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9 March 1999 Implementation and performance considerations of hybrid digital/optical correlator configurations
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Two-dimensional correlation between a reference template and an input scene is a powerful pattern recognition technique but is demanding of computational power. Coherent optical correlators, exploiting the Fourier transforming properties of a lens and the capability to impart a phase modulation on a wavefront with an appropriate spatial light modulator (SLM), hold the promise of real-time implementation of two- dimensional correlation for realistic pattern recognition problems. However, their practical use has been delayed in many applications by the lack of availability of suitable SLM devices with the required speed and dynamic range, with different needs for input and frequency plane modulators. It is now possible to compute a two-dimensional Fourier transform at video-rates with various digital signal processing chip sets. Thus a hybrid correlator is proposed in which the input scene is digitally Fourier transformed at video-rate, and multiple templates searched during the next video frame interval by optical mixing and Fourier transformation at a speed at least two orders of magnitude faster than possible with digital methods. In this way, the input SLM is avoided and a precise spectrum is available for subsequent digital or optical mixing with the stored templates. The speed advantage over all-digital processing allows unconstrained pattern recognition problems to be tackled that require many template searches to match the input with a reference function. Different hybrid correlator configurations are considered, together with discussion of the various digital chip sets available to perform the video-rate FFT, as well as the SLM devices currently available that are suitable as frequency domain phase modulators.
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Rupert C. D. Young, Shiping Huang, Gongde Li, Triantafillos Koukoulas, Malcolm I. Heywood, David M. Budgett, and Christopher R. Chatwin "Implementation and performance considerations of hybrid digital/optical correlator configurations", Proc. SPIE 3715, Optical Pattern Recognition X, (9 March 1999);

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