introduction to optical networking

7/27/2019 Introduction to Optical Networking

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Introduction to Optical Networking:From Wavelength Division Multiplexing

to Passive Optical Networking

Dr. Manyalibo J. Matthews

Optical Data Networking Research

Bell Laboratories, Lucent Technologies

Murray Hill, NJ 07974 USA

University of Tokyo Visit March 22, 2004


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T.Harris A.Harris M.Matthews1997 2000

AT&T Lucent Uber Alles Lucent A la Carte

1996 2001

spectroscopy,NSOM,Confocaldevice physics network subsystems!

Evolution of Lucent and Matthews/Harris Lab:

Akiyama MatthewsTunableLasers

TelecomLasersSemiconductor Laser

Device Physics

QuantumWire Lasers


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Outline

Introduction Overview of Optical Networking

Types of Networks Fiber, Lasers, Receivers

Coarse Wavelength Division

Multiplexing Ethernet Passive Optical Networks Conclusions & Future


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Emergence of Optical Networks

Optical

L

ineSystem

OLS 40/80GOLS 400G800G/1.6T

Mesh

BackboneNetwork Regional

Pointof

Presence

CO-1

CO-n

Core/Backbone/LongHaul

Regional/Metro

Access/Enterprise

EPON

node

Metro

DMX

LocalServiceNodeMetro

Edge

Switch

Metro

Edge

Switch

OpticalCross

Connect

Metro

DMX

Access

Node

PassiveW

DM

PassiveWDM

C/DWDM

C/DWDM

C/DWDM

MetroEdge

Switch

DSL,FTTH

PON


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Wavelength Division Multiplexed (WDM)

Long-Haul Optical Fiber Transmission System

Transmitter

Transmitter

Transmitter

Receiver

Receiver

Receiver

M

UX

D

E

M

U

XOptical Amplifier

1

2

3

WDM Routers Erbium/Raman Optical Amplifier


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Categorizing Optical Networks

Who Uses

it?

Span

(km)

Bit Rate

(bps)

Multi-

plexing

Fiber Laser Receiver

Core/

LongHaul

Phone

Company,

Govt(s)

~103 ~1011

(100s of

Gbps)

DWDM/

TDM

SMF/

DCF

EML/

DFB

APD

Metro/

Regional

Phone

Company,

Big Business

~102 ~1010

(10s of

Gbps)

DWDM/

CWDM/

TDM

SMF/

LWPF

DFB APD/ PIN

Access/

LocalLoop

Small

Business,

Consumer

~10 ~109

(56kbps

- 1Gbps)

TDM/

SCM/

SMF/

MMF

DFB/ FP PIN

DWDM: Dense Wavelength Division Multiplexing (


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Optical Fiber Attributes

Attenuation: Due to Rayleigh scattering and chemical absorptions,the light intensity along a fiber decreases withdistance. This optical loss is a function of wavelength(see plot).

Dispersion: Different colors travel at different speeds down the

optical fiber. This causes the light pulses to spreadin time and limits data rates.

Types of Dispersion

Chromatic Dispersionis caused mainly by thewavelength dependence of the index ofrefraction (dominant in SM fibers)

Modal Dispersionarises from the differences ingroup velocity between the modes travelling

down the fiber (dominant in MM fibers)

t

t t

t

launch receive


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Non-Linear Effects in Fibers

Self-Phase Modulation: When the optical power of a pulse isvery high, non-linear polarization termscontribute and change the refractiveindex, causing pulse spreading and delay.

Four-wave Mixing: Non-linearity of fiber can cause mixingof nearby wavelengths causinginterference in WDM systems.

Stimulated BrillouinScattering: Acoustic Phonons create sidebands that

can cause interference.

Cross-Phase Modulation: Same as SPM, except involving more than

one WDM channel, causing cross-talkbetween channels as well.


