Research on thermal effects of beam-splitter mirror in high-power laser system

Abstract

High-power laser propagation through the inner optical path will produce a significant thermal effect on the beam-splitter mirror which will cause phase aberrations. Based on the three-dimensional transient heat conduction equation and the thermal elastic stress-strain equation, a simulation model of reflector mirror was built with three-dimensional finite element method (FEM). The temperature increase and thermal displacements of two kinds of mirror substrates (Al₂O₃ crystal and spinel) were especially investigated with different laser intensity, output duration and absorption coefficient. The effects of mirror thermal distortion on laser beam phase aberrations were also evaluated on both reflection and transmission directions. The experiments of high–power laser propagation through two kind materials of beam-splitter mirrors samples with diameters of 50mm and thicknesses of 5mm were carried out to measure the thermal effects induced by the absorbed laser energy. Both two kinds mirror samples were deposited the same film layer of same reflectance. A high power semiconductor laser was expanded to a beam of 35mm diameter, and double Shack-Hartmann wavefront sensors were used to detect both reflection and transmission thermal distortions of the mirror samples. The measurements showed that reflection aberrations of spinel mirror sample were larger than those of Al₂O₃ crystal mirror sample while its transmission aberrations were slightly less than Al₂O₃ crystal mirror sample. The results of experiments and simulations showed a very good consistency.