Optical Bistability: Controlling Light with Light PDF
Preview Optical Bistability: Controlling Light with Light
Academic Press Rapid Manuscript Reproduction QUANTUM ELECTRONICS - PRINCIPLES AND APPLICATIONS A Series of Monographs EDITED BY PAUL F. LIAO PAUL KELLEY Bell Telephon,e Laboratories Lincoln Laboratory Murray Hill New Jersey Massachusetts Institute of Technology Lexingtont Massachusetts A list of books in this series is available from the publisher on request. Optical Bistability: Controlling Light with Light Hyatt M. Gibbs Optical Sciences Center University of Arizona Tucson, Arizona 1985 ACADEMIC PRESS, INC. Harcourt Brace Jovanovich, Publishers Orlando San Diego New York Austin London Montreal Sydney Tokyo Toronto Copyright © 1985 by Academic Press, inc. ALL RIGHTS RESERVED. NO PART OF THIS PUBLICATION MAY BE REPRODUCED OR TRANSMITTED IN ANY FORM OR BY ANY MEANS, ELECTRONIC OR MECHANICAL, INCLUDING PHOTOCOPY, RECORDING, OR ANY INFORMATION STORAGE AND RETRIEVAL SYSTEM, WITHOUT PERMISSION IN WRITING FROM THE PUBLISHER. ACADEMIC PRESS, INC. Orlando, Florida 32887 United Kingdom Edition published by ACADEMIC PRESS INC. (LONDON) LTD. 24-28 Oval Road, London NW1 7DX LIBRARY OF CONGRESS CATALOGING-IN-PUBLICATION DATA Gibbs, Hyatt M. Optical bistability. Bibliography: p. Includes index. 1. Optical bistability. I. Title. QC446.3.065G55 1985 535'.2 85-48069 ISBN 0-12-281940-3 (alk. paper) PRINTED IN THE UNITED STATES OF AMERICA 85 86 87 88 9876 5 4321 To Pip and Maa Lethia, Alex, and Vanetta and all of my bistability friends PREFACE This is a research book intended for new entrants and active workers in the field of optical bistability. The treatment interprets optical bistability broadly to include all of the steady-state and transient characteristics of nonlinear optical systems which exhibit bistability under some operating conditions. It is restrictive in placing the emphasis on passive (non-laser) systems which exhibit reversible bistability with input intensity as the hysteresis variable. The book is motivated by the desire to summarize the beauty of the physics and to describe the potential applications of such systems for nonlinear optical signal processing. This book may even be useful as a reference for advanced specialized courses. The first draft was written in the fall of 1981 as the author taught what may have been the only course on optical bistability. Parts of it were used again in the spring of 1984 as part of a course on the semiclassical description of coherent optical phenomena. The history of optical bistability in lasers and passive systems is summarized in Chapter 1. Steady-state theories of optical bistability are presented in Chapter 2. Both intrinsic and hybrid experiments are described in some detail in Chapters 3 and 4, respectively. Light-by-light control is the focus of Chapter 5 which treats pulse reshaping and external switching. In contrast, the transient phenomena of Chapter 6 occur with a steady input, i.e., they are intrinsic instabilities. Consid erations important for applications to optical signal processing and computing are discussed in Chapter 7. This is a research book about a very rapidly expanding field. This fact has made it difficult to decide when to publish. Much of the underlying physics of the behavior of these nonlinear optical systems is now understood, even though intense studies on instabilities, transverse effects, and nonlinearity mechanisms will surely result in many new breakthroughs. Work on applications to very-high-speed switching and to massively parallel processing has just begun. Hopefully this book captures the beautiful and diverse behavior of nonlinear optical systems in a manner that will serve both as a basis for further physics research and as a ready reference for nonlinear optical signal processing. The author has had many rewarding collaborations in twelve years of working on optical bistability. Thanks to Sam McCall for introducing the possibility of seeing bistability to me and convincing me that collaborating with him was essen tial to avoid a catastrophic explosion during one of his sodium-oven cleaning oper ations! Thanks to Dick Slusher for earlier laser technology transfer and fruitful collaborations on coherent optical phenomena. To Sam, Venky Venkatesan, George Churchill, and AI Passner for sharing the excitement of those first Bell Labs experiments in Na, ruby, and GaAs; somehow 2 a.m. data are special! And without the beautiful GaAs samples of Art Gossard and Bill Wiegmann, I would have never made the gas-to-solid transition. In 1980 the Justice Department frightened me into establishing "Bell Labs West" at the Optical Sciences Center, resulting in a greatly expanded effort with the help of a fantastic group of gradu ate students: Shin-Sheng Tarng, Jack Jewell, Ed Watson, David Kaplan, Doreen xi xii Preface Weinberger, Mike Rushford, Kuo-Chou Tai, Shlomo Ovadia, Lois Hoffer, Matt Der- stine, Mial Warren, Yong Lee, Greg Olbright, Arturo Chavez, John Valley, Hans Kulcke, George Gigioli, Lon Wong, Ruxiang Jin. Thanks to Professors Fred Hopf, Jerry Moloney, Dror Sarid, and Rick Shoemaker for bistability collaborations and to them, George Stegeman, and many others for helping in the formation of the Opti