Characterization of absorption losses in deep UV optical materials

Abstract

Comprehensive calorimetric absorption measurements were performed for CaF₂ crystals at irradiation wavelengths 193 nm and 157 nm. By using samples with different thickness a separation of surface and bulk absorptance could be achieved and thus, single- and two-photon absorption coefficients could be determined. For the surface absorptance, a dependence of the polishing grade of the sample was observed at 193 nm. The presented results support earlier proposed models of the absorption mechanisms in wide band-gap materials. For an assessment of the optical quality of DUV optics, a high-sensitivity wavefront analyzer system based on the Hartmann-Shack principle is employed. The device accomplishes precise online monitoring of wavefront deformations of a collimated test beam transmitted through the laser-irradiated site of a sample. Due to the achieved sub-nm resolution, it can be used as an alternative to interferometric measurements for 'at wavelength' testing of optics, e.g. for on-line registration of thermal lensing effects or compaction in fused silica.