Narrow spectral response of a Brillouin amplifier

We investigate the spectral response of a Brillouin amplifier in the frequency regime within the stimulated-Brillouin-scattering (SBS) bandwidth. This is done by amplitude modulating the pump with a low frequency; therefore, unlike in previous studies, the spectrum of the modulated pump is, in all cases, smaller than the SBS bandwidth. We show both theoretically and experimentally that, unlike phase modulation, amplitude modulation increases the Brillouin amplifier gain and that this effect has a very narrow bandwidth. Only modulation frequencies that are lower than a certain cutoff frequency increase the gain. This cutoff frequency can be arbitrarily small.