high-power ultra-broadband mode-locked yb3+-fiber laser with 118 nm bandwidth

High-power ultra-broadband mode-locked Yb3+-fiber laser with 118 nm bandwidth

P. Adel and C. FallnichLaser Zentrum Hannover e.V., Hollerithallee 8, D-30419 Hannover, Germany

[email protected]

Abstract: A diode-pumped mode-locked double clad Yb3+-fiber ring laserwith up to 118 nm bandwidth, a repetition rate of 7.6 MHz and a maximumoutput power of 2.8 W is presented. The broad bandwidth is achieved bysuppressing the long wavelength components in the dispersive grating lineof the resonator.© Optical Society of AmericaOCIC codes: (140.3510) Lasers, fiber; (140.4050) Mode-locked lasers; (060.2380) Fiberoptics sources and detectors

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References

1 M. Bashkansky, M. D. Duncan, L. Goldberg, J. P. Kopolow, J. Reintjes, „Characteristics of a Yb-dopedsuperfluorescent fiber source for use in optical coherence tomography,“ Opt. Express 3, 305-310 (1998),http://www.opticsexpress.org/abstract.cfm?URI=OPEX-3-8-305

2 T. Dresel, G. Häusler, and H. Venzke, "Three-dimensional sensing of rough surfaces by coherence radar,"Appl. Opt. 31, 919-925 (1992)

3 V. Cautaerts, D. J. Richardson, R. Paschotta, D. C. Hanna, “Stretched pulse Yb3+-silica fiber laser,” Opt.Lett. 22, 316-318 (1997)

4 U. Morgner, F.X. Kaertner, S.H. Cho, Y. Chen, H.A. Haus, J.G. Fujimoto, E.P. Ippen, V. Scheuer, G.Angelow, T. Tschudi, "Sub-two-cycle pulses from a Kerr-lens mode-locked Ti:sapphire laser," Opt. Lett.24, 411-413 (1999)

5 M. Auerbach, D. Wandt, C. Fallnich, H. Welling, S. Unger, “High-power tunable narrow line widthytterbium-doped double-clad fiber laser,” Opt. Commun. 195, 437-441 (2001)

6 H. Zellmer, A. Tünnermann, H. Welling, V. Reichel, “double-Clad Fiber Laser with 30 W Output Power,”in: Optical Amplifier and their Applications (Optical Society of America, Washington D.C.), paper WC7-1, 251 (1997)

7 L. E. Nelson, D. J. Jones, K. Tamura, H. A. Haus, E. P. Ippen, “Ultrashort-pulse fiber ring lasers,” Appl.Phys. B 65, 277-294 (1997)

8 G. P. Agrawal, Nonlinear fiber optics, (Academic Press, 1995), Chap. 4.1 and 10.3.9 P. Weßels, M. Auerbach, C. Fallnich, “Narrow-linewidth master oscillator power amplifier system with

very low amplified spontaneous emission,” Opt. Commun. 205, 215- 219 (2002)

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1. Introduction

Fast and accurate measurements with optical metrology systems like optical coherence tomo-graphy (OCT) [1] or optical coherence radar [2] require broad-bandwidth high-power opticalsources. Superluminescent diodes, which are the common light sources for OCT systems,show about 40 nm bandwidth but only a few mW output power. For many non-medical appli-cations this low output power limits the signal to noise ratio and the probe penetration depthon OCT measurements. As an alternative light source Bashansky et al. presented a Yb3+-doped superfluorescent fiber source with up to 700 mW output power with a bandwidth of40 nm [1]. The output power of this system is sufficient for many applications. But the reso-lution of OCT and coherence radar measurements are still limited to about 12.5 µm as thebandwidth determines the minimum achievable resolution of such metrology systems. There-

(C) 2002 OSA 15 July 2002 / Vol. 10, No. 14 / OPTICS EXPRESS 622#1377 - $15.00 US Received June 18, 2002; Revised July 08, 2002

http://www.opticsexpress.org/abstract.cfm?URI=OPEX-3-8-305

fore there is a demand for sources with broader emission bandwidth and mode-locked lasersystems are attractive candidates for such broad-bandwidth high-power light sources. Themost prominent representative of those sources is the Ti:Sapphire laser, which is able toprovide up to 400 nm bandwidth in combination with output powers of about 200 mW [4] oreven higher output powers on the expense of smaller bandwidth. However, these laser systemsare too bulky and too expensive for most medical and industrial applications, because theyrequire argon-ion or frequency-doubled solid-state lasers as pump sources. Mode-lockedYb3+-fiber lasers do not suffer from those disadvantages, but typically the maximum band-width of these systems is below 40 nm with an output power of only a few mW [3], althoughYb3+-fibers are able to provide gain over more than 60 nm [5,6]. As a consequence those fiberlaser systems are up to now not used as light sources for metrology systems.

