Adaptive laser-induced damage detection

Abstract

Laser-Induced Damage Threshold (LIDT) measurements are typically performed in order to characterize the optical resistance of laser components. However the sensitivity of online damage detection techniques is often limiting factor for the accuracy and reproducibility of so called S-on-1 and 1-on-1 measurements. In fact the sensitivity of damage detection has the biggest impact to precision of these experiments. In this paper we describe the idea of making improvements on scattered light registration based damage detection. It was learned, that scattered light intensity is linearly proportional to incident energy of laser pulses while material is not damaged. However in case of induced damage the linear proportionality becomes nonlinear: this feature is used in order to detect optically induced surface and bulk changes in material. According to the base of those theoretical considerations the adaptive scattering detector was proposed and made up. It was put into practice by projecting it on discrete element schematics. The improved sensitivity of laserinduced damage detection was reached. This technique helps to avoid degradation of damaged site and pollution of surrounding area due to laser ablation during the S-on-1 tests since it allows blocking the repetitive irradiation immediately when damage appears. Numerous tests were made, that shows, that adaptive scattering detector can precisely detect damages in their initiation state, independently from material ability of light scattering. Calibration of this detector can be automated, therefore the influence of human factor is minimized. This fact opens up the possibility to run whole damage threshold measurement procedure automatically.