Study On The Feasibility Of Quasi-Brewster Angle Method To Characterize The Surface Integrity Of Laser Crystal During Polishing

The surface and subsurface damage of laser crystal is inevitable in the process of machining.These damages have a vital effect on the performance and service life of the device.Compared with the grinding process,the polishing process gradually reduces the damage of the material to an extremely small degree.Residual damage will eventually accompany the full life of the optical element.It is necessary to study the change process of surface integrity in polishing stage to master the process quality of polishing.However,it is difficult to detect surface integrity,especially subsurface damage.In response to this problem,this paper firstly analyzes the principle and advantages of quasi-Brewster angle method in detecting surface integrity by using mathematical model,after summarizing the damage forms in polishing stage.Then,using gadolinium gallium garnet laser crystal as the research object,the change of sample surface integrity under different polishing processes was studied experimentally with ellipsometer.Using quasi-Brewster angle method,white light interferometer and atomic force microscope,the change of surface quality during polishing process was accurately reflected.The non-damage and high sensitivity of quasi-Brewster angle method are demonstrated.The main research contents of this paper include:1.The surface roughness test method is relatively mature.Therefore,when investigating the research status of this topic,this paper mainly summarizes the commonly used subsurface damage testing methods,and discusses the application conditions,advantages and disadvantages of different subsurface testing methods.2.The theory of elliptic polarization measurement is expounded.Based on the understanding of the surface structure and composition of laser crystal in the polishing process,the corresponding optical model is established.The theoretical basis of quasi-Brewster angle method for detecting the surface integrity of laser crystal is analyzed and the feasibility of the method is verified by simulation.3.Detailed and perfect laser crystal processing scheme was designed,and samples with different surface integrity in the whole polishing process were obtained.In order to achieve the purpose of measurement,different test methods were used to characterize the samples.4.The test results obtained by different test methods are given,and the change process of the surface integrity of the laser crystal during polishing is analyzed from two aspects of surface roughness and subsurface damage.The feasibility of quasi-Brewster angle method to detect the surface integrity of laser crystal is verified.5.The advantages and disadvantages of optical structure of different ellipsometers and their applicable scenes are analyzed.According to the measurement requirements of this subject,the phase modulated optical structure is selected for the ellipsometer built by ourselves.The prototype of ellipsometry measurement system is preliminarily built,which lays a foundation for the following system development and integration.