Phasing of Stokes radiation under shock excitation of stimulated Brillouin scattering

Interaction of counterpropagating waves in Brillouin-active media has been analysed by numerical computation. The dynamics of the development of acoustic waves is described using a secondorder equation. It is shown that in the case of counterpropagating waves with sufficiently steep leading pulse edges (τ_f ≤ 3T₂, where T₂ is the acoustic phonon lifetime), SBS begins from the level of acoustic waves induced by shock excitation in the bulk of the active medium rather than from the spontaneous noise level. This mechanism determines the phase of the output Stokes wave, which is generated in the backward direction to the wave with the highest input intensity, irrespective of the ratio of counterpropagating-wave frequencies.