Suppression of Stimulated Brillouin Scattering in Single-Frequency Fiber Raman Amplifier Through Pump Modulation

Achar Vasant Harish,Johan Nilsson

doi:10.1109/jlt.2019.2914081

Abstract

We propose and implement the use of an intensity-modulated pump, counter-directional to a forward-propagating signal for suppressing unwanted stimulated Brillouin scattering (SBS) of spectrally narrow signals in optical fiber Raman amplifiers. The modulated Raman pump cross-phase modulates and thus spectrally broadens the parasitic Brillouin Stokes wave traveling in the same (backward) direction. The parasitic SBS then decreases when the broadening exceeds the Brillouin gain bandwidth. By contrast, the modulated pump does not induce any significant cross-phase modulation on the signal, which therefore can remain spectrally narrow. We study the effect of the pump modulation frequency and modulation format and experimentally obtain nearly 5 dB of SBS threshold enhancement and over 50 $\%$ pump-to-signal conversion efficiency.

Highlights

S TIMULATED Brillouin scattering (SBS) [1], [2] is a major obstacle for power scaling of fiber amplifiers with signal linewidths narrower than the SBS bandwidth, typically in the range 10-100 MHz in silica fibers
In fiber Raman amplifiers (FRAs), the problem of unwanted SBS is exacerbated by the relative weakness of stimulated Raman scattering (SRS) and the resulting need for large intensity-length product often achieved through the use of long fibers [3]
At 0.4 W of pump power, the XPM phase shift induced by one pump pulse at 1 nJ of energy within the walk-off length Lwo is given by φXP M = (4/3)γLwoEp/τp = (4/3)γτp[D(λs − λp)]−1PpT /τp = (4/3)γ[D(λs − λp)]−1PpT = 6 rad, independently of the duty cycle

Summary

Introduction

S TIMULATED Brillouin scattering (SBS) [1], [2] is a major obstacle for power scaling of fiber amplifiers with signal linewidths narrower than the SBS bandwidth, typically in the range 10-100 MHz in silica fibers This includes so-called singlefrequency signals, e.g., as generated by lasers operating on a single longitudinal mode. In fiber Raman amplifiers (FRAs), the problem of unwanted SBS is exacerbated by the relative weakness of stimulated Raman scattering (SRS) and the resulting need for large intensity-length product often achieved through the use of long fibers [3]. Both SBS and SRS are thirdorder nonlinear effects which scale in the same way with fiber

Methods

Results

Conclusion