Spatial light rebroadcaster bit-slice word-addressableholographic memory

Alastair D. McAulay and Junqing Wang

A heteroassociative memory is proposed that allows a key word in a dictionary of key words to be used to recallan associated holographic image in a database of images. A novel bit-slice search approach finds the wordsought in a dictionary of key words and generates a beam that selects the corresponding holographic imagefrom a directory of images. Spatial light rebroadcasters are proposed for the key-word database. Opticalexperimental results demonstrate that spatial light rebroadcasters may be used in loops with image intensifi-ers and optically addressable liquid-crystal light valves and that the approach can be used to select hologramsbased on key-word matching. In the case considered, a holographic image having more than 40,000 bits isselected from 8 by using a key word from a dictionary of 8 words.

Key words: Associative memories, optical storage, holographic memory, key-word search, spatial lightmodulators, luminescence, spatial light rebroadcasters.

1. Introduction

Optics is ideal for large databases because of its inher-ent parallelism, matching capabilities, and high reso-lution.1 In this paper we describe research into data-bases that use a key word to reference a correspondingsegment of memory containing text and images.2Such databases are heteroassociative memories be-cause the input and output are related only by associa-tion. An example would be the association of thetextural word tree with an image of a tree. We areassociating key words with holograms (Fig. 1).

We propose a system to perform this word-hologramassociation by combining a new technique, bit-slicesearch, with a holographic memory. Bit-slice search isused to find a key word and produce an orthogonalpattern in which a single beam is available to select theassociated hologram for reconstruction.

In Section II we discuss problems in attempting touse approaches proposed previously for this applica-tion. The basic operation of spatial light rebroadcas-

A. D. McAulay is with the Department of Computer Science andEngineering, Wright State University, Dayton, Ohio 45435. J.Wang is with the Department of Computer Science and Engineering,Wright State University, 3171 Research Boulevard, Kettering, Ohio45420.

Received 3 December 1990.0003-6935/91/202885-05$05.00/0.© 1991 Optical Society of America.

ters (SLR's) is also explained. Section III provides adescription of the proposed method in which a bit-slicesearch using a key word is used to select a correspond-ing hologram for reconstruction. Two optical experi-ments are designed to verify feasibility. The firstdemonstrates how SLR's may be used in a loop. Thesecond demonstrates that a bit-slice search can be usedto select a hologram by using a key word. The resultsof these experiments are described in Section IV.

11. Background

A. Difficulties with Alternative Approaches forHeteroassociative Memory

Many researchers have investigated approaches to he-teroassociative memory that use multiplication of aninput vector by a matrix to obtain the output associat-ed vector.3-5 Many different approaches to comput-ing or updating the matrix have been proposed. In allthese cases, the output is a single word. In this paperwe are interested in recalling a large amount of data byreferencing from a single key word.

Previously6 7 we showed experimental results for anoptical orthogonal heteroassociative memory in whichsums of outer products for the associations were storedon a SLR. In that case, the input vectors representingthe key words had to be orthogonal. The recall of acorresponding associated output for an input word wasdemonstrated by using optical matrix-vector multipli-cation. In this paper we are interested in removing therestriction to orthogonal key words as well as in obtain-ing an output dataset.

Another approach is to perform a parallel correla-

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C,B,A Blue (Write)Input

Word31r

Orange+~~~,( (Incoherent Read

Output) PIS

R,Q,P Infrared (Read)

Memory Hologram Reconstructedof Array Image for

Words Selected InputWord

Fig. 1. Associating key words and holograms.

tion between the word sought and the database. Thisis easily accomplished in optics. However, it is thennecessary to select the largest of the correlation coeffi-cients, set it to a fixed high value, and then set theothers to zero.8 Unfortunately this winner-take-allcircuit is not easily accomplished in optics. In thiscase the process is similar to a classifier that classifiesthe input as one of a number of words in a database.After classification, the output is used to generate theassociated output. A holographic memory could beused in this situation, in the same manner as in theproposed system.

The winner-take-all circuit may be avoided by usingnonlinearity, such as a threshold, in a feedback loop.9

An autoassociative memory has been demonstratedwith holograms and phase-conjugate mirrors.10

Symbolic substitutional 2 can be used to search for akey word in a dictionary of words and then convert itinto an associated pattern. However, the conversionto an associated pattern is not practical if a largeamount of data is to be associated with the key word, asproposed here.

B. Holographic Memory

It is convenient to store multiple holograms adjacentto one another or one on top of another with differentreference angles.13"14 Holographic memories that usethese principles have been constructed15 with 254 ho-lograms in an approximately 2.5-cm square, each holo-gram having four images of 32 X 32 bits. The means ofselecting holograms was mechanical for one of fourgroups and by mirror for one out of 63 in a group; acamera was used to separate the one required image offour in a set. In the application that we consider, wewish to select a hologram by means of a key word, so adifferent mechanism is needed.

