Enabling Devices using Enabling Devices using MicroElectroMechanicalMicroElectroMechanical System (MEMS) System (MEMS)

Technology for Optical NetworkingTechnology for Optical Networking

Dan MaromDan MaromApplied Physics DepartmentApplied Physics DepartmentThe Selim and Rachel Benin School of Engineering The Selim and Rachel Benin School of Engineering and Computer Scienceand Computer ScienceHebrew UniversityHebrew Universityhttp://http://aph.huji.ac.il/maromaph.huji.ac.il/marom

December 17, 2007Workshop on Optical Communications

Tel Aviv University

22

OutlineOutline

From Optical Communication Systems to Optical NetworkingFrom Optical Communication Systems to Optical NetworkingExemplary Optical MEMS Devices for Optical NetworkingExemplary Optical MEMS Devices for Optical Networking

Switching WDM Channels in Optical Mesh NetworksSwitching WDM Channels in Optical Mesh NetworksWavelengthWavelength--Insensitive SwitchingInsensitive SwitchingWavelengthWavelength--Selective Switches (WSS) Selective Switches (WSS) Hybrid FreeHybrid Free--Space & GuidedSpace & Guided--Wave WSSWave WSS

Chromatic Dispersion Trimming for Performance EnhancementChromatic Dispersion Trimming for Performance EnhancementConclusionsConclusions

33

Optical Networking Optical Networking –– The Old ViewThe Old View

Traffic demand is met by optical networks built with highTraffic demand is met by optical networks built with high--capacity capacity WDM line systems, optical add/drop nodes, and digital crossWDM line systems, optical add/drop nodes, and digital cross--connects.connects.

Disadvantages:Disadvantages:High High OpExOpEx..Long provisioning time.Long provisioning time.Many O/E/O conversions.Many O/E/O conversions. Inventory.Inventory.

44

Optical Networking Optical Networking –– The New ViewThe New View

Transparent optical mesh network with no end terminals.Transparent optical mesh network with no end terminals.Optical path switching and reconfigurable optical add/drop.Optical path switching and reconfigurable optical add/drop.Integrated electrical switches for add/drop interface, grooming,Integrated electrical switches for add/drop interface, grooming, and and electrical regeneration.electrical regeneration.WavelengthWavelengthservices.services.

Challenges:Challenges:Wavelength managementWavelength managementWavelength blockingWavelength blockingPerformance monitoringPerformance monitoring

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Optical crossOptical cross--connect with O/E/O conversionconnect with O/E/O conversion

N N –– number of WDM channelsnumber of WDM channels

K K –– number of I/O line systemsnumber of I/O line systems KK××NN –– switch fabric sizeswitch fabric size

Resolves the challenges of transparent optical networking (wavelResolves the challenges of transparent optical networking (wavelength ength management, blocking, and performance monitoring).management, blocking, and performance monitoring).However, solutionHowever, solutionis inefficient:is inefficient:

Lose effectiveness of Lose effectiveness of ultraultra--long haullong haultransmission.transmission.High cost due to High cost due to o/e/oo/e/oconversions.conversions.Full cost incurred onFull cost incurred ondeployment.deployment.

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Large Transparent Optical CrossLarge Transparent Optical Cross--ConnectConnect

Disadvantages:Disadvantages:Switch fabric overkill / complexity.Switch fabric overkill / complexity.DemuxDemux and and muxmux required.required.Full cost on deployment.Full cost on deployment.

Eliminate O/E/O conversions in optical crossEliminate O/E/O conversions in optical cross--connect to lower connect to lower deployment cost and introduce transparency to network.deployment cost and introduce transparency to network.

Supported switching states: Supported switching states:

((KK××NN))22

Allowable switching states:Allowable switching states:

KK××((KK××NN))

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WavelengthWavelength--Modular CrossModular Cross--Connect SolutionConnect Solution

Solution to lower deployment cost: modular crossSolution to lower deployment cost: modular cross--connect connect architecture. architecture.

WavelengthWavelength--modular crossmodular cross--connects use an array of small crossconnects use an array of small cross--connects, each dedicated to a specific WDM channel:connects, each dedicated to a specific WDM channel:

•• When using fixed channel transmitters, When using fixed channel transmitters, need to install additional switch module at need to install additional switch module at each network node on channel turn on.each network node on channel turn on.

