Towards high-optical-strength, fluorine-resistant coatings for intracavity KrF laser optics

Intracavity optics of e-beam-pumped high-energy KrF lasers should survive in a hostile environment of simultaneous irradiation by intensive UV laser light, scattered electrons, bremsstrahlung X-ray radiation, and permanent etching by a fluorine-containing working gas mixture. The present paper is focused on the research and development of high-optical-strength and fluorine-resistant AR coatings capable of protecting the fused silica windows and HR rare mirrors of these lasers against fluorine attack. The most promising coatings deposited by e-beam evaporation were ${{\rm NdF}_3}$NdF3-based compositions, which have low water content, good resistance to fluorine, and demonstrated the highest damage thresholds for AR coatings ${29.8}\,\,{{\rm J/cm}^2}$29.8J/cm2 in a large irradiation spot. Outstanding “water-free” coatings of ${{\rm MgF}_2}$MgF2 and ${{\rm Al}_2}{{\rm O}_3}$Al2O3 were deposited by ${{\rm CO}_2}$CO2 laser evaporation with preliminary laser treatment of substrates.