Wavelet And Fractal Analysis About Surface Topography Of KDP Crystal And Its Effect On Transparent Performance

In the Inertial Confinement Fusion (ICF) project, KDP crystal component is very important and the optical performance of it directly impacts on the development of ICF. The basic requirements to the KDP crystal optics of ICF solid laser driver are: high accuracy face shape quality (Transmission wave frontλ/6 PV), high laser damage threshold value (15J/cm~2), good surface roughness (≤5nm). But the relation between the surface topography and oprical performace has never been studied completely. Therefore completely evaluating the surface topography of KDP and analyzing the influence on the optical performance are very important to optimize the machining process of KDP and enhance the optical performance of it. Based on these reasons, the relations of surface topography with machining process and optical performance are studied in this thesis.After completely analyzing the different analytic methods of surface topography, the wavelet, fractal and power spectral density (PSD) are selected to analyze the surface topography of machined KDP crystal. The ability to characterize surface roughness using scale-independent parameters is a specific feature of fractal approach; because fractal dimension is never changed for a certain surface and can completely use the measurement data, fractal dimension is a substance property of surface topography. The PSD method could sufficiently use the measured data and completely reflect the influence of frequency information on the surface topography. The composition of wavelet and PSD could extract the information of any micro-scale waveness from the machined surfaces.Through the Fourier model method, the relation between the surface frenquency and intensity of optical field is analyzed. And through calculation of influence of surface topography on optical deviation, the regularity between fractal dimension and efficiency of secondary harmonic is obtained. The 2D wavelet method is used to realize multi-scale decomposition of original machined KDP surface in the thesis. 1D and 2D PSD methods are adopted to reflect the frequency information of machined KDP surface. Com PSD and 2D continuous wavelet transform (CWT2D) will be used together to achieve the extraction of the spacial frequency. And the PSD method could calculate the impact degree of the particular spacial frequency on the surface morphology. Then find out the frequency information what is the main factor impacting the quality of the surface and optical performance. Fractal method is used to analyze fractal and anisotropic features. Fractal features include the calculation of fractal dimension and the relation between fractal dimension and surface roughness. The anisotropic features can be analyzed through the circle profile fractal dimension distribution method.The influence of feed rate, cutting depth and spindle speed on the fractal features and frequency features of surface topography could be analyzed in the different topography obtained by the cutting experiments. Through analysis of machined surfaces by wavelet, fractal and PSD methods, the feed rate has the obvious influence on the fractal dimension and frequency feature of machined surfaces. The cutting depth has only the obvious influence on the fractal dimension but no influence on the frequency feature. The spindle speed has no obvious influence on the fractal dimension and frequency feature.At last the influence of different surface topography on the optical performance can be obtained by optical experiments. Combining the surface analysis method, the influence of fractal and frequency features on the optical performance can be obtained, and both of them have the obvious influence on the optical performance. The results are nearly accordant with the theoretical analysis. Through the optical experiments, fractal and frequency features can impact optical performance of KDP crystal, and frequency features have larger effect than fractal features. And though the analysis results of influence of different cutting parameters on the fractal and frequency features, optimized plan of machining process parameters that could be obtained to improve surface topography and enhance optical performance of KDP crystal component.