三倍频光学系统中近场调制增长规律研究

高功率固体激光装置的光束质量控制既要满足精密物理实验的苛刻要求,又要保证激光装置在高负载条件下的安全运行,是建造激光器的核心环节。三倍频光学系统承担着将1.053 μm的基频光转换成0.351 μm的三倍频光并聚焦到靶上的任务,是影响激光装置光束质量的重要环节。基于B-T理论研究了非线性传输条件下三倍频光学系统中光束调制的增长规律,基于典型的KDP晶体面形数据建立的中高频相位屏模型,通过数值计算获得了三倍频光学系统不同B积分下光束近场的变化规律。研究表明,三倍频光学系统中最快增长频率和相应的最快增长因子都是为基频光系统的三倍,空间调制周期在0.1～1 mm之间的空间频率成分增长最快。当三倍频光学系统B积分的设计值为1.5,要求三倍频光近场调制度小于1.15时,中高频噪声的RMS值必须控制在1.6 nm以下。

Increase Law of Near-Field Beam Modulation in 3ω Optics System

Beam quality control is the most important segment in high power solid state laser facility construction, for the rigid requirement of precise physics experiments and the operation safety of laser facility in high capacity. The 3ω optics system converts the 1ω beams to the third harmonic, focuses them onto the target and is an important segment influencing beam quality. Based on the B-T theory, increase of beam modulation in 3ω optics was derived under the condition of nonlinear propagation. By the medium-high phase plate constructed according to the surface profile of typical KDP crystals, influence of ∑B3ω on beam near-field quality was investigated. The results show that the fastest increasing frequency and the corresponding fastest increasing factor in 3ω optics system are triple of those of 1ω system, and periods of spatial frequency between 0.1 mm and 1 mm increase fastest. The root-mean-error (RMS) of medium-high frequency aberration has to be less than 1.6 nm to acquire ∑B3ω of 1.5 and 3ω near-field contrast below 1.15.