Simulation analysis of polarization smoothing based on full Poincaré beam modulated by single transmission element

Abstract

Polarization smoothing is a means to increase the uniformity of the focal spot by using polarization characteristics of light. It can theoretically reduce the contrast of focal spot to 0.707 times, and because of its real-time modulation characteristics, it can significantly suppress backscattering and improve the beam-target coupling performance of high-power laser facilities. However, in the traditional polarization smoothing scheme, the crystal used for polarization smoothing (largeaperture KDP crystal) will produce transverse stimulated Raman scattering in the high-power ultraviolet band, which will cause crystal damage. To prepare for the exploration of new single beamlet polarization smoothing schemes, we conduct a simulation study on the scheme of modulating the full Poincaré beam with a single transmission element. The results show that a single waveplate with a fast axis that rotates with polar angle and a phase retardation that increases with radius can modulate a full Poincaré beam, and we analyze the properties of this full Poincaré beam. Then combined with the engineering parameters, we analyzed the polarization smoothing performance of the full Poincaré beam with square aperture laser input. The performance was quantified by several target focal spot intensity evaluation indexes, and the results show that the use of the full Poincaré beam for polarization smoothing can reduce the contrast of the focal spot effectively, and the reduction range is close to the theoretical maximum value. And there are advantages in suppressing high energy density hot spots and optimizing the spatial frequency of the focal spot.