Abstract

This paper addresses the noise processes which initiate stimulated Brillouin scattering (SBS). The SBS process is described by a set of coupled equations for the laser and Stokes field amplitudes and the amplitude of the density oscillations. The quantum or thermal noise which initiates the SBS process is described by Langevin forces which drive the density variations at all points in space. Previously, simple models of the initiation of SBS have assumed that noise is present only at the boundary of the interaction region. In the limit where the laser field is undepleted by the interaction we have solved these equations analytically. We thereby obtain an expression which shows how the laser intensity at threshold depends on the laser frequency and on the material parameters and temperature. In this limit, the system acts as a noise amplifier. The optical spectrum shows gain narrowing with increasing laser intensity, and the output is very noisy, showing 100% intensity fluctuations. In the regime where the laser intensity is depleted by the interaction, the equations are solved numerically using the method of characteristics. We find that the intensity fluctuations are strongly (but not completely) quenched for laser intensities near and above the threshold intensity. We have studied these effects in an experiment performed using an optical fiber. Gain narrowing of the output spectrum is observed. Also, we have found that the threshold intensity and optical spectrum are strongly modified by the presence of a reflecting boundary at one end of the interaction region.

© 1989 Optical Society of America