C. Spatial Light Rebroadcasters

SLR's consist of a crystalline layer of alkalai earthsulfide containing selected impurities and depositedupon a sapphire or other substrate. Devices have beenproduced with coating thicknesses varying from 5 to250 ,gm. The submicrometer resolution and the possi-bility of nanosecond speed16 make the device an idealcandidate for storing and processing high-resolutiondata.'7

Electrons are trapped at those points in the materialthat are exposed to light of a given frequency, forexample, blue in Fig. 2. The number of electrons

KInfrared (CoherentRead Output) P-S

Electron TrappingMaterial

Fig. 2. Operation of spatial light rebroadcaster.

trapped is proportional to the light intensity over awide range of intensities. On exposure to a secondfrequency, infrared in Fig. 2, incoherent luminescenceat a third frequency, orange in Fig. 2, occurs at thosepoints where electrons were trapped. Further, forthin samples the luminescence at each point is propor-tional to the product of the number of electronstrapped and the intensity of the second frequency.The device can perform a sequence of OR operations ona sequence of binary arrays by writing the arrays se-quentially onto the device with sufficient energy tocause the device to saturate. The device can alsoperform a sequence of AND NOTs by successive readswith binary arrays such as B, C, and D, assuming that Ais an array of ones:

S = B C * D... (1)

As B - C is equivalent to B + C or NOR, the system iscapable of performing a sequence of NORs.

111. Approach for Word Associative Holographic Memory

The bit-slice search approach is described first. Theoptical setup is then described for the proposed bit-slice holographic memory, in which the output of thebit-slice search for a key word is used to select from anumber of holograms. Results of two experiments toshow feasibility are then presented.

A. Optical Bit-Slice Search and Replacement

A novel bit-slice search and replacements is proposedthat avoids the difficulty of implementing a winner-take-all circuit.2 It differs from symbolic substitutionin two ways. First, it permits the search word to beentered serially, which is more suitable for interfacingwith electronics. Second, it requires less powerfulsources because only one-dimensional instead of two-dimensional illumination is used.

Figure 3 shows an optical implementation for bit-slice search in which we illuminate the least-significantbit (rightmost bit) of a search string, s, set on a one-dimensional spatial light modulator (SLM) labeledSLM1, with a narrow beam. Light expands verticallyon passing through the vector s at SLM, such that itilluminates the rightmost column of the matrix po seton SLM2. If 1 is assumed transparent and 0 opaque,light will pass through the 1 region. The output right-hand column of the matrix p0 is fed back through BS2and mirror M1. Mirror Mlis displaced horizontally sothat the column of light is focused through the second-

2886 APPLIED OPTICS / Vol. 30, No. 20 / 10 July 1991

> _

BS2 OutputBS, SLM1 SLM,

Offset one column

Fig. 3. Illustration of optical bit-slice word search.

from-right element of s on SLM1. It expands to coverthe second-from-right column of po on SLM2. Thisprocedure is repeated for three iterations, and only acomplete string of I's will generate a 1 in the leftmostcolumn at the output. Only the leftmost column of theoutput plane is observed. Search for an arbitrarystring is performed by a second setup, similar to that inFig. 3 but using complementary arrays for s and po.The results are merged into mirror Ml for feedback.

In an optical experiment we first perform a bit-slicesearch on the database, using the input key word. Inthe case of a single match, the single optical outputcorresponds to the position of the match and may beused to read out associated data optically.

Figure 4 shows the optical setup for performing bit-slice search. Figure 5 is a photograph of the opticalsetup. A database of key words is written, using bluelaser light onto SLR2, where each key word occupies arow. The complement of the key-word database iswritten onto SLRl. The optical system to write ontoSLRl and SLR2 is not shown but is essentially imagingfrom a two-dimensional electronically addressablespatial light modulator (ESLM).

The key word whose association is sought is enteredonto the ESLM, or ESLM2 , and the complement ofthis word onto ESLMl. Collimated infrared light isintroduced at Cl, at a frequency that can read storedenergy out of the SLR's. The input beam has a verti-cal spread, and in the horizontal direction it is narrowand in alignment with the leftmost pixel of ESLM2.Cylindrical lens Cl1 focuses the beam onto the leftmostpixel of ESLM2. (Note that it makes no differencewhether you start with the leftmost or rightmost pixel,as long as the appropriate shift direction is used.)

If the leftmost pixel of ESLM2 is set clear, afterpassing through ESLM2, the beam expands in thevertical direction and is then collimated by C14. Whenthe beam strikes SLR2, orange luminescence is gener-ated at any point in the leftmost column that hasstored data. This column vector of light is imagedonto an optically addressable SLM. Mirror M3 is setso that the vertical column is moved one column to theright of the normal alignment. Coherent infraredlight is reflected off beam splitter BS3 onto the front

Fig. 4. Optical setup for bit-slice search of the key word.

