optical communication systems nitin kumar asst professor electronics & comm. engineering deptt...

Optical CommunicationOptical Communication Systems Systems

NITIN KUMARNITIN KUMARAsst Professor Asst Professor

Electronics & comm. Engineering DepttElectronics & comm. Engineering DepttSDEC, GHAZIABADSDEC, GHAZIABAD

OVERVIEWOVERVIEW

Information Systems Evolution & What is it ?Information Systems Evolution & What is it ? Why there is Demand of Large bandwidth ?Why there is Demand of Large bandwidth ? Why Optical Fiber Technology ?Why Optical Fiber Technology ? Optical Transmission fundamentals.Optical Transmission fundamentals. How to Explode the optical fiber bandwidth ?How to Explode the optical fiber bandwidth ? Data rate requirements for high speed Data rate requirements for high speed

networks.networks. Optical Fiber Solutions for today’s Systems Optical Fiber Solutions for today’s Systems

& Networks& Networks..

An Information ModelAn Information Model

Definition:Definition:Delivering information to an Delivering information to an authorized user authorized user when it is when it is neededneeded, , wherever it is neededwherever it is needed i.e, regardless of the physical i.e, regardless of the physical location of the user or of the location of the user or of the information, and information, and whatever whatever form it is neededform it is needed in a secure in a secure way.way.

Information Systems Information Systems EvolutionEvolution

Compared to legacy systems today’s Compared to legacy systems today’s Systems are:Systems are:- - Data oriented, large, and complexData oriented, large, and complex- - On-line, interactive with strong emphasis On-line, interactive with strong emphasis on user interface e.g. Graphical User on user interface e.g. Graphical User InterfaceInterface- - Global, distributed and extensive in their Global, distributed and extensive in their reachreach- More volatile and subjective to constant - More volatile and subjective to constant changechange

Today’s systems often require reuse of Today’s systems often require reuse of components of existing systems and components of existing systems and building new systems to deal with changes building new systems to deal with changes

Needs For Today’s Optical SystemsNeeds For Today’s Optical Systems

Increase capacity of transmission Increase capacity of transmission (bit/sec).(bit/sec).

Minimize insertion loss (dB).Minimize insertion loss (dB).

Minimize polarization dependent loss Minimize polarization dependent loss (PDL).(PDL).

Minimize temperature dependence of the Minimize temperature dependence of the optical performance (a thermal solutions).optical performance (a thermal solutions).

Minimize component packaging size Minimize component packaging size (integrability).(integrability).

Modularity of components is an Modularity of components is an advantage (versatility)advantage (versatility)

TrendsTrends

Internet:Internet: A Deriving force A Deriving force

SOME ACTUAL FACTSSOME ACTUAL FACTS 12 Million email12 Million email messages in next minute messages in next minute 0.5 Million voice0.5 Million voice mail messages in next minute mail messages in next minute 3.7 Million3.7 Million people log on the net today people log on the net today Next 100 daysNext 100 days, Internet traffic doubles , Internet traffic doubles 100 Million100 Million additional internet users every year additional internet users every yearData based on the survey at Bell Laboratories, USA in Nov., 2000.Data based on the survey at Bell Laboratories, USA in Nov., 2000.

DEMAND FOR MORE BANDWIDTH DEMAND FOR MORE BANDWIDTH ONLY SOLUTION ISONLY SOLUTION IS

OPTICAL COMMUNICATIONOPTICAL COMMUNICATION

The Race for BandwidthThe Race for Bandwidth19951995 20012001

World Wide World Wide Web UsersWeb Users

6 Million6 Million 300+300+ MillionMillion

World Wide World Wide Web Web

ServersServers

100K100K 17+17+ MillionMillion

Monthly Monthly Internet Internet TrafficTraffic

31 31 TerabytesTerabytes

350,000350,000 TerabyteTerabyte

ss

Internet Internet Backbone Backbone DemandDemand

DoubleDoubles Every s Every

6 6 MonthsMonths

Exploding Demands for Exploding Demands for BandwidthBandwidth

Optical Fiber Bandwidth as a function of timeOptical Fiber Bandwidth as a function of time40 X OC– 92 denotes 40 wavelength channels40 X OC– 92 denotes 40 wavelength channels

