Nonsinusoidal phase modulations for high-power laser performance control: stimulated Brillouin scattering and FM-to-AM conversion

High-power lasers, such as the Laser MegaJoule (LMJ), have to be phase modulated to avoid stimulated Brillouin scattering (SBS) that may strongly damage optics at the end of the laser chain. Current spectral broadening on LMJ is performed with a sinusoidal phase modulation. This pure sinusoidal phase modulation leads to inhomogeneous spectral power densities (SPD). Thus, for a same SBS power threshold, the sinusoidal phase-modulated spectrum has to be larger than the equivalent ideal SPD with isoenergetic peaks. We present in this paper a technique to generate energy-balanced Dirac peaks spectra thanks to nonsinusoidal phase modulations. Thus, we can build a narrower spectrum with a nonsinusoidal phase modulation that has the same SBS threshold as a sinusoidal phase modulation, and we show that FM-to-AM conversion can be strongly reduced, which is of great interest for LMJ laser performance, with reductions up to 40%.