Ronald A. Nordin

Anthony F. J. Levi

A System Design Perspective onOptical Interconnection Technology

Demands for increased interconnection density and higher bandwidth, cou­pled with constraints on the costofadvanced wide-bandwidth telecommuni­cation switching and high-throughput computer architectures, areexhaust­ing the capabilities ofconventional electrical interconnection. Advances inintegrated circuit technologies and enhanced digital services have created a"bottleneck" at the circuit-board-to-eircuit-board level ofthe interconnectionhierarchy. Toalleviate thisproblem, AT&T is developing new techniques inparallel optical interconnection. Inserting parallel optical interconnectiontechnology into advanced electronic processing systems notonly meets pro­jected performance requirements, butalso potentially offers them ata com­petitive cost. This paper describes how future high-performance paralleloptical interconnect technology might be packaged and used.Introduction

Technological advances in integratedcircuits (Ics) and demand forenhanced digi­tal telecommunication services andadvancedcomputer services are creating a needforincreased interconnection density andhigherbandwidths, while maintaining stringentrelia­bility specifications. Enhanced digital tele­communications services, which includeBroadband Integrated Services Digital Net­work (BISDN), high-definition television(HDTV) , video conferencing, cellular services,and microcell interconnection, havespurredresearchand design ingraphics, real-timeimage processing, patternrecognition, model­ing,speechprocessing, and parallel process­ing. (See Panel 1fordefinitions ofabbrevia­tions, acronyms, and terms.)

Increased interconnection density andbandwidth at alllevels ofthe interconnectionhierarchy are shapingsystemdesign advancesin traditional electronic packaging andopticaltechniques.1-4 End-to-end optical datalink(ODL) products (e.g., ODL® 40 and ODL50) havebeen used successfully in high-performancecomputer and telecommunication switchingsystems, such as the AT&T Network Systems5ESS® switch. To meetfuture systemneeds,AT&T is developing newtechniques in paralleloptical interconnection.

Parallel optical interconnection willmostlikely be achieved usinga generic com­ponentdesigned to communicate overdis­tancesbetween 1meter (m) and 10kilome­ters (km). These devices are known as one­dimensional onts (lD-ODL).

Figure 1ashows the conventionalelectronic packaging technology forlargedig­ital switching andcomputer systems. Thelengthofthe corresponding interconnectionhierarchy is roughly bounded byits packag­ingtechnology, as shown in Figure lb. Elec­trical interconnections are efficient andsup­port the density and bandwidth needed tointerconnect lengthsofless than 1m. DiscreteODLs are efficient forinterconnection lengthsofmorethan 25m, but theycannot supportthe density or cost constraints imposed byhigh-performance system design in the 1- to100m range. In this range, weexpect the 10­ODL to be a competitive solution.

Tofulfill the needsofmany small­volume users,which in total createa largedemand for 1O-0DLs, the industry needsastandard, such as the IEEE 1596-1992 Scal­ableCoherent Interface (SCI), that will notneed to be customized fordifferent applica­tions. Although it supplies computer-bus-likeservices, it uses a collection offastpoint-to­point links (ofsize16N +2), instead ofa

AT&TTECHNICAL JOURNAl. 0 SEPTEMBER/OCTOBER 1993 37

Panel 1. Abbreviations, Acronyms, and Tenns

1O-oDL - one-dimensional optical datalinkAlGaAs - aluminum gallium arsenideAu-goldBER - biterror rateBH-GRINSCH - buried-heterostructure graded-index

separate-confinement heterostructureBISDN - broadband Integrated Services Digital Net-

workcw - continuous wavedBm - decibels referenced to a milliwattDFB - distributed feedbackECL-emittedcoupkrwgicEEL - edge-emitting laserEMI - electromagnetic interferenceFP - Fabry PerotGaAs - gallium arsenideGaInAsP - gallium indium arsenide phosphideGbit/s - gigabit(s) per secondIC - integrated circuitInP- indium phosphidekm- kilometerLED -light-emitting diode

physical bus, to reachdatarates as highas 1gigabit persecond (Gbit/s) per link. This standard provides a conve­nientframework fordeveloping the 1O-GDL

