spie proceedings [spie international conference on lasers, applications, and technologies '07 -...

30 W dual active element Yb:KGW regenerative amplifier for amplification of sub - 500fs pulses

Darius Stučinskas, Roman Antipenkov, Arūnas Varanavičius

Department of Quantum Electronics, Vilnius University, Saulėtekio 9, LT-10222 Vilnius, Lithuania

ABSTRACT

We report on dual active element Yb:KGW regenerative amplifier that provides ultrashort pulses in the 0.3 mJ range at high repetition rates. Each active element was pumped by two 50 W diode pump modules at 980 nm. 30W average power was obtained at 100 kHz repetition rate. Measured spectrum width of 5,4 nm allows for compression of amplified pulses to sub-300 fs pulse duration. As output power increases above 15 W nearly symmetrical Gaussian profile beam becomes slightly elliptical, although remains Gaussian in transverse directions. The measured beam quality parameter M2 is below 1,5 even at maximum output power.

Keywords: Yb:KGW, laser, regenerative amplifier, diode pumped.

1. INTRODUCTION Femtosecond laser sources became promising tool for various scientific and industrial applications. High average power and repetition rates are required to speed up operation processes. Recently increased interest in Yb3+ doped Potassium Gadolinium Tungstate KGd [WO4]2 (KGW) for the development of high power ultrashort pulse lasers has been shown [1, 2]. Yb:KGW exhibits an attractive set of parameters which makes it as one of the best choices for lasers operating around 1 µm. Small quantum defect together with absence of upconversion and excited-state absorption gives high slope efficiencies [3]. Intrinsic Yb:KGW birefringence allows for low intra-cavity losses caused by thermally induced birefringence. The broad gain bandwidth obtainable in these crystals makes available the femtosecond pulse generation. Another attractive feature seems to be existence of athermal crystal cut direction, that helps to reduce thermal lens in active element of laser [4]. Although thermal conductivity in Yb:KGW is about three times lower than in Yb:YAG, it is, however, significantly higher as compared to Yb:glass, making the use of Yb:KGW quite promising for femtosecond pulse average power scaling. Average output power levels of ~ 130 W using femtosecond fiber lasers was demonstrated [5]. Main drawback of fiber laser design is small aperture of fiber core, that limits the maximum pulse energy to some 100 µJ [6]. The possibility to achieve high doping levels makes Yb:KGW attractive for thin disk laser applications. Thin disk lasers are well suited for high pulse energies due to large apertures. It has been demonstrated that such lasers are well suitable for sub-500 fs pulses in 100 µJ energy range [7]. Such systems can be easily scaled to 1 mJ range. However, such designs tend to be quite complex due to multiply passes of short active element limiting in this way the use of such lasers in industrial applications. End-pumping schemes offer in most cases more compact and less expensive laser designs. Highest slope efficiencies have been reported for end-pumped configurations [1]. To our knowledge, highest average output power up to date is demonstrated with commercial chirped pulse amplification system Pharos [8], and reaches 6 W at repetition rates above 50 kHz. Pulse energies of ~1 mJ are available at repetition rates of 1 kHz. Although similar average output powers are available from demonstrated directly from power oscillators [9], which offer greater simplicity compared to chirped pulse amplification systems, but the finite TEM00 mode size limits power scaling of these lasers or results in multimode operation with lower beam quality. The promising method for average power scaling is the implementation of symmetric laser cavity with two active elements [10, 11]. Such an approach allows decreasing thermal load for laser active medium and stabilizes the laser operation over broad range of pump power. In this paper we present the results on development of high average power Yb:KGW regenerative amplifier based on dual active element architecture for femtosecond pulse amplification.

