convergence to the InternetProtocol (IP). Last, bit rates ofindividual services are rapidlyrising, with 2.5- and 10-Gbit/sprivate-line services mush-rooming, owing to the suddenemergence of IP router inter-faces at those rates. Over thepast five years, these threechanges have driven the mostfundamental transformationthat optical-fiber systems haveyet experienced: Transmissionfibers have gone from simple,unamplified, single-channel2.5-Gbit/s pipes to opticallyamplified, terabit/s systemstransporting hundreds ofwavelength-division-multi-plexed (WDM) light signals. Itis chiefly this sudden prolifera-tion of wavelengths in WDMtransport systems that is justnow beginning to drive theneed for optical subsystemsthat can switch, wiggle, andadjust—thus the sudden re-cent interest in MEMS.
In 1995, the inaugural yearfor commercial WDM trans-mission-system deployment, atotal of eight 2.5-Gbit/s chan-nels were painfully coaxedthrough recalcitrant amplifiedsystems, achieving a capacityof 20 Gbit/s per fiber. Apartfrom optical gain, no equaliza-tion technologies were used tocombat the effects of transmis-sion impairments. The art hassince rapidly accelerated. Theyear 2001 will witness thecommercialization of 2.4-Tbit/s systems transporting240 10-Gbit/s channels. It willalso see the first tentative de-ployments of WDM systems operating at40 Gbit/s per wavelength.
As a result of this acceleration, total op-tical power levels traversing fiber links aregrowing precipitously while spectral chan-nel spacing and bit period shrink. This inturn means that optical-amplifier noise,fiber nonlinearity, and dispersion becomemajor factors requiring meticulous trans-mission engineering, thus creating oppor-tunities for any technology that can miti-gate accumulating optical performanceimpairments. Micromachines are onesuch promising technology.
Gain equalizationOne of the earliest and strongest opportu-nities for MEMS is the mitigation of spec-tral gain nonuniformities. These resultfrom the erbium-doped fiber amplifiersthat have made long-haul WDM transportfeasible. Such amplifiers generate gainspectra of the form:
G(�) ~ exp0
L
�[�e (�)n2(z)-�a(�)n1(z)]dz),
where �e and �a , the erbium emission andabsorption cross-sections, are weighted bythe excited and the ground-state popula-
tions n2 and n1. The resultingragged set of physically feasiblegain shapes, generated by thelumpy cross sections of erbiumatoms in aluminosilicate glass,is shown in Fig. 2.1
Owing to saturation effects,the above gain expression im-plies that amplified WDMlinks are by their naturestrongly coupled systemswhose end-to-end transmis-sion spectra are sensitively de-termined by essentially all ma-jor parameters in the system—channel-count, pump and sig-nal power levels, interamplifierloss—all of which impact thepopulations n2 and n1 and thusthe gain shape. It is this strongcoupling that generates largevariations in end-to-end trans-mission spectra that are diffi-cult to predict and control.1
This ultimately requires dy-namically adjustable gainequalization.
This need is exacerbatedboth in systems that incorpo-rate wavelength add–drop andin wideband (~80-nm) sys-tems, in which parasitic Ramangain generates large channel-loading-dependent gain varia-tions.2,3 Since high-channel-count systems inevitably re-quire both wavelength add–drop, to avoid stranding capac-ity, and large optical band-widths, owing to four-wavemixing and optical filter-reso-lution constraints, such sys-tems also inevitably require dy-namic gain equalization. Thisset of transmission considera-tions in practice becomes acute
as channel counts rise from tens to hun-dreds—a transition that is underway nowin deployed systems. Thus the need for dy-namic gain equalization in WDM trans-port systems is becoming urgent.
