Detection of the laser-damage onset in optical coatings by the photothermal-deflection method

Abstract

Photo-thermal deflection (PD) method is one of the typical pump-probe techniques for absorption evaluation. In this study, the PD method was used for detection of a signature of laser-induced damage prior to critical destruction. A setup for the PD technique was incorporated into the nanosecond damage testing system and the absorption of some optics was measured in combination with N-on-1 damage test. The absorption decreased with pulse-laser irradiation at first. Then, the absorption increased at some point. Finally, the laser damage was caused and the absorption decreased. The relaxation times of the absorption were also evaluated to reveal the reason for the variation. The measured results indicated that the decrease came from the disposal of initial contamination and the effect might be considered as laser conditioning. On the other hand, the increase of the absorption might be attributed to generation of laser-induced defects (self-trapped excitons).