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

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.