We report what we believe to be the first demonstration of cw and passively mode-locked Yb3+:YAlO3(Yb:YAP) laser operation under diode pumping. Spectroscopic properties of a 0.6 at.%Yb3+-doped YAP single crystal were investigated. Output power up to 1.2 W with slope efficiency of 64.5% in the cw regime and 225 fs pulse duration with average power of 0.8 W from a mode-locked Yb:YAP laser were demonstrated

Grigoriev, S. V.

Kisel, V. E.

Kuleshov, N. V.

Kurilchik, S. V.

Smirnova, S. A.

Yasukevich, A. S.

Repository of Belarusian National Technical University (BNTU)

2194 OPTICS LETTERS / Vol. 33, No. 19 / October 1, 2008Spectroscopy and femtosecond laser performanceof Yb3+:YAlO3 crystalViktor E. Kisel,1 Sergey V. Kurilchik,1,* Anatol S. Yasukevich,1 Sergey V. Grigoriev,1 Sofya A. Smirnova,2and Nikolay V. Kuleshov11Institute for Optical Materials and Technologies, Belarusian National Technical University,65 Nezavisimosti Avenue, Building 17, Minsk 220013, Belarus2Russian Research Institute for the Synthesis of Materials, 1 Institutskaya Street, Alexandrov 601650,Vladimir Region, Russia*Corresponding author: sergio-vk@me.byReceived June 30, 2008; revised August 22, 2008; accepted August 23, 2008;posted August 28, 2008 (Doc. ID 98152); published September 25, 2008We report what we believe to be the first demonstration of cw and passively mode-locked Yb3+:YAlO3(Yb:YAP) laser operation under diode pumping. Spectroscopic properties of a 0.6 at.% Yb3+-doped YAP singlecrystal were investigated. Output power up to 1.2 W with slope efficiency of 64.5% in the cw regime and225 fs pulse duration with average power of 0.8 W from a mode-locked Yb:YAP laser were demonstrated.© 2008 Optical Society of AmericaOCIS codes: 140.3380, 140.3480, 140.3615, 140.4050, 140.5680.Present widespread use of efficient InGaAs laser di-odes and favorable electronic structure of ytterbiumions stimulate interest in Yb3+-doped solid-state ma-terials for diode-pumped high-power lasers emittingin the spectral range near 1 m. Broad gain band-width of these media has stimulated their applica-tions for generation of ultrashort pulses in mode-locked regime.So far, the highest average output power of 60 Wwith ultrashort pulses has been obtained with a thin-disk Yb:YAG laser, passively mode-locked with asemiconductor saturable-absorber mirror (SESAM)[1]. The thermal conductivity of YAG is about13 W/ mK. However, the emission bandwidth ofYb:YAG limits the pulse duration to only 700–800 fsin a thin-disk laser, or to 340 fs in a bulk laser [2].Much shorter pulse durations were reported for anumber of crystals with broader emission bands,such as Yb-doped YVO4, CaGdAlO4, Y2SiO5,KGdWO42, KYWO42, KLuWO42, SrY4SiO43O,Ca4GdOBO33, Sr3YBO33, etc. [3–12], as well asphosphate and silicate glasses [13]. However, thesematerials have low thermal conductivity of approxi-mately 2–5 W/ mK, which limits their potential forhigh-power operation. With Yb-doped CaF2, whichhas thermal conductivity of 9.7 W/ mK, averagepower up to 880 mW with pulses as short as 150 fswas reported [14]; however, a rather low absorptioncross section of this crystal makes it not suitable forthin-disk lasers. Sesquioxides, such as Lu2O3 andSc2O3, that have high thermal conductivity of morethan 12 W/ mK are particularly promising materi-als to date. 370 fs pulses with 20.5 W average powerwith a mode-locked Yb:Lu2O3 thin-disk laser wererecently demonstrated [15]. Here we publish resultson spectroscopy and efficient cw and mode-locked la-ser operation of a Yb3+:YAlO3 single crystal as prom-ising material for thin-disk laser applications.Perovskite-like yttrium aluminate, YAlO3 (YAP), is16a biaxial crystal with the orthorhombic D2h space0146-9592/08/192194-3/$15.00 ©group. It has high thermal conductivity of11 W/ mK [16]. Some spectroscopic properties aswell as an energy-level diagram of Yb:YAP crystalswith Yb3+ concentration of 2 at.