Optische netwerken - SNE/OS3 Homepage [OS3 Website] netwerken SNE opleiding - 19 maart 2009 Roeland Nuijts, ... optical fiber transmission systems were “loss-limited”, ... which

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  • Optische netwerkenSNE opleiding - 19 maart 2009

    Roeland Nuijts, SURFnet, The Netherlands

    roeland.nuijts@surfnet.nl

  • Outline

    - Introduction

    - Optical transmission fiber

    - Optical transmitters and receivers

    - DWDM enabling technologies

    EDFA (E bi D d Fib A lifi )- EDFAs (Erbium Doped Fiber Amplifiers)

    - Optical multiplex and demultiplex filters

    - 10Gb/s transmission and Dispersion compensationp p

    - High-speed transmission (40Gb/s and 100Gb/s)

    - All optical switching WSS (Wavelength Selective Switches)

    2

  • Optical fiber - Historical perspective

    - Basic principle of internal reflection

    known from 19th century (John

    Tyndall, 1870)

    E l fib ith l ddi t l - Early fibers with cladding extremely

    lossy ~1000dB/km (1960)

    - Progress in fabrication (MCVD)

    leads to low loss fibers (0.2dB/km

    at 1550nm wavelength, limited by

    fundamental limit of Rayleigh

    scattering) around 1979

    3

  • Fiber absorption

    - Fiber loss is wavelength dependent, minimum is around 1550nm- Current fiber loss is close to fundamental limit determined by Rayleigh

    scattering, proportional to l-4 therefore dominant at short wavelengths- Loss at long wavelengths (l > 1625nm) dominated by infra-red absorption

    4

    - Peak at 1400nm arises from OH impurities, can be removed (AllWave fiber)

  • Fiber dispersion - Refractive index varies with wavelength which leads to a wavelength dependence of the group delay, tg, (delay for different gwavelengths) in ps/km

    - Dispersion coefficient, D, is the derivative of the group delay, tg , with respect to wavelength per unit length (ps/nm km)tg

    (ps)

    1 2

    1,4 Optical pulse shape at Tx output

    (nm)0,0

    0,2

    0,4

    0,6

    0,8

    1,0

    1,2

    4100 4300 4500 4700 4900 5100 5300 5500

    Opt

    ical

    Pow

    er (A

    .U.)

    D(ps/nm km)

    0-30 -20 -10 0 10 20 30

    Frequency (G Hz)

    4100 4300 4500 4700 4900 5100 5300 5500

    Time (ps)

    0 (nm) -5

    -10

    -15Pow

    er(d

    B)

    Optical spectrumat Tx output

    l0

    5

    -30

    -25

    -20

    15

    Optic

    alP

    No distortion at zero-dispersion wavelength, l0 Distortion at other wavelengths

  • Optical fiber Historical perspective Standard SMF Optical fiber Historical perspective Standard SMF (G.652)

    - Initial (80s) optical components for transmission through single mode fiber operated at the 1.3 mmwavelength, therefore fiber was developed which had zero-dispersion at this wavelength. For this wavelength, therefore fiber was developed which had zero dispersion at this wavelength. For this reason, this type of fiber is often referred to as standard fiber, conventional fiber or ITU G.652 fiber.

    - Installed fiber base in the world is mainly comprised of this standard (1.3m zero-dispersion wavelength) SMF (Single Mode Fiber)

    - This embedded base represents an enormous investment, strong incentive to use it

    - Development and commercialization of sources and detectors operating in the 1550nm wavelength region, where the minimum fiber loss is achieved, were developed later, more specifically in the 80s

    - Dispersion-Shifted Fiber (zero-dispersion at 1550nm) later developed and deployed, predominantly in Japan

    17

    6

  • NZDSF (Non-Zero Dispersion Shifted Fiber) optimizes dispersion in the EDFA region

    7

  • Introduction Traditional digital point-to-point Introduction Traditional digital point to point optical fiber transmission systems

    Transmission fiber

    Tx Rx

    a (dB/km)

    Tx Rx

    PT (dBm) PR (dBm)

    T itt d l i l d b t i li ht d ff i

    Transmission distance = (PT-PR) / a (km)

    - Transmitter sends logical ones and zeros by turning light on and off, receiver converts received optical power to electrical signal, retrieves clock signal and determines on decision moment whether one or zero was sent

    - Initial, low-speed, optical fiber transmission systems were loss-limited, transmission distance was limited by the thermal noise in the optical receiverdistance was limited by the thermal noise in the optical receiver

    - Increase in transmission bit rate to high speeds (bit rate 2.5Gb/s) has made fiber dispersion, D, an important system parameter which limits the achievable transmission distance

    8

  • Decibel scale versus li llinear scale

    Power levels and loss scales in optical systems cover a hugh dynamic range Power levels and loss scales in optical systems cover a hugh dynamic range

    Losses in fibers and filters are multiplication factors in linear domain, additions and subtractions in dB

    Typically power levels and losses on logarithmic scale in decibels, more practical

    Power is mW in linear domain -> dBm in logarithmic domain

    Loss dimensionless in linear domain -> dB in logarithmic domain

    P (mw) P (dBm) P (mw) P (dBm)

