Origin of spectral hole burning in Brillouin fiber amplifiers and generators

We show theoretically that the spectral hole burning recently observed in Brillouin fiber amplifiers and generators can be interpreted without invoking an inhomogeneous broadening of the Brillouin line. By using the three-wave model of stimulated Brillouin scattering (SBS), we first investigate the linear response of a Brillouin fiber amplifier to a weak amplitude modulation of the injected signal wave. The transfer functions that fully characterize this response are analytically calculated. The fact that they may exhibit a dip is shown to be due both to the gain saturation and to the coupling between the pump and signal perturbations that counterpropagate around the steady-state intensity profiles. These transfer functions also appropriately characterize the way through which noisy perturbations are filtered in SBS fiber generators. The spectral hole burning observed in generators is thus simply connected to spectral hole burning studied in SBS amplifiers.