Tunable MEMS Devices for Optical Networks

42 Optics & Photonics News ■ March 2003

March 2003 ■ Optics & Photonics News 43

Tunable

MEMSDevices

For Optical Networks

Jill D. Berger and Doug Anthon

Early deployment of tunable lasers and filters

in optical networks that use dense wavelength division

multiplexing has been driven by the inventory advantages of

universal line cards and multiwavelength sparing. But the

greatest benefits can be achieved by using tunable devices

for dynamic wavelength provisioning in reconfigurable

transparent optical networks. A prerequisite for the

realization of dynamic optical networks is the availability

of tunable devices that meet the rigorous performance

requirements of fixed-wavelength transceivers—at

comparable manufacturing costs. Widely tunable

external-cavity diode lasers and diffraction-grating

filters based on silicon microelectromechanical

actuators meet these criteria.

1047-6938/03/03/0042/8-$0015.00 © Optical Society of America

Widely tunable external-cavity diode laser, based on a silicon MEMS actuator, for opticalnetwork applications.

between multiple channels. More gener-ally, tunable filters can be used in thereconfigurable optical add-drop multi-plexers (ROADM) that are the centralswitching element of transparent opticalnodes. A ROADM can be constructedentirely from single channel tunable fil-ters using a broadcast-and-select archi-tecture. In this case, the incoming signalis amplified and split into N independentoutputs that are divided into groups ofpass-through channels, branched chan-nels and received channels. Although thenumber of channels of each type is deter-mined by the hardware, the use of tun-able filters allows for control of thenetwork configuration by making it pos-sible to modify decisions over which sig-nal is routed where.

Because most optical networks are stillopaque, with a set of point-to-point linksconnected through a mesh of electricalswitches, full advantage cannot be takenof the benefits offered by tunability.

ing, but not carrying revenue-generatingtraffic. In agile optical networks, such asthe system depicted in Fig. 1, bandwidthcan be remotely provisioned in only min-utes instead of weeks or months, and sub-sequently reprovisioned as revenue-generating traffic demands change. Withappropriate network management, fre-quency contention problems can be min-imized and optical routing becomes anefficient tool for dynamically reconfigur-ing networks.2

Tunable transmitters based on tunablelasers are analogous to fixed-wavelengthtransmitters. But in a transparent opticalnetwork, tunable filters can play severaldifferent roles. Single-channel tunablebandpass filters are used in amplifiedtransmitters for the suppression of ampli-fied spontaneous emission (ASE) and inreceivers as adaptive prefilters for noisereduction. Tunable filters can reduce thecosts of optical performance monitoringby allowing one monitor to select

S ignificant advances in optical tech-nology, particularly the develop-ment of optical amplifiers and

dense wavelength division multiplexing(DWDM), have transformed the designof metro, regional, long-haul, and under-sea telecommunications networks. Overthe course of the past decade, the numberof channels that can be carried on a sin-gle optical fiber has risen from one tomore than 100, while data rates per chan-nel have increased from 622 Mbits/s to 10 Gbits/s. Single-channel systems thatonce required optical-electrical-optical(OEO) regeneration at 40-km intervalshave been supplanted by optically ampli-fied DWDM systems with regenerationintervals of up to 3000 km. Theseadvances tend to drive networks awayfrom electronically switched “opaque”architectures, with frequent OEO conver-sions, and towards transparent, opticallyswitched designs.

Rapid technological progress,combined with the readily available capi-tal in the late 1990s, generated an abun-dance of bandwidth that has yet to befully employed. Although the marginalcost of providing bandwidth is rapidlyfalling, carriers are still having difficultyachieving profitability, in part because ofthe low prices being paid for data ser-vices. For carriers seeking to maximizerevenue and minimize both capitalinvestments and operating expenses,dynamic provisioning may be one solu-tion. Opaque networks contain a numberof costly redundancies that can be elimi-nated with appropriate network design.For example, it has been estimated thatan agile transparent network, one thatpermits nonterminating channels to passthrough nodes in optical form, can elimi-nate more than half the OEO conversionsfound in an opaque network.1

End-user bandwidth demand changesdynamically, because of both trafficgrowth and traffic churn. In fixed-wave-length networks, traffic churn results instranded, unusable bandwidth.Provisioning time frames of up to severalmonths—without benefit of preprovi-sioning to meet projected future trafficdemands—make it difficult to respond totraffic growth. And when forecastinginaccuracies occur, the result is equip-ment that is both deployed and operat-


TUNABLE MEMS DEVICES

Figure 1. (a) Optical network based on widely tun-able lasers and filters. At node A, signals from anarray of tunable transmitters, each composed of atunable laser and modulator capable of transmittingany one of 100 channels, are combined by a wave-length-insensitive power combiner and transmittedthrough an amplified fiber link with transparentnodes B and C. Signals arriving at B and C may belocally terminated, or passed through to other nodes. Locally added or passed-throughsignals can be dynamically switched to any other transparent node. At node D, the signalsare demultiplexed by a wavelength-insensitive power splitter and detected by an array oftunable receivers, each composed of a tunable filter and photoreceiver. (b) Dynamic trafficrouting in an agile network can be accomplished by changing the tunable transmitter/receiver wavelength or the optical switch configuration. Traffic traveling from A to C isrerouted from A to D by changing the transmitter wavelength at A. Traffic travelling fromG to D is rerouted from G to E by changing the optical switch configuration at F.