    (mW)] [P Log 10=(dBm) P 10

    [L]Log10=(dB)L 10

    1000 30 1 0800 29 0.8 -1500 27 0.5 -3400 26 0.4 -4250 24 0.25 -6200 23 0 2 7[L]Log 10(dB)L 10 200 23 0.2 -7100 20 0.1 -1080 19 0.08 -1150 17 0.05 -1340 16 0.04 -1425 14 0 025 -1625 14 0.025 1620 13 0.02 -1710 10 0.01 -208 9 0.008 -215 7 0.005 -234 6 0.004 -24

    x 2 3dBx 5 7dBx10 10dB

    9

    2 3 0.002 -271 0 0.001 -30

    0 0d

  • 10Gb/s optical transmitter technologies

    {0,1,1,0,1,1,,0,1,0}

    DFB

    DM-DFB (Directly Modulated Distributed Feed Back laser) Cheap, small, low power consumption Chirped, i.e. different wavelength during ones and zeros which leads to a wide optical spectrum and associated transmission impairments Used for short reach transmission

    EML (El t Ab ti M d l t L )

    {0,1,1,0,1,1,,0,1,0}

    DFB EA

    EML (Electro-Absorption Modulator Laser) Monolithically integrated laser and modulator combination Potentially cheap, small, medium power consumption Chirped, i.e. different wavelength during ones and zeros Used for intermediate and long reach transmissiong

    {0,1,1,0,1,1,,0,1,0}CW-DFB (Continuous Wave DFB laser) and MZ (Mach-Zehnder) combination

    DFB

    (Mach Zehnder) combination External modulator Expensive, relatively large, high-power drivers (high power consumption) Low (or deterministic) chirp, excellent

    f

    10

    Mach-ZehnderLiNbO3 modulator

    performance Used for long reach and DWDM (Dense Wavelength Division Multiplexing) transmission

  • Typical 10Gb/s optical receiver setup

    preamp AGCdecisioncircuit

    data

    CLK

    (A)PD

    Photodetector converts optical signal to electrical signal. PIN or APD (Avalanche Photo Detector) for improved receiver sensitivity

    P id hi h i l i

    BER

    Preamp provides high gain, low noise

    AGC (Automatic Gain Control) amplifies signal at output of preamp to rail-to-rail voltage of decision circuit

    Decision circuit, usually D-flip-flop, signal at input is clocked to the output on rising edge of clocksignal distortion is removed

    10-9

    10-6

    rising edge of clocksignal, distortion is removed

    BER (Bit-Error Rate) performance limited by thermal noise in receiver, receiver performance is usually specified in terms of receiver sensitivity, i.e. the amount of optical power needed to achieve a BER of 10-12

    10-12

    Psens

    11

    Prec (dBm)

  • WDM enabling technologies I: WDM enabling technologies I: EDFAs (Erbium Doped Fiber Amplifiers)

    Fiber doped with Er3+ ions be excited by 980nm or 1480nm photonsp y p spontaneous emission generates noise Excited state Erbium ions can be stimulated to decay to ground state via stimulated emission by a 1550nm signal

    12

  • Erbium Doped Fiber Amplifier

    Erbium DopedFiber

    isolator isolator

    1480nmoror

    980nm

    13

  • ASE (Amplified Spontaneous Emission)

    () = 2 h n sp (G() 1)

    - Amplifiers are used to overcome fiber losses.- Optical Noise is added by each amplifier.

    14

    - Engineering rules usually defined for equal spans (e.g. 20 x 20dB) which is not the case in the real fiber networks

    Slide courtesy of Kim Roberts, Nortel

  • Initial two-stage EDFA configuration, Initial two stage EDFA configuration, example

    High-gain, low-noise first stage followed by high-power second stage

    Current designs state of the art designs are wideband, 1520nm-1560nm (C-band) or 1565nm-1605nm (L-band), can be used for simultaneous amplification of multiple channels at different wavelengths

    Bitrate transparent

    15

    Fiber loss no longer limiting factor

  • WDM enabling technologies II: WDM enabling technologies II: Multiplex/Demultiplex filters

    somewhat analogous to prism input white beam, seperates it spatially onto output fibers works both ways, demux and mux other technologies possible (e.g. thin film filter)

    16

    g p ( g )

  • WDM system configuration (one-way)C

    MD 1

    CM

    D

    1TxTxTxTx

    1

    RxRxRxRx

    Add/drop siteG

    MD

    CM

    D 2

    OA OA OA OA

    GM

    D

    CM

    D2

    Tx Rx

    OA

    GM

    DG

    MD

    OA

    G

    CM

    D 9

    D

    CM

    D

    9TxTxTx

    RxRxRx

    DG

    CMD CMD

    Tx = 10Gb/s Optical transmitter(OM5200/OME6500)

    Rx = 10Gb/s Optical Receiver (OM5200/OME6500)

    36

    C

    DTxTx

    RxRxT

    xTx

    Tx

    Tx

    Rx

    Rx

    Rx

    Rx

    GMD = Group Multiplexer/Demultiplexer

    CMD = Channel Multiplexer/Demultiplexer

    OA = Optical amplifier

    S