KDP晶体相位匹配角理论预测模型及其验证分析

在高功率固体激光器的终端光学组件内, 大口径薄型KDP (KH₂PO₄)晶体的精密装配和校准是实现惯性约束核聚变的关键技术之一. 为了达到晶体在线安装高效高精度的要求, 需要测量高功率激光三次谐波转换效率达到最高时的晶体相位匹配角分布. 本文针对Ⅰ/Ⅱ类大口径薄型KDP晶体三次谐波转换的方式, 根据晶体的非线性光学属性获得了晶体不同位置相位匹配角之间的关系; 根据激光束在晶体内的传输路径分析得到了晶体面形、相位匹配角与激光三次谐波转换效率达到最高时 晶体最佳偏转角之间的相互关系. 在此基础上, 建立了Ⅰ/Ⅱ类KDP晶体相位匹配角的理论预测模型, 并利用实验进行了验证和分析. 实验结果表明, 晶体相位匹配角的预测值与实验值之差在10.0 μrad以内, 验证了Ⅰ/Ⅱ类KDP晶体相位匹配角理论预测模型的正确性, 为获得晶体全口径相位匹配角分布提供了简单、高效的预测方法.

Theoretical mo del of phase-matching angles for KDP crystals and its verification analysis

In final optics assembly of high-power solid-state laser, in order to improve the third harmonic generation efficiency, the accurate assembly and calibration of ultra-thin KH₂PO₄ (KDP) crystal with large-aperture is one of the key technologies to realize inertial confinement fusion. In order to meet the requirements for high efficiency and precision crystal of online installation, it is necessary to measure crystalline phase matching angle for achieving the highest third harmonic conversion efficiency of high power laser. In this paper, for the third harmonic conversion by ultra-thin type Ⅰ/Ⅱ KDP crystals with large-aperture, the relationship between phase matching angles at different locations on the crystal is obtained according to the nonlinear optical properties of the crystal. Based on the analysis of the propagation path of the laser beam in the crystal, the relationship among the crystal surface shape, the phase matching angle and the best deflection angle is given. On this basis, the theoretical model for phase-matching angle of type Ⅰ/Ⅱ KDP crystal is proposed, and verified by the experimental results. The results show that the difference in phase matching angle between the prediction values and the experimental results is within 10.0 μrad, showing that the theoretical model for phase-matching angles of type Ⅰ/Ⅱ KDP crystals is valid. This model provides a simple and efficient prediction method to obtain the phase matching angle distribution in full aperture of KDP crystal.