Fluorescence-induced Enhancement Detection Technology Of Subsurface Defects In Fused Quartz

Fused quartz is an important part of high power laser equipment.During processing of fused quartz,uneven force or impurities will cause sub-surface defects(such as cracks,scratches,etc.).Due to the strong laser absorption by sub-surface defects,when a laser irradiates fused quartz optical elements the sub-surface defects will absorb laser energy rapidly,and then the local temperature will rise sharply.As a result,fused quartz will be damaged by thermal melting and explosion,which is the so called laser-induced damage.It is an urgent problem to detect the subsurface defects of fused quartz optical elements,improve the laser-induced damage resistance of fused quartz optical elements and prolong the service life of optical elements.Fluorescence nondestructive detection is a commonly used method for nondestructive detection of fused quartz sub-surface defects.However,since the fluorescence signal of fused quartz subsurface defect is relatively weak,the tiny subsurface defects are not easy to be detected,which increases the difficulty of accurate analysis of sub-surface defects.In this paper,the variation and depth range of fused quartz subsurface defects were studied by peeling off fused quartz layer by layer using HF acid and ion beam etching.According to the variation and depth range of fused quartz subsurface defects,the fluorescent agent is added to the sub-surface defects of fused quartz by polishing method to enhance its fluorescence intensity and hence facilitate the detection of sub-surface defect of fused quartz.The main results are as follows:1)Fused quartz optical elements were etched layer by layer in HF acid solution with the same mass fraction.The etching depths of fused quartz optical elements are 300,1,3,5,10 and 15 microns,respectively.The experimental results show that the damage threshold of fused quartz increases with the increase of HF acid etching depth,and finally tends to be stable.The fluorescence nondestructive detection method was used to test the sub-surface defect of fused quartz.It was found that there was a scratch in the upper right corner of the fluorescence image when the etching depth of fused quartz element reached 300 nm,and the scratch disappeared when the etching depth reached 1 micron,indicating that the depth of the scratch was between 300 and 1 micron and the intensity of fluorescence signal was also weakened.The intensity of fluorescence signal was enhanced when the etching depth changed from 3 micron to 5 micron,while it decreased to near zero when the etching depth is from 5 to 15 microns,indicating that the sub-surface defects have been almost removed.2)The surface of fused quartz optical elements was peeled off layer by layer using ion beam etching.The relationships between scattering,fluorescence defects,surface morphology,surface roughness,laser damage threshold and peeling depth were studied in detail.3)Comparing the fluorescence image measured by fluorescence non-destructive detection equipment with the in-situ morphology image of optical microscope,it can be found that the density of fluorescent spot defects decreases gradually with the increase of etching depth induced by HF acid;the width,length and depth of scratches in fused quartz sub-surface defects increase with the increase of etching depth,and the scratches are passivated when etched to a certain extent.Which confirms the reliability and scientificity of this method for detecting sub-surface defects.The fluorescence enhancement of fused quartz sub-surface defect is realized by adding fluorescent agent,and the fluorescence-induced enhancement detection technology for detecting fused quartz sub-surface defect is developed,which provides a guarantee for the safe use of fused quartz in high power laser devices.