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Low-order adaptive optics forfree-space optoelectronic interconnects

James Gourlay, Tsung-Yi Yang, Masatoshi Ishikawa, and Andrew C. Walker

The concept of adaptive optics for improving the cost–performance of free-space optoelectronic intercon-nects is discussed. Adaptive optics as a design option for optical interconnect systems is presented anddiscussed. A practical demonstrator that performs low-order correction was built and tested. Slowlyvarying misalignments, including thermal effects, were compensated for in a 622-Mbitys free-spaceoptical data link. © 2000 Optical Society of America

OCIS codes: 200.0200, 010.1080.

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1. Introduction

Free-space optoelectronic interconnects have beenidentified as a solution to the future communicationneeds of electronic systems.1 It has been estab-lished that, over the next decade, one of the mainlimitations for computing systems will be intercon-nection. Standard electrical wires have a band-width penalty over distance that relates to the skineffect at high data rates. Optical fibers have alreadyreplaced copper wire for long-haul and short-haultelecommunications. As silicon-chip densities, datarates, and electrical pin-out requirements increase,the distances over which optics becomes an attractiveinterconnect technology reduce. Within the next de-cade optical connections directly to and between sil-icon chips will become commonplace. The questionnow is how practical optical interconnections will beimplemented.

There are a number of optical technologies that canbe applied to the problem of interconnection over dis-tances of 1 cm to 1 m. The three main technologiesare optical fibers, planar waveguides, and free space.We investigate free-space optics because we believe ithas a number of additional advantages over fiber andwaveguide solutions. First, whereas fibers and

J. Gourlay ~j.gourlay@hw.ac.uk!, T.-Y. Yang, and A. C. Walkerre with the Department of Physics, Heriot-Watt University, Ed-nburgh EH141 4AS, UK. M. Ishikawa is with the Department of

athematical Engineering and Information Physics, University ofokyo, Tokyo 113, Japan.Received 24 May 1999; revised manuscript received 30 August

999.0003-6935y00y050714-07$15.00y0© 2000 Optical Society of America

714 APPLIED OPTICS y Vol. 39, No. 5 y 10 February 2000

waveguides can be viewed as wirelike, the greaterparallelism of optics is not limited by the number oflight guides. Second, the connection to the chip it-self is nonmechanical in nature, allowing easiercircuit-board reinsertions without detrimental wearand tear. Third, global reconfiguration is possiblethrough the use of beam-steering elements such asdiffractive gratings. Finally, because componentsare separated by free space, there are possibilities ofeasier thermal management of systems, and thechips are optically isolated, hence less susceptible toelectromagnetic interference. These features com-bined raise the prospect of a range of novel computingarchitectures, for example, flat-memory models.

However, free-space optics has a major drawback:alignment. As the optical connections themselvesare nonmechanical, a peripheral optomechanical sys-tem must be used to mount the various components~e.g., lenses! and ensure that the light beams aremaged accurately from the transceivers to the re-eivers. There has been much effort recently di-ected at solving this problem.2–4 These studies

mostly used passive- or active-alignment techniquesto align arrays of transceivers with receivers in thedevice planes. In this paper, we investigate adap-tive alignment, or, more accurately, adaptive optics.

2. Adaptive Optics

Adaptive optics is not a new solution to achievingcommercial, feasible, free-space optical systems.CD players use adaptive optics to maintain the focus-ing and the detection of a light spot on a rapidlyrotating optical disk.5 The adaptive-optics systemin a CD player performs the measurement and thecorrection of focusing and positional errors in realtime. If adaptive optics were not used, the mechan-

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ical precision of the optical system and of the diskitself would have to be much higher, and it would beunlikely that the players and the CD’s would be asaffordable as they are. We investigate this approachin the context of optoelectronic interconnects: Adap-tive optics could dramatically reduce the specifica-tions and the tolerances of the optical componentsand the optomechanics; hence it could improve thecost–performance trade-off for free-space optoelec-tronic interconnects.

