Long-duration CW laser testing of optical windows with random antireflective surface structures on both interfaces: preliminary results

Abstract

High power laser optical systems can suffer damage from uncoated optics due to undesirable Fresnel reflections. With high power 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 random anti-reflective surface structures (rARSS) have been shown to have a higher laserinduced damage threshold than traditional AR coatings. Previously published work detailed nanosecond-pulsed laserinduced damage on rARSS on both single surface and dual surface of optical quality, planar, fused silica substrates at 1064 nm. This study details laser fatigue testing of double-sided rARSS samples via continuous wave, 2 kW ytterbiumfiber- laser irradiation (1075 nm). Laser output was focused to increase incident intensity at the initial interface. The laser spot was focused to ~ 60-μm-diameter (1/e2) yielding a maximum power density of 70.7 MW/cm2. Laser power, test duration, and testing grid were controlled externally via LabVIEW software. Damage testing sites maintained the same laser power density (70.7 MW/cm2) and varied by irradiation time, incrementally from one minute up to one hour. It was determined that double-sided rARSS substrates have a higher damage threshold than thin-film AR coatings, showing no damage of the AR structures at power densities up to 70.7 MW/cm2 and laser irradiation times up to one hour at each interrogation site, while thin-film samples have been reported to fail with power density as low as 2 MW/cm2.