Onset of the optical damage in CaF2 optics caused by deep-UV lasers

The exterior sides of calcium fluoride (CaF2) outcoupling mirrors are damaged by ArF laser light irradiation with high pulse-energy densities (80 mJ/cm2). The damage is generated by a partial alteration of the CaF2 substrate to calcite. The CaF2 decomposition is driven by photochemical processes due to the UV light and the presence of water vapor and is supported by elevated temperatures within the laser beam transmitting area. Small filaments act as starting points for the decomposition process, where kerogenous carbon and calcite can occur.