optik logic gate

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99 2 IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. QE-18, NO. 6, JUNE 19

Opt ica l AND Gate

Abstract-We have successfully demonstrated a new type of logic circuit which provides an optical output pulse that is the AND function oftwo wavelength multiplexed optical nput signals. The active comp o-nents o f this op tically coupled logic gate are a triggerable sem icon duc -tor laser and a novel photod etector consisting of two eries photod iodeswhich are sensitive in different wavelength bands.

A OSSIBLE technique for he realization of high-speedlogic and memory circuits is the use of optical couplingbetween logic gates. O ptical coupling offers the potentialadvan-tage of very low gate propagation delays because gate inter-connect capacitance, the principal limitation on the speed ofconv entional circuits, can be eliminated . Initial work in this areahas relied on the creationof a nonlinear interaction between twolasers. The approaches which have been used include: 1) thebistable po larization of a dye laser in a birefringe nt cavity [ I ] ,2 ) the quenching of one semiconductor laser with anotherlaser [2], [3],3) he interaction of two laser beams in a satur-able absorber [4], 4) optical excitation of intracavity saturableabsorbers in a semiconductor laser [ 3 ] , nd 5) the coupling oftwo parallel pyroelectric photodetectors to a bulk electroopticmodulator [ 5 ] . More recently, Taylor [6 ] has proposed thatoptical logic circuits could be fabricated by interconnectingintegrated optical devices such as interferome tric mod ulatorsand directional cou pler switches. In this paper we report on anew ty pe of optical logic gate which utilizes two high-speedelectrooptic devices, a dual p-i-n pho todetec tor [7] and a trig-gerable sem iconductor laser.Fig. 1 shows a circuit diagram of an AND gate for which bot hthe inpu ts and the outp ut are in he form of optical pulses.There are three active com ponents: two p-i-n photod iodes anda triggerable semicon ductor laser (TSL). A characteristic ofTSLs is that for a particular bias current, the light ou tpu t in-creases abruptly (almost discontinuously) [9], [ l o ] . t hasbeen shown that appreciable gain can be achieved by biasingthe TSL just below this point and triggering the laser with asmall additional current pulse [ lo] . After triggering, light isemitted in short (lo0 mW) pulses witha period of a few nanoseconds. The two series-connected pho to-diodes in Fig.1 constitute an AND gate in the sense that theirelectrical output is the AND function of the optical inputs. Iflight is incidenton bo th photod iodes simultaneously (i.e.,logical 1s at bo th inputs), and if the TSL is biased near its

Manuscript received December 21 , 1981.Theauthorsare with Crawford Hill Laboratory,BellLaboratories,Holmdel, NJ 07733 .

O P T I C A LAN0 GATE-U T PU T ( A . B ) -S EM ~ C O N DU C T O R$ LASER 1R IG G ER A BL EIN PU T A -1) PHOTODIODESIN PU T B U

Fig. 1. Circuitdiagram of an opticallycoupled AND gate.Theactivecomp onents of thiscircuitare two discrete photod iodes connectin series and a triggerable semiconductor laser.trigger point, the photocurre nt will be sufficient to trigger tlaser. As long as the duration of the current pulse from tphotod iodes is less than the pu lsation p eriod of the TSL, thgate ou tpu t will be a single optical pulse. However, if one the np ut signals smissing i.e., a logical 0 at one or boinputs), the off diode(s) will act as a blocking element awill prevent th e laser from triggering. Thus, in this circuit, tseries photodiod e comb ination performs the logic and the TSprovides gain pulse com pression and the conversion of hsignal back t o an op tical format.Fig. 2 shows a schematic cross section of an AND gaphotodetector which integrates the two photodiodes in Fig.on to a single chip. The wophotodio des in this structuare sensitive in different wavelength bands, thuspermittinthe use of w avelength multiplexed inp uts instead of two spcially separated beams. This approach permits the reductiof he device area as wellas the minimization of he nter-connect capacitance. Thestruc ture consists of five epitaxlayers grown by LPE on a (10 0) oriented InP substrate. Tfirst layer grown is an n-typ e InP buffer layer. This is followin succession by an n-: Ino.7oGa0.30AS0.66P0.34 qua terna(e) ayer (Eg N- 0.92 eV), a p+ : nP layer, an n: Id layer, aan n-: Ino.47Gao.53As ternary T) layer (Eg-0 .75 ev). Ding crystal growth some Zn diffuses from the p+:JnP laycreating a p-n junction in the Q layer. After crystal grow thsecond p-n junction abo ut 3 pm deep is formed in the Tlayby Zn diffusion (- 2 h at 500C) in a closed ampoule. C ontaare fabricated by pulse electroplating Au-Zn to th e Zn-diffusT region, Au-Sn to the substrate and then alloying at 425OIndividual mesas (approximately 75 pm in diameter) are etchin a dilute solution of bromine-methanol. Prior to mounti

