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Source: Master 7_4 WDM and DWDM Multiplexing

DWDM Study

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DWDM study

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  • Source: Master 7_4WDM and DWDM Multiplexing

  • MultiplexingMultiplexinga process where multiple analog message signals or digital data streams are combined into one signal over a shared mediumTypesTime division multiplexingFrequency division multiplexingOpticallyTime division multiplexingWavelength division multiplexing

  • 197519801985199019952000Optical FibreSDHDWDMTimeline2005CWDM2008

  • Problem: Demand for massive increases in capacity

    Immediate Solution: Dense Wavelength Division Multiplexing

    Longer term Solution: Optical Fibre NetworksProblems and Solutions

  • Wavelength Division Multiplexing

  • Dense WDM

  • Multiple channels of information carried over the same fibre, each using an individual wavelengthA communicates with X and B with Y as if a dedicated fibre is used for each signalTypically one channel utilises 1320 nm and the other 1550 nmBroad channel spacing, several hundred nmRecently WDM has become known as Coarse WDM or CWDM to distinguish it from DWDMWavelength Division MultiplexerWavelength Division Demultiplexerl1Al2Bl1Xl2Yl1 + l2FibreWDM Overview

  • Multiple channels of information carried over the same fibre, each using an individual wavelengthAttractive multiplexing techniqueHigh aggregate bit rate without high speed electronics or modulation Low dispersion penalty for aggregate bit rateVery useful for upgrades to installed fibresRealisable using commercial components, unlike OTDM Loss, crosstalk and non-linear effects are potential problems Wavelength Division MultiplexerWavelength Division Demultiplexerl1Al2l3BCl1Xl2l3YZl1 + l2 + l3FibreWDM Overview

  • Types of WDM

  • Wavelength multiplexer types include: Fibre couplersGrating multiplexersWavelength demultiplexer types include:Single mode fused taper couplersGrating demultiplexersTunable filtersGrating Multiplexer DemultiplexerGratingGrin Rod LensFibresl1l2l1 + l2WDM Multiplexers/Demultiplexers

  • WDM systems require sources at different wavelengthsIrish researchers at U.C.D. under the ACTS program are developing precision tunable laser sourcesObjective is to develop a complete module incorporating:Multisection segmented grating Distributed Bragg Reflector Laser diodeThermal and current driversControl microprocessorInterface to allow remote optical power and wavelength setting ACTS BLISS AC069 ProjectTunable Sources

  • Multiplexer Optical Output SpectrumArt O'Hare, CNET, PTL May 1996160 Gbits/s8 channels, 20 Gbits/s eachGrating multiplex/demultiplex4 nm channel spacing1533 to 1561 nm band238 km span3 optical amplifiers usedEarly DWDM: CNET 160 Gbits/sec WDM

  • Buffered Fibre on ReelsOptical TransmittersEarly DWDM: CNET WDM Experimental Setup

  • Dense Wavelength Division Multiplexing

  • Multiple channels of information carried over the same fibre, each using an individual wavelengthDense WDM is WDM utilising closely spaced channelsChannel spacing reduced to 1.6 nm and less Cost effective way of increasing capacity without replacing fibreCommercial systems available with capacities of 32 channels and upwards; > 80 Gb/s per fibreWavelength Division MultiplexerWavelength Division Demultiplexerl1Al2l3BCl1Xl2l3YZl1 + l2 + l3FibreDense Wavelength Division Multiplexing

  • Multiple channels of information carried over the same fibre, each using an individual wavelengthUnlike CWDM channels are much closer togetherTransmitter T1 communicates with Receiver R1 as if connected by a dedicated fibre as does T2 and R2 and so onWavelength Division MultiplexerWavelength Division Demultiplexerl1T1l2lNT2TNl1R1l2lNR2RNSource: Master 7_4l1 + l2 ... lNFibreSimple DWDM System

  • Multiplexer Optical Output Spectrum for an 8 DWDM channel system, showing individual channelsSource: Master 7_4Sample DWDM Signal