•• Using tunable transmitters, lose Using tunable transmitters, lose advantage of wavelength routing due to advantage of wavelength routing due to partially deployed switch modules.partially deployed switch modules.

Disadvantages:Disadvantages:DemuxDemux and and muxmux required.required.Operational expenses.Operational expenses.

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MEMS Switch FabricsMEMS Switch Fabrics

InputFiberarray

OutputFiberarray

99

PortPort--Modular CrossModular Cross--Connect SolutionConnect Solution

PortPort--modular crossmodular cross--connects can be constructed using connects can be constructed using wavelengthwavelength--selective 1selective 1××KK switches. These switches distribute switches. These switches distribute the the NN WDM channels across the WDM channels across the KK output ports.output ports.

FreeFree--spaced based WSS provide spaced based WSS provide wide passbands, again facilitatingwide passbands, again facilitatingcascadabilitycascadability. .

Broadcast capability enabled.Broadcast capability enabled.

Least hardware requirement.Least hardware requirement.

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WavelengthWavelength--Selective 1Selective 1××K Switch: ArchitectureK Switch: Architecture

(1 input,K outputs)

•• Performs functions of demultiplexing, switching, and multiplexinPerforms functions of demultiplexing, switching, and multiplexing in one low loss unit.g in one low loss unit.•• Mirror tilt angle determines selected output portMirror tilt angle determines selected output port

1111

Fringe Field Activated SOI Fringe Field Activated SOI MicromirrorsMicromirrors

•• MonolithicMonolithic•• No No snapdownsnapdown•• No contactNo contact•• No No stictionstiction•• No chargingNo charging

1212

64 channel wavelength64 channel wavelength--selective 4selective 4××1 switch1 switch

1313

Switch CharacterizationSwitch CharacterizationTypical voltage response:Typical voltage response: Spectral response:Spectral response:

Losses inclusive of isolators on input ports.Losses inclusive of isolators on input ports.Switch provides switching, 10 dB dynamic Switch provides switching, 10 dB dynamic equalization range, and wavelength blocking.equalization range, and wavelength blocking.

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Mirror Actuation Options: Are They Equivalent?Mirror Actuation Options: Are They Equivalent?

MEMS mirrors tilting in the MEMS mirrors tilting in the dispersion direction (DD)dispersion direction (DD)

MEMS mirrors tilting in the MEMS mirrors tilting in the direction orthogonal to direction orthogonal to

dispersion (DOD)dispersion (DOD)

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Problems Due to Rotation in Dispersion DirectionProblems Due to Rotation in Dispersion Direction

Interchannel spikes in coupling efficiency

Dynamic equalization by detuning mirror angle, results in “Rabbit Ears”

Wavelength (nm)Tr

ansm

issi

on (d

B)

1542.5 1543 1543.5 1544 1544.5 1545

-20

-15

-10

-5 0 dB

-5 dB

-10 dB

-15 dB

Mirror edge diffraction results in coupling to adjacent channelsMirror edge diffraction results in coupling to adjacent channels and rabbit ear phenomenaand rabbit ear phenomena

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Mirror Curvature Effect on WSSMirror Curvature Effect on WSS

Δλ

Reflected beam will strike diffraction grating at a displaced position, resulting in a different optical path length.

Path length changes as a function of wavelength offset i.e., chromatic dispersion:

( )2 2

0

8 tanfD

Rcφ

λ=C

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MEMS Channelized Dispersion CompensatorMEMS Channelized Dispersion Compensator

φ

MEMS Mirrorswith variable curvature Grating

LensFoldMirror

0.2 0.4 0.6 0.8 1.0 1.20

1

2

3

R= 3.9 mm

R= -160mm

Z(

m)

X (mm)

μ

0 mA

24 mA

25 mA

26 mA

27 mA

28 mA

29 mA

30 mA--400 400 psps/nm/nm

400 400 psps/nm/nm

0 0 psps/nm/nm

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WSS + Dispersion Compensator WSS + Dispersion Compensator

An ideal device would provide not only wavelengthAn ideal device would provide not only wavelength--selective selective switching, but also add the ability to perform dispersion trimmiswitching, but also add the ability to perform dispersion trimming.ng.This required a tilting mirror with adjustable curvature; not eaThis required a tilting mirror with adjustable curvature; not easy!sy!An alternative to MEMS technology, An alternative to MEMS technology, LCoSLCoS, can perform this trick., can perform this trick.

