Recent developments in computer systems design andarchitectures have highlighted deficiencies with thecurrent electrical interconnect systems. Opticalinterconnects are promising new replacements for

future optical system buses. These buses may be implementedmost efficiently by using an electrically controlled steerablelaser. We review current state-of-the-art techniques forachieving electrically steerable devices, with a particularemphasis on a novel steerable laser based on wide aperturevertical cavity surface emitting lasers (VCSELs). Our device iscapable of steering over a wide range and exhibits very lowcrosstalk levels and narrow output beam. We show how thisdevice may be used in a free-space optical interconnect system.Finally a model is presented to explain this phenomenon.

Speed Makes a DifferenceNew clock frequency records in high-speed processors arebeing set regularly. It is now common to clock CPUs fasterthan 2 GHz, and 4 GHz appears to be quite possible with cur-rent technology. This trend of high clock rates for CPUs hasbeen hampered by the physical limitations imposed on the

interconnecting circuits and buses involved.The system bus is the main interconnect responsible for

delivering data between different resources, such as CPU andmemory to different peripherals, such as hard disks, networkconnections, and video processors. Traditional computershave an electrical bus consisting of copper tracks on a dielec-tric motherboard.

When one considers that at current CPU clock rates, thephysical extent of electrical buses approaches the dimensions ofthe electromagnetic wavelength of the clock signals, the task ofproperly managing radiation and electromagnetic crosstalkbetween electrical interconnects is becoming more and morechallenging. In practice bus speeds have not been able to matchthe equivalent increases in CPU speed and currently run at10–15% of the CPU speed. This factor leads to bottlenecks typi-cally associated with low memory bandwidth and limited I/Ospeeds.Optical communications have become part of everyday lifesince the early to mid 1980s. Throughput bit rates over opti-cal links commonly exceed 2–3 Gbit/s. In addition, the fallingmanufacturing costs of solid-state optoelectronic components

� 38 IEEE CIRCUITS & DEVICES MAGAZINE � JANUARY/FEBRUARY 20048755-3996/04/$20.00 ©2004 IEEE

M.I. Cohen and C. Jagadish

make board to board photoniccommunications more viable.

Increasingly optical commu-nications are starting to domi-nate medium to short rangecommunications. Modern com-puter systems are currentlyseeing the emergence of high-speed optical buses for periph-erals, such as used for fiberchannel hard disk drives. Theseshort- to medium-range appli-cations typically operate in thenear infrared region (850 nm or 980 nm) due to the highefficiency and low cost of GaAs technology.

Optical Interconnect ApplicationsA topic of intense research in the last few years had been theimplementation and viability of optical interconnects forinterboard and interchip communications. This type of inter-connect is seen as a promising technology for implementingfuture high speed buses in computers.

VCSELs have a number of attractive properties for opticalinterconnect applications. They can be mass produced verycheaply since no manual cleaving is required. In addition,VCSELs have been produced with very lowthreshold currents and high quantum effi-ciency. These factors make VCSELs verysuitable to applications where a largenumber of lasers need to be packed into asmall space with acceptable power con-sumption requirements.

In addition, the beam quality of theVCSEL is superior to the more conven-tional edge emitting heterojunction laser.Far-field divergence angles are muchsmaller and are typically symmetrical.These properties make the VCSEL attrac-tive for free-space interconnect applica-tions, since VCSELs typically requiremuch simpler focusing optics and havelower beam spill over.

There are three traditional approach-es for implementing optical buses .These are shown in Figure 1. The fiber channel bus (a) haseach resource and peripheral connected to every other onevia an optical fiber. The light from all these fibers is thenmixed onto a central coupler. This approach requires thatonly one resource/ peripheral communication occurs atany one time since the channel is occupied. Also sincepower is distributed equally to all peripherals at once, thetotal number of peripherals, or the fan out, is limited.

The use of free-space optical interconnects (b) dispenseswith the need for fibers by splitting the output light into anumber of beams, each aligned with a particular peripheral.The advantages of free-space interconnects include lower

cost, less complex connections,and the ability to be packedinto smaller space. These comeat the cost that the micro-optics are difficult to produceaccurately. Simultaneous datatransfer operations are notpossible with either busdesign, and the total numberof output devices depend onthe laser power.

