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

V. Balaswamy; Roopa Prakash; Vishal Choudhury; Santosh Aparanji; B. S. Vikram; V. R. Supradeepa

doi:10.1117/12.2510317

4 March 2019 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

V. Balaswamy, Roopa Prakash, Vishal Choudhury, Santosh Aparanji, B. S. Vikram, V. R. Supradeepa

Author Affiliations +

Proceedings Volume 10902, Nonlinear Frequency Generation and Conversion: Materials and Devices XVIII; 109021G (2019) https://doi.org/10.1117/12.2510317
Event: SPIE LASE, 2019, San Francisco, California, United States

Abstract

Power scaling of narrow-linewidth, continuous-wave, fiber lasers with near-diffraction-limited beam quality is primarily limited by stimulated Brillouin scattering (SBS). Among several SBS mitigation techniques, line broadening by phasemodulation has been widely used. Recently, enhanced SBS seeding (threshold reduction) due to spectral overlap between the backscattered, line-broadened signal and the SBS gain spectrum has been reported. Backscattering of the signal is composed of the Rayleigh component and reflections from the end termination. However, in high power amplifiers 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 to substantially 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 fiber terminations are utilized. In the first case, the amplifier is terminated by a high-power laser cable with an end-cap and anti-reflection coating and in the second case, by an angle cleaved passive delivery fiber. Back-reflections from the angle cleaved facet (<80) providing ~70dB isolation (ideal case) was enough to enhance SBS. We analyzed the threshold differences between the two cases as a function of linewidth from 4.91GHz to ~10GHz. At smaller linewidths, the difference was negligible while at larger linewidths, there was a substantial difference in thresholds (<20%). This linewidth dependent difference in thresholds was accurately simulated by the backward seeding of SBS by the linebroadened signal, thus conclusively proving this effect.

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V. Balaswamy, Roopa Prakash, Vishal Choudhury, Santosh Aparanji, B. S. Vikram, and V. R. Supradeepa "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", Proc. SPIE 10902, Nonlinear Frequency Generation and Conversion: Materials and Devices XVIII, 109021G (4 March 2019); https://doi.org/10.1117/12.2510317

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