Power scaling of narrow linewidth single-mode fiber amplifiers is hampered by the onset of stimulated Brillouin scattering (SBS) process at power levels in the order of a few Watts. The threshold for the onset of SBS in the above amplifiers may be increased by chirping the source laser linewidth in a controlled manner by implementing external phase modulation. It has been reported that different phase modulation formats such as noise and pseudo-random bit sequences (PRBS) provide reasonable levels of SBS threshold enhancement with increased linewidth. However, increasing the laser linewidth to larger value may compromise the overall efficiency in applications such as coherent beam combination (CBC). Therefore, there is a clear need to find the phase modulation format which can provide the best SBS threshold enhancement for a given linewidth.

In this work, we explore the use of multi-parameter Pareto-optimization with the objective of finding the optimum phase modulation format for a high-power fiber amplifier. A key aspect of our work is the accurate modeling of the SBS process in a fiber amplifier using numerical techniques. We perform experiments with optimized waveforms in a 100 W fiber amplifier and find its performance to be at least 30% better than noise modulation. Furthermore, we discuss the simulation results for extending our work to kW power levels. We will also discuss a novel method based on Karl-Pearson correlation coefficients to detect the onset of SBS in a fiber amplifier.