Multipass cell compression of an ultrafast Tm-fiber laser delivering 1.3 mJ, 130 W, sub-two cycle pulses at 1.9 µm

Intense few-cycle pulses in the short-wave infrared (SWIR, 1.3-3.0 µm) region have become popular tools for driving nonlinear frequency conversion processes. Compared with the well-established near-infrared laser sources, the longer driving wavelength can enhance conversion efficiency in two-color plasma terahertz generation [1], and enable efficient mid-infrared generation above 5µm employing non-oxide crystals [2]. Moreover, SWIR serve balanced trade-off between the phase matching cutoffs and the single emitter efficiency, as the driving wavelength for high harmonic generation towards the soft X-ray water window region (300 eV-500 eV) [3]. Laser sources in these three spectral regimes with high brightness and high photon flux can benefit numerous subsequent biological and medical applications [4,5]. Besides optical parametric chirped pulse amplification (OPCPA) laser systems [6,7], a promising alternative is post compression of ultrafast laser emissions, based on rare earth elements like chromium (Cr), holmium (Ho), and thulium (Tm). Recently, the multipass-cells (MPCs) have emerged as a propitious post-compression approach. Excellent throughputs and down to two- to single-cycle regime compressed pulses duration have been reported in the near-infrared [8,9]. On the other hand, only a few experiments have been reported in the SWIR region [10-13] with no shorter than ~20 fs output pulse duration. In this contribution, we report on a post pulse compression based on a gas-filled MPC delivering mJ-level, sub-two cycle pulses (11fs) with 130 W average power at 1900 nm wavelength. It is enabled by a high-power ultrafast thulium-doped fiber CPA system, employing coherent combination of four rod-type fiber amplifiers [14]. For this experiment, the driving laser provides approximately 90 fs pulses with 150 W average power at a repetition rate of 101kHz and a central wavelength of 1920 nm. The Herriott-type MPC consists of two concave mirrors arranged nearly concentrically in a gas-tight chamber. The chamber is filled with krypton as nonlinear medium for SPM-induced spectral broadening. The dispersion compensation is achieved by fused silica plates, offering negative GDD in this spectral region.