KDP晶体前表面修复轮廓的光强调制能力影响

Influence of front-surface mitigation pits on distribution of light intensity inside KDP crystal

在高功率激光系统中,精密微机械修复是减缓磷酸二氢钾(KDP)晶体表面缺陷增长的有效方法,使用精密微铣削机床可以加工出球面型与高斯型修复轮廓。为得到最优的修复结构参数,建立了晶体前表面球面型与高斯型修复轮廓的电磁场有限元模型,通过改变轮廓的宽度、深度等参数,对两种修复轮廓的光强调制能力进行对比研究。仿真结果表明光强调制能力主要是由修复轮廓的衍射效应及入射光在修复界面处的二次入射所引起的干涉作用共同决定;针对初始损伤点,建议采用宽深比大于5的修复轮廓,从而有效提高KDP晶体表面缺陷点的激光损伤阈值,对于宽深比大于10的修复轮廓建议选用高斯型;对宽1000μm,深20μm的两种修复表面的激光损伤实验表明,高斯型修复轮廓具有较高的抗损伤能力,实验与仿真结果相一致。

Micro-machining is the most effective method for KDP crystal to mitigate the surface damage growth in highpower laser systems. In this work, spherical and Gaussian mitigation pits are fabricated by micro-milling with an efficient machi-ning procedure. The light intensification caused by front-surface features after mitigation is numerically modeled based on the fi-nite element method （FEM） for acquiring the optimal structural parameters of mitigation pits. The results indicate that the occur-rence of diffraction from the pits is principally responsible for the light intensification inside the crystal, and the interference fromthe second incident laser on the pit＇s fringe results in the light intensification, which unsteady increases with the decreasing of thepit＇s wide-depth ratio. For spherical and Gaussian pits after mitigation, it is suggested that the width-depth ratio of spherical mit-igation pit should be devised to be larger than 5.0 to avoid the second incident light, achieving the minimal light intensification forthe mitigation of surface damage growth When the wide-depth ratio of the mitigation pit is larger than 10.0, Gaussian contour ispreferred to achieve better repairing quality. The laser damage test on the mitigation pits with 1000 /μm-width and 20 /μm-depthhas effectively verified the simulation results.