In this work we present a mode-locked diode-pumped Yb3+-fiber laser system whichovercomes the above mentioned restrictions. The laser generated pulses with up to 118 nmbandwidth and showed a maximum average output power of 2.8 W. For this system the influ-ence of the pump power and of the cavity dispersion on the pulse bandwidth was investigated.Broad-bandwidth operation was enabled by suppression of the long-wavelength componentsof the pulse spectrum inside the dispersive grating line of the cavity.

2. Experimental setup

Figure 1 shows the laser setup in detail, which consists of six modules: a 26 m long Yb3+-doped double-clad fiber that provides the gain and the nonlinearity, quarter- (QWP) and half-wave-plates (HWP) for polarization control, an optical isolator for unidirectional laser opera-tion, a 50% output coupler (M3), a dispersive delay line with two parallel mounted diffractiongratings (1200 lines/mm) for compensation of the normal fiber dispersion and an adjustableblade inside the grating line. This blade was used for suppression of the long-wavelengthcomponents and was orientated parallel to the vertical grating lines. The double-clad fiber hada pump and laser core diameter of 4.2 µm and 400 µm, respectively, a Yb2O3-doping concen-tration of 6500 ppm and a cut-off wavelength of about 950 nm. The fiber was pumped at975 nm by a 25 W fiber-coupled laser diode through the dichroic mirror M1 and was “kidney-shaped” coiled for improved pump-light absorption [6].

The output light of the fiber was steered by mirrors M1 and M2 to the optical isolator.The polarizing beam splitter at the input of the isolator rejected one polarization and, there-fore, acted as an additional output port (port 2). Behind the isolator, the signal propagated

Fig. 1. Setup of the broadband fiber laser system

grating 1

grating 2

port 1

isolator

M1

QWP

QWP

HWP

M2

90°prism

M4

M5

port 2

filteringblade


below mirror M4 to the output coupler M3 and then to the dispersive grating pair unit. Thetotal dispersion of this unit was adjusted by the grating separation. After passing the secondgrating, the different wavelengths of the signal propagated spatially parallel and were sepa-rated laterally. Therefore, the horizontally adjustable (vertical) blade behind the second gra-ting acted as a spectral filter. The portion of the spectral pulse components which passed thisblade was then back reflected below the incoming beam by a 90° prism and then coupled intothe Yb3+-fiber via mirrors M4 and M5. The polarization at the fiber input was determined bythe settings of the zeroth-order wave-plates (1.064 µm). For appropriate wave-plate settingsthe polarization beam splitter (port 2) in combination with the nonlinear polarization rotationin the fiber rejected low intensity signals like a saturable absorber and self-starting mode-locking with a repetition rate of 7.6 MHz was achieved [7].

3. Results and discussion

The pump power for self-starting mode-locking of our fiber laser depended on the wave-platesettings, on the total cavity dispersion and especially on the blade position inside the disper-sive grating line. Without spectral filtering the mode-locking started typically at about 4.5 Wpump power. The output signal showed a center wavelength of 1.1 µm and a maximum band-width of about 40 nm. Moving the blade into the beam inside the grating line suppressed thelong-wavelength components and resulted in a shift of the center wavelength of the circulatingpulse to shorter values. If the blade was moved further into the beam the bandwidth of theoutput signal increased dramatically for appropriate wave-plate settings. At an optimum posi-tion an extremely broadband output signal with up to 118 nm bandwidth (FWHM) extendingfrom 1060 to nearly 1240 nm was achieved. In order to determine the cut-off wavelength atthat specific blade position, we measured the ASE-signal of the Yb3+-fiber behind the gratingline with and without the blade. As a result, only wavelengths below 1080 nm (13 dB) aretransmitted, which is a surprisingly small part of the broad bandwidth signal.

Moving the blade further into the beam reduced strongly the signal bandwidth as well asthe center wavelength. It should be noted that moving the blade from the other direction intothe beam in order to suppress the short-wavelength components shifted only the pulses tolonger wavelength and no spectral broadening was observed.

In Figure 2a the normalized output spectra of the two ports of the laser system are shown,adjusted for maximum bandwidth at port 2. At a pump power of 18.8 W we achieved outputpowers of 0.55 W at port 1 and 1.31 W at port 2, respectively. The bandwidth (FWHM) of theoutput spectra at port 1 was 103 nm and 118 nm at port 2, which is about 3 times larger thanfor a Yb3+-fiber laser system without spectral filtering. Both curves are similar in shape, ex-cept around 1175 nm, where the spectral intensity measured at port 1 was negligible, whereasthe output of port 2 showed a local maximum. This was probably caused by the limited band-width of the zeroth order (1064 nm) QWP in front of the isolator, since the signal polarizationat the input of the isolator determines the power splitting ratio between port 1 and port 2.

