Photooptical Storage and Processing of Information

Gilbert W. King

Information

All the papers in the feature section of this issue dis-cuss the acquisition, storage, processing, or display ofinformation which falls more or less, into four classes.Digital information, the staple of the computer frater-nity, can have an easy binary representation in photo-optical contrivances as light or no light, black or trans-parent areas. Symbolic information is what humanshave become accustomed to over the last 8000 years inhieroglyphics, Chinese ideographs, in letters of alpha-bets, and in numerals. Although these are representedin a two-state form, either ink or no ink, they are farmore complex than the binary digits so easy for amachine to handle. Symbolic notation introduces themystery of pattern recognition, which is hardly mech-anized at all. Nevertheless most of our data today(and the kind which needs processing most of all) arestrings of symbols, such as the text of this journal. Aconsiderably less formal although still binary (black andwhite) representation of information is graphics in theform of line drawings, such as circuit diagrams, en-gineering tracings, or illustrations in patents. Fi-nally, we have photographs of which there is now anenormous quantity-Tiros and Nimbus pictures of theatmosphere, aerial photographs of the earth, and tele-scopic observations of the moon. In photographs, weencounter the new features of a gray scale and of vaguebut recognizable forms.

Processing

Why should data be processed? It is not because ofan information explosion which is perhaps a form ofopulence, like the explosion of agricultural produce inthe United States. What is needed are automaticmeans of eliminating the dull human labor of scanningfiles, records, indexes and abstracts, interpretingphotographs, etc., which humans must perform in orderto discover real information.

The author is with the Itek Corporation, Lexington 73,Massachusetts.

Received 19 January 1965.

To bring out the flavor of the area of optics featuredin this issue, we should mention kinds of processing inthe background and relate them to the types of informa-tion. Aerial photographs contain information thatcan be used with great benefit to route roads, planagricultural development, discover minerals, and makemaps. But photogrammetry and photointerpretationare painstaking arts, impossible as yet to mechanize.Even the drawings in patents, which are highly stylizedby law, are still completely incomprehensible tomachines.

With respect to information in symbolic form, ma-chines are better off, having been proved capable, atleast, of recognizing stylized printed letters. Binarydigits, of course, are easily recognized in a variety offorms, can be recorded with high density and manipu-lated at very high speed, although still in simple pat-terns. It is not yet possible, for example, to scan textto isolate topics or to translate one language to an-other. Perhaps in the future, machines may be able todigest an article such as this and render it clearer andmore concise.

What can be the contribution of optics in each ofthese areas? While we have seen the steady develop-ment of optical techniques for recording graphic imagesand reducing them physically, little has been done tomake the information in images more directly acces-sible. We can expect a continuation of the work tosuppress and enhance data and to sense and manipulatedata patterns.

So far as symbolic data are concerned, optical tech-niques have begun to be used for its transformation todigital form. Clearly, optical methods will become im-portant in this application (and also in the transforma-tion back from this form) to achieve high computer out-put speeds and more complex types of outputs. Be-cause of the large masses of information being gen-erated in or converted to digital form, both by humansand by instruments, high speed digital recording, massstorage, and high speed search and processing are themain features of tomorrow's data processors. Thepoint in question in the two seminars on optical process-ing of information and in this issue is "what can opticsoffer in these areas?".

April 1965 / Vol. 4, No. 4 / APPLIED OPTICS 369

At the present time, it is not entirely clear that opticscan invade the province now pre-empted by magnetics,for storage, and by electronics for processing. Cur-rent activity is essentially probing, but by no meansdiscouraging. One obstacle has been that although theexecution of logical operations, essentially nonlinear,could be done at high speeds, it could not be done in areasonable volume per bit. The only high speedcomponents are cathode-ray tubes, crystals of variouskinds, and photomultiplier tubes, all essentially cum-bersome transducers of electrons into photons and viceversa. This picture may be radically changed bycombinations of injection lasers and photodiodes.Photon exchange decouples input and output, and pho-ton streams, unlike electron beams, do not interact.Indeed, they can pass through each other. Fiber opticsand fiber lasers (one fiber triggering another), however,may revolutionize the assessment of the volume-per-bitfactor.

