Mid-IR Kerr-lens mode-locked polycrystalline Cr2+: ZnS lasers

This paper summarizes recent improvements of output characteristics of polycrystalline Cr:ZnS/Se master oscillators in Kerr-Lens-Mode-Locked regime: 1.9 W average power at 41 fs pulse duration, 24 nJ pulse energy and 515 kW peak power with efficiency of 19% with regards to 1567 nm pump power from linearly polarized Er-fiber laser. A simple design of mid-IR fs Cr:ZnS MOPA enabled power scaling to 6.8 W at 79 MHz repetition rate. This was accompanied by a 2 fold spectral broadening to 600 nm at -10 dB level, pulse compression from 44 to <30 fs, and overall 25 % optical to optical efficiency. Improved dispersion management of the resonator enabled pulse duration of Cr:ZnS master oscillator approaching 2 optical cycles (<26 fs) and 500 nm (27 THz) bandwidth of the spectrum at half-maximum. Further improvements of the optical coatings will result in octave-spanning polycrystalline Cr2+:ZnS/ZnSe lasers. In this work we also report on recent progress in spinning ring gain element technology and show new unprecedented output power levels for Cr:ZnSe laser gain media: similar to 40 W at 2400-2500 nm spectral range and similar to 32 W at 2940-2950 nm in CW regime of operation. High gain of the spinning ring Cr:ZnSe power amplifier demonstrated in this work may potentially enable scaling up the femtosecond mid-IR Cr:ZnS MOPA up to 70-100W.