A simple systems-oriented model of image correlation by nondegenerate four-wave mixing in photorefractive media is discussed, and its computational implementation is described. The model is based on a linear-incomplex modulation index formulation and is capable of describing the correlation product in a wide range of experimental conditions. Bragg matching effects on the test and reference images are explicitly included. The comparison of calculated results with experimental results is excellent, and a discussion of higher-order theories of the photorefractive effect is given. The model is currently being used to optimize the performance of experimental correlators.
A technique for accurate measurement of the linear electro-optic coefficient(s) in crystals that are simultaneously optically active and birefringent is described. It is shown that this technique provides a direct method of measuring field-induced birefringence and so can form the basis of a Bi(12)SiO(20) (or Bi(12)GeO(20)) sensor arrangement. The high degree of accuracy obtained in the measurements of the linear electro-optic coefficient in Bi(12)SiO(20) and Bi(12)GeO(20) crystals indicates the particular suitability of these materials in electric-field and voltage-sensor systems with a high dynamic range and for possible waveguiding applications.
This paper describes initial work on the design and application of a versatile optical image processor based on the combination of degenerate four wave mixing (DFWM) and an optically addressed spatial light modulator. This has demonstrated for the first time a processor which can be both programmed and addressed in real -time and which through the use of a low power HeNe laser, has enabled an appreciable reduction in the size and power requirements of such a system.The combination of SLM and DFWM technologies enables a number of processing operations to be implemented. The light valve can be used both as a means of implementing incoherent to coherent image conversion and for certain preprocessing operations such as contrast modification and image subtraction.Degenerate four wave mixing is an all -optical phenomenon whereby three input waves mix within a non -linear medium to generate a fourth wave.By spatially modulating two of the input waves, and including Fourier transforming elements, the output wave can, under certain conditions, represent the correlation product of the two inputs.The design specification for the processor and progress towards its practical implementation will be considered.An assessment of the performance of the system will be described and initial results for both phase conjugate imaging and pattern recognition (for simple test images) will be given.
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