The computers based on electronics and magneticshave been outstandingly successful in digital opera-tions. Analog computers are still useful, especiallyfor real-time applications. In trying to adapt opticaltechniques to processing, we must soon face up to thedecision of whether to go the digital or the analog route.The rather fantastic operations, which can be done byspace filtering, on millions of bits simultaneously in notime at all, certainly are attractive (as shown in severalpapers in this issue), yet the only digital photoopticalprocessor in operation is the Air Force An/GSQ-16Language Translator. Possibly the ability to convertfrom digital to analog through Fourier transfer (diffrac-tion theory) may allow us to have the best of bothworlds.

Storage

Temporary and permanent storage of information isan intrinsic feature of data-processing systems. It ap-pears that we are still casting about for a really satis-factory way of using optical phenomena for temporarystorage. Optics and photography, however, can makea great contribution to permanent storage, because ofthe ease of recording and high density of packing in-formation on an area basis. This is important, sincethe higher the density the faster the access. Therestill remains much to be desired in finding a photosensi-tive material which is fast, has high resolution, and iseasy to develop and fix. The high intensity of lasersmay ease this situation.

In the future, there may be other physical phe-nomena permitting higher packing density of and fasteraccess to stores of information, involving atomic ornuclear orientations. Photographic systems have aproperty which may fend off such an invasion, namely,the irreversibility of their change of state, in normaltemperature or electromagnetic environments, over ex-tremely long intervals of time.

The capacity of an optical storage device, for a givenstate-of-the-art packing density, depends on the fieldof the optical system. This is limited by laws ofphysics and to increase the area to be scanned, either the

light source or the storage medium has to be moved.There is an amazing number of favorable opticalqualities of a laser, but the one most needed in informa-tion storage is the one it lacks, namely, an easy methodof moving the light source rapidly and asynchronously.

In storage, optical systems must rely on mechanicalmovements and call on advanced principles and tech-niques. The use of airbearing for precise movementsand the Benouilli principle for stabilization, e.g., of theworking distance of a lens, offer new possibilities to theoptical designer.

Input

A major characteristic of our society is its fantastictechnical performance in information storage (as on theprinted pages of our libraries) and retrieval (a manreading a newspaper on a dimly lit, crowded subway).In modern terms, though, with the vast quantity ofacquired data and need for processing (better perhapsdigestion), the man-storage interface must be elim-inated.

Specifically, a data processing system has other sub-systems. Excluding so-called scientific computations,which start with very little input data (the funda-mental equations, boundary conditions, etc.) and re-quire little output (often "yes" or "no"), data process-ing is concerned with large volumes of symbols, graph-ics, or pictures. There exists the intractable problemof injecting data into a machine system. There aretwo fairly distinct types: identification of symbolsbelonging to a predefined set, and recognition. Theformer is tractable with the aid of information theoryof signal-to-noise ratios and discrimination techniques.On the other hand, pattern recognition, even of lettersof the alphabet, is still unsolved in the general sense.

In the case of digital data, the picture is muchbrighter. The advent of the laser now permits thewriting of digital information on photographic materialat much higher speeds than any other technology.Here again, it is interesting to note that the advanceof optics into information processing (in the sense offinding a use for lasers) is in the digital field, ratherthan in the two-dimensional graphic or photographicdomain which is the heart of optics.

Output

The output of a data-processing machine requires aman-machine interface. Phototypesetting and re-productions of photographs, drawings, and maps havehad a long, successful history. There are still engineer-ing problems in producing high quality output at lowcost related to over-all system economics, i.e., numberof bits per symbol as shown in the concluding papersof the feature section of this issue.