cal Circuitry Cooperative. A special thanks to Assistant Professor Nasser Peyg- hambarian for his hard work and close friendship during the last three years. Although Sam McCall never found time to coauthor this book as first planned, much of what I know about bistability has come from him and I hope I have cap tured much of his insight. Thanks to David Holm and Jerry Moloney for writing Appendices F and G, respectively. Thanks to Jeannette Gerl and Lisa DuBois for early versions of the book and to Kathy Seeley and Norma Laguna for the tedious and seemingly endless preparation of the camera-ready manuscript. Thanks for helpful comments on the manuscript from Professors Howard Carmichael, Luigi Lugiato, Pierre Meystre, Des Smith, and Doreen Weinberger and from Drs. Martine LeBerre, Elisabeth Ressayre, and Andree Tallet. CHAPTER 1 INTRODUCTION TO OPTICAL BISTABILITY Optical bistability is a rapidly expanding field of current research because of its potential application to all-optical logic and because of the interesting phe nomena it encompasses. Since the first observation of optical bistability in a pas sive, unexcited medium of sodium (Na) vapor in 1974 (McCall, Gibbs, Churchill, and Venkatesan, 1975), bistability has been observed in many different materials including tiny semiconductor etalons. Current applied research is focused on opti mizing these devices by decreasing their size, switching times, and operating power, and operating them at room temperature. Both improved nonlinear materi als and more efficient device configurations are being sought. Current fundamen tal research centers on the interesting physical behavior of simple bistable sys tems. Many bistable devices consist of a nonlinear medium within an optical reso nator, just as do lasers, except the passive bistable devices are excited only by the incident coherent light. The counterparts of many of the phenomena studied in lasers, such as fluctuations, regenerative pulsations, and optical turbulence, can be observed in passive bistable systems, often under better controlled conditions. Optical bistability in lasers, which was seen prior to passive bistability, is treated briefly in Section 1.3 although it is not the main subject of this book. 1.1. DEFINITION AND TYPES OF OPTICAL BISTABILITY A system is said to be optically bistable if it has two output states Ij for the same value of the input Ιχ over some range of input values. Thus a system having the transmission curve of Fig. 1.1-1 is said to be bistable between I* and If. Such a system is clearly nonlinear, i.e., Iy is not just a multiplicative constant times Ιχ. In fact, if Ιχ is between I4, and If, knowing Ιχ does not reveal Ij. Non- linearity alone is insufficient to assure bistability. It is feedback that permits the nonlinear transmission to be multivalued, i.e., bistable. It is this restricted defini tion of optical bistability defined by Fig. 1.1-1, with the nonlinear medium unex cited, that is adopted here. This definition implies that the bistable system can be cycled completely and repeatedly by varying the input intensity. Systems that exhibit hysteresis as a function of some other parameter but not light intensity are not of interest here. This restricted definition rules out "bistable" optical systems that cannot be reset merely by reducing the input intensity, such as a burglar alarm or a card in a laser beam powerful enough to burn through the card. Even an optical damage device that can be restored by irradiation with light of a dif ferent wavelength is not in the spirit here of an all-optical completely recyclable passive system. 1 2 [1.1] INPUT INTENSITY Ij Fig. 1.1-1. Characteristic curve for an optical bistable system. An example of a system exhibiting optical bistability is a Fabry-Perot inter ferometer containing a saturable absorber; see Fig. 1.1-2. A simple analysis of such a nonlinear etalon reveals the possibility of bistability. For weak input in tensity, Ιχ, the intracavity absorption spoils the finesse of the cavity even though the laser frequency v and cavity frequency vpp of peak transmission are coinci dent. Therefore the intracavity intensity Iq at z = 0 is simply Ιχ times the input mirror transmission T. At the cell exit led) = e"“*- Tl! (1.1-1) and the transmitted intensity is IT = β-β ίτ*ΐχ. (1.1-2) ETALON Ii 0 Ic L It Fig. 1.1-2. Etalon intensities for an intracavity intensity much less than the saturation intensity. Introduction 3 Equation (1.1-2) holds as long as the saturation intensity Ig of the medium is large compared with the intracavity intensity, i.e., if Is > ΤΙχ (1.1-3) is satisfied sufficiently. Figure 1.1-3 depicts the case of strong input intensity in which the medium is bleached, the finesse is high, and the etalon transmits per fectly; i.e., Tj s and « Ij/T. This clearly holds for I^· » I$, i.e., if II > TIS (1.1-4) is satisfied sufficiently. The possibility of bistability is suggested by noting that both Eqs. (1.1-3,4) can be satisfied by the same input intensity. For example, take Ij = Is, then both inequalities require that T be lesst han 1 as it always is. This physical argument iss ubstantiated by the morer igorous deriivnaS teiocnti on . . 