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 μm pumped by a 10 kHz Q-switched Ho:LLF MOPA system at 2 μm. The beam quality factor increases from nearly diffraction-limited at OPO threshold to a M² = 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. M² 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.

中文翻译：

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泵浦脉冲能量和重复频率对大功率中红外ZnGeP ₂ OPO中光束质量的影响

我们报告了在非平面腔中的高功率中红外ZnGeP ₂（ZGP）OPO中，光束质量随泵浦脉冲能量和重复频率的升高而变化的情况。所述ZGP OPO生成3 25 W - 5 μ米×10千赫的Q开关何泵浦：LLF MOPA系统在2 μ微米。光束质量因数从OPO阈值附近的几乎衍射极限增加到M ²=在最大泵浦功率下为1.8。退化可归因于非线性晶体中的热透镜效应和增益引导效应。为了区分这些影响，我们使用Pockels单元作为脉冲选择器，在最高泵浦脉冲能量下，根据重复率来监视光束质量。因此，引起热负荷的热透镜线性增加，而增益导引几乎不受影响。M ²在1 kHz时显示最小值1.2，从100 Hz时的1.3开始到10 kHz时的1.8。这些发现已通过数值模拟再现。进一步的研究表明光束质量的瞬态演变，揭示了热透镜积累过程达到稳态。