Random and pseudo-random phase modulations for FM-to-AM reduction in high power lasers

Frequency modulation to amplitude modulation (FM-to-AM) conversion is an important issue that can prevent fusion ignition with high power lasers such as with the Laser Megajoule (LMJ). A way to reduce FM-to-AM conversion is to change the phase modulation (currently sinusoidal). In this paper, we study the case of random phase modulation. We show, with numerical simulation modeling, FM-to-AM conversion induced by different transfer functions, in which such a modulation format, in some cases, may significantly reduce FM-to-AM conversion, particularly in the case of amplitude filtering. Phase filtering may create high overshoots, but the occurence probability is low, and, on average, random phase modulation is advantageous compared with sinusoidal modulation. We also demonstrate an equivalent smoothing efficiency with the LMJ facility with an important reduction of FM-to-AM conversion compared with the current solution. Taking particular random draws (pseudo-random modulation), we exhibit optimal performance.