International Conference on Lasers, Applications, and Technologies 2007: Advanced Lasers and Systems, edited byValentin A. Orlovich, Vladislav Panchenko, Ivan A. Scherbakov, Proc. of SPIE Vol. 6731, 67312Y, (2007) ·

0277-786X/07/$18 · doi: 10.1117/12.753021

Proc. of SPIE Vol. 6731 67312Y-1

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2. EXPERIMENTAL SETUP The scheme of regenerative amplifier that employs two slab shaped Yb:KGW active elements is presented in Fig. 1 . The Yb:KGW crystals were cut for amplified pulse propagation along the c crystallographic direction. We have used slightly different active elements (AE1 length 5 mm, aperture 6x1,4 mm, doping rate 2% and AE2 length 6mm, aperture 6x1mm, doping rate 1,6%) each pumped by two 50W pump modules produced by Light Conversion, Ltd., and ensuring pump intensity on the crystal up to 70 kW/cm2. Resonator consists of flat mirrors V2, V5, V6 (reflectivity R=100%), V1 (R=98%) and spherical mirrors V3, V4 (radius r= -300 mm, R= 100%). Other components are f=150 mm lens (L), λ/4 phase plate FP1, λ/2 phase plate FP2, BBO Pockels cell PC and pair of the thin-film polarizers P. The polarization plane of pump radiation in all the cases was parallel to a crystallographic direction, while the plane of polarization of radiation oscillating in cavity was set by changing the angular orientation of phase plate FP2. Output coupling of cavity we varied by rotation of the phase plate FP1.

Fig. 1. Experimental setup.

3. CW OPERATION At the first stage of experiments we examined the simplified experimental scheme using single laser head with active element AE1. In this case the lens L was replaced by HR spherical mirror with r= 300 mm. Laser output characteristics for different polarizations were measured. When polarization of radiation oscillating in the cavity is set to be parallel to b crystallographic axis of Yb:KGW crystal , the ~26 W output power was reached. For perpendicular polarization (cavity supported polarization is parallel to a crystallographic axis) approximately 25 % lower output power was observed (see Fig. 2). The laser output power measurements were performed optimizing the transmission of the effective output coupling at the every value of pump power. The value of optimum transmission of the output coupler is rising along with increase of pump power and at maximum output power was evaluated as 22%. This value was calculated by comparing laser output power with reference to the power of radiation that was leaking through the mirror V1 (R=98%).



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Fig. 2. Single active element laser output power versus incident pump power in CW mode.

While examining laser operation during startup both laser beam profile and output power experience dramatic changes. Laser output power changes in time were observed with oscilloscope instantly opening blocked laser cavity. Output power peak was observed, witch gradually settles down (see Fig. 3). Ratio between settled down and peak values rise with increase of pump power. After startup, laser output power decrease exponentially to steady value.

Fig. 3. Laser output power during startup. Zero in time scale corresponds to moment when cavity is unblocked.

Such laser behavior indicates considerable thermal lens influence. Initial thermal lens strength in active element is conditioned only by absorbed pump. In Yb:KGW case, pump absorption saturation is reached in several kW/cm2 intensity range, thus most of pump passes the crystal without being absorbed. As cavity is unblocked and laser starts, population inversion decreases and pump absorption increases at the same time. The mean fluorescence wavelength (993 nm) is much lower than the laser wavelength (1030 nm) as well. That means that a stimulated photon generates more heat than a spontaneous photon. This leads to larger thermal lens strength in lasing conditions. In current experimental setup, laser cavity is optimized for lower thermal lens strength, thus output power peak is observed at startup. Such thermal lens influence on laser performance corresponds well to published data of Yb:KGW thermal lens



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measurements [12]. As well at some certain pump powers rapid output power fluctuations were observed. We believe those fluctuations are associated with transient processes of laser mode shaping

We also have examined the dynamics of the output beam profile formation. The beam profiles registered with CCD camera at different time moments after laser startup that are shown in Fig. 4 clearly show a significant beam profile alteration due to transitional thermal lens forming processes.

Fig. 4. Laser output beam profile at different moments after laser startup. Zero corresponds to moment when cavity is

unblocked.

Next step in our experiments was the examining of laser CW operation using full experimental set up, i.e. with both laser heads. As in previous experiments, laser output power versus pump was measured for different polarizations of radiation oscillating inside laser cavity and it was found out that output power was approximately 20-30% larger when polarization is set to be parallel to active elements b crystallographic axis (see Fig. 5). Maximum output power reached in CW mode was 44 W. Once again, optimum output mirror for each pump power value was measured, by optimizing output power with phase plate FP1. Optimum mirror dependency on laser output power is shown in Fig. 6. Comparison of these results and the ones obtained in single active element case shows that due to increased gain the optimum mirror transmission value is approximately twice bigger, and reaches 50-60% at the highest pump level.