Adaptive optics should be viewed as only one de-sign option in our goal of developing feasible optoelec-tronic systems. The approach has been studiedwidely for military laser systems and astronomicalinstrumentation for which adaptive-optics systemsare used to improve the imaging performance of largeground-based telescopes in which optical wave frontsfrom distant astronomical objects are aberrated asthey pass through the upper atmosphere.6 Figure 1shows an example of a typical astronomical adaptive-optics scheme.

3. Limitations of Free-Space Optical Interconnects

For the purposes of optical interconnects adaptiveoptics comprises the measurement and the manipu-lation of the optical wave front~s! in real time toimprove system cost–performance. If one takes afree-space optoelectronic interconnect system out ofthe laboratory and into the real world a number ofpotential problems and limitations can be foreseen.

Typically, the free-space optoelectronic intercon-nect system attempts to image an array of light spotsfrom a transmitter array in one plane to a receiverarray in the other. Figure 2 shows two cases: theideal case on the right-hand side and the more likelyreal-world case on the left-hand side. Adaptive op-

Fig. 1. Astronomical adaptive-optics system for a large ground-based telescope. Optical aberrations are introduced into the wavefront as it passes through the upper atmosphere. The aberrationsare measured with a wave-front measurement system and usuallycorrected by a deformable mirror by means of a closed-loop controlsystem.

tics can be used to approach the ideal case. Imagingcan be performed by microlenses, minilenses, bulklenses, diffractive lenses, or any combination of these.Performance of the system is optimized if ~i! as muchight as possible is imaged onto the correct receiverreducing overall power requirements! and ~ii! as lit-le light as possible is imaged onto the neighboringeceiver ~minimizing cross talk!. Performance is en-

hanced by the minimization of the detector area toincrease the data rate ~relating to detector capaci-tance!; hence small light spots are preferred. Thisrelation is consistent with system requirements thatwill lead to large arrays of dense channels for increas-ing overall communication bandwidth.

Therefore, for practical systems, one should be con-cerned with

• Components: All the components in the sys-tem will have a particular tolerance relating to thecost. The higher the tolerances in the system, thehigher the costs will be.

• Manufacture: Components have to be alignedinitially, possibly through passive or active tech-niques, and it is likely that they will have to be re-placed from time to time.

• Mechanics: Components will be mounted in amechanical system that may suffer from long-termcreep, may experience thermal effects ~computing-system heat up and cool down!, and may be influ-nced by mechanical vibrations, possibly from otheromponents, e.g., disk drives or cooling fans.

• Environment: The system itself may be usedin harsh environments such as in avionics or auto-motives, should be portable, and, in some extremecases such as critical medical systems, should be in-susceptible to natural effects, for example, earth-quakes.

Adaptive optics can help to achieve these perfor-mance requirements because it permits 6 degrees offreedom ~x, y, focus, rotation, tip, and tilt! and somelow-order-aberration ~astigmatism, coma! measure-

ent and correction at the transceiver and the re-eiver arrays. We do not envision a requirement toorrect higher-order aberrations, as is commonly

Fig. 2. Simple definition of the problem: light spots imaged ontodetectors. Realistically, our system is likely to produce spots suchas those shown on the left-hand side that change in real time.Ideally, we would wish for the situation shown on the right-handside.

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done, for example, in astronomy. The time scales ofthe variations vary from static effects ~for example, ininitial alignment! to seconds ~for example, for ther-mal effects! to milliseconds @for example, mechanicalvibrations ~the military gun-shot test has a band-width of ;10 kHz!#.