(a)

(b)

Wavelength-InsensitivePower Multiplexing

Optical Add/DropMultiplexer

Optical Amplifier

Optical SwitchFiber

Tunable Transmitter

TX1�1

�2

�N

�1

�1 ... �N�2

�N

TX2

TXN

N:1 1:N

M:1 1:M

RX1

RX2

RXN

Tunable ReceiverA

A

B

B

C

C

D

D

E

F

G

TerminalTerminal

Transparent Node

MUX

Wavelength-InsensitivePower Demultiplexing

DEMUX


Nonetheless, tunability offers a solutionto the increasingly difficult problem ofinventory stock and sparing for DWDMtransmitters and receivers used in opaquenetworks capable of carrying more than100 wavelengths per fiber. In case ofhardware failure, a complete inventory offixed-wavelength transmitters andreceivers must be carried at numerouslocations throughout the country so thatspares can be deployed into the network.Widely tunable lasers and filters offer acost-effective solution to this problem byallowing the creation of universal linecards that can be dynamically tuned tooperate at any one of 100 wavelengths.

Tunable transceivers have to meet orexceed the rigorous telecom performancedemands made on fixed-wavelengthtransceivers—and at comparable costs.Today, DWDM channel spacing, whichhas already migrated from 100 GHz to 50 GHz, is moving toward 25 GHz. Long-haul systems are now being designed totake advantage of both the C-band(~1530-1565 nm) and the L-band(~1570-1610 nm) with 100 channels ineach band. Long-distance, transparenttransmission over distances greater than3000 km requires tight specifications onlaser parameters (linewidth, frequencystability and output power) and on filterparameters (pass bandwidth, loss anddispersion). Metro systems are now beingdesigned with 32 wavelengths, a reach ofseveral hundred kilometers, and datarates of 2.5 Gbits/s per channel. Althoughmetro systems use very low-cost directlymodulated lasers, they can still benefitfrom tunable lasers and filters that reduceinventory costs and enable agile opticalnetworking to handle unstable demandpatterns.3 Because network reconfigura-tions are infrequent on a 1-s timescale,acceptable network performance can beachieved by use of lasers and filters withtuning times in the tens of millisecondsrange, within the requirements ofSONET restoration. These tuning timesare difficult for thermally tuned devices,but are achievable using MEMS devices.Telecom components must also meetTelcordia reliability criteria and be capa-ble of operating in a variety of environ-mental conditions.

The widely tunable laser technologiesavailable for telecommunications appli-

cations include: MEMS tunable external-cavity diode lasers (MEMS-ECL)4,5;distributed Bragg reflectors (DBR)6-8;temperature-tuned distributed-feedbacklasers (DFB)9,10; and MEMS tunable verti-cal-cavity surface-emitting lasers(VCSELs).11 The technologies employedfor widely tunable filters include MEMSdiffraction-grating filters12 and fiber-Bragg-grating,13 thin-film-dielectric14,15

and fiber-Fabry-Perot filters.16 Each ofthese technologies has advantages anddisadvantages. Yet the fact that multiplevendors are bringing a variety of compo-nents to market has enabled the deploy-ment of tunable devices to begin.

MEMS-based external-cavity diodelasers and diffraction-grating filters offeroptical performance and environmentalstability rivaling that of high-performancefixed-wavelength DFB lasers and fiber-Bragg-grating or thin-film dielectric fil-ters. These devices support full C- or L-band tuning at 25, 50 or 100 GHz chan-nel spacings, with accurate wavelengthcontrol over the required environmentalconditions. MEMS external-cavity diodelasers offer high power, high side modesuppression, narrow linewidth and lowrelative intensity noise (RIN); for use incost-sensitive metro networks, they canalso be directly modulated at 2.5 Gbits/s.MEMS diffraction-grating filters offernarrow bandwidths for high channel iso-lation, as well as low insertion loss, low

polarization dependent loss (PDL) andlow dispersion. The manufacturing costsof MEMS-based agile optical componentsare on par with the scalable cost struc-tures of their fixed-wavelength counter-parts, so that direct replacement offixed-wavelength devices can be justifiedpurely on the basis of reduced inventoryand sparing. This means that dynamicoptical networking with photonic layerintelligence comes almost for free.

MEMS tunable external-cavity diode lasersThe tunable laser shown in Fig. 2 is aLittman/Metcalf external-cavity diodelaser17,18 tuned by a silicon MEMS actua-tor.4,5 The gain medium is based on ahigh-power, 1.55-�m InGaAsP/InP mul-tiple-quantum-well laser diode.Reflections at the intracavity facet of thelaser diode are suppressed by use of a lowreflectance (< 2 � 10-3) antireflectivecoating in combination with an angledfacet; this makes possible an effectivefacet reflectance of less than 10-4. The lowintracavity facet reflectance enabled by anangled facet is highly stable over the life-time of the device. The laser output beamis collimated at the opposite diode facet,which serves as the output coupler of theresonator. Light emerging from the intra-cavity diode facet is collimated by adiffraction-limited, micro-optical lens


GlossaryAgile optical network—An optical network in which data traffic can be dynamically routed by changing either the tunable transmitter or receiver wavelength or the optical switch configuration.

