Optimization of radial smoothing scheme for improving illumination uniformity of laser quad

Abstract

Radial smoothing scheme based on the optical Kerr effect can add periodical spherical wavefront modulation to the laser beam and make the speckles within the focal spot in far field sweep periodically in radial direction, which improves the illumination uniformity of the laser beam on target in inertial confinement fusion facilities. In order to analyze the smoothing performance of radial smoothing scheme in a laser quad, the corresponding theoretical model was established and the temporal characteristic parameters of the pump laser were further analyzed and optimized. The results show that the fluctuation of the smoothing performance with time can slow down by appropriately adjusting the delay time between the laser beam and pump laser pulses in a laser quad. Furthermore, the different sweep period of the incoherent beams can be obtained by altering the pulse duration of pump laser pulses in a laser quad. Consequently, the group velocity of intensity envelope becomes faster with the increase of the frequency difference between incoherent beams. What’ more, a better smoothing performance can be achieved by applying the appropriate spatial deflection between the laser beam and pump laser pulses in a laser quad, and the deflection angle of the pump laser in a laser quad was further optimized to improve the illumination uniformity on target significantly. Radial smoothing with obliquely incident pump laser can add a spherical wavefront to the laser beam, and further makes the speckles redistribute rapidly both in radial and transverse directions. With appropriate incident direction and angle, RS can achieve a much better final smoothing performance than 2D-SSD.