Cheng Jian, Chen Mingjun, Liao Wei, Wang Haijun, Xiao Yong, Li Mingquan
Center for Precision Engineering, Harbin Institute of Technology, Harbin 150001, China.
Opt Express. 2013 Jul 15;21(14):16799-813. doi: 10.1364/OE.21.016799.
Micro-machining is the most promising method for KH(2)PO(4) crystal to mitigate the surface damage growth in high power laser system. In this work, spherical mitigation pit is fabricated by micro-milling with an efficient machining procedure. The light intensification caused by rear surface features before and after mitigation is numerically modeled based on the finite-difference time-domain method. The results indicate that the occurrence of total internal reflections should be responsible for the largest light intensification inside the crystal. For spherical pits after mitigation, the light intensification can be greatly alleviated by preventing the occurrence of total internal reflections. The light intensification caused by spherical mitigation pit is strongly dependent on the width-depth ratio and it is suggested that the width-depth ratio of spherical mitigation pit must be devised to be larger than 5.0 to achieve the minimal light intensification for the mitigation of surface damage growth. Laser damage tests for KH(2)PO(4) crystal validate that the laser damage resistance of initially damaged surface can be retrieved to near the level of ideal surface by replacing initial damage site with predesigned mitigation pit.
微加工是磷酸二氢钾（KH₂PO₄）晶体减轻高功率激光系统中表面损伤扩展的最具前景的方法。在这项工作中，通过高效的微铣削加工工艺制造出球形减轻坑。基于时域有限差分法对减轻前后晶体后表面特征引起的光强增强进行了数值模拟。结果表明，全内反射的发生是晶体内部光强增强最大的原因。对于减轻后的球形坑，通过防止全内反射的发生可大大减轻光强增强。球形减轻坑引起的光强增强强烈依赖于宽深比，建议将球形减轻坑的宽深比设计大于5.0，以实现减轻表面损伤扩展的最小光强增强。对KH₂PO₄晶体的激光损伤测试证实，通过用预先设计的减轻坑替换初始损伤部位，初始损伤表面的抗激光损伤能力可恢复到接近理想表面的水平。