+/+/-- 60 60 psps/nm dispersion/nm dispersion

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Is Is LCoSLCoS an Allan All--Capable MEMS Replacement?Capable MEMS Replacement?

A A LCoSLCoS device replaces a continuous mirror with a diffractive device replaces a continuous mirror with a diffractive version, comprised of many independent phaseversion, comprised of many independent phase--controlled pixels.controlled pixels.Routing to different fiber ports requires setting a phase ramp.Routing to different fiber ports requires setting a phase ramp.Dispersion compensation requires setting a quadratic phase Dispersion compensation requires setting a quadratic phase distribution. distribution.

LCoSLCoS performance is primarily challenged in diffraction efficiency performance is primarily challenged in diffraction efficiency dependence on tilt angle and crosstalk to adjacent ports.dependence on tilt angle and crosstalk to adjacent ports.

2020

Hybrid Integration of Planar Lightwave Circuits Hybrid Integration of Planar Lightwave Circuits with Freewith Free--Space OpticsSpace Optics

Optical system consists of multiple elements, increasing cost and assembly difficulty.

2121

Planar Lightwave Circuit Designed for Planar Lightwave Circuit Designed for Generating Angular DispersionGenerating Angular Dispersion

Waveguide array with length increasing by ΔL.

ΔL=m·λ0/n

Gratingorder

Designwavelength

λ0 λ < λ0λ > λ0

Accumulated phase: K·ΔL = 2πnΔL/λ = 2πmλ0/λ

Output facet phase:

2222

WavelengthWavelength--Selective 1Selective 1××2 Switch2 Switch

Port 1

Input

Port 2Cylindricalcollimators

PLC 1 and PLC 2

Fourierlens

MEMSmicro-mirrorarray

PLC’s designed for telecom C-band (1530-1560 nm), and support 40 channels at 100 GHz separation.MEMS micromirror array was reused from previous WSS, therefore mirror pitch did not match AWG parameters.

2323

Switching of WDM ChannelsSwitching of WDM Channels

Switching with bulk mirror

Switching with micro-mirror array

Port 1 signal

Port 2 signal

Switch to Port 1 Switch to Port 2

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Hybrid WavelengthHybrid Wavelength--Selective 1Selective 1××3 WSS with 3 WSS with Integrated Integrated Mux/DemuxMux/Demux FunctionalityFunctionality

-50

-45

-40

-35

-30

-25

-20

-15

-10

-5

0

1528 1533 1538 1543 1548 1553 1558 1563

Wavelength [nm ]

Tran

smis

sivi

ty [d

B]

Out 1 Out 2 Out 3

Advantages: • Less discrete components• Lower losses from input to drop• Maintain multiplexed path switching ports

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Spatially demultiplexed

spectrum

Hybrid WavelengthHybrid Wavelength--Selective 1Selective 1××3 WSS with 3 WSS with Integrated Integrated Mux/DemuxMux/Demux FunctionalityFunctionality

~125 mm

Channel demux PLC input

Spatially demultiplexedspectrum

Matched spatial dispersions

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Tunable Dispersion Compensation (TDC) with a Tunable Dispersion Compensation (TDC) with a Deformable MirrorDeformable Mirror

TDC combining PLC and deformable mirror with ±500 ps/nm tuning range for all channels on 100 GHz grid.

• Single knob tuning• Compact design• Low-power consumption

( )2 2

0

8 tanfD

Rcφ

λ=C

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Testing TDC at 42.7 Testing TDC at 42.7 Gb/sGb/s Data RateData Rate

Eye diagram of 42.7 Eye diagram of 42.7 Gb/sGb/s CSRZ signal with CSRZ signal with 425 425 psps/nm dispersion./nm dispersion.

Signal compensated.Signal compensated.

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ConclusionsConclusions

Photonic devices based on optical MEMS actuators can meet the functional requirements of optical networking.The efforts behind telecom optical MEMS in the last 8 years has resulted in significant progress in:

• MEMS device fabrication• Micro-optics• Packaging• Control

To successfully compete in the telecom device space, MEMS-based components are introducing higher degrees of integration into more compact and robust packages such as planar lightwavecircuits.