Fig. 5. Dual active element laser output power versus incident pump power.



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Fig. 6. Optimum output coupler transmittance versus output power: (a) cavity supported polarization is parallel to b

crystallographic axis; (b) cavity supported polarization is parallel to a crystallographic axis.

Laser output beam profiles at different values of laser output power that was measured using CCD camera at the distance of 2 m from laser output mirror are presented in Fig. 7. It was found that when laser output is increased up to ~15 W the laser output beams profile is symmetric and nearly Gaussian. We note that slight modulation of registered beam profiles is not a real beam characteristic and is caused by Fabry-Pero cavity effects in the camera. Increasing of the output power above 15 W results in gradual raise of distortions of beams profile and the beam cross-section transforms into the slightly elliptical one. However, as seen in figure, distribution remains nearly Gaussian in traverse directions. The far field laser output beam profiles measured in focus of f=100 mm lens are presented in Fig. 8. At low power far field intensity distribution is very close to Gaussian. At high powers some background radiation around the central spot appears, however the energy content of this background is of order of only several percents. The measurements of beam quality parameter M2 indicated that even at the highest laser output powers the M2 values are below 1.5.

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Fig. 7. Laser output beam profile at different values of laser output power.



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Fig. 8. Beam profile at focus of f=100mm lens when laser output power is (a) 9W and (b) 40 W.

4. OPERATION IN Q-SWITCH MODE Q-switched operation of dual active element Yb:KGW laser was obtained using BBO crystal based Pockels cell LightGate 3 (Cleveland Crystals, Inc.). It was driven by Pockels cell driver PD4 (Bergmann Messgeräte Entwicklung KG ). Switching of the cavity to low-loss conditions were performed by applying ~ 2,2 kV pulse to Pockels cell. The moment of cavity dumping was regulated by pulsewidth of high voltage driving pulse and during the measurements was set to match the moment of the amplification saturation. In this work the Q-switched operation was investigated at 100 kHz repetition rate. Although lower repetition rates were available, the measurements were carried out only at 100 kHz, in order to prevent optical damage of active medium by milijoule energy pulses. Maximum average output power reached in Q-switched mode was ~34W (Fig. 9 a) at pump power of ~200 W. As in case of CW mode operation laser output power in Q-switch mode was approximately 25% larger when polarization is set to be parallel to b crystallographic axis of Yb:KGW crystal. As the pump power increases, the radiation build-up time and consequently the optimum cavity dump delay value decreases (Fig. 9 b).

Fig. 9. a) Laser output power versus incident pump in Q-switched mode. b) Optimal delay versus incident pump



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The output pulse duration corresponded to the roundtrip of laser cavity and was around 16 ns at full width of half maximum (FWHM). Nanosecond pulse spectra are shown in Fig. 10. Central wavelength of nanosecond pulse spectra for different polarizations differs by ~15 nm. In case when cavity supported polarization is parallel to crystallographic axis a we observed slight drift of the spectrum toward short wavelengths, while for polarization parallel to b crystallographic axis this effect was negligible. In both cases FWHM of the measured spectra was 3,5-4 nm.

Fig. 10. Output spectra of laser operated in Q-switch mode: (a) cavity supported polarization is parallel to b crystallographic

axis. b) cavity supported polarization is parallel to a crystallographic axis..

5. FEMTOSECOND PULSE AMPLIFICATION The dual active element Yb:KGW laser was tested in operation regime of regenerative amplification, seeding it by femtosecond pulses from commercial diode-pumped Yb:KGW oscillator of PHAROS laser system (Light Conversion, Ltd.). In our experiment the oscillator generated ~ 200 fs pulses duration and ~ 40 nm spectrum width pulses with energy of 3 nJ. These pulses were stretched to 200 ps by the diffracting grating based stretcher with positive group velocity dispersion and were directed to input of regenerative amplifier. The seed laser is protected against feedback by an optical isolator. A thin-film polarizer in combination with a Faraday rotator and a half-wave plate are used to separate the amplified pulses from the input pulses. Regenerative amplifier output dependence on pump power in case when the amplified pulse polarization was set parallel to pump polarization plane is presented in Fig. 11. Output power close to 30 W was reached at 200 W of incident pump.

