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28 November 2011 Absorption measurement of HR coated mirrors at 193nm with a Shack-Hartmann wavefront sensor
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All dielectric HR mirror coatings consisting of AlF3/LaF3/Oxide layers were deposited on DUV grade fused silica and CaF2. A novel technique was employed to measure the absorption of these mirrors during irradiation by a 193nm 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. Gage R&R (Repeatability & Reproducibility) measurements were done to show that this is a practical test technique for use in production. 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 spatial resolution is sufficient to make wavefront distortion measurements both inside and outside of the laser beam footprint. 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.
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Byungil Cho, Edward Danielewicz, and J. Earl Rudisill "Absorption measurement of HR coated mirrors at 193nm with a Shack-Hartmann wavefront sensor", Proc. SPIE 8190, Laser-Induced Damage in Optical Materials: 2011, 81901P (28 November 2011);

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