<title>Implementation and performance considerations of hybrid digital/optical correlator configurations</title>

Abstract

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 videorate FFT, as well as the SLM devices currently available that are suitable as frequency domain phase modulators.

Influence of perturbations in a hybrid digital-optical correlator

“…Young and his co-workers in 1993 outlined an opticaldigital hybrid correlator system that allows the potential for a multikilohertz reference template search on data acquired at video rates [5][6][7][8]. In this hybrid digital-optical correlation approach, the input data is digitally Fourier transformed and is multiplied with the digitally presynthesized distortion-invariant filters, and the product function is displayed over an electrically addressed spatial light modulator (SLM).…”

“…Pattern recognition using optical techniques, one of the most important and promising applications, has generated considerable amount of interest in the optics community for last few decades [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15]. As a result, optical correlator systems have now been started appearing in the market place [15].…”

Fault detection and feature analysis in interferometric fringe patterns by the application of wavelet filters in convolution processors

“…There are commercial chip sets available, which perform a two-dimensional correlation of 128ϫ128 spatial bandwidth product at 200 Hz or more. 1 Still, there is a processing speed advantage of certain optical systems using light modulating materials of high switching speed. But all optical systems suffer from the nonideal modulation behavior of all optical components, which leads to noise-superimposed output signals.…”

“…But all optical systems suffer from the nonideal modulation behavior of all optical components, which leads to noise-superimposed output signals. 1 Current optical correlators use various kinds of materials to modulate the incident light wave with image and filter functions which leads to various resolutions and frame rates. In general it is not necessary to update the input scene with a higher frame rate than 25-30 Hz achieved by standard video sources.…”

“…1 They are based on the ability to realize massively parallel convolution operations via an optical Fourier transform. 2,3 These systems combine optical and digital operations to achieve a high processing performance.…”