Research On Subsurface Defect Detection Technology Of Fused Silica Optical Components

In large-scale high-power laser devices,precise fused silica optical components are widely used,and the subsurface defects of optical components will directly affect the laser damage threshold and the stability of the device.In recent years,with the advancement of processing technology,there are almost no large-area defects in optical components,but some optical components have some microscopic defects on their surface and subsurface to varying degrees.Because these defects are mainly distributed in the depth range of several microns to hundreds of microns,and have the characteristics of small size,low density and wide distribution range,it is difficult to detect them non-destructively,effectively and quickly by conventional methods.In view of the above problems,in order to obtain relatively high-quality original images,based on the principle of dark-field scattering and photoluminescence effect,the excitation spectrum and emission spectrum of the fluorescence signal were measured,and the illumination method and illumination angle were optimized.The scheme of combining EMCCD and common CCD for microscopic imaging was determined.Finally,a detection experimental platform was built,which included a laser illumination system,a microscopic imaging system,a three-dimensional scanning system and an image processing system.The scattered light signal images of the optical components under the 520 nm wavelength laser and the fluorescence under the 351 nm wavelength laser were simultaneously obtained.Due to the inherent characteristics of optical components,the obtained original scattered light signal images and fluorescent signal images generally have the characteristics of sparse signal,weak intensity,low contrast,and uneven background,which cause problems for subsequent segmentation and detection.Firstly,image enhancement processing is performed on the two original images;secondly,the sub-aperture images are registered,and then the sub-aperture scattering image and the fluorescence image are spliced respectively to obtain a full-aperture image,and then the image is subjected to binarization segmentation;and then screen and detect individual subsurface defects.For the existing methods,it is not considered that there will always be a certain dislocation phenomenon in the spliced full-aperture images,and due to different imaging principles,the images formed by the same defect area are quite different,resulting in poor screening results and improved extraction methods.In order to verify the effectiveness of the method,the samples to be tested were etched with HF acid,and then compared with the results of the detection method proposed in this thesis.The results show that the method can detect weak defects on the surface and sub-surface of optical components,and can effectively separate the sub-surface defects,which verifies the reliability of the detection method proposed in this thesis,and the method has the advantages of non-destructive,rapid and high detection accuracy.At the same time,using a large number of experimental data obtained,some important factors affecting the imaging and image processing of optical components,as well as the distribution characteristics,generation mechanisms and causes of microscopic defects on the surface and subsurface of optical components are studied and summarized.Finally,using the laser damage threshold test platform,the triple frequency laser damage thresholds of fused silica optical components from different manufacturers were tested,and the data were analyzed.It can provide certain guidance for the evaluation of laser damage threshold and processing technology of optical components in high-power laser devices.