Determination of sub-surface damage depth applying atmospheric plasma jet and ion beam etching

The performance of optical systems can be affected by thermal instabilities and light scattering that can occur due to sub-surface damage (SSD), even though the optical lenses may have the desired surface quality and refractive properties. A reliable inspection of the SSD is desirable before a large number of lenses are mounted in an optical system. Non-destructive methods such as Optical Coherence Tomography could be helpful here. To validate this relatively new measuring technology in this application, destructive methods are required. Plasma jet etching and ion beam etching are used as destructive methods to prepare SSD defects on ground and polished fused silica samples to make them visible as topography. The samples were etched step by step and the depth of the defect was accurately measured after each step. The two techniques are compared with respect to their capability of revealing SSD depth. Plasma jet etching was found to be valuable for determining SSD depth, whereas ion beam etching is less specific for deeper defects, so a good correlation can only be determined for smaller defects.