4. Adaptive Alignment in Demonstrator Systems

Some studies have already been carried out by otherresearchers in the field of active alignment in opto-electronic interconnects. NTT has investigated theuse of a variable-angle prism ~VAP!—described inSection 5—and liquid-crystal prisms to correct grossx–y misalignment on low-density free-space links inlarge rack-mounted ATM systems.7 Researchers atMcGill University ~Montreal, Quebec, Canada! haveinvestigated both the use of a mechanically rotatedpair of Risley prisms for correcting x–y misalign-ment8 and configurations for the in situ measure-ment of misalignment.9 In the field of adaptiveoptics itself a number of attractive low-cost solutionsare being researched, particularly by use of liquid-crystal devices10 and micromachined-membrane mir-ors.11 These methods may become important in

future optoelectronic interconnect applications.At Heriot-Watt University, Edinburgh, Scotland,

we developed a number of free-space optoelectronicinterconnect demonstrators.12 Our systems arebased on the so-called slotted-base-plate optome-chanics initiated by AT&T13 in 1991 and developedfurther at Heriot-Watt University14 and elsewhere.15

With care, we typically can achieve a 10-mm lateralanual alignment in these systems. Through our

esearch on these systems, we came to realize that,lthough our systems are excellent proof-of-principleemonstrators, they fall short of what would be re-uired for a commercial prototype. However, thisptomechanical packaging technique allowed usreat insight into the problems and the difficulties ofree-space systems. Therefore we decided to base

Fig. 3. Schematic diagram showing the experimental setup includposition of a reference beam to control the VAP correction devieedback controller.

16 APPLIED OPTICS y Vol. 39, No. 5 y 10 February 2000

our initial adaptive-optics demonstration on this typeof system.

The research presented here differs from studieselsewhere7,8 in that we ultimately want to demon-strate high-bandwidth adaptive optics on high-density channels to improve our standardoptomechanics ~which is also used by other groups!.

his is the important difference between our researchnd that performed at NTT.7 At present, we are

concentrating on the optomechanics ~not on the de-vices! and are investigating adaptive alignment in avery different optomechanical environment and on adifferent scale. Our requirements are for the align-ment of many dense channels ~;1000! within an ac-tive device area of approximately 1 cm 3 1 cm that isenabled by use of our slotted base plate and customcompound-lens–based optomechanical hardware.We used only discrete optoelectronic devices at thisstage, as the same principles hold when we introducelarge arrays of optical channels. Alignment-stabilization measurements on a single channel canbe extrapolated to give an indication as to the futurebehavior of dense multichannel systems.

Research by Naruse and co-workers16 suggeststhat certain of the degrees of freedom are more sig-nificant than others. Their theoretical study sug-gests that lateral misalignment should be tackledfirst, then focus, next rotation, and last tip–tilt.Therefore our initial demonstrator is based onlateral-misalignment correction only.

5. Demonstrator-System Description

We constructed a simple free-space optoelectronic in-terconnect: a point-to-point link between two smart-pixel arrays that uses a 4f imaging system ~Fig. 3!.Off-the-shelf optoelectronic components mounted oncustom, high-speed printed circuit boards were used tomimic smart-pixel devices: vertical-cavity surface-emitting lasers ~VCSEL’s! ~Honeywell, ModelHFE4080-321, with l 5 850 nm and a 10-mm facet!

n external influence. A quad-cell detector is used to monitor therough a proportion-integration–differentiation ~PID! closed-loop

ing ace th

and Vitesse metal–semiconductor–metal ~MSM! detec-tors @an integrated GaAs MSM–transimpedance am-plifier ~Vitesse, Model VSC7810, with a diameter of theactive area of 100 mm and a low cutoff of 1.8 MHz!#. A30-mm pinhole was glued above the 100-mm-diameterMSM detector to mimic a more realistically sized op-tical receiver. At this stage only a single VSCEL anda single MSM were used to provide a high-speed opti-cal data channel.

Because only lateral misalignment was studied,one optical channel was sufficient for a misalignment

Fig. 4. Eye diagram showing the 622-Mbitys free-space opticaldata transfer between the transceiver and the receiver planes.

