CCD-Based Thermal Lensing for Fast Localization of Microscale Absorptive Defects on Large-Sized Laser Components

Large-sized laser components with diameters over 100 mm are widely used in high-power/high-energy laser systems. In these laser systems, one of the key factors leading to laser damage is the strongly absorptive defects (usually with microscale sizes) on the surface/sub-surface of laser components. In previous studies, these absorptive defects were detected by thermal lensing (TL) technique with modulated focused pump beam and raster-scanning. This raster-scanning TL method is time-consuming and impractical for fast defect detection in full diameter of large-sized laser components. In this paper, a CCD-based pulse-pump pulse-probe TL technique, in which a high-energy pulse at the pump wavelength with a large diameter is used for excitation and a relatively low-energy pulse at a short wavelength is used for probe, is proposed for fast localization of absorptive defects in full diameter of large-sized laser components. A theoretical model is developed and employed to optimize the experimental parameters. The feasibility of this proposed TL technique for defect detection is quantitatively analyzed by theoretical simulations.