  • Dense WDM is WDM utilising closely spaced channelsChannel spacing reduced to 1.6 nm and less Cost effective way of increasing capacity without replacing fibreCommercial systems available with capacities of 32 channels and upwards; > 80 Gb/s per fibreAllows new optical network topologies, for example high speed metropolitian ringsOptical amplifiers are also a key componentSource: Master 7_4DWDM: Key Issues

  • 1.1 Tbits/sec total bit rate (more than 13 million telephone channels)55 wavelengths at 20 Gbits/sec each1550 nm operation over 150 km with dispersion compensationBandwidth from 1531.7 nm to 1564.07 nm (0.6 nm spacing)Terabit Transmission using DWDM

  • Expansion Options

  • Install more fibreNew fibre is expensive to install (Euro 100k + per km)Fibre routes require a right-of-wayAdditional regenerators and/or amplifiers may be requiredInstall more SDH network elements over dark fibreAdditional regenerators and/or amplifiers may be requiredMore space needed in buildingsCapacity Expansion Options (I)

  • Install higher speed SDH network elementsSpeeds above STM-16 not yet trivial to deploySTM-64 price points have not yet fallen sufficientlyNo visible expansion options beyond 10 Gbit/sMay require network redesignInstall DWDMIncremental capacity expansion to 80 Gbits/s and beyondAllows reuse of the installed equipment baseCapacity Expansion Options (II)

  • DWDM Advantages and Disadvantages

  • Greater fibre capacityEasier network expansionNo new fibre neededJust add a new wavelengthIncremental cost for a new channel is lowNo need to replace many components such as optical amplifiersDWDM systems capable of longer span lengthsTDM approach using STM-64 is more costly and more susceptible to chromatic and polarization mode dispersionCan move to STM-64 when economics improveDWDM Advantages

  • DWDM can give increases in capacity which TDM cannot matchHigher speed TDM systems are very expensiveDWDM versus TDM

  • Not cost-effective for low channel numbersFixed cost of mux/demux, transponder, other system componentsIntroduces another element, the frequency domain, to network design and managementSONET/SDH network management systems not well equipped to handle DWDM topologiesDWDM performance monitoring and protection methodologies developingDWDM Disadvantages

  • DWDM installed on a large scale first in the USA larger proportion of longer >1000km linksEarlier onset of "fibre exhaust" (saturation of capacity) in 1995-96 Market is gathering momentum in EuropeIncrease in date traffic has existing operators deploying DWDMNew entrants particularly keen to use DWDM in EuropeNeed a scaleable infrastructure to cope with demand as it growsDWDM allows incremental capacity increasesDWDM is viewed as an integral part of a market entry strategyDWDM: Commercial Issues

  • DWDM StandardsSource: Master 7_4

  • ITU Recommendation is G.692 "Optical interfaces for multichannel systems with optical amplifiers"G.692 includes a number of DWDM channel plansChannel separation set at:50, 100 and 200 GHz equivalent to approximate wavelength spacings of 0.4, 0.8 and 1.6 nmChannels lie in the range 1530.3 nm to 1567.1 nm (so-called C-Band)Newer "L-Band" exists from about 1570 nm to 1620 nmSupervisory channel also specified at 1510 nm to handle alarms and monitoringSource: Master 7_4DWDM Standards

  • Wavelength in nm120017001300140016001500S BandC BandL Band5th WindowE Band2nd WindowO BandOptical Spectral Bands

  • Optical Spectral Bands

  • Trend is toward smaller channel spacings, to incease the channel countITU channel spacings are 0.4 nm, 0.8 nm and 1.6 nm (50, 100 and 200 GHz)Proposed spacings of 0.2 nm (25 GHz) and even 0.1 nm (12.5 GHz)Requires laser sources with excellent long term wavelength stability, better than 10 pmOne target is to allow more channels in the C-band without other upgradesWavelength in nm1550155415511552155315530.8 nmChannel Spacing

