IMAGING OF TRACE SPECIES DISTRIBUTIONS BY DEGENERATE 4-WAVE-MIXING - DIFFRACTION EFFECTS, SPATIAL-RESOLUTION, AND IMAGE REFERENCING

We describe the theory of imaging by degenerate four-wave mixing (DFWM ) using a standard diffraction theory of imaging by coherent light. We demonstrate that, even with the phase-conjugating geometry, no aberra tion correction can be achieved by DFWM imaging. We demonstrate the co herent nature of DFWM image formation using spatially modulated signal s generated in flame OH in the phase-conjugating geometry. The intensi ty distribution in the Fourier plane of a telecentric lens system is s hown to be the spatial Fourier transform of the object distribution ch aracteristic of coherent imaging The brightness of the DFWM signals ex ceeds that of similar laser-induced fluorescence signals that can be d iscriminated by restricting the aperture of the imaging system while s till allowing a spatial resolution of approximately 70 mu m. DFWM imag ing with the forward-folded boxcars geometry is demonstrated and used in a simple referencing scheme to compensate for structure on the imag es imposed by nonuniformity of the laser beams employed. Images formed in NO are used to illustrate that structure on a scale of less than 1 00 mu m arising from beam inhomogeneity can be removed by this referen cing technique. (C) 1997 Optical Society of America.