Threshold reduction for stimulated Brillouin scattering using a multipass Herriott cell

Abstract

We present an analysis of stimulated Brillouin scattering (SBS) threshold reduction through periodic refocusing of the pump laser within the Brillouin medium. The maximum achievable threshold-reduction factor is limited to ~0.85/(1 − T), where T is the transmission between foci. We also analyze SBS gain in a multifocus system for a pump consisting of two narrow-band modes. If Ω is the angular frequency of the mode spacing, maximum gain will result from a periodic focal spacing of integral multiples of πc/Ω, i.e., one half the laser coherence length. We show that a Herriott cell, a two-mirror system capable of periodically refocusing a beam in a relatively small volume, is a convenient device for producing multiple foci for this application. We present results of experiments designed to measure spontaneous SBS threshold with as many as 24 sequential foci in high-pressure xenon. A pulsed hydrogen fluoride chemical laser, operating at 2.910 μm on mainly two adjacent axial modes, was used as the pump. The ratios of the measured thresholds compared with that for a single focus are in reasonable agreement with those expected from the theory developed. Finally, we demonstrate that the Herriott cell is capable of high-fidelity optical phase conjugation, providing good reflectivity and correction of added phase aberrations under conditions in which the aberrated pump power is below SBS threshold for a single focus.