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800 900 1000 1100 1200 1300 1400 1500 1600 1700

0.5

1.0

1.5

2.0

2.5

3.0

FirstWindow Second

Window

ThirdWindow

ATTENUATION

(dB/km

)

WAVELENGTH (nm)

1310nm 1550nm

Attenuation/Loss in Optical Fiber

First Window @ 850nm

High loss; First-gen. semiconductor diodes (GaAs)

Second Window @ 1310nm

Lower Loss; good dispersion; second gen. InGaAsP

Third Window @ 1550nm

Lowest Loss; Erbium Amplification possible

850nm

First window, second window,third window correspond(roughly) to first, second andthird generation opticnetwork technology


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Dispersion Characteristics*

1310nm 1550nm850nm

800 900 1000 1100 1200 1300 1400 1500 1600 1700

-120

-90

-60

-30

0

3.0

FirstWindow

SecondWindow

ThirdWindow

DISPERSIO

NC

OEFF

,D

(ps

/km-n

m)

WAVELENGTH (nm)

Standard SMF has zero dispersion at 1310nm

Low Dispersion => Pulses dont spread in time

Dispersion compensation needed at 1550nm

Limits data transmission rate due to ISI (inter-symbolinterference)

Dispersion not so important at 850nm

Loss usually dominates

* Modal dispersion notincluded


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Characterization of System Quality

Bit Error Rate: input known pattern of 1s and 0s and see how many

are correctly recongnized at output.Eye Diagram: Measure openness of transmitted 1/0 pattern using

scope triggered on each bit.

Eye opening


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Effect of Dispersion and Attenuation on Bit Rate

30

10

1

Bit rate (Mb/s)

Distance

(km

)

0.1 10 100 1000 10,0001

1550nm

1310nm850nm

Dispersion limitedAttenuation limited

sing

le-m

odefib

er

multi-m

odefiber

Coaxialcable

For short reaches (1-2 km), all optics are Gigabit capable

For longer reaches (~10 km), only 1310/1550 nm optics are Gigabit capable

20

x x

Cat 3limit

Cat 7limit

Cat 5limit

x

Twisted Pair


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Technology Trends

850nm & 1310nm Preferred by high-volume,

moderate performancedata comm manufacturers

1310nm & 1550nm Preferred by high performancebut lower volume (today)

telecomm manufacturers

Reason? You need lots of them, they dont need to go far,and youre not using enough fiber ($) to justify wavelength

division multiplexing (WDM), I.e. low-quality lasers are OK.

Reason? You dont need lots, but they have to be goodenough to transmit over long distances cost of fiber (andTDM) justifies WDM 1550nm is better for WDM


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DFB vs. FP laser

Simple FP

mirror

gain

cleave

+

-mirror

gain

AR coating

+

-Etchedgrating

DFB

FP: Multi-longitudinal Mode

operation Large spectral width

high output power

Cheap

DFB: Single-longitudinal Mode

operation Narrow spectral width

lower output power

expensive


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Fiber Bragg Grating External CavityLaser for Access/Metro Networks

SHOW PLOTS OF FBG-ECL DATA

SHOW PICTURE OF XPONENTS EXTENDED REACH FP

Typical FBG-ECL:

Bell Labs FBG-ECL:

HR AR

gainFBGLensed

tip

T=25C

T=85C

HR AR

gainFBG

XB regionT=25,85C

1-2nm grating


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Fiber Bragg Grating External Cavity Laser

FBG-ECLoutput

TypicalFP output

1305 1310 1315 1320 1325-70

-60

-50

-40

-30

-20

Power(dB)

wavelength (nm)

Narrow FBG bandwith limitsoutput ~1nm for extended

reach or WDM applications.

Simple design (AR-coated FP,XBR, butt-coupled FBG)

Mode-hop free operation

over 0-70C


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20 30 40 50 60 70 80

1310.3

1310.4

1310.5

1310.6

1310.7

1310.8

1310.9

1311.0

ave

dependence 0.008nm/C

Wavelength(nm)

Temperature (oC)

Wavelength Stability of FBG-ECL

CW, ~40mA bias

DFB drift ~ 0.1nm/oC

FP drift ~ 0.3nm/oC


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Filter bandwidths ofWDM Mux/Demux

0.8nm (100GHz)

>100 channels (C+L+S)

20nm

18 channels (O,E,S,C,L)

3.2nm (400GHz)

32-64 channels (C+L+S)

DWDM: High channel count, narrow channel spacing Temp-stablized DFBs required Temp-stablized AWGs required (typically)

CWDM: Low channel count, large channel spacing Uncooled DFBs can be used Filters can be made athermal

xWDM?: Moderate channel count, moderate channel spacing FBG-ECL or Temp-stablized DFBs required Filters can be made athermal suitable for athermal WDM PON!