Fig. 5. Photograph of the optical setup for the key-word search.

face of the SLM. The reflected energy now provides acoherent replica of the column vector in infrared,which is needed for further loops. Cylindrical lens C13focuses the light onto the second-leftmost pixel ofESLM2 and ESLMl.

If at any time ESLM2 is not set clear, then its com-plement on ESLM, will be clear, and the light will passthrough the upper path rather than the center path.In this case, the column vector from the complementwill be fed back. The loop is repeated until lightreaches the rightmost column in the feedback path.Only those rows of the database that exactly match theincoming key word will have light in the last column atthe right on the final feedback loop. If unique keywords are chosen there will be only one spot of light inthe rightmost column. The position of the spot in thevertical vector represents the number of the row corre-sponding to the matched word. Consequently the bit-slice search is equivalent to orthogonalizing the key

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- - - U -Ar

SH2

SH1

(a)IR

SH3

SLR2

Fig. 7. Optical module with a SLR.

(b)Fig. 6. Experimental inputs: (a) key word for recall, (b) key-word

database.SLR

Fig. 8. Photograph of a SLR module.

words. The single beam illuminates one of a numberof holograms at H to provide reconstruction of theassociated image at the output.

B. Optical Experiments and Results

Two experiments were conducted to verify feasibilityof the proposed approach. As the signal level fromSLR's is low, the SLR is followed by an image intensifi-er and an optically addressable liquid crystal lightvalve (OLCLV, manufactured by the Hughes Compa-ny). For the purpose of associative memory experi-ments, Fig. 6(a) shows a word used for search and Fig.6(b) shows a database of key words arranged by row.The input word can be seen to match the word in thesecond row of the database. To verify that a loop maybe constructed, as shown in the approach proposed inFig. 4, we place the database at P1 in the optical setupshown in Fig. 7. A photograph of the setup (Fig. 8)shows the three devices. Argon light maps the patternonto SLR1 via shutter SH,; SH1 is then closed, andshutters SH2 and SH3 are opened to allow light to pass.Infrared passes through SH3 to read SLR1 . The out-put passes through the image intensifier and writes onthe OLCLV. Argon light, passing through SH2, readsout the OLCLV and transfers the information to SLR2.A high-pass spatial filter in the form of a spot at theFourier-transform plane P2 is used to remove back-

Fig. 9. Output of a SLR cascadable module.

ground noise generated in the image intensifier and theOLCLV. The image at SLR2 is shown in Fig. 9. Thesignal level at the output is sufficient to write on anoth-er device. The gain around the loop is -1000 in theimage intensifier and 30 in the OLCLV. This compen-sates for the loss in the SLR between write and outputenergy and the loss in the high-pass filter.

In the second optical experiment, film is used inplace of the SLR's. Eight images are stored as holo-grams on a piece of film. Each hologram is '1 mm indiameter, and the holograms are stored one under

2888 APPLIED OPTICS / Vol. 30, No. 20 / 10 July 1991

L2

Fig. 10. Output of the bit-slice search experiment.

Fig. 11. Reconstructed holographic image selected by key word.

another in a column. For simplicity of experimenta-tion we chose this arrangement of holograms instead ofplacing the holograms on top of one another and ad-dressing them at different angles. The film with holo-grams is placed at H in Fig. 4. The holograms show upas dark spots in Fig. 10, which was obtained by lookingback from the position of H in Fig. 4. The light spotsin Fig. 10 show the match between the input word andthe word database (Fig. 6) as the loop in Fig. 4 istraversed seven times during the bit-slice search. Oneach loop, the output was displaced one column to theright. The result in Fig. 10 shows that the key wordmatches rows 2, 5, 6, and 8 of the database for the firsttwo bits at the left. This can be verified by inspectionof Fig. 6. At the third loop, only the words in row 2 and6 match the key word. After seven loops, only thesecond row of the database still matches the input keyword, so that only the second hologram from the top isilluminated. The reconstructed image from the sec-ond hologram is shown in Fig. 11. Therefore we havedemonstrated how key words may be used to addresslarge blocks of data stored in the form of holograms.

IV. Conclusion

The ability to recall large segments of data or imagesby using key words is an important capability that wewish to achieve in optics because of the high-densitystorage possible with holograms. We proposed aheteroassociative memory that uses a new technique,bit-slice search, to find a key word and then associate it

with a hologram. Further, SLR's are proposed forstoring the key-word database. We performed twooptical experiments. The first shows that SLR's maybe used in a loop with image intensifiers and OLCLV'sto restore gain and wave-length for cascadability.However, OLCLV's that can be read out with infraredlight are required. The second optical experimentshows that bit-slice search permits selection of holo-grams from key words without requiring a winner-take-all circuit, as would be required with correlation.The complete memory remains to be demonstrated.

The support of the U.S. Air Force Office of ScientificResearch (grant AFOSR-89-0525) is gratefully ac-knowledged.

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