This paper concerns binary digital computing systems in which the information-carrying medium consists entirely or primarily of photons. The paper begins with a review of analog, discrete, and binary methods of representing information in a computer, followed by a survey of many techniques for implementing binary combinatorial and sequential logic functions with individual optical devices and arrays of devices. Next is a discussion of communication, interconnection, and input-output problems of digital electronic and optical computers at the gate, chip, and processor level. A particular architecture for implementing general sequential optical logic systems including digital optical processors is described. This architecture avoids some of the interconnection problems of electronic integrated circuits and VLSI systems, and offers the potential of non von Neumann parallel digital processors. Finally, the current limitations and future needs of optical logic devices and digital optical computing systems are outlined.

Digital optical computing

In this paper we identify and discuss some fundamental physical mechanisms that will provide limits on the speed, power dissipation, and size of optical switching elements. Illustrative examples are drawn primarily from the field of bistable optical devices. We compare the limits for optical switching elements with those for other switching technologies, and present a discussion of some potential applications of optical switching devices. Although thermal effects will preclude their wide application in general-purpose computers, the potential speed and bandwidth capability of optical devices, and their capability for parallel processing of information, should lead to a number of significant applications for specific operations in communication and computing fields.

On the physical limits of digital optical switching and logic elements

An efficient symbolic substitution scheme for modified-signed digit arithmetic operations is introduced. In this technique, an additional bit is used along with each pair of the input bits so that the nth additional bit is a characteristic of the (n - 1)th pair of input bits. The truth table minimization thereby shows that relatively fewer minterms are to be included in the corresponding optical content-addressable memory.

Modified-signed digit arithmetic using an efficient symbolic substitution.

A new conditional symbolic substitution rule for modified signed-digit arithmetic computation is introduced. Using this substitution rule, the numbers to be added or subtracted are first replaced by a pair of new equivalent strings, which in a second step are then subject to another substitution to generate both the addition or subtraction result and its complement. For an optical implementation, a holographic contentaddressable memory is used. Correspondingly, the input encoding, the logic reduction, and the optical processing techniques are described.

Conditional symbolic modified signed-digit arithmetic using optical content-addressable memory logic elements.

An optical computer based on the residue number system employs a plurality of computational modules which may be programmed using electronic pulses to perform addition, subtraction, multiplication and division operations. Each module comprises an array of optical switches in the form of directional wave guide couplers interconnected by optical transmission paths to form a series of maps corresponding to a given modulus. The optical switches are arranged in rows and groups of rows may be selectively activated to adapt the module to perform various arithmetic operations. One residue is input to the module in the form of light pulses and is spatially arranged to automatically select the proper map to perform a desired function. The optical switches in each row thereof are electrically interconnected to a bi-stable electrical switch that simultaneously switches all of the logical switches in that row in accordance with a programming input pulse delivered to one of the bi-stable switches. The light output from the last row of optical switches correspond to the function of two residues whose function has been computed and is delivered in spatially oriented form. The inputs of the electrical switches associated with one module may be connected with the light output of another module so as to permit various functions, such as polynomials, to be computed using chain processing techniques. Arrangements for encoding, decoding, and scaling are also disclosed.

Optical residue arithmetic computer having programmable computation module

Using residue arithmetic it is possible to perform additions, subtractions, multiplications, and polynomial evaluation without the necessity for carry operations. Calculations can, therefore, be performed in a fully parallel manner. Several different optical methods for performing residue arithmetic operations are described. A possible combination of such methods to form a matrix vector multiplier is considered. The potential advantages of optics in performing these kinds of operations are discussed.

Yoshito Tsunoda

Papers

Optical computation using residue arithmetic

Combined optical AD conversion and page composition for holographic memory applications.