Chen Yuchuan, Gao Hang, Wang Xu, Guo Dongming, Liu Ziyuan
Key Laboratory for Precision and Non-traditional Machining Technology of Ministry of Education, Dalian University of Technology, Dalian 116024, China.
Materials (Basel). 2018 Mar 13;11(3):419. doi: 10.3390/ma11030419.
Laser induced damage threshold (LIDT) is an important optical indicator for nonlinear Potassium Dihydrogen Phosphate (KDP) crystal used in high power laser systems. In this study, KDP optical crystals are initially machined with single point diamond turning (SPDT), followed by water dissolution ultra-precision polishing (WDUP) and then tested with 355 nm nanosecond pulsed-lasers. Power spectral density (PSD) analysis shows that WDUP process eliminates the laser-detrimental spatial frequencies band of micro-waviness on SPDT machined surface and consequently decreases its modulation effect on the laser beams. The laser test results show that LIDT of WDUP machined crystal improves and its stability has a significant increase by 72.1% compared with that of SPDT. Moreover, a subsequent ultrasonic assisted solvent cleaning process is suggested to have a positive effect on the laser performance of machined KDP crystal. Damage crater investigation indicates that the damage morphologies exhibit highly thermal explosion features of melted cores and brittle fractures of periphery material, which can be described with the classic thermal explosion model. The comparison result demonstrates that damage mechanisms for SPDT and WDUP machined crystal are the same and WDUP process reveals the real bulk laser resistance of KDP optical crystal by removing the micro-waviness and subsurface damage on SPDT machined surface. This improvement of WDUP method makes the LIDT more accurate and will be beneficial to the laser performance of KDP crystal.
激光诱导损伤阈值（LIDT）是用于高功率激光系统的非线性磷酸二氢钾（KDP）晶体的一项重要光学指标。在本研究中，KDP光学晶体首先采用单点金刚石车削（SPDT）进行加工，随后进行水溶超精密抛光（WDUP），然后用355nm纳秒脉冲激光进行测试。功率谱密度（PSD）分析表明，WDUP工艺消除了SPDT加工表面上对激光有害的微波纹空间频率带，从而降低了其对激光束的调制效果。激光测试结果表明，WDUP加工晶体的LIDT提高，与SPDT加工晶体相比，其稳定性显著提高了72.1%。此外，建议后续的超声辅助溶剂清洗工艺对加工后的KDP晶体的激光性能有积极影响。损伤坑调查表明，损伤形态呈现出熔芯的高热爆炸特征和周边材料的脆性断裂，这可以用经典的热爆炸模型来描述。比较结果表明，SPDT和WDUP加工晶体的损伤机制相同，WDUP工艺通过去除SPDT加工表面的微波纹和亚表面损伤，揭示了KDP光学晶体真正的体激光抗性。WDUP方法的这种改进使LIDT更加准确，将有利于KDP晶体的激光性能。
