Exterior surface damage of calcium fluoride outcoupling mirrors for DUV lasers

Abstract

Damage of optical components due to laser irradiation reduces reliability and limits durability. Calcium fluoride (CaF(2)) is commonly used for deep UV laser optics because it shows a very low tendency of color center formation as, compared to other UV-X optical materials. Here, we report on the exterior damage of CaF(2) UV-X optics due to radiation with high pulse-energy densities (80 mJ/cm(2)) from an ArF laser. At such high energy densities, damage occurs on the external resonator side. The damage is generated by a partial alteration of the CaF(2) substrate to crystalline CaCO(3) (calcite). The decomposition of CaF(2) is mainly driven by photochemical processes in the presence of water vapor, which are induced by the UV-laser light and the elevated temperature within the beam profile.