps to avoid component damage and non-linear ef-

Yb:KYW (ref.) Yb:CALGO Yb:CaF2

Absorption cross section [10-20 cm2] 6.30 (@981 nm) 1.56 (@979 nm) 0.54 (@979 nm)

Emission cross section [10-20 cm2] 3.00 (@1025 nm) 0.75 (@1025 nm) 0.25 (@1045 nm)

Fluorescence lifetime [ms] 0.23 0.42 2.40

Thermal conductivity [W/mK] 3.3 6.6 9.7

fects due to the pulse stretching per round-trip and hence the reduction of the pulse peak intensity. This stretch-ing can additionally be supported by introducing dispersive mirrors into the resonator. Once the amplified pulse has left the resonator, it is compressed to its original pulse duration prior to amplifi-cation by means of dispersive elements, mostly a pair of diffraction gratings, with opposite dispersion compared to the stretching.

Experimental results

In one experiment, we integrated a Yb:CALGO crystal into our stan-dard 63 MHz oscillator and generated 650 mW output power in 93 fs long pulses. The spectrum was centered at 1050 nm with a broad bandwidth of 12.5 nm. These pulses were amplified in a Yb:CALGO regenerative amplifier to 36 W. The wavelength shifted slightly to 1046 nm and the bandwidth was re-duced to 9 nm due to gain-pulling and -narrowing. After compression with an efficiency of 78 % (28 W of true output power), a record-high pulse energy of 56 µJ at 500 kHz, a pulse du-ration of 217 fs and a beam quality of M2 < 1.15 required for tight focusing of the laser beam was obtained. Interest-ingly, only the inherent material dis-persion of the amplifier was sufficient for pulse stretching. Simulations show

Table 1 Comparison of optical and thermal properties of Yb-doped host materials for ultra-short pulse generation and amplification.

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46 Laser Technik Journal 3/2014 © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

High Q Laser GmbHRankweil, Austria

Founded in 1999, High Q Laser GmbH has evolved to a leading supplier of OEM femto- and picosecond, diode-pumped, solid-state laser products for materials processing as well as medical applications. The portfolio comprises of both high-repetition rate as well as high-ener-gy oscillators and of high-power amplifier sys-tems up to repetition rates of 1 MHz and output power levels of 16 W in the infrared, plus fre-quency conversion to the green and ultraviolet spectral range. The company expanded in 2009 moving into new facilities with state-of-the-art clean rooms. Since 2011, High Q Laser GmbH is part of the Newport/Spectra-Physics family.

www.highqlaser.atwww.spectra-physics.com

Company

that a more optimized arrangement of stretcher and compressor should help to obtain sub-200-fs pulses (already shown in previous experiments at lower power levels) and mJ-level pulse energies.

In another experiment, the same Yb:CALGO oscillator was used as the front-end for a Yb:CaF2-based regen-erative amplifier. However, instead of seeding the Yb:CaF2 amplifier with femtosecond pulses right away, we con-structed a grating-based pulse stretcher to get 400 ps long pulses, which was re-quired to make possible the pulse am-plification to energies > 1 mJ at 5 kHz without triggering non-linear effects. Furthermore, the beam diameter in-side/on all intracavity optics was kept at a maximum to reduce the intensity. At

this pulse energy, high-fidelity pulses at a center wavelength of 1045 nm, with a duration of 324 fs and an M2-value of approximately 1.1 were produced af-ter compression. This corresponds to a peak power of about 3 GW.

Applications

Fig. 1 shows some images from the mul-titude of applications that is possible with laser systems as described above. The materials were processed with either our standard, commercially available Spirit laser system or with a modified version thereof. As shown in Fig. 2, it is a compact, ultrafast re-generative amplifier system with a repetition rate up to 1 MHz for micro-machining processes in industrial ap-plications.

Fig. 1a shows a silicon carbide micro-gearwheel machined with very high precision and burr-free. Silicon carbide is difficult to machine due to its very high hardness. Owing to the high peak power and pulse energy of femtosecond lasers, silicon carbide can be machined

with minimal defects in the process area.

Figures Fig. 1b and 1c show medical stents produced from biodegradable (PLGA) and shape metal alloy (nickel titanium) materials, respectively. Due to the short pulse duration, the excel-lent beam quality (enabling small focus diameters) and therefore high peak in-tensity, the polymer-based PLGA ma-terial is not affected by heat deposition but can be readily cut down to a sub-200 µm wall thickness. Even super-fine structures like nickel titanium stents with a wall thickness and strut width of 45 µm and 35 µm, respectively, can be produced with the Spirit laser. Medical devices and their geometrical structures have become smaller and more com-plex in recent years. For this reason, thermally induced damage can only be avoided by using femtosecond pulses as the interaction time between laser pulse and material is short enough that hardly any heat is transferred from the ablated particles to the substrate.

Besides processing of materials for medical devices, one can also use fem-

Fig. 1 Application examples of high-ener-gy femtosecond Spirit laser system: a) 3D micro-machining of silicon carbide, b) cutting of biocompat-ible stent material PLGA with a wall thickness of 180 µm, c) cutting of super-elastic Nickel Titanium (shape memory alloy) stent with tube diameter of 4.5 mm, wall thick-ness of 45 µm and strut width of 35 µm, d) burr-free cutting of Xensation glass used e.g. for smartphones touch screens.

Fig. 2 a) Spirit regenerative amplifier laser system, b) image of High Q Laser application lab setup for cutting and dicing of transparent glass material.

a) b)

c) d)

© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Ultrafast Laser Sources

Laser Technik Journal 3/2014 47

tosecond pulses to optimize the yield in production lines for flat panel displays. In this application, the advantage of minimal heat affected zones (HAZ) can be used to darken pixels or repair color filters without destroying the small structures. Femtosecond lasers are be-coming more important for this kind of applications as pixel sizes are getting smaller to reach higher resolution for screens and portable device displays. Especially new technologies like OLED or flexible displays require a minimum of heat absorption inside the processed material. Another application currently in the center of interest is the cutting process and scribe-and-break pro-cess of very hard and scratch-resistant transparent glass materials for the use in the display and touch-screen indus-try. Shown in Fig. 1d is a comparison between Xensation glass being cut with nanosecond and femtosecond laser pulses. Clearly, femtosecond pulses gen-erate a much more defined cut without any burr due to the above mentioned advantages.

Conclusion

In summary, Yb:CALGO and Yb:CaF2 appear to be convenient laser gain me-dia that are capable to support sub-100 fs and sub-400-fs operation in oscil-lators and amplifiers, respectively, and to withstand the extreme thermal con-ditions without compromising laser sta-

Authors

Florian KienleDr. Florian Kienle joined High Q Laser GmbH late in 2011 working as a scientist and project manager on novel technologies for femtosecond pulse generation and ampli-fication. Since autumn

2013, he is also leading the photonics R&D activities.

Daniel Achenbachentered High Q Laser GmbH early in 2007 and was part of the R&D group for ultrafast oscillators. In 2011, he changed position taking care of ultrafast oscillators as a project manager. Since sum-

mer 2012, he is product marketing manager and key account manager of High Q Laser’s ultrafast amplifiers.

Daniel Achenbach, High Q Laser GmbH, Im Feldgut 9, 6830 Rankweil, Austria, E-Mail: daniel.achenbach@spectra-physics.com

bility and robustness. Both crystals have the potential to allow for further scaling of power and energy in future femtosec-ond laser products. This will enable the wide-spread use of femtosecond laser sources in many industrial and medi-cal applications beyond those that are already established.

DOI: 10.1002/latj.201400032