0018-9197/82/0600-0992$00.75 0 1982 IEEE


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CAMPBELL et 01 . : OPTICAL AND GAT E 99 3

D U AL - WA V E L E N GTH A N D G A TE

L ~ N GSHORT X INPUT f3INPUT A

Fig. 2. Schematic cross section of the AND gate photodetector.the devices on headers, a window is etched in the back contactto permit illumination through the InP substrate.Wavelength discrimination is achieved in a similar manner asthe dual wavelength demultiplexing photodetector [121, [131 ;however, the AND gate photodetector described in this paperdiffers from the demultiplexing ph otod etect or bo th tructurallyand functionally. The two primary differences in the tructuresare that the AND gate photodete ctor uses an n-type substrateinstead of p-type and has a p : nP layer and a thick (> 3 pm)n+ : nP layer separating the Q and T layers in place of a thin,lightly doped n- : nP layer. The function of the demu ltiplex-ing photodetector is to separate signals in two different wave-leng th bands. This is accom plished using a parallel configu r-ation w hich requires three terminals. (We note here that anoptical OR gate could be fabricated by coupling the demulti-plexing photodiode t o a triggerable laser.) The AND gatephotodetector, on theotherhand, is a wo terminal devicewhich, ow ing to the fact tha t the photodiod es are connectedin series, provides an output only when both optical inputs areon simultaneously.To illustrate the blocking action of the off diode, we con-sider the case where initially both photodiodes are off andsuddenly one diode receives an inpu t pulse. Initially, the biasvoltage is divided equally between the Q and T photodiodes.When the input signal turns one of the, photodiodes on, thephotogeneratedcurrent in thatphotod iode will exceed itsreverse saturation cu rrent. To compensate there will be a flowof charge from theo t h e r ( ~ f f ~ )iode. Thiswill result inan increase in the potential drop across the o ff diode anda corresponding decrease in the bias across the on diode.This process will continue un til the 0 d 7diode is sufficientlyforward biased to generate a current equal to and opposite tothe photocurrent. At that point, the net current through thedevice will be zero and the potentials across the o ff diodeand on diode will be approxim ately the bias voltage and theopen circuit voltage, respectively.

These devices have been tested by using a beam splitter tocomb ine he light from twodifferent wavelength emitters.The results are shown in Fig. 3 . The upper trace is the drivecurrent for the short wavelength source, an LED which has anemission peak at 1.03pm. The middle trace is the drive current

TIME ( 5 p r / d i v ) --..)Fig. 3. Experimental operation of the AND gate photod etector. The up-per trace and middle traces are the drive currents for the short ( h21.03 pm) nd long ( h 1.3 pm) wavelength sources, respectively. Thelower trace is the output current of the AND photodetector.

