Multiscale optical design for global chip-to-chip optical...

548 IEEE JOURNAL ON SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 9, NO. 2, MARCH/APRIL 2003

Multiscale Optical Design for Global Chip-to-ChipOptical Interconnections and Misalignment

Tolerant PackagingMarc P. Christensen, Member, IEEE, Predrag Milojkovic, Associate, IEEE,

Michael J. McFadden, Student Member, IEEE, and Michael W. Haney, Member, IEEE

Abstract—As transistor densities on integrated circuits (ICs)continue to grow, off-chip bandwidth is becoming an ever-in-creasing performance-limiting bottleneck in systems. Electronicmultichip module and printed circuit board packaging technologyhas not kept pace with the growth of interchip interconnectionrequirements. Recent advances in “smart pixel” technology offerthe potential to use optical interconnects to overcome the interchipinput/output bottleneck by linking dense arrays of vertical cavitysurface emitting lasers and photodetectors. For optical intercon-nections to be relevant to real systems they must be able to bemanufactured and packaged inexpensively and robustly. Thispaper introduces an optical design and packaging approach thatutilizes multiple sizes (or scales) of optical elements to simplify thedesign of the optical interconnection and coupling while providingan enhanced degree of insensitivity to misalignments inherent inthe packaging of these systems. The scales of the optical elementsdescribed are: the size of the IC (termed macrooptical); the size ofthe pitch of optical I/O (termed microoptical); and sizes in between(termed minioptical) which are smaller than the size of the ICbut cover several optical I/O. This paper describes the utilityof elements of each of these scales and shows that, through thecombination of them, simple robust systems can be constructed.Two case studies for applying this multiscale optical design areexamined. The first case study is a global chip-to-chip optical in-terconnection module (termed free-space accelerator for switchingterabit networks) that uses a macrolens array and mirror to effectthe all-to-all optical interconnection pattern among an array ofICs on a single board. Micro- and miniscale optical elementssimplify the design of the macro-lens by performing correctionsat scales where they are more effective. In this system, over11 000 optical links are implemented across a five inch multichipmodule with diffraction limited RMS spot sizes and registrationerrors less than 5 m. The second case study analyzes designsfor board-to-board optical interconnections with throw-distancesranging from one millimeter to several centimeters. In this case,micro- and miniscale optical interconnections provide insensitivityto misalignments. The results show the feasibility of an opticalcoupler that can tolerate the typical packaging misalignments of5 to 10 mil without placing rigid constraints on the angular sensi-tivity of the modules. The multiscale optical interconnection andcoupling concept is shown to provide an approach to simplifyingdesign and packaging—and, therefore, the costs—associated withimplementing optical interconnection systems.

Manuscript received January 9, 2003. This work was supported in part by theDefense Advanced Researched Projects Agency through a grant from the AirForce Research Laboratory.

M. P. Christensen is with the Department of Electrical Engineering, SouthernMethodist University, Dallas, TX 75205 USA (e-mail: [email protected]).

P. Milojkovic is with Applied Photonics, Fairfax, VA 22030 USA.M. J. McFadden and M. W. Haney are with the Photonic Architectures Center,

College of Engineering, University of Delaware, Newark, DE 19716 USA.Digital Object Identifier 10.1109/JSTQE.2003.814414

Index Terms—Optical design, optical interconnections, verticalcavity surface emitting lasers (VCSELs).

I. INTRODUCTION/BACKGROUND

T HE combination of free-space optical interconnections(FSOI) with smart pixel technology [based on the inte-