OC-48= 2.5Gb/s, OC-192=10Gb/s, OC-OC-48= 2.5Gb/s, OC-192=10Gb/s, OC-768=40Gb/s768=40Gb/s

# W

DM

-cha

nnel

s

4

16

64

256

0.01 0.1 1 10 100

Channel bitrate (Gb/s)

1

Trunk transmission capacityTrunk transmission capacity

•‘97

10 Gb/s

1 Tb/s

0.1 Gb/s

1 Gb/s

100 Gb/s

•‘98

•‘98•

‘99

•‘00

•‘02?

•‘86

•‘96

•‘89

•‘83

•‘80

Do We Need Terabits ?Do We Need Terabits ?

Information SystemsInformation SystemsComputing ShiftComputing ShiftThe InternetThe InternetLigthwave Capacity Ligthwave Capacity

TrendsTrendsGlobal NetworkingGlobal Networking

Facts Regarding Optical Facts Regarding Optical TransmissionTransmission

BIT RATE INCREASING

TRANSMISSION DISTANCE INCREASING

Capacity Growth of Optical Capacity Growth of Optical Fiber Each YearFiber Each Year

YearYear Capacity (Gb/s)Capacity (Gb/s)1980 1980 0.10.11985 1985 1 1 19901990 331995 1995 552000 2000 100 100 (40 practically (40 practically

shown)shown)2005 2005 1,000 1,000 (If (If

limitations due to Dispersion &limitations due to Dispersion & Nonlinearities are overcome)Nonlinearities are overcome)

The optical world is approaching The optical world is approaching towardstowards

1. 50 THz1. 50 THz Transmission WindowTransmission Window 10001000 Channel WDMChannel WDM 100 Gb/s100 Gb/s TDMTDM 1000 km1000 km Repeater less Repeater less

transmissiontransmission

If Nonlinearities can be If Nonlinearities can be controlled, transmission controlled, transmission window will be window will be 300THz300THz

Optical Fiber ApplicationsOptical Fiber Applications

Fiber to the HomeFiber to the Home

OFC Backbone CapacityOFC Backbone Capacity

Bandwidth-What is it ?Bandwidth-What is it ?

Bandwidth is the a Bandwidth is the a measure of information measure of information carrying capacity of a mediumcarrying capacity of a medium..

To the digital word, it is translated into a To the digital word, it is translated into a maximum bit rate at which signals can be maximum bit rate at which signals can be sent without significant signal degradationsent without significant signal degradation

Fiber bandwidth is typically quoted in Fiber bandwidth is typically quoted in frequency and normalized to fiber length frequency and normalized to fiber length (MHz-Km)(MHz-Km)- - As length increases bandwidth decreasesAs length increases bandwidth decreases

A fiber bandwidth is determined by its A fiber bandwidth is determined by its pulse spreading propertiespulse spreading properties

Bandwidth-What is it ? Bandwidth-What is it ?

The The difference between the difference between the highest and lowest frequencies of highest and lowest frequencies of a banda band that can be passed by a that can be passed by a transmission medium without transmission medium without undue distortion.undue distortion.