0.8 0.9 1.0 1.1 1.2.3 1.4 1.5.6.7.8WAVELENGTH l p m )Fig. 4 . Spectral responses of the quaternary Ino.7oGa0.30AS0.66P0.34(dark solid lin e) and ternary Ino.47Gao.53As (light solid line) pho to-diodes which comprise he AN D gate photodetector.for the long wavelength emitter which, in this figure, is a 1.3 pmsource, b ut 1.55 pm sources have also been used. The lowertrace is the detector outp ut. In addition to confirming thatthe electrical output of this device is the AND function of theoptical inputs, this figure also reveals two sources of the triggernoise characteristic of this type of detecto r; namely, crosstalk,which is seen as a small residual signal when one of the inputsignals is off, and transient spikes, which o ccur w henever oneof the inputs changes states.In our better devices we find that the total crosstalk, whichcan be bo th optical and electrical in origin, is 15 dB below thesignalevel. Optical crosstalk occurs when light from theinputs is absorbed in the wrong layer.Todetermine hemagnitude of this compo nent, a third terminal was added to afew devices so that the spectral response of each photodiodecould be measured separately. In Fig. 4 thedarkand ightcurves are the resp onsivities of the Q and T , respectively. Theshort wavelength cutoff of the Q layer is due to absorption inthe InP substrate nd the long wavelength cutoff of bo th curvescorresponds to the bandgap energies of the Q an d T ayers. Ina properly designed structure the wavelengths of the emittersshould be well t o each side of the region where the responsitivitycurves cross. The wavelength at w hich this crossover occurscan be adjusted to minimize the optical crosstalk by changingthe crystal composition of the Q layer. The shape of the tworesponsitivity curves should be symm etrical at ab out the cross-over poin t, but in Fig. 4 we observe that t he T layer exhibitssome response at shor t wavelengths (X


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994 IEEE JOURNAL OF Q U A N T U M ELECTRONICS, VOL. QE-18, NO. 6 , J U N E 198

due to incomplete absorption in the Q layer. Our experiencewith demultiplexing photodetectors has shown that this sourceof crosstalk can be eliminated by making the Q layer thicker.The other possible cause for optical crosstalk, namely, the ab-sorptiqn of lmg.w avelength photons in the Q layer, appearsto be insignificant in these devices.There are two m echanisms for electrical crosstalk. The firstis the leakage current of an o ff diode. Since there is alwaysa certain amount of leakage current in a reverse biased photo-diode, this mechanism will set a lower limit on the crosstalk.The reverse leakage current density of the T and Q diodes areplotted in Fig. 5 . We observe that the leakage curre nt of th eQ diode is much less than that of the T diode. Some differenceis expected due to the lower bandgap energy of the T layerand to variations in he background do ping levels but thesetwo factqrs alone do not seem t o be sufficient to explain thedifference observed here. The lower leakage current of theQ diode may also be due in part t o the difference in the waythe jun ctions are fabricated. Electrical crosstalk can also occurif the reverse biased Q and T diodes are not adequately isolatedfrom m inority carriers injected by the forward-biased junctionseparating them. How ever, by growing the p:I nP and n: InPseparation layers sufficien tly th ick, we have successfully elimi-nated this source of electrical crosstalk.Transient pulses represent a more serious source of triggernoise than crosstalk. Whenever there is a change in the state ofon e of the optical inpu ts, there w ill be a curr ent pulse whilecharge stored on the photodio des reaches a new steady statedistribution. In the worst case, the charge in the ransientcurrent pulse will be V ,C where V, is the bias voltage and C isthe diode capacitance. A transient pulse and a superimposeds i p d pulse are shown in the photograph in ig. 6 . For a rigger-able laser to successfully discriminate between these tw o pulses,the height of the signal pulse shou ld be about three times tha tof the transient. We find that the AND gate photodetector canachieve this level of discrim ination with as little as 5 pJ of inputenergy.The rise time of the A N D gate pho todete ctor is approximately3 ns. In our present devices the rise time is diffusion limitedbecause the absorbing layers are not com pletely depleted. T hefall time, on the other hand, is RC limited with R being thesum hf the dy nam ic, series, and load resistances. We observefall times of -12 ns. Improvements in both the rise time andfall time should be achievable by further optimization of thedevice parameters such as layer thicknesses and doping levels.We haveuccessfully demonstratedhe AND gate pro-posed in Fig. 1 by coupling the amplified output of the ANDgate photodetector described above to a protonbombardedGaAs-Al,Ga,-,As trigg erab le laser . Fig. 7 shows the outputof the laser fordifferent np utstates. The top trace showsthat during the 20 ns that both optical input pulses are on simultaneously the laser em its light in a series of sho rt pulses.By contrast, the middle and lower traces show that if eitherof the optical inputs is of f the laser is no t triggered. H ence,if th e presence of a light pulse is equated t o a logic 1 and itsabsence to a logic 0, this combination of photodiodes and atriggerable laser provides an optical output which is the ANDfunction of twowavelength multiplexed optical inputs.In conclusion, we have fabricated and characterized a novelphotodetector consisting of two series photodiodes which are

