Absorption measurement of high-reflectance coated mirrors at 193 nm with a Shack–Hartmann wavefront sensor

Abstract

All dielectric high-reflectance (HR) mirror coatings consisting of AlF3/LaF3/oxide layers were deposited on deep-ultraviolet-grade fused silica and CaF2. A novel technique was employed to measure the absorption of these mirrors during irradiation by a 193-nm ArF excimer laser source. The method involves the application of a photothermal measurement technique. The setup uses a Shack-Hartmann wavefront sensor to measure wavefront deformation caused by the heating of the coating by the ArF beam. Laser calorimetric measurements of absorption were used to calibrate the wavefront sensor. The new test setup was used to investigate HR mirror coatings both before and after exposure to high average power ArF laser beams. HR mirror samples were irradiated by a 193-nm kilohertz laser source for either 500 million or 18.6 billion pulses. The differences between wavefront distortion measured inside the beam footprint compared to measured outside the beam footprint can be explained by compaction of the coating in the area heated by the ArF laser. Interesting wavefront-distortion results from testing mirrors with either fused silica or CaF2 substrates can be explained by considering the figure of merit of these materials for excimer-laser mirror substrates.