Smoothing scheme for intensity sweep and polarization rotation at a subpicosecond timescale

A novel scheme for decreasing the scattering levels of stimulated Raman scattering (SRS) and stimulated Brillouin scattering (SBS), as well as for improving illumination uniformity of focal spots, is proposed. In this scheme, four beamlets in a laser quad are frequency shifted, spatial phase shaped by conjugate phase plates, and counter-rotating circularly polarized by polarization control plates before being focused by wedged lenses. Consequently, the intensity sweep and the polarization rotation of the focal spot in the timescale of a subpicosecond are achieved, resulting in the reduction of the scattering level of SRS and SBS, and also the improvement of the illumination uniformity when the frequency shift among beamlets is comparable to the parametric backscattering growth rate and the filamentation instability growth rate. The theoretical model of the novel smoothing scheme has been established, and the influences of some key parameters including spatial modulation type, peak-to-mean value of phase distortion, and frequency shift have been discussed. Results show that the uniformity of the focal spot of this novel scheme is improved much faster than that of the conventional temporal smoothing scheme, i.e., smoothing by spectral dispersion. The parametric instabilities are significantly reduced and both phase distortion and modulation depth of the laser quad have a large tolerance, opening a new dimension for controlling the incident light in laser plasma interaction.