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We present what is to our knowledge the first demonstration of a potentially low-cost refractive-index sensor based on UV processing. A channel waveguide and a Bragg grating are defined in a single UV processing step, resulting in a buried structure with a well-defined grating period. A subsequent wet etch process located over the Bragg grating opens a sensing window in the device and reveals the grating structure. Sensitivity of as much as 5 x [10.sup.-6] was inferred from our device. OCIS codes: 130.0130, 130.6010.

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 aberration correction can be achieved by DFWM imaging. We demonstrate the coherent nature of DFWM image formation using spatially modulated signals generated in flame OH in the phaseconjugating geometry. The intensity distribution in the Fourier plane of a telecentric lens system is shown to be the spatial Fourier transform of the object distribution characteristic of coherent imaging. The brightness of the DFWM signals exceeds that of similar laser-induced fluorescence signals that can be discriminated by restricting the aperture of the imaging system while still allowing a spatial resolution of approximately 70 [[micro]meter]. DFWM imaging with the forward-folded boxcars geometry is demonstrated and used in a simple referencing scheme to compensate for structure on the images imposed by non-uniformity of the laser beams employed. Images formed in NO are used to illustrate that structure on a scale of less than 100 [[micro]meter] arising from beam inhomogeneity can be removed by this referencing technique.