CSC/ECE 778: Optical Networks Rudra Dutta, Fall 2007 Fiber-Optical Communication and Switching.

CSC/ECE 778: Optical NetworksCSC/ECE 778: Optical NetworksRudra Dutta, Fall 2007Rudra Dutta, Fall 2007

Fiber-Optical Communication and Switching

Copyright Rudra Dutta, NCSU, Fall, 2007 2

OutlineOutline We want/need to understand effect on

networking– What components are possible, limitations

Quick overview of representative technology– Optical Connection and Power Budget– Fundamentals of Fiber Optic Transmission– Transmission Impairments and Solutions– Lasers and Photodetectors– Other Optical Components (Couplers, Filters,

Multiplexers, Switches, OADMs, Amplifiers)


Layering and Optical ServicesLayering and Optical Services Generalized protocol layering can create

complicated multi-layer networks In this context, “optical layer” is another layer

close to physical layer, but possibly implementing network semantics of its own

NetworkData Link

Physical

Optical

SONET

ATM

IP

User Apps

NetworkData Link

PhysicalPhysicalNetworkData LinkPhysical


Why Fiber?Why Fiber? Huge bandwidth: 30-50 THz Low losses (intrinsic): 0.2 db/Km Low bit error rates (BER): 10-11

Low power requirements: 100 photons/bit Immunity to electromagnetic interference (EMI) Low cross-talk Repeater-less amplification (EDFAs) Low cost, maintenance


Optical EndpointOptical Endpoint


Optical Power BudgetOptical Power Budget Finite power available at source (laser) Minimum detectable receiver power Must account for all losses between source and

receiver Optical networks are power-budget limited, not

bandwidth limited


Optical Power Budget (cont'd)Optical Power Budget (cont'd)


Wavelengths of ImportanceWavelengths of Importance


Optical FiberOptical Fiber Optical waveguide Cylindrical core surrounded by cladding (+ protective

covering)– made of same transparent material (glass, plastic)– difference is value of refractive index n = c / v

Single-mode vs. multimode fiber– single-mode: core diameter 8-12µm, link length > 2Km– multimode: core diameter 50µm, link length < 2Km

Step-index vs. graded-index fiber– step-index: refractive index constant across core diameter– graded-index: refractive index varies along core diameter


Refractive Index ProfilesRefractive Index Profiles


Geometric Optics: Snell's LawGeometric Optics: Snell's Law

n1 sin i = n2 sin t


Geometric Optics: Total ReflectionGeometric Optics: Total Reflection

Critical angle: c = sin-1 (n2 ÷ n1)


Maximum Cone of AcceptanceMaximum Cone of Acceptance


Transmitter-to-Fiber CouplingTransmitter-to-Fiber Coupling


Modes: The Wave PictureModes: The Wave Picture


Allowed Ray AnglesAllowed Ray Angles

Only allowed ray angles result in guided modes AB = d sin m = m /2 leads to half wavelength in the

core– m : integer, : optical wavelength in the core

Mode: one possible path that a guided ray can take


Transmission ImpairmentsTransmission Impairments Factors affecting transmission distance and bandwidth:

– attenuation– dispersion– non-linear effects

Must minimize their effects for high performance– improvement and redesign of fiber itself– compensating for these factors

Attenuation problem solved dispersion effects significant

Dispersion effects reduced non-linear effects dominant


AttenuationAttenuation Decrease in optical power along the length of

the fiber Varies with wavelength Attenuation coefficient: adB = - 10/L log10 (PR÷PT)

(dB/Km)– L : length of fiber

– PT : power launched into the fiber

– PR : power received at end of fiber


Power LossesPower Losses Material absorption: due to

– resonances of silica molecules– impurities -- most serious is peak at 1390 nm due to OH ions

Rayleigh scattering: medium is not absolutely uniform– refractive index fluctuates light is scattered– scattering proportional to -4 dominant at < 800 nm

Waveguide imperfections: relatively small component– nonideal fiber geometries– due to bending, manufacturing imperfections


Low Loss Region of An Optical FiberLow Loss Region of An Optical Fiber


Erbium-Doped Fiber AmplifiersErbium-Doped Fiber Amplifiers


EDFA Principle of OperationEDFA Principle of Operation

Ei : energy level

Ni : population of erbium ions at energy level Ei

– normally (no pump/signal): N1 > N2 > N3

– pump/signal present: population inversion N2 > N1


EDFA PropertiesEDFA Properties Emission:

– stimulated amplification– spontaneous noise amplified spontaneous

emission limit on number of EDFAs along the fiber Energy levels are narrow bands each

transition associated w/ a band of wavelengths amplify wide band around 1550nm

Replace expensive and complicated electronic units

Signal remains in optical form transparency “Distributed” amplifiers


Semiconductor Optical Amplifiers (SOAs)Semiconductor Optical Amplifiers (SOAs) Similar to semiconductor laser Consist of active medium (p-n junction) Energy levels of electrons confined to 2 bands

EDFA E1, E2

Mobile carriers (holes, electrons) play the role of erbium ions

Has several disadvantages compared to EDFAs Useful when combined with other components

into optoelectronic integrated circuits (OEICs)– preamplifier in optical receiver– power amplifier in optical transmitter


DispersionDispersion

A narrow pulse spreads out as it propagates along the fiber

Intersymbol interference:– pulse overlaps neighboring pulses– sharply increases the BER

Dispersion imposes a limit on the bit rate that can be supported

Intermodal vs. chromatic dispersion


Intermodal DispersionIntermodal Dispersion Most serious form of dispersion Occurs in multimode fibers Different modes of a wavelength travel at

different speeds Multimode fibers limited to low bitrate-distance

products Solutions:

– use single-mode fibers for large bitrate-distance products(8 µm < 2a < 10 µm only one mode is guided)

– use graded-index fibers


Graded Index FibersGraded Index Fibers


Propagation in Graded Index FibersPropagation in Graded Index Fibers

Rays are bent as they approach the cladding

Rays further from core travel faster (due to lower n)

Intermodal dispersion reduced by several orders of magnitude


Chromatic DispersionChromatic Dispersion Two sources of chromatic dispersion:

– material dispersion, DM

– waveguide dispersion, DW

Chromatic dispersion: D = DM + DW


Material DispersionMaterial Dispersion The physical effect that allows raindrops to form

rainbow Refractive index of a material changes with

wavelength different wavelengths travel at different speeds along the fiber

Different delays cause spreading of output pulse, depending on:– wavelength span of source– length of fiber


Waveguide DispersionWaveguide Dispersion DW is a function of fiber geometry

Dispersion-shifted fibers:– DW causes zero-dispersion point to shift to 1550 nm

range– min dispersion range coincides with min loss range

Dispersion-flattened fibers: dispersion profile close to zero for a wide spectral range


Dispersion Profile of Single-Mode FiberDispersion Profile of Single-Mode Fiber


Non-Linear EffectsNon-Linear Effects Stimulating Raman Scattering (SRS):

– light interacts with fiber medium inelastic collisions– not important in single-channel systems (thresh. about 500mW)– involves transfer of power: hi freq. wave lo freq. wave– introduces cross-talk in multiwavelength systems

Stimulating Brillouin Scattering (SBS):– no cross-talk, low threshold power (few mW for 20-Km fiber)

Four-Wave Mixing– three signals present at neighboring freq: f1, f2, f3

– new signal produced, e.g., f4 = f1 + f2 - f3


SolitonsSolitons

Distortion, non-linearities: distort, broaden a propagating pulse

Right combination of distortion, non-linearity:– compensate each other– produce a narrow, stable pulse (soliton)– solitons travel over long distances without any distortion– solitons in opposite directions pass thru transparently

Ideal situation for long-distance communication EDFAs needed to maintain solitons over long distances


Lasers Lasers Light amplification by stimulated emission of

radiation Schawlow and Townes, 1958 First solid-state laser by Maiman, 1960 Today, lasers exist in myriad forms


Semiconductor Energy State DiagramsSemiconductor Energy State Diagrams


Fabry-Perot CavityFabry-Perot Cavity

Part of light leaves cavity through right facet, part is reflected

Resonant wavelengths: L = m /2


Single-Wavelength OperationSingle-Wavelength Operation FP laser cavity supports many

modes/wavelengths of operation Monochromatic light needed for high bitrate-

distance products Geometry is modified to achieve single-

wavelength operation Distributed Bragg Reflector (DBR) lasers Distributed Feedback (DFB) lasers Expensive, widely used in long-distance

communication


TunabilityTunability Laser tunability important in WDM network

applications:– slow tunability (ms range): set up lightpaths in

wavelength routing networks– fast tunability (µs or ns range): multiple access (T-

WDMA) applications


Tunability (cont'd)Tunability (cont'd) Mechanically tuned: change FP cavity length

– (tuning range: 10-20 nm, tuning time: 100-500 ms)

Injection current tuned: change refr. index in DFB/DBR lasers– (tuning range: 4 nm, tuning time: 10s of ns)