Dynamic wavelength provisioning/routing—Remote provisioning or routing of traffic by changing the tunable transmitter or receiver wavelength in an agile optical network.

Multiwavelength sparing—Use of a single tunable transceiver as a spare to replace many different fixed-wavelength transceivers in case of hardware failure.

Opaque network—Optical links connected through a mesh of electrical switches,where the optical signals are detected, converted to electrical, and retransmittedas optical (OEO).

Provisioning—Deployment/allocation of bandwidth into an area of the network.

Traffic churn—Changing bandwidth demand patterns in a telecom network.

Transparent network—Optical links connected through optically transparentnodes without OEO conversion.

Universal line card—Hardware with transceivers that can be remotely controlledto operate at any one of many optical telecommunications wavelengths.

of a new class of devices fortelecommunications systems.21

Many of the large arrays of opticalswitches have been built by use ofsurface micromachining, the tech-nique whereby sensors and actua-tors are made from patterning thinfilms, such as polysilicon, on thesurface of a silicon substrate andsacrificially etching another mate-rial, such as silicon dioxide, to freethe mechanical structure. Mostaccelerometers for deploying airbags are now made in this way.Actuators made by use of this tech-nique are poorly suited for translat-ing or rotating relatively large,externally fabricated elements—such as mirrors or lenses—because

their actuator force is low. For these pur-poses, higher force actuators made bydeep-reactive-ion etching (DRIE) of a silicon substrate are preferred.19,20

Although thermal actuation and mag-netic actuation are used for some devices,electrostatic actuation is still preferred.Devices based on electrostatic actuatorscan be accurately positioned and easilydriven with up to 150 V at very low current.

The actuator fabrication process is rel-atively simple, requiring only five masks(see Fig. 3). First, alignment marks areformed on the front and back surface ofan oxidized carrier wafer. Shallow cavitiesare then plasma etched into the silicon onthe front surface. These cavities willdefine what portions of the actuators willbe free to move and which will beattached to the carrier. A second wafer isfusion bonded to this front surface andthen ground and polished to the desiredactuator device thickness, in this case 85 �m. The alignment marks from thebottom surface are then transferred tothe new top surface to allow alignmentwith the now enclosed cavities in thedevice. After oxidation, contact-hole etch,and metallization deposition and pat-terning, the DRIE etch is performedthrough the thickness of the top waferuntil it intersects the cavity. Those por-tions of the device that are above the cav-ities are then suspended, typically bynarrow flexural elements, to parts of thedevice still fusion bonded to the carrierwafer. Electrical connections to the mov-



and then diffracted at grazing inci-dence from a high-efficiency, free-space diffraction grating. The firstdiffracted order travels to an externalmirror mounted out-of-plane on arotary silicon microactuator.Wavelength tuning is achieved byapplying a voltage to the microactu-ator, which rotates the mirror toallow a specific diffracted wavelengthto couple back into the laser diode.The actual wavelength of the laseroutput is determined by the gainbandwidth of the diode, the gratingdispersion and the external-cavitymode structure. The laser-diode gainbandwidth is greater than 80 nm,while the external-cavity mode spac-ing is only 0.16 nm. For this reason, alarge number of external-cavity modesare supported by the diode-gain medium,although the narrow spectral passband ofthe diffraction-grating filter introducesmore than 1 dB loss at the adjacent sidemodes. This spectral filter works in com-bination with gain-saturation mecha-nisms to suppress the adjacent side modesand preferentially select a single external-cavity mode. Design criteria, including aspectrally narrow grating-filter passband,a high external-cavity feedback level, lowintracavity diode-facet reflectance and awell-confined single-spatial-mode diodewaveguide are all key to ensuring stable,single-mode laser operation.18

The laser is tuned by applying voltageto the comb elements of the MEMS actu-ator to produce an electrostatic force thatrotates the mirror about its virtualpivot.19,20 The angular range of the actua-tor determines the tuning range; a varietyof 150-V actuators, with ranges of up to ± 2.8 degrees, have been used to tune overa wavelength range of up to 42 nm. Themirror’s geometrical pivot can bedesigned to adjust the cavity lengthbecause the laser tunes in a way thatmaintains constant cavity phase at allwavelengths across the tuning range.The laser wavelength determined by thediffraction angle then scans syn-chronously with the grating-filter pass-band, and the laser tunes continuouslywithout mode hops. The pivot point canalso be designed to provide for a limitedcontinuous tuning range between mode

Figure 2. Schematic drawing of the packaged MEMS-ECL-WLL, showing a scanning electron microscope(SEM) image of the MEMS-ECL on the left and the iso-lator, WLL, shutter/VOA and fiber coupling on the right.

