Optical communication system

Introduction to Optical Communication system

BY: ENG. MOHAMED HAMDY NAEEMSUPERVISOR: DR.MOSTAFA FEDAWAYCOURSE PHOTONICS DEVICES E414

E414

1-OPTICAL COMMUNICATION SYSTEM OVERVIEW2-HISTORY AND MATHEMATICS OF OPTICAL COMMUNICATION3-SYSTEM CONSTRUCTION IN DETAILI-CABLE CONSTRUCTION AND OPTICAL FIBER TYPESII-TRANSCEIVER CONSTRUCTIONIII-OPTICAL AMPLIFIER4-APPLICATIONS OF THE OPTICAL COMMUNICATION SYSTEM5-COMMISSIONING AND EXTENSION OF OPTICAL FIBER CABLE6-CABLE SPLICING7-TEST AND MEASURES8-CONCLUSION

By: Eng. Mohamed Hamdy Naeem

Contents

OPTICAL COMMUNICATION SYSTEM OVERVIEW

Transmissionchannel

Tx EO RxO

E

ReceiverConverterTransmitter Converter

25 ps

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1 0 1 1 0 1 0 1

25 ps

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use

1 0 1 1 0 1 0 1

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tempsPuiss

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use

Receiver Rx1Tx 1, bit-rate B

Tx N, bit-rate B Receiver RxN

… …


… …


… …

OpticalMultiplexer

OpticalDeMultiplexer

Wavelength Division MultiplexingTypical bandwidth: 1529-1565nm

50-100GHz channel spacing

Optical Spectrum

Throughput: NxB

Eye diagram

Optical FiberSection

OpticalAmplifier

Wavelength(nm)

2-History and Math of optical communicationA Short History of Optical Telecommunications

Circa 2500 B.C. Earliest known glassRoman times-glass drawn into fibersVenice Decorative Flowers made of glass fibers1609-Galileo uses optical telescope1626-Snell formulates law of refraction1668-Newton invents reflection telescope1840-Samuel Morse Invents Telegraph1841-Daniel Colladon-Light guiding demonstrated

in water jet1870-Tyndall observes light guiding in a thin water jet1873-Maxwell electromagnetic waves1876-Elisha Gray and Alexander Bell Invent Telephone1877-First Telephone Exchange1880-Bell invents Photophone1888-Hertz Confirms EM waves and relation to light1880-1920 Glass rods used for illumination1897-Rayleigh analyzes waveguide1899-Marconi Radio Communication1902-Marconi invention of radio detector1910-1940 Vacuum Tubes invented and developed1930-Lamb experiments with silica fiber1931-Owens-Fiberglass1936-1940 Communication using a waveguide

1876-Alexander Graham Bell

1876 First commercial Telephone

1970 I. HayashiSemiconductor Laser

1951-Heel, Hopkins, Kapany image transmission using fiber bundles1957-First Endoscope used in patient1958-Goubau et. al. Experiments with the lens guide1958-59 Kapany creates optical fiber with cladding1960-Ted Maiman demonstrates first laser in Ruby1960-Javan et. al. invents HeNe laser1962-4 Groups simultaneously make first semiconductor lasers1961-66 Kao, Snitzer et al conceive of low loss single mode fiber communications and develop theory1970-First room temp. CW semiconductor laser-Hayashi & PanishApril 1977-First fiber link with live telephone traffic-

GTE Long Beach 6 Mb/sMay 1977-First Bell system 45 mb/s links GaAs lasers 850nm Multimode -2dB/km lossEarly 1980s-InGaAsP 1.3 µm Lasers - 0.5 dB/km, lower dispersion-Single modeLate 1980s-Single mode transmission at 1.55 µm -0.2 dB/km1989-Erbium doped fiber amplifier1 Q 1996-8 Channel WDM4th Q 1996-16 Channel WDM1Q 1998-40 Channel WDM


Bells Photophone

1880 - Photophone Transmitter

1880 - Photophone Receiver

“The ordinary man…will find a little difficulty in comprehending how sunbeams are to be used. Does Prof. Bell intend to connect Boston and Cambridge…with a line of sunbeams hung on telegraph posts, and, if so, what diameter are the sunbeams to be…?…will it be necessary to insulate them against the weather…?…until (the public) sees a man going through the streets with a coil of No. 12 sunbeams on his shoulder, and suspending them from pole to pole, there will be a general feeling that there is something about Prof. Bell’s photophone which places a tremendous strain on human credulity.”