A term used to indicate the A term used to indicate the amount of transmission or amount of transmission or processing capacity possessed by processing capacity possessed by a system or specific location in a a system or specific location in a systemsystem (Usually a network system) (Usually a network system)

Copper Versus Fiber: Copper Versus Fiber: RepeatersRepeaters

Eliminate the dangers found in Eliminate the dangers found in

areas of high lightning-strikeareas of high lightning-strike

Fiber links offer over 1,000 times Fiber links offer over 1,000 times as much bandwidth and distances as much bandwidth and distances

over 100 times over 100 times

DistancDistancee

BandwiBandwidthdth

Voice Voice ChannelChannel

ss

CopperCopper 2.5 km2.5 km 1.5 Mb/s1.5 Mb/s 2424

FiberFiber 200 KM200 KM 2.5+ 2.5+ Gb/sGb/s

32,000 32,000 ++

Electromagnetic SpectrumElectromagnetic Spectrum

Introduction to Optical Introduction to Optical COmmunicationCOmmunication

The first practical scheme of optical The first practical scheme of optical communication, was invented by Alexander communication, was invented by Alexander Grahm Bell, in 1880, the Photophone.Grahm Bell, in 1880, the Photophone.

Photophone:Photophone: Device in which speech can be Device in which speech can be transmitted on a beam of light, using mirrors transmitted on a beam of light, using mirrors & selenium detectors.& selenium detectors.

Present optical communication systems use Present optical communication systems use Laser & Optical Fiber technologies.Laser & Optical Fiber technologies.

Optical frequency is typically 10Optical frequency is typically 101414 Hz, which Hz, which can support wideband modulation. Compared can support wideband modulation. Compared to microwave frequencies 10to microwave frequencies 1099 Hz, the optical Hz, the optical career can offer 10career can offer 105 5 times more bandwidth.times more bandwidth.

Basics of Fiber Optic CommunicationBasics of Fiber Optic Communication Fiber Optics is a revolutionary Fiber Optics is a revolutionary

development that has changed the face development that has changed the face of telecommunications around the worldof telecommunications around the world

Transmission of data as a light pulses Transmission of data as a light pulses through optical fiber (first converting through optical fiber (first converting electronic binary signals to light and then electronic binary signals to light and then finally converting back to electronic finally converting back to electronic signals)signals)

Elements of Fiber OpticsElements of Fiber OpticsTransmissionTransmission

Light Source (such as Infrared LED converts Light Source (such as Infrared LED converts pulses and sends into optical fiber)pulses and sends into optical fiber)

850 nm, 1300 nm850 nm, 1300 nm Low cost, easy to useLow cost, easy to use Used for multi mode fiberUsed for multi mode fiber Special edge emitting LEDs for single mode fiberSpecial edge emitting LEDs for single mode fiber

Basics of Fiber Optic Communication Basics of Fiber Optic Communication (Contd..)(Contd..)

Laser Source having propertiesLaser Source having properties CoherenceCoherence MonochromaticityMonochromaticity DirectionalityDirectionality High Specific IntensityHigh Specific Intensity 850 nm, 1300 nm, 1550 nm850 nm, 1300 nm, 1550 nm Very high power outputVery high power output Very high speed operationVery high speed operation Very expensiveVery expensive Need specialized power supply & circuitryNeed specialized power supply & circuitry

ReceptionReceptionPhoto detector converts back to electrical pulsesPhoto detector converts back to electrical pulses

PIN DIODESPIN DIODES 850, 1300, 1550 nm850, 1300, 1550 nm Low costLow cost

APDs (Avalanche Photodiodes)APDs (Avalanche Photodiodes) 850, 1300, 1500 nm850, 1300, 1500 nm High sensitivity, can operate at very low power levels High sensitivity, can operate at very low power levels expensiveexpensive


Propagation in FiberPropagation in Fiber Light propagates by mans of total internal reflection. Light propagates by mans of total internal reflection. Optical Fiber consists of two concentric layersOptical Fiber consists of two concentric layers

Core – inner layerCore – inner layer Cladding – outer layerCladding – outer layer

Refractive index of core is greater than cladding, Refractive index of core is greater than cladding, necessary for total internal reflectionnecessary for total internal reflection

Light entering with acceptance angle propagates Light entering with acceptance angle propagates through fiberthrough fiber

Strikes core cladding interface > critical angle and Strikes core cladding interface > critical angle and gets reflected completely.gets reflected completely.

Zig-zags down length of core through repeated Zig-zags down length of core through repeated reflections.reflections.

