Revisiting laser optical smoothing in weakly-damped plasmas: from spectral dispersion to spectral distribution

Optical smoothing techniques are widely used in large high-power laser facilities designed for inertial confinement fusion research in order to improve the laser-energy deposition. However, those techniques are not sufficient and may produce deleterious effects in the laser chain, leading to the need of new laser smoothing schemes such, as those using large bandwidth laser, like the StarDriver laser [D. Eimerl et al. , J. Fusion Energy 33, 2014]. In order to reduce the unwanted conversion of frequency modulation into intensity modulation, it has been shown [M. Duluc et al. , Phys. Rev. Applied 12, 2019] that it is possible to divide by two the spectral width and distribute it on sub-beams, while maintaining smoothing performance for the reduction of the Stimulated Brillouin Scattering (SBS). We revisit optical smoothing by pursuing the spectral distribution along several colors. We reduce the total spectrum bandwidth by dividing the total spectrum in k segments distributed in k sub-beams. We succeed in decreasing the spectral bandwidth up to a monochromatic spectrum leading to the same SBS level. We also show that this configuration allows obtaining a more homogeneous spectrum leading to an even better beam smoothing.