Millijoule femtosecond pulses at 5 kHz from cw-pumped Ho:YAG regenerative amplifier

Summary form only given. The gain bandwidth of Ho-doped crystals is sufficiently broad to support femtosecond operation, which would allow pumping of ultrashort-pulse mid-IR optical parametric amplifiers (OPAs) [1,2] and, upon reaching adequate peak powers, eye-safe filamentation in the >2-μm transparency window of the atmosphere. In this contribution, we report on, to our knowledge, the first demonstration of a femtosecond cw-pumped Ho:YAG chirped pulse amplifier (CPA) system operating at room temperature and producing broadband pulses supporting ~440 fs pulse durations at the energy level of up to 3 mJ at a 5-kHz repetition rate. The CPA system consists of a seed source, stretcher, Ho:YAG regenerative amplifier (RA) and compressor. The RA is based on an AR coated Ho:YAG rod end-pumped by 70-W Tm fiber laser (IPG Photonics). Due to the unavailability of Faraday rotators capable to withstand high peak intensities, we have designed a ring-shaped RA cavity, in which the injection and output beam ports are separated and the amplification of the injected beam is unidirectional. Because of the ring-cavity design, the Pockels cell (RTP) is operated in the λ/2 regime.The RA is seeded with 2.1-μm pulses generated in a white-light-seeded two-stage OPA based on type I BBO and type II KTA crystals. In early experiments the OPA was pumped by a commercial 0.4-mJ Yb:KGW DPSSL system. More recently, we have demonstrated that the 10-μJ, 190-fs pulses from our monolithic Yb fiber amplifier [3] are sufficient to generate the 0.7-μJ, 260-fs pulses at 2.1 μm in the OPA. The output of the OPA is directed to the positive dispersion Öffner stretcher. When seeded with an unshaped spectrum at low pump powers, the RA generates narrowband, 2.4-nm FWHM, pulses centered at around 2090 nm. For higher pump powers, a weaker second band appears which is centered at the 2097 nm gain peak of Ho:YAG (Fig.1a). In order to compensate for gain- narrowing in the RA cavity, we have installed a simple mechanical amplitude shaper in the Fourier plane of the stretcher. The shaping, although at a cost of 85% of the seed pulse energy, results in a substantially broadened, > 12 nm FWHM, amplified spectrum. The amplified pulses are recompressed with a grating compressor based on a 600 l/mm grating pair. The pulse duration inferred from frequency-resolved optical gating (FROG) is 530 fs (Fig. 1b), whereas the calculated spectrum-limited pulse duration is ~440 fs. The output of the RA exhibits a rather good spatial profile (inset in Fig.1d) with the beam quality factor M2=1.3. In the ring cavity, picosecond pulses propagate only in one direction, while the Q-switched radiation develops in both directions, which allows monitoring of the Q-switched background in the direction opposite to the seed propagation and, consequently, evaluating the contrast of the amplified picosecond pulses (Fig. 1c). Measurements reveal that for achieving high contrast a strong seed is essential. Measurements using the full available pump power, presented in Fig. 1d, have proven the scalability of the RA toward a multi-mJ operation. The saturation of the output energy seen in Fig. 1d is due to worsened pump beam quality at high pump powers.