Cambridge Studies in Modern Optics: Design Issues in Optical Processing

Abstract

This recent addition to the Cambridge Studies in Modern Optics series aims to present the characteristic engineering problems encountered in the design of electro-optical systems concerned with signal processing. It does this through the presentation of the work of six groups of workers in the field. The book provides a good insight into the general nature of the design problems of electro-optical systems through the presentation of specific problems and their resolutions. Chapter 1 deals with the design of coherent optical systems concerned with image processing (e.g. feature extraction using Fourier and Hough transforms). The design of the the system is shown to involve the characteristics of available optical devices, architectural considerations (e.g. of optical correlators) and the algorithms employed. Set against this scheme are the practical purposes for which the system is to be designed and constructed. The case study examined shows how novel algorithms can produce new architectures which, in turn, lead to new operations. The potential of optical processors lies in their analogue operation: noise is thus a problem. Chapter 2 is devoted to an analysis of noise in matrix - vector multipliers, from which design criteria for optical linear-algebra processors are derived. The theoretical work is then augmented by experimental work on different types of light-emitting devices. Measurements of the fluctuating light intensities emitted from a number of different light sources which were subjected to a range of systematically varied drive currents were carried out. By this means design trade-off information was obtained. Chapter 3 examines CAD techniques for use in the design of optical processors. Commercially available CAD packages are reviewed. Usually, the complexity is such that no single package can completely simulate the performance of a design; consequently, more than one CAD package is likely to be required for exacting designs. Additionally, packages are required to deal with limitations in performance deriving from the effects of diffraction, small-angle optical scatter and stray light. CAD packages which are capable of handling such factors over large dynamic ranges are required. Further design constraints include fabrication limitations such as assembly accuracy, glass homogeneity etc, though some CAD packages accomodate tolerancing in the design process. Design examples discussed include acousto-optic (Bragg cell) microwave spectrum analysers and space-integrating acousto-optic temporal signal correlators. It should be emphasized that from the standpoint of design it is the approaches adopted by the CAD packages that are of importance. Chapter 4 examines the application of optical interconnections between large arrays of digital microprocessors as a means of increasing the computational capabilities of such systems. Three schemes are considered; the mesh, the hypercube and the fully-connected scheme. Using existing theory, comparisons are made between guided-wave and free-space connections in terms of density of connections, power dissipation etc. As the number of processors and the number of interconnects increases, free-space interconnects possess advantages over guided-wave approaches. Chapter 5 describes the design of a Van der Lugt correlator which uses electrically addressable spatial light modulators. After discussing various types of SLM, those which incorporate electronic circuitry at each pixel to create arrays are concentrated on. Two types are examined: the ferroelectric liquid SLM and the silicon SLM. Important design issues are the influence of performance of pixel and device size, pixel fill factor, contrast ratio, flatness and uniformity and framing speed. Experimental results from a designed correlator are presented and conclusions drawn. Chapter 6 is devoted to the design of a free-space digital optical logic system. After discussing the potential of the system, an overview is given of the characteristics of three optical logic devices: a Fabry - Perot etalon, the self electo-optic effect device (SEED) and light-emitting devices, such as the laser diode. The characteristics are then related to design objectives, which are essentially system-related issues, such as size of detector and modulator/emitter elements, element pitch, switching energy, power and speed. A system design using a symmetric SEED device (S-SEED) is presented, as well as test results of an operating system based on the design. The book as a whole, through the description of different approaches to the design problems arising in different aspects of the field, presents a comprehensive view of the important issues which have to be confronted in the design of optical systems for signal processing. It can be recommended to optical researchers and designers and as an excellent introduction to the basic techniques of optical processing. B O'Connor