gration of Silicon ICs with arrays of vertical cavity surfaceemitting lasers (VCSELs) and photodetectors] is projectedto enable chip-to-chip interconnection fabrics that achievebandwidth densities on the order of a terabit per second percm [1]. Scalableglobal (i.e., chips-to-chips) interconnectionfabrics that achieveminimum bisection bandwidthsin themultiterabits per second regime may be implemented usingmultiple optoelectronic integrated circuits linked to each otherin the manner depicted in Fig. 1 [2]. This approach is thebasis for a global chips-to-chips interconnection approachtermed free-space accelerator for switching terabit networks(FAST-Net) [3], [4]. In this approach, the optical input/output(I/O) from any single smart pixel array (SPA) chip, located at alens’s focal plane, are linked to portions of the I/O arrays of allchips in the system. To achieve this, clusters of VCSELs andphotodetectors are imaged onto corresponding clusters on otherchips. Multiple point-to-point links are established betweencluster pairs on different SPAs. The clusters are interleaved toachieve a global interconnection pattern across the multichipplane, thus effecting a high-density bidirectional data pathbetweenevery pairof SPA chips on the electronic multichipmodule (MCM). SPA chips with integrated VCSEL/detectorarrays that have emitter and receiver elements sizes of 10 and50 m, respectively, and with element-to-element spacing assmall as 125 m, have been evaluated in a prototype inter-connection fabric [3], [4]. To fully exploit the smart pixel I/Odensity, the global optical interconnection module must provideflat, high resolution, near distortion-free image fields, across awide range of ray angles, with low optical loss.

Modern optical design and manufacture techniques achievewide-field imaging systems with high resolution. Low loss isachievable by optimizing lens designs that minimize the numberof elements and employ antireflection coatings. Simultaneouslyachieving high registration accuracy across the entire field, how-ever, is more challenging and can lead to complex multielementsolutions for the lens design. The initialFAST-Netprototype em-ployed a set of matched 7-element off-the-shelf lenses. The pro-totype performed well in terms of registration and resolution [3],

1077-260X/03$17.00 © 2003 IEEE

CHRISTENSENet al.: MULTISCALE OPTICAL DESIGN FOR GLOBAL CHIP-TO-CHIP OPTICAL INTERCONNECTIONS 549

Fig. 1. Multichip interconnection fabric achieves a high-density globalmultichip interconnection across an array of chips, thereby leveraging both thehigh bandwidth and high minimum bisection bandwidth ability of smart pixeltechnology and FSOI.

[4] with SPAs that had small (1 mm in diameter) VCSEL/pho-todetector clusters separated by several millimeters. The firstgeneration prototype system, however, was not suitable for theeventual large scale systems, which will require larger clustersof optical input/output that are more closely spaced. We defineregistration accuracy here as the difference between the locationof the image of a VCSEL and the location of its correspondingdetector. In the system, registration must be maintained at a levelmuch less than the size of the detector m across theentire multichip plane ( 10 cm). A comprehensive approach todesigning the linking lens array, which maximally exploits theunique features of theglobal multichipVCSEL-based architec-ture, was required.

In this paper, key elements of a new design approach for theoptical interconnection modules are reviewed. The approach iscentered on the optimization of a new hybrid lens concept thatemploys elements at three scales: micro-, mini-, and macroop-tical. These three scales are matched to the three physical scalesof the VCSEL/photodetector arrays used in theglobal multi-chip interconnectionconcept: individual VCSEL/photodetectorelement, cluster, and chip, respectively. This paper describesthe utility of elements of each of these scales and shows that,through the combination of them, simple robust systems can beconstructed. This paper will examine two case studies applyingthis multiscale optical design. The first case study is a globalchip-to-chip optical interconnection module (FAST-Net) thatuses a macrolens array and mirror to effect the all-to-all opticalinterconnection pattern among an array of ICs on a singleboard. Micro- and miniscale optical elements simplify thedesign of the macro-lens by performing corrections at scaleswhere they are more effective. In this system, over 11 000 op-tical links are implemented across a five inch multichip modulewith diffraction-limited RMS spot sizes and registration errorsless than 5 m. The second case study analyzes designs forboard-to-board optical interconnections with throw-distancesranging from one millimeter to several centimeters. In thiscase, micro- and miniscale optical interconnections provideinsensitivity to misalignments. The results show the feasibilityof an optical coupler, which can tolerate the typical packaging

Fig. 2. FAST-Netcluster layout consists of five rows of VCSELs (top half) andfive rows of photodetectors (bottom half).

misalignments of 5–10 mil without placing rigid constraintson the angular sensitivity of the modules. Multiscale opticalinterconnection and coupling design is shown to provide anapproach to simplifying design and packaging and, therefore,the costs associated with implementing optical interconnectionsystems.

