Modeling And Verification Of Scattering Properties Of Particulate-Contaminated Optical Mirror

Optical mirrors can be easily contaminated by particles in alignment and operation. Particulate contamination deposited on an optical mirror can affect its scattering properties. More stray lights will be generated and the SNR of the optical system will decrease if the mirror is heavily contaminated. The main content of this paper is to analyze the scattering properties of the particulate-contaminated optical mirror. The scattering properties are quantitatively described by Bidirectional Reflectance Distribution Function(BRDF).Firstly, three scattering models which suits clean rough mirror are introduced. When solving the particle scattering with theoretical methods, there will be requirements of the diameter and the separation distance of the particulates. So three numerical methods, Mo M, FDTD, FEM are compared and analyzed, then FEM is selected for numerically calculating particle electromagnetic scattering characteristics. The mathematical expression between electric field vector and irradiance is provided.Secondly, it is supposed that BRDF of particulate-contaminated mirror is composed by BRDF of clean mirror and particles. COMSOL is a software which can solve electric field vector with FEM. The BRDF of particulates is calculated at Shortwave and Mediumwave Infrared through COMSOL. Diameter of the particle is between ?/10 and 10 ?. Then mathematical expression of BRDF is deduced. Relationship between particulate scattering and incident wavelength, diameter of the particulate, number of the particulate is calculated and discussed. Then discrete data of the particulate scatter is fitted in ABg model so that a complete BRDF curve of the randomly distributed particulates is acquired.Finally, a verification experiment is performed. Al2O3 polishing powder of 2.5μm is selected as researched particulate and a microscope is used to count its numbers. The BRDF of the sample is measured and compared with the one calculated through COMSOL. Results indicate that the magnitude and tendency of BRDF are identical.