Compact and efficient sub-10 ps pump sources at 2 µm for the generation of coherent mid-infrared radiation

Abstract

Ultrashort pulse laser systems in the 2 µm wavelength region featuring high pulse energies are powerful tools for driving a multitude of different applications in industry, medicine, and fundamental science. The implementation of such laser sources remains challenging and usually relies on the chirped-pulse amplification (CPA) in regenerative amplifiers. Here, a much more simplified concept based on a CPA-free multipass amplification scheme operating at room temperature has been investigated. I show that optical pulses with moderate sub-10 ps duration can be amplified up to the millijoule energy level without the onset of nonlinear effects in holmium-doped crystals. The laser system consists of an ultrafast all-fiber mode-locked oscillator and power amplifier based on holmium-doped silica fiber. It has been spectrally tailored to efficiently seed subsequent amplifiers based on holmium-doped YLiF4 crystals. A multipass amplification concept was used to amplify the nJ-level seed pulses from the fiber front-end up to 100 µJ of pulse energy at a pulse repetition frequency of 50 kHz. The maximum pulse energy was limited only by the laser-induced damage threshold of the amplifier crystals. Further pulse energy scaling has been achieved in a final single-pass booster amplifier generating 1.2 mJ at 1 kHz. The overall gain in the Ho:YLF crystals amounts to > 51 dB. Taking into account a measured pulse duration of 8.3 ps, this yields a pulse peak power of 136 MW. These results have been supported by numerical simulations based on a modified Frantz-Nodvik formalism, which is capable of modeling chromatic effects as well as a detailed description of the energy built-up in such amplifiers. Up to 50 µJ at 100 kHz from the multipass amplifier have been used to pump an optical parametric generator/amplifier tandem configuration based on the highly nonlinear non-oxide crystal ZnGeP2. The phase-matching condition has been set to achieve a signal and idler center wavelength of 3 µm and 6.5 µm, respectively. The maximum signal and idler pulse energy was 7.7 µJ and 2.5 µJ. Considering a measured pulse duration of 4 ps for both wavelengths, a peak intensity of about 2 MW (signal) and 0.5 MW (idler) was reached. The mid-IR coherent source is wavelength tunable covering the spectral range from 2.5 to 8 µm under appropriate phase-matching conditions and has shown long-term stability of less than 1.25%rms low-frequency power noise.