2 1 ETALON Fig. 1.1-3. Etalon intensities for an intracavity intensity much larger than the saturation intensity. There are two useful classifications of bistable systems. A system may be absorptive or dispersive, and it may be intrinsic or hybrid. For example, the nonli near Fabry-Perot interferometer just discussed is an absorptive intrinsic system. A system is absorptive or dispersive depending on whether the feedback occurs by way of an intensity-dependent absorption or refractive index. Clearly this dis tinction is not sharp, since both absorptive and refractive mechanisms may be sig nificant simultaneously. The distinction between intrinsic (all-optical) and hybrid (mixed optics and electronics) i£ sharp. In an intrinsic system the intensity de pendence arises from a direct interaction of the light with matter. In a hybrid system the intensity dependence arises from an electrical signal from a detector monitoring the transmitted intensity, usually applied to an intracavity phase shifter. Experimental embodiments of intrinsic and hybrid systems are described in Chapters 3 and 4, respectively. For further reading: a simple introduction to optical bistability is Gibbs, McCall, and Venkatesan (1979) and recent collections of papers are: Bowden, Ciftan, and Robl (1981); Bowden, Gibbs, and McCall (1984); and A. Miller, Smith, and Wherrett (1984). Apart from this book, the most extensive review of optical 4 [1.2] bistability, both theory and experiment, is Abraham and Smith (1982a)· Lugiato (1984) gives a more recent and thorough review of the theory of optical bistabil ity. Goldstone (1984) is a good introduction, especially for dynamic effects. 1.2. OPTICAL LOGIC WTTH BISTABLE DEVICES The transmission of information as signals impressed on light beams traveling through optical fibers is replacing electrical transmission over wires. The low cost and inertness of the basic materials of fibers and the small size and low loss of the finished fibers are important factors in this evolution. Furthermore, for the very fast transmission systems, for example, for transmitting a multiplexed composite of many slow signals, optical pulses are best. This is because it is far easier to gen erate (Höchstrasser, Kaiser, and Shank, 1980; Shank, Ippen, and Shapiro, 1978) and propagate (Bloom, Mollenauer, Lin, Taylor, and DelGaudio, 1979) picosecond opti cal pulses than electrical pulses. With optical pulses and optical transmission a reality, the missing component of an all-optical signal processing system is an op tical logic element in which one light beam or pulse controls another. The optical bistable systems described in this book have many desirable properties of an all- optical logic element. Hopefully they are the forerunners of tiny, low-energy, subpicosecond, room-temperature devices. The high frequencies of optical elec tromagnetic radiation give optical devices a potential for subpicosecond switching and room-temperature operation unavailable to Josephson junctions or electronics. The fact that electrical charges are not used or are used only in tiny beam-inter acting regions makes an all-optical system much more immune to electromagnetic interference from electrically noisy industrial environments or the electromagnetic pulses from a nuclear explosion. If this book aids and accelerates the understand ing and development of such all-optical systems, it will have served its purpose. Bistable devices have already performed a host of logic functions. Both two- state (Fig. 1.2-1) and many-state (Fig. 1.2-2) optical memories have been demon strated. The amount of transmitted light reveals the past history of the input light; i.e., the system "remembers" whether or not the input ever exceeded a par ticular threshold value. By modifying the operating conditions, an optical transis tor or transphasor mode of operation is achieved as in Fig. 1.2-3. A small modu lation on the input (or on a second signal beam) is amplified. An optical discrimi nator (Fig. 1.2-4) transmits pulses with intensities above the threshold and suppresses those below. An optical limiter (Fig. 1.2-5) shows little change in the transmitted power. This function could serve to limit the power reaching some thing or someone or to decrease the percentage noise level. Figure 1.2-6 illus trates optical discrimination in which a signal is faithfully transmitted whenever it exceeds the threshold level. Changes in pulse shape can be accomplished with nonlinear etalons. For example, in Fig. 1.2-7 the etalon initially transmits the light well, but the energy absorbed from the pulse heats the etalon, detunes it from the laser frequency, and terminates the transmission long before the input pulse turns off. Sometimes the transmitted signal oscillates when the input is per fectly constant, resulting in an optical oscillator (Fig. 1.2-8). Or an etalon placed in a continuous wave (cw) beam and initially detuned from the laser frequency can be swept using a pulse from a second laser to gate out a short pulse from the cw beam (Fig. 1.2-9). These examples illustrate that existing bistable optical devices have the desired characteristics of many all-optical operations. Nonetheless, smaller, faster, cheaper, room-temperature, efficient devices are needed.
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