DiffractionGrating

Isolator

Beamsplitter

WavelengthLocker

PM FiberPigtail

Shutter/VOA

Lenses

SiliconMirrorMEMS

Actuator

LaserDiode

hops by allowing the cavity phase tochange by several wavelengths across thefull tuning range. In this case, adding anindependent cavity-length control actua-tor can optimize performance at anychannel by adjusting the laser frequencywith respect to the grating-filter pass-band. The actuator voltage determines thelaser frequency with an open-loop accu-racy of approximately 10 GHz, and thefrequency is then stabilized to ± 1.25 GHzby use of the error signal from a wave-length locker (WLL) etalon in a servo thatadjusts the mirror position.

The collimated output beam from theECL, with optical power of up to 70 mW,passes through an isolator and is coupledinto a polarization-maintaining (PM)fiber pigtail with 65% coupling efficiency.A beamsplitter reflects 5% of the lightinto an integrated wavelength locker. TheMEMS shutter is used to block the outputfor dark tuning; with suitable externalfeedback, it can also be used as a voltage-controlled attenuator (VOA). TheMEMS-ECL-WLL is assembled on aceramic substrate, bonded to a thermo-electric cooler (TEC), and packaged, asshown in Fig. 2, in a hermetic, 18-pin but-terfly package. The result is a compact,environmentally robust, high-perfor-mance tunable laser that is suitable fornetwork applications.

MEMS design and fabricationThe availability of high-performanceMEMS actuators has led to the creation


ing elements are made through theconductive silicon flexures. Electricalinsulation is provided by a combina-tion of the oxide layer between themoving or fixed parts of the deviceand the carrier wafer, and by etchedtrenches that can surround devicefeatures.

Tunable laser assemblyThe network tunable laser modulesavailable today all use the same fun-damental package structure, com-bining a tunable laser, free-spaceisolator and wavelength locker on aTEC in a butterfly package with aPM fiber pigtail and drive electron-ics. Some tunable laser modules alsoincorporate a semiconductor opticalamplifier (SOA) to reach 20-mWoutput power.6 Tunable laser trans-mitters combine a tunable lasermodule with an external Mach-Zhender or an integrated electroab-sorption (EA) modulator for10-Gbits/s data rates, and use eitheran EA modulator or direct modulation ofthe laser diode current for metro applica-tions at 2.5 Gbits/s.

The MEMS-ECL takes advantage ofdramatically reduced cost and complexityat the laser-diode level compared to othertunable laser technologies. The MEMS-ECL is based on a simple, high-powerFabry-Perot laser diode that is readilyavailable from a variety of vendors. Theother cornerstone of the MEMS-ECL isthe silicon MEMS actuator. With onlyfive mask levels, standard silicon process-ing, hundreds of devices per wafer andhigh yield, these actuators can be fabri-cated at low cost. For this reason, thechallenge of MEMS-ECL fabricationresides not at the chip level but at theassembly level, where high-level precisionautomation is essential. The laser assem-bly process relies on automated roboticvision systems and optical alignment sys-tems for accurate component placementand attachment, resulting in highthroughput and yield, which translatesinto low manufacturing costs.

Tunable laser performanceThe optical performance of the MEMS-ECL matches or exceeds that of the fixed-wavelength distributed feedback (DFB)

lasers currently being used in metro andlong-haul systems. The MEMS-ECLtunes over 42 nm in the C or L band,with fiber-coupled output powers of upto 40 mW. Figure 4 shows the laser powervariation and frequency deviation for alaser locked sequentially to 100 L-bandchannels spaced by 50 GHz. Similar performance is obtained with channelspacings of 25, 50 or 100 GHz. TheMEMS-ECL has a side-mode suppressionratio (SMSR) of more than 55 dB, a RINof less than -155 dB/Hz from 10 MHz to22 GHz, a polarization extinction ratiobetter than 20 dB and a spontaneousemission background of less than -50 dBc/nm. Due to its longer cavitylength, the MEMS-ECL has a narrowerlinewidth than a DFB, with a 3-dBinstantaneous linewidth of 125 kHzinferred from phase-noise spectral den-sity measurements. Submegahertz 1/fand actuator-motion-induced phasenoise produces a time-averaged effectivelinewidth of 2 MHz, measured by a 25-�sdelayed homodyne measurement. Aswith a DFB, this linewidth can be broad-ened for suppression of stimulatedBrillouin scattering (SBS) by adding asinusoidal dither to the laser current. A200-kHz current dither, giving a 3%

peak-to-peak intensity modulation,for example, produces a linewidth of200 MHz. The linewidth can also bereduced to near the instantaneousvalue by use of a current servo tolock the laser to an external etalon.The MEMS-ECL can be directlymodulated with low chirp at datarates of up to 2.7 Gbits/s by use ofan appropriate low-capacitance laserdiode. Direct modulation at up to2.7 Gbits/s provides a cost-effectivetunable transmitter solution formetro applications. The laser can betuned to another channel and lockedwithin 15 ms. Figure 5 shows a side-by-side comparison of the MEMS-ECL and a high-performance,fixed-wavelength DFB in an exter-nally modulated, 40-Gbits/s, 500-kmamplified link. The MEMS-ECL andDFB exhibit nearly identical bit-error ratio (BER) performance.Similar results have been obtained at2.5 and 10 Gbits/s, and with ampli-fied link lengths up to 3200 km.

