Seeding High Brightness Fiber Amplifiers With Multi-Phase Coded Signal Modulation for SBS Effect Management

We propose a new modulation approach based on multi-phase coded signal (MPCS) to increase stimulated Brillouin scattering (SBS) threshold in high power narrow-linewidth fiber amplifiers. To investigate the SBS suppressing capacity of the proposed modulation technique, the dynamics of SBS process with different spectra are simulated in active fibers with different dopants, core diameters, lengths and pumping lights based on three typical types of pumping sources (976 nm laser diodes, 915 nm laser diodes and 1018 nm fiber lasers). Before simulation, a spectral-correlated combining efficiency criterion based on coherent beam combining (CBC) system is introduced and is set to be identical in different spectra. As a result, compared with the typical white-noise-signal (WNS) and pseudo random bit sequence (PRBS) modulation techniques, the MPCS modulation yields a higher SBS threshold in 915 nm and 1018 nm pumped fiber amplifiers with relatively longer active fiber. Besides, in fiber amplifier constructed by general 20/
$400~\mu \text{m}$
active fiber and with 976 nm pumping strategy, MPCS modulation performs better than WNS and PRBS modulation as well. We believe that this modulation signal is expected to be applied in narrow-linewidth, large-mode-area (LMA) high-brightness fiber laser systems to balance the SBS effect and thermal mode instability (TMI) phenomenon.