Two-dimensional imaging of OH in a flame by using degenerate four-wave mixing in a forward geometry

The use of degenerate four-wave mixing in a three-dimensional, forward four-wave mixing geometry for two-dimensional imaging of combustion species is reported. In this configuration, a beam of circular cross section interacts in the flame the crossing area of two beams that have been focused to form thin sheets of light. The generated signal beam propagates in the direction that fulfills the phase matching condition and, therefore, is spatially separated from the other beams. The thin intersection volume of the three beams is imaged onto a detector. Thus, information on the signal distribution in one plane in the sample can be recorded. The method is demonstrated with experiments performed on OH molecules in a premixed acetylene-oxygen flame.