As the need for dynamic gain equaliza-tion has accelerated, technologies rangingfrom acousto-optics to liquid crystals towaveguide grating routers and microma-chines have begun to show promise for ad-dressing it. Among MEMS approaches, themost successful to date is almost certainlythe continuous micromechanical attenua-tor array based on the well-known MARS
A s data-networking interfaces approach the per-wavelengthline rate, while core-transport line rates steadily rise, thereis an emerging need for reconfigurable networking at the
granularity of an individual wavelength, currently 2.5-10 Gbit/s.This need places severe strainson an already-war-weary arse-nal of conventional optoelec-tronics and thus offers strikingopportunities for lightwavemicromachines, or micro-electromechanical systems(MEMS), that were implausi-ble only a year or two ago.There have been two nearlyimmediate results. Microma-chines have emerged as a gen-uinely disruptive technol-ogy whose communicationspromise has few precedents.Concurrently, widespread hy-perventilation about thepromise and ubiquity of mi-cromachines for optical net-working (cf. Fig. 1) has itselfreached a level with few recentprecedents.
The resulting tension seems almost certain to produce a mixedoutcome in which the genuinely sweeping impact of lightwavemicromachines over the long-term is tempered by some disap-pointment with the timescale over which this impact is felt. Inlong-haul transmission-system impairment-mitigation, where thearsenal of conventional optoelectronics offers few strong alterna-
tives, MEMS represent serious candidates for satisfying the net-work’s rapidly mounting needs for tunable equalization of gain,chromatic dispersion, and polarization-mode-dispersion, and forreconfigurable wavelength-add/drop. Here, some near-term im-
pact is likely. On the otherhand, in large core-networkswitching systems, where mi-cromachines face far deepertechnical and competitive chal-lenges, they nonetheless offerthe long-term promise of raw,aggregate switch capacity that isunmatched by other technolo-gies. Still, notwithstanding a ris-ing stream of tour-de-forceconcept-demonstrators, the se-rious deployment of MEMS ascore-network switches will al-most certainly occur overtimescales that disappoint cur-rent overheated expectations.We here summarize the gen-uinely disruptive promise oflightwave micromachines incore-transport optical network-
ing, as well as the substantial challenges they have yet to overcome.
Lightwave micromachines for transmissionGlobal core long-haul communications networks now face pres-sures from three diverse sources. Aggregate demand is growingquickly. This demand is rapidly being transformed from time-multiplexed circuit-switched traffic to packetized data, with swift
LightwaveMicromachines for Optical NetworksVast Promise Amid Vaster Promises
Figure 1. Optical-layer networking with lightwave micromachines: the vision.
Figure 2. Erbium amplifier gain spectra-origin of the tight parameter couplingin core-transport networks. Curves depict the family of feasible erbium ampli-fier gain spectra, exhibiting their sensitive dependence on population-inver-sion. n2, excited-state population; n1, ground-state population.
Figure 3. MEMS-based dynamic gain equalizer. (a) Continuous-membrane at-tenuator array. (b) MEMS attenuator array embedded in a free-space wave-length multiplexer to yield a dynamically tunable spectral shaper. I/O, input-output. (c) Input signal constellation after erbium amplifier. (d) Output signalconstellation after gain equalizer (dashed curve) and reamplification (solidcurve). Inset, flattened spontaneous-emission spectrum. (Courtesy of LucentTechnologies.)
EVAN L. GOLDSTEIN, LIH-YUAN LIN, AND JIM A. WALKER
March 2001 ■ Optics & Photonics News 61
structure.4 This is shown in Fig. 3. The sil-icon-nitride membrane plus air gap serveas a spatially variable, tunable multilayerdielectric mirror. When the incident signalis spectrally dispersed along the axis of thedevice defined by an array of strip elec-trodes, as in Fig. 3(b), one obtains a simpleand compact tunable spectralshaper. Such devices have beenshown to be capable of gener-ating a variety of filter shapesthat quite effectively flatten outthe lumpy gain spectra of er-bium amplifiers, as indicatedin Figs. 3(c) and 3(d).
