A simple method for ultrafast smoothing and polarization rotation by interfering beamlets with spatial phase difference

Abstract

The coherent superposition of beamlets with proper frequency shift, spatial phase difference and polarization control can smooth the irradiation uniformity and rotate the polarization of focal spot on picosecond timescale. In our previous work, the introduction of spatial phase difference requires insertion of additional phase plates that might increase the laser cost and complexity. In this paper, we propose a simple method to introduce the required spatial phase difference without inserting any additional optical elements in the beamline, but good-working in improving irradiation uniformity and rotating polarization of the focal spot. This novel method takes full advantages of continuous phase plate to explore its potential in introducing the spatial phase difference among beamlets. Two representative approaches have been presented, i.e., the first is designing individual CPP for each beamlet which can generate focal spot with same intensity envelope but different speckles individually, and the second is mounting the identical CPP for each beamlet in different orientations. Results indicate that, this simple method can effectively improve the irradiation uniformity within the sub-picosecond timescale and bring about polarization rotation to suppress the parametric backscattering.