% were reported al-ready in 1971 [17]. Unpolarized absorption and emis-sion spectra of ytterbium-doped yttrium aluminateand comparison with other ytterbium-doped materi-als were made in [18]. More recently, comparison ofspectroscopic parameters of Yb3+-doped YAP andYAG single crystals was carried out, and self-absorption phenomena was studied [19,20]. It wasshown that Yb3+15 at.%  :YAP crystal is a potentialcandidate for compact, efficient thin-chip lasers; inaddition, its broad emission band allows the genera-tion of ultrashort pulses. The latest description andanalysis of Yb:YAP crystal growth, structural charac-terization, and spectroscopic properties were pub-lished in [21]. In this study, Boulon et al. reported ex-perimental decay-time dependence on Yb3+-dopantconcentrations, and room-temperature absorptionand emission cross section spectra along the a axiswere given. Radiative lifetime of an upper-laser levelwas estimated there to be 0.6 ms, which has a largediscrepancy with the decay time published in [19].In our study polarized absorption spectra ofYb3+0.6 at.%  :YAP at room temperature were mea-sured by a Varian CARY-5000 spectrophotometer.The content of ytterbium ions in crystal was mea-sured by means of a Tescan VEGA II LMU scanningelectron microscope with Oxford INCA Energy 350energy dispersive x-ray analyzer and was estimatedto be 1.171020 cm−3. The cross-section spectra forthree polarizations are shown in Fig. 1. Strong ab-sorption is found for E c light polarization. A peakabsorption cross-section at 978.2 nm is about 6.610−20 cm2, however, with comparatively narrowbandwidth of about 4 nm. For a and b polarizationsmaximal absorption cross sections near 978.5 nm are0.9710−20 cm2 and 1.3810−20 cm2, respectively.The stimulated emission cross sections are also givenin Fig. 1. They were calculated by use of a modified2008 Optical Society of AmericaOctober 1, 2008 / Vol. 33, No. 19 / OPTICS LETTERS 2195reciprocity method [22]. Lifetime measurementswere carried out with fine powderedYb0.6 at.%  :YAP crystal immersed in glycerol in or-der to suppress reabsorption [23]. The sample wasexcited by about 20 ns pulses at 982 nm. The emittedluminescence was measured with the use of a 0.3 mmonochromator, fast Ge-photodiode with a rise timeof 20 ns and a 500 MHz digital storage oscilloscope.All the samples exhibited single exponential decay(see inset in Fig. 2). With the decrease of weight con-tent of crystalline powder in suspension measuredlifetime also decreased (see Fig. 2). After considerabledilution of the powder and thus elimination of self-trapping effect the emission lifetime of ytterbium ex-cited state in YAP crystal was determined to be500±10 s. This value is even smaller than the re-sults published in [21] 0.6 s, which is to our beliefowing to more careful reduction of reabsorption influ-ence.Fig. 1. Polarized absorption and stimulated emissionspectra of Yb0.6 at.%  :YAlO3 crystal.Fig. 2. Measured lifetime for different weight content ofYb:YAP crystalline powder in glycerol suspension. Inset, ki-netics of luminescence decay.For a cw laser, experiments set up with a foldedcavity design were used. It consisted of two curvedmirrors with 100 mm and 300 mm radii of curvature,a plane output coupler (OC), and a plane end mirrorhighly reflecting at 1020–1100 nm. For the experi-mental assessment a 3-mm-long 0.6 at.% Yb:YAPBrewster-angled crystal cut perpendicular to thecrystallographic a axis was used and was oriented inthe cavity for polarization parallel to the b axis,where the stimulated emission cross section isslightly higher than that for c polarization (Fig. 1).For heat dissipation a gain medium was mountedonto a copper heat sink kept at 20°C. A cw fiber-coupled (=105 m, NA=0.15) InGaAs laser diodewith a maximum output power of about 6 W at980 nm wavelength was used for longitudinal pump-ing of the gain medium through the mirror with100 mm radius of curvature. The pump beam was fo-cused inside a Yb:YAP laser crystal into a 105 mspot with a confocal length of about 2.5 mm. Thecavity-mode diameter for TEM00 transversal mode atthe active element was calculated to be 98 m.Input-output diagrams for the Yb:YAlO3 CW laserwith OC transmissions of 3.5% and 5.5% are pre-sented in Fig. 3. Maximum output power