  • Speed of Light assumed to be 2.99792458 x 108 m/s All Wavelengths in nm1552.521552.931553.331553.731554.131554.541554.941555.341555.751556.151556.551556.961557.361557.771558.171546.521546.921547.321547.721548.111548.511548.911549.321549.721550.121550.521550.921551.321551.721552.121540.561540.951541.351541.751542.141542.541542.941543.331543.731544.131544.531544.921545.321545.721546.121534.641535.041535.431535.821536.221536.611537.001537.401537.791538.191538.581538.981539.371539.771540.16

    1528.771529.161529.551529.941530.331530.721531.121531.511531.901532.291532.681533.071533.471533.861534.251558.581558.981559.391559.791560.201560.61So called ITU C-Band

    81 channels defined

    Another band called the L-band exists above 1565 nm ITU DWDM Channel Plan0.4 nm Spacing (50 GHz)

  • Speed of Light assumed to be 2.99792458 x 108 m/s1552.52

    1553.33

    1554.13

    1554.94

    1555.75

    1556.55

    1557.36

    1558.171546.52

    1547.32

    1548.11

    1548.91

    1549.72

    1550.52

    1551.32

    1552.121540.56

    1541.35

    1542.14

    1542.94

    1543.73

    1544.53

    1545.32

    1546.121534.64

    1535.43

    1536.22

    1537.00

    1537.79

    1538.58

    1539.37

    1540.16

    1528.77

    1529.55

    1530.33

    1531.12

    1531.90

    1532.68

    1533.47

    1534.251558.98

    1559.79

    1560.61 All Wavelengths in nmITU DWDM Channel Plan 0.8 nm Spacing (100 GHz)

  • G.692 Representation of a Standard DWDM System

  • DWDM Components

  • TransmittersDWDM MultiplexerPower AmpLine AmpLine AmpReceive Preamp200 kmDWDM DeMultiplexerEach wavelength behaves as if it has it own "virtual fibre"Optical amplifiers needed to overcome losses in mux/demux and long fibre spansReceiversOptical fibreDWDM System

  • TransmittersDWDM MultiplexerPower AmpLine AmpLine AmpOptical fibreReceive PreampDWDM DeMultiplexerReceivers

  • Passive Components:Gain equalisation filter for fibre amplifiersBragg gratings based demultiplexerArray Waveguide multiplexers/demultiplexersAdd/Drop CouplerActive Components/Subsystems:Transceivers and TranspondersDFB lasers at ITU specified wavelengthsDWDM flat Erbium Fibre amplifiersDWDM: Typical Components

  • Mux/Demuxes

  • lnlnl + lTravelling on two different paths, both waves recombine (at the summer, S)Because of the l path length difference the waves are in-phaseComplete reinforcement occurs, so-called constructive interferenceSourceSBAA + BConstructive Interference

  • lnlnl + 0.5 lTravelling on two different paths, both waves recombine (at the summer, S)Because of the 0.5l path length difference the waves are out of phaseComplete cancellation occurs, so-called destructive interferenceSourceSBAA + BDestructive Interference

  • Two different wavelengths, both travelling on two different pathsBecause of the path length difference the "Red" wavelength undergoes constructive interference while the "Green" suffers destructive interferenceOnly the Red wavelength is selected, Green is rejectednlnl + DlSourceSBAA + BUsing Interference to Select a Wavelength

  • l1 .... l5Constant path difference = DL between waveguidesWaveguidesOutput fibresInput fibreAll of the wavelengths l1 .... l5 travel along all of the waveguides. But because of the constant path difference between the waveguides a given wavelength emerges in phase only at the input to ONE output fibre. At all other output fibres destructive interference cancels out that wavelength. l1l5CouplerArray Waveguide Grating Operation: Demultiplexing

  • Array Waveguide Grating Mux/Demux

  • An Array Waveguide Demux consists of three parts : 1st star coupler, Arrayed waveguide grating with the constant path length difference 2nd star coupler. The input light radiates in the 1st star coupler and then propagates through the arrayed waveguides which act as the discrete phase shifter. In the 2nd star coupler, light beams converges into various focal positions according to the wavelength.Low loss, typically 6 dBArray Waveguide Operation

