Stimulated Brillouin scattering, phase noise, and dynamical evolution in a passive fiber laser coherent beam combination

Abstract

A theoretical model is proposed to study a passive fiber laser coherent beam combination (CBC) with an all-optical ring-cavity feedback loop. The dynamic evolution of a frequency filter is carefully studied and is shown to be sensitive to stimulated Brillouin scattering (SBS) and phase noise. SBS plays an important role in the process of mode selection or CBC because the linewidth of the longitudinal mode is narrowed continuously in the ring-cavity feedback. Numerical studies show that the SBS process is significantly influenced by phase noise, optical path length difference, and four-wave mixing. Our results indicate that SBS would be a remarkable problem in this type of CBC unless the phase noise causes a rapid mode jump in the frequency domain. Since the mode jump worsens the efficiency of the coherent combination or beam quality, a trade-off consideration is necessary for the SBS, phase noise, and beam quality. The maximal SBS-allowed CBC efficiency is obtained numerically for output laser power of 700 W–3.5 kW.