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7/28/2019 DWDM Basics http://slidepdf.com/reader/full/dwdm-basics 1/50 Paramatma Bhattarai Min Bahadur K.C. Telecom Engineer Nepal Telecom TPID, DWT DENSE WAVELENGTH DIVISION MULTIPLEXING

DWDM Basics

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    Paramatma BhattaraiMin Bahadur K.C.Telecom Engineer

    Nepal TelecomTPID, DWT

    DENSE WAVELENGTH DIVISION MULTIPLEXING

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    Agenda

    Background of DWDM

    DWDM Transmission Media

    Key Technologies DWDM Network Design

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    Limitation of TDM

    TDM Up gradation effects Service

    Lack of rate upgrade flexibility

    Higher Cost

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    Traditional TDM/SDM

    RX

    EDFA

    TX

    TX

    TX

    TX

    TX

    TX

    TX

    TX

    RX

    RX

    RX

    RX

    RX

    RX

    RX

    120 km 120 km 120 km

    DWDM

    EDFA

    TX RXRegReg

    TX RXRegReg

    TX RXRegReg

    TX RXRegReg

    TX RXRegReg

    TX RXRegReg

    TX RXRegReg

    TX RXRegReg

    120 km 120 km 120 km

    DWDM is the Main Expansion Scheme

    Current DWDM Technology

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    Operation Modes

    Two Fiber Bi-directional Transmission

    Single Fiber Bi-directional Transmission

    Optical Add Drop Multiplexing

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    Application Modes of DWDM

    Open DWDM

    Integrated DWDM

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    Advantages of DWDM

    Ultra-large Capacity

    Transparent Data rates

    Cost Effect Up gradation

    Flexible and Reliable Network

    Compatible with AON

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    Future of DWDM

    1996 2000 2004 2008

    OADM

    Point-point DWDM

    Fixed wavelength

    add/drop

    OADM

    OADM

    OADM

    OADM

    Re-configurable

    wavelength add/drop

    OXC

    OXC

    OXC

    OXC

    OXC

    Optical cross-connect

    and mesh networking

    More capacity,

    less fibers

    Flexible

    bandwidth

    assignment and

    protection

    schemes

    Wavelengthconversion and

    routing

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    Agenda

    Background of DWDM

    DWDM Transmission Media

    Key Technologies DWDM Network Design

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    Optical Fiber Structure

    Multi Mode Fiber(Step/Graded Index)

    Core Diameter: 50/62.5 Micron

    Light follows multiple paths in the fiber coreNot Recommended for Long haul transmission

    Single Mode Fiber(Step Index)

    Core Diameter: 8~10 Micron

    Light can follow only a single path in the fiber core

    Suitable for long haul transmission

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    Fiber Characteristics affecting OSNR

    Absorption Loss

    Scatter Loss

    Bend Loss

    Fiber Loss

    Fiber Dispersion

    Chromatic Dispersion

    Polarization Mode Dispersion

    Fiber Non Linearity

    Four Wave Mixing

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    Fiber Loss Spectrum and EDFA Gain Spectrum

    EDFA Gain

    Single-Mode Fiber

    1.2 1.3 1.4 1.5 1.6 1.7Wavelength (m)

    Loss(dB/km

    )

    0.1

    0.2

    0.4

    0.8

    1.0

    5000GHz

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    Fiber Characteristics: No linearity

    Generating new frequencies with the interaction of the channels

    Related parameters including the number of channels, the spacing

    of channels and the power of channels

    w1 w2ww1 w22w1-w2 2w2-w1

    w

    Fiber

    FWM Effect

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    ITU-T Defined Optical Fiber Types

    G.652 Single Mode Fiber

    G.653 Dispersion Shifted Fiber

    G.654 Cut-off Wavelength Shifted Fiber

    G.655 Nonzero Dispersion Shifted Fiber

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    G.652 Single Mode Fiber

    also called Dispersion Unshifted Fiber

    Extensively Used

    Optimal for 1310nm Window

    Based on refractive index cross-sectioncan be divided into two categories

    a. Matched Cladding Fiberb. Depressed Cladding Fiber

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    G.653 Dispersion Shifted Fiber

    1550nm Window Proprietary Optimal Fiber

    Zero Dispersion shifted to 1550nm Window

    Can implement Ultra high speed and Ultralong distance optical transmission

    Not suitable for DWDM Application due to

    FWM phenomena

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    G.654 Cut-off wavelength Shifted Fiber

    Designed to reduce attenuation at 1550 nm

    Zero Dispersion Point in 1310 nm window

    Dispersion at 1550nm relatively high(i.e.18ps/nm/km)