Evanescent data from a system, however, are noteasy to display for human consumption. MonochromeCRT's in radar presentation and digital-graphic proc-essing are inadequate, and hard copy is still not cheapenough to "throw away". Generally speaking, com-puter output does not have the graphic arts qualitythat increases ease of reading, which, after all, is the

370 APPLIED OPTICS / Vol. 4, No. 4 / April 1965

objective of data processing. Colored displays wouldbe desirable, and some old ideas in optics of half-waveplates coupled with the Pockles' effect should beexamined again.

Summary

Photons may win over electrons in nonnumerical dataprocessing, since so much input and output has a visual

Symposium on Optical and Electrooptical In-

formation Processing Technology, 9-10 November

1964, Boston*

Reported by W. V. Smith, IBM Watson Research Center, York-

town Heights, New York

I have found this a most interesting conference, and one in whichI learned of new advances in some areas with which I had pre-viously been relatively unfamiliar. It has been most worthwhile.

I shall address this critique to seeing what answers this sym-posium has given us to two questions: (1) Can optical devicesand interconnections replace existing magnetic and electric de-vices and interconnections in conventionally organized com-puters? Are they (a) faster? (b) more compact? (c) cheaper?

Do they consume less power etc.?; and (2) Are there particulartypes of data processing activities for which optical techniquesare more suited than other techniques? If so, what are theyand how important are they? Most important, do any of theseareas have the potential for use in drastically reorganized com-puter designs, possibly capable of displacing conventional com-puters?

In considering the first question, conventional computers,neither the results presented at this conference, nor my inde-pendent considerations of the physics involved, suggest obvioussimple reasons why light is advantageous. Its speed of propaga-tion is essentially the same as that of electrical signals on trans-mission lines. Also, though the concepts of optical fibers and

the high resolution of photographic images suggest the possibilityof compact optical systems, the ultimate resolution of the wave-length of light is actually much larger than the correspondinglimit for the wavelength of electrons. We may remember thatelectron microscopes resolve images to a few angstroms. In-deed, a paper, by K. F. Wallace Ampex Corp. on electron beamreadout of thermoplastic recordings served to remind us of theadvances being made in this competitive technology. (I will nottry to make specific comparisons. I only note that here, as inother areas, optical techniques must be compared realisticallywith other approaches to doing the same job.) Finally, the ad-vantage of trading interconnection problems of wires or depositedmetal strips for those of optical fibers is far from obvious. In-deed, we remember that in the paper by E. H. Cooke-Yarborough,P. K. Gibbons, and P. Iredale AERE-Harwell, the first author,after analyzing an optoelectronic system that seemed potentiallycompetitive with conventional systems from power-time constant

* This critique will be published, together with all the otherpapers given at this meeting, in the proceedings of the sympo-sium (ed. James T. Tippett, Lewis C. Clapp, David Berkowitz,Charles J. Koester, and Alexander Vanderburgh, Optical andElectro-Optical Information Processing; M.I.T. Press, Cam-bridge, 1965); it is printed here with the publisher's permission.

form, and photons have attractive features of mutualnoninteraction for internal processing. They will belimited by their wavelength or the statistical nature oflight, and possibly someday they will give way to evennewer technologies involving nuclear properties, butthere are still great potentialities in the science of opticsfor new technologies in information storage processingand retrieval.

considerations, felt that a major difficulty remaining was toachieve an optical fiber equivalent of the back panel wiring mazein conventional computers. This is not to say that optical fibersdo not have important applications but only that wiring com-puters may not be one of them. On a more positive note, how-ever, we were reminded by Mr. Jacobs, who presented the paperby T. E. Bray, G. Jacobs, and H. Raittard GE-Syracuse, that asingle lens can serve to connect arrays of diode light emitters on afixed one-to-one relation to arrays of detectors.