Dispersion compensationAs per-channel bit rates rise,WDM transport systems re-quire tunable equalization notonly of gain shapes but also ofvarious forms of dispersion,which causes pulse distortionand thus intersymbol interfer-ence. At ~40 Gbit/s the needarises for dynamically tunablecompensation of the transportsystem’s chromatic dispersion.Here, too, MEMS have alreadybegun to show substantialpromise. MARS-type struc-tures, when employed as tun-able mirrors in Fabry–Perotcavities, are an effective meansof subjecting the various spec-tral components of an inci-dent signal to adjustable Fab-ry–Perot lifetimes, thus pro-ducing dynamically tunabledispersion values.5 These canthen be used for algebraic can-celing of chromatic dispersionimposed by fiber, multiplexers,and other transmission-sys-tem components.
Other forms of dispersionalso require dynamic compen-sation. Because of departuresfrom precise radial symmetryin the cores of real fabricated and de-ployed optical fibers, the two orthogonalpolarizations travel in fact with differentgroup velocities, resulting in distortiondue to polarization-mode dispersion(PMD). In digital systems one needs tohold the time-averaged differential delaybetween these two polarizations,
to values smaller than ~0.1 T, where T isthe bit period, DPMD(k) is the PMD pa-rameter of the kth fiber segment, and L(k)is that segment’s length. Since the PMDparameter is DPMD ~ 1 ps/(km)1/2 for typ-
ical embedded fiber, this implies maxi-mum transmission lengths of ~1600 km at2.5 Gbit/s, 100 km at 10 Gbit/s, and 7 kmat 40 Gbit/s. Thus, whereas PMD is inpractice not a factor at 2.5 Gbit/s, its com-pensation becomes important at 10 Gbit/sand rapidly grows critical at higher rates.Because the orientations of the fiber’s slowand fast axes vary in time, compensationmust be dynamic. Moreover, because thecompensation must be accomplished on a
channel-by-channel basis, PMD compen-sators, if they are to compete with the al-ternative of simply replacing older in-stalled PMD-afflicted fiber, must be bothcompact and cheap. Here again is a naturalopportunity for micromachines.
No mature MEM-based PMD-com-pensating technology has yetbeen reported, even in the re-search literature. However, thekernel of such a compensatorwould be a polarization con-troller, which, in combinationwith an appropriate birefrin-gent element, provides first-or-der PMD compensation. Thebasic building block, a polar-ization rotator, has been imple-mented with the MEMS-baseddevice shown in Fig. 4.6 It con-sists simply of a Mach–Zehn-der interferometer fabricatedby means of surface-microma-chining in polysilicon, with theChronos MUMPS process.7
Since the micromachined poly-plates can be held in preciseposition with resolution farbetter than an optical wave-length, the interferometer is ca-pable of creating precise, stablepolarization-rotation by ad-justment of the phase differ-ence between |TE> and |TM>waves. This is achieved by elec-trostatic adjustment of thephase-shifting mirror positionshown in the scanning-elec-tron-microscopy photographof Fig. 5(a). Applying a sinu-soidal voltage to this phaseshifter should in principle gen-erate output polarization stateslying in a circle on the Poincarésphere (Fig. 4). As seen in Fig.5(b), this is precisely what oc-curs, with operation that is sta-ble and nearly ideal, except fora small amount of phase-to-amplitude coupling at the
phase-shifter mirror, evident in slight de-partures from circular trajectories.
In both these applications—as dynam-ic gain equalizers and as dispersion com-pensators—micromachines have with re-markable speed begun to move out of thedomain of the research lab and into con-tention as candidates for deployment inWDM transmission links. MEMS-baseddevices of similar complexity, generally
fabricated by surface-micromachining ofpolysilicon, have also emerged as candi-date means of reconfigurably adding anddropping wavelengths in WDM transmis-sion links8,9—a capability that becomescritical to avoiding stranding capacity aswavelength channel countsgrow into the hundreds.
Lightwave micromachinesfor switchingThe discussion thus far hasbeen restricted to microma-chines for transmission. Yetwith the maturation of thesources, amplifiers, compen-sators, filters, and high-speedelectronics that have emergedthis year in the first commercialterabit/s WDM links, core-net-working challenges are swiftlyshifting from transmission toswitching. Transporting enor-mous information capacityfrom point to point is no longerproblematic; the difficulty is inparting out information anddisposing of it.
