Downstream light intensification induced by Gaussian mitigation pits using micro-milling on rear KDP surface

Micro-machining has been proved the most effective method to mitigate the laser-induced surface damage growth on potassium dihydrogen phosphate (KDP) crystal in high power laser systems. However, the phase contrast of outgoing laser beam, introduced by the mitigated KDP surface, would cause light propagating turbulence and downstream intensification with the potential to damage downstream optics. In this work, a Gaussian mitigation pit with width of 800μm and depth of 10μm is fabricated on KDP rear surface by micro-milling. The effect of the mitigation pit on downstream light intensification is analyzed through propagation calculations based on Fresnel diffraction integral theory. The light intensity modulations reach a peak value at the position of 10mm downstream from the rear surface, decrease sharply subsequently and get stable eventually. The results indicate that the modulations induced by Gaussian mitigation pits would change with various downstream locations. It is essential to notice the unacceptable downstream intensification and reduce the risk of laser damage on other optics by choosing an appropriate installation location.