Implementation of optical and optoelectronic circuits

Navjot Singh and Harpreet Singh

This paper is concerned with the design of optical and optoelectronic circuits. In particular new designs foran exclusive-OR circuit, set-reset trigger flip-flop, and a programmable logic array (PLA) are proposed. ThePLA utilizes optical properties and performs computation on binary images using optical parallel logicdevices, lenses, etc. This work is then extended to the design of erasable programmable logic devices.

1. Introduction

The increased number of interconnections and high-er density of components on present chip designs leadto problems of propagation delays and communicationbarriers. The parallelism and noninterfering proper-ties of optics overcome some of these problems. Themain building blocks of digital circuits are logic gatesand memory devices. Optical gates, using shadow-casting techniques, were proposed by Ichioka and Tan-ida.1 An optical NOR gate using a laser diode has beenpresented by Ojima et al.2 Okumura et al.3 haveshown the implementation of optical AND, inverter,and flip-flop circuits. However this circuitry has toomany feedback links, which reduces the overall speedof these logic devices. In summary, all the above cir-cuits are either complex or are difficult to realize asreplacements for present electronic circuitry.

Liu and Chen4 implemented simple optoelectronicgates and flip-flops using optoelectronic switches andpolarization bistable semiconductor laser diodes.This work shows the feasibility of cascading opticaland electronic circuits to obtain higher package densi-ty. The limiting factor in these circuits is the switch-ing speed of the polarization bistable laser diode.5

The object of this paper is to consider some designaspects of optical and optoelectronic circuits. In par-ticular the following elements are considered here: anexclusive-OR circuit using optoelectronic switches anda light emitting diode (LED); and a set-reset trigger(SRT) flip-flop using optoelectronic switches in con-junction with polarized bistable semiconductor laser

The authors are with Wayne State University, Electrical & Com-puter Engineering Department, Detroit, Michigan 48202.

Received 30 March 1988.0003-6935/89/010178-04$02.00/0.© 1989 Optical Society of America.

diodes.5 Then an optical programmable logic array(PLA) is illustrated using optical parallel logic (OPAL)devices, detectors, and lenses. The optical PLA ac-cepts data in the form of images. The concept of aPLA is further extended to develop an erasable pro-grammable logic device (EPLD).

II. Implementation of Optical and Optoelectronic Circuits

To fully utilize the power of optics, the gates andmemory devices, which can handle the speed of opticsat low operating power levels, are required. Extensiveresearch is ongoing in developing optical and optoelec-tronic devices to achieve this speed of computation.Most of the devices proposed to date receive input atsome wavelength and polarization and provide an out-put at some other wavelength and polarization. Somegates require coherent light for their operation. Tocombine more than one such gate, frequency matchingis therefore required at every stage. This frequencymatching reduces the packaging density of the device.The main design consideration for achieving low powerconsumption and reduced physical size involves mono-lithically integrating these elements. The gates andflip-flops proposed here can be fabricated on GaAssubstrates along with laser diodes. The designs con-sidered in this paper are a gate, a memory device, and aPLA. One example from each of the different gates,memory devices, and arrays, etc. is discussed. Theprinciples discussed here can readily be applied toother gating circuits, flip-flops, read-only memories,arrays, etc.

Many optoelectronic gates are proposed in the liter-ature2-4 6-8 using various devices. We consider here adesign of an exclusive-OR circuit. The proposed ex-clusive-OR circuit consists of four optoelectronicswitches and one laser diode as shown in Fig. 1. Theoptoelectronic switches work as input devices and re-ceive the optical data as a stream of optical pulses. Inoptics, binary values 1 and 0 are equivalent to thepresence or absence of a light beam, respectively. In

178 APPLIED OPTICS / Vol. 28, No. 1 / 1 January 1989

I -v

Fig. 1. Optoelectronic exclusive-OR circuit.

-sv

Fig. 2. Implementation of the exclusive-OR circuit.

the circuit in Fig. 1, switches SW3 and SW4 are con-nected in series, whereas SW1 and SW2 are in parallel.The positive voltage Vis applied across SW1 and SW2,and negative voltage V across SW3 and SW4. Thelaser diode is biased just below its threshold value.Input X is applied to switches SW1 and SW3, andinput Y to SW2 and SW4. If both X and Y are 0, noswitch conducts and therefore the laser diode stays inits 0-state. When X is 1 and Y is 0, SW1 and SW3conduct. A positive voltage is applied across the laserdiode and it goes to the 1-state. When Xis 0 and Yis 1,only SW2 and SW4 conduct. Therefore positive volt-age is applied across the laser diode and it goes to the 1-state. When both X and Y are 1, all the switches,SW1-SW4, conduct. The positive and negative volt-ages cancel and so the laser diode stays in the 0-state.The circuit has been implemented using optoisolatorsas shown in Fig. 2. In this case optical input is provid-ed by the LEDs inside the optoisolators. The photo-transistors inside the optoisolators replace the optoe-lectronic switches in Fig. 1. The implemented circuitis simple and does not have any feedback links.

