Influence of aero-optic effect on laser beam expanding system

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 0.317m to about 0.078m. 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.