Effects of pump pulse energy and repetition rate on beam quality in a high-power mid-infrared ZnGeP2 OPO

We report on changes of beam quality with rising pump pulse energy and repetition rate in a high-power mid-infrared ZnGeP2 (ZGP) OPO in a non-planar cavity. The ZGP OPO generates 25 W in 3 - 5 mu m pumped by a 10 kHz Q-switched Ho:LLF MOPA system at 2 mu m. The beam quality factor increases from nearly diffraction-limited at OPO threshold to a M-2 = 1.8 at maximum pump power. The degradation can be attributed to thermal lensing and gain guiding effects in the nonlinear crystal. To discriminate between those effects, we monitored the beam quality as a function of the repetition rate at the highest pump pulse energy using a Pockels cell as a pulse picker. Therefore, the heat load inducing thermal lensing increases linearly, whereas gain guiding remains nearly unaffected. M-2 shows a minimum of 1.2 at 1 kHz, starting from 1.3 at 100 Hz and rising to 1.8 at 10 kHz. Those findings have been reproduced by numerical simulations. Further investigations show the transient evolution of beam quality, revealing the thermal lens build-up process to the steady-state. (C) 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Automated summary

The study investigates changes in beam quality in a high-power mid-infrared ZGP OPO with increasing pump pulse energy and repetition rate, attributing degradation to thermal lensing and gain guiding effects. The findings were reproduced by numerical simulations, showing a transient evolution of beam quality and the build-up process of thermal lensing.