大口径KDP晶体加工相位扰动与三次谐波转换

Phase Perturbation of Large Aperture KDP Crystal Manufacture and Effects on Third Harmonics Generation

根据神光Ⅱ三次谐波转换中的各种现象,分析了晶体中高频周期相位调制的影响。目前国内大口径KDP晶体在中高频段存在强烈周期性相位调制,相位调制周期约20mm,调制幅度约35nm,导致三次谐波近、远场产生明显的强度调制,实验测得三倍频的近场周期条纹对比度在0.1～0.3之间,周期约12mm,理论分析该周期相位调制导致光束下游元件产生自聚焦风险明显增大,并且会引起三倍频远场畸变分裂,可聚焦能力下降。中高频段的周期调制可能来自于晶体加工过程中真空吸附,需要进一步实验判断并在加工中消除周期性的相位扰动。

The effects of period phase perturbation of large aperture KDP crystal in middle spatial frequency are analyzed based on the results of experiments of Shenguang II. The period of phase perturbation of crystal reflection wavefront is near 20 ram, and the modulation amplitude is about 35 nm, the period of the third harmonics near field is about 12 mm, the contrast of strip is between 0.1 and 0.3, which leads to 3ω far field distortion and the probability of self-focusing increase. This cycle modulation at medium high frequency is possible caused by vacuum siphon in the process of crystal manufacture, and it needs to be verified in the follow-up experiment.