Steerable LasersSteerable lasers are ideal for free-space optical buses. Eachtransmitter is capable of aligning with a single peripheral, asrequired, leading to low crosstalk with other peripherals andallowing simultaneous connections to be carried. Since thesteerable laser is capable of steering smoothly through a rangeof angles, electronic feedback may be used to dynamically alignthe beams, placing far lower tolerances on the micro-optics.This idea is illustrated in Figure 1(c).

Traditional optical interconnects are made using edge emit-ting lasers. These lasers are grown epitaxially and are thencleaved to form mirrors. The output beam of these lasers is typ-ically stigmated since the epitaxial thickness is far smaller than

39 �IEEE CIRCUITS & DEVICES MAGAZINE � JANUARY/FEBRUARY 2004

1. Proposed optical bus solutions. (a) Fiber channel bus, (b) free-space optical interconnectswith fixed optics, and (c) free-space, dynamically reconfigurable optical interconnects using

steerable lasers.

Peripherals Peripherals Peripherals

CouplerMicro-Optics

Resources

(a)

Resources

(b)

Resources

(c)

2. Waveguide-based beam steering. The pumping ratio between thetwo contacts controls the position of the guided beam.

I1 I210-µm Strip Contacts

Active Region

Substrate

Back Contact

A topic of intense researchin the last few years had been

the implementation andviability of optical interconnects

for interboard andinterchip communications.

the ridge width. This makes theirapplication in free-space opticalinterconnects difficult. Typicalthresholds for edge-emittinglasers are much higher thanVCSELs, a fact which becomesmore important as more lasersare packed into tighter space.

VCSELs, on the other hand, have a symmetric aperturewith a symmetric output beam. The beam characteristics arewell suited to free-space interconnect applications.

There are a number of important properties in a steerablelaser that are crucial for its implementation in free-space opti-cal buses. The laser must have a wide steerable range, so alarge number of peripherals may be accessed by the same laser.

In addition, the level of crosstalk experienced by two adja-cent peripherals must be low, since this crosstalk is respon-sible for limiting the maximum transmission speed of thebus. The level of crosstalk is related to the beam width,which ideally should be very narrow. If a steerable laser has anarrow beam and a low level of crosstalk, it becomes possibleto place peripheral detectors closer together and henceincrease the number of peripherals on the bus.

Although the advantages of steerable lasers are many, theirimplementation has been slow. To create an electrically steer-able device, the output beam needs to be influenced by thecontrolling current in some way. An early attempt to do thisrelied on the manipulation of current injection profiles withinan active waveguide structure. This is shown in Figure 2.

The device is controlled by controlling the relative pump-ing currents between the two contacts. This results in theshifting of the active gain guide closer towards the strip withthe higher current injection.

This technique proved very effective in accurately controllingthe near field spot of the guided beam. This technique is verysuitable for integrated optoelectronic applications; however, it isnot ideally suited for free-space applications. To steer the far

field beam pattern, the phaseacross the aperture must be con-trolled.

This has been done by inte-grating optical beam routers atthe exit aperture [1], [2]. Thistechnique is shown in Figure 3.The top aperture of a VCSEL is

divided into two separately powered halves. A half wavelengthdielectric layer is deposited over one of these halves. Whenonly one of these halves is powered, the beam is emitteddirectly normal to the device.

If both contacts are powered, however, the final emissionconsists of the interference of the contributions from the twodistinct halves. This effect causes the beam to steer [3].

This technique allowed the device to be switched very

� 40 IEEE CIRCUITS & DEVICES MAGAZINE � JANUARY/FEBRUARY 2004

3. Beam steering technique achieved by integrated beam routers.

Optical BeamRouter

4. Optical microscope images of the device: (a) top contact in focus and(b) focus is shifted to aperture/active region.

(a)

(b)

Steerable lasers areideal for free-space

optical buses.

rapidly at up to 2 GHz. Howev-er, the switching range was lim-ited to a few degrees, with asignificant level of crosstalk.These deficiencies made thistechnique nonappealing forfree-space optical interconnects,where low crosstalk and largeswitching range is far moreimportant than fast switching.

Microelectromechanical SystemsRecently microelectromechanical systems(MEMS) opened many new possibilities foroptoelectronic devices. In particular somevery promising work is underway to createa steerable VCSEL with both narrow beamwidth, as well as low crosstalk and accu-rately controllable far field angle [4].