There are two sources ofacute switching pressure in thecore long-haul network. Thefirst is restoration: Traffic mustbe automatically rerouted inthe event of failures over timeintervals of the order of ~100ms. The second is provisioning:New circuits must be estab-lished in response to service-layer requests over time inter-vals of the order of a few min-utes. Both functions are nowcarried out with electronicswitches and add–drop multi-plexers that operate typicallyon signals at 45-155-Mbit/sdata rates—an arrangementthat becomes both unmanage-able and unaffordable in thenewly emerging core networks whose larg-er nodes transport hundreds of gigabit/sof traffic. Moreover, as mentioned above,service-layer vehicles—IP routers—arenow sprouting interfaces, at 2.5-10 Gbit/s,that are well matched to the per-wave-length bit rates of transport systems. Thusthere is an emerging need to provision andrestore traffic at much coarser granularity,at or approaching the wavelength. What isneeded, then, is a switch scalable to thou-sands of ports, with each port transport-
ing 2.5-10-Gbit/s signals, scalable to 40Gbit/s, and with switching times of <10ms. This challenge greatly outstrips the ca-pabilities of embedded networking tech-nology. The result has been a frenzieddrive, over the course of the past two years,
to develop coarse-granularity circuitswitches with capacities that stretch intoterabits/s and beyond.
Among optical technologies, microma-chines constitute the most promising wayof meeting the challenge. Moreover, theonly promising MEMS-based approachesare those that employ free-space propaga-tion, since waveguide structures, in prac-tice confined to shallow bends in theplane, have thus far proven incapable ofachieving the dense interconnection of
optical signal paths needed to accomplishhigh-port-count strictly nonblockingswitching.
There are two emerging approaches tobuilding such switches with lightwave mi-cromachines. The first is the two-dimen-
sional cross-bar switch of Fig.6. Here the (i, j) mirror of asquare array, when raised,serves to couple the i th port ofthe input fiber array with thej th port of the output.10-12 Theadvantage of this approach isthat the mirror positioning isbinary—each mirror is eitherup or down, but not in be-tween. This vastly simplifiescontrol. However, the disad-vantage is loss. In such aswitch, for a given Gaussianbeam diameter w0, and a fixedmirror diameter R, the opticalpath length grows linearly withthe number of ports. Switchloss due to clipping of Gauss-ian beams thus grows rapidlywith port count, as indicatedin Fig. 7. Although switch lossin principle declines monoton-ically as beam width is in-creased, beyond 32 ports theMEMS die size rapidly growsimpractical, as indicated in Fig.7. This has the effect of limit-ing two-dimensional cross-barswitches to roughly 32 inputand 32 output ports.
To scale beyond this limit,one must free the beams fromthe constraint of lying in aplane and use the full three di-mensions as an interconnec-tion region. This is achieved inthe three-dimensional steered-beam configuration of Fig. 8.Here a given input or outputport is associated with aunique two-axis steerable mir-ror.13-15 The great virtue of this
approach is that, for a given beam width,the optical path length scales only as √N,where N is the number of input ports. Portcounts of several thousand, with losseswell below 10 dB, are feasible. But the greatdisadvantage of three-dimensional steeredbeams is that they are systems containingthousands of elements—microlenses,fibers, micromirrors—each of which isconstrained by truly oppressive alignmenttolerances of the order of micrometers
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62 Optics & Photonics News ■ March 2001 March 2001 ■ Optics & Photonics News 63
Figure 4. Operating principle of micromachined polarization rotator.
Figure 5. (a) Scanning-electron micrograph of surface-micromachined poly-silicon polarization rotator. (b) Polarization-rotation trajectories on the Poin-caré sphere for sinusoidal applied voltages of various amplitude.
Figure 7. Scaling of loss, due to clipping of Gaussian beams, with port countin two-dimensional MEMS cross-bar switches.
���� = �DPMD (k)2 * L(k)M
k=1
and hundreds of microradians. Such sys-tems require exquisite engineering of mir-ror-control servo algorithms; fiber bun-dles; lens arrays; micromirror mechanicalresonances, optical flatness, and electricalproperties; and thermal and optomechan-ical packaging. These imposing technicaldifficulties continue to makehigh yield at low loss problem-atic in practice, although thesolutions to these problems aresteadily becoming better un-derstood.
