SBS Suppression Capability of Optimized Pseudo-Random Binary Sequence Phase Modulation in Multi-Stage Fiber Amplifiers

We demonstrate the capability to suppress stimulated Brillouin scattering (SBS) in a high-power all-fiber laser amplifier system using filtered and amplified pseudo-random binary sequence (PRBS) phase modulation techniques. Based on the time-dependent three-wave coupled SBS interaction equations in an amplifier model consisting of active fiber and passive fiber and spectral calculation of phase modulation, we numerically simulate the dependence of the normalized SBS threshold and the root-mean-square (RMS) linewidth on both the filter cutoff frequency and the phase modulation depth for filtered and amplified PRBS phase modulation at a fixed clock rate with different pattern lengths. PRBS9 is superior to other investigated patterns. A set of optimal pattern lengths, RMS modulation depths, and the ratio of the filter cutoff frequency to the clock rate are determined. Specific time-domain details of the variation of the RF signal with experimentally measured RMS modulation depth are shown. The dependence of different time-domain waveforms and their corresponding spectra and SBS thresholds on the RMS modulation depth is illustrated by theoretical predictions and experimental measurements, and the optimal value of the RMS modulation depth is demonstrated. Then, both the RMS linewidth of the optical spectra and the maximum normalized SBS threshold under the optimized parameters increase linearly with clock rate. While, with the further increase of the clock rate to ∼14 GHz, the SBS threshold reaches a saturation point when the maximum effective spectral linewidth is reached, where the spectral line spacing is half of the FWHM Brillouin linewidth, and the optimal spectral line spacing is not affected by the fiber length of the system. Eventually, a laser power output of 2.78 kW at an FWHM linewidth of 9.95 GHz is experimentally obtained.