Optimization of filter frequency ratio in pseudo random modulated fiber amplifiers for maximally suppressing stimulated Brillouin scattering

A filtered pseudorandom binary sequence (PRBS) phase modulation scheme to broaden the linewidth of a fiber laser has previously been shown to simultaneously enhance stimulated Brillouin scattering (SBS) suppression and spectral brightness relative to a fiber laser under an unfiltered modulation scheme. We examined in detail the unwanted distortions to the broadened spectrum of an optical signal due to the low-pass filtering of the PRBS modulation signal, and show how one can navigate these deleterious effects for an optimal filtering strategy for SBS suppression. Through numerical simulations of the filtering effects on the optical spectrum, we determined the optimal ratios of the filter bandwidth to the PRBS modulation frequency to be 0.63, 0.56, 0.50, and 0.44 for bit numbers k = 7, 8, 9,10, respectively. A cursory experimental study for the k= 7 case verified the approach, which provides a more precise filtering-to-modulation frequency ratio for improved SBS suppression than the more general guideline of filtering at approximately half the modulation frequency reported in the literature. Applying this optimization strategy with a filtered 4.6 GHz PRBS scheme, we demonstrated a 1 kW, M-2 similar to 1.27, all-fiberized narrow linewidth amplifier. (C) 2022 Optical Society of America

Automated summary

Filtered PRBS phase modulation scheme can enhance SBS suppression and spectral brightness compared to unfiltered scheme, but optimization of filtering strategy is required to avoid adverse effects. Numerical simulations suggest optimal filter bandwidth to PRBS modulation frequency ratios can be adjusted based on bit numbers.