Introductory Lecture on

Photonic Networks

Motivation and ObjectivesThe idea behind the all-optical networking is to maximize the transmission distance and deliver transparent and flexible connections. Keeping it all optical will result in cost reduction both in capital as well as in operations. It will lead to an ultimate realization of the Optical Internet.

Limiting Optical-Electrical-Optical (OEO) conversions to mainly the ingress and egresspoints of the network, and thus reducing the amount of equipment that needs to be installed to add capacity to the network, should achieve the economic gains. A number of Ultra-Long-Haul (ULH) system vendors have promoted such a benefit but none to date have fulfilled the promise.

Some HistoryOptical Communication is not new!

Alexander Graham Bell’s Photophone, 1880

.The Network Life Cycle

.*Source: From Bell to Broadband, Professor Keith Ward of University College London

TelegraphNetwork

ManualTelephoneNetwork

SwitchedAnalog

TelephoneNetwork

DigitalNetwork

1880 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000

BroadbandNetwork

VF TelexMessaging Magneto

AudioAmplifiers

SleeveControl

Switchboard

ElectricalExchange

CoaxSystems

VF Signaling PCM

ISDN

SONET/SDH

Intelligent Network

#7 Signaling

ATMIP

DWDM

Transmission Media• Transmission Medium, or channel, is the actual physical

path that data follows from the transmitter to the receiver.

• Copper cable is the oldest, cheapest, and the most common form of transmission medium to date.

• Optical Fiber is being used increasingly for high-speed applications.

Fiber Replaces Copper

Fiber Optics

.Fiber Optic Transmission Bands

.

Near InfraredFrequency

Wavelength1.6

229

1.0 0.8 µm0.6 0.41.8 1.4

UV

(vacuum) 1.2

THz193 461

0.2

353

Longhaul Telecom

Regional Telecom

Local Area Networks850 nm

1550 nm

1310 nmCD Players780 nm

HeNe Lasers633 nm

C=νλ /n

Why Optical ?

• Growing demand for faster and more efficient communication systems

• Internet traffic is tripling each year• It enables the provision of Ultra-high

bandwidth to meet the growing demand• Increased transmission length• Improved performance• etc.

Demand for Bandwidth

BandwidthDemand

1990 2000 2010

• Raw text = 0.0017 Mb• Word document = 0.023 Mb• Word document with picture = 0.12 Mb• Radio-quality sound = 0.43 Mb• Low-grade desktop video = 2.6 Mb• CD-quality sound = 17 Mb• Good compressed (MPEG1) video = 38 Mb

Typical data bandwidth requirement

20,000 x

Light Provides more BandwidthBandwidth is the range of frequencies employed to transmit (communicate) a signal (information)More bandwidth = more information.

Typical Signals:Audible Sound-waves (20 to 20,000 cycles/s)AM radio uses 10kHzFM radio uses 100kHzTV uses 6MHz rangeSignals: sound = a variation in air pressure

loudness = the amplitude of the variationpitch = the frequency of the variationwavelength = speed of propagation

pitchΛs: 1100ft/sec = 2.5 ft.

440Hz

•• After After ShoaShoa--Kai LiuKai LiuDirector Director -- Network Technology Development; WorldComNetwork Technology Development; WorldCom

All-Optical Network(Terabits ⇒ Petabits)

TDM DWDM

0

5

10

15

20

25

30

35

40

Ban

dwid

th

8λ @OC-484λ @OC-192

4λ @OC-48

2λ @OC-48

2λ @1.2Gb/s(1310 nm, 1550 nm)

10 Gb/s

2.4 Gb/s1.2 Gb/s565 Mb/s

1.8 Gb/s810 Mb/s405 Mb/s

EnablersEDFA + Raman AmplifierDense WDM/FilterHigh Speed OptoelectronicsAdvanced Fiber

1982

1984

1988

1994

1996

1998

2000

2002

1990

1986

1992

16λ @OC-192

40 Gb/s

32λ @OC-192

176λ @OC-192

2004

2006

TDM (Gb/s)

EDFA

EDFA +Raman Amplifier

80λ @ 40Gb/s

Bandwidth Evolutionary LandmarksBandwidth Evolutionary Landmarks

Point-to-Point Optical Linka building block for backbone and all-optical networks.

