Nearly degenerate four-wave mixing using phase-conjugate pump beams

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A novel configuration has been developed for performing degenerate four-wave mixing (DFWM) experiments using optical phase conjugation from stimulated Brillouin scattering in a cell. The typical geometry for DFWM requires that two pump beams counterpropagate through the measurement volume, which is accomplished either by splitting off a portion of the beam and directing it in from the opposite direction or reflecting the first pump beam back onto itself. In both approaches it can be difficult to maintain alignment between the counterpropagating pump beams, particularly in combustion systems that present thermal and density gradients. DFWM has been demonstrated for OH in a flame by using counterpropagating pump beams that are phase conjugate. The optical phase conjugation is achieved by focusing the first pump beam into a cell containing hexane, which produces a counterpropagating beam through stimulated Brillouin scattering. This approach greatly simplifies DFWM experiments by relaxing alignment constraints and sensitivities to index-of-refraction gradients in the flow.