Surface defects on optical components may strongly modulate the incident light field. The local intensity increases significantly, and even exceeds the Laser Induced Damage Threshold (LIDT) of optics. The operation of high-power laser systems is threatened seriously. A three-dimensional topography measurement technique for defects on large-aperture element surface based on the principle of transient interferometry is proposed. Based on the location measurement results of surface defect by the fast-scanning method, the transient interferometry system can focus on the measurement of regions including surface defects. The sub-aperture stitching algorithm is utilized to acquire the complete morphology of surface defects. The algorithm of defect depth extraction from 3D topography is proposed. Moreover, surface defect’s depth and width are statistically analyzed preliminarily, which lays a foundation for the manufacturing process improvement and evaluation for laser system applications. Surface defect and contaminant are distinguished effectively by the three-dimensional topography measurement system for surface defect of large-aperture optics. The established technique can effectively compensate the shortcomings of existing methods, such as microscopic scattering dark-field imaging.
Large optics are the key components of high power laser systems. The defects existing on the surface, under the surface and in the bulk are the main limitation to improve the output power of high power laser systems. The structural defects with micron or sub-millimeter size include surface scratches, digs, subsurface crack, bulk inclusions and bubbles, which are required to be removed during the manufacturing processes. The measurement precision and efficiency are the main challenges of developing novel measurement techniques due to their small size and random distribution on the surface with meter in length and within the materials with centimeters in depth. This paper presents our recent progress of measurement techniques based on laser scattering technique and microscopy for structural defects measurement of large optics. The laser scattering technique can quickly find the positions of these defects both on the surface and within the material. The microscopy is employed to precisely measure the size of the defects. The measurement strategy can not only improve the measurement efficiency but also obtain a considerable precision. Finally, the applications of these developed techniques for Nd-glass, laser crystals, optical coatings will also be presented, which has provided useful information both for manufacturing processes but also for laser systems design.
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