l I I l 1 1 1 1 1 1 l / , 1 1 1 / , I , 1 1 1 l I l I I I I I I I I- 5 -10 - 15 -20 -25 -30

R E V E R S E B I A S ( V !Fig. 5 . Leakage current density of the quaternaryndernaryphotodiodes.

Fig. 6 . A transient noise pulse (smaller pulse) and a superimposed signpulse (larger pulse).

sensitive in different wavelength bands. By coupling this photodetector to a triggerable semiconductor laser, we have demonstrated a gate having an o ptical ou tput which is the logical ANDfunction of tw o optical input signals. If sufficiently high speedcan be achieved, integrated devices of this type could becomthe building blocks of optically co upled logic circuits.AC KNOW LEDGMENT

We are grateful to S . E. Miller for helpful discussions. Wthank F. J. Favire, J. C . Centanni, J. F . Ferguson, G. J. Quaand W. B. Sessa for capable techn ical assistance and M. Dixofor supp lying the triggerable laser.


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CAMPBELL et al . : OPTICAL AND GATE 995

Fig. 7 . Lght out put of a triggerable laser which has been coupled to the AN Dgate photodetector. The top trace shows thatthe laser is triggered and emits light in a series of short pulses when both optical inputs are on. The two lower tracesshow the laser output for first one and then the other optical input off.

REFERENCES[ l ] E. J. Johnson, L. A. Riseberg, A. Lempicki, and H. Samuelson,Completely optical coincidence logic employing a dye laser,Appl. Phys. Lett., vol. 26, pp. 444-447, Apr. 15, 1975.[21 G. J. Lasher and A. B. Fowler, Mutually quenched injectionlasersas bistable devices, IBM J. Res. Develop., vol. 8,pp. 471-475,

Sept. 1964.[3 ] N.G. Basov,V.V. Nikitin, and A. S . Semenov, Dynamics ofsemiconductor injection lasers, Sov.Phys. Usp.,vol. 12, pp. 219-239, Sept. 1969.[4 ] 0.A. Reimann and W. F. Kosonocky, All-optical computer tech-niques, IEEE Spectrum, vol. 2, pp. 181-183, Mar. 1965.[ 5 ] A. M. Glass and T. J. Negran, Optical gating and logic with pyro-electriccrystals,Appl. Phys. Lett., vol. 24 , pp. 81-82, Jan. 1974.[6 ] H. F. Taylor, Guided wave electro-optic devices for logic andcomputation,Appl. Opt., vol. 17 , pp. 1493-1498, May, 1978.[7 ] J. A. Copeland, J. C. Campbell, A. G. Dentai, and S . E. Miller,Wavelength-multiplexed AND gate; A building block for mono-lithic optically-coupled circuits, Appl. Phys. Lett ., vol. 39 , pp.[SI J. A. Copeland, Triggerable semiconductor lasers and light-coupled logic, J . Appl. Phys., vol. 5 1 , pp. 1919-1921, Apr.1980.[ 9 ] T. L. Paoli, Saturable absorption effects in the self-pulsing(A1Ga)As junction laser, Appl. Phys. Lett., vol. 34 , pp. 652-655, May 1 5 , 1 9 7 9 .[ l o ] J.A. Copeland, S. M. Abbott,and W. S. Holden, Triggerablesemiconductor lasers, IEEE J . Quantum Electron., vol. QE-16,[111 C. A. Burrus, Pulse electropolating of high-resistance materials,poorly contacted devices, and extremely small areas, J . Elec-trochem. Soc., vol. 188, pp. 833-834, May 1971.[12] J. C. Campbell, T. P.Lee, A. G . Dentai, and C. A. Burrus, Dual-wavelength demultiplexing InGaAsP photodiode, Appl.Phys.Lett., vol. 34 , pp. 401-402, Mar. 1 5 , 1 9 7 9 .[13] J. C. Campbell, A. G. Dentai, T. P. Lee, and C. A. Burrus, Im-proved two-wavelength demultiplexing InGaAsP photodetector,IEEEJ. QuantumElectron.,vol. QE-16, pp. 601-603, June 1980.

