When Potassium Dihydrogen Phosphate (KDP) crystal is irradiated by nanosecond laser with fluences exceeding its damage threshold, laser-induced damage occurs in the bulk or on the surfaces of crystal components. Such damage process is a multi-physical coupling process which is composed of energy deposition stage, temperature/pressure rising stage and subsequent micro explosion stage. So far, great efforts have been made in modeling the energy deposition and temperature/pressure rising stages of the damage process, but little attention has been paid on the subsequent micro explosion event. As a result, it is still impossible to reproduce the laser damage phenomena such as damage crater formation and shockwave propagation with the existing damage models. To address this concern, equivalent explosion simulation model for studying the laser-induced damage process of KDP crystals has been constructed by finite element method (FEM). According to the model, explosion energy leading to damage, formation of damage craters and propagation of shockwave can be obtained. Moreover, laser damage experiments combined with time-resolved techniques have been utilized to investigate the impact of laser fluences on the shockwave speed. Experiment results agree well with the simulated phenomenon, which has proved the validity of the simulation model.
The ultra-precision single point diamond flycutting is an effective way for finishing potassium dihydrogen phosphate (KDP) crystals. However, the dynamic performance and motion precision of the machine tool would introduce the errors and have an adverse effect on the surface quality of elements. In this study, a significant method is presented to trace error sources by combining monitoring the rotary errors and analyzing the machined surface. On the one hand, forward recursion method is adopted. Based on the online measuring platform, the radial error, axial error and inclination error of spindle rotation can be obtained. Through the Fourier transform algorithm, the frequency information in each direction is known. Furthermore, the waviness along the feed direction is identified, which is determined by the rotation fluctuation. On the other hand, a novel method for errors tracing is presented by a reverse thinking mode based on the surface topography. The wavelet analysis is carried out along the cutting direction, and the obvious waviness appears in the 4th layer, of which the dominant frequency is 594Hz. Through the knocking test for the spindle system, the waviness error along the cutting direction is determined as the modal vibration of the spindle system.
To improve the surface quality and laser damage resistance of Potassium Dihydrogen Phosphate (KDP) crystal processed by single point diamond fly-cutting (SPDF) technique, formation and suppression of KDP surface defects are investigated. Firstly, multiple measuring methods have been utilized to characterize KDP surface defects. According to their structure and chemical characters, KDP surface defects were classified into four categories and forming reason of each was analyzed. Based on these analysis results, theoretical model for describing the formation process of KDP surface defects was established and conditions for achieving defects-free KDP surfaces were proposed. Finally, flycutting experiments were carried out to verify the effect of the defects suppression measures. Experiment results indicate that smooth KDP surface with roughness less than 2 nm can be obtained and KDP surface defects can be suppressed effectively by applying the proposed methods.