Environmental performance and reliabilityThermal and vibrational sensitivities areimportant issues in a telecom tunablelaser. In the MEMS-ECL, they areaddressed by a combination of thermo-mechanical design and servo control.Mounting the ECL and WLL componentson a TEC eliminates the majority of ther-mal issues. Figure 4 shows how TEC ther-mal control of the ECL and WLL, incombination with active servo control ofthe MEMS mirror position and cavityphase, stabilizes channel-locking perfor-mance. The laser power stability (± 0.1dB) and frequency stability (± 0.5 GHz)are shown as the laser is randomly locked1500 times between 100 channels whilethe case temperature varies from -10 to70° C. Accelerated aging tests at 70° Ccase temperature also show excellentpower and frequency stability while thelaser is locked to a single channel for over 2000 hours. Coupling to external shockand vibration is minimized using a zero-moment balanced MEMS actuator. Theuse of active servos to position the tuningmirror allows the device to operate underrigorous shock and vibration conditionsfar exceeding the combined European


Figure 3. (a) Scanning electron microscope (SEM)image and (b) cross-sectional view of the fabricationprocess for the DRIE-MEMS actuators used to tunethe lasers and filters.

(a)

(b)

Etch shallow cavity in carrier wafer to createfuture movable areas.

Fusion bond devicewafer to carrier, polish tofinal thickness.

Oxidize; open contactholes; deposit and pat-tern pad metal.

DRIE etch throughdevice wafer.

Telecommunications Standards Institute(ETSI) and Telcordia specifications forearthquake, office and equipment rackenvironments, with a maximum fre-quency deviation of less than 2.5 GHz.5

Reliability of the MEMS-ECL has beenproven through over 150,000 devicehours in Telcordia testing.

MEMS tunable diffraction-grating filterThe tunable MEMS-diffraction gratingfilter shown in Fig. 6 is analogous to theMEMS-ECL tunable laser, but withoutthe gain medium. The tunable filter is aminiature monochromator based on afree-space Littrow diffraction grating, abeam expander and a silicon MEMSactuator. Optical devices using MEMSactuators work best with relatively low-mass moving parts, such as silicon mir-rors, to optimize speed and range ofmotion. While traditional Littrowmonochromators tune by grating rota-tion, the MEMS-grating filter—as in thecase of the tunable laser—tunes byreflecting light from a rotating siliconmicromirror to change the beam inci-dence angle at the grating. The optics aredesigned to maintain a constant filterbandwidth as the mirror rotates acrossthe tuning range. High-efficiency diffrac-tion gratings, optimized for a single



Figure 5. BER versus received optical powerand eye diagram for single-channel transmis-sion at 40 Gbits/s in an amplified 500-km link.The MEMS-ECL is compared to a fixed-wave-length DFB. [Courtesy Lucent Technologies].

Figure 4. Laser power (upper curves a-c) and frequency (lower curves d-f) of the MEMS-ECLare stabilized by a combination of thermomechanical design and servo control. Powerdeviation of less than +/- 0.1 dB and frequency deviation of less than +/- 0.5 GHz areshown in (a,d) versus laser frequency as the laser is tuned and locked sequentially across100 channels; in (b,e) versus case temperature from -10 to 70° C as the laser performs1500 random channel switches; and in (c,f) over a 2000-h accelerated aging test at 70° Ccase temperature with the laser locked to a single channel.

Laser Frequency Case Temperature Time(THz) (C) (hours)

(a) (b) (c)

(d) (e) (f)

Freq

uenc

yD

evia

tion

(GH

z)Po

wer

(dB

m)

Sequential locking Random locking Locked to singleto 100 channels to 100 channels channel at 70°C

13.2

13.1

13.0

12.9

12.81.0

0.5

0.0

-0.5

-1.0187 188 189 190 191 -10 0 10 20 30 40 50 60 70 0 500 1000 1500 2000

Received Optical Power (dBm)

MEMS-ECLDFB

BER

-4

-5

-6

-7

-8

-9

-10

-11-36 -34 -32 -30 -28 -26

polarization state, have inherently highpolarization-dependent loss (PDL). Theinsertion loss and PDL of the MEMS-grating filter are minimized using acopackaged micro-optical circulator toimpart light of a single polarization stateon the grating.