This technique relies on the position-ing of microlenses above the VCSEL with-in a MEMS structure. These microlensesare then decentered by using electrostaticvoltage signals to divert the angle of thethe far field as shown in Figure 4. By usinga complex MEMS structure the authorshave demonstrated beam steering in bothdimensions with low crosstalk and excel-lent beam quality.

Creating a SingleOutput Beam

In general the above methods have triedto create a single output beam that can besteered through the entire steering range.To create a single output beam, one mustcontrol the phase across the aperture—whether by controlling the injection cur-rent profile or by physically moving lensesabout.

We have recently demonstrated a novel steerable VCSEL.Our device has a large steerable range of 30◦. The laser wasgrown by MOCVD and processed using standard lithographicsteps, details of which can be found elsewhere [5]. Figure 5shows an optical micrograph of the device as taken under avery small operating current. Figure 5(a) shows the laserwith the top facet in focus. The actual aperture is buriedabout 3 µm below the surface. The microscope is adjusted inFigure 5(b) so the aperture is in focus. As can be seen, themajority of the current is pumped around the edge of thedevice [6].

Figure 6 shows the far-field power profile for a numberof driving currents. This figure also contains a psuedo colorimage showing the far-field intensity along the major emis-sion axis (color varies from black through red, yellow, andwhite as a function of driving current). The full far-field

profile for a number of selectedcurrents is shown in Figure 7.The laser is initially emittinglight in two main lobes, situat-ed about 60◦ apart. These lobesmove towards the normal asthe driving current isincreased. When the pumpingcurrent is very high, the farfield pattern shows lasing at

higher order Gaussian modes.A two-channel optical switch was constructed using two

41 �IEEE CIRCUITS & DEVICES MAGAZINE � JANUARY/FEBRUARY 2004

5. The beam profile as measured along the major axis plotted using various driving currents.The beam width in the orthogonal direction varied from 15 to 20 ◦.

6. Far field of steerable laser as measuredunder a number of different driving currents.

10 mA 14 mA

30 mA20 mA

65° 60°

22°

15°

46°

–30° –20° –10° 0° 10° 20° 30°Angle from Normal

5 mA7 mA9 mA

11 mA13 mA15 mA17 mA19 mA21 mA23 mA25 mA

Driv

ing

Cur

rent

579

1113151719212325

false color image goes her e

Inte

nsity

(a.

u.)

To ensure maximum flexibilityand reconfigurability, opticalinterconnects must involve

steerable lasers.

detectors placed above the VCSEL. The first detector D1 wasplaced at a distance of 6 cm above the laser and 4 cm awayfrom the normal. A second detector D2 was placed at the same

height but 12 mm closer to the normal. Both detectors had anaperture of diameter 3 mm. This setup is illustrated in Figure8. A crucial property of the optical switch is the level ofcrosstalk. In this particular switch configuration, the crosstalkpower is that power which is measured at detector D2, whilethe beam is aligned with detector D1.

This power was measured at detector D2 while modulatingthe driving current about the same average level which alignsthe laser with D1, and changing the modulation depth(Imax − Imin). This relationship is shown in Figure 9. As can beseen if the modulation frequency was too low the crosstalkpower at detector D2 was quite high, while increasing the mod-ulation frequency significantly lowered the crosstalk power.

This behavior indicates that the steering effect in theVCSEL is a thermal effect. If the laser is modulated too slowly,the beam oscillates about the average level, increasing theaverage crosstalk power.

Modeling a Steerable LaserTo better understand the processes involved in this form ofsteerable laser, a model was developed as shown in Figure 10.

The model illustrates current crowding effects in a topemitting VCSEL. The top DBR is approximated by a Laplacian(isotropic and charge free) slab of width 3 µm.

The current density is shown in Figure 11. As can beseen the current consists of a constant component which isuniform across the aperture as well as a non-uniform orcurrent crowding component. This is maximized at theperimeter of the aperture. For small aperture devices, thecurrent density is expected to be relatively uniform. Howev-er, for larger area devices, current crowding effects aremore significant. This general behavior is observed clearlyin Figure 5, where the device was imaged whilst under a lowcurrent pumping level.

The temperature distribution within the DBR itself isthen calculated from the heat equation. This has been solvednumerically to produce the temperature distributions illus-trated in Figure 12. This figure shows the temperature dis-tributions for 3 representative currents. As can be seen, atlow pumping currents, two distinct peaks occur in the tem-perature profile. Since refractive index increases withincreasing temperature over the range of temperatures of

interest, this results in two distinct ther-mal lenses.