of about1.2 W at 1040 nm for TEM00 mode with a slope effi-ciency of 64.5% with respect to the absorbed pumppower was obtained with 5.5% OC. For the laser withOC transmission of 3.5% the slope efficiency de-creased to 58% with output power of 1.12 W at1041 nm. Laser thresholds for the 3.5% and 5.5% out-put couplers were determined to be 0.85 W and1.05 W of the absorbed pump power, respectively.For the experiments in a mode-locked regime thesame cavity design was used with a SESAM havingmodulation depth of 1.6% and saturation fluence of30 J/cm2 substituted for a high-reflecting mirrorand two chirped mirrors, each providing second-orderdispersion of nearly −920 fs2 in the spectral rangefrom 1030 to 1065 nm, for obtaining negative group-delay dispersion in the cavity (Fig. 4). Stable cwmode locking was obtained for OC transmission of3.5%. Average output power up to 0.8 W at 1041 nmwith pulse duration of 225 fs and pulse repetitionFig. 3. Average output power of cw Yb:YAP laser versusabsorbed pump power for output couplers with differenttransmissions.2196 OPTICS LETTERS / Vol. 33, No. 19 / October 1, 2008rate of 70 MHz occurred, resulting in peak power ashigh as 44.5 kW. Intensity autocorrelation with asech2 fit and optical spectrum of the mode-lockedYb:YAP laser are presented in Fig. 5. During autocor-relator scanning a time range of 130 ps occurred af-ter laser pulse, and no other pulses were observed.They were not also seen on the oscilloscope with timeresolution less than 1 ns. The time-bandwidth prod-uct was estimated to be 0.32, which was very close tothe transform limit for soliton pulses p=0.315[24].In conclusion, for the first time, to our knowledge,efficient cw and mode-locked laser operation withYb3+:YAlO3 single crystal was demonstrated. Polar-ized absorption and stimulated emission cross-section spectra and luminescence lifetime of Yb:YAPwere determined. The cw operation of a Yb:YAP laserat 1040 nm with 1.2 W output power and slope effi-ciency as high as 64% with respect to the absorbedpump power were achieved. In a mode-locked regimesoliton-like pulses with duration of 225 fs and aver-age power as high as 0.8 W around 1041 nm were re-alized. We believe that the increase of output powerand pulse shortening could be achieved by optimiza-tion of Yb-dopant concentration and group-velocitydispersion compensation in the cavity. It is prospec-tive to grow samples with higher Yb ions doping con-centration and find out the maximum doping level forYAP. Yb:YAlO3 looks very promising for thin-disk ul-trafast lasers owing to high thermal conductivity,strong absorption at 985 nm, and comparativelybroad emission bandwidth. In the future it also is in-teresting to measure its nonlinear refractive indexand determine the possibility of using Yb:YAP forFig. 5. Intensity autocorrelation (left) with a sech2 -fit andoptical spectrum (right) of the mode-locked Yb:YAlO3laser.Fig. 4. Experimental setup of passively mode-lockeddiode-pumped Yb3+:YAP laser.Kerr-lens mode-locked ultrashort Yb lasers.References1. E. Innerhofer, T. Südmeyer, F. Brunner, R. Härring, A.Aschwanden, R. Paschotta, C. Hönninger, M. Kumkar,and U. Keller, Opt. Lett. 28, 367 (2003).2. C. Hönninger, R. Paschotta, M. Graf, F. Morier-Genoud, G. Zhang, M. Moser, S. Biswal, J. Nees, A.Braun, G. A. Mourou, I. Johannsen, A. Giesen, W.Seeber, and U. Keller, Appl. Phys. B 69, 3 (1999).3. V. E. Kisel, A. E. Troshin, V. G. Shcherbitsky, N. V.Kuleshov, V. N. Matrosov, T. A. Matrosova, M. I.Kupchenko, F. Brunner, R. Pashotta, F. Morrier-Genoud, and U. Keller, Opt. Lett. 30, 1150 (2005).4. Y. Zaouter, J. Didierjean, F. Balembois, G. L. Leclin, F.Druon, P. Georges, J. Petit, P. Goldner, and B. Viana,Opt. Lett. 31, 119 (2006).5. F. Thibault, D. Pelenc, F. Druon, Y. Zaouter, M.Jacquemet, and P. 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Spectroscopy and femtosecond laser performance of Yb3+:YAlO3 crystal

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Spectroscopy and femtosecond laser performance of Yb3+:YAlO3 crystal

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