Improvement of surface laser damage resistance of KDP crystal under combined machining process

Abstract

The nature of potassium dihydrogen phosphate (KDP) crystals leads to a hard manufacture for optical surface, especially in the application of an intensive laser system. We use single-point diamond turning (SPDT), magnetorheological finishing with water-dissolution (MRFWD), and ion beam figuring (IBF) to accomplish high surface quality and to inhibit surface defects meanwhile. According to the intensification of light field of 355-nm wavelength calculated by finite-difference time-domain method, the surface ripples of 2-μm spatial period created by SPDT process should be strictly treated. On the basis of SPDT, a series of processing and testing experiments are carried out on the samples for the best laser damage performance. The MRFWD developed with the measures of pressure depth control, fluid temperature control, and fluid PH value adjustment etc. can partly smooth out the ripples of critical frequency and maintain lower iron pollutions at the same time. These metal particles induce stronger light field modulations compared with the residual ripples from the simulation. As for IBF processing, the sample with removal depth of 100 nm exhibits the best laser damage performance in the experiment. The cleaning effects of IBF under optimized parameters combined with the better full aperture uniformity scanned by a photothermal absorption system after MRFWD finally decide the optimal processing procedure. And the surface damage threshold of KDP crystal increases from 4.98 J / cm2 after SPDT processing to ultimate 9.2 J / cm2 under the irradiation of 355 nm, 10 ns pulsed laser.