For many years, there has been considerable research in optical digital computing systems with the objective of developing and implementing parallel processing systems. The advantages of using optical systems allowing high throughput parallel processing with very high space-bandwidth products are well known to the persons skilled in the art. Nevertheless, most optical systems heretofore used have been analog in nature and have therefore sacrificed some of the accuracy and flexibility that digital computing can provide. Furthermore, analog optical systems have severe limitations in the sizes of the operational gates. Continuing efforts in which the object of the present invention partakes have been made to combine optical systems and digital algorithms in order to obtain the benefits of both.
In the article entitled "Coherent optical implementation of generalized two-dimensional transforms", by James R. Leger and Sing. H. Lee, Optical Engineering, Vol. 18, no. 5, 1979, there is disclosed a coded phase coherent optical processor capable of performing two dimensional linear transformations. The set of two-dimensional Walsh functions is chosen as a transform basis. The coherent optical correlator performs a correlation function between a two-dimensional object image and the Walsh-Hadamard transform of that image. The optical output plane consists of delta functions which represent the maximum of eight discrete levels of the Walsh-Hadamard transformation. However, without the accurate computation of spectral expansion functions and accurate data encoding of computer generated holograms, the coded phase optical correlator can only play a limited role for optical image processing and optical pattern recognition. This article is hereby incorporated by reference.
In U.S. Pat. No. 4,318,581 to Guest, published in March 1982 and in the article entitled "Two proposed holographic numerical optical processors", by C. C. Guest and T. K. Gaylord, SPIE, Vol. 185, 1979, there are disclosed two holographic numerical optical processors based on EXCLUSIVE OR processing and NAND-OR-OR processing. Input data words of the truth table perform either EXCLUSIVE OR or NAND-OR-OR processing on partially recorded truth table input data word array with truth values "1". After logical operations have occurred, the output row photoconductor arrays are filled with the necessary patterns such that the truth table output data words can be reconstructed through the photoconductor arrays. The optical processors described in the two references hereabove mentioned, which are incorporated by reference, provided an advance in the prior art of the time insofar as they had a simple structure. Furthermore, the mapping relation existed for both the location addressable memory and the content addressable memory. Additionally, the truth table look up processing could be performed for either numerical arithmetic operations or boolean logic operations. However, in the Guest patent, the digital inputs and digital outputs are one-dimensional. There is no teaching in the Guest patent of any possibility to extend the computational methods to a two-dimensional input. Futhermore, the hologram recorded in the storage medium comprises solely the pattern of the corresponding truth table input pattern corresponding to the value 1 as an output value. Such hologram recording saves considerable space but can only allow logic operations on the digital inputs. Guest illustrates it with the use of the XOR and the NAND-OR-OR logical operations. The XOR operation, for instance, is performed by appropriately phase shifting between the reference beam and the object beam. When the number of digital inputs becomes large, severe problems arise in the recording of large truth-tables in the hologram, thereby limiting the use of the Guest processor to modest input sizes.
Another recent approach to optical truth table look-up processing has been disclosed in an article by Shing-Hon Lin, Thomas F. Krile and John F. Walkup, entitled "Optical triple-product processing in logic design", Applied Optics, Vol 25, no. 18, 1986, which is hereby incorporated by reference. In this article, triple-product processing is used to perform a generalized bilinear transform on two different input functions to produce the outputs of the truth table. Yet, the triple-product processor utilizes two input planes, thereby preventing the two-dimensional space bandwidth product from being fully used. An additional drawback resides in the lack of accuracy in the computation of the spectral mapping coefficients by the generalized bilinear transform for truth table mapping.
All the disadvantages hereabove described in connection with optical systems of the prior art are overcome by the optical system of the present invention. The system architecture according to the present invention is intrinsically a massively parallel logic device which can run at the highest possible speed permitted in operations of basic micro-level computing functions. The optical system of the present invention is equivalent to an optical two-dimensional, large-scale integrated circuit with at most a one- or two-gate delay. Not only can it run at extremely high speeds, but it obviates the problems caused by radiation and the spurious effects produced by electromagnetic field interferences.