A solution to the design of optical and optoelectronicflip-flops has been proposed3 49 using numerous prin-ciples. The bistability in semiconductors leads to themost promising solution.4 The circuit for a SRT flip-flop consists of six optoelectronic switches,1 0 two feed-back lines, and one polarization bistable semiconduc-tor laser diode as shown in Fig. 3. The semiconductorlaser diode is biased just below its polarization switch-ing current.5 CLK represents the optical clock pulsewhich activates the SW6 optoelectronic switch. Theoutput through SW6 affects the state of the polarizedbistable semiconductor laser. In the absence of theclock pulse there is no voltage applied across the laserso its state remains unaffected, i.e., it stays in its previ-ous state. The TE and TM modes5 represent outputsQ and Q', respectively, of the flip-flop. The feedbacklines Q and Q' are connected across SW2 and SW1,respectively. The truth table for a SRT flip-flop isgiven in Table I. The X terms represent don't-careconditions. Qn designates the present state and Qn +1 corresponds to the next state. Atthe moment a clockpulse arrives, if S = 0, R = 0, and T = 0, no voltage is ap-plied to the laser diode. Therefore it stays in itspresent state Qn. If S = 0, R = 1, T = 0, and the clockpulse is present, a positive pulse propagates through

SW3------- Ij DC BIAS

SW3TE MODE (Q)

T MODE (Q)

SW4

Fig. 3. Optoelectronic set-reset trigger flip-flop.

SW6 and the output is switched from the TE to theTM mode. If the laser is already in the TM modebefore the positive pulse arrives, it stays in the TMmode after the positive pulse has arrived. If S = 1, R =0, and T = 0, a negative pulse is applied through SW6across the laser diode and it is triggered from the TM tothe TE mode. The conditions S = 1, R = 1, and T =0for a SRT flip-flop is an indeterminate state. If S = 0,R = 0, and T = 1, switch SW5 conducts. If the laser isin the TM mode, switch SW1 conducts and a negativepulse is applied across the laser diode. This pulsetriggers the laser diode from the TM mode to the TE

Table 1. Truth Table for the SRT Flip-Flop

S R T Qn+l(Next Stage)

0 0 0 Qn

o 0 1 Qn

o 1 0 0

o 1 1 X

1 0 0 1

1 0 1 X

1 1 0 X

l 1 1 X

1 January 1989 / Vol. 28, No. 1/ APPLIED OPTICS 179

y

,

/

y

/

C

,

/ \ I m : I 1-7=1 _~~~~~~

"I ___

At v T 1 I

FUSIBLE LINK

Fig. 4. Reproduction of a generalized programmable logic array.

mode. However, if the laser is operating in the TEmode when the clock pulse comes across SW5, a feed-back pulse switches SW2. A positive pulse triggersthe laser diode from the TE mode to the TM mode. S= 0, R = 1, and T = 1, or S = 1, R = 0, and T = 1, or S =1, R = 1, and T = 1 correspond to the don't-careconditions in a SRT flip-flop.

A PLA is a semicustom device which can be pro-grammed to implement a desired logical function.The PLA has a set physical design, thus reducing pro-duction costs. The conventional PLA contains onlyAND-OR-INVERT planes as shown in Fig. 4. At thetime of fabrication fuses are placed at all the intersec-tion points between the input and the AND gates, andbetween the AND and OR gates. So every possibleconnection is made at the time of fabrication. Unde-sired connections in the PLA are removed by blowingthese fuses at the time of programming. The general-ized PLA which consists of AND-OR-INVERT planescan be used for implementing any combinational cir-cuit. PLAs have created a great impact on the designof digital circuits. Some interest has recently beenshown in the design of optical PLAs. The develop-ment of optical gates, full adder, and PLA using aBragg grating for combinatorial logic is given in Ref. 6.This PLA has a packaging problem, as it requires aminimum distance between the Bragg grating and thedetector at the output. High power requirements alsomake this PLA an unsuitable candidate for an opticalPLA.

