Monostatic Mueller matrix measurement and theory

Abstract

Monostatic Mueller matrix measurements of aluminum plates of various roughnesses are presented using a Mueller matrix polarimeter with a dual rotating retarder. The measurements are compared with a theoretical Mueller matrix model derived from the vector Kirchhoff diffraction equation. The wavelength of the laser is 1.55 μm. The rms roughness depths are provided by surface profilometer measurements and the roughness correlation length is estimated by finding the best match between the measured and the model reflectance for varying roughness correlation length. Except one smooth surface, all other aluminum samples studied have roughness ratio ( = roughness correlation length/rms roughness depth) less than 5. We compare the Mueller matrices between the lab measurement and the theoretical model. The model results show that the off-diagonal elements of the matrix have a symmetry relation and the magnitudes of diagonal elements are nearly 1, implying negligible depolarization for angles less than 30°. The lab measurements show that the off-diagonal elements have a symmetry relation for a smooth sample but the symmetry relation is weaker for rougher samples (lower roughness ratios). The lab data also show that depolarization is about 2% for the smooth sample but larger than 25% for the rougher samples for angles near 0°. The smooth surface shows reasonable agreement between the lab data and the model result except higher depolarization shown by the lab data for angles larger than 30°. On the other hand, the rough samples do not show similar agreement as the smooth surface shows. Possible causes of discrepancies are discussed and improvements for the lab measurement and model are suggested.