Secondary peak of downstream light field modulation caused by Gaussian mitigation pits on the rear KDP surface

Abstract

Micro-milling has been proved to be the most effective method to mitigate the growth of laser-induced surface damage on potassium dihydrogen phosphate (KDP) crystals used in high power laser systems. However, the secondary peak of downstream light field modulation caused by Gaussian mitigation pits on the rear KDP surface would cause potential risk to damage downstream optics. In order to explore the effect of the mitigation pits on the secondary peak, we numerically calculated the downstream light field modulations caused by Gaussian mitigation pits on the rear KDP surface based on the angular spectrum diffraction theory. The results suggest that the secondary peaks are dependent on the parameters of the width, depth, depth error and title error. Among them, the tilt error and depth have greater influence on the mitigation effect. To reduce the laser damage risk caused by the secondary peak, the depth of the pre-designed mitigated contour should be optimized according to the actual operating conditions. The tilt error and depth error are proposed to be controlled within 1′ and 2 μm, respectively, during the micro-milling. Also, the experiments verified the calculation results of downstream modulations and the effects of these parameters on the secondary peak. This work can not only provide available models for evaluating the laser damage risk of secondary peak caused by mitigation pits on the KDP surface but also contribute to the development of optimal micro-milling parameters for laser damage mitigation as well as the installation strategy of optical components employed in the high power laser systems.

Fractal Analysis on Machined Surface Morphologies of Soft-Brittle KDP Crystals Processed by Micro Ball-End Milling

“…Potassium dihydrogen phosphate (KDP) crystals, known as an excellent non-linear optical material [1,2], have been widely adopted to generate harmonics for Nd: YAG laser systems [3,4]. While working under continuous laser irradiation, laser-induced damage defects (e.g., micro-cracks and micro pits [5][6][7]) are vulnerable to generate on the KDP surfaces, thus restricting the sustainable operations of inertial confinement fusion (ICF). In order to recycle and use these valuable KDP optics, the most cost-efficient way is to repair the damaged defects as quickly as possible to avoid them further growing to scrap the entire optics.…”