Appl. Phys. B 68, 169–175 (1999) Applied Physics B Lasers and Optics  Springer-Verlag 1999 High-peak-power femtosecond Cr:forsterite laser system T.Togashi, Y. Nabekawa, T. Sekikawa, S. Watanabe Institute for Solid State Physics, University of Tokyo, 7-22-1 Roppongi, Minato-ku, Tokyo 106-8666, Japan (Fax: +81-3/3478-4496, E-mail: tadashi@evans.issp.u-tokyo.ac.jp) Received: 25 June 1998 Abstract. We have developed a high-peak-power Cr:forsterite laser based on a regenerative ampliﬁer. The output energy was 0.4 mJ with a pulse duration of 77 fs at a 10 Hz repetition ratio, resulting in a peak power of 5.2 GW, which as far as we know is the highest power ever obtained from Cr:forsterite laser. To switch the regenerative ampliﬁer rapidly, we de- veloped a Pockels cell driver based on fast high-voltage tran- sistor switches. We also investigated the effects of excited- state absorption on the gain numerically and experimentally. The way to optimize the regenerative ampliﬁer is indicated by the analytical results obtained using a model including absorption in the crystal and excited-state absorption. PACS: 42.60; 42.55; 42.79 Chromium-doped forsterite (Cr 4+ :Mg 2 SiO 4 ) was the ﬁrst near-infrared tunable solid-state laser material that employed Cr 4+ as an active ion. Since the operation of Cr 4+ :forsterite lasers was ﬁrst demonstration several years ago [1–3], the spectroscopic and laser properties of forsterite have been in- vestigated extensively, [4–9] and mode-locked picosecond and femtosecond operation [10–14] has resulted in pulses as short as 20 fs [13]. This material offers a broadband gain near 1.3 μ m, a near-infrared wavelength region not accessible by Ti 3+ :sapphire and Cr 3+ -based lasers. Furthermore, the sec- ond harmonic of a Cr:forsterite laser ﬁlls the gap between the fundamental and the second harmonic of a Ti 3+ :sapphire laser [15]. Recently, high-peak-power, ultrashort pulse laser sys- tems have been developed by using Ti 3+ :sapphire as a gain medium [16–20]. These systems were made possible by the chirped-pulse ampliﬁcation (CPA) technique [21], and we have applied this technique to the Cr 4+ :forsterite laser system. Cr 4+ :forsterite offers potential advanstages over Ti 3+ :sapphire with regard to the scaling up of output power. The quantum efﬁciency (laser wavelength/pump wavelength) is higher than that of Ti 3+ :sapphire if we use Nd:YAG lasers as a pump source. The maximum available pump energy is twice that of Ti 3+ :sapphire. Nonetheless, extensive efforts to obtain higher peak power have not yet been made. An output energy of 0.1 mJ was obtained before pulse compression of a CPA system [22]. The main limitation to the peak power that can be obtained using Cr 4+ :forsterite seems to be the excited-state absorption (ESA) at high-ﬂuence pumping. In the work described in this paper, we found the inﬂuence of ESA on the gain obtained experimentally was in good agree- ment with the result of numerical calculation. We constructed a CPA system based on a regenerative ampliﬁer, and obtained 5.2 GW peak power with a pulse duration of 77 fs. This is the highest peak power ever obtained from a Cr 4+ :forsterite laser. The results obtained experimentally are in good agreement with the results calculated. An optimum design of the regen- erative ampliﬁer is proposed on the basis of the model used in the above-mentioned analysis. 1 Kerr-lens mode-locked oscillator and pulse stretcher The Kerr-lens mode-locked (KML) oscillator we used was an asthmatically compensated Z-fold cavity [23]. A 2-cm-long Brewster-cut Cr:forsterite crystal was set between concave mirrors with a 10 cm radius of curvature. The crystal was mounted in a heat sink cooled to −15 ◦ C by an antifreeze so- lution and was pumped by 6W CW Nd:YAG laser (Spectra- Physics’ model 3800) focused by a lens with a focal length of 9 cm. A 1% output coupler was used at one end of the cavity, and a pair of SF6 prisms 17.5 cm apart was inserted in order to compensate the group velocity dispersion in the cavity. This ocsillator produced approximately 0.3 nJ, 53 fs pulses at a 118 MHz repetition rate. The duration of the pulses was measured by the slow-scan autocorrelation, assuming a sech 2 intensity proﬁle (Fig. 1). The spectral width of the pulses was 50 nm (FWHM) (Spectrum C in Fig. 10), which gave the time–bandwidth product of 0.50. The pulse duration was stretched from 53 fs to ∼ 50 ps by a conventional stretcher [24] consisting of a grating with a groove density of 600 lines/mm, a lens with a focal length of 700 mm, and two plane mirrors. All the reﬂective elem- ents, including a grating and mirrors, were coated with gold to make them highly reﬂective. The incident angle of the grat-