Here we propose a PLA using OPAL devices11 whichcan perform computation on binary images. ThisPLA receives optical input and is capable of providingboth optical or electronic output. The devices usedfor our proposed PLA, shown in Fig. 5, are OPALdevices, lenses, beam splitters, and detectors, etc. AnAND operation on the incoming data is performed us-ing an array of OPAL devices. To understand the

*1S

AND PLANE I OR PLANE

Fig. 5. Optical programmable logic array.

principle of operation of an OPAL device, OPALI inFig. 5 is considered. Two signals A and B are appliedacross the OPALl device. If the voltage across theliquid crystal (LC) is below a threshold level, the mole-cules of the LC affect the polarization of signal B insuch a way that B is rotated by 7r/2 when it passesthrough the LC. The potential applied across the LCis controlled with the help of signal A and the photo-conductor. When signal A is not present the photo-conductor does not conduct so it stays in its high-impedance state. Due to this high impedance most ofthe voltage is dropped across the photoconducting ma-terial and the voltage across the LC stays above itsthreshold value. But when signal A is present, thephotoconductor starts conducting, and it goes to itslow-impedance state. Due to this drop in impedancethe voltage across the LC goes below its threshold level.This drop in voltage affects the polarization of signalB. To achieve AND operation an analyzer orientedparallel to the polarization of signal B is used. Alogical 1 is achieved only when both signals A and B arepresent. To achieve the OR operation an array oflenses and detectors is used. The output of the ANDplane goes to the lenses in the OR plane. The lensessum the rays and focus the output on the detectors.The detectors receive optical input and provide elec-tronic output. Fibers can also be used instead oflenses for the summation of optical rays in the ORplane.

Figure 6 shows how an erasable programmable logicdevice12 (EPLD) can be implemented using the aboveconcepts. The input to this circuit is in the form ofbinary images. The inversion of any input is achievedin preprocessing. The optical clock controls the oper-ation of this EPLD. The IO architecture can havestatic or dynamic interconnections. To realize theseinterconnection optical space-invariant7 or space-vari-ant 7 techniques can be utilized.

Ill. Summary

The proposed exclusive-OR circuit in Fig. 1 is simpleand does not have any feedback links. The speed ofthis circuit is only dependent on the switching speed ofthe optoelectronic switches and not on the light emit-

180 APPLIED OPTICS / Vol. 28, No. 1 / 1 January 1989

. ,hK hK i< OK I I

}' I * * I * *~~~~~~~~~~..I I iK §< I I

1,II

I

�11 I I

AND PLNE OR P-ANE

Fig. 6. Optical erasable programmable logic device.

ting diode. On the other hand the speed of the SRTflip-flop is mainly limited by the switching speed of thepolarized bistable semiconductor laser. These cir-cuits can be monolithically integrated and are capableof operating at the same or at different wavelengths atthe input and output, as desired. The optical input,output, and clock pulses are not directly coupled sothey can be cascaded without matching the frequencyat every stage.

The proposed PLA is better than the one given inRef. 6 as it uses the global property of optics andperforms computation in parallel using OPAL devices,i.e., computation in three dimensions can be per-formed. This PLA is capable of providing both opticaland electronic output. Therefore, it can be used inpure. optical systems or in cascading optical with elec-tronic systems.

The authors are grateful to R. L. Thomas and theInstitute of Manufacturing Research at Wayne StateUniversity for supporting this work and J. Meisel foradvise and encouragement. The authors are alsograteful to a reviewer for helpful suggestions.

References

1. J. Tanida and Y. Ichioka, "Optical Logic Array Processor Using

Shadowgrams," J. Opt. Soc. Am. 73, 800 (1983).2. M. Ojima et al., "Optical NOR Gate Using Diode Laser Sources,"

Appl. Opt. 25, 2311 (1986).3. K. Okumura, Y. Ogawa, H. Ito, and H. Inaba, "Optical Bistabili-

ty and Monolithic Logic Functions Based on Bistable Laser/Light-Emitting Diodes," IEEE J. Quantum Electron. QE-21,377 (1985).

4. J. Liu and Y. Chen, "Digital Optical Signal Processing with

Polarization-Bistable Semiconductor Lasers," IEEE J. Quan-tum Electron. QE-21, 298 (1985).

5. Y. C. Chen and J. M. Liu, "Polarization Bistability in Semicon-

ductor Lasers," Appl. Phys. Lett. 46, 16 (1985).6. R. Arrathoon, E. R. Schroeder, and F. Westervelt, "Integrated

Optical Combinational Logic Using Electro-Optic Bragg Gra-ting," Proc. Soc. Photo-Opt. Instrum. Eng. 578, 207 (1985).

