Degenerate Four Wave Mixing techniques for Combustion Diagnostics

Degenerate Four Wave Mixing, DFWM, is a third order nonlinear optical process which has been used for high resolution spectroscopy and, by virtue of its phase conjugating property, for adaptive optics(1). The process is closely related to Coherent Anti-Stokes Raman Scattering, CARS, which is widely used for temperature and concentration measurements of majority species in combustion studies(2). Minority species, on the other hand are usually detected using the more sensitive technique of Laser Induced Fluorescence, LIF. DFWM shares the coherence property of CARS and the sensitivity of LIF and thus provides a coherent technique for the detection of minority species such as radicals and combustion intermediates. This new technique has the added advantageous property of phase conjugation. DFWM signals have been generated from atomic Na in seeded flames(3,4) and the potential of the technique for combustion diagnostics was demonstrated by the detection of the OH radical in a methane/air flame(3). More recent developments have included two-dimensional single-shot imaging of atomic and molecular distributions(5,6), accurate flame temperature measurements based on rotationally resolved DWFM spectra(7), broadband or multiplex DFWM in flames(8) and two-dimensional mapping of temperature fields(9).