Study On The Detection And Control Techniques Of Subsurface Damage In Optical Fabrication

With the ever-increasing demands on system performances, plentiful large optics are adopted in space detailed reconnaissance cameras, astronomical telescopes, and laser fusion systems, moreover, the technical requirements are more than traditional, which have challenged the capability of optical manufacturing unprecedentedly. Besides the rigorous limits of large optics in production cycle, wavefront errors, and manufacturing cost, researchers have paid more and more attentions to subsurface quality. Subsurface damage (SSD) increases the amount of material removal and decreases directly operational life, secular stability, coating quality, transmission performance, and laser-induced damage threshold of optics. The improvement of working efficiency and realization of surface integrity targets via SSD inspecting, evaluating, and controlling become vital in optical fabrication. This thesis is dedicated to resolving the key problems of SSD produced in optical grinding, lapping, and polishing processes, in order to improve the efficiency of rough machining and quality of finishing processing. The major research efforts include the following points.1. A non-linear model of correlation between surface and subsurface quality is established, then, a detection method of subsurface cracks (SSC) depth based on surface roughness (SR) is developed according to the model. A set of SSD detection methods suitable for grinding and lapping processes is proposed. Then accurate inspection of SSD is realized. Utilizing indentation fracture mechanics and considering the contribution of elastic stress field to the median crack propagation, a non-linear model of relationship between SSC depth and SR is established. According to the model, a non-destructive, rapid, and low cost detection method of SSC depth based on SR is developed.2. Influences of grinding parameters on SSC depth are investigated. The influences of grinding parameters on SSC depth are disclosed through systematical experiments. And an empirical equation with equivalent grinding thickness is constructed according to the influences, in order to predict SSC depth effectively. Then a grinding procedure based on SSC depth controlling is proposed to improve grinding efficiency. Finally an integrated SSD space distribution is obtained through the analysis and detection of surface/subsurface residual stress.3. Characterization and depth prediction methods of lapping SSC are proposed. Three parameters (cluster depth, maximum depth, and depth distribution) are presented to describe the features of SSC. Then a model of correlation between maximum and cluster depth is constructed through analyzing the interaction of adjacent indentations. A prediction model of SSC depth based on lapping parameters is established with indentation fracture mechanics and statistics. Then influences of lapping parameters on SSC depth are obtained, combined with the model and systematical experiments. Finally a lapping procedure based on the influences is provided to improve lapping efficiency.4. An SSD model of traditional polishing is established based on the experimental results of fused silica. According to the model we conclude that hydrolyzed layer contains superficial flow layer and polishing contaminations with descending concentration in depth. The model validates that polishing process is responsible for the corporate effects of hydrolyzed action, mechanical removal, and plastic flow. Then plastic scratches and hydrolyzed layer remained after traditional polishing process are removed effectively by magnetorheological finishing (MRF) and ion beam figuring (IBF) techniques, respectively.5. Research findings are applied to the high efficiency production of O500mm large optics and ultramicro-damage production of fused silica. Grinding and lapping procedures of 0500mm large paraboloid optics (f/3, K9 glass) on the basis of the influences of machining parameters on SSC depth and performances of machine tool are proposed to improve the machining efficiency. A combined method (MRF+IBF) is adopted to gradually reduce the SSD remained after traditional polishing process based on the accurate detection of SSD depth. The method improves the wavefront precision and laser-induced damage threshold of optics in the mean time.