Effects of substrate etching on laser-induced damage of 355nm space coatings

Abstract

355nm UV lidars are of high spectral resolution and are demanded in space-borne applications. As a result of the vulnerability of the coatings in the laser systems and coatings irradiated by 355nm UV laser are more prone to be damaged than visible or infrared lasers. Thus, the study of the processing on substrate surface will contribute to the improvement of the performance of optical coatings. The dual ion beam sputtering can provide compact and stable layers for space applications. Thus, Al2O3/SiO2 high reflection(HR) and anti-reflection(AR) coatings deposited on acid etched substrate were prepared by dual ion beam sputtering. Damage characteristics on the surface were analyzed after 355nm laser irradiation on the coatings. The damage morphology measured by focused ion beam-field emission scanning electron microscopy (FIB-FESEM) and depth measurement of damage pits hint that the damage of HR coatings occurs in the coating layers rather than the substrate. The HR and AR coatings deposited on acid etched substrate are of larger changes on laser-induced damage thresholds(LIDT) than these on the non-etched substrate. The damage mechanisms of coatings are attributed to thermal absorption and the damage area is a molten pool surrounded by mechanical ejection. The etched area with nicks caused the damage on the HR coatings deposited on etched substrate. And due to the elimination of defects on the subsurface of the AR coatings by etching method, the LIDT of AR coatings was improved.