Measurement of the instability threshold in Brillouin enhanced four-wave mixing

Abstract

Four-wave mixing mediated by Brillouin scattering in the transient regime exhibits an instability which results in large phase conjugate reflectivities [A. M. Scott and K. D. Ridley, IEEE J Quantum Electron. QE-25, 438 (1989)]. In this paper we compare measurements of the threshold for the instability both as a function of the ratio of the second pump beam to the first pump beam and as a function of the phase mismatch. Phase conjugation in Brillouin enhanced four-wave mixing occurs through the interaction of a signal with two counterpropagating pump beams. In our experiments, the frequencies of all three input beams are different, and a small phase mismatch results. The magnitude of this mismatch was varied both by changing one of the input frequencies and the interaction length. The frequency of the second pump beam was changed by using stimulated Brillouin scattering in various mixtures. Thus we were able to vary the phase mismatch from four to thirteen resulting in a 50% change in the threshold intensity, in agreement with theoretical models. The ratio of the backward-to-forward pump beam intensities also affects the threshold, and measurements of this agree with the model.