The keytechnological developments neededbythe 1O-ODL are laserdiode arrays, detector diode arrays,solderbumpbonding, electronic laser driver arrays, elec­tronic receiver arrays, optical submount technologies(e.g., micro-machined silicon with waveguide capability),optical fiber ribbon cables, andoptical fiber connectoriza­tion. Figure 2ashows a schematic diagram ofAT&Ts1O-ODL technology. System design advantages ofthistechnology include low-eost optical datalinks (packageand manufacturing costare sharedovermany channels);highbandwidth; high reliability; piece parts ruggedenough formanufacturing environments; high opticalinterconnection density; laser-to-Iaser optical delay differ­ences, or skew, low enough to be used in synchronousdesigns; a small module footprint (approximately 6.5crrr'): andlow electromagnetic interference (EM!). This

38 AT&TTECHNICALJOURNAL. SEPTEMBER/OcroBER1993

LGA -laser gatearrayUNb0 3 -lithium niobatem-meterMAC - multi-fiber arrayconnectorMo- molybdenumNRZ - nonretumto zeroNPN - transmitteroc- optical carrierODL - optical data linkOEIC - opto-electronic integrated circuitPiN-diodePN- diodePOLYFIC - thin-film polymer film integrated circuitPOLYHIC - thin-film polymer hybrid integrated circuitPQFP - plastic-quad flat packps- picosecondR1V - room-temperature vulcanizationSCI - scalable coherentinterfaceSEED - self-electro-optic effect deviceS.SEED - symmetric SEEDSEL - surface-emitting laserSn-tinW-watt

represents a significant cost/performance advantageovera serialODL scheme, which uses discrete devices, oran all-electronic interconnection scheme.

Figure 2bshows how1O-ODLs may be incorpo­ratedin a systemdesign. Theycanbe mounted to thesurface ofa board, andthen,at the appropriate point inthe manufacturing process, the multi-fiber arrayconnec­tors (MAC)-IIs canbe mounted andconnected to the 10­ODL Figures 2candd shows 181O-ODL modules (18 wide)mounted ona circuit board. Each module has 18fibers,andeachfibertransmits at 1Gbit/s, collectively yieldinga bandwidth of324 Gbits/s. Forhigh-bandwidth connec­tions, this represents a significant improvement over"traditional" connections between a circuit board andbackplane. The slim profile andlow power dissipation of1O-ODLs allow themto supportmoremodules percircuitboard andmorecircuit boardsper equipment shelf.

Critical to the development ofthe ID-ODL pro­gramis the adoption ofan appropriate packaging

Switching/computer system

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scheme, in which the transmitter and receiver modulesarecompatible with an existing fiberribbon connector.One such fiberribbon connector isAT&Ts MAC-II,which consistsof 12or 18multimode (orsingle-mode)glassfibers placed inaccurately machined silicon v­grooves on 25G-micrometer {J..Lm) centers.Guide pinsensure that the polished ends ofthe two connectorhalves canbe accurately attached with less than a O.!HI.Bcoupling loss.For the MAC-II, the standard deviation for

fibermisalignment is less than 2 urn. Bonding the laserarray (ordetectorarray) on a silicon submount thatincorporates v-grooves andguidepinsmakesit compati­blewith the MAC-II connector, and usable ina manufac­turable, self-aligned packaging scheme.

Backplane Interconnection RequirementsPanel 2 listsgeneralsystemspecifications foran

advanced synchronous high-performance system based

AT&TTECHNICALJOURNAL. SEPTEMBER/OCTOBER 1993 39

Figure 2. 1D-ODLtech­nology. (a) Schematicdiagram of a trans­mitter connected to areceiver by 18-wldeoptical fiber ribbon.The physical design ofthe system Is shownIn (a) a side view,(b) an end view, and(c) and (d) a topview.

Transmitter module

Driver array

(a)

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on environments that contain genericcentralofficeswitching equipment. To attain the requiredperformancewithin acceptable constraints, the electronic driverandreceiver mustbe integrated intoan arrayform. Althoughintegrating veryhighgain/bandwidth amplifiers is diffi-

40 AT&TTECHNICAL JOURNAL • SEPTEMBER/OCTOBER 1993

cult, because ofcrosstalk and power/ground "noise,"integrating low-gain transimpedance amplifiers (e.g.,300n) is muchmorefeasible. The evendistribution ofpower dissipation between the transmitter and receiveris also desirable, from a systemperspective. Key to the

Panel 2. System Specifications for an Advanced Synchronous HIgh­Performance System

