Low absorption ion beam sputtered coatings on large optics for high energy laser systems

In this work we introduce several developments of unique optics with specified reflected wave front error (RWE) of λ/10 (63nm P.V) before exposure to at least 40 seconds 200kilowatt laser power at 1064nm as well as during exposure. A 10μm thick broadband dielectric high reflection (HR) coating was deposited on a 400mm diameter fused-silica substrate. A 0.4% coating uniformity was achieved which is equivalent to 40nm P.V RWE. To compensate the RWE introduced by the stressed HR coating, a unique anti-reflection (AR) coating was deposited on the backside resulting in total RWE of less than λ/10 (63nm P.V). A post deposition thermal process resulted in a 1.2% spectrum red shift, absorption reduction from 20ppm to 3ppm and no change in the RWE. In a second case, a 130mm dichroic mirror with rigid tolerance limiting the acceptable spectral red shift post thermal treatment, was exposed to low temperature process, reducing absorption from ~25ppm to 6ppm. In a third case, a scanning mirror (R>99.995%) with high specific stiffness substrate material (absorbing) and λ/10 RWE measured during exposure to 72kW/cm² laser for 40 seconds is discussed. Keywords: Ion beam sputtering, laser damage threshold, high refection coating, anti-reflection coating, stress