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Fig. 5. ~a! Schematic that shows the operation of the VAP. ~b!Photograph of the VAP.

reference. This reference consisted of a singleVCSEL and a quadrant ~or quad-cell! detector. Theposition of the light spot from this channel was mea-sured in the receiver plane with a quadrant detector~Hamamatsu, Model S1557-03, with a diameter of theactive area of 1 mm and a gap between the elementsof 20 mm!. The separations between the twoVCSEL’s and the MSM and the VCSEL’s and thequadrant detector on both boards was 6 mm, corre-sponding to a typical field of view on our demonstra-tor systems.

A VAP, supplied by the University of Tokyo andCanon, was used as a correction device. An analogelectronic circuit was used as the feedback-controllersystem ~instrumentation amplifier, integrator, poweramplifier–biasing, and variable gain! to supply thedrive signals for the VAP by use of the positionalerror signals from the quadrant detector. The sig-nal on a 622-Mbitys channel between a VCSEL and aMSM detector was used as a figure of merit for sys-tem performance. The eye diagram for the 622-Mbitys data channel is shown in Fig. 4.

The VAP, shown in Fig. 5, consists of two glassplates between which is a layer of liquid. Two elec-tromagnetic actuators permit the control of the tilt ofone of the glass plates with respect to the other inboth the x and the y directions. Therefore an elec-trically controllable VAP is created in two directions.This prism can be used to introduce a variable devi-

Fig. 6. Graph of the frequency response of the VAP. The band-width is 30 Hz.

Fig. 7. Computer-aided-design layout of the optomechanical sys-tem used in the experiments. The VAP is located in the middle,and the two printed circuit boards are mounted at each end.

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ation in the direction of a light beam ~beam steering!in both the x and the y directions. The clear aper-ure has a diameter of 28 mm, the optical throughouts 92% at 850 nm ~the device is optimized for visibleavelengths!, the beam deviation is 62° for 61 V,nd the bandwidth is 30 Hz ~Fig. 6 shows the mea-ured frequency response!.Figure 7 shows a schematic of the optical system

nd the closed-loop control system used in the exper-ments. Figure 8 shows the computer-aided-designayout of the optomechanical system, which was ma-hined in aluminium. The printed circuit boards forhe transceiver and the receiver planes were attachedo mounts with 5 degrees of freedom ~focusing waserformed by the movement of the lens along thelot!. The mounts were separated by 30 cm,hereby a slot allowed the positioning of two triplet-

ompound custom bulk lenses, forming the 4f imag-ng relay. The lenses were designed in house with a

Fig. 8. Photograph of the experimental system with all the com-ponents and the peripheral kit in place.

Fig. 9. Voltage applied to the VAP compared with the channel simovement is shown above.

18 APPLIED OPTICS y Vol. 39, No. 5 y 10 February 2000

ocal length of 42 mm, three lens elements, an-number of 5.25, a barrel diameter of 25 mm, aength of 70 mm, and a diagonal-field size of 7.4 mm.he lenses were optimized for an 850-nm wavelengthperation. The VAP is located between the twoenses at the Fourier plane of the 4f system. Twoeam splitters permitted the illumination of the re-eiver plane with a white-light source and the obser-ation of this plane with a CCD camera for grosslignment purposes. The gross alignment of the ref-rence spot onto the quadrant detector is performedanually at this stage—accurate alignment is

chieved with the closed-loop control system. A pho-ograph of the complete system is shown in Fig. 8.