The generalsystemspecifications foran advanced synchronoushigh-performance systembasedon environments that contain gen­ericcentraloffice switching equipment include:- Cost/interconnection - Less than$50, which includes driver,

receiver, connectors, and backplane media- Module reliability - 105 hours < (J

- Zero systeminterconnect error - Appropriate digital coding(e.g., parity) to ensure systemintegrity

- Ambient temperature - O°C < T < 70°C- Absolute humidity - Lessthan 0.026 (5percent< relative humi-

dity< 95percent)- Interconnect length (L) - 1< L < 100m- Compatible with standardEeL input/outputlevels- Module electrical differential inputsandoutputs- Power supplies - +5, ground, -2, and-4.8- Bitrate (BR) - 0 < BR< 1Gbit/s- Average groupinterconnect delay variation - Lessthan 200

picoseconds (ps)- Skew (delay variation within a group) - Total interconnect skew

less than 400 ps (toavoid clock recovery circuitry or anyjitteraccumulation)

- Small package size- 1" xl" (e.g., integrated 18channels)- External to module dc coupling- Low module power dissipation ofless than 2W.

successofthe lD-ODL is an appropriate lightsource,which weaddress in the nextsection.

Optical Transmitter Source &electionThe three competitive optical transmitter tech­

nologies that provide an optical backplane strategy! forbothone- or two-dimensional arraysare:- Light-emitting diode (LED) arrays- Optical modulator arrays- Laser diode arrays.Thesystemperformance and costofeach is discussed inthe sections that follow.

LED Amlys. LEO arrays" have beenregardedasattractive, economical optical transmitters becausetheyare inexpensive to manufacture; highly reliable; easilyextendible to large, two-dimensional (i.e., surface­emitting) arrays; and morestablein temperature thanotheroptical methods. Unfortunately, LEOs have a fewdisadvantages. Their low external power efficiency (a

coupled power efficiency ofabout0.2 percent, with anintegral lens) manifests itselfin unwanted chippowerdissipation, therebylimiting the width ofthe LEO array.LEOS also havea longcarrier recombination lifetime,which controls their modulation bandwidth (neglectingparasitic capacitance). Resonant cavity ("super") LEOsrepresenta possible upgrade in LEO performance. Theyimprove spectral width byan order ofmagnitude,therebyreducing the chromatic dispersion infiber.Quantum wells alsoimprove LED bandwidth. Despite allthis, LEOs havea fundamentally longercarrierlifetimethan injection lasers,and,moresignificantly, cannot sup­ply the lightpower necessary for the lD-OOL.

OptIcal Moduletor Amlys. Another ofthe opticaltransmitter technologies is the classofoptical devicescalled modulators. These devices havean input lightpower sourcedirected to each modulator andan outputconnection that directs lightto the receiver. The lightismodulated either with an electrical signal (e.g., IiNb0 3

AT&T TECHNICAL JOURNAL • SEPTEMBER/OCTOBER 1993 41

signal, or theycan operate transparently. Examples ofdata-transparent optical modulators include deformablemirrordevices," LiNbO 38 liquid crystal devices (electro­optic effect),9 and magneto-optic devices. 10 The self­electro-optic effect device (SEED) 11 is an example ofthelatter type oflatchable modulator.

Although a modulator-based lD-ODL isunlikelyto be available soon, researchon modulators andaddi­tional related topics is underway, and may offer futureadvantages.

LaHr Arrays. In recentyears, laserarrays havebeen considered foruse as optical transmitters. One­dimensional arraysofedge-emitting lasers (EELs) 12.13

and two-dimensional arraysofsurface-emitting lasers(SELs) ,14.15 in both GaAs and InPmaterial systems, havestimulated the searchfornewnonlinear devices withbeneficial characteristics for lD-ODL applications. Lasersare preferred over LEDs becausetheyhave:- Higherexternal power efficiency- Anarrow output radiation conethat allows simple butt

coupling offibers- Modulation capabilities that extend into the giga­

hertz16 range (reduction ofcarrier lifetime with stimu­latedemission).

There are three fundamental ways ofmodulatinga laser: directcurrent drive (with or without pre-bias),external cavity modulation (e.g., LiNbO 3 or other electro­optic devices), or intra-eavity modulation. Different systemspecifications (orperformance specifications) anddevicelimitations dictate the type ofmodulation that canbe used.Ifdesigners use pre-bias modulation, or an external modu­lation schemein which the contrastis low, theymust"code" the transported data to set the properthresholdadjustment forthe receiver. This resultsinan ac-eoupledsystem. Foran ac-eoupled systemto work, the thresholdlevel at the receiver mustbe adjusted automatically usingautomatic gaincontrol. Aback-facet monitor at the trans­mittercontrols the sourcepower. To reducethe complex­ityand associated cost,a moredigital approach (i.e., de­coupled systems) is desirable. Direct modulation pro­ducesa dc-eoupled system (i.e., logic 0 equals notrans­mitted photons, which results ina highcontrast).

