The USC Andrew and Erna Viterbi School of Engineering USC Signal and Image Processing Institute USC Ming Hsieh Department of Electrical Engineering University of Southern California

Technical Report USC-IPI-1110

“Optical Logic Systems: Implementation and Architectural Implications”

by Brian Keith Jenkins

April 1984

A general technique is presented for implementing sequential logic circuits optically. The optical system consists of a spatial light modulator (SLM) to provide a nonlinear threshold response and one or more computer generated holograms (CGHs) to provide interconnections between logic gates. A two-dimensional array of logic gates with binary inputs and outputs is formed on the active surface of the SLM. Three different optical systems for interconnecting these gates are given.

Two basic interconnection methods, space-variant and space- invariant, are described. A hybrid technique having both space-variant and space- invariant elements provides a compromise between these two methods. The maximum number of gates that can be implemented is limited primarily by the space-bandwidth product of the CGHs and the SLM, and is calculated for each of the three interconnection techniques. Methods for implementing the interconnections of arbitrary circuits with each of these optical interconnection techniques are studied. The advantages and limitations of each interconnection method are discussed, and various types of processors that take advantage of each interconnection technique are described.

The concept is demonstrated experimentally using a liquid-crystal light valve (LCLV) as the SLM. A test circuit has been implemented that includes a synchronous master-slave flip-flop and an oscillator consisting of five inverters in a feedback loop. Experimental results of this test circuit are presented.

Because the SLM response is critical in determining system performance, the characteristics of LCLVs are analyzed. Two methods for implementing parallel arrays of logic gates are presented, and the relevant LCLV characteristics are considered. For operation in a sequential circuit, the temporal response of the SLM becomes important. Both experimental characteristics and theoretical models are discussed.

In contrast with semiconductor electronic circuitry, optical logic systems allow very flexible interconnections between gates and between subsystems. Because of this, certain processor architectures which are difficult or inefficient to implement with semiconductor electronics can be implemented with the optical system. Similarly, some processing algorithms can be performed on the optical system that are difficult to perform using semiconductor architectures. The types of algorithms and processor architectures which can be implemented most easily with the optical logic system depend on the interconnection technique.

A variety of optical devices which could permit the optical logic system to realize its potential are being developed by others. The use of these devices in the optical logic system is considered. Some general device requirements that ensure correct system operation are given. In addition, the fundamental limitations on the development and performance of these devices are evaluated and compared with those of their semiconductor electronic counterparts.

To download the report in PDF format click here: USC-IPI-1110.pdf (19.8Mb)