Novel dynamic wavefront control scheme for ultra-fast beam smoothing

Abstract

In inertial confinement facilities, the irradiation uniformity of the lasers is highly required to suppress the laser plasma instabilities. In order to realize the ultrafast smoothing of the focal spot, a novel scheme by using an optical Kerr medium and a high-power pump laser is proposed. The principle of the ultrafast beam smoothing scheme is to change the refractive index of the Kerr medium with the pump laser, which appends a spatiotemporal wavefront to the main laser beam in the beamline. The dynamic wavefront modulation of the main laser beam further makes the speckles within the focal spot redistributed rapidly and complicatedly, which contributes to the smoothing of the focal spot. A Gaussian beam with a temporal profile of a Gaussian pulse train is obliquely incident on the optical Kerr medium at a small angle. As a result, the spherical wavefront of the main laser beam is rapidly changed in the direction perpendicular to the propagation direction of the main laser beam. Thus the transverse and the radial redistribution of the speckles within the focal spot are both generated simultaneously. Comparing with the simple radial smoothing scheme, the spherical phase of the main laser beam always changes perpendicularly to the propagation direction in the novel scheme, and thus achieving a more stable beam smoothing effect. Besides, the phase gradient in the center region of the main laser beam changes greatly over time, making the irradiation uniformity on the focal plane further improved. The optimal deflection angle in the optical Kerr medium of the pump laser is obtained. By controlling the deflection angle of the pump laser, the spatial period of the pump laser in the transverse direction is set to be equal to the waist diameter of the main laser, which is identical with one color cycle in the typical smoothing by spectral dispersion technique. Moreover, a relatively low control precision of the deflection angle of the pump laser is required.