Laser damage testing of windows with anti-reflection structured surfaces for high power continuous-wave near-infrared laser applications

High power continuous-wave (CW) and high pulse energy laser optical systems can suffer damage from untreated optics due to undesirable Fresnel reflections. With high energy pulse lasers, traditional anti-reflection (AR) thin-film coated optics are susceptible to localized field enhancement regions, due to multiple boundaries, and experience laser induced damage on both entry and exit interfaces. Sub-wavelength anti-reflective structured surfaces (ARSS) have been shown to have a higher laser-induced damage threshold than traditional AR coatings. Previously published work detailed nanosecond-pulsed laser-induced damage on planar fused silica (FS) substrates with random ARSS (rARSS) treatment (λ=1064 nm). This study details and compares laser damage and fatigue testing of rARSS treated FS via continuous-wave, 2 kW irradiation (λ=1075 nm). Gaussian laser output was focused to 60-μm-diameter (1/e²), to increase incident intensity, yielding a maximum power density of 70.7 MW/cm². Laser power and duration were controlled while monitoring the window's optical transmission and entry/exit surface temperatures. CW durations at maximum power up to 5 min were used. Peak temperatures were recorded for untreated FS, as well as double side treated rARSS samples.