197-199, Aug. 1 , 1 9 8 1 .

pp. 388-390, Oct. 1979.

J o e C. Campbell (SY73-M74)was born in Gorman, TX, on January 11,1947. He received the B.S. degree in physics from the University ofTexas, Austin, in 1969, and the M.S. and Ph.D. degrees in physics fromthe University of Illinois, Urbana-Champaign, in 1971 and 1973,respectively.From 1974 to 1976 he was employed by Texas Instruments, Inc.,where he worked on integrated optics in GaAs/AlGaAs. Since 1976he has been at he Crawford Hill Laboratory, Bell Laboratories,Holmdel, NJ, where he has worked on electrooptic devices for fiberoptics applications.Dr. Campbell is a member of Sigma Xi, he American Physical Society,and the Optical Society of America.

Andrew G. Dentai (76) was born in Budapest,Hungary, in 1942. He received the Dipl. Engi-neer of Chemistry degree from the Universityof Chemistry, Veszprem, Hungary, in 1966, andthe M.S. and Ph.D. degrees in ceramic sciencefrom Rutgers University, New Brunswick, NJ,in 1973 and 1974, respectively.From 1969 to 1972 he worked at Bell Labora-tories, Murray Hill, NJ, on high punty aluminamaterials, application of freeze-drying tech-niques tohe preparation of ceramic rawmaterials, and fluorescent ceramics. In 1974 he rejoined Bell Labora-tories, Holmdel, NJ, as a member of the Guided Wave Research Labora-tory. Currently he is involved with epitaxial crystal growth of 111-Vsemiconductors for the preparation of a variety of electroluminescentdevices and photodetectors covering the wavelength range of 0.8-1.65 pm.

John A. Copeland (M67-SM76) attendedGeorgia Institute of Technology, Atlanta,from 1958 to 1965, receiving the B.S., M S . ,and Ph.D.egrees in physics. His doctoraldissertation was on the quantumheory offerromagnetism.He is currently Head of the Repeater ResearchDepartment, Bell Laboratories, Holmdel, NJ.Since joining Bell Laboratories in 1965, hiswork has included theoretical studies of space-charge dynamics andhermal noise genera-tion as well as experimental studies of devices for high-speed logic andmillimeter-wave power generation. He has also contributed to the fieldof magnetic-bubble domains and participated in the design of the MAC-8 microprocessor. His recent work is on semiconductor lightwave sys-tems and optically coupled logic.Dr. Copeland is a member of the American Physical Society. In 1970he received the IEEE Morris N. Liebmann Award or his work on gallium-arsenide microwave devices. He was Editor of the IEEE TRANSACTIOON ELECTRON EVICESrom 1971 to 1973.

Wayne S. Holden (60) was born n Kingston, PA, on February 2,1947. He graduated from RCA Insti tute in 1970.In 1970 he joined Bell Laboratories, Holmdel, NJ, where he has beeninvolved in the evaluation of optical fiber parameters and the design ofelectronic circuitry foroptical fiber communication systems. Hespresently engaged in the processingand evaluation ofemitters anddetectors used in fiber optical communications systems.

optik logic gate

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