气动光学效应对激光扩束系统的影响

Influence of aero-optic effect on laser beam expanding system

激光系统往往需要应对复杂的环境气流,同时由于激光扩束系统口径增大,其晶体窗口难以实现,环境气流更容易进入系统内部,从而影响光束质量。针对环境气流对激光扩束系统带来的气动光学效应问题,借助流体力学软件FLUENT对系统内部流场进行CFD求解,得到不同风力等级和进风角度下流场的各种参数分布,再通过Gladstone-Dale关系将流场密度场映射为折射率场,运用变折射率流场的光线追迹法,得到光束在该非均匀流场中的传输路径;最后结合数值分析方法,计算得到湍流场所带来的光学像差。结果表明,环境气流会给主次镜和反射镜周围引入较多涡流,因此不能忽略其光学效应,从而提出了一种增加扩束系统镜筒长度的方法来降低这一影响。镜筒加长0.5 m之后,扩束系统内部的涡流团可以避开光束传输的主要路径,其出口处波像差的RMS值从最初的0. 317μm下降到0.078μm左右。研究证明了避免环境气流带来的气动光学效应的必要性,同时为扩束系统的流控和结构设计提供了参考数据和思路。

Laser systems often cope with complex ambient air flow.As the size of the laser beam expanding system increases,its crystal window is difficult to achieve.Then the ambient air flow can easily enter the system,thus it affects the beam quality.CFD was used to solve the problem of aero-optical effects caused by ambient air flow.With the aid of fluid mechanics software FLUENT CFD model,the internal flow field of the laser beam expander was simulated.Various parameters of the internal flow field of the system were obtained under different inlet angles.Through the Gladstone-Dale relationship,the refractive index could be calculated from the flow density field.Using the ray tracing method,the beam transmission path could be got in the non-uniform refractive index flow field.Finally,the optical aberration caused by turbulence was calculated by numerical analysis.The results show that the ambient airflow will introduce more vortices to the flow field of the primary mirror,the secondary mirror and the deflecting mirror.The influence of the optical effect cannot be ignored,which must be considered in the overall design.Therefore,a method to increase the length of the mirror tube of the beam expanding system was proposed to reduce this effect.After the length of the mirror tube was lengthened by 0.5 m,the eddy current in the beam expanding system can avoid the main path of beam propagation.Meanwhile,the RMS can be reduced from the 0.317μm to about 0.078μm.The study proves the necessity of avoiding the aero-optical effect caused by ambient airflow.And it provides reference data and ideas for the optical and structural design of the beam expanding system.