Fig. 11. Regenerative amplifier output versus incident pump

The spatial characteristics of amplified radiation were basically the same as in the case of CW mode operation. As pump power is raised, the amplified pulse build-up time decreases and consequently the output pulses spectrum experiences less gain narrowing. Fig. 12 presents the ~ 5.4 nm FWHM spectrum of amplified pulses registered at maximum pump power. Relying on spectrum measurement in Q-switch mode we were expecting even broader by 15% spectrum width for the case when cavity supports amplified pulse polarization parallel to a crystallographic axis. Unfortunately, after



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several days of operation the active element with 1.4 mm thickness has fractured. We believe that 100 W of pump power is the limit for current experimental conditions crystal mount and cooling setup. It is planed that this work will be continued optimizing spectral bandwidth of femtosecond pulse regenerative amplification and performing recompression of amplified pulses down to sub-300 fs pulses durations.

Fig. 12. Amplified pulse spectrum.

6. CONCLUSIONS In this work we have demonstrated the potential of power scaling of femtosecond Yb:KGW regenerative amplifier by implementation of dual active element cavity configuration. 30W average output power at 100 kHz repetition rate was obtained, that corresponds to 0.3 mJ amplified pulse energy. Operation at higher repetition rates is feasible because in our case were limited only by choice of Pockels cell driver. Maximum spectrum width of 5,4 nm was measured in experiments that indicates the possibility to compress the amplified pulses down to sub-300fs durations. As output power increases, nearly symmetrical Gaussian profile beam becomes slightly elliptical, although remains Gaussian in traverse directions. The measured beam quality parameter M2 is below 1,5 even at maximum output power.

REFERENCES

1. J.H.Hellstrom, S.Bjurshagen, V.Pasiskevicius, J.Liu, V.Petrov, U.Griebner, „Efficient Yb;KGW laser end-pumped by high power diode bars“, Appl. Phys.B, 83, 235-239 (2006). 2. G.R.Holtom, „Mode-locked Yb:KGW laser longitudinally pumped by poliarization-couples diode bars“, Opt.Lett., 31, 2719-2721(2006). 3. F. Balembois et al. New Yb-doped solid-state laser materials for high power and femtoseconds lasers, EPS-QEOD Europhoton Conference, Solit-state and Fiber Coherent Light Sources, 2004 4. J.H.Hellstrom, S.Bjurshagen, V.Pasiskevicius, „Laser performance and thermal lensing in high-power diode-pumped Yb;KGW with athermal orientation“, Appl.Phys.B, 83, 55-59 (2006). 5. F.Roser et al ,“131 W 220 fs fiber laser system” Opt. Lett., 30, 2754-2756 (2005). 6. L.Shah, Z.Liu, I.Hartl, G.Imeshev, G.C. Cho and M.E. Fermann „High energy femtosecond Yb cubicon fiber amplifier“, Optics Express, 13, 4717-4722 (2005). 7. A.Beyertt, D. Nickel, A.Giesen, “Femtosecond thin-disk Yb:KYW regenerative amplifier”, Appl.Phys.B 00, 1-6 (2005). 8. http://www.lightcon.com/index.php?id=29,0,0,1,0,0 9. http://www.amplitude-systemes.com/produitv.php?mode=specifications&id_rub=179&id_prod_selec=0 10. J.Z.H.Yang, B,C.Walker, “0.09-terawatt pulses with a 31% efficient, kilohertz repetition-rate Ti:sapphire regenerative amplifier”, Opt.Lett., 26, 453-455 (2001). 11. E.C.Honea, R.J.Beach, S.C.Mitchell, J.A.Skidmore, M.A.Emanuel, S.B.Sutton, S.A.Payne, P.V. Avizonis, R.S.Monroe, and D.G.Harris “High-power dual-rod Yb:YAG laser”, Opt.Lett., 25, 805-807 (2000). 12. S.Chenais etal. Thermal lensing measurments in diode-pumped Yb-doped GdCOB, YCOB, YSO, YAG and KGW, Optical materials, 22, 129-137(2003).



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