Currentresearchon digital applications oflaserarraysfocuses on direct-drive lasers," driven from alogic family such as emitted coupler logic (ECL) , withoutcurrent pre-bias. The philosophy behind the transmittermodule electronic design centerson the directdigital

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or InPdirectional coupler), an optical signal, a magneticsignal, a mechanical signal, or a combination ofallofthese. Usually, the lightpower sourceis carriedinafiber-optic cable. Onceinside the lD-ODL, the lightispower divided and routed to each modulator. This allowsthe continuous wave (cw) high-power lightsourceto bephysically separate, unconstrained by sizeor power, andcapable ofproviding reliability through redundancy. Theincident cwlightis directed on the optical device, whichcan then reflect or transmitlight. The modulated outputlightis carriedoutofthe lD-ODL in waveguides. Thesemodulators can either interactwith, or latch, the data

Figure 3. A simulated "optical eye diagram" using two dif­ferent current pulse shapes to support a 1-Gbltjs datarate. The bright red line represents a step function currentpulse with t r =t f =250 ps and a 2D-mA magnitude. Theother line Is a shaped current pulse with a 4D-mA Initialspike settling back down to the 2D-mA level. Pulse shapingrelaxes oscillation peaks and reduces settling time. Thisdelay, together with relaxation oscillations, dictates thelaser bandwidth.

42 AT&T TECHNICAL JOURNAL • SEPTEMBER/OCTOBER 1993

drive ofthe laser.The electronic current driverdeliversa stepofcurrent to the laser, in which the magnitude ofthe current pulse in the "on"state is usually twoto tentimes greater than the lasingthreshold.This is donewithout feedback (i.e., back-facet monitors).

Twodesignconstraintsimmediately arisewhenconsidering this strategy.First, the magnitude ofthe cur­rent pulsemust be large enough to deliver the necessaryamount ofoptical powerin the worst-case conditions(i.e., high temperatureand near-end of life), and largeenoughto minimize the turn-on delay. Second, becausethe laser is not pre-biased above threshold, the turn-ondelay and jitter can be excessive. This obstaclemaybeaverted usingappropriately shaped current pulsesandengineering the laser propertiesto achieve the optimalcoating design,cavity length, active area design, etc.Maintaining a lowlaser threshold current (while stillmaintaining high outputpowerlevels) maximizes signalfidelity and minimizes transmitterpowerdissipation.

Figure3 showsa simulated "optical eye dia­gram" using twodifferent current pulseshapes to sup­porta 1-Gbit/s data rate. Pulseshapingreduces oscilla­tionpeaksand settlingtime.This delay, together withrelaxation oscillations, dictatesthe laser's usefulband­width. The uniformity ofturn-on delayacross the array(i.e., skew) alsolimitsthe usablebandwidth in syn­chronousapplications. Decreasing the laser's cavitylength and improving facetreflectivities decrease theturn-on delay. Ahigh-bandwidth, direct-drive laser arraydeveloped byAT&T Bell Laboratories consistsofa 12­laser InP/GaInAsP array18 in which the lasingthresholdis controlled to 9.9±0.9 rnA, the optical poweris 8.6±0.4mW per facet, the bandwidth is greater than 4 gigahertz(GHz) (at3 mWofoptical power), and the crosstalkisless than -26 dB. Similar reports on InP-based laserarrayshavebeen madeby IBM 19 and ORTEL in GaAs,andby NTI20 and lincoln LabS.21The conclusion drawnfrom this simulation is that direct-current modulation,with pulseshaping, can result in an excellent eye dia­gram at 1 Gbit/s, in which a digital logiczero corre­spondsto no emittedphotons.

