Influence of front-surface mitigation pits on distribution of light intensity inside KDP crystal

Micro-machining is the most effective method for KDP crystal to mitigate the surface damage growth in high power laser systems. In this work, spherical and Gaussian mitigation pits are fabricated by micro-milling with an efficient machining procedure. The light intensification caused by front-surface features after mitigation is numerically modeled based on the finite element method (FEM) for acquiring the optimal structural parameters of mitigation pits. The results indicate that the occurrence of diffraction from the pits is principally responsible for the light intensification inside the crystal, and the interference from the second incident laser on the pit's fringe results in the light intensification, which unsteady increases with the decreasing of the pit's wide-depth ratio. For spherical and Gaussian pits after mitigation, it is suggested that the width-depth ratio of spherical mitigation pit should be devised to be larger than 5.0 to avoid the second incident light, achieving the minimal light intensification for the mitigation of surface damage growth. When the wide-depth ratio of the mitigation pit is larger than 10.0, Gaussian contour is preferred to achieve better repairing quality. The laser damage test on the mitigation pits with 1000 μm-width and 20 μm-depth has effectively verified the simulation results.