Observation and compensation of frequency detuning in high-reflectivity Brillouin enhanced four-wave mixing

Abstract

Details of the frequency relationships among interacting beams in Brillouin enhanced four-wave mixing (BEFWM) are examined. A new technique using two individually injection-seeded single-mode lasers allows for precise control and knowledge of the relative wavelengths of the interacting beams. This is compared with the standard frequency-shifting technique employing a single laser in conjunction with stimulated Brillouin scattering (SBS) frequency shifters. A continuously tunable frequency difference ranging over many gigahertz is achieved, resulting in a valuable tool for the investigation of nondegenerate four-wave mixing processes such as BEFWM. Two variations of BEFWM are studied: ωpump1 − ωpump2 = 0 with noncollinear pumps; and ωpump1 − ωpump2 = ωSBS with collinear pumping. Resonance spectra of BEFWM gain versus probe–pump frequency shift show a significant detuning in the frequency for the gain maximum from the expected Brillouin shift for the former case. When the probe–pump frequency is tuned in compensation for this detuning, improvements in BEFWM phase conjugation are realized in comparison with more conventional techniques using SBS frequency shifting.