Atemperature-stable laser - one that does notrequirea thermal electriccooler- costs less and is morereliable. To increasethe temperaturestability, designerscan lowerthe threshold currents and/or change the mate­rialsystemto one witha widerbandgap. GaAs lasers aremore temperaturestablethan InP lasers, owing to the

improved To constant, that is Ithoc exp(TITo), and typi­calvaluesfor InGaAsP and AlGaAs are 50°C and 120°C,respectively. In addition, the gain ofa GaAs laser islarger than that ofan InP laser, indicating that GaAslaser arrays shouldbe superiorto InP laser arrays. How­ever, InP lasers are currentlymore reliable than GaAslasers,22 because they sufferless facetdeterioration withtime,fewer dark linedefects, and less nonradiativerecombination in the cavity. As GaAs laser technologyimproves, the reliability difference shoulddisappear.Future switching systems, which will havelower-costoptical connectorsand plastic fibers, will use evenshorter-wavelength optical sources.Although the opticalinterconnection applications being described inthispaperare for distancesofless than 100m, and for modestbit rates (1 Gbit/s), the wavelength used need notbe dic­tated by minimum loss or dispersion in the fiber. Thefundamental concernsfor interconnect designersincludelowcost and minimum skew (matched delay) in the fiberribbon. Eventhough minimum loss and dispersion in thefiberdo not controlthe wavelength, wavelength doesbecomean issue at bit rates greater than 1 Gbit/s,and/or at distancesgreater than 1km.Here,InGaAsP/lnP lasers operating at a minimum l.3-mmfiberdispersion havea distinct advantage.

Device and circuitdesignersare currentlydebating the use ofn-substrate versus p-substratelasers.Sincethere are no differences between them infactors such as device processing costs,yields, or char­acteristics, p-substrate lasers would be preferable, froma circuitdesignperspective, becausethe electronicdriver:- Uses NPN transistors, which havea higherIt- Has a less complex circuitdesign- Has a lowerpowerdissipation.

Recent research in SELs for optical interconnec­tionapplications alsosuggest that they are an attractivecandidate for the lD-GDL lightsource.First, they canbetested at the wafer level, which lowers the device costand provides a naturalway to evaluate defectmaps.Contrary to popular opinion, however, this does not rep­resent a significant advantage forhigh-volume arrayproduction.

The secondadvantage ofSELs is their poten­tially high coupling efficiency to fiber, the result ofasingle-mode gaussianoutputprofile. The couplingefficiency can be increasedbecauseofthe single-mode

AT&T TECHNICAL JOURNAL. SEPTEMBER/OCroBER 1993 43

FIgure 4. A schematic and a photomicrograph of a lasergate array.

gate (absorber section) is voltage modulated. The high­inputimpedance gate is terminated at 500. Achange inthe gate voltage affects the magnitude ofthe loss (orgain) in the absorbersection, which modulates the out­put lightintensity. The lightoutputis either in the spon­taneous state,wherethe gate voltage is low, or the stimu­lated emission state,wherethe gate voltage is high.Whenthe anode is de-biased (e.g., I, = 50rnA), there is alargenonlinear regime. Figure 4 shows howsmallchangesin the gate voltage cancausea largechange inlightoutputintensity, producing a largecontrastratio.The absorbersection becomes electrically saturated, andalmost lOO-percent modulation canoccur. Thisenablesthe electrical or optical (with integrated PiN) input pulseto be reconditioned, provide de coupling, andsupport awider temperature change.

These devices alsoexhibitpower gain, and,assuch,may be considered to have transistor-likeamplification characteristics. Forexample, a 3O-~Wchangeofelectrical input power candeliver 7 mWofoptical outputpower. The laser described is anInGaAs/InP buried-heterostructure graded-indexseparate-eonfinement heterostructure (BH-GRINSCH) laserdiode with fourquantum wells.

Conventional lasersoutperform LED andmodula­tor devices and cost less (if the costofthe receiver func­tionis included). Compared with direct-drive, low­thresholdlaserarrays, lasergate arrays (LGAs) have:- Wider bandwidth, because ofthe high input

impedance ofthe gate- Lower power dissipation, owing to the lower effective

drive currentand lackofpower dissipation from an ICdriver

- Direct ECL compatibility- Reconditioned pulse shape,resulting inan improved

bit error rate (BER)- Less intersymbol interference- Wider temperature margins (nothermal crosstalk)- Lower delay times, to minimize skew.

Currently, the only disadvantage that LGAs havecompared with direct-drive low-threshold lasers is a mar­ginally increased processcomplexity. Isolating absorberregions and defining gate contact padsare the only addi­tional processing required.

