Abstract
We investigate the origin of line-shape induced enhancement of stimulated Brillouin scattering (SBS) in narrow linewidth, noise broadened, high-power fiber lasers. A polarization-maintaining seed laser with continuously tunable linewidth (single frequency to >10 GHz), based on white noise modulation was developed for this study. With increasing linewidths, a substantial difference in SBS thresholds was observed depending on the end termination utilized. This observation can be explained by the line-broadened source, having significant power in the Stokes frequency at larger linewidths, seeding the SBS process. Here, SBS threshold for the system terminated with an anti-reflection coated delivery cable is compared with a simple angle cleaved end termination. The influence of end termination on SBS threshold becomes significant with increased linewidths, showing >20% difference in output power between the two cases at ~10 GHz linewidth. The experiments are complemented by simulations, analyzing relative contributions of Rayleigh scattering and fiber end-facet reflections to SBS. At larger linewidths, due to substantial overlap between laser line-shape and SBS Stokes, with low end-facet reflectivity, Rayleigh is the dominant mechanism, which gives way to end-facet reflections with increasing reflectivity. The Rayleigh contribution is negligible at smaller linewidths, and end-facet reflectivity has a weaker influence than with larger linewidths.
Highlights
Polarized sources with narrow spectral width and high power are crucial for beam combining applications [1], [2], nonlinear frequency conversion [3], and remote detection
At large linewidths (∼10 GHz), which are required for higher power scalability, the use of AR coated delivery cable minimizes end-facet reflections, providing the best possible power scaling
We investigated line-shape induced enhancement of stimulated Brillouin scattering (SBS) in narrow linewidth, noise broadened, high-power fiber lasers
Summary
Polarized sources with narrow spectral width and high power are crucial for beam combining applications [1], [2], nonlinear frequency conversion [3], and remote detection. Linewidth broadening by phase modulation has become the primary technique due to the development of fiber-coupled lithium niobate phase modulators with low drive voltages [12]. Various phase modulating RF waveforms such as sinusoid, pseudo-random binary sequence (PRBS) [12], optimized arbitrary waveforms [14], and filtered white noise [12], [15], [16] have been demonstrated. Sinusoidal phase modulation produces a comb with poor flatness, causing the SBS threshold to be determined only by the comb line with the highest power, resulting in poor enhancement. Though PRBS and arbitrary waveform generators can tailor the spectrum, they are more complex and resource-heavy, making white noise source (WNS) the simplest way to implement line broadening
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