Introduction to Optical Networking: From Wavelength Division

Multiplexing to Passive Optical Networking

Dr. Manyalibo J. MatthewsOptical Data Networking Research

Bell Laboratories, Lucent TechnologiesMurray Hill, NJ 07974 USA

University of Tokyo Visit – March 22, 2004

T.Harris A.Harris M.Matthews1997 2000

AT&T Lucent ‘Uber Alles’ Lucent ‘A la Carte’1996 2001

spectroscopy,NSOM,Confocal…device physics… network subsystems!

Evolution of Lucent and Matthews/Harris Lab:

Akiyama Matthews TunableLasers

TelecomLasersSemiconductor Laser

Device PhysicsQuantum

Wire Lasers

Outline• Introduction• Overview of Optical Networking

– Types of Networks– Fiber, Lasers, Receivers

• Coarse Wavelength Division Multiplexing

• Ethernet Passive Optical Networks• Conclusions & Future

Emergence of Optical NetworksO

ptic

alLi

ne S

yste

m

OLS 40/80GOLS 400G800G/1.6T

MeshBackboneNetwork Regional

Pointof

Presence

CO-1

CO-n

Core/Backbone/LongHaul

Regional/Metro

Access/Enterprise

EPONnode

MetroDMX

LocalServiceNodeMetro

EdgeSwitch

MetroEdge

Switch

OpticalCross

Connect

MetroDMX

Access

Node

Passive W

DM

Passive WDM

C/DWDM

C/DWDM

C/DWDM

MetroEdge

Switch

DSL,FTTH

PON

Wavelength Division Multiplexed (WDM)Long-Haul Optical Fiber Transmission System

Transmitter

Transmitter

Transmitter

Receiver

Receiver

Receiver

MUX

DEMUXOptical Amplifier

1

2

3

WDM “Routers” Erbium/Raman Optical Amplifier

Categorizing Optical NetworksWho Uses it?

Span (km)

Bit Rate(bps)

Multi-plexing

Fiber Laser Receiver

Core/LongHaul

Phone Company, Gov’t(s)

~103 ~1011

(100’s of Gbps)

DWDM/TDM

SMF/ DCF

EML/ DFB

APD

Metro/Regional

Phone Company, Big Business

~102 ~1010

(10’s 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 (<1nm spacing)CWDM: Coarse Wavelength Division Multiplexing (20nm spacing)TDM: Time Division Multiplexing (e.g. car traffic)SCM: Sub-Carrier Multiplexing (e.g. Radio/TV channels)SMF: Single-Mode Fiber (core~9m)MMF: Multi-Mode Fiber (core~50m)LWPF: Low-Water-Peak FiberDCF: Dispersion Compensating FiberEML: Externally modulated (DFB) laserDFB: Distributed Feedback LaserFP: Fabry-Perot LaserAPD: Avalanche PhotodiodePIN: p-i-n Photodiode

Optical Fiber Attributes

Attenuation:Due to Rayleigh scattering and chemical absorptions, the light intensity along a fiber decreases with distance. 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 spread in time and limits data rates.

Types of DispersionChromatic Dispersion is caused mainly by thewavelength dependence of the index of refraction (dominant in SM fibers)Modal Dispersion arises from the differences in group velocity between the “modes” travelling down the fiber (dominant in MM fibers)

t

t t

t

launch receive

Non-Linear Effects in Fibers

Self-Phase Modulation: When the optical power of a pulse is very high, non-linear

polarization terms contribute and change the refractive index, causing pulse spreading and delay.

Four-wave Mixing: Non-linearity of fiber can cause ‘mixing’ of nearby wavelengths causing interference in WDM systems.

Stimulated Brillouin Scattering: Acoustic Phonons create sidebands that

can cause interference.

Cross-Phase Modulation: Same as SPM, except involving more than one WDM channel, causing cross-talk

between channels as well.

