A high-energy 100-Hz optical parametric oscillator (OPO) based on a confocal unstable resonator with a Gaussian reflectivity mirror was demonstrated. A KTA-based OPO with a good beam quality was obtained when the magnification factor was 1.5, corresponding to the maximum signal (1.53 µm) energy of 56 mJ and idler (3.47 µm) energy of 20 mJ, respectively. The beam quality factors (M²) were measured to be M²_x = 5.7, M²_y = 5.9 for signal and M²_x = 8.4, M²_y = 8.1 for idler accordingly. The experimental results indicated that the beam quality positively changed with the increase of magnification factors, accompanied by an acceptable loss of pulse energy.

Beam quality improvements by a large margin for signal and idler beams of a high energy 100 Hz KTiOAsO4 (KTA) non-critical phase matching (NCPM) optical parametric oscillator (OPO) were demonstrated using an unstable resonator configuration instead of a plane-parallel one. Theoretically, influences of cavity lengths and transmission of an output coupler on the OPO conversion efficiency for both were numerically simulated. For OPO based on an unstable resonator with a Gaussian reflectivity mirror, the maximum pulse energies at the signal (1.53 µm) and idler (3.47 µm) were about 75 mJ and 26 mJ, respectively. The corresponding beam quality factors of the signal were M_x² = 9.8 and M_y² = 9.9, and M_x² = 11.2 and M_y² = 11.5 for the idler. As a comparison, 128 mJ of signal and 48 mJ of idler were obtained with the plane-parallel resonator, and the M² factors of the signal were M_x² = 39.8 and M_y² = 38.4, and M_x² = 32.1 and M_y² = 31.4 for the idler. Compared with a plane-parallel cavity, over eight times and three times brightness improvements were realized for the signal and idler light, respectively.

2 μm、中波红外3~5 μm及长波红外8~12 μm波段的激光处于大气传输窗口，在激光医疗、环境监测、激光雷达、化学遥感和红外对抗等领域有着非常广阔的应用前景。基于非线性频率转换技术，采用非线性光学晶体在实现中长波红外固体激光输出方面具有结构简单、宽调谐和高功率等技术优势。尤其是使用2 μm单掺Ho固体激光器泵浦ZnGeP₂晶体，在3~5 μm和8~10 μm中长波红外输出中性能优异。在平均输出功率方面，目前可达到102 W@3~5 μm、12.6 W@8.2 μm以及3.5 W@9.8 μm的输出水平，光束质量M²均小于3，其中中波的光光转换效率可达60%。文中针对2 μm单掺Ho固体激光器及ZnGeP₂晶体在中长波输出方面进行了总结。

We demonstrate a Fe:ZnSe laser gain-switched by a ZnGeP₂ optical parametric oscillator (OPO) under the pulse repetition frequency of 1 kHz at room temperature. The 2.9 μm signal light of the OPO is employed as the pump for the Fe:ZnSe laser. The maximum output power of the Fe:ZnSe laser is 58 mW with the pulse duration of 2.7 ns under the incident pump power of 280 mW, corresponding to a peak pulse power of 21.5 kW and an optical-to-optical efficiency of 20.7%. The spectrum of the Fe:ZnSe laser has a range of 4030.2–4593.6 nm with a dip at 4187.1–4340.4 nm due to the absorption of CO₂.