• High Power Laser Science and Engineering
  • Vol. 8, Issue 4, 04000e37 (2020)
Hao Yang1、2, Jian Cheng1、*, Zhichao Liu2, Qi Liu1, Linjie Zhao1, Chao Tan1, Jian Wang2, and Mingjun Chen1、*
Author Affiliations
  • 1State Key Laboratory of Robotics and System, Harbin Institute of Technology, Harbin150001, China
  • 2Research Center of Laser Fusion, China Academy of Engineering Physics, Mianyang621900, China
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    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.

    1 Introduction

    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[14]. Laser damage sites on the surface of optical components would reduce their resistance to high power lasers, accelerate the scrapping of components and deteriorate the quality of laser beams[5,6]. For high-cost optics like KDP crystal, NIF proposed a ‘recycling’ strategy to mitigate the growth of laser-induced surface damage and maximize its lifetime[7,8]. The main method uses an optics damage inspection system to determine whether laser damage to the optics surface has occurred[5,9]. Tiny defects such as surface fractures and laser ablation would then 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.