Tx/RxE

Tx/RxE

DLaserAPDA

ModFThresh

DLaserAPDA

ModFThresh

The Need for Photonic Networks

Node A

Node J

Node C

Node BRegeneratoror amplifier

Circuit

Path

Media

• Optics is used only for point to point links• All other functionality is in the electronics

– frequent switching enables high utilisation of transmission link

The Need for Photonic Networks

Transmitter ReceiverTransmitter Receiver

Transmitter Receiver

Transmitter Receiver

Transmitter Receiver

Transmitter ReceiverTransmitter Receiver

Transmitter Receiver

WDM + amps saves regenerators + fibre

Increasing transmission capacityIncreasing transmission capacity by by Wavelength Division MultiplexingWavelength Division Multiplexing

(WDM)

The Need for Photonic Networks

Capacity(log scale)

1990 2000

2Mb/s8Mb/s

34Mb/s

140Mb/s565Mb/s

2.5Gb/s

10Gb/s 4λ

8λ16λ

32λ

64λ

128λ

1980 1985 1995

Year

1 Tb/s

TDM

WDM now

WDM soon

Growth of transmission capacity with time

The Need for Photonic Networks

40x2.5 16x10 40x1032x2.5 80x2.5 32x10

Capacityper fibre

(Gb/s)

64x10

Compiled from information in vendor web sites

16x2.50

100200300400500600700

•OADM and optical protection features also available

•40Gb/s systems around the corner?

Capacity of existing or announced WDMCapacity of existing or announced WDM productsproducts

Bandwidth and Capacity

First Generation

Wideband

35nm; 80ch

Ultra-Wideband

80nm; 200chEnables TerabitSystems !

Optical Technology - Advantages

• High data rate, low transmission loss and low bit error rates• High immunity from electromagnetic interference• Bi-directional signal transmission• High temperature capability, and high reliability• Avoidance of ground loop• Electrical isolation• Signal security• Small size, light weight, and stronger

448 copper pairs5500 kg/km

62 mm

21mm648 optical fibres

363 kg/km

Importance of Optical Networks

• The services provided by Optical Technology are:– Next Generation Internet Internet 2– IPTV– Video Conferencing– Online Gaming– And many more

Applications

Optical Communication SystemsHigh Speed Long Haul Networks (Challenges are transmission type)

Metropolitan Area Network (MAN) ?Access Network (AN)?Challenges are:

- Protocol- Multi-service capability- Cost

Electronics and ComputersBroad OptoelectronicMedical ApplicationInstrumentation

Optics

is he

re to

stay f

or

a lon

g tim

e.

Evolution of Photonic Networks• Further increase in transmission capacity of TDM lines to 40

Gb/s (ETDM) and above (OTDM)• Further increase in number of wavelengths above 100• Intelligent electrical at border of transparent optical core• Wavelength switching devices introduced in network core• IP routers with optical interfaces deployed• IP routers combined with OADMs and OXCs equipped with

optical interfaces enabling optical bypass

Evolution of Photonic Networks• New kinds of network elements optimized for

packet switching/routing. UNI between electronic routers and optical network elements and NNIbetween optical network elements with control and signalling capability

• Integrated protocols and algorithms for traffic engineering, path establishment, protection and restoration in the optical layer coordinated with IP layer will be developed

Evolution of Photonic Networks

Fibre level

Wavelengthchannel

level

Sub-wavelength

level

Incr

easi

ng re

solu

tion

Manualprovisioning

Remoteprovisioning

Dynamicprovisioning

Fibrepatchpanels

Opaque‘Oparent’

Transparent

Digital cross-connect

Tbitrouter

‘Oparent’packet router

Transparentphotonic

packet router

Network Evolution

– High Capacity Transmission

– Automated Connection Provisioning – Flexible Adjustment of Bandwidth– Network Self-Healing/Restoration

WDM/Point-to-Point Transport

– Fixed Sharing Between Multiple Nodes – Passive Access of Wavelength Channels

Fixed WDM/Multipoint Network

Photonic XC and WADMReconfigured WDM Network

Fiber Amplifier

Wavelength Mux/DMux

Wavelength Add/Drop

Wavelength Cross-Connect

.