Receiver Design. The receiver module design phi­losophy contradicts that oftraditional telecommunicationreceiver design strategies. Here, high-incident optical

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natureofthe outputpower. SEL technology has pro­gressed enormously to date,especially incertainperfor­mance specifications. ButforSELs to play an active rolein lD-ODLs, they mustdemonstrate:- Direct, dc-eoupled modulation- Highersingle-mode outputpower- Higher"wall plugefficiency"- Proven reliability performance- High performance overa wider temperature rangethan that ofa comparable edge emitter. Althoughresearch on SELs and related topics continues, a SEL­based lD-ODL is technologically premature.

Figure 4 showsanother type ofedge emitter,one that uses an intra-eavity loss rnodulator-" to controllightoutput. This FP lasergate is a three-terminal device.The anode (gain section) is de biasedabove its lasingthreshold. The cathode is at groundpotential, and the

44 AT&TTECHNICALJOURNAL. SEPTEMBER/OCTOBER 1993

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Figure 5. A com­parison of lIghtsource technologiesand receiver charac­teristics for anoptimal data-lInkdesign. The receiverperformance Indicesshown on the left aretranslmpedance gainrequirements to pro­duce a so-mv outputsignal, the translm­pedance amplIfierpower dissipation,and the minimumpower/ground supplycrosstalk allowed toprevent oscillation.The receiver powerdissipation was deter·mined by simplyassuming 20 mW pergain stage (at a gainof 10 per stage).

power on the detectorarrayproduces a largecurrent sig­nal to the transimpedance amplifier array IC.Sucha largesignal significantly relaxes the gainand dynamic rangerequirements on the transimpedance amplifier. The tran­simpedance gaincan be relaxed (e.g., to 3oon), to inte­gratemultiple amplifiers ontoa single tc, This, in tum,will reducesignal delay variations and minimize cost.Otherwise, interchannel crosstalk resulting from the useofhigher-gain amplifiers would prevent this level ofinte­gration. Forexample, 200 fJWoflightpower absorbed inthedetectorat an 85-percent quantum efficiency will pro­duce an outputcurrent of170 J,!A Atransimpedance gainof300n will produce an outputvoltage of51 millivolts(mY). Generally, the outputofa transimpedanceamplifier is connected to a decision circuit, wheretheinput signal is "regenerated." The 51-mV signal in thisexample is enough to drive the decision circuit. Weesti­mate a per-ehannel power dissipation of 100 mW on the

module, which generatesa total power dissipation of1.8W on the receiver module.

Figure 5compares plotsofdifferent light sourcetechnologies. The receiver power dissipation was deter­mined bysimply assuming 20mW pergainstage (atatransimpedance gainof 10per stage).The crosstalk wasscaled linearly with gain, anda maximum crosstalk of-30 dBwasused as a reference at a transimpedance gainof2000n (asper private discussions with Dr. R. N. Not­tenburg on transimpedance amplifier arraywork at theUniversity ofSouthern California).

The EEL power dissipation wascalculated with85-percent detectorefficiency, 3O-percent quantumefficiency, 3-dB coupling efficiency at the detector andtransmitter, 5-mA threshold current,twice calculated cur­rent pulseheight (tominimize turn-on delay), andcur­rent derived from a 5V power supply. For the calculatedSEL power dissipation, the assumptions varied from the

AT&T TECHNICAL JOURNAL. SEPTEMBER/OCTOBER 1993 45

Figure 6. 1D-ODL

transmitter. (a) Opti­cal silicon subassem­bly connecting toMAC-II connectorplug. (b) Modulepackaging scheme.

Wirebonds

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(a)

Laser array chip

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EELonly in two areas: a la-percentquantum efficiencyanda 6-dB coupling loss,to account forspacial hole­burningeffects. These parameters wereplotted againstthe output current exiting a detectorarray. With a detec­tor current closeto 500~ the receiver and transmitter(EEL) power dissipation balance to approximately 90mW.

Clearly, it is desirable to lower the amount ofdetectorcurrent,eventhoughthis results ina slightimpairment ofthe power balance with respect to the EEL.The evidence suggeststhat EELsshould be the predomi­nantlightsourcetechnology for lD-GDL applications.They provide a lower power dissipation and an over­whelmingly widertolerance ofpower/groundcrosstalkthan do other technologies at this time.