Fairly lossless propagation through bends also.Fairly lossless propagation through bends also. Optical fiber Optical fiber

Multimode (Graded Index 50/125Multimode (Graded Index 50/125 & 62.5/125 & 62.5/125 ) ) Single mode (8.7 /125 Single mode (8.7 /125 ) )


Major Advantages of FOCMajor Advantages of FOCLarge Bandwidth (Extremely high information carrying Large Bandwidth (Extremely high information carrying

capacity)capacity) Carrier frequency – Light – 10Carrier frequency – Light – 101414 Hz Hz Makes possible widespread long distance communication Makes possible widespread long distance communication

of high bandwidth signalsof high bandwidth signals Color videoColor video High speed networkHigh speed network

High degree of Multiplexing, without much interference High degree of Multiplexing, without much interference among them.among them.

Low Loss (Long repeaterless link length/repeater Low Loss (Long repeaterless link length/repeater spacing)spacing)

Loss as low as 0.1 dB/KmLoss as low as 0.1 dB/Km Repeater spacing of over 100 Km possible over land & Repeater spacing of over 100 Km possible over land &

under sea.under sea.EMI immunity (Even in noisy or harsh environments-EMI immunity (Even in noisy or harsh environments-

Lightning, factory floor, high voltage lines, broadcast Lightning, factory floor, high voltage lines, broadcast towers)towers)


Major Advantages of FOC (Contd..)Major Advantages of FOC (Contd..)Compact and light weightCompact and light weight

Single fiber can easily replace 1000 pair copper Single fiber can easily replace 1000 pair copper cable of 10 cm dia.cable of 10 cm dia.

Security (impossible to tap)Security (impossible to tap)Safety (insulator & no sparks – ideal for Safety (insulator & no sparks – ideal for

hazardous environment)hazardous environment)Can be used inCan be used in

Oil explorationOil explorationOil refineriesOil refineriesMinesMinesExplosivesExplosivesPetrochemicalPetrochemicalOther hazardous chemicalOther hazardous chemical


Some practical disadvantages of FOC Some practical disadvantages of FOC Fiber is expensiveFiber is expensiveConnectors very expensive (due to degree Connectors very expensive (due to degree

of precision involved)of precision involved)Connector installation time consuming & Connector installation time consuming &

highly skilled operationhighly skilled operationJoining (splicing) of fibers requires Joining (splicing) of fibers requires

expensive equipment & skilled operatorsexpensive equipment & skilled operatorsConnections & joints are relatively lossyConnections & joints are relatively lossyDifficult to tap in & out (for bus Difficult to tap in & out (for bus

architectures) need expensive couplersarchitectures) need expensive couplersRelatively careful handling requiredRelatively careful handling required

Advances in Optical CommunicationAdvances in Optical CommunicationFirst Generation Support:First Generation Support:

Operating at: 850 nmOperating at: 850 nmBit Rates: 50 -100 MbpsBit Rates: 50 -100 MbpsRepeater Spans: 10 KmsRepeater Spans: 10 KmsSources & Detectors made of InGaAsP compound Sources & Detectors made of InGaAsP compound

semiconductorsemiconductor

Second Generation Support:Second Generation Support:Operating at: 1300 nmOperating at: 1300 nmBit Rates: 1-2 GbpsBit Rates: 1-2 GbpsRepeater Spans: 40 -50 Kms Repeater Spans: 40 -50 Kms Sources & Detectors made of InGaAsP compound Sources & Detectors made of InGaAsP compound

semiconductor semiconductor

Third Generation Support:Third Generation Support:Operating at: 1550 nmOperating at: 1550 nmBit Rates: 2.4 GbpsBit Rates: 2.4 GbpsRepeater Spans: 100 KmsRepeater Spans: 100 Kms

Advances in Optical Communication Advances in Optical Communication (Contd..)(Contd..)

