The concept of "slow light" and nature of Stokes pulse delay in stimulated Brillouin scattering

Abstract

This work addresses the nature of the delay experienced by pulsed Stokes radiation when amplified by stimulated Brillouin scattering (SBS), topically referred to as “slow light in SBS”. The term “slow light” refers to the propagation of a light pulse in a medium in which the group velocity of the pulse is considerably lower than the phase velocity of light. A comprehensive review of the literature on “slow light” has revealed a range of inconsistencies in attributing experimentally observed pulse delays to the group velocity effect. For the case of SBS the controversies are resolved through analytic solutions of the basic coupled SBS equations in both the frequency and time domains. The solutions provide the first mathematically rigorous and physically non-contradictory description of the temporal, spectral and energy characteristics of the Stokes radiation and of the induced acoustic wave in an SBS amplifier. Based on these solutions, a theoretical model of Stokes pulse propagation through a CW-pumped SBS medium is developed, the so called “inertial” model. The solutions are verified experimentally through study of the Stokes pulse dynamics in a set of fibers with different inhomogeneous SBS bandwidths and acoustic wave relaxation times. The results obtained confirm that the delay, shape and amplitude of the output Stokes pulse follow the predictions of the “inertial” model and that, contrary to popular opinion, the phenomenon of group delay, or “slow light”, is irrelevant to the observed delays.