The filtering blade acted as a short-pass filter. The measured spectral attenuation of thisfilter was about 40 dB at 1087 nm and increased further with wavelength. Therefore the long-wavelength components of the output spectra could not circulate inside the cavity and must begenerated in a single pass through the Yb3+-fiber by various nonlinear effects like self-phasemodulation, four-wave mixing and stimulated Raman scattering [8].

At shorter wavelengths a steep decrease around 1060 nm was observed. This behaviorwas nearly independent of the wave-plate settings, the pump power and the output port. Thislimited emission of short-wavelength components was caused by the reflection characteristicsof the dichroic mirror M1, which was highly transmittive for wavelengths below 1055 nm andadditionally, reabsorption in the Yb3+-fiber is increasing with shorter wavelengths, what mayact as an additional long pass filter.


The influence of the pump power on the measured bandwidth at the two output ports isshown in Figure 2b. For pump powers of up to 15 W the bandwidths increased nearly linearlywith similar values for both ports. This can be easily be understood in terms of the intensitydependent nonlinear effects, which are broadening the spectral bandwidth. In the pump powerrange from 15 to 19 W the maximum signal bandwidth was achieved. At these power levelsthe maximum bandwidth of the spectrum at port 2 was at least more than 10 nm broader thanat port 1. For pump powers beyond 19 W the bandwidth of both output signals decreased tovalues below 100 nm.

A more detailed investigation, by using a fast photodiode (2 GHz) and an oscilloscope(500 MHz), showed typically only one short pulse with a repetition rate of 7.6 MHz for pumppowers below 20 W. For higher pump powers typically two closely spaced short pulses wereobserved. The decreasing bandwidth with increasing pump power at this power level wasprobably caused by multiple pulse operation which reduced the peak intensity inside the fiber.

In Fig. 3 the measured autocorrelation of a 116 nm bandwidth signal from port 2 isshown. It is noticeable that the autocorrelation function shows not the typical 8:1 ratio near theinterference pattern. Even for a time delay of 30 ps the autocorrelation signal intensity wasstill about 25% of the interference maximum. This large pedestal is probably caused by a largechirp of the output signal. Taking the large bandwidth of the signal and the fiber dispersioninto account a pulse duration of up to 100 ps could therefore be expected at the output.

Fig. 2. a) Output spectra of the ports for maximum bandwidth. For wavelengths above 1087 nm(right of the vertical dashed line) the attenuation of the spectral filter is above 40 dB.b) Influence of the pump power on the spectral bandwidth of the output spectra at the two ports

0

0,5

1

1040 1140 1240

Wavelength [nm]

Inte

nsity

[a.u

.]

port 1

port 2

a)

Fig. 3. Autocorrelation function of the output signal (signal bandwidth 116 nm)

0

4

8

-100 0 100

Time [fs]

Inte

nsity

[a.u

.]

0

40

80

120

0 10 20Pump power [W]

FW

HM

band

wid

th[n

m]

port 1port 2

b)


It should be noted, that in our experiments we observed no influence of the total cavitydispersion on the spectral bandwidth between –1.4 x 105 fs² and 1.6 x 105 fs². Measurementsof the output spectra at four different values of the second order dispersion between thisvalues showed always the extreme spectral broadening with similar signal bandwidths.

In Fig. 4 the output power of both ports are plotted versus the pump power for two differ-ent adjustments of the wave-plates and the filtering blade. In Fig. 4a these results are shownfor an optimized spectral bandwidth, spanning up to 118 nm at port 2. For this bandwidth anoutput power of 1.3 W at port 2 was achieved. A further increase of the pump power enabled amaximum output power for this set-up of 1.02 W at port 1 and 2.16 W at port 2 but at reducedsignal bandwidth (see Fig. 2b). At the bandwidth maximum of 118 nm the output power was1.3 W at port 2. In Fig. 4b the laser system was optimized in respect of the output power,which have maximum values of 0.8 W and 2.8 W, respectively, with a bandwidth of 85 nm. Inboth figures the gradients of the curves were smaller at lower pump power levels. This powercharacteristics were mainly caused by a shift of the pump wavelength with pump power. Thisresulted in a variation of the pump light absorption since the absorption peak of Yb3+ at975 nm is relatively narrow [9].

0

1

2

3


Out

putp

ower

[W]

port 1

port 2

a)

0

1

2

3


Out

putp

ower

[W]

port 1

port 2

b)

Fig 4. Output power of both output ports versus pump power: a) set-up adjusted for maximumbandwidth (see Fig. 2). b) maximum output power (maximum bandwidth of 85 nm above22 W).