WDM Adds a New Dimension

.

Power

Wavelength

Time

• Cross-phase modulation

• Laser power• Fiber attenuation• Component and system losses

• Fiber PMD• Fiber dispersion• Jitter• Transmission rate

• EDFA bandwidth• Central wavelength

stability • BW stability

• EDFA ASE• EDFA gain• Crosstalk• 4W mixing• Raman FS• Brillouin BS

• Laser chirp• Chromatic dispersion• DGD

• Modulation• Self-phase mod.• BER

.

Anatomy of a DWDM system

All Optical Network

SDH

ATM

IP

SDH

ATM

IP

Open Optical Interface

SDH ATM IP Other

All Optical Networks

Challenges ahead:• Network routing• Network routing • True IP-over-optics• True IP-over-optics• Network protection• Network protection

Undersea Cables

System Block Diagram

Architecture of Photonic Networks

Tx-1

Tx-2

Tx-N

mux

OADM

OXC

Rx-1

Rx-2

Rx-N

demux

transmitterswavelength defined,

high speed?

multiplexergratings,couplers,

filters

fibredisp compensation?

nonlinearities?

optical amplifierEDFA?

gain flattened?

optical add-dropmultiplexer:

couplers, filters, circulators

optical cross -connect

couplers, amplifiers,filters , switches

demuxgratings,couplers,

filters

receivers

Network devices:

Key Functional Blocks

• Transceivers (LD, PD, Mod.)DFBs, VCSELs

• Optical Amplifiers (EDFAs, SOAs,Raman)

• DWDM (Sources, Mux/ de-Mux)

• Tunable Filters / OADMFBGs, FPs

• Switching/Routing

• Optical Circulators

Tx/Rx

E

OA

Mux

1 2

3

TransceiversCrucial building blocks

• Transmitters (LDs, LEDs, Drivers/ Modulators)(Precision Tunable Laser arrays needed for DWDM)

• Modulators (External, data source)

• Receivers (PDs, Pre-Amp, Filters)

signal outD Laser

APDA

Mod

FThreshsignal in

Data in

Source

SourcecodingSourcecoding ModulationModulation MultiplexingMultiplexing ModulationModulation

ExternalExternal InternalInternal

• Analogue• Digital• Analogue• Digital

• Frequency• Time• Frequency• Time

• Pulse shaping• Channel coding• Encryption• etc.

• Pulse shaping• Channel coding• Encryption• etc.

Receiver

1st-stageamplifier1st-stageamplifier

2nd-stageamplifier2nd-stageamplifier

Pre-detectionfiltering

Pre-detectionfiltering

Sampler&

detector

Sampler&

detector

DemultiplexerDemultiplexer

• Equalizer DemodulatorDemodulator

Output signalOutput signal

DecoderDecryptionDecoder

Decryption

Light Sources

Semiconductor lasers, LEDs and particularly VCSEL’s remain an important area for further work.

Communication Window and Er

Nature’s gift to optical communications: Erbium gain spectrum and transmission fiber minimum loss wavelengths coincide.

Gain

Absorption

EDFA: A Key Enabler for WDM

Data OutWDM Point-to-Point

XMTR OMUX

Data In

OAXMTR

XMTR

OA

RCVR

RCVR

RCVR

ODMUX

•••••

•••••

λ1

λ2

λN

λ1

λ2

λN

OAOA

• High power• Low noise figure • Bit-rate transparent• No cross-talk• Wide bandwidth• Excellent mech. property • and more …

Optical Amplifiers (EDFA)Erbium doped fiber amplifiers permit the boosting of optical signals within the fiber, without the o-e-o conversion that limits bit rates!

These work exactly in the range of frequencies, 1525 - 1620 nm, cover whole third window !

This technology permits long haul transmission and, more importantly, enables:

Wavelength Division Multiplexing (WDM)

Today EDFAs Exist in S-band, C-band and L-band

Optical Isolators and Circulators

.

1 2

3

0o 45o 45o

PT

PR

Pi

Faraday Rotator

90o

Combination of Isolators make up a Circulator

.

.