Multiwavelength laser arrays– built in single chip– one or more lasers can be activated simultaneously– light from each laser fed to star coupler


Optical ReceiversOptical Receivers


PhotodetectorsPhotodetectors


FiltersFilters Various technologies:

– Fabry-Perot filters– Multilayer interference (MI) filters– Mach-Zehnder interferometers– Arrayed waveguide grating– Acousto-optic tunable filter

Tunability important Can be used as MUX/DEMUX, wavelength

routers


MI FiltersMI Filters

Bandpass filter

Passes thru particular wavelength, reflects all other

Cascade multiple filters to create a MUX/DEMUX


MI Filters as MUX/DEMUXMI Filters as MUX/DEMUX


MUX/DEMUX: Logical ViewMUX/DEMUX: Logical View


Directional CouplersDirectional Couplers

Coupling possible when waveguides placed close together

Coupling ratio controlled by voltage


Couplers: Logical ViewCouplers: Logical View

P1’ = a11 P1 + a12 P2, P2’ = a21 P1 + a22 P2

For ideal symmetric couplers:

a11 = a22 = a, a12 = a21 = 1-a


CouplersCouplers Star Coupler:

– a = 1/2, 2x2 star coupler (3-dB coupler)– Cascade 2x2 couplers to build NxN star coupler

Power Splitter:– P2 = 0, a = 1/2

Switches:– a = 0,1; 2x2 switch– cascade 2x2 switches to build NxN switch

Real devices are lossy:– a11 + a12 < 1, a21 + a22 < 1


Internal Structure of Star CouplerInternal Structure of Star Coupler


GratingsGratings


Gratings: Principle of OperationGratings: Principle of Operation Multiple narrow slits spaced equally apart on the

grating plane Light incident on one side of grating transmitted

through slits Diffraction: light through each slit spreads out in

all directions Different s interfere constructively at different

points of imaging plane separate WDM signal into constituent wavelengths


Bragg GratingsBragg Gratings Bragg grating: any periodic pertrubation in

propagating medium Perturbation is usually periodic variation of

refractive index Bragg gratings used in many photonic devices:

– DBR lasers: Bragg gratings written in waveguides– Fiber Bragg gratings (FBG): written in fiber– Acousto-optic tunable filters: Bragg grating formed by

propagation of an acoustic wave in the medium


FBG as Add-Drop MultiplexersFBG as Add-Drop Multiplexers


OADM: Logical ViewOADM: Logical View


Optical SwitchesOptical Switches Mechanical switches

– directional couplers, ratio modified by bending (ms range)– MEMS mirrors moved in and out of path (100s of ns range)

Bubble-Based switches– bubbles in optical fluid reflect beam (10s of ms range)

Electro-Optic switches– couplers, ratio modified by changing refr. index (ns range)

Thermo-Optic switches– refractive index function of temperature (ms range)

Semiconductor Optical Amplifier (SOA) switches– SOA, change in voltage to use as on-off switch (ns range)


MEMS Optical SwitchingMEMS Optical Switching MEMS = micro-electro-mechanical system Movable mirrors to reflect light 2D MEMS: a 2-state pop-up MEMS mirror

– state ``0'': popped up position light reflected– state ``1'': flat (folded) position light passes through


2D MEMS Switches2D MEMS Switches


Analog Beam-Steering MirrorAnalog Beam-Steering Mirror

Mirror can be freely rotated on two axes to reflect a light beam


3D MEMS Switch3D MEMS Switch


Static Optical SwitchesStatic Optical Switches


Reconfigurable Optical SwitchesReconfigurable Optical Switches


Wavelength ConvertersWavelength Converters


Spectrum PartitioningSpectrum Partitioning c = f, f - c/2 100 Ghz is about .8 nm at 1,550 nm range 10-Ghz spacing:

– very dense by current standards– can accommodate 1 Gbps digital bit rates– can accomodate 1 Ghz analog bandwidths– OK for receivers, but too close for wavelength routing

100 Ghz spacing OK for optical switches– WDM limit today

Waveband routing alleviates throughput loss– But better switching technology nullifies advantage– However, continue to be useful because needs “coarser” filters


Spectrum Partitioning (cont'd)Spectrum Partitioning (cont'd)


Waveband vs. WavelengthWaveband vs. Wavelength

CSC/ECE 778: Optical Networks Rudra Dutta, Fall 2007 Fiber-Optical Communication and Switching.

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Transcript of CSC/ECE 778: Optical Networks Rudra Dutta, Fall 2007 Fiber-Optical Communication and Switching.