PlICkage De..... for Perellel Optics. To successfullyintroduce newconcepts intoproduction-grade products,it is essential to design and develop an appropriate pack­agingscheme. It is advantageous to blendcomponentscurrently in production (orextensions thereof) with aminimum oftechnological development, so products canbe developed fasterand with less risk.The lD-ODL pack­age designis described in the next two sections. Thefirst section details the subassembly, wherethe optical

46 AT&TTECHNICALJOURNAL. SEPTEMBER/OcroBER 1993

components are housed, and the second sectiondescribes how the optical subassembly is packaged.

Silicon Submount De...... The lD-ODL package wasdesigned-' as a connectorized transmitter (orreceiver)module. The package wasassembled within the toler­ancesneededto couple lightfrom a laserarrayinto amultimode fiberarray. Astrictly passive alignmentscheme25•26 wasused,one inwhich the module wasdesigned to fit inside a standard plastic-quad flat pack(PQFP). To makethe design compatible with existing rib­bonconnectors, designers choseAT&Ts MAC-II,27 an18-wide fiberribbon connector. Consistent with the fiberarrayconnector, the laser (and PiN) subassemblies usedsilicon. Silicon also provides a good thermal pathforheatdissipation andcanbe manufactured to lithographicaccuracies.P Figures6aand b, respectively, show thealignment methodology andthe lD-GDL transmitter mod­ulepackaging scheme. The alignment between the multi­mode fiberarrayand the laserwasobtained by springloading the alignment pinsin the fiberarray (MAC-IIplug) intothe alignment v-grooves in the transmittermodule. The alignment v-grooves, located along theedges ofthe transmitter module, were formed by

Rgure 7. (a) Assem­bled transmittermodule without Its RTV

or lid attachment.(b) Eye diagram of apseudo-random pattem(2 15 - 1) NRZ Inputsignal transmittedthrough the 1D-ODLat622 Mblts/s.

precisely etchingthe silicon submount and the cap. Thebonding padson the submount (corresponding to the p­contacts ofthe laser array) are located within litho­graphic accuracies ofthe etchedv-grooves.

The laserarraychipconsistsof 18lasers locatedon25O-Ilm centers. It is bondedontothe silicon sub­mount usingAu/Sn solder, with the epitaxial sidefacingthesubmount. The solderbonding padson the siliconsubmount in Figure6a,located at the end ofelectricalinterconnection lines, are used fordriving the lasers.Thesolder bumpheightwasoptimized to provide a goodmechanical joint, to dissipate the heat generatedby thelasers, and to allow the laserarrayto self-align duringthesolder reflow process. Aresidue-free active atmospheresolder reflow process.F' which does not requireanypost-eleaning, wasused to keep the laser's surface free ofcontamination. Figure7ashows a photograph ofthecompletely assembled transmitter module.

HybrId De....... The package forthe lD-ODL isbased on a family ofmultilayer hybrid integrated circuits,called POLYHICS,30,31 developed byAT&T. Precisionthin-film resistors, fabricated on the ceramic substrate,

(8)

(b)

are placed closeto the ends ofsignal traces to provideexcellent termination ofhigh-data-rate lines. The leadframe supports data ratesgreater than 1 Gbit/s. Much ofthe POLYHIC package wasdesigned to minimize develop­mentcostand interval, which wascontrolled byincorpo­rating pre-existing, premolded leadframe anda quad­flat-pack geometry. For an initial predevelopment prod­uct, the package configuration wassatisfactory.

lD-ODL Performance. How well anyinterconnec­tiontechnology performs ina largedigital systemdesignis a function of:- Signal density (on/offofthe PCB and its footprint)- Signal bandwidth- Power dissipation- Interconnection lengthlimitations- Costper interconnection- Skew- Signal groupdelay variations- Biterror rate- Reliability.The next two sections describe the performance ofthetransmitter and receiver modules in the lD-ODL.

AT&TTECHNICALJOURNAL.SEPTEMBER/OCfOBER 1993 47

Transmitter. The 18-wide laser diode arraysusedin the lD-ODL wereN-substrate, FP, InGaAs/InP (A= 1.3urn), bulk-active arrays, with PN junction-blocking layers.Laserfacets, on 250-J..lm centers (tomatch the fiber-ribbonconnector), havea cavity lengthof250 urn, The -3 dBbandwidth ofthese lasers is greater than 2 GHz. Since thearraysare "hightemperature" lasers,there is no needforthermal electric coolers. These lasers,whose thresholdcurrent is in the 10- to 15-rnA range, with reasonable uni­formity, can supply several milliwatts oflightpower at85°C. "Butt" coupling ofthe lasers to the MAC-II connectorreducesthe amount ofpower to the fiberby no morethan3 dB. About 1mWis the the optimal amount ofopticalpower that should be coupled to the fiber.