  • Typical Demux Response, with Temperature Dependence

  • DWDM Systems

  • TransmittersDWDM MultiplexerPower AmpLine AmpLine AmpReceive Preamp200 kmDWDM DeMultiplexerEach wavelength behaves as if it has it own "virtual fibre"Optical amplifiers needed to overcome losses in mux/demux and long fibre spansReceiversOptical fibreDWDM System

  • TransmittersDWDM MultiplexerPower AmpLine AmpReceive Preamp200 kmDWDM DeMultiplexerEach wavelength still behaves as if it has it own "virtual fibre"Wavelengths can be added and dropped as required at some intermediate locationReceiversAdd/Drop Mux/DemuxOptical fibreDWDM System with Add-Drop

  • Manufacturer&System

    Number of ChannelsChannel SpacingChannel SpeedsMaximum Bit RateTb/sNortel OPtera 1600 OLS160

    0.4 nm2.5 or 10 Gb/s1.6 Tbs/sLucent402.5AlcatelMarconiPLT40/80/16040/80/1600.4, 0.8 nm2.5 or 10 Gb/s1.6 Tb/sTypical DWDM Systems

  • Different systems suit national and metropolitian networksTypical high-end systems currently provide:40/80/160 channelsBit rates to 10 Gb/s with some 40 Gb/sInterfaces for SDH, PDH, ATM etc. Total capacity to 10 Tb/s + C + L and some S band operationSystems available from NEC, Lucent, Marconi, Nortel, Alcatel, Siemens etc.DWDM Performance as of 2008

  • up to 600-700 kmLLRP160-200 kmRP700 + kmRLLP3R RegenAnimationPower/Booster AmpReceive PreampLine AmpPRLOptical AmplifiersDWDM System Spans

  • ITU Recommendation is G.692 "Optical interfaces for multichannel systems with optical amplifiers"G.692 includes a number of DWDM channel plansChannel separation set at:50, 100 and 200 GHz equivalent to approximate wavelength spacings of 0.4, 0.8 and 1.6 nmChannels lie in the range 1530.3 nm to 1567.1 nm (so-called C-Band)Newer "L-Band" exists from about 1570 nm to 1620 nmSupervisory channel also specified at 1510 nm to handle alarms and monitoringDWDM Standards

  • Aggregate span capacities up to 320 Gbits/sec (160 Gbits/sec per direction) possibleRed band = 1547.5 to 1561 nm, blue band = 1527.5 to 1542.5 nm Nortel S/DMS Transport SystemNortel DWDM

  • 8 wavelengths used (4 in each direction). 200 Ghz frequency spacingIncorporates a Dispersion Compensation Module (DCM)Expansion ports available to allow denser multiplexingRed band = 1547.5 to 1561 nm, blue band = 1527.5 to 1542.5 nmNortel DWDM Coupler

  • 16 wavelengths used (8 in each direction). Remains 200 Ghz frequency spacingFurther expansion ports available to allow even denser multiplexingRed band = 1547.5 to 1561 nm, blue band = 1527.5 to 1542.5 nmSixteen Channel Multiplexing

  • 32 wavelengths used (16 in each direction). 100 Ghz ITU frequency spacingPer band dispersion compensationRed band = 1547.5 to 1561 nm, blue band = 1527.5 to 1542.5 nm32 Channel Multiplexing

  • DWDM Transceivers and Transponders

  • Transmission in both directions needed.In practice each end has transmitters and receiversCombination of transmitter and receiver for a particular wavelength is a "transceiver" DWDM MultiplexerPower AmpLine AmpReceive PreampDWDM DeMultiplexerReceiversTransmittersDWDM MultiplexerPower AmpLine AmpReceive PreampDWDM DeMultiplexerTransceiverDWDM Transceivers

