Journal of the Korean Physical Society, Vol. 49, No. 1, July 2006, pp. 424∼429 Development and Characterization of 100-kHz High-Peak-Power Femtosecond Laser Based on Downchirped Regenerative Amplification Kyung-Han Hong, * Sergei Kostritsa, Tae Jun Yu, Jae Hee Sung, Il Woo Choi, Young-Chul Noh, Do-Kyeong Ko † and Jongmin Lee Femto Science Laboratory, Advanced Photonics Research Institute, Gwangju 500-712 (Received 25 January 2006, in final form 25 April 2006) We report on the development of a GW-class femtosecond Ti:sapphire laser operating at the repetition rate of 100 kHz using downchirped pulse amplification (DPA) technique. A regenerative amplifier, acousto-optically switched and pumped by a 100-kHz Q-switched green laser, allows the high-energy amplification with a high repetition rate. The DPA-based dispersion compensation has helped us obtain 28 μJ, 39 fs laser pulses with a compression efficiency as high as 95 %. Temporal and spatial characterization of the pulses is presented. PACS numbers: 42.60.By, 42.65.Re Keywords: Femtosecond laser, Downchirped pulse amplification I. INTRODUCTION Since Kerr-lens mode-locking was successfully demon- strated with a Ti:sapphire laser in 1991 [1], Ti:sapphire laser technology has provided practical solutions of solid- state femtosecond light source with high average power as well as high peak power. Especially, compact high- average-power femtosecond lasers with a high repetition rate (kHz range) are good tools for basic and applied sci- ence. Typical high-repetition-rate Ti:sapphire amplifica- tion lasers using electro-optic pulse selection method op- erate at at the range from 10 Hz to 20 kHz [2–4] with an energy of ∼mJ, whereas those using acousto-optic pulse selection method can operate at 100 - 250 kHz with an energy of ∼μJ. As for the 100 - 250 kHz Ti:sapphire am- plification system, the regenerative amplifier setup sug- gested by Norris [5] has been a conventional configura- tion. Several features of the lasers based on Norris’s am- plifier configuration can be described as follows. First, they are pumped by a continuous-wave (CW) green laser and intracavity Q-switched for the suppression of prelas- ing. Second, the high-repetition-rate pulse injection and dumping is achieved by the acousto-optic cavity dumper composed of a Brewster-angle Bragg cell and an rf driver. Third, the dispersion control of the amplification sys- tem has been achieved either without [5,7] or with [8] a chirped-pulse amplification (CPA) technique [6]. In the case of non-CPA systems, pulses are naturally broad- ened and compressed by use of prism pairs, whereas, in the case of CPA system, transmission gratings are used for both stretching and compression. Adaptive control * E-mail: khhong@gist.ac.kr; Fax: +82-62-970-3389; † E-mail: dkko@gist.ac.kr has been also applied for the non-CPA system to com- press the laser pulse down to 35 fs [9]. The compression efficiency for those systems is 60 - 80 %. Despite higher repetition rate, 100 - 250 kHz systems have no advantage with average power because of much lower energy per pulse or even lower peak power : the highest energy and peak power reported at 100 kHz is 7 μJ and 0.2 GW [9] so far. Thus, the enhancement of the energy per pulse will make this kind of lasers be a more valuable tool not only for high-average-power ap- plications such as micromachining and medical surgery but also for high-peak-power applications such as high- order harmonic generation [10] and above-threshold ion- ization study. For the generation of higher peak power at 100 kHz, we demonstrate an efficient acousto-optically switched regenerative amplification system, which is modified from the conventional scheme in terms of pump- ing source and dispersion compensation method. First, we have employed a Q-switched pump laser, instead of a CW laser, to obtain higher pumping fluence and simplify the cavity by removing an intracavity Q-switcher. Use of a pulsed source allows more efficient pumping than CW source because the portion of the pump energy partici- pating in the population inversion process is higher than that of CW source. Also, accumulated dispersion and spectral narrowing due to an intracavity Q-switcher can be reduced. Second, we have applied downchirped-pulse amplification (DPA) technique [11] for higher compres- sion efficiency. In a DPA configuration, the laser pulse is stretched with negative dispersion elements (down-chirp), such as prism pair and grating pair, and then compressed by positive dispersion element which is just an optical glass block. The concept of DPA is essentially the same as -424-