Receiver. The 18-wide detectorarraysused in thelD-ODL are N-substrate, bottom-illuminated, InGaAs PiNdiodes, whoseoptically active region is 75urn in diame­ter on 250-J..lm centers.Detectorarraysare mounted ontopofa turningmirror (anisotropically etched on the sili­consubmount), to "up-reflect" the lightfrom the butt­coupled MAC-II connector. Loss ofoptical power from theMAC-II to the detectorarrayis less than 3 dB.

Figure 7bshowsan eye diagram fora channelusing lD-ODL modules. The receiver function wasimple­mented with discretedevices (Hewlett Packard trans­impedance amplifiers and decision circuits), usinganinput synchronous optical network datarate of622Mbits/s (OC-12). The patterngeneratorproduces apseudo-random patternlengthof215 - 1 ofnon-return tozero (NRZ) data.

ConclusionsAdvanced switching (and computer) system

architecture designsrequirehigher performance andlower-eost packaging technologies to produce newprod­uctsandenhanced services. Electronic interconnectiontechniques - usingtechnologies such as controlled col­lapsechipconnect (C4) and multichip modules (MCMs)- offer performance superiorto other methodsat a costthat is lower for short-length interconnections (e.g.,chip-to-chip packaging). Optical interconnection strate­gies are useful at higher levels ofpackaging (i.e., board­to-board, shelf-to-shelf, andframe-to-frame level ofinter­connections). Furthermore, parallel optical datalinkswith high datarates now offer greater flexibility andlower cost than time-multiplexed serialdatalinks.

Ofthe numerous techniques used to implement

48 AT&TTECHNICALJOURNAL. SEPTEMBER/OCTOBER 1993

a parallel datalink, the lD-ODL may be the first to pro­vide high performance at low cost. Designers agreethat the keyconcepts of lD-ODL technology include anECL-eompatible dc-eoupled system, highoptical outputpower andcontrastthat vastly simplify the receiver cir­cuitdesign, improved skew, lower total power dissipa­tion, small PCB footprint, and low thermal impedance athigh temperatures.

AcknowledgmentsWeespecially thankthese people from AT&T

Bell Laboratories: M.Cappuzzo andJ. Shmulovich(Murray Hill), forpreparing the silicon submounts;J. Zilko (Solid StateTechnology Center), forprovidinglaserarraychips; R. Frahm (Murray Hill), forprovidingdetectorarraychips; D. Buchholz and R. Huisman(Indian Hill), forhybrid design andexperimental mea­surements; N. Basavanhally, M.Brady, H. Nguyen, andR. Roll (Engineering Research Center), fordesigningand performing the silicon submount bonding andassembly operations; andother members ofthe lD-ODLteam, toonumerous to name, whose efforts weregreatly appreciated.

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(Manuscript approved October 1993)

RonaldA. Nordin is a distinguished member of technical staffin the Broadband and Photonic Switching Department at AT&TBell Laboratories in Indian Hill Park (Naperville), Illinois. Hedevelops optical interconnection equipment within the switchand to the subscriber for broadband systems. Mr. Nordinreceived a B.S.E.E. from Purdue University, West Lafayette,Indiana, and an M.S.E.E. and Ph.D. from Northwestern Univer­sity, Evanston, Illinois, all in electrical engineering. He holdsfour U.S. patents, in the areas of optics and high-data-ratepackaging. Mr. Nordin joined AT&T in 1977.AnthonyF. J. Levi was appointed Professor of ElectricalEngineering at the University of Southern California in LosAngeles in 1993. From January 1984 to mid-1993, Mr. Leviworked in the Physics Research Division of AT&T Bell Labora­tories in Murray Hill, New Jersey, first as a member of tech­nical staff, and then, in 1988, as a distinguished member oftechnical staff. Mr. Levi's research interests include non­equilibrium phenomena in semiconductors, the scaling ofelectronic and photonic devices to submicron dimensions,and integration of electronic and photonic devices. He holdsnine U.S. patents in these research areas. Mr. Levi receiveda B.Sc. from the University of Sussex, England, and a Ph.D.from Cambridge University, England, both in physics.

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