Copying computer-generated-holographicinterconnects by the use of partially coherent light
A. Fimia, R. Fuentes, 1. Pascual, A. Bel6ndez, J. J. Egozcue, and M. T. Navarro
Dichromated gelatin, bleached photographic emulsion, photoresist, and silver halide-sensitived gelatinare studied as holographic recording materials for holographic optical interconnects. We make theanalysis by using a copying process with a computer-generated interconnect, which produces 64 diffractedbeams on axis, as a master. The experimental results obtained confirm that it is possible to obtain copiesin which we improve the performance of the master.
1. IntroductionThe use of holographic optical elements (HOE's) asan interconnect distribution network for both opticalcomputing and switching systems has been discussedby various authors in the past few years.1-3 Anoptical interconnection system requires sources, inter-connects, and detectors, and a HOE can implementthe pattern of connections from the sources to thedetectors. Use of computer-generated HOE's pro-vides distinct advantages over optically recorded orsynthesized holograms. Computer-generated holo-grams (CGH's) have been demonstrated to be anexcellent way to design holographic interconnectsbecause it is possible to control the profiles of theinterferenctial patterns, thereby optimizing the signal-to-noise ratio (SNR) and the diffraction efficiency 4
(DE). In previous papers it has been suggested thatit is possible to mass produce holographic opticalelements through a copying process by the use ofpartially coherent light.5-7
The main drawback to the use of CGH's for makingoptical interconnects in their limited DE, a factor of
A. Fimia, R. Fuentes, I. Pascual, and A. Belendez are with theUniversidad de Alicante, Apartado 99, Alicante 03080, Spain; A.Fimia, R. Fuentes, and I. Pascual are with the Laboratorio deOptica, Departamento Interuniversitario de Optica, and A. Belen-dez is with the Departamento de Ingenieria de Sistemas y Comuni-caciones. J. J. Egozcue and M. T. Navarro are with the Departa-mento de Matemdtica Aplicada III, Universidad Polit6cnica deCatalufia, Catalufla, Spain.
Received 1 June 1993; revised manuscript received 27 August1993.
great impoortance in optical computing systems.In this paper we show that this fact may be overcomeby copying the CGH interconnects onto differentholographic recording mediums in which higher effi-ciency interconnects can be produced. We presentexperimental results obtaioned when copying free-space computer-generated holographic interconnectsby using partially coherent light, and we analyze thepossibilities of increasing the SNR and the DE of themaster interconnect by using the copying technique.Experiments were carried out with different record-ing materials and photochemical techniques, andresults obtained demonstrate the feasibility of thiscopying method for obtaining holographic intercon-nects.
Basically, the copying process consists of storingthe interference patterns generated in the masterhologram in a recording material with sufficientspatial resolution. We place the master interconnectin direct contact with the recording material we wantto use to make the copy, with the photosensitivelayers of the master and the copy placed together.We work with partially coherent light, which enablesus to work with more economical sources and devices,provides stability conditions that are not as strict asthose used in conventional devices, and eliminatesnoise gratings when the recording material has agranular structure, as photographic emulsions do, forexample.8
2. Master InterconnectIn order to obtain the computer-generated holo-graphic interconnect we calculated the interferencepattern for 64 divergent beams with an on-axisreference beam. As can be seen in Fig. 1, the set of
emitter points Pi (with i = 1, . . , 64) was placed on aplane (the object plane) parallel to the holographicplate, and we assumed that each pi point emitted aspherical wave whose amplitude was equal to Ai.A plane reference wave with an amplitude equal to Arentered orthogonal to the hologram plane. Further,the intensity distribution I (q) of the points Pi at eachpoint q on the holographic plate is given by
I(q)=A 2 + 2 As D(qApi)r|
(1)
where D (q, pi) stands for the distance between pi andq, and represents the light wavelength. In Eq. (1)we have eliminated the intermodulation terms thatare due to the interference between different emitterpoints in order to avoid the intermodulation noisethat is an important source of noise in the generationof holographic interconnects.9' 10
The interference pattern was calculated by usingEq. (1) for 1000 x 1000 points on the hologramsurface. The results were drawn on a 400 x 400mm2 computer screen by means of a range of 100 graytones. The black and white tones were associatedwith the minimum and the maximum values of theintensity, respectively.
