Optimization of continuous phase plate for reducing near-field diffraction modulation of back-stimulated Brillouin scattered light

A continuous phase plate (CPP) is often employed to shape and smooth laser beams to improve focal spot quality in inertial confinement fusion facilities; however, near-field modulation resulting from back-stimulated Brillouin scattering (SBS) and the CPP may damage optical devices. In this study, we first explored the relationship between the back-near-field diffraction intensity and the CPP phase structure based on SBS and CPP diffraction models. Subsequent theoretical analysis and simulations showed that strong modulation of the back-near-field diffraction occurs near the extremum of the CPP phase Laplacian. Then, the probability distribution of the CPP phase Laplacian was calculated to determine optimize threshold in real-time, and CPP phase local spatial filtering was performed to reduce phase fluctuation to constrain the near-field modulation in the iteratively designed CPP. The improved CPP can effectively reduce the back-near-field modulation, including the local maximum intensity and overall contrast over a long diffraction distance, while the uniformity of the far-field focal spot is slightly reduced within an acceptable range of 2% to satisfy the high requirements for the focal spot uniformity. The optimization of the CPP is of great significance for reducing the damage of high-power laser devices and practical application of CPP.