Research on the light intensity modulation and characterizing methods of surface texture on KDP optics generated in fly-cutting and micro ball-end milling processes

In the Inertial Confinement Fusion facilities, single-point diamond fly-cutting and micro ball-end milling are the main techniques to fabricate the large-aperture KDP (KH₂PO₄) optics and repair the surface defects on KDP, respectively. These machining processes inevitably generate various surface textures on the machined optics while the effect of these surface textures on the optical performance of machined optics has not been clarified yet. In this work, the light intensity modulation inside KDP machined by single-point diamond fly-cutting and micro ball-end milling processes was firstly analyzed and the characterizing methods of these machined surface textures were then investigated. It is found that the machining-caused surface textures with higher amplitude and smaller spatial period could result in more severe light intensity modulation inside KDP optic which has been acknowledged as one of the most important factors causing laser-induced damage to KDP. Moreover, the relative directions (i.e., parallel or perpendicular) between these textures on the front and rear surfaces of KDP could have a distinct effect on their light intensity modulation. For characterizing the texture structures, especially, texture direction, power spectrum density (PSD), and fractal dimension were adopted. The PSD analysis results indicate that the dominated textures on single-point diamond fly-cutting surfaces are mainly along the feed direction while the dominated textures on micro ball-end milling surfaces are perpendicular to the feed direction. These textures on micro ball-end milling surfaces have been identified as the residual tool marks whose period equals the step over of the milling process. The fractal dimension can precisely evaluate the machined surface roughness and texture directions. But in comparison, the angular spectrum of PSD has higher characterizing accuracy than the circumferential profile of the fractal dimension approach.