Laser conditioning mechanism revealed by defect and absorption variation in the bulk and at the surface of KDP/DKDP crystals

Abstract

Previous works proved that laser conditioning process was able to improve laser induced damage thresholds (LIDTs) in the bulk of KDP/DKDP crystals. In this paper, it's also demonstrated that laser conditioning process was an effective method to improve LIDTs at their surface. The variation of scattering defects and absorption in the bulk of DKDP crystals during laser pre-exposure was investigated by combining light scattering technique and on-line transmittance measurement technique. Laser-induced disappearance of scattering defects and decrease of absorption revealed the mitigation process of laser damage initiators in the bulk of KDP/DKDP crystals. At the surface of KDP/DKDP crystals, most of damage initiators were the invisible defects. Laser conditioning process could mitigate the invisible defects, but it's hard to mitigate the indentation with fractures. Therefore, it's admitted that laser conditioning process could help to improve the optical properties of crystal material, but it's hard to improve the properties of optical finishing.