7. A. A. Sawchuk, "Digital Logic and Computing with Optics,"Proc. Soc. Photo-Opt. Instrum. Eng. 456,41 (1984).

8. G. Eichmann et al., "Optical Computing and Logic Using Sag-

nac Interferometric Switches," Soc. Photo-Opt. Instrum. Eng.625, 79 (1986).

9. C.-H. Lee, T.-H. Yoon, and S.-Y. Shin, "Optical Flip-Flop UsingLight-Emitting Diodes and Photodetectors," Appl. Opt. 25,2244 (1986).

10. F. J. Leonberger and P. F. Moulton, "High-Speed InP Optoelec-tronic Switch," Appl. Phys. Lett. 35, 712 (1979).

11. R. A. Athale and S. H. Lee, "Development of an Optical ParallelLogic Device and a Half-Adder Circuit for Digital Optical

Processing," Opt. Eng. 18, 513 (1979).12. Data Book, Altera Corp., 5525 Monroe St., Santa Clara, CA, pp.

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Patents continued from page 153

4,763,998 16 Aug. 1988 (Cl.350-427)Compact zoom lens.S. TSUJI, M. SUGIURA, K. TANAKA, and M. KATO. Assigned toCanon K.K. Filed 23 June 1986 (in Japan 26 June 1985).

A compact conventional four-component zoom lens is described, containinga three-element positive front unit for focusing, a three-element negativevariator, a three-element positive compensator, and a fixed positive rearsystem containing five to seven elements. The diaphragm is between thesecond and third units to maintain a small diameter. Seventeen examples aregiven, with apertures lying between f/1.2 and f/2, with focal-length rangesfrom 8.3-46 mm and 10.8-82 mm in the various examples. The image diame-ter varies from 8 to 11 mm. R.K.

4,764,001 16 Aug. 1988 (Cl.350-465)Retrofocus-type objective for an endoscope.A. YOKOTA. Assigned to Olympus Optical Co., Ltd. Filed 1 Dec.1987 (in Japan 13 June 1984). Continuation of Ser. 743,356,10 June1985.

An objective for an endoscope is described, consisting of a negative frontcomponent containing the stop, a positive intermediate lens group which ismoved for focusing, and a positive rear field lens adjacent to the fiber bundle.The focusing range goes from a magnification of 0.058 to 0.566. Four exam-ples are given controlled by six conditions. R.K.

4,765,722 23 Aug. 1988 (Cl.350-427)Zoom lens.Y. AONO. Assigned to Nippon Kogaku K.K. Filed 25 Apr. 1986 (inJapan 11 May 1985).

A high-aperture (f/1.8-f/2.8) zoom lens for a video camera is described,covering a focal length range from 24 to 275 mm. The lens contains five

groups, in order (+ - - + +), the first being used for focusing; the second isthe variator moving linearly with focal length; the third group is the compensa-tor; and the fourth is fixed, yielding an afocal beam. The rear lens group ismoved in and out to maintain a fixed image plane. There are twenty-two ortwenty-three elements in all in the five groups as follows: 4 or 5, 5, 2, 6, and 5.Two examples are given. R.K.

4,765,736 23 Aug. 1988 (Cl.356-300)Frequency modulation spectroscopy using dual frequencymodulation and detection.T. F. GALLAGHER, G. R. JANIK, and C. B. CARLISLE. Assignedto Electric Power Research Institute. Filed 24 July 1986.

The invention consists of the use of a single modulator in FM laser spectros-copy with two modulation frequencies, producing three groups of sidebands onthe laser beam at the laser frequency plus and minus the two modulationfrequencies and at plus and minus the difference between the two modulationfrequencies. Improved demodulation and practical high-frequency FM laserspectroscopy (for observing pressure broadened spectral features) are theclaimed advantages. The five claims granted cover the method and theappropriate apparatus. J.A.D.

4,767,192 30 Aug. 1988 (Cl.350-342)Light activated light valve with a silicon control element.I. F. CHANG and E. I. GORDON. Assigned to International Busi-ness Machines Corp. Filed 31 Oct. 1986.

Where the usual light-addressed light valves for projection displays use ahomogeneous CdS or SE photoconductive layer as the control element for aliquid crystal cell, this patent discloses a silicon photodiode array to make aneffective control element capable of applying a spatially varying ac voltageacross a liquid crystal light valve. There are ten claims. D.C.G.

1 January 1989 / Vol. 28, No. 1/ APPLIED OPTICS 181