  • In a "classic" system inputs/outputs to/from transceivers are electricalIn practice inputs/outputs are SDH, so they are optical, wavelength around 1550 nmIn effect we need wavelength convertors not transceiversSuch convertors are called transponders1550 nmSDH1550 nmSDHC band signal lC band signal lTransceivers .V. Transponders

  • 1550 nmSDHC band signal lC band T/XSDH R/XElectricallevels1550 nmSDHC band signal lSDHT/XC Band R/XElectricallevelsTransponders are frequently formed by two transceivers back-to-backSo called Optical-Electrical-Optical (OEO) transponders Expensive solution at presentTrue all-optical transponders without OEO in developmentDWDM Transponders (I)

  • Full 3R transponders: (power, shape and time)Regenerate data clock Bit rate specificMore sensitive - longer range2R transponders also available: (power, shape)Bit rate flexible Less electronics Less sensitive - shorter rangeLuminet DWDM TransponderDWDM Transponders (II)

  • Bidirectional Transmission using WDMSource: Master 7_4

  • Source: Master 7_4Local TransceiverDistant TransceiverFibres x2TransmitterReceiverReceiverTransmitterMost common approach is "one fibre / one direction"This is called "simplex" transmission Linking two locations will involve two fibres and two transceiversConventional (Simplex) Transmission

  • lAlBWDMMux/DemuxAlAlBReceiverTransmitterLocal TransceiverWDMMux/DemuxBlAlBReceiverTransmitterDistant TransceiverFibreSignificant savings possible with so called bi-directional transmission using WDMThis is called "full-duplex" transmissionIndividual wavelengths used for each direction Linking two locations will involve only one fibres, two WDM mux/demuxs and two transceiversBi-directional using WDM

  • Different wavelength bands are used for transmission in each directionTypcially the bands are called: The "Red Band", upper half of the C-band to 1560 nmThe "Blue Band", lower half of the C-band from 1528 nmRed BandFibreBlue BandReceiverl1BReceiverl2BReceiverlnBl1RTransmitterTransmitterlnRTransmitterDWDMMux/Demuxl2RReceiverl1RReceiverl2RReceiverlnRl1BTransmitterTransmitterlnBTransmitterDWDMMux/Demuxl2BBi-directional DWDM

  • To avoid interference red and blue bands must be separated This separation is called a "guard band" Guard band is typically about 5 nmGuard band wastes spectral space, disadvantage of bi-directional DWDMBlue channel bandRed channel band1528 nm1560 nmGuard

    BandThe need for a Guard Band

  • TRANSMITTERAFibre, connectors and splicesRECEIVERBTRANSMITTERBRECEIVERAFibre CouplerFibre CouplerFrequency FaFrequency FbTransmitter A communicates with Receiver A using a signal on frequency FaTransmitter B communicates with Receiver B using a signal on frequency FbEach receiver ignores signals at other frequencies, so for example Receiver A ignores the signal on frequency Fb Bi-directional Transmission using Frequency Division Multiplexing

  • TRANSMITTERAFibre, connectors and splicesRECEIVERBTRANSMITTERBRECEIVERAWDM Mux/DemuxWDM Mux/Demux1330 nm1550 nmTransmitter A communicates with Receiver A using a signal on 1330 nm Transmitter B communicates with Receiver B using a signal on 1550 nmWDM Mux/Demux filters out the wanted wavelength so that for example Receiver A only receives a 1330 nm signal Bi-directional Transmission using WDM

  • DWDM Issues

    Spectral Uniformity and Gain Tilt

  • In an ideal DWDM signal all the channels would have the same power. In practice the power varies between channels: so called "gain tilt"Sources of gain tilt include:Unequal transmitter output powersMultiplexersLack of spectral flatness in amplifiers, filters Variations in fibre attenuation 1520 1530 1540 1550 15603020100Gain (dB)EDFA gain spectrumDWDM Test: Power Flatness (Gain Tilt)

