Depth determination of critical fluence-limiting defects within planarized and non-planarized mirror coatings

Multilayer interference optical mirror coatings are traditionally fluence-limited by nodular inclusions. Planarization of these defects modifies the geometrically and interference-induced light intensification to increase the laser resistance of mirror coatings. Previous studies using engineered defects on the substrate or buried in the middle of the coating stack have focused only on understanding the improvement in laser resistance. However, real coating defects are distributed throughout the coating. To better understand differences between the critical fluence-limiting defects of both planarized and non-planarized mirror coatings, laser damage pit depths were determined as a function of laser fluence.