II. CASE 1: FAST-Net GLOBAL MULTICHIP

INTERCONNECTIONMODULE

In the design for the second generationFAST-Netprototype,there are 704 bidirectional channels on each of the 16 SPAs, for atotal of 11 264 FSOI links across the MCM. There are, therefore,16 spatially separated clusters of 44 VCSELs and 44 photode-tectors on each SPA. Fig. 2 depicts the layout for an individualcluster. The cluster is divided into spatially separate arrays ofVCSELs (depicted as small dots) and photodetectors (depictedas squares). As can be seen in the figure, the shape of the clusteris octagonal, which results from the optimum circular shape assampled by a regular square grid with a pitch of 175m. Thecircular apertures of the VCSELs used in the prototype are ap-proximately 5 m in diameter. The photodetectors (squares inthe figure) have a dimension of 60m on a side. The maximumvertical/horizontal dimensions of the cluster are 1.575 mm.

Fig. 3 shows the full array of clusters on one of the SPAs intheFAST-Netprototype. As can be seen, the 16 clusters are ac-tually achieved by selectively utilizing VCSEL/photodetectorsfrom a regular grid of VCSELs and detectors that are arrayed ina repeating pattern of six rows of VCSELs, six rows of photode-tectors, and one row of unused elements, all on a 175-m grid.The clusters are formed by sampling adjacent sets of five rows ofVCSELs and photodetectors to achieve the desired cluster con-figuration depicted in Fig. 2. The optical arrays are area bumpbonded to a matching array of driver and receiver circuits onthe underlying Silicon SPA IC. The distance between the cen-ters of adjacent clusters on the SPA is 2.275 mm and thereforethe overall SPA chip size is 10 mm.

The registration and resolution design goals for the secondgenerationFAST-Netoptical system prototype derive from anoverall goal of 90% light-capture efficiency at the detector,meaning that the blur spot of any VCSEL image should be


Fig. 3. I/O layout of theFAST-Netchip. There are 16 clusters, each with 88 OE elements on a 175�m pitch. The overall I/O real estate utilization in thisconfiguration is greater than 50%.

confined and registered so that its corresponding 60-m-widesquare detector captures 90% of the light energy. This levelof performance will ensure that the receiver detects sufficientlight from the VCSEL and optical crosstalk between adjacentchannels will be negligible. The combined levels of distortionerror and blur size should be small enough to ensure this levelof performance. The overall optical transmission efficiency forthe optical system should be maximized by minimizing thenumber of elements in the overall lens and employing antire-flection coatings to minimize reflection losses. To minimize theoverall size of the MCM and achieve good SPA chip real estateutilization, a maximum lens diameter of 3 cm and an f-numberof less than 1.25 were desired.

The overall goal of the design is to effect the requiredglobal interconnection pattern across a 44 chip array whileminimizing the complexity (i.e., number of elements, cost,etc.) of the optics. Fig. 4 is a schematic depiction of one of the16 custom-designed lenses in the system. It consists of threedistinct types of optical elements referred to as “micro” (oneper VCSEL or detector), “mini” (one per cluster of parallelVCSEL links), and “macro” (one per SPA). In this design,the microoptical elements are solely responsible for reducingthe numerical aperture (divergence angle) of the VCSEL

Fig. 4. Multiscale optical interconnection design for the global multichipsystem.

beam, thereby reducing the overall required lens complexity asdiscussed below. The mini-optical elements effect a distortioneliminating beam-steering function that was recently proposed[5] and evaluated [6]–[8]. The concept uses fixed microoptical


Fig. 5. Lens barrel containing multiscale optical elements for global multichipinterconnections. Macrooptical elements are the size of the barrel, whereas anarray of minioptical elements is mounted to an optical flat in the barrel.

Fig. 6. Nominal spot size for multiscale global multichip lens design.

beam-steering elements to achieve symmetrical, and hencedistortion-free, ray paths through the global optical intercon-nection system. This approach exploits the inherent small NAof VCSELs to eliminate distortion by achieving holosymmetryfor each pair of lenses. The macrooptical elements (four in eachlens) implement the global optical interconnection pattern.The complexity of these macroelements is greatly reduced bythe presence of the microoptical and minioptical elements.The macro- and minielements are contained in a single barrelas shown in Fig. 5. The microelements, which reduce thenumerical aperture of the VCSEL beam and therefore simplifythe remainder of the lens design [9], [10], are integrated directlyon the VCSEL/detector array, via mounting to a transparentsuperstrate, as depicted in the blow-up in Fig. 4.