Present Standards Supported:Various multiplexing techniques for enhanced capacity utilization, use of optical amplifiers & Soliton – based transmission systems developed.Speed & Repeater spacing due to fiber optic systems, newer standards such as:

•FDDI (Fiber Distributed Data Interface)•DQDB (Dual Queue Distributed Bus)•SONET (Synchronous Optical Network)•SDH (Synchronous Digital Hierarchy)

Advances in Optical Communication Advances in Optical Communication (Contd..)(Contd..)More Advanced Systems:

Era of high capacity Trans Atlantic Telecommunication (TAT) began as under:

TAT - 2 in 1959TAT – 6 in 1976TAT – 7 in 1983 (offered a capacity of about 4000 analog circuits)Optical fiber based TAT – 8 in 1989 (offered 40,000 circuits, 64,000 Km long, 280 Mbps, 40 Km repeater distance )TAT - 12/13 with many new features is now operationalSome other fiber systems include HAW – 4 (Hawaiian Cable 4), TPC – 3(Trans – Pacific Cable –3)

Advances in Optical Communication Advances in Optical Communication (Contd..)(Contd..)Further achievements include

Fiber losses 0.16 dB/Km (at 1550 nm)Laser with threshold currents of few milli-amperes and life time of over a million hoursRepeater spans of more than 200 Kms.Transmission rates in excess of 2 GbpsAdvent of EDOFA (Erbium-Doped optical fiber amplifier), using dispersion compensating Soliton transmission techniques or the use of dispersion compensating fibers (DCF) and the improvements made in the attenuation & dispersion characteristics of the modern optical fiber have led to the demonstration of data transmission in experiments with repeaterless spans of over 10,000 Km and bit rates in excess of 10 GbpsMore complex coherent optical communication, wavelength routed, dense wavelength division multiplexing (DWDM) links are available.

Advances in Optical Communication Advances in Optical Communication (Contd..)(Contd..)Coherent communication systems make use of:

Sources & detectors made of quantum well structures with high directional properties.Single mode single polarization optical fiber having very low loss and very low dispersion.Has superior SNR capabilities, long repeater spans & high bit rates.

WDM (Wavelength Division Multiplexing)Provides an easy way to increase the utilization of the high channel channel capacity of the optical fiber.

Integrated OpticsDeals with the miniaturization & integration on a single substrate optical components such as

- electro optic modulator- polarization controller- splitters / combiners- directional couplers- lenses


-Optical MEMs make use of silicon micro machining to realize micro-opto-mechanical elements-Soliton Propagation in Optical Fibers

-Initially launched pulse may propagate with ultra-low dispersion over thousands of Kilometers-Active devices within fibers EDFA (Erbium Doped Fiber Amplifiers) are now available.-Photonic switching architectures (which use integrated optic switches) & optical MEMs provides data – rate transparent switching services to optical fiber based trunks


S i g n a l s V o i c e , D a t a , V i d e o , I n t e g r a t e d S e r v i c e s

S y s t e m s P o i n t - t o - P o i n t , M u l t i p o i n t , S h o r t - h a u l ,L o n g - H a u l ( U n d e r s e a )

S t a n d a r d s S O N E T / S D H , F D D I , I S D N , B I S D N , A T M

D e p l o y m e n t L A N , M A N , W A N , C A T V , H F C , F T T C ,F T T H

P h o t o n i cT e c h n o l o g y

P h o t o n i c s w i t c h i n g , W D M / T D M / O F D M ,A l l o p t i c a l / p h o t o n i c n e t w o r k s , S o l i t o nS y s t e m s , O p t i c a l a m p l i f i c a t i o n

Features of Present Optical Communication


System Design Issues

Source Receiver Fiber

LED Laser Diode

Detector Amplifier

Quantum Quantum NoiseNoise

Quantum Noise Optical: Spontaneous emission noise

Mode partition noise

Bandwidth Mode Limit Partition noise Bandwidth Limit

Shot noise Electronic: Shot & thermal noiseBandwidth BandwidthLimit Limit

Dispersion limit Non-linear effects

Information Transmission Information Transmission SequenceSequence

Optical Communication Optical Communication SystemsSystems

First Generation, ~1975, 0.8 mMM-fibre, GaAs-laser or LED

Second Generation, ~1980, 1.3 m, MM & SM-fibreInGaAsP FP-laser or LED

Third Generation, ~1985, 1.55 m, SM-fibreInGaAsP DFB-laser, ~ 1990 Optical amplifiers