This unusual and hardly expected result of extreme spectral broadening of the Yb3+-fiberlaser signal by nearly a factor of 3 compared to previous reported systems can be interpretedin the following way:

The long wavelength spectral components at the output above about 1090 nm must begenerated inside the fiber each pass, since these components were blocked by the spectralfilter before the fiber input. Therefore these spectral components were obviously generatedinside the fiber by nonlinear effects like self-phase modulation and four wave-mixing andprobably in a smaller extent by intra-pulse stimulated Raman scattering. In addition to thegeneration of new spectral components by nonlinear effects the generated long-wavelengthcomponents were partially further amplified by the fiber gain. On the other hand short wave-lengths have a lower gain or were even absorbed by the Yb3+-fiber. Nevertheless, this lasersystem showed not such a extreme spectral broadening without spectral filtering.

The spectral filtering reduced the pulse duration at the fiber input, reduced the influenceof the initial chirp, the gratings and the highly uncompensated third order dispersion on theinput signal and increased the gain in the fiber. The combination of these effects raises thepeak intensity inside the fiber. Because of the higher peak intensity the above mentionedintensity dependent nonlinear effects generate the observed additional spectral components incombination with the spectral filtering.


The spectral filtering reduces the pulse duration at the fiber input since the signal is therestrongly chirped. As this is in contrast to the common known pulse lengthening with band-width reduction, we want to explain it in more detail. The large chirp is attributed to thepassive mode locking based on the nonlinear Kerr-effect in the fiber and the large fiber andgrating dispersion. A result of the passive mode-locking mechanism is that the circulatingpulse has the lowest chirp roughly in the middle of the fiber and is strongly chirped at theinput and the output of the fiber. For example the estimated pulse duration at the fiber input isroughly about 400 times above the bandwidth limit for a 40 nm bandwidth signal, taking themeasured fiber dispersion of 2.35 x 104fs²/m into account. Therefore the different spectralcomponents of the strongly chirped pulse are spread over the large pulse length and the time-bandwidth product is far above the theoretical limit. Under these circumstances a reduction ofthe spectral bandwidth by the above mentioned spectral filtering reduces the pulse duration atthe fiber input, since this separates excessive spectral components and therefore one of thepulse wings. This pulse width reduction by spectral filtering can be driven until the time-bandwidth product is approaching the theoretical Fourier limit.

If the filtering is further increased the pulse approaches the Fourier limit and the pulseduration increases with decreasing bandwidth. At this regime the remaining pulse is nearlytime-bandwidth limited. Because of the relatively small bandwidth the influence of the initialchirp, the gratings and the high uncompensated third-order dispersion in the cavity of about2.7 x 106 fs³ was also substantially reduced. Hence the effects of the initial chirp were blurredand the pulse duration depends mainly on the remaining bandwidth. Probably this also is thereason why the grating dispersion showed no influence on the spectral broadening. It shouldbe noted that without spectral filtering a bandwidth limited pulse is not expected somewhereinside the fiber laser because of the highly uncompensated third order dispersion.

Furthermore the spectral filtering increased the cavity loss, in particular for the longwavelength components and reduced therefore the power at the fiber input. As in laseroperation the gain equals the losses, the spectral filtering resulted in a higher gain.Furthermore the center wavelength of the input signal is shifted to the short-wavelength sideof the gain maximum. Therefore the gain for long-wavelength components of the pulse isadditionally increased. However our experiments with high cavity loss without spectralfiltering showed, that a high gain is not sufficient for an extreme spectral broadening.

The above described relations and effects explain qualitatively the observed behavior.Nevertheless, the detailed interaction between the various nonlinear effects, the Yb3+-gain andthe fiber parameters is quite complicated and has to be further investigated by numericalsimulations and experiments.

4. Conclusion

To our knowledge, we have demonstrated the largest bandwidth (118 nm) of a mode-lockedYb3+-fiber laser. This high power laser source (up to 2.8 W) is well suited for opticalmeasurement techniques, like optical coherence tomography or optical coherence radar,because of the broad bandwidth signal generated in a single mode fiber (950 nm cut-off). Byusing Yb3+-fibers with larger mode field diameters further power scaling of this broadbandwidth system should be possible.

Acknowledgements

This research is supported by the German Ministry of Science, Education, Research andTechnology under contract 13N7799. We gratefully acknowledge the Institut fuer physika-lische Hochtechnologie Jena (IPHT) for preparing the Yb 3+-double-clad fiber.


high-power ultra-broadband mode-locked yb3+-fiber laser with 118 nm bandwidth

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