Study On The Method Of Measuring Subsurface Damage Of Laser Crystal Based On Polarization Effect

Optical components based on laser crystals are the basic elements in high-power optical engineering.The laser damage threshold greatly determines the operational life,performance and stability of the overall system.However,the laser crystal is a hard and brittle material.The traditional contact processing not only changes the surface topography,roughness and other surface topography,but also causes different types of micro-damage such as crystal dislocation,plastic deformation,fracture,crack for the subsurface.It is difficult to avoid damage during the processing,which affects the damage threshold of the laser crystal device and then affects the use of high-energy laser systems.This paper focuses on the surface/subsurface damage detection of laser crystals.Firstly,the physical mechanism of surface/subsurface damages of grinded and polished laser crystal is studied.Secondly,combining the interaction between polarized light and laser crystal,the response of the variation of the quasi-Brewster angle to different surface/subsurface damage is explored.Finally,Subsurface damage(SSD)characterization is achieved by the characteristics of phase shift before and after polarization modulation.The main works and achievement in this paper are listed below:1.The mechanism and manifestation of SSD are summarized.The detection methods for SSD are investigated and classified.Besides,the principle and application characteristics of typical detection methods are illustrated.2.The polarized quasi-Brewster angle method is applied to characterize the SSD of laser crystals.To this end,the qualitative relationship between the surface/subsurface damage structure and the phase change of polarized light is established.The mathematical transfer model of polarized light in the system is established in MATLAB.The feasibility of the presented model is verified by simulation.3.The Nd:GGG laser crystal is used as the samples.The influence of processing technology on the surface/subsurface damage is studied using polarized quasi-Brewster angle method.In the measurement,different grinding and polishing process parameters are used.4.These samples are also characterized by other methods,including 3D profilometer,atomic force microscope and Raman spectroscopy.The experimental results reveal the correlation between the processing technology and the surface quality of the optical components.The polarized quasi-Brewster angle method is proved to be an efficient method to characterize SSD of laser crystal.Furthermore,the optimal processing technology is also explored and a set of processes for the laser crystals with Ultra-smooth surface and low internal damages are established.