1-λOADM

Out

Drop Add

In1 2

3 2 31

Opticalcirculator

Opticalcirculator

FBGλBragg = λ3

Fiber Bragg Gratings (FBG)

Example

OADM Technologies

Basics of Optical Mux/De-Mux

Prism based λ1

λ2

λ1+ λ2+..λn ..

λn

Grating based

Sub-Multiplexing and Multiplexing

.

.

.

DWDM ComponentsMEMS Switch

Technology

Opaque / Translucent Optical Switch

.Transponders1310 nm Optical Switching Fabric

Add traffic

Transponders

Output Fibers

Input Fibers

Optical Layer X-connect Opto-

electronic

Drop traffic

Output Fibers

Transparent Optical Switching Fabric without λ−Conversion

Optical Space Switch

λ2

Input Fibers

Optical Space Switch

λ1

Optical Space Switch

λn

Transparent Optical Switch Architecture

Optical Cross Connects (OXCs)

Chromatic dispersion PMD Non-linear effectsDispersion and non-linear effects

Fiber Related Problems

P

λ

Chromatic Dispersion

60 Km SMF-28

4 Km SMF-28

40 Gbps

t

10 Gbps

t

• It causes pulse distortion, pulse "smearing" effects

• Higher bit-rates and shorter pulses are less robust to Chromatic Dispersion

• Limits "how fast“ and “how far” data can travel

Polarization Mode Dispersion (PMD)

• The optical pulse tends to broaden as it travels down the fibre; this is a much weakerphenomenon than chromatic dispersion and it is of some relevance at bit rates of 10Gb/s or more

nx

nyEx

Ey

Input pulse Spreaded output pulse

Challenges AheadModulation and detection and associated high speed electronicsMultiplexer and demultiplexerFibre impairments:

. Loss

. Chromatic dispersion

. Polarization mode dispersion

. Optical non-linearity

. etc.

Optical amplifier . Low noise . High power . Wide bandwidth. Longer wavelength band S

Challenges Ahead

Dedicated active and passive componentsOptical switches managementAll optical regeneratorsNetwork protectionInstrumentation to monitor QoS

Optical Transport Network

Global Network

Wide Area Network

Metropolitan/Regional Area Optical Network

Corporate/Enterprise Clients

Cable modemNetworks

Client/Access Networks

FTTHMobile

SDH/SONET

ATM

PSTN/IP

ISPGigabit Ethernet

Cable

FTTB

ATM

< 10000 km< 10 Tbit/s

< 100 km< 1 Tbit/s

< 20 km100M - 10 Gbit/s

Types of MPLS NodesThere are three types of MPLS nodes as shown in diagram and perform the following

functions.

EgressLSR UserUser

IngressLSR

TransitLSR

TransitLSR

TransitLSR

TransitLSR

Ingress LSR receives native-mode traffic (e.g. IP datagram) and classifies into FEC. It then generates Generates MPLS header and assign it initial label. IP datagram encapsulated into MPLS packet with MPLS header attached to datagram. Integrated with QOS operations (if applicable).

Transit, Interior, or core LSR receivers the packet and use MPLS header to make forwarding decision. It processes label header only and performs label swapping.

Egress LSR receives performs decapsulation operations (it removes the MPLS header).

Resources & facilities @ NIIT• Optsim• Access to vast online digital data

– IEEE (Comm Magazine, Comm letters, Photonic TechnlogyLetters (PTL), Journal of Lightwave Technology (JLT), Journal of Selected Areas in Communications (JSAC), etc

– ACM• Focussed Networking Research group with strong

international Collaborations (Stanford (PNRL), UNCC (Center for optoelectronics and optical communications), North Carolina Research Triangle, TTU, US

• Participation in International GLIF project• HONET Workshop/Symposia

Concluding Remarks• We have given a selective overview of Optical Networks

architecture from physical layers perspective

• Optical components technologies will undergo a major revolution for next generation all-optical internet !!

• Demand for low-cost high-quality active and passive components and modules,e.g. DWDM Interleavers, ROADMs, Optical Circulators, Mux/de-mux, Dispersion-compensating elements, OAs and Dynamic gain-flattening filters, and so on...is felt

• GMPLS supports the evolving OTN that will accommodate all type of traffic with a high level of reliability and transparency