In the tunable filter schematic ofFig. 6, the collimated beam from a single-mode input fiber passes through a polar-ization recovery module (PRM), reflectsfrom a mirror mounted on a MEMSactuator and diffracts from a 95% effi-ciency, Littrow-mounted diffraction grat-ing. The PRM is a micro-opticalcirculator consisting of a polarizingbeamsplitter, Faraday rotator and twohalf-wave plates. The PRM minimizespolarization-dependent loss (PDL) andpolarization-mode dispersion (PMD) inthe filter by converting the randomlypolarized input light into p-polarizedlight incident at the grating, and then cir-culating the return light to the outputfiber. The Gaussian output fiber mode isthe wavelength-selective aperture, result-ing in a Gaussian filter passband set bycoupling loss versus lateral beam shift atthe output fiber. The filter center wave-length is selected by the diffracted wave-length that returns to the center of theoutput fiber. The filter bandwidth is gov-erned by the grating period and the pro-

jected beam size, as determined by thegrating incident angle and the prismmagnification. Different bandwidths areobtained by changing prisms, and theangular dependence of the prism magni-fication is such that the beam sizeremains constant at the grating even asthe incident angle changes with tuning. Aconstant filter bandwidth is maintainedacross the entire tuning range. The filtercenter wavelength is tuned over 40 nm byapplying ± 140 V to the comb elementsof the MEMS actuator to produce anelectrostatic force that rotates the mirror± 1.8 degrees about its center pivot.22

Similar to the tunable laser, the micro-optical filter assembly and MEMS actua-tor are assembled on a ceramic substrateand packaged in a hermetic, 18-pin but-terfly package with dual single-modefiber pigtails. The tunable filter packagedesign can be extended to create a tun-able receiver module in which the outputbeam is coupled into an integrated spatialfilter and detector, mounted on the sameceramic platform.

Tunable filter performanceThe MEMS-grating filter offers severalperformance advantages compared toother widely tunable filter technologies.The filter supports full C or L band tun-ing and narrow bandwidths for high


channel isolation at 25, 50, or 100 GHzchannel spacing with a wide range ofbandwidths achievable using the sameoptical components. The filter has lowinsertion loss and PDL, low dispersion(enabling use at 10-Gbits/s data rates)and accurate wavelength control.

Figure 7(a) shows the Gaussian filterpassband at a single C-band channel,with 32 GHz -3 dB bandwidth, 76 GHz -20 dB bandwidth and 1.3-dB insertionloss. The measured 50 GHz adjacentchannel isolation is 30 dB, and the non-adjacent channel isolation is greater than40 dB. Figure 7(b) shows the PDL of lessthan 0.2 dB within ± 20 GHz of the filtercenter frequency. Dispersion in the grat-ing filter is governed by the free-spaceoptical path difference between inputpolarization states (PMD) and wave-lengths (CD). The grating tunable filterhas PMD of less than 0.2 ps, chromaticdispersion (CD) of less than ± 20 ps/nm,and chromatic dispersion ripple of lessthan 10 ps/nm on all channels across the40-nm tuning range.

Figure 7(c) shows the filter tuningover 100 C-band channels spaced by 50 GHz. Figure 7(d) shows a constant 32 GHz -3dB filter bandwidth across the40-nm tuning range. With appropriateselection of the prism beam expansion,-3 dB filter bandwidths ranging from 16

to 64 GHz can be achieved to support 25,50 and 100 GHz channel spacings usingthe same components. The Gaussian fil-ter bandwidth can be similarly optimizedto enable optically matched filtering forenhanced optical signal-to-noise ratio(OSNR)23 under various modulation for-mats and data rates, provided the filtercenter wavelength is accurately con-trolled. The actuator voltage determinesthe filter center wavelength, which can bestabilized to within ± 5 GHz by use of theerror signal from a position sensor in aservo that adjusts mirror angle.

High accuracy wavelength tracking, towithin ± 1.25 GHz of the incoming signalwavelength, is possible by dithering thefilter mirror position to maximize trans-mitted output power. Adaptive wave-length tracking to the incoming signal, incombination with a matched Gaussianfilter passband as narrow as 16 GHz,enables improved system performance.23

The filter can be tuned to another chan-nel and wavelength-stabilized in less than 15 ms.

ConclusionWidely tunable ECLs and diffraction-grating filters based on silicon MEMSactuators offer the performance requiredin DWDM networks at costs comparableto equivalent fixed-wavelength modules.

As tunable devices approach cost paritywith fixed-wavelength devices, the eco-nomic benefits of reduced inventory andsparing alone are sufficient to justify thereplacement of fixed components withtunables in metro and long-haul net-works. This conversion, which startedwith deployment of narrowly tunableDFBs, is expanding to widely tunabledevices that enable universal line cardscapable of dynamically tuning to any oneof 100 channels. These are some of thefirst steps towards realizing all-opticalnetworks that promise to increase carrierrevenues and lower operating expensesby streamlining traffic patterns andallowing bandwidth to be dynamicallyreprovisioned as traffic patterns change.

AcknowledgmentsThe authors gratefully acknowledge theIolon development team, John D. Grade,Hal Jerman and Kevin Y. Yasumura, forthe MEMS material, John McNulty forthe reliability data, and LucentTechnologies for the BER test data.

ReferencesPlease see OPN Feature Article References,page 62.

Jill D. Berger ([email protected]) and Doug Anthon([email protected]) are with Iolon, Inc., in SanJose, California.


Figure 6. (a) Center wavelength of diffraction-grating filter selected by mirror angle, whichdetermines the wavelength of diffracted lightcoupled to output fiber. (b) Micro-optics andsilicon MEMS actuator are assembled on acommon substrate mounted in a hermeticallysealed butterfly package.