    Mainly used in submarine application

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    G.655 Nonzero Dispersion Shifted Fiber

    Similar to G.653 fiber

    Two Types

    1. NZDSF+

    2. NZDSF-

    Preserves some Dispersion in 1550nm Window to

    avoid FWM

    Recommended for DWDM Application

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    Fiber Type and Loss

    EDFA

    bandwidth

    1.2 1.3 1.4 1.5 1.6 1.7 Wavelength (nm)

    Loss(dB/km)

    0.1

    0.2

    0.4

    0.81.0

    0

    -20

    -10

    10

    20

    Di

    spersion(ps/nm-km)

    SMF

    DSF

    NZDF+

    NZDF-

    G.652

    G.653

    G.655+

    G.655-

    Dispersion/Loss for Various types of Fibers

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    Agenda

    Background of DWDM

    DWDM Transmission Media

    Key Technologies

    DWDM Network Design

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    LASER

    Regeneration Distance increased from 50~60to 500~600 Km.

    Wavelength spacing from several

    nanometer to sub-nanometer. Requirement for DWDM system.

    a. Relatively large Dispersion tolerance

    b. Standard and stable wavelength DWDM system uses Semiconductor LD

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    LASER Modulation Modes

    Two Types

    Direct Modulation Indirect Modulation

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    Direct Modulation(Internal Modulation)

    Changing the laser intensity by controlling thedriving current

    Extensively used in traditional PDH/SDH systems Simple Structure,Low Loss, Low Cost With passage of time it changes the length of laser

    resonant cavity,hence varying the modulationcurrent will cause linear variation in emitting laserwavelength.This phenomena is called ModulationChirp.

    Modulation Chirp is inevitable for Direct Modulation Broadens the emitting spectrum BW, deteriorates

    spectrum characteristics, hence limiting transmissiondistance/rate

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    Indirect Modulation(External Modulation)

    Constant

    Light

    Source

    Optical

    Modulator

    Electric

    Modulation signal

    Output

    Signal

    Modulator acts as a Switch

    Constant Laser is highly stable, is not effected by electric

    modulation current during emission.

    Spectrum characteristics of light wave will not be effected, hence

    guaranteeing spectrum quality. High cost, but modulation chirp is very low

    Can be classified into two categories

    1. Integrated External Modulated Laser

    2. Separated External Modulated Laser

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    Integrated External Modulated Laser

    Becoming the developing trend for DWDM Lasers

    Commonly used Modulator is EA Modulator

    EA Modulator is compact and integrated with laser

    1 o

    Absorption

    Region

    2o

    Absorption

    Region

    Biased

    Unbiased

    Can support transmission of 2.5Gb/s up to 600Km

    1 = Absorption side wavelength of unbiased modulator

    2 = Absorption side wavelength of biased modulator

    0 = Operating wavelength of constant light source

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    Separated External Modulator Laser

    Uses LiNbO3 MZ External Modulator

    Separates light into two equal signals, entering twobranches

    Each branch uses electro-optic material,whose refractive

    index changes with the magnitude of electrical modulationcurrent

    Change in the refractive index of the optical branches willresult in variation of signal phase

    When the signal at the output of two branches combine

    together, the optical signal is an interfering signal withvarying phase characteristics

    The frequency chirp of this type of modulation can be zero

    Relatively lower cost compared to EA modulation

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    Wavelength Stability and Control

    According to ITU-T G.692 deviation from centralwavelength should not be greater than one fifth (1/10) ofoptical channel spacing,

    i.e. wavelength deviation should not be greater than 20

    GHz for a system with channel spacing of 0.8nm 0.5 nm variation of wavelength can shift an optical channel

    to another, critical for UDWDM

    Practically the variation should be controlled within 0.2nm

    Fine tuning for EML is implemented by adjusting

    temperature Temperature sensitivity of wavelength is 0.08nm/C

    DFB Laser is used to control the temperature of laser chipto achieve wavelength stability

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    Wavelength Stability and Control

    Preferable to utilize wavelength sensitive componentfor feedback control of the Laser.