  • Gain Tilt and Gain Slope

  • Gain Tilt Example for a 32 Channel DWDM System

  • DWDM Issues

    Crosstalk between Channels

  • With DWDM the aggregate optical power on a single fibre is high because:Simultaneous transmission of multiple optical channelsOptical amplification is usedWhen the optical power level reaches a point where the fibre is non-linear spurious extra components are generated, causing interference, called "crosstalk"Common non-linear effects:Four wave mixing (FWM)Stimulated Raman Scattering (SRS)Non-linear effects are all dependent on optical power levels, channels spacing etc. Non-linear Effects and Crosstalk

  • With DWDM the aggregate optical power on a single fibre is highWith the use of amplifiers the optical power level can rise to point where non-linear effects occur:Four wave mixing (FWM): spurious components are created interfering with wanted signalsStimulated Raman Scattering (SRS)Non-linear effects are dependent on optical power levels, channels spacing etc: FWMFWMFWMChannel SpacingDispersionOptical PowerSRSSRSSRSChannel SpacingDistanceOptical PowerDWDM Problems

  • Four Wave Mixing (FWM)

  • Four wave mixing (FWM) is one of the most troubling issuesThree signals combine to form a fourth spurious or mixing component, hence the name four wave mixing, shown below in terms of frequency w:Spurious components cause two problems:Interference between wanted signalsPower is lost from wanted signals into unwanted spurious signalsThe total number of mixing components increases dramatically with the number of channels

    Four Wave MixingNon-Linear Optical Mediumw1w3w2w4 = w1 + w2 - w3

  • The total number of mixing components, M is calculated from the formula:M = 1/2 ( N3 - N ) N is the number of DWDM channelsThus three channels creates 12 additional signals and so on.As N increases, M increases rapidly.....FWM: How many Spurious Components?

  • l1l2l3l1l2l3l123 l213l113l112l223l221l332l331l312 l132l321 l231Original plus FWM components

    Because of even spacing some FWM components overlap DWDM channelsOriginal DWDM channels, evenly spacedFWM Components as Wavelengths

  • Channelnml11542.14l21542.94l31543.743 ITU channels 0.8 nm spacing For the three channels l1, l2 and l3 calculate all the possible combinations produced by adding two channel l's together and subtracting one channel l.For example l1 +l2 - l3 is written as l123 and is calculated as 1542.14 + 1542.94 - 1543.74 = 1541.34 nm Note the interference to wanted channels caused by the FWM components l312, l132, l221 and l223

    FWM mixing components Equal spacing Four Wave Mixing example with 3 equally spaced channels

  • Reducing FWM can be achieved by:Increasing channel spacing (not really an option because of limited spectrum)Employing uneven channel spacingReducing aggregate powerReducing effective aggregate power within the fibreAnother more difficult approach is to use fibre with non-zero dispersion:FWM is most efficient at the zero-dispersion wavelengthProblem is that the "cure" is in direct conflict with need minimise dispersion to maintain bandwidthTo be successful the approach used must reduce unwanted component levels to at least 30 dB below a wanted channel. Reducing Four Wave Mixing

  • Channelnml11542.14l21542.94l31543.843 DWDM channelsAs before for the three channels l1, l2 and l3 calculate all the possible combinations produced by adding two channel l's together and subtracting one channel l.Note that because of the unequal spacing there is now no interference to wanted channels caused by the generated FWM components FWM mixing components Channelnml1231541.24l2131541.24l3211544.64l2311544.64l3121543.04l1321543.04l1121541.34l1131540.44l2211543.74l2231542.04l3311545.54l3321544.74unequal spacing Four Wave Mixing example with 3 unequally spaced channels

  • 3 channels 1.6 nm spacing For the three channels l1, l2 and l3 shown calculate all the possible FWM component wavelengths.Determine if interference to wanted channels is taking place.If interference is taking place show that the use of unequal channel spacing will reduce interference to wanted DWDM channels.Problem:Sample FWM problem with 3 DWDM channels