(a)

(b)

(a) (c)

(d)

(b)

Figure 7. (a) MEMS-diffraction-grating filter passband centered at 193.8 THz, with 32 GHz-3 dB bandwidth, 76 GHz -20 dB bandwidth and 1.3 dB insertion loss. A Gaussian fit to thedata is indicated by the dashed line. (b) PDL of less than 0.2 dB within ± 20 GHz of the filter center frequency. (c) Tuning the filter to 100, 50 GHz ITU channels in the C band. (d) The 32-GHz, -3-dB-filter bandwidth is constant across the 40-nm tuning range.

DiffractionGrating

Diffraction Grating

Ceramic SubstrateMEMS Actuator

MEMS Actuator

Silicon Mirror

Beam Expanders

-3 d

b BW

(G

Hz)

Inse

rtio

n Lo

ss (

dB)

Inse

rtio

n Lo

ss (

dB)

PDL

(dB)

Inse

rtio

n Lo

ss (

dB)

Frequency (THz) Frequency (THz)

PolarizationRecoveryModule

Input Fiber

Output Fiber

RotatingMirror

Polarization RecoveryModule (PRM)

0-2-4-6-8

-10-12-14-16-18-20-221.81.61.41.21.00.80.60.40.20.0

0

-2

-4

-6

-8

-10

-12

-14

-16

-18

-20

38

36

34

32

30

28

26193.76 193.76 193.76 193.76 193.76

31.6 GHz(-3 dB BW)

40 GHz (PDL < 0.2 dB)

75.6 GHz (-20 dB BW)

192 193 194 195 196


Feature Article ReferencesNote: References in OPN feature articles are published exactly as submitted by the authors.

Novel Fiber Lasers and Applications 38L.A. Zenteno and D.T. Walton

1. L.A. Zenteno, J.D. Minelly, A. Liu, A.J.G. Ellison,S.G. Crigler, D.T. Walton, D.V. Kuksenkov andM.J. Dejneka, “1W single-transverse-mode Yb-doped double-clad fibre laser at 978 nm,”Electron. Lett. 37 (13) 819-20 (2001).

2. A. Liu, J.A. Garcia, M.D. Skeldon, W. Chen andL.A. Zenteno, “High power fiber laser proto-type packaging and characterization,” unpub-lished report.

3. K. Oh, P.S. Westbrook, R.M. Atkins, P. Reyes,R.S. Windeler, W.A. Reed, T.E. Stockert, D.Brownlow and D. DiGiovanni, “UV photosensi-tive response in an antimony-doped opticalfiber,” Opt. Lett. 27 (7) 488-90 (2002).

4. J.K. Sahu, C.C. Renaud, K. Furusawa, R. Selvas,J.A. Alvarez-Cordero, D.J. Richardson and J.Nilson; “Jacketed air-clad cladding-pumpedYb-doped fibre laser with wide tuning range,”Electron. Lett. 37 (18) 1116-7 (2001).

5. A. Liu, C.M. Truesdale, L.A. Zenteno, N.Venkataraman, M.T. Gallagher, G.E. Burke andW. Chen, “Holey double-clad fiber lasers,”unpublished report.

6. M.D. Mermelstein, C. Headley, J.C. Bouteiller,P. Steinvurzel, C. Horn, K. Feder and B.J.Eggleton, “Configurable three-wavelengthRaman fiber laser for Raman amplification anddynamic gain flattening,” IEEE Photon.Technol. Lett. 13 (13) 1286-8 (2001).

7. D. Kuksenkov and K. Hoover, unpublishedwork

Tunable MEMS Devices for Optical Networks 42Jill D. Berger and Doug Anthon

1. R. Nadon, “Wavelength provisioning with anagile photonic network control plane,” inNational Fiber Optic Engineers Conference,Technical Proceedings, 2002, pp. 1377-1386.

2. S. Meyer, “Quantification of wavelength con-tention in photonic networks with reach varia-tion” in Optical Fiber CommunicationConference, OSA Technical Digest, (OpticalSociety of America, Washington, DC, 2002),paper TuG3.

3. J. Berthold, “WDM in the Metro” in EuropeanConference on Optical Communication,Technical Digest, 2002, Tutorial Session 7.

4. D. Anthon, J. D. Berger, J. Drake, S. Dutta, A.Fennema, J. D. Grade, S. Hrinya, F. Ilkov, J. H.Jerman, D. King, H. Lee, A. Tselikov, and K.Yasumura, “External cavity diode lasers tunedwith silicon MEMS” in Optical FiberCommunication Conference, OSA TechnicalDigest, (Optical Society of America,Washington, DC, 2002), paper TuO7.

5. D. Anthon, J.D. Berger, K.Cheung, A. Fennema,S. Hrinya, H. Lee, and A. Tselikov, “Frequencyand mode control of tunable external cavitysemiconductor lasers,” in Optical FiberCommunication Conference, OSA TechnicalDigest, (Optical Society of America,Washington, DC, 2003), paper MF61.