Fourth Generation, 1996, 1.55 mWDM-systems

1.80.8 1.0 1.2 1.4 1.60.9 1.1 1.3 1.5 1.7Wavelength (m)

Att

en

ua

tion

Fiber StructureFiber Structure

A A Core Carries most of the lightCore Carries most of the light, surrounded , surrounded byby

A A Cladding, Which bends the light and Cladding, Which bends the light and confinesconfines it to the core, covered by it to the core, covered by

A primary buffer coating which A primary buffer coating which provides provides mechanical protectionmechanical protection, covered by, covered by

A secondary buffer coating, which A secondary buffer coating, which protects protects primary coating and the underlying fiberprimary coating and the underlying fiber..

Fiber Structure Cont…Fiber Structure Cont…

Types Of Optical Fibre Types Of Optical Fibre

Single-mode step-index fibre

Multimode step-index fibre

Multimode graded-index fibre

n1 coren2 cladding

no air

n2 cladding

n1 core

Variablen

no air

Lightray

Index porfile

Multimode Step Index FiberMultimode Step Index Fiber

Core diameter range from Core diameter range from 50-100050-1000mm Light propagate in many different ray Light propagate in many different ray

paths, or modes, hence the name paths, or modes, hence the name multimodemultimode

Index of refraction is same all across Index of refraction is same all across the core of the fiberthe core of the fiber

Bandwidth range 20-30 MHzBandwidth range 20-30 MHz

Multimode Graded Index Multimode Graded Index FiberFiber

The index of refraction across the The index of refraction across the core is gradually changed from a core is gradually changed from a maximum at the center to a maximum at the center to a minimum near the edges, hence minimum near the edges, hence the name “Graded Index”the name “Graded Index”

Bandwidth ranges from Bandwidth ranges from 100MHz-100MHz-Km to 1GHz-KmKm to 1GHz-Km

Pulse SpreadingPulse Spreading

time

Pulse from zero-order mode

Pulse from highest-order mode

Pulses from other modes

Resulting pulse

T

T

T

T

T

Calculation of Pulse SpreadCalculation of Pulse Spread

C C

x

y/2 y/2

Cyx cos

Modes of Vibration of a StringModes of Vibration of a String

Lowest order Lowest order modemode

Second order Second order modemode

Third order modeThird order mode

)sin( 01 tA

)2sin( 02 tA

)3sin( 03 tA

Single-Mode Graded Index Single-Mode Graded Index FiberFiber

The Core diameter is The Core diameter is 8 to 98 to 9mmAll the multiple-mode or All the multiple-mode or

multimode effects are eliminatedmultimode effects are eliminatedHowever, pulse spreading However, pulse spreading

remainsremainsBandwidth range Bandwidth range 100GHz-Km100GHz-Km

Typical Core and Cladding Typical Core and Cladding Diameters (Diameters (m)m)

Acceptance Cone & Numerical Acceptance Cone & Numerical ApertureAperture

n2 cladding

n2 cladding

n1 coreAcceptance

Cone

Acceptance angle, c, is the maximum angle in whichexternal light rays may strike the air/fibre interfaceand still propagate down the fibre with <10 dB loss.

22

21

1sin nnC Numerical aperture:NA = sin c = (n1

2 - n22)

C

Multiple OFCMultiple OFC

Standard Optical Core SizeStandard Optical Core Size

•The standard telecommunications core sizes in use today are:8.3 µm (single-mode), 50-62.5 µm (multimode)

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optical communication systems nitin kumar asst professor electronics & comm. engineering deptt...

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