Laser damage evolution by defects on diamond fly-cutting KDP surfaces

Abstract

The unavoidable surface defects on the practical processed potassium dihydrogen phosphate (KDP) crystals, especially the most common protuberance defects, which are prone to damage under intense laser irradiation, seriously limit the application of KDP optics in laser fusion facilities. In this work, a defect-induced laser damage explosion model that couples the electromagnetic field and stress field was constructed to reproduce the dynamic damage process and reveal the laser-induced damage mechanisms innovatively. First, the near-field optical modulation to incident lasers was considered to describe the effect of structural parameters of surface defects on the optical transmission performance. A multi-physics field coupled failure model was then established for the first time to explore the laser damage mechanism through the dynamic behaviors of laser damage that reproduced by this model. It was proved that the protuberance defects on KDP crystal surface would lead to severe laser damage with the characteristics of large transverse spalling and longitudinal cracks due to the Rayleigh and stress waves, which well agrees with the results of laser damage experiments. The established damage model is capable of revealing the hazards of protuberance defects, and providing effective guidance for the suppression of surface defects introduced in the optical ultra-precision machining processes.