Experimental analysis of Stimulated Brillouin enhancement in high power, line-broadened, narrow-linewidth fiber amplifiers due to spectral overlap between the Brillouin gain spectrum and the signal back-scatter from the fiber termination

摘要

Power scaling of narrow-linewidth, continuous-wave, fiber lasers with near-diffraction-limited beam quality is primarilylimited by stimulated Brillouin scattering (SBS). Among several SBS mitigation techniques, line broadening by phasemodulationhas been widely used. Recently, enhanced SBS seeding (threshold reduction) due to spectral overlapbetween the backscattered, line-broadened signal and the SBS gain spectrum has been reported. Backscattering of thesignal is composed of the Rayleigh component and reflections from the end termination. However, in high poweramplifiers with small lengths of optical fiber used, the Rayleigh component of the backscatter is anticipated to be small.Here, we report conclusive experimental evidence that even very small reflections from the output facet are enough tosubstantially reduce the SBS threshold due to spectral overlap. We demonstrate this in a 500W, white noise phasemodulated,narrow-linewidth, polarization-maintaining power amplifier operating at 1064nm. Two commonly used fiberterminations are utilized. In the first case, the amplifier is terminated by a high-power laser cable with an end-cap andanti-reflection coating and in the second case, by an angle cleaved passive delivery fiber. Back-reflections from the anglecleaved facet (>80) providing ～70dB isolation (ideal case) was enough to enhance SBS. We analyzed the thresholddifferences between the two cases as a function of linewidth from 4.91GHz to ～10GHz. At smaller linewidths, thedifference was negligible while at larger linewidths, there was a substantial difference in thresholds (>20%). Thislinewidth dependent difference in thresholds was accurately simulated by the backward seeding of SBS by the linebroadenedsignal, thus conclusively proving this effect.