Challenges to the visionAlthough MEMS has greatpromise for the long-term,substantial challenges lie be-tween the cheerful vision artic-ulated above and the readinessof the technology for deploy-ment in reliable core-transportnetworks. The most prominentof these challenges concern de-vice reliability, ease of manu-facture, and packaging, all ofwhich are almost completelyunexplored in the publishedliterature to date. And yet we are propos-ing to deploy, in the heartbeat positions ofthe core-transport network through whichall high-speed links pass, network ele-ments whose underlying technology hasjust been invented over the past one to twoyears. This suggests that the widespreadacceptance of MEMS switching systemswill likely be gated by the emergence of acompelling reliability story—somethingthat, despite growing efforts, has not yetbegun to happen.
The second major challenge comesfrom rapid progress in electronic switch-ing systems. Three years ago electronicswitches offering just eight 2.5-Gbit/sports seemed well beyond reach. By con-trast, the current state of the art being ad-
vanced by multiple vendors offers switch-ing systems with 512 2.5-Gbit/s ports, for atotal capacity well in excess of 1 Tbit/s, re-siding in four equipment bays. These sys-tems provision and mesh-restore wave-lengths at a granularity of 155 Mbit/s to2.5 Gbit/s. They also provision and mesh-
restore 10-Gbit/s wavelengths by inversemultiplexing down to the basic switchrate, with the capability of grooming suchsubrate signals within a given 10-Gbit/spipe.
Moreover, electronic technology doesnot yet show signs of diminishingprogress.The aggregate information-han-dling capacity of a monolithic electronicswitch chip continues to double roughlyeach year, as summarized in Table 1, withno signs yet of tapering off. In four distinctdevice technologies, 128 x 128 chips oper-ating at 2.5 Gbit/s have just become avail-able. When used in conventional three-stage switch structures, these permit theconstruction of strictly nonblocking 8000x 8000 2.5-Gbit/s switches or 2000 x 200010-Gbit/s switches. Given this scaling,
such switches will likely maintain a marginover the needs of the largest core-switch-ing nodes for some years to come. The ef-fect of this appears to be a pushing-off ofthe era of MEMS optical fabrics by at leastone to two years or more.
The final challenge comes from theneed to incorporate intelli-gence into the core-networkswitch—a need that owes tothe fact, long lamented by net-work-operators, that it costsmore to operate a core-trans-port network than it does tobuy it in the first place. Be-cause of this, network opera-tors continue to demand a va-riety of management featuresfrom their switches, rangingfrom performance monitoringto connection verification tothe ability, by means of readingand writing signal overheadbytes, to automatically uncov-er the identity of neighboringswitches and construct net-work topology maps. In thelong run, significant transpar-
ent-optical-switch innovation in these ar-eas seems likely; but over the next severalyears the practical approach to these chal-lenges will be to seek raw capacity fromoptical switch fabrics while recovering so-phisticated management and network-monitoring functions by use of optoelec-tronic interfaces.
In conclusionThe rapid proliferation of wavelengths
in WDM systems has created a suddenneed for compact, high-density subsys-tems that can switch, tune, and adjust. Inlong-haul transmission systems, lightwavemicromachines represent relatively near-term candidates for satisfying needs—justnow rising to the level of urgency—fortunable equalization of gain, chromaticdispersion, and PMD, as well as for imple-menting reconfigurable wavelength add–drop. In core-network switching systems,however, in which micromachines facesubstantially deeper challenges, theynonetheless offer promise in the long-term of aggregate switch capacity that isunmatched by other technologies. Sincethe global appetite for transmitted bits asyet shows no sign of being sated, makinguse of the eventual raw capacity of MEMScore switches will probably not, after all,require the imagination of the poet; it will
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Evan L. Goldstein, Lih-Yuan Lin, and Jim A.Walker are
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Table 1. State of the Art in Monolithic Electronic Strictly-Nonblocking Switch Chipsa
