Real-time investigation of UV nonlinear absorptance in anti-reflective coated LBO crystals

Absorptance is often considered a static feature of an optical element that is determined via standardized measurement procedures. Although such measurements are often performed using optical instruments with low light intensity, in high power laser applications irradiation conditions are considerably different. Optics might become unstable due to highly intense light: optical properties change in a nonlinear way and might eventually lead to laser-induced damage. To study these effects we employed the common-path interferometry technique in combination with a high energy and high average power laser source, operating at 1 MHz repetition rate and delivering 10 ps pulses at 355 nm wavelength. We investigated an anti-reflective (AR@355 nm) coating deposited using ion beam sputtering on a lithium triborate (LBO) crystal. Our preliminary results indicate both strong nonlinear absorptance and fatigue near the damaging fluence, however, damage events were not directly related to the critical absorptance level. An attempt is made to predict the lifetime of an AR coated optics by establishing a numerical model of nonlinear absorption.