  Micro-milling can effectively remove laser damage sites on a KDP (potassium
dihydrogen phosphate) surface and then improve the laser damage resistance of
the components. However, the repaired KDP surface could cause light propagating
turbulence and downstream light intensification with the potential risk to
damage downstream optics. In order to analyze the downstream light field
modulation caused by Gaussian mitigation pits on KDP crystals, a computational
model of the downstream light diffraction based on the angular spectrum theory
and the Gaussian repair contour is established. The results show that the phase
offset caused by the repaired surface produces a large light field modulation
near the rear KDP surface. The modulation generated in the whole downstream
light field is greater than that caused by the amplitude change. Therefore, the
phase characteristics of the outgoing light could be suggested as a vital
research topic for future research on the downstream light field modulation
caused by mitigation contours. Significantly, the experimental results on the
downstream light intensity distribution have good agreement with the simulation
ones, which proves the validity of the established downstream light diffraction
model. The phase characterization of the outgoing light is proposed as an
evaluation tool in the repair of KDP crystals. The developed analytical method
and numerical discrete algorithm could be also applicable in qualifying the
repair quality of other optical components applied in high-power laser systems.
