The present invention relates to the field of electro-optical signal processors. The inherent parallelism associated with optical signal processing techniques allows such systems to process a large image just as rapidly as a small one; whereas, for digital methods, the processing time tends to increase exponentially with image size. For example, a single liquid crystal television can be driven at 60 frames per second. Thus, 60 2D Fourier transforms (Fraunhofer diffraction patterns) could be formed per second. This is much faster than current state-of-the-art digital techniques. Moreover, other spatial light modulators such as ferro-electric devices, which are more expensive, can be driven at even faster rates of 2,000 frames per second.
Current optical signal processors, using spatial light modulators as programmable transparencies, process a single image at a time. Since very high resolution spatial light modulators are commercially available, often the images to be processed do not require the full TV screen. Hence the "single image" optical systems discussed above don't take full advantage of the inherent parallelism associated with optical processing signal processing. It is thus desirable to partition the TV screen or other SLM into separate regions so that several Fourier transforms can be optically computed in parallel to significantly increase the throughput rates for many optical signal processing applications.
Using lenslet arrays has been suggested for spatial multiplexing. However, using lenslet arrays suffer from serious disadvantages. Most importantly, for high resolution spatial light modulators, small diameter lenses must be used. Such small lenslets tend to be poor in quality and consequently have aberrations that significantly degrade the diffraction pattern. Moreover, even if perfect small lenslets could be manufactured, their small diameter, relative to the partitioned region to be optically processed, also causes serious degradation in the resulting diffraction patterns. In order to avoid this problem, the lenslet diameter would need to be several times larger than the partitioned region; but this severely limits the number of regions that can be partitioned and thus reduces the number of Fourier Transforms that can be computed in parallel. Secondly, each of the lenslets must be aligned and spaced very accurately to within a few microns. No such lenslet arrays exist and consequently, haven't been used to perform spatial multiplexing. Thirdly, lenslet arrays are very expensive. The present invention does not suffer from any of these drawbacks.