This thermal lensing is responsible forforming two distinct beams at low cur-rents. Clearly, however, as the pumpingcurrent is increased, the current distribu-tion becomes more uniform across theaperture, as is shown in Figure 11, thisresults in a single thermal lens. As thepumping current increases, the absolutevalue of the temperature increase is alsogreater, leading to stronger thermal lens-ing refocusing the beam into a singlebeam.

� 42 IEEE CIRCUITS & DEVICES MAGAZINE � JANUARY/FEBRUARY 2004

8. Lines of constant crosstalk power. Hatched region representscrosstalk power of better than −18 dB.

0 10 k 20 k 30 k 40 k 50 kModulation Frequency (Hz)

10

15

20

25

Mod

ulat

ion

Dep

th (

mA

pp)

–5.5 dB–11.5 dB

9. Current flow model for the steerable VCSEL.

Contact Aperture

Oxide

Active Region

d

7. Experimental setup used for measuring steerable VCSELs as optoelectronic switched lasers.

D1 D2

VCSEL

6 cm

4 cm

Light Beams

ConclusionEngineering limitations in cur-rent computer systems architec-ture limit the operating speedsof electrical interconnects. Apossible solution for this prob-lem was presented by usingoptical interconnects. To ensuremaximum flexibility and recon-figurability, the optical inter-connects must involve steerablelasers. Recent progress had been made on producing steerablelasers that have both a high steering range as well as having anarrow beam width. Both these properties result in lowercrosstalk between closely spaced detectors.

A number of different solutions are currently being pur-sued, including MEMS. We demonstrated a solution based onthe formation of thermal lenses in a resonant cavity. Weshowed that our device can steer up to a range of 30 ◦ from thenormal at threshold to near normal incidence at higher cur-rents. The emergence of these novel devices should the devel-opment of novel optical free-space interconnects which in turnshould serve to increase the clocking rates of computer buses.

M.I. Cohen is with the Australian Department of Defense,Canberra, Australia. C. Jagadish is with the Department ofElectronics Materials Engineering, Australian National Uni-

versity, Canberra, Australia. E-mail: cjagadish@ieee.org.

References[1] L. Fan, M. Wu, H. Lee, and P.

Grodzinski, “Dynamic beam switching of vertical-cavity surface-emittinglasers with integrated optical beamrouters,” IEEE Photon. Technol.Lett., vol. 9, pp. 505–507, Apr. 1997.

[2] L. Fan, M. Wu, H. Lee, and P.Grodzinski, “Novel vertical-cavitysurface-emitting lasers with inte-

grated optical beam router,” Electron. Lett., vol. 31, pp. 729–730, Apr.1995.

[3] T. Ide, M. Shimizu, S. Mukai, M. Ogura, and T. Kikuchi, “Continuousoutput beam steering in vertical cavity surface emitting lasers with twop-type electrodes by controlling injection current profile,” Japanese J.Applied Phys., vol. 38, pp. 1966–1970, Apr. 1999.

[4] A. Tuantranont, V.M. Bright, J. Zhang, J.A. Neff, and Y.C. Lee, “Opticalbeam steering using MEMS-controllable microlens array,” Sens. Actua-tors A, Phys., vol. 91, pp. 363–372, July 2001.

[5] M. Cohen, A. Allerman, K. Choquette, and C. Jagadish, “Electricallysteerable lasers using wide-aperture VCSELs,” IEEE Photon. Technol.Lett., vol. 13, pp. 544–546, June 2001.

[6] W. Nakwaski and M. Osinski, “Current spreading in proton-implantedvertical-cavity top-surface-emitting,” Int. J. Optoelectronics., vol. 10, pp.119–127, Mar. 1996.

43 �IEEE CIRCUITS & DEVICES MAGAZINE � JANUARY/FEBRUARY 2004

10. Calculated current density for VCSEL.

1.2

1

0.8

0.6

0.4

0.2

0

Cur

rent

Den

sity

(a.

u)

–10 –8 –6 –4 –2 0 2 4 6 8 10Distance from Center (µm)

11. Temperature distribution for three representative driving currents.

Tem

pera

ture

Incr

ease

(au

)

–30 –20 –10 0 10 20 30

Low Pump CurrentMedium Pump CurrentHigh Pump Current

Distance Across Aperture (µm)

Recent progress had beenmade on producing steerablelasers that have both a high

steering range as well as havinga narrow beam width.