Mid-infrared, single crystal, linear cavity optical parametric oscillators based on ZnGeP2

Abstract

We present our efforts to power-scale the output from a single crystal, linear cavity Zinc Germanium Phosphide (ZnGeP2) optical parametric oscillator (OPO). Our initial doubly-resonant OPO produced a maximum total output power of 1.1 W while double-pass pumped with a 7 W 150 ns pulsed Ho:YLF laser. Next, we developed a pump source with higher power (38 W) and shorter pulse lengths (65 ns). This resulted in the demonstration of a maximum 5 W output from the OPO. However, the higher pump power could no longer pass through an optical isolator (limited damage threshold) and single pass pumping had to be used to prevent feedback to the pump laser. In addition, a longer folded OPO cavity had to be used to rid the cavity of the transmitted pump light using available optics. Overall, this resulted in a slightly less efficient optical to optical conversion (14 % vs. 15 %). To improve efficiency the OPO cavity was subsequently optimised in terms of pump and cavity mode sizes and output coupler mirror reflectivity (50 % vs. 80 % at 3 to 5 μm). The output coupler was also specified for high transmission of the pump allowing a short cavity length of only 20 mm with a ZnGeP2 crystal length of 15 mm. Operating near degeneracy, a total output power of 14 W was achieved when single-pass pumped with 48.8 W (conversion efficiency of 29 %).