1260nm 1610nm

1480nm 1610nm

1480nm 1610nm


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Example 1: 10Gbps Coarse WDM

-Used currently in Metro systems (rings, linear, mesh)-Spacing of CWDM grid determined by DFB wavelength drift-Current systems limited to 2.5Gbps due to cheaper optics-Possible upgrade to 10Gbps?


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CWDM Lasers

16 uncooled, directly modulated CWDM lasers (DMLs)

rated for2.5 Gb/s direct modulation (cheap! - $350 a piece)

NRZ-modulation at 10 Gb/s (careful laser mounting; no device selection)

2.5-Gb/s DML 50 line

47 chip resistor


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CWDM System Improvement usingElectronic Dispersion Compensation


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Example 2: Ethernet Passive OpticalNetworks

NO Active Elements in Outside Plant

Enable triple-play services

Simple & cheap

IP Video

Services

PSTN

Internet PON

Headend/COHomes/Businesses

Outside Plant


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Choices of PONs

Architecture/Layout Upstream Multiplexing

OLT

ONU

ONU

OLT

WDM:simple, expensive

TDM: simple, cheap

SCM: complex, expensive

Linear Bus: lossy, fiber lean

Ring: lossy, protected

OLT

ONU

Simple or Cascaded Star: low loss

ONU

ONU

ONU

ONU

ONU

ONU

ONU

ONU

ONU

OLT=Optical Line Termination (head-end)ONU=Optical Network Unit (user-end)


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EPON Access Platform

Video/IPTelevisionVoice/IP POTS serviceHigh-speed data

Residence

MetroEdge

Voice/IP

Services

Business

Broadcast Video

VOD

Management

MetroNetwork

Data

10G Ethernet

Or up to 61GbE

EPON

opticalsplitter

opticalsplitter

32 subscribers

Per EPON

Panther EPON OLT Chassis

1232384 subscribersDynamic bandwidthGuaranteed QOS

premium access

.

..

12 EPONS

Lucent

EPON ONU

+ Gateway

Note on Lasers:-Use DFB at headend (shared)

-Use FP at Homes (not shared)

DFB

FP


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ONU Design

ReportGenerator

PacketMemory

TX

RX

ControlParser

Demux

watchdog0

watchdog1

discoveryPeriodicReport

generatorEPON driver

EPON core

RX

TX

EPON MAC

Mux

TimestampCRCLLIDMemory

managerQueue

manager

GMII

SERDES&

Optics

CPUFPGA

SerialPort

GigE uplink

Packetmemory

1.25G BMBiDi Xcvr

Flash (CPU)memory

10/100bTdiagnosticport

SERDES(w/CDR)

PON

FPGA w/EmbeddedProcessor

CHILD

BOARD

PARENT

BOARD


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ONU

GrantList

GateGenerator

Packet

Memory

RTT table

TX

RX

ControlParser

Demux

watchdog0

watchdog1

discovery Keepalive scheduler

EPON driver MPCP driver

EPON core MPCP core

RX

TX

EPON MAC

Mux

TimestampCRCLLIDMemory

managerQueue

manager

RTT Processor

Report processor

GMII

SERDES&

Optics

Reporttable CPUFPGA

OLT Design

SerialPort

GigE uplink

Packetmemory

1.25G BMBiDi Xcvr

Flash (CPU)memory

10/100bTdiagnosticport

SERDES(w/CDR)

PON

FPGA w/EmbeddedProcessor


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Downstream: continuous, MAC addressed Uses Ethernet Framing and Line Coding Packets selected by MAC address QOS / Multicast support provided by Edge Router

Upstream: Some form of TDMA ONU sends Ethernet Frames in timeslots Must avoid timeslot collisions Must operate in burst-mode BW allocation easily mapped to timeslots

EPON downstream/upstreamtraffic

1 2 3 2

1

2 2

3

1 2 3 21

2 2

3

1

23

2

12

32

1

2

3

2

1

2

2

OLT

OLT

3

3

33

O

NU

O

N

UO

N

U

O

N

UO

NUO

N

U

EdgeRouter

ONU: Optical Network Unit

OLT: Optical Line Termination

EdgeRouter

Control Gates

Control Reports


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PON TDMA BURSTMODE OPTICS

Because upstream transmissions must avoid collisions, each ONU musttransmit only during allowed timeslot

Transmitting 0s during quiet time is not allowed!