  • 3 channels 1.6 nm equal spacing FWM mixing components Channelnml11530.00l21531.60l31533.20Channelnml1231528.40l2131528.40l3211534.80l2311534.80l3121531.60l1321531.60l1121528.40l1131526.80l2211533.20l2231530.00l3311536.40l3321534.803 channels unequal spacing FWM mixing components Channelnml11530.00l21531.60l31533.40Channelnml1231528.20l2131528.20l3211535.00l2311535.00l3121531.80l1321531.80l1121528.40l1131526.60l2211533.20l2231529.80l3311536.80l3321535.20Solution to FWM problem

  • Traditional non-multiplexed systems have used dispersion shifted fibre at 1550 to reduce chromatic dispersionUnfortunately operating at the dispersion minimum increases the level of FWMConventional fibre (dispersion minimum at 1330 nm) suffers less from FWM but chromatic dispersion risesSolution is to use "Non-Zero Dispersion Shifted Fibre" (NZ DSF), a compromise between DSF and conventional fibre (NDSF, Non-DSF) ITU-T standard is G.655 for non-zero dispersion shifted singlemode fibresReducing FWM using NZ-DSF

  • Provides small amount of dispersion over EDFA bandNon-Zero dispersion band is 1530-1565 (ITU C-Band)Minimum dispersion is 1.3 ps/nm-km, maximum is 5.8 ps/nm-kmVery low OH attenuation at 1383 nm (< 1dB/km)Dispersion Characteristics Lucent TrueWave NZDSF

  • One way to improve on NZ-DSF is to increase the effective area of the fibreIn a singlemode fibre the optical power density peaks at the centre of the fibre coreFWM and other effect most likely to take place at locations of high power densityLarge effective Area Fibres spread the power density more evenly across the fibre coreResult is a reduction in peak power and thus FWMReducing FWM using a Large Effective Area Fibre NZ-DSF

  • Corning LEAF has an effective area 32% larger than conventional NZ-DSFClaimed result is lower FWMImpact on system design is that it allows higher fibre input powers so span increasesSection of DWDM spectrum

    NZ-DSF shows higher FWM components

    LEAF has lower FWM and higher per channe\l powerDWDM channelFWM componentCorning LEAF

  • Wavelength Selection

  • Conventional DSF (G.653) is most affected by FWMUsing equal channel spacing aggravates the problemITU-T G.692 suggests a methodology for choosing unequal channel spacings for G.653 fibreITU suggest the use equal spacing for G.652 and G.655 fibre, but according to a given channel planNote that the ITU standards look at DWDM in frequency not wavelengthITU Channel Allocation Methodology (I)

  • ITU Channel Allocation Methodology (II)

  • Basic rule is that each frequency (wavelength) is chosen so that no new powers generated by FWM fall on any channelThus channel spacing of any two channels must be different from any other pairComplex arrangement based on the concept of a frequency slot "fs"fs is the minimum acceptable distance between an FWM component and a DWDM channelAs fs gets smaller error rate degradesFor 10 Gbits/s the "fs" is 20 GHz. ITU Channel Allocation Methodology (III)

  • Wavelength Introduction MethodologiesBecause of non-linearity problems wavelength selection and introduction is complexNOT just a matter of picking the first 8 or 16 wavelengths!Order of introduction of new wavelengths is fixed as the system is upgradedTable shows order of introduction for Nortel S/DMS system

  • High Density DWDM

  • DateManufacturerChannel CountTotal CapacityApril 2000Lucent823.28 Terabits/secSeptember 2000Alcatel1285.12 Terabits/secOctober 2000NEC1606.4 Terabits/secOctober 2000Siemens1767.04 Terabits/secMarch 2001Alcatel25610.2 Terabits/secMarch 2001NEC27310.9 Terabits/secNote: Single fibre capacity is 1000 x 40 Gbits/s = 40 Tbits/s per fibreRecent DWDM capacity recordsExploiting the Full Capacity of Optical Fibre

  • At present commercial system utilise typically 32 channelsCommercial 80+ channel systems have been demonstratedLucent have demonstrated a 1,022 channel systemOnly operates at 37 Mbits/s per channel 37 Gbits/s total using 10 GHz channel spacing, so called Ultra-DWDM or UDWDMScaleable to Tbits/sec?Ultra-High Density DWDM