One must next proceed from generalities to specific numbers.Perhaps on detailed examination some of the specific optical de-vices examined may prove superior to their specific electricalcounterparts. Furthermore, to be fair in the analysis, one mustbe prepared to allow marriages between electronic and opticaldevices, using the best features of each where advantages seempossible. Certainly many examples of such optoelectronic mar-riages have been examined in this conference. Fortunately,numerical comparisons have been included in several cases. Ifind the comparisons in the paper by H. Ruegg Stanford Uni-versity between optoelectronic and all electronic logic circuitsparticularly useful, and his conclusion that for the same timeper logical decision the power required by solid-state optoelec-tronic circuits seems to be about two orders of magnitude greaterthan that required by all electronic circuits is a sobering one.Indeed, the only examples where optoelectronic circuits were po-tentially competitive in this respect with all electronic circuitswere ones that utilized high power gain stages in which electronswere accelerated in vacuum by high voltages (E. H. Cooke-Yar-borough, G. B. Jacobs). Thus, optoelectronic devices seem onlysuited to those parts of conventional computers that requireoptical effects, such as visible display, and we have had some in-teresting papers dealing with that and other input-output appli-cations. Indeed, we may use these to compare some electroopticand electronic approaches to similar problems by noting the at-tention here given to the deflection of light. It is obvious thatthe deflection of electron beams is far easier-an electric or mag-netic field acts directly on a moving charge but only indirectly,through its effect on the polarization of a dielectric, on a lightbeam. It is only for the reason that one is interested in applica-tions requiring intense light that one even considers deflectingbeams of photons instead of beams of electrons.

Next, let us consider the approach of all optical, fairly conven-tional (i.e., serial) logic. Several papers have been devoted tothis subject. I include the neuristor because it, too, is serialand can be implemented in nonoptical components. Here too,I feel that the numbers, although not presented as quantitativelyat this symposium as in the case of optoelectronic logic, are un-favorable. One notes, for example, that those optical nonlineari-ties that arise from nonlinear susceptibilities, harmonic gen-eration and frequency mixing, are generally associated withgiant laser pulses (megawatts). This is another example ofthe indirect way in which light beams interact with each other.One sometimes advances the optical isolation between two cross-ing light beams passing through lenses as an advantage of opticalinterconnections-and indeed we have already cited this example.Such weak interactions cannot simultaneously be an advantagewhen one tries to utilize nonlinear optical phenomena. Ofcourse, one selects different materials for lenses than for har-

April 1965 / Vol. 4, No. 4 / APPLIED OPTICS 371

monic generators. Nonetheless, the basic problem remains.Thus, I question the projection of optical parametric amplifica-tion as a useful logic device.

The optical fields required to saturate optical transitions or toquench lasers are not as phenomenal as those required for para-metric amplification. Nonetheless, as pointed out in the paperby W. F. Kosonocky RCA Laboratories general relations existbetween the time constants of the processes involved and thefields-and these are hardly comforting if we are interested in fastdevices. I agree with Mr. Kosonocky that these relations aremore favorable for GaAs lasers than for neodymium glass lasersby three or four orders of magnitude, essentially because of thelarger optical cross sections of the allowed optical transitions in-volved in the latter case. Even so, it would require considerableminiaturization to get the power-per-logical-decision, in thensec time constant range, below 1 W, still far from the requiredgoal of a few mW. And there are still the problems of low-temperature operation and optical interconnections remainingIndeed, as one of my colleagues has remarked, if we had beenable to achieve an all optical serial computer before achieving oneelectronically, we would very likely be grateful if someone theninvented transistors, copper wire, printed circuits, etc.

