基于中红外光参量振荡器光束质量优化的90°像旋转四镜非平面环形谐振腔型设计与分析

Design and analysis of 90° image rotating four-mirror nonplanar ring resonator based on mid-infrared optical parametric oscillator beam quality optimization

为改善中红外光参量振荡器(OPO)激光输出光束质量,设计了一种90°像旋转四镜非平面环形腔型结构.通过建立单位球等效计算方法,对此种特殊腔型结构存在的像旋转角进行计算,并由此确定了适用于中红外OPO运行的90°像旋转谐振腔结构相关参数.在此基础上进一步建立了非对称轴环形腔中光场模式自再现模型,分析得出随着像旋转角由0°向90°变化,谐振腔内光场模式逐渐均匀化,当旋转角为90°时,基模以及高阶模都表现出非常好的中心对称性.基于此采用中红外ZnGeP2 OPO对所设计的腔型参数进行实验测量,实现了光束质量MX2=1.81和MY2=1.61.由此可以证明所设计的90°像旋转四镜非平面环形腔对中红外OPO激光系统的输出光束质量的优化有显著效果.

Mid-infrared optical parametric oscillator(OPO)operating in the mid-infrared transmission window(3-5μm wavelength range)is one of hot issues in the field of laser system.It has many applications in environmental detection,remote sensing,and medicine.Besides,this laser system is used as a key component of infrared countermeasures.The optical damage limit of nonlinear crystal is a great challenge to the mid-infrared OPO which is pumped by a nanosecond laser source.Therefore,the pump beam diameter should be appropriately increased to avoid damaging the crystal when scaling a nanosecond OPO to high pulse energy.The result of this design is that the Fresnel number in the cavity is increased and the beam quality is deteriorated.In order to improve the beam quality of mid-infrared OPO laser,a 90°image-rotating four-mirror non-planar ring resonator structure is designed.The advantages of this design include the general ring resonators,such as greatly reduced feedback into the pump laser and the avoidance of optical damage caused by standing wave cavity structure.Most importantly,the image rotating cavity can uniform the beam in the cavity and improve the beam quality.In this paper,the equivalent sphere representation of a four-mirror nonplanar ring resonator is established,and the image rotation angle of this special cavity structure is calculated.Based on this method,the parameters related to the 90°image rotating resonator structure suitable for mid-infrared OPO operation are designed.The self-reproduction of the transverse mode in the axially-asymmetric resonator is further established.It is found that the transverse mode in the resonator is gradually uniformed as the rotation angle of the image changes from 0° to 90°.When the rotation angle is 90°,the fundamental mode and the high-order mode both exhibit very good central symmetry.Finally,the mid-infrared ZnGeP2 OPO laser with the 90° image rotating resonator structure is used to verify the improvement of beam quality.The beam quality of M_X^2=1.81 and M_Y^2 = 1.61 are achieved.It can be proved that the 90° rotating four-mirror non-planar ring resonator has a significant effect on the optimization of the output beam quality of the mid-infrared OPO laser system.