200-fs, 2-mJ pulses from 1-kHz Yb 3+ ,Na + :CaF2 Cryogenic Amplifier G. Andriukaitis 1 , A. Pugžlys 1 , L. Su 2 , J. Xu 2 , R. Li 3 , W. J. Lai 4 , P. B. Phua 4 , A. Marcinkevičius 5 , M.E. Fermann 5 , L. Giniūnas 6 , R. Danielius 6 , A. Baltuška 1 1. Photonics Institute, Vienna University of Technology, Gusshausstrasse 27-387, A-1040, Vienna, Austria. 2. Shanghai Institute of Cemaics, Chinese Academy of Sciences, Address Јє1295 DingXi Rd., Shanghai,200050,P.R.China. 3. Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, 390# Qinghe Rd., Shanghai, 201800, P.R. China. 4. Nanyang Technological University, 21 Nanyang Link, Singapore 637371. 5. IMRA America Inc., 1044 Woodridge Ave.,Ann Arbor, MI 48105, USA. 6. Light Conversion Ltd., P/O Box 1485, Saulėtekio av. 10, LT-10223 Vilnius, Lithuania. Yb 3+ -doped CaF2 is in the focus of attention for developing high peak- and average power tunable femtosecond oscillators and amplifiers [1,2] because of a low quantum defect, high damage threshold, low linear and nonlinear refractive indices and suitability for direct pumping with laser diodes. Recently, a modified host, Yb 3+ ,Na + :CaF2 was introduced, where Na + is presumed to act as a charge compensator [3]. Cooled to cryogenic temperatures (the interest in cryogenically cooled amplifiers is determined by a number of advantages expected at low temperatures [4]), both Yb 3+ : and Yb 3+ ,Na + :CaF2 crystals exhibit a significant increase of absorption cross-section in the vicinity of zero phonon line (ZPL), although ZPL preserves it’s width, and disappearance of the ground state absorption beyond 1000 nm [5]. In this contribution, we present the latest developments of Yb fiber laser seeded, cw pumped, cryogenically cooled kHz-repetition-rate Yb 3+ ,Na + :CaF2 regenerative amplifier (RA). Previously, several LN2-cooled Yb RAs at 10 Hz were developed by Yamakawa and coworkers [6]. In the RA cavity an AR-coated 2%Yb 3+ 3%Na + :CaF2 4-mm long slab is mounted inside a cryogenic chamber cooled to temperature of 110K by a closed-loop refrigerator (CryoTiger). The crystal is pumped in a double-pass configuration by two 60-W laser bars. The output of the RA has excellent spatial profile (Fig. 1a) and rather high energetic efficiency (Fig. 1b): cw lasing at the level of ~14 W while pumping with ~ 60 W can be achieved. In Q-switched regime 3-mJ ns pulses at a 1-kHz repetition rate were generated while in the ps operation mode seed pulses were amplified to 2 mJ level. The incident pump power on the crystal was about 60 W and 36 W respectively. At 20 kHz repetition rate ps pulses were amplified to 300 µJ level, i.e. an average power of 6 W was extracted from the RA. Neither pulse train saturation nor bi-stable behavior of the pulse train was observed at 2-mJ energy. Further amplification was preveted by the optical damage of AR coatings of the crystal. In the ps operation the RA was seeded with stretched in a positive dispersion stretcher pulses from ~20-nm- FWHM femtosecond Yb fiber laser. Seeded with an unshaped spectrum, the amplifier supports ~ 5 nm bandwidth centered at 1031 nm. By installing in the Fourier plane of the stretcher an amplitude shaper we were able to broaden the spectrum of the RA output to 12 nm FWHM (solid curve, Fig.1c). Amplified pulses were recompressed with a grating compressor (1700 lines/mm, grating separation ~ 200 mm). The results of pulse characterization with SHG FROG are presented in Fig.1 c-f and reveal a 195-fs duration, whereas the calculated spectrum-limited pulse duration is ~130 fs, suggesting that a more careful higher-order dispersion control is required to handle the rather broad bandwidth obtained from Yb,Na: CaF2 amplifier. Fig. 1. Overview of amplifier performance. (a), spatial profile of the beam, (b), dependence of the output vs. pump power in the case of seeded amplifier operation, (c-f), characterization of recompressed pulses with SHG FROG. In conclusion, by merging Yb fiber oscillator and cryogenically cooled DPSS Yb 3+ ,Na + : CaF2 RA we have generated 195-fs 2-mJ pulses at a 1-kHz repetition rate. Amplification substantially beyond 2 mJ is feasible by improving the quality of surface polishing and AR coatings, as well as by optimizing the RA cavity. Spectral shaping of the seed is crucial for amplification of broad spectra and shows potential for attaining ~100-fs pulses. 0 1 1010 1020 1030 1040 1050 -4 -2 0 2 4 -1000 0 1000 0 1 -3 -2 -1 0 1 2 3 Measured Retrieved from FROG Spectral Intensity Phase [rad] Wavelength [nm] Retrieved from FROG TL Temporal Intensity 195 fs Phase [rad] Time [fs] (d) (c) -2000 0 2000 505 507 509 511 513 515 517 519 521 Delay [fs] Wavelength [nm] -2000 0 2000 506 510 514 518 522 Delay [fs] Wavelength [nm] (f) (e) Measured Retrieved 20 30 40 50 60 70 80 0 1 2 3 4 5 6 Output power, W Pump Power, W 1 kHz 20 kHz Coating damage at 1 kHz (b) (a) M 2 = 1.05 References [1] M. Siebold, M. Hornung, S. Bock, J. Hein, M. C. Kaluza, J.Wemans, and R. Uecker, "Broad-band regenerative laser amplification in ytterbium-doped calcium fluoride (Yb:CaF2)", Appl. Phys. B 89, 543 (2007) and references therein. [2] M. Siebold, M. Hornung, R. Boedefeld, S. Podleska, S. Klingebiel, C. Wandt, F. Krausz, S. Karsch, R. Uecker, A. Jochmann, J. Hein, and M. C. Kaluza, "Terawatt diode-pumped Yb:CaF2 laser", Opt.Lett. 33, 2770 (2008). [3] L. Su, J. Xun, H. Li, W. Yang, Z. Zhao, J. Si, Y. Dong, and G. Zhou, "Codoping Na+ to modulate the spectroscopy and photoluminescence properties of Yb3+ in CaF2 laser crystal", Opt. Lett. 30, 1003 (2005). [4] D. C. Brown, "The promise of cryogenic solid-state lasers", IEEE J. Sel. Top. Quantum Electron. 11, 587 (2005). [5] A. Pugzlys, D. Sidorov, T. Ali, A. Baltuska, L. Su, J. Xu, R. Li, L. Giniunas, and R. Danielius, "Spectroscopic and lasing properties of cryogenically cooled Yb,Na:CaF2", in Advanced Solid-State Photonics, OSA Technical Digest Series (CD), paper MF4 (2008). [6] J. Kawanaka, K. Yamakawa, H. Nishioka, and K. Ueda, "30-mJ, diode-pumped, chirped-pulse Yb:YLF regenerative amplifier", Opt. Lett. 28, 2121 (2003). 978-1-4244-4080-1/09/$25.00 c 2009 IEEE