This is the negative side of the picture. I will now turn tothe more positive aspect of considering those types of dataprocessing for which optical techniques are more suited thanother techniques. Clearly we have seen examples of such appli-cations in several of the papers presented here. I have alreadymentioned the obvious categories where either input or outputdata are required by some particular problem to be optical innature. I will pass on to the question of how much progresshas been made in using optics as an approach to parallel organizedcomputers, and on the somewhat related problem of using holo-grams and coherent light for image processing. I am rather in-experienced both in the fields of computer organization andhologram optical Fourier transforms. Consequently, it is withconsiderable restraint that I refrain from a long discourse on allmy recently acquired understanding of these subjects. Nonethe-less, I shall analyze briefly the general problem as I see it. Massesof data, in batches, are to be sorted for relevant information.The size of the batch may be fairly small, as a single a-numericcharacter, or large, as an aerial photograph. A printed page is,however, a sequence of batches for most purposes, though onecould be interested merely in selecting one page from many, inwhich case it would be a batch. One may make the generalthesis that the larger the batch and the simpler the selectioncriteria (the fewer the alternatives), the more appropriate theproblem is for Fourier transform (hologram) analysis. Also, forhologram analysis the space location of the unknown in the batchis neither known nor desired. If, however, one were sortingaerial photographs of a ship at sea as a target, to see how wellcentered they were on target, one obviously would comparepictures directly rather than as holograms. Holograms are dif-fraction patterns; hence, just as with familiar x-ray diffractionpatterns, they reveal regularities in small scale structure at theexpense of losing large scale structure. We remember the strik-ing aerial photographs of parked cars in regular patterns and thehologram picture revealing the regular parking structure. If wehad been interested in locating a winding road going through theparking lot, it would be lost in the hologram.

Examples were given of image processing, by parallel (co-herent light) techniques or by sampling and digital analysis ofthe samples. The discussion following Dr. Ledley's description(R. S. Ledley, L. S Rotolo, T. J. Golab, J. D. Jacobsen, M. D.Ginsberg, and J. B. Wilson National Biomedical Research F ounda-tion) of digitally processing and recognizing chromosome photo-micrographs emphasized the flexibility of the latter approachwhen the recognition criteria were difficult and required modifica-

tion as a result of experience. Thus it seems difficult to general-ize on the pros and cons of parallel versus serial image processing.It seems that both approaches have their areas of applications:parallel, essentially analog computer type processing wherethe recognition criteria are clear and simple and only smallamounts of data are to be retrieved from a large background ofunused data, and serial processing, where large amounts of com-plex data are to be retrieved and perhaps reexamined underchanging rules of analysis. We have seen that the laser hascontributed substantially to the progress achieved in coherentlight image processing-and certainly the three-dimensionalhologram reproductions of Leith and Upatnieks University ofllichigan are most impressive. I project a growing future forthese techniques, but not one that will seriously impact digitalcomputer applications. I feel it is too early to comment on thepossible extension of these techniques to the x-ray region as sug-gested by G. W. Stroke University of Michigan. This would bean exciting development, if feasible.

Finally, I found the discussions of adaptive learning machinesinteresting, but I consider them a separate subject, not re-stricted to the field of optical devices, and not likely to affectgreatly the applications of optical and electrooptical phenomena.

In closing, let me say again that I found the symposium quiteworthwhile, and I wish to thank J. T. Tippett DOD for invitingme to give this critique.

Radiation Symposium, AMAP, Leningrad, 5-16August 1964Reported by John N. Howard, Air Force Cambridge ResearchLaboratories, Bedford, Massachusetts

Every three years the Radiation Commission of the InternationalAssociation of Meteorology and Atmospheric Physics (IAMAP)holds a symposium on atmospheric radiation. We reported onthe previous symposium in Vienna in 1961 [Appl. Opt. 1, 379(1962)]. The IAMAP is the association of the various nationalor regional meteorological societies, and its symposia are held toencourage interaction and exchange of ideas between researchersin such fields as meteorology, climatology, and geophysics. Manyof its commissions, such as the Ozone Commission and the Radia-tion Commission, are joint commissions with the WMO (theWorld Meteorological Organization), which is the association ofthe weather bureaus of the various countries and which concernsitself primarily with improving the coverage and rapidity ofweather reporting and forecasting (but not particularly withresearch). Until about ten years ago the IAMAP and WM(Otypes of people were much the same men, often spending part oftheir time making routine observations and then turning to

continued on page 382

J. N. Howard AFCRL (left), author of this report and A. AdelArizona State College.

372 APPLIED OPTICS / Vol. 4, No. 4 / April 1965