  • Lucent demonstration (circa April 2000)3.28 Tbits/s over 300 km of Lucent TrueWave fibrePer channel bit rate was 40 Gbits/s40 channels in the C band and 42 channels in the L bandUtilised distributed Raman amplification3.28 Terabit/sec DWDM

  • NEC demonstration in March 200110.9 Tbits/sec over 117 km of fibre273 channels at 40 Gbits/s per channelUtilises transmission in the C, L and S bandsThulium Doped Fibre Amplifiers (TDFAs) used for the S-bandThulium Doped Amplifier Spectrum (IPG Photonics)10.9 Terabit/sec DWDM

  • Wavelength Division Multiplexing in LANs

  • Still in its infancyExpensive by comparison with single channel 10 Gbits/sec proposalsSinglemode fibre onlyTypical products from ADVA networking and Nbase-XyplexProducts use a small numbers of channel such as 4 (Telecoms WDM is typically 32 +)Wavelengths around 1320 nm, Telecoms systems use 1530-1570 nmNbase-Xyplex SystemWDM in LANs

  • Coarse Wavelength Division Multiplexing

  • WDM with wider channel spacing (typical 20 nm)More cost effective than DWDMDriven by:Cost-conscious telecommunications environment Need to better utilize existing infrastructureMain deployment is foreseen on:Single mode fibres meeting ITU Rec. G.652.Metro networksCoarse Wavelength Division Multiplexing

  • First announced in November 2003, as standard for CWDMSets optical interface standards, such as T/X output power etc.Target distances of 40 km and 80 km. Unidirectional and bidirectional applications included.All or part of the wavelength range from 1270 nm to 1610 nm is used. CWDM Standards: Recommendation G.695

  • 127012901310133013501370139014101430145014701490151015301550157015901610ITU-T G.694 defines wavelength grids for CWDM ApplicationsG.694 defines a wavelength grid with 20 nm channel spacing: Total source wavelength variation of the order of 6-7 nm is assumed Guard-band equal to one third of the minimum channel spacing is sufficient. Hence 20 nm chosen18 wavelengths between 1270 nm and 1610 nm. ITUCWDMGrid(nm)CWDM Wavelength Grid: G.694

  • In principle installation possible on existing single-mode G.652 optical fibres and on the recent 'water peak free' versions of the same fibre. Issues remain about viability of full capacity because of water peak issue at 1383 nmCDWM Issues: Water peak in the E-Band

  • Flexible and scalable solutions moving from 8 to 16 optical channels using two fibres for the two directions of transmission Up to 8+8 optical channels using only one fibre for the two directions. Support for 2.5 Gbit/s provided but also support for a bit rate of 1.25 Gbit/s has been added, mainly for Gigabit-Ethernet applications. . Two indicative link distances are covered in G.695: one for lengths up to around 40 km and a second for distances up to around 80 km8 Ch Mux/Demux CWDM cardCDWM Details

  • CWDM is a cheaper and simpler alternative to DWDM, estimates point to savings up to 30%Why is CWDM more cost effective?Less expensive uncooled lasers may be used - wide channel spacing. Lasers used require less precise wavelength control, Passive components, such as multiplexers, are lower-cost CWDM components use less space on PCBs - lower costDFB laser, typical temperature drift 0.08 nm per deg. CFor a 70 degree temperature range drift is5.6 nm Why CWDM?

  • DWDM Demultiplexer Spectral Response

  • 4 Channel CWDM Demultiplexer Spectral Response

  • 8 Channel Unit: AFW ltd, AustraliaCWDM Mux/Demux Typical Specifications

  • A clear migration route from CWDM to DWDM is essentialMigration will occur with serious upturn in demand for bandwidth along with a reduction in DWDM costsApproach involves replacing CWDM single channel space with DWDM "band"May render DWDM band specs such as S, C and L redundant?CWDM Migration to DWDM

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