6. Y.A. Akulova, C. Schow, A. Karim, S. Nakagawa,P. Kozodoy, G.A. Fish, J. DeFranco, A. Dahl, M.Larson, T. Wipiejewski, D. Pavinski, T. Butrie,and L.A. Coldren, “Widely-TunableElectroabsorption-Modulated Sampled GratingDBR Laser Integrated with SemiconductorOptical Amplifier,” in Optical FiberCommunication Conference, OSA TechnicalDigest, (Optical Society of America,Washington, DC, 2002), paper ThV1.

7. D. A. Ackerman, J. Johnson, L. Ketelsen, J.M.Geary, W.A. Asous, F.S. Walters, J.M. Freund,M. Hybertsen, K.G. Glogovsky, C. Lentz, C.L.Reynolds, R. Bylsma, E.J. Dean, and T. Koch,“Fully Functional Wavelength SelectableDWDM Transmitter,” in IEEE/LEOS SummerTopical Meeting on Advanced SemiconductorLasers and Applications, Technical Digest,2001, paper TuA2.3.

8. G. Busico, N.D. Whitbread, P.J. Williams, D.J.Robbins, A.J. Ward, and D.C.J. Reid, “A widelytunable digital supermode DBR laser with highSMSR,” in European Conference on OpticalCommunication, Technical Digest, 2002, paper3.3.2.

9. Y. Kotaki and K. Morito, “Wavelength TunableDFB Laser Array for WDM Applications,” inEuropean Conference on OpticalCommunication, Technical Digest, 2002, paper3.3.1.

10. B. Pezeshki, E. Vail, J. Kubicky, G. Yoffe, S. Zou,P. Epp, S. Rishton, D. Ton, B. Faraji, M.Emanuel, R. King, X. Hong, M. Sherback, V.Agrawal, and T. Razazan, “12 element multi-wavelength DFB arrays for widely tunable lasermodules,” in Optical Fiber Communication

Conference, OSA Technical Digest, (OpticalSociety of America, Washington, DC, 2002),paper ThGG71.

11. P. Kner, D. Sun, J. Boucart, P. Floyd, R. Nabiev,D. Davis, W. Yuen, M. Jansen, and C.J. Chang-Hasnain, “VCSELs go the distance,” Optics andPhotonics News, March 2002, pp. 44-47.

12. J.D. Berger, F. Ilkov, D. King, A. Tselikov, and D.Anthon, “Widely tunable, narrow optical band-pass Gaussian filter using a silicon microactua-tor,” in Optical Fiber CommunicationConference, OSA Technical Digest, (OpticalSociety of America, Washington, DC, 2003),paper TuN2.

13. M.R. Fernald, T.J. Bailey, M.B. Miller, J.M.Sullivan, M.A. Davis, R.N. Brucato, M.A.Putnam, A.D. Kersey, and P.E. Sanders,“Compression-tuned Bragg grating and laser,”U.S. Patent No. US6363089, March 26, 2002.

14. JDS Uniphase product data sheet, “Voltage-Controlled Optical Filters, VCF050/100 series”from www.jdsu.com.

15. Santec product data sheet, “Optical TunableFilter OTF-655,” from www.santec.com.

16. C.M. Miller and J.W. Miller, “Temperaturecompensated fiber Fabry-Perot filters,” U.S.Patent No. US5289552, February 22, 1994.

17. M. Littman and H. Metcalf, “Spectrally narrowpulsed dye laser without beam expander,”Appl. Opt. 17, 2224-2227 (1978).

18. P. Zorabedian, “Tunable External CavitySemiconductor Lasers,” in “Tunable LaserHandbook, “ (F.J. Duarte Ed.), Academic Press,San Diego, 1995.

19. J.H. Jerman and J.D. Grade, “BalancedMicrodevice and Rotary ElectrostaticMicroactuator for use therewith”, Int. Pub. No.WO 01/43268, June 2001.

20. H. Jerman and J.D. Grade, “A Mechanicallybalanced, DRIE rotary actuator for a high-power tunable laser,” Technical Digest 2002Solid State Sensor and Actuator Workshop,Hilton Head, SC, June 2002, pp. 7-10.

21. E.L. Goldstein, L.Y. Lin, and J.A. Walker,“Lightwave Micromachines for OpticalNetworks,” Optics and Photonics News, March2001, pp. 60 -65.

22. J.D. Grade and H. Jerman, “MEMS ElectrostaticActuators for Optical Switching Applications,”in Optical Fiber Communication Conference,OSA Technical Digest, (Optical Society ofAmerica, Washington, DC, 2001), paper WX2.

23. D.O. Caplan and W.A. Atia, “A quantum-lim-ited optically-matched communication link,”in Optical Fiber Communication Conference,OSA Technical Digest, (Optical Society ofAmerica, Washington, DC, 2001), paper MM2.

REFERENCES

www.osa-opn.org

Search feature articles from January 2002

Information for advertisers

Author guidelines

OPN online

Date post:	05-Oct-2016
Category:	Documents
Upload:	doug
View:	212 times
Download:	0 times

Tunable MEMS Devices for Optical Networks

Documents