Abstract
Abstract To determine whether a potassium dihydrogen phosphate (KDP) surface mitigated by micro-milling would potentially threaten downstream optics, we calculated the light-field modulation based on angular spectrum diffraction theory, and performed a laser damage test on downstream fused silica. The results showed that the downstream light intensification caused by a Gaussian mitigation pit of 800 μm width and 10 μm depth reached a peak value near the KDP rear surface, decreased sharply afterward, and eventually kept stable with the increase in downstream distance. The solved peak value of light intensification exceeded 6 in a range 8–19 mm downstream from the KDP rear surface, which is the most dangerous for downstream optics. Laser damage sites were then induced on the fused silica surface in subsequent laser damage tests. When the distance downstream was greater than 44 mm with a downstream light intensification of less than 3, there were no potential damage threats to downstream optics. The study proves that a mitigated KDP surface can cause laser damage to downstream optical components, to which attention should be paid in an actual application. Through this work, we find that the current manufacturing process and the mitigation index still need to be improved. The research methods and calculation models are also of great reference significance for related studies like optics mitigation and laser damage.
Highlights
The issue of laser damage to potassium dihydrogen phosphate (KDP) crystal is an important factor restricting the development of high-power laser facilities for inertial confinement fusion (ICF) such as the National Ignition Facility (NIF) in the United States, the Laser MegaJoule in France and the Shenguang Laser Facility in China[1,2,3,4]
In order to determine whether mitigation pits on KDP surfaces would reduce the laser damage resistance of downstream optical components, we built a model to calculate downstream light-field modulation caused by mitigation pits on the KDP rear surface based on the angular spectrum diffraction theory
Gaussian mitigation pits on the rear surface of KDP crystal would cause disturbance of incident laser propagation and obvious downstream diffraction phenomena accompanied by severe light-intensity enhancement at various distances from a KDP rear surface
Summary
The issue of laser damage to potassium dihydrogen phosphate (KDP) crystal is an important factor restricting the development of high-power laser facilities for inertial confinement fusion (ICF) such as the National Ignition Facility (NIF) in the United States, the Laser MegaJoule in France and the Shenguang Laser Facility in China[1,2,3,4]. Tiny defects such as surface fractures and laser ablation would be replaced with specific smooth contours[7,10] It is a very effective and feasible method for improving laser damage resistance of optical components during subsequent laser irradiation. The strategy and process for laser damage mitigation on KDP crystal surface have made great progress, the effect of KDP surface mitigation contours upon downstream far-field light modulation is unclear. In order to determine whether mitigation pits on KDP surfaces would reduce the laser damage resistance of downstream optical components, we built a model to calculate downstream light-field modulation caused by mitigation pits on the KDP rear surface based on the angular spectrum diffraction theory. The results are not just used as evidence of the potential damage threats to downstream optics caused by Gaussian mitigation pits on a KDP rear surface, and provide technical indexes for the installation of downstream optical elements, such as continuous phase plates and wedged focus lenses
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