Effect of radial polarization modulation on smoothing and polarization properties of focal speckle

Numerical calculation model of beam smoothing is established based on scalar diffraction theory. The model takes account of smoothing by spectral dispersion (SSD), random phase plate (RPP), and polarization smoothing (PS). Effect of radial polarization modulation on smoothing and polarization properties of focal speckle is studied based on the model. Speckle contrast and polarization degree are used to characterize smoothing and polarization properties, respectively. The results show that contrast and polarization degree of focal speckle can be reduced by radial polarization modulation. Approximately radially polarized beam can be generated by polarization control plate (PCP) made by jointing several half-wave plates. The influence on smoothing and polarization properties of the number of half-wave plates is relatively small. Actually, the effect of an approximately radially polarized beam generated by 8 half-wave plates is nearly the same as that of a perfectly radially polarized beam. Three methods for polarization smoothing (radial polarization modulation, orthogonal polarization modulation and using birefringent wedge) are analyzed and compared, and the results show that the effects of the three methods are similar.