6. Experimental Results

The performance of the system was compared withand without the adaptive alignment engaged. Be-cause of the nature of the closed-loop feedback-control circuitry, which was a simple analogintegrator circuit ~operational amplifier, capacitor,and resistor!, the overall bandwidth of the adaptive-lignment system was limited to only a few hertz.herefore only relatively slow misalignments ~of therder of seconds! could be tracked and corrected byhe system. Figure 9 shows a short sequence of mis-lignments and spot realignments to the center of theuadrant detector. Included is a graph of the chan-el signal level ~used as a figure of merit! and theuad-cell positional error signals plotted versus thepplied VAP voltage.Two experiments were performed: manual mis-

lignment of the system to investigate the range oforrection that could be achieved and thermal mis-lignment of the system in which it was heated fromoom temperature to typical computing-system oper-ting temperatures. In the first experiment mis-lignment was induced manually to simulateechanical creep, initial alignment error, or other

lowly varying effects. Micrometers were attached

and the quadrant-detector error signal. The corresponding spot

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to the receiver board to permit calibrated movementin the x and the y lateral directions. Starting withthe system aligned, we displaced the board with theadaptive-alignment system disengaged and then en-gaged. Figure 10 shows for one such experiment thevariation of the 622-Mibitys signal level as a functionof board displacement. With the feedback systemdisengaged there is only a very narrow range ~;100mm! in which the data channel has high integrity.However, with the alignment system compensatingfor the board movement, this range is increased~.800 mm!, so the system now has a much lowertolerance ~by almost an order of magnitude! to initialmisalignments and slowly varying effects.

In the second experiment the system was heated toinvestigate compensation for thermal expansion inthe various components. Temperature sensitivitywill be a major factor for free-space systems, as spec-ifications for both military and commercial tempera-ture ranges are greater than 100 °C. With thesystem initially aligned the temperature of the sys-tem was increased to 100 °C, as measured in theregion of the VAP, in approximately 10° steps andallowing approximately 20 s for thermal stabiliza-tion. The heating of the system was quite crude andwas performed by the surrounding of the system witha box and the introduction of heated air from a heat-shrink gun. Figure 11 shows typical results. Withthe adaptive-alignment system disengaged the signallevel rapidly dropped below the noise level, i.e., the

Fig. 10. Typical results of the manual-misalignment experiment.

Fig. 11. Typical results of the thermal experiment.

channel failed. With the adaptive-alignment sys-tem engaged the signal integrity was maintained andonly started to fall off in a gentle slope at tempera-tures higher than 90 °C.

7. Discussion and Conclusions

We have demonstrated that, by use of adaptive-alignment methods, free-space optoelectronic inter-connects can be made less susceptible to initialalignment problems and thermal effects. There areways this system can be improved: ~i! increase theresponse speed and ~ii! increase the numbers of de-grees of freedom or the aberrations being corrected.To allow high-bandwidth correction in the kilohertzregime, it is likely that the response time of the cor-rection device will be the limit. In this experimentthe VAP had a bandwidth of 30 Hz, which means, inconjunction with an optimized closed-loop feedback-control system, corrections at bandwidths approach-ing 300 Hz could be achieved. Therefore a newcorrection device with a bandwidth of hundreds ofhertz is required. A VAP designed with a smalleraperture, a thinner liquid layer, and a lighter glassmaterial could reach this bandwidth regime.

We are investigating faster devices and dedicateddigital hardware that would permit the implementa-tion of more sophisticated control algorithms. Toinvestigate further degrees of freedom, we are study-ing measurement techniques and correction algo-rithms that are compatible with our smart-pixelplane geometry, e.g., the integration of a number ofquadrant detectors at the receiver plane and the in-ferrence from the two-dimensional data of the three-dimensional alignment and aberrations.

Free-space optoelectronic interconnects have to im-prove cost–performance to become more generallyacceptable to the commercial community. In the fu-ture it is likely that connections will be made betweenmany silicon chips, which means that the complexityand the degrees of freedom on the optomechanics willincrease. We have shown that low-order adaptiveoptics has a role to play as a design option. The aimof the research is to allow the development of realisticand usable systems within the next 10 years.

We thank G. Smith ~Heriot-Watt University! forhe design of the demonstrator optomechanics and. A. P. Tooley ~Heriot-Watt University! and M. Na-

ruse ~University of Tokyo! for useful discussions.

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