A. Lens Performance

The multiscale lens-design effectively partitions the criticalVCSEL cluster imaging requirements into: numerical aperturecontrol, beam steering, and off-axis imaging. The combina-tion of the micro-, mini-, and macroscale optical elementsprovides an effective solution for the stringent optical systemrequirements. Fig. 6 shows a histogram of the root mean square(RMS) spot sizes for each VCSEL/detector link in the system

Fig. 7. Nominal misregistration (distortion) for multiscale global multichiplens design.

as determined by ray tracing. The spot sizes for all of thelinks are 3–4 m. Fig. 7 shows a histogram of distortion foreach VCSEL/detector link. The multiscale lens design correctsdistortion from 8% (optimized without minilens elements)to 0.08%. This greater than 100reduction in distortionreduces misalignments from 580m to 4 m. This lack ofdistortion is highly unusual for off-axis imaging systems andit is achieved through the beam steering (miniscale opticalelements) of the low-resultant NA VCSELs (created by themicroscale elements). Without the combination of these threescales of optical components, a lens would be unreasonablycomplex in order to meet the wide field global off-axis imagingrequirements dictated by the system. The design presented inthis paper enables global optical interconnection modules, suchas the one depicted in Fig. 1, to fully exploit the anticipated ter-abit per second per cmcapabilities of smart pixel technology.

B. Misalignment Tolerance

Since the multiscale optical interconnection system (halfof which—for any pair of chips—is depicted in Fig. 4) isimplemented as two infinite-conjugate-ratio systems in animaging configuration, one would expect misalignments of thelens barrels to directly translate into misalignments of imagespots. However, this is not the case as the multiscale designprovides a measure of immunity to lens misalignments. Recallthat in the design described above, the mini- and macroopticalelements are mounted in the same barrel, whereas the microop-tical elements are directly integrated onto the superstrate of theoptoelectronic devices. As the lens barrel is translated, due tosome source of alignment or operational environment error,only the mini- and macrooptical elements are displaced. Figs. 8and 9 compare the performance of the nominal system (nomisalignments) to that with a 10-m displacement of the lensbarrel. Note that the image location (measured by distortion)remains relatively unchanged, where it would have been ex-pected to translate a corresponding 10m. Also, note that thespot sizes have only increased slightly—up to a 9-m radius.Fig. 10 depicts the worst case encircled energy plot for one ofthe VCSEL detector links. Since the image is still well centeredon the detector, the larger detector area will readily capture theenergy. Figs. 11, 12, and 13 depict the equivalent data for a20- m displacement of the lens barrel. In this configuration,


Fig. 8. Spot size for multiscale global multichip lens design under a 10-�mdisplacement as compared to the nominal design.

Fig. 9. Misregistration (distortion) for multiscale global multichip lens designfor a 10-�m displacement as compared to the nominal design.

Fig. 10. Encircled energy plot for the worst case link at a 10-�m displacement.

the distortion is still negligible, but the spot size for some ofthe links is beginning to be problematic. These links are atthe edge of the cluster and under this displacement, are hittingnear the edge of optical elements. Systems requiring additionaldisplacement accuracy could be designed with smaller clustersallowing more margins at the lens edges. Placing the miniop-tical elements in the lens barrel breaks the rotational symmetry

Fig. 11. Spot size for multiscale global multichip lens design under a 20-�mdisplacement as compared to the nominal design.

Fig. 12. Misregistration (distortion) for multiscale global multichip lensdesign for a 20-�m displacement as compared to the nominal design.

Fig. 13. Encircled energy plot for the worst case link at a 20-�m displacement.

of the macrooptical elements so rotational misalignments ofthe barrels must be considered as well. Figs. 14, 15 a¸ nd 16depict similar results for a rotational misalignment of the lensbarrel. The symmetry provided by the beam steering of theminioptical elements provides a well-balanced point aboutwhich the effects of misalignments are mitigated by the use ofmicrooptical scale elements.


Fig. 14. Spot size for multiscale global multichip lens design under a 1rotational misalignment as compared to the nominal design.

