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

Abstract

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%.

Spectral evolution and stimulated Brillouin scattering suppression in phase-modulated fiber amplifier

“…The ideal top-hat shaped spectrum cannot be obtained with a pure phase modulation, as shown in paper [15], but it is possible to come close by optimizing the modulation signal. The principle of the approaches to achieve nearly top-hat shaped spectrum is judiciously select the modulation signal with optimal frequencies and amplitudes.…”

“…Phase modulating to signal on time scales shorter than the phono lifetime can decrease the Brillouin gain and increase the SBS threshold. The prevalent phase modulation seeds consist of sinusoidal [7,10], white noise signal (WNS) [11][12][13], pseudo-random binary sequence (PRBS) [14] and nearly top-hat shaped spectrum seed [15,16]. The theoretical models predicting and comparing the SBS enhancement capacity of sinusoidal, WNS and PRBS phase modulation techniques have been reported [17][18][19].…”

Spectral and temporal properties of optical signals with multiple sinusoidal phase modulations

“…This is the useful range for application to high-energy lasers since the impairments and amplitude modulation must be minimized. Signals with commensurate frequencies have been studied in the context of providing a uniform spectral density over a given bandwidth, but the resulting on-target smoothing might be impacted by resonances [23]. These signals should be the subject of a separate study if they find an application for beam smoothing.…”

“…Inspection of Eqs. (16) and (23) shows that the corresponding modulations occur in quadrature, each of them being either a sine or a cosine of the argument Ω j t. The PV AM at frequency Ω j resulting from the in-quadrature modulations with respective PV amplitudes 2lm j Ω j and 2φ 2 m j Ω 2 j is 2φ 2 m j Ω 2 j 2 2lm j Ω j 2 q , i.e.,…”

SBS management in Yb-fiber-amplifiers using multimode seeds and pulse-shaping

“…Plus or minus efficient techniques may be used to cancel out or reduce SBS effects, using Large-Mode-Area fibers. A very common option is based on the use of active phase-modulation [5,6], to be selected for single-frequency operation [2,3]. Other options [7][8][9][10] either consist of specific fiber core-clad designs or of the application of thermomechanical constraints by external means, when fibers are long enough.…”