800 900 1000 1100 1200 1300 1400 1500 1600 1700

0.5

1.0

1.5

2.0

2.5

3.0First

Window SecondWindow

ThirdWindow

ATTE

NUAT

ION

(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 and third generation optic network technology

Dispersion Characteristics*

1310nm 1550nm850nm

800 900 1000 1100 1200 1300 1400 1500 1600 1700

-120

-90

-60

-30

0

3.0

FirstWindow

SecondWindow

ThirdWindow

DISP

ERSI

ON C

OEFF

, D (

ps/k

m-n

m)

WAVELENGTH (nm)

• Standard SMF has zero dispersion at 1310nm– Low Dispersion => Pulses don’t spread in time

• Dispersion compensation needed at 1550nm– Limits data transmission rate due to ISI (inter-

symbol interference)• Dispersion not so important at 850nm

– Loss usually dominates

* Modal dispersion not included

Characterization of System QualityBit Error Rate:input known pattern of ‘1’s and ‘0’s 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’

Effect of Dispersion and Attenuation on Bit Rate

30

10

1

Bit rate (Mb/s)

Dist

ance

(km

)

0.1 10 100 1000 10,0001

1550nm

1310nm850nm

Dispersion limitedAttenuation limited

single-mode fiber

multi-m

ode fiberCoaxialcable

• 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 3 limit

Cat 7 limit

Cat 5 limit

x

Twisted Pair

Technology Trends850nm & 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 don’t need to go far, and you’re not using enough fiber ($) to justify wavelengthdivision multiplexing (WDM), I.e. low-quality lasers are OK.

Reason? You don’t need lots, but they have to be good enough to transmit over long distances… cost of fiber (and TDM) justifies WDM… 1550nm is better for WDM

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

Fiber Bragg Grating External Cavity Laser for Access/Metro Networks

• SHOW PLOTS OF FBG-ECL DATA• SHOW PICTURE OF XPONENT’S EXTENDED REACH FP

Typical FBG-ECL:

Bell Labs FBG-ECL:

HR AR

gainFBGLensed

tipT=25C

T=85C

HR AR

gainFBG

XB region T=25, 85C

1-2nm grating

<1nm grating

1309.0 1309.5 1310.0 1310.5 1311.0 1311.5 1312.0-80

-60

-40

-20

0

Wavelength (nm)

T=20C

Opt

ical

Pow

er (d

Bm

)

(3dB) typ<0.5nmddnm/oC

?

(from Xponent Photonics, Inc.)

Fiber Bragg Grating External Cavity Laser

FBG-ECLoutput

TypicalFP output

1305 1310 1315 1320 1325-70

-60

-50

-40

-30

-20

Pow

er (d

B)

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

20 30 40 50 60 70 801310.3

1310.4

1310.5

1310.6

1310.7

1310.8

1310.9

1311.0

ave

dependence 0.008nm/C

Wav

elen

gth(

nm)

Temperature (oC)

Wavelength Stability of FBG-ECL

CW, ~40mA bias

DFB drift ~ 0.1nm/oCFP drift ~ 0.3nm/oC

Filter bandwidths of WDM 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

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?

CWDM Lasers 16 uncooled, directly modulated CWDM lasers (DMLs)

rated for 2.5 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 50line

chip resistor

CWDM System Improvement using Electronic Dispersion Compensation

Example 2: Ethernet Passive Optical Networks

• NO Active Elements in Outside Plant• Enable “triple-play” services• Simple & cheap

IP VideoServices

PSTN

InternetPON

Headend/COHomes/BusinessesOutside Plant

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

OLTONU

Simple or Cascaded Star: low loss

ONUONUONU

ONUONUONU

ONUONUONU

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

EPON Access Platform

Video/IP TelevisionVoice/IP POTS serviceHigh-speed data

Residence

Metro Edge

Voice/IPServices

Business

Broadcast Video VOD

Management

Metro Network

Data

10G EthernetOr up to 6 1GbE

EPON

opticalsplitter

opticalsplitter

32 subscribersPer EPON

Panther EPON OLT Chassis1232 384 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

ONU Design

ReportGenerator

Packet Memory

TX

RX

ControlParser

Dem

ux

watchdog0

watchdog1

discoveryPeriodicReport

generatorEPON driver

EPON core

RX

TX

EPON MAC

Mux Timesta

mpCRC LLIDMemory

managerQueue

manager

GMII

SERDES&

Optics

CPUFPGA

Serial Port

GigE uplink

Packet memory

1.25G BM BiDi Xcvr

Flash (CPU)memory

10/100bTdiagnosticport

SERDES(w/CDR)

