Double phase-conjugate four-wave mixing of oh in flames

Abstract

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 counter-propagate 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 on itself. In both approaches it can be difficult to maintain alignment between the counter-propagating, pump beams, particularly in combustion systems which present thermal and density gradients. DFWM has been demonstrated for OH in a flame using counter-propagating 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 counter-propagating beam via stimulated Brillouin scattering. This approach greatly simplifies DFWM experiments by relaxing alignment constraints and sensitivities to index of refraction gradients in the flow. The method has been applied to temperature measurements in flames and to the imaging of a turbulent diffusion flame simplifying, the spatial ambiguites which arise from signal beam steering. Additionally, it is, observed that at high laser intensities the spectral profiles display a dip at line center. The effect is attributed to saturation of the molecular transition.