Continuous wave stimulated Brillouin scattering phase conjugation and beam cleanup in optical fiber

Abstract

A review of research into stimulated Brillouin scattering (SBS) phase conjugation and beam cleanup conducted at the Air Force Institute of Technology is presented. CW phase conjugation was demonstrated using SBS in short lengths of multi-mode, step-index fiber. A fidelity of 0.8 was achieved using a 0.06-NA fiber of 40 m length and a 0.13-NA fiber at 15 m length. In modeling and experiment, the fidelity declined as fiber length or numerical aperture increased. A relationship was established empirically between step-index fiber parameters and the phase conjugation fidelity. In addition, increased fidelity was related to an increase in the effective Brillouin gain coefficient in step-index fibers. Beam cleanup was observed in both graded-index and step-index fibers. The Stokes beam generated in long, multimode, graded-index fibers was analyzed using spatial and phase methods and found to be a low-order mode of the fiber instead of a phase conjugate of the pump. SBS in long, graded-index fibers was used to combine multiple beams and to improve the beam quality of a single aberrated beam.