Study On The Speckle In Deep Fresnel Diffraction Region And The Calibration Of Random Scattering Surface

There will be a complex random light field when a coherent light is transmitted(or reflected)by a rough surface or a medium rich in scattering particles.This light field is called speckle.Generally speaking,the characteristics of speckle are determined by the rough surface,random medium(scatterer)and the optical system which the scattered light waves pass by.Among them,the initial random phase fluctuation range of the light wave caused by the scatterer has an important influence on some basic statistical characteristics such as the probability distribution of the speckle field.According to the roughness of the scatterer,it can be divided into strong scatterer and weak scatterer.According to the distance between the speckle field and the scatterer(the scattering distance),the speckle field is divided into far-field speckle(the scattering distance is more than a dozen wavelengths)and near-field speckle(the scattering distance is within one wavelength).And the scattering distance between one wavelength and dozen wavelengths is called the deep Fresnel speckle.If the weaker secondary scattering is ignored,part of the light waves will be inevitably affected by the shadowing of the adjacent micro-surface undulations and cannot reach the observation surface when the scattered light propagates from the scattering surface to the observation surface,that is,the shadowing effect.The smaller the scattering distance is,the more obvious the shadowing effect is.Therefore,after considering the shadowing effect,the information of the scattering surface contained in the simulated deep Fresnel region speckle is closer to the actual situation,which has certain theoretical reference significance for the study of random scattering of light waves and surface non-contact calibration.In this thesis,a three-dimensional self-affine fractal random surface is simulated and generated,and the influence of statistical parameters such as roughness,correlation length and fractal index on the surface topography is analyzed.The shadowing effect is introduced into the deep Fresnel region speckle simulation based on Kirchhoff's approximation,and the statistical distribution of speckles and the relationship with the surface topography and size are studied.The theory of the superposition of direct transmission light and Gaussian speckles is put forward,and the deep Fresnel region speckles are used to calibrate statistical parameters such as the roughness,correlation length and fractal index of random surfaces.This article is divided into the following six chapters:In chapter 1,the random surface parameters and their statistical characteristics,the calibration methods,the basic principles of light scattering,the effect of scattering and the scatter shadowing effect are reviewed.The various parameters of random surfaces and their statistical characteristics on the surface are introduced in detail,and various calibration methods of surface parameters are discussed.Finally,the calculation and research progress of the shadowing effect are introduced.In chapter 2,we introduce the concept of speckle and explain the description of the "random walk" model in the speckle field.Some statistical properties of speckle,such as the intensity probability density function,contrast,signal-to-noise ratio and other statistical components of the speckle field are introduced.We also explain the concept of Gaussian speckles,and at the same time prove that the real and imaginary parts of the complex amplitude of Gaussian speckles have circular symmetry.According to the relationship between the scatterer and the incident wavelength,the length of the scattering distance,and whether the scatterer moves relative to the observation surface,the speckle field is classified from these three aspects.Finally,the development and application of speckle in various fields are introduced.In chapter 3,we introduce the application of random surfaces in various fields and the methods of generating random surfaces,and propose a self-affine fractal surface with different fractal index.According to the generated several self-affine fractal surfaces,the actual simulation parameters are carried out to verify and analyze the given parameters.The results show that the actual parameters of the self-affine fractal surface produced are relatively stable,which has a certain influence on the research and development of random surfaces in various fields.In chapter 4,based on the more stringent shadowing effect of random surfaces,the deep Fresnel region speckles are simulated by using optical scattering theory.The phenomenon of the antisymmetric relationship between the shape of the weak scattering and the speckle intensity distribution in the deep Fresnel region is studied.It is noted that the probability density function of intensity is closer to the experimental results,and both belong to non-Gaussian speckle.Combined with the scattering and shadowing effect,the characteristics of the effective scattering region of the random surface with the scattering surface and the scattering distance are studied.As the scattering area increases,the change trend of the light intensity of the speckle field corresponding to different positions on the random surface is explored.In chapter 5,we introduce a non-Gaussian speckle approximation model formed by a new weak scatterer while the scattering distance is extremely close.This model is a non-Gaussian speckle composed of a superposition of direct transmission light and Gaussian speckle.We extract the statistical parameters of the scattering surface based on the statistical characteristics of the deep Fresnel speckle.And compared with the original approximate model of non-Gaussian speckle which is superimposed by uniform background light and Gaussian speckle,it is found that the new non-Gaussian speckle model is closer to the real scattering situation.On this basis,from the antisymmetric relationship between the surface topography and the light intensity distribution of the speckle field which we discussed in chapter 4,we find the approximate model of the non-normal speckle can be used to extract the various parameters of the surface.And it is found that the extraction of the complex amplitude of the deep Fresnel region speckle can obtain a large amount of information of the scatterer,which has a certain influence on the study of the optical calibration method of the surface.In chapter 6,we summarize the main work of this dissertation,the various results obtained,the innovations of this article,and finally briefly outline the work and direction to be carried out in the next step.