Characterization of manufacturing-induced microcracks in optical components

Microcracks occur during the conventional manufacturing chain (grinding, polishing), but also as a result of ultra-short pulse laser processing of hard-brittle materials, such as those frequently used in the optical industry. Non-destructive methods, such as optical coherence tomography, are being discussed in order to understand the development of nearsurface damage and to enable process control without sample consumption. Destructive testing methods will be necessary to validate those methods. Beam-based etching applied to Fused Silica by plasma jet or ion beam is presented. The application of reactive plasma jet etching causes process-induced roughness. Nevertheless, significant holes and defects can be detected. No additional roughening occurs with ion beam etching. Both applied beam-based etching processes ensure that small defects are enlarged, allowing these microcracks to be detected even if they are not directly visible on the initial surface. For an accurate depth determination of prominent, sharp-edged defects, white-light interferometry is limited. Confocal laser scanning fluorescence microscopy is shown as an alternative measurement technique to determine and visualize the development of the extent of defects with increasing etching attack.