Fig. 15. Misregistration (distortion) for multiscale global multichip lensdesign for a 1 rotational misalignment as compared to the nominal design.

Fig. 16. Encircled energy plot for the worst case link at a 1rotationalmisalignment.

III. CASE 2: BOARD-TO-BOARD OPTICAL INTERCONNECTION

Although the multiscale optical approach was originally de-veloped for global chips-to-chips optical interconnection mod-ules, its misalignment insensitivity in that domain makes it an

Fig. 17. Macrooptical interconnection perfectly aligned (top) and with250-�m displacement in the plane, 250-�m displacement along the optical axisand 1 rotational (out of plane) displacement (bottom).

interesting candidate for other interconnection problems. Aninterconnection application in which an array of emitters islinked to an array of detectors or guided-wave channels overa short (1 mm–2 cm) throw distance is both of great interestand is plagued by the effects of misalignments which results inan increased packaging cost. In such a configuration, the lackof “global” interconnections causes degeneracy between thescales of minioptical and macrooptical elements, i.e., often thecluster size is the same as the array size. When this is the case,we use the more commonly used macrooptical designation todescribe the scale of the element. In order to quantify thebenefits of the multiscale design approach, we compare it tothe macrooptical-only approach. Microoptic approaches havebeen studied in detail but are limited in throw distance anddo not provide the tradeoff between angular and translationalmisalignments which we will show in the multiscale approach.In this analysis, both approaches image an array of VCSELs(with a 3-mm field) onto an associated array of detectors. Theanalysis assumes that the link is broken into two halves: trans-mitting plane with its associated optics and receiving planewith its optics. All misalignments happen between these twohalves.

A. Macrooptical Interconnection Approach

The first optical interconnection approach evaluated was amacrooptical one. In this case, the size of the optical ele-


Fig. 18. Spot diagram of on-axis point and off-axis point, aligned and misaligned for macrooptical interconnection. Square depicts boundary of the 75-�m sidephotodetector.

Fig. 19. Schematic diagram depicting misregistrations due to misalignments in macrooptical only (top) and multiscale (bottom) approaches.

ments will be on the order of the optical array size (e.g.,several millimeters). An expanded beam (infinite conjugateratio) interconnection between planes will be utilized. Thisoptical interconnection approach provides maximum toleranceto misalignments in the x and y directions [i.e., perpendic-ular to the optical axis (z)], as shifts between the two systemhalves do not affect the angle of the beam between them.The increased tolerance to x and y misalignments comes at aprice of increased sensitivity to angular misalignments. We canbound the best performance of such a system by consideringthe lenses to be perfect elements (i.e, there is a tangentialrelationship between angle and position). While the x and ytolerances are on the order of the lens radius, the tolerancesto angular rotations out of the plane and within the plane

are extremely tight

(1)

and

(2)

where is the displacement of the image due to the misalign-ment (in either or ), f is the focal length of the macrolens,x is the radial off-axis distance of the VCSEL, is theangular rotation out of the plane in the direction of the xaxis, and is the rotation within the plane (about the optical

axis). Fig. 17 depicts a typical macrooptical interconnectionapproach when perfectly aligned (top) and under a 250-my-displacement, 250-m z-displacement and 1angular mis-alignment. Fig. 18 (bottom) shows associated predicted spotdiagram for on-axis and full field object points in the alignedand misaligned systems in relation to a 75-m detector width.As the figure shows, slight rotations between the planes wouldcause link failure in a macrooptically interconnected system.

B. Multiscale Micromacrooptical Interconnection Approach

In the multiscale approach, macrooptical interconnectionlenses and microoptical elements are combined with the hopeof achieving the similarly increased system performance to theglobal multichip system. By including microoptical elementsin a macrooptical expanded beam-interconnection approach theoverall sensitivity to angular misalignments can be reduced.This comes at the expense of increasing the positional (x, y)insensitivity inherent in the macrooptical approach. The goal,therefore, is to design the interconnections optics which pro-vide the best tradeoff in angular and positional tolerances, asdetermined by the application packaging requirements and con-straints. As before, in assuming a perfect lens element, a boundon the sensitivities to positional and angular misalignments ofthe multiscale approach yields

(3)


Fig. 20. Multiscale optical interconnection perfectly aligned (top) and with250-�m displacement in the plane, 250-�m displacement along the optical axisand 1 rotational (out of plane) displacement (bottom).

