Combined modulation of incident laser light by multiple surface scratches and their effects on the laser damage properties of KH2PO4 crystal

Manufacturing-induced surface defects are deemed as a potential source, leading the laser-induced damage threshold (LIDT) of the actual KDP crystal optics to be much lower than the intrinsic one. However, the underlying mechanisms have not been fully recognized. We explore the combined modulation of incident laser light by multiple scratches and their effects on laser damage performance of KDP optics by modeling the light intensifications and performing a laser damage test. Under the combined modulation of multiple scratches, enhanced hot spots are generated due to the focusing effects of convex lens profiles surrounded by the neighboring scratches. The combined modulation actions are much stronger than that of a single scratch. The relative light intensities (I R s) caused by multiple scratches can reach up to two times, and the number of hot spots (IPs) are four times as large as those by a single scratch. The I R s exhibit a general, increasing tendency as the scratch density increases. But for the case of double total reflections of rear-surface scratches, the totally reflected lights are transmitted through neighboring scratches, resulting in decreasing tendency of I R s. The tested LIDTs and optical transmittances of multiple scratches present a gradual, decreasing tendency with the increase of scratch density, which agrees with the simulation results. Besides, the simulated light intensifications could well explain the locations of laser damage, which further verify the role of multiple scratches in lowering the laser damage resistance.