    LASER Aging

    LD Splitter

    Wavelength

    Sensitive

    Component

    Optical

    Output

    Signal

    Processing

    LD Control

    Circuit

    For Wavelength monitoring

    For

    Wavelength

    Control

    Standard wavelength control using feedback fromwavelength sensitive components is under research

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    EDFA Erbium Doped Fiber Amplifier

    Key component of new generation OpticalCommunication Systems

    Advantages High Gain

    Large Output Power

    Wide operating Bandwidth

    Polarization independence

    Amplification independent of bit rate and data format

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    Erbium DopedSilica Fiber

    Isolator

    IsolatorIsolator

    Pump Laser980 nm or 1480 nm

    CouplerWDM

    Pin Pout

    EDFA Erbium Doped Fiber Amplifier

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    Principle of Light Amplification

    980 nm pump

    1480 nmpump

    Fast non-radiation decay

    E1

    E3

    E2

    1530 - 1560 nmSimulated

    Emission andSpontaneousEmission

    1550 nmSignals

    SimulatedAbsorption

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    Types of EDFA

    Transmitter Receiver

    Boost Amplifier Line Amplifier Pre Amplifier

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    Characteristics of Different types of EDFA

    BoostAmplifier

    LineAmplifier

    PreAmplifier

    Gain Low High High

    OutputPower

    High Moderate/High

    Low

    Noise Not Critical Low Noise Low Noise

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    Gain Control for EDFA

    EDFA Gain Flatness EDFA based on Pure Silica has a very narrow flat gain

    range 12 nm i.e. (1549~1561nm)

    Gain fluctuation between 1530~1542 up to 8db

    Solution: Use EDFA based on aluminum doped siliconFiber

    EDFA Gain locking When channels are dropped, their energy is transferred to

    un-dropped channels due to gain competition

    Solution: Control gain of the pump through internalfeedback monitoring circuit

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    DWDM Components MUX/DEMUX

    Should meet requirements defined by ITU-TG.671

    Four types of widespread DWDM

    Components

    Diffraction Grating type WDM Component Filter type WDM Component

    Coupler type WDM Component Integrated Wave guide type WDM

    Component

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    Diffraction Grating Type WDM Component

    Angular Dispersion Component

    When light reaches the grating

    surface, signals at differentwavelengths are reflected atdifferent angle, due to angulardispersion of grating

    Diffraction grating type WDM

    Signals then converged to different fibers via lenses

    High resolution wavelength selection function can beimplemented using this technique

    Extensively applied in DWDM systems

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    Filter Type WDM Component

    Filters based on Dielectric Films

    According to the Film interference

    type, the Filter can be designed asBand pass for some wavelengths,and Band stop for otherwavelengths

    Flat pass-band, polarization independence and structural

    stability No. of wavelengths added or dropped cannot be large

    Relatively longer design and manufacturing process andlow volume of production

    Filter type WDM

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    Coupler Type WDM Component

    A device capable of splittingwavelengths; two wavelengthson a single optical fiber into twofibers, or vice versa

    Optical couplers are made byfusing/tapering two fiberstogether so that the cores areclose enough to each other forthe optical power to betransferred from one fiber to theother.

    Extensively applied in DWDM and EDFA applications

    Coupler type

    1234

    5678

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    Integrated Wave guide type WDM Component

    A plane wave guide componentbased on optical integrationtechnology

    Can be fabricated into matrixstructure to add/drop opticalchannels (used in OADM)

    Typical wave guide type is AWG type manufactured by NTT

    Advantages: Small wavelength spacing, large no. of channels,flat pass band

    Suitable for ultrahigh speed/capacity DWDM

    AWG type WDM

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    Agenda

    Background of DWDM

    DWDM Transmission Media

    Key Technologies DWDM Network Design

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    Network Element types of DWDM

    OTM Optical Terminal Multiplexer

    OLA Optical Line Amplifier

    OADM Optical Add Drop Multiplexer

    REG Electrical Regenerator

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    OTM Optical Terminal Multiplexer

    Transmitter End Multiplex STM-N signals to M wavelengths 1 to M Amplify Optical power

    Add Optical Supervisory Channel s (generally 1510 nm)

    Receiver End Extract and process OSC

    Amplify Optical channels and Demultiplex into M STM-N signals

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    OLA Optical Line Amplifier

    Extract and process OSC

    Amplify the main Optical channel

    Multiplex OSC back to the Optical channel

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    OADM Optical Add Drop Multiplexer

    Static/Fixed OADM

    Can be implemented via a single board/unit

    Board/Unit capable of wavelength conversion

    Capable of adding/dropping 1~8 wavelengths Back to back OTM

    More flexible compared to fixed OADM

    Can Add/Drop all M wavelengths at certain node

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    REG Electrical Regeneration Unit

    No capability to Add/Drop service

    Used to elongate Dispersion limited distance

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    DWDM Network Design

    Point-to-point Network

    Chain Network

    Ring Network

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    Point to Point Topology

    SDH OTM OLA SDH OTM

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    Chain Topology

    SDH OTM

    SDH OTM

    OADM SDH

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    Ring Topology

    OADM

    OADM

    OADMOADM

    1~8

    1~8

    1~8

    1~8

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    THANK YOU!