(4)

(5)

where is the distance from the microlens to the old imageplane, is the distance from the microlens to the new imageplane, is the lateral shift of the lens system (misalignment),and is the shift in image position due to. Fig. 19 depictsthe resulting misregistrations due to the various misalignments.The top of the figure represents those of the macrooptical-onlysystem, whereas the bottom of the figure represents those ofthe multiscale micro- and macrooptical approach. Note thatthe macrooptical approach does not suffer under small trans-lational misalignments (upper left) but is sensitive to angularmisalignments. Some of this translational insensitivity is tradedoff for angular sensitivity in the multiscale approach. The termsin the brackets of (4) and (5) are the previous results for themacrooptical interconnection approach. Notice that the shiftof the image plane due to the presence microoptical elementsyields a direct (and inverse) tradeoff between sensitivities tomisalignments due to angles within and out of the image plane.Fig. 20 depicts a typical multiscale macrooptical interconnec-tion approach when perfectly aligned (top) and under a 250-my-displacement, 250-m z-displacement and 1angular mis-alignment (bottom). Fig. 21 shows the associated spot diagram

Fig. 21. Spot diagram of on-axis point and off-axis point, aligned andmisaligned for multiscale optical interconnection. A comparison with Fig. 18shows the benefits of the hybrid approach in reducing misregistrations.

for on-axis and full field spots in the aligned and misalignedsystems in relation to a 75-m detector. As the figure shows,the slight rotations which plagued the macrooptical approachare readily handled by this system.

The multiscale micromacrooptical interconnection approachallows for a fluid trade space between sensitivities in positionsand rotations between planes. Its main limitation is in itsmacroscale: if the throw distance is reduced to an extremelysmall distance, then the focal lengths of the macroopticalelements will necessarily become very short. Combining thiswith a manufacturing constraint of avoiding lenses with anexcessively low f# would limit the field size of the interconnec-tion (and, therefore, the number of links behind the macrolens).For throw distances of 1 cm or more, the multiscale approachwill work well. For smaller center-to-center spacings, themicrooptical interconnection approach may be more practicaland would be allowable as the diffraction limits would nothinder them in this domain.

IV. CONCLUSION

This paper introduced a hybrid optical design and packagingapproach that utilizes multiple sizes (or scales) of optical ele-ments to simplify the design of the optical interconnection andcoupling while providing an enhanced degree of insensitivity tomisalignments inherent in the packaging of these systems. Theutility of elements of each of these scales was described and itwas shown that through the combination of them simple robustsystems can be constructed. This paper examined two casestudies in which the this multiscale optical design approach wasapplied. The first case study involved a global chips-to-chipsoptical interconnection module which uses a macrolens arrayand mirror to effect the all-to-all optical interconnection patternamong an array of ICs on a single board. Micro- and miniscaleoptical elements were shown to simplify the design of themacrolens by performing corrections at scales where they aremore effective. In this system, over 11 000 optical links are im-plemented across a five inch multichip module with diffractionlimited RMS spot sizes and registration errors less than 5m.The second case study analyzed designs for board-to-boardoptical interconnections with throw-distances ranging fromone millimeter to several centimeters. In this case, micro- andmacroscale optical interconnections provide insensitivity tomisalignments. The results show the feasibility of an opticalcoupler that can tolerate the typical packaging misalignmentsof up to 250 m without placing rigid constraints on the angularsensitivity of the modules. Multiscale optical interconnectionand coupling design were shown to provide an approach to


simplifying design and packaging, and therefore the costs,associated with implementing optical interconnection systems.

ACKNOWLEDGMENT

The authors gratefully acknowledge the contributions ofOptical Research Associates in optimizing and tolerancing theglobal multichip design for manufacturing and of CommOpticsfor the fabrication of the lens assemblies.

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[10] P. Milojkovic, Ph.D. dissertation, George Mason Univ., Fairfax, VA,2001.

Marc P. Christensen (M’95) received the B.S. de-gree in engineering physics from Cornell University,Ithaca, NY, in 1993 and the M.S. and Ph.D. degreesin electrical engineering from George Mason Univer-sity, Fairfax, VA, in 1998 and 2001 respectively.