For the master interconnect we assumed that thereference wave had an amplitude A = 0.5, theamplitudes of the object beams were Ai = 1 for i =1, ... , 64, and the light wavelength was 633 nm.The size of the interconnect was 0.805 x 0.805 cm.The plane on which the 64 emitter points were foundwas placed 30 cm from the holographic plate, and the64 points were arranged so as to form a square gridmeasuring 1.65 cm + 1.65 cm.
The image on the computer screen was properlyphotoreduced in order to achieve a holographic inter-connect measuring 1 cm + 1 cm on Agfa 8E56 HDphotographic emulsion with a substrate of acetate,and the plate was processed as an absorption holo-gram. Figure 2 is an enlarged image of the interfer-ential figure we obtained.
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carried out with dichromated gelatin (DCG) fromKodak 649 F emulsion, positive photoresist fromShipley. AZ-1350 J and B, bleached photographicemulsion from Agfa-Gevaert plates, and silver halide-sensitized gelatin (SHSG), also from Afga plates.
In the course of our experiments a collimated beamfrom a high-pressure mercury lamp, filtered at 405nm and incident to the master and copy, was used toexpose the recording materials. For bleached emul-sion two types of bleach bath were used: one was asolvent bleach and the other was a rehalogenatingbleach bath, both without fixation step. The ex-posed plates were developed, and detalls ofthe process-ing schedule are given in Ref. 6 (for DCG andphotoresist), 11 (for bleached emulsion), and 12 (forSHSG). The incident exposure was varied for eachcase and already-optimized photochemical processeswere always used.
4. Experimental Analysis of the CopiesIn this study both processing and recording materialhave been studied, and therefore we would like tocompare the results obtalned in the copy with thedata that correspond to the master. The DE wasmeasured for each on the 64-diffracted beams, andthe SNR was calculated by measuring light that wasdiffused among the different diffracted beams accor-diong to the following equation:
(DE)i SNR = z D" 2
i=1 j~ (DX (2)
where n = 64, (DE)i is the light diffracted by the i
beam, and (DE)ij is the light diffracted between the iamd thej beams. We also evaluated the differencesbetween the DE that correspond to the differentinterconnect beams by using the expression
DEm. -DEminL9(DE) =DEm + DEmi (3)
However, these measurements do not allow us tocompare the different holographic recording materi-als, so a Q factor, which is defined as the value of themaximum DE multiplied by the SNR and divided bythe dispersion that exists between the maximum and
minimum values for the DE, gives us the overalloptimal characteristics of the copy interconnects:
(DE)m (SNR)
In Table 1 we give a summary of the experimentalresults obtained. The measurements were done withan He-Ne laser (X = 633 nm).
In the first place we can observe that when photore-sists are used, results depend on the energy used, andtherefore, by correctly selecting layer thickness andexposure time, we can significantly improve the re-sults we obtain. In our case the best results wereachieved with a thickness of 1.7 lm. When bleachedemulsions were used, the results obtained with reha-logenating bleach were better than those obtainedwith solvent bleach. Finally, if we compare theresults obtained with DCG with those obtained withSHSG, we must conclude that the latter has beenshown to be the best recording material for copying.One important aspect is that the copying processmodifies the deviation parameter between the dif-fracted beams, which in some cases allows for theoptimization of the distribution of DE between thedifferent interconnect beams.
As we can see, the, copy interconnects have betterdiffraction efficiency than the original, but only SHSGgive us an experimental Q factor better than themaster.
5. Conclusions
Different conventional holographic recording materi-als and processing techniques for copying holographicinterconnection systems have been analyzed experi-mentally. The analysis presented shows that it ispossible to copy low efficiency absorption CGH inter-connects with partially coherent light -onto phasematerials that achieve high DE from the resultinginterconnects. Of the different recording materialsanalyzed, SHSG has proven to be the optimummaterial for the manufacturing of this type of inter-connection system given that it has an excellent SNR,high DE, and excellent sensitivity and light-stabilitylevels. Excellent results can be achieved with thiscopying process if the masters used are correctlyoptimized and manufactured by the use of techniquesthat can ensure an exact reproduction of the originalprofile. Finally, when the copying method describedin this paper is used, it is possible to mass produceholographic interconnects.
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