PON

FPGA w/EmbeddedProcessor

“CHILD” BOARD

“PARENT”BOARD

ONU

GrantList

GateGenerator

Packet Memory

RTT table

TX

RX

ControlParser

Dem

ux

watchdog0

watchdog1

discovery Keepalive scheduler

EPON driver MPCP driver

EPON core MPCP core

RX

TX

EPON MAC

Mux Timesta

mpCRC LLIDMemory

managerQueue

manager

RTT Processor

Report processor

GMII

SERDES&

Optics

Report table CPUFPGA

OLT Design

Serial Port

GigE uplink

Packet memory

1.25G BM BiDi Xcvr

Flash (CPU)memory

10/100bTdiagnosticport

SERDES(w/CDR)

PON

FPGA w/EmbeddedProcessor

• 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/upstream traffic

1 2 3 2

1

2 2

3

1 2 3 21

2 2

3

1 2 3 2

1 2 3 21

23

2

12

2

OLT

OLT

3

3

3 3

ONU

ONUO

NU

ONU

ONUO

NU

Edge Router

ONU: Optical Network UnitOLT: Optical Line Termination

Edge Router

Control “Gates”

Control “Reports”

PON TDMA BURSTMODE OPTICS

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

• Transmitting “0”s 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

BURSTMODE TRANSMITTERS

Tx FIFO Encoder Serializer TransmitterData

ClockPrebias

Physical Media

currentIth

Optical output

“0”

“1”

Modulationcurrent

“off”

• Driving LD belowThreshold causesJitter• Off-state ~ -40dBm

BURST-MODE RECEIVERS

• PROBLEM OF FAST CDR LOCKING• GAIN LEVELING & DYNAMIC

RANGE OF OPTICAL RECEIVER

Rx FIFO CDR LimitingAmp ReceiverData

Clock

DeserializerDecoder

Reset

IMPACT ON EFFICIENCY

~1460 Bytes64 Bytes

CRC

DMAC

SMAC

VLAN

HLEN

TOS

LEN

ID

OFF

ST

TTL

PROT

CHK

SM

SIP

DIP

ACK

HLEN

FLA

GS

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

Laser on

AGCsettle

CDRlock

Bytesync

ONU1 payload(Ethernet Frames)

Laser off

Throughput Efficiency

0.70.75

0.80.85

0.90.95

11.05

0 1000 2000 3000

AGC+CDR+LASER ON/OFF (ns)

Util

isat

ion

Our current situation Standard GE transceivers

Burst-mode transceivers

Conclusions• Optical Networking getting closer and

closer to end user• For Metro, CWDM is lowest cost solution,

but must be improved to handle 10Gbps• PON systems could deploy ‘in mass’ over

next 1-2 years, with EPON one of the leading standards

• Lasers dominate cost, therefore useful to study physics of low-cost laser structures!

THANK YOU VERY MUCH!(Domo Arigato Gozaimashita!)

Spare Slides

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 (about 1W)• Typical 1310nm FP lasers 0dBm output power (about 1mW)

dBBDLISI 8.2)(14 2

(for worst case, D=6ps/nmkm, L=20km, B=1.25Gbps, =3nm

dBlossesotherLNloss 22.log10

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

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()~S0, 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 particular FP!D

(ps/

nm.k

m)

(nm)0

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 isn’t quite fair… statistical model shows most fiber-laser combinations, D<3ps/nmkm, k<0.5.

2ln1

mpnkkBDL

2

12

ekmpn

BDL

Power penalty due to MPN given by(Ogawa 1985):

221log5 mpnmpn Q

Where k is the MPN coeficient, dependent on mode power correlations. 0.0 0.5 1.0 1.5 2.0 2.5 3.0

0

2

4

6

8

10

12

14

16

18

20

Q~6.7 (BER 10-11)2dB penalty

Rea

ch (k

m)

Bit Rate (Gbps)

k=0.5 k=0.7 k=0.9

REDUCING MPN

• Dispersion Compensation at OLT– Additional Loss, some cost– One-size won’t 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

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

Inputwaveguides

Outputwaveguides

Arrayedwaveguides Star coupler

Types of Lasers & Receivers used for Telecommunications