Average 0 power ~ -10 to 5 dBm

Summing over 16 ONUs would result in a ~1dBm noise floor

Distinct from Bursty nature of Ethernet TRAFFIC Ethernet transmitters never stop transmitting (Idle characters)

CDR circuit at receiver stays locked even when no data is transmitted

Besides PONs, other systems use burstmode

Wireless Shared buses/backplanes

Optical burst switched (OBS) systems


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BURSTMODE TRANSMITTERS

Tx FIFO Encoder Serializer TransmitterData

Clock

Prebias

PhysicalMedia

currentIth

Opticaloutput

0

1

Modulationcurrent

off

Driving LD belowThreshold causes

Jitter Off-state ~ -40dBm


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BURST-MODE RECEIVERS

PROBLEM OF FAST CDR LOCKING

GAIN LEVELING & DYNAMIC

RANGE OF OPTICAL RECEIVER

Rx FIFO CDRLimiting

AmpReceiverData

Clock

DeserializerDecoder

Reset


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IMPACT ON EFFICIENCY

~1460 Bytes64 Bytes

CRC

DMAC

SMAC

VL

AN

HLEN

TOS

LEN

ID

OFF

ST

TTL

PROT

CHK

SM

SIP

DIP

ACK

HLEN

FL

AGS

WSZE

CHK

SM

URG

SPT

DPT

SEQ Data

1:4OLT

ONU 1

1:8ONU 2

...

Upstream BurstsCascaded PON

guardband

ONU 1ONU 2

Ethernet IP TCP

Laseron

AGCsettle

CDRlock

Bytesync

ONU1 payload(Ethernet Frames)

Laseroff

Throughput Efficiency

0.7

0.75

0.8

0.850.9

0.95

1

1.05

0 1000 2000 3000

AGC+CDR+LASER ON/OFF (ns)

Utilisation Our current situation

Standar

d GE

transcei

vers

Burst-mode transceivers


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Conclusions

Optical Networking getting closer andcloser to end user For Metro, CWDM is lowest cost solution,

but must be improved to handle 10Gbps

PON systems could deploy in mass overnext 1-2 years, with EPON one of theleading standards

Lasers dominate cost, therefore useful tostudy physics of low-cost laser structures!

THANK YOU VERY MUCH!(Domo Arigato Gozaimashita!)


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Spare Slides


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SYSTEM PENALITIES in PONs

Attenuation in PONs dominated by power splitters:

Dispersion penalty for MLMs (Agrawal 1988)

Typical p-i-n receivers w/ ~150nA current noise, 1.25Gbps, R~1

-27dBm (about1W) Typical 1310nm FP lasers 0dBm output power (about1mW)

dBBDLISI 8.2)(142++=

(For N=32, L=20km; typically ~ 24-26dB w/ connectors, splices, etc.)


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MODE PARTITION NOISE EFFECT

Mode Partition Noise is due to fluctuations

in individual Fabry Perot modes coupled

with optical fiber dispersion.

Due to uncontrolled temperature and

wavelength drift in FP diodes, d/dT ~0.3nm/oC, and D()~S

0, the magnitude of

this penalty will change with time.

Due to lack of screening of FP mode

partition coefficient, k, the magnitude of

this penalty will also depend on particularFP!

D(ps

/nm.km)

(nm)0


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Bit Rate and Reach Limits due to MPN

Reach dependent on quality of laser (k factor)

(another) Reason why asymmetry in PONs (e.g., 155/622Mbps) are favored GigE?

Worst-case isnt quite fair statistical model shows most fiber-laser combinations, D


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REDUCING MPN

Dispersion Compensation at OLT Additional Loss, some cost

One-size wont fit all, SMF 0~ 1300-1325nm

High-pass filtering using SOA

Low frequency MPN components are partially removed

Very low noise FP LD driver

Replace FP w/ narrow-line source DFB is current solution

1310nm VCSEL (high-power) Fiber Bragg Grating ECL also a possibility if cost/integration

improves


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Structure of WDM MUX/DEMUX(Arrayed Waveguide Grating)

(100) Si

B,P-doped v-SiO2

Thermal v-SiO2

P-doped v-SiO2 core

} core layer

TM, y

TE, x

Input

waveguides

Output

waveguides

Arrayed

waveguides Star coupler


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Types of Lasers & Receivers used for

Telecommunications

introduction to optical networking

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