From 1991–1998, he was a Staff Member andTechnical Leader in the Sensors and PhotonicsGroup, BDM International, Inc., where his workranged from developing optical processing andinterconnection architectures, to infrared sensormodeling and analysis. In 1997, he cofounded Ap-

plied Photonics, where he was responsible for several prototype developmentsthat incorporated precision optics and microoptoelectronic arrays into systemlevel demonstrations. In 2002 he joined Southern Methodist University, Dallas,TX, where he is currently an Assistant Professor of electrical engineering.He has coauthored 12 journal papers and holds two patents in the field offree-space optical interconnections.

Prof. Christensen was the Workshop Chair for the IEEE CommunicationsSociety Workshop on High-Speed Interconnections within Digital Systems in2002. He is a member of the Optical Society of America and the IEEE Commu-nications and Lasers and Electrooptics Societies.

Predrag Milojkovic (S96–A’01) received theDiploma degree in electrical engineering from theUniversity of Belgrade, Belgrade, Yugoslavia, in1972 and the Ph.D. degree in electrical engineeringfrom George Mason University, Fairfax, Virginia in2001.

From 1978 to 1988, he was with the ElectronicsIndustry Corporation, Belgrade, Yugoslavia, wherehe worked on microwave radio system development.From 1988 to 1993, he was with the Institute forMicrowave Techniques and Electronics (IMTEL)

in Belgrade, Yugoslavia, where he was in charge of the development of theinstantaneous frequency measurement receiver and associated microwavecomponents. In 1996, he joined the Photonics Group, George Mason Uni-versity. From 1997 to 1998, he was with the Sensors and Photonics Group,BDM International, Inc., working on the design of free-space interconnectsystems. In 1998, he joined Applied Photonics Fairfax, VA, where he iscurrently a Principal Engineer. He has coauthored several journal papers andhas one patent. His primary research interests center on the tradeoffs betweenelectronics, optics, and mechanics that evolve in smart pixel-based opticalinterconnection concepts.

Dr. Milojkovic was Tutorial Chair for the IEEE Communications Society’sWorkshop on Interconnections within High-Speed Digital Systems, and is cur-rently its Program Co-chair. He is a member of the Optical Society of America.

Michael J. McFadden (S’02) received the B.S.degree in electrical engineering from George MasonUniversity, Fairfax, VA, in 2000 and is currentlypursuing the Ph.D. degree in electrical engineeringat the University of Delaware, Newark.

He is currently a Research Assistant in the Pho-tonic Architectures Center, University of Delaware.His research is currently focused on multiscale op-tical architectures and intrachip optical interconnects.

Mr. McFadden received the Defense AdvancedResearched Projects Agency MTO PWASSP Out-

standing Achiever Award in 2001, for which he was recognized for bench-levelwork done on the ACTIVE-EYES project. He is a member of the HKNEngineering National Honor Society.

Michael W. Haney (M’80) received the B.S. degreein physics from the University of Massachusetts,Amherst, in 1976, the M.S. degree in electricalengineering from the University of Illinois, Ur-bana–Champaign, in 1978, and the Ph.D. degree inelectrical engineering from the California Instituteof Technology, Pasadena, in 1986.

From 1978 to 1986, he was with GeneralDynamics, where his work ranged from the de-velopment of electrooptic sensors to research inphotonic signal processing. In 1986, he joined

BDM International, Inc., where he became a Senior Principal Staff Memberand the Director of photonics programs. In 1994, he joined George MasonUniversity, Fairfax, VA, as an Associate Professor of electrical and computerengineering. In 2001, he joined the University of Delaware, Newark, as aProfessor of electrical and computer engineering, where he is currently theDirector of the Photonics Center, College of Engineering. He has contributedto approximately 100 journal and conference papers in optical informationprocessing. His research activities are focused on the application of photonicsto new computing, switching, and signal-processing architectures.

Prof. Haney has chaired and co-chaired several technical conferences and isa previous chairman of the IEEE Communications Society’s Technical Com-mittee on Interconnections within High-Speed Digital Systems. He is a fellowof the Optical Society of America.

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