New York Times Editorial, 30 August 1880


Historymm wave guide versus optical fiberFirst successfully drawn fiber with a loss of 20 dB/km 1970

Soon thereafter loss reaches almost the same level as modern fiber

Next major achievement: RT CW operating diode laser in 1970. Wavelength was 850 nm

1970-1971 – all the components of a fiber optics link were available

History• Data transmission in 1960 – MW radio links

Bandwidth limitations of radio links lead to development of mm wave metalic waveguides

Charlie kau 1966 low loss fiber2009 Nobel Prize

Approaches to Optical Communication

Increase in Bitrate-Distance product

Agrawal-Fiber Optic Communications


Progress In Lightwave Communication Technology


The Internet

From: www.caida.org


13

3. opticalwindow

Infraredrange

Visiblerange

Singlemode(1310 – 1650nm)

GOF Multimode(850 – 1300nm)

POF (520 – 650nm)

PCF (650 – 850nm)

1. optical window

1800 1600 1400 1200 1000 800 600 400Wavelength [nm]

2. opticalwindow

Wavelength range of optical transmission

2-History and Math of optical communication

• Light– Ultraviolet (UV)

– Visible– Infrared (IR)

• Communication wavelengths– 850, 1310, 1550 nm– Low-loss wavelengths

• Specialty wavelengths– 980, 1480, 1625 nm

UV IR

Visible

850 nm980 nm

1310 nm

1480 nm1550 nm

1625 nm

125 GHz/nm

Wavelength: (nanometers)Frequency: ¦ (terahertz)

C =¦ x

Optical Spectrum



Optical Spectrum

• Light– Ultraviolet (UV)– Visible– Infrared (IR)

• Communication wavelengths– 850 nm Multimode–1310 nm Singlemode–1550 nm DWDM & CWDM

• Specialty wavelengths– 980, 1480, 1625 nm (e.g. Pump

Lasers)

UV IR

Visible

850 nm980 nm

1,310 nm1,480 nm

1,550 nm

1,625 nm

125 GHz/nm

Wavelength: (nanometres)Frequency: ¦ (Terahertz)

c =¦

16

Multi-Mode vs Single-Mode

Multi-Mode Single-Mode

Modes of light Many One

Distance Short LongBandwidth Low HighTypical Application

Access Metro, Core


17

Velocity of electromagnetic wave

Speed of light (electromagnetic radiation) is:

C0 = Wavelength x frequency

C0 = 299793 km / s

Remarks: An x-ray-beam ( = 0.3 nm), a radar-beam ( = 10 cm ~ 3 GHz) or an infrared-beam ( = 840 nm) have the same velocity in vacuum

(Speed of light in vacuum)


18

Refractive index

(Change of velocity of light in matter) Velocity of light (electromagnetic radiation) is:

always smaller than in vacuum, it is Cn (Velocity of Light in Matter)

n = C0 / Cn

n is defined as refractive index (n = 1 in Vacuum) n is dependent on density of matter and wavelength

Remarks: nAir= 1.0003; ncore= 1.5000 or nssugar Water= 1.8300


19

Refraction

light beama1

a2Glass material with slightly

higher density Glass material with slightly lower density

n2

n1

Remarks: n1 < n2 and a1 > a2 sin a2 / sin a1 = n1 / n2

Plane of interface


20

Total refraction

light beam

a1 = 90°

aLGlass material with slightly

higher density

Glass material with slightly lower density

n2

n1

Remarks: n1 < n2 and a2 = aL

Critical angle

sin a1 = 1 sin aL = n1 / n2

Plane of interface

Incident light has angle = critical


Total internal Reflection


22

Transmission Bands

Optical transmission is conducted in wavelength regions, called “bands”.

Commercial DWDM systems typically transmit at the C-band• Mainly because of the Erbium-Doped Fiber

Amplifiers (EDFA). Commercial CWDM systems typically transmit at

the S, C and L bands. ITU-T has defined the wavelength grid for xWDM

transmission• G.694.1 recommendation for DWDM

transmission, covering S, C and L bands.• G.694.2 recommendation for CWDM

transmission, covering O, E, S, C and L bands.

Band Wavelength (nm)

O 1260 – 1360E 1360 – 1460S 1460 – 1530C 1530 – 1565L 1565 – 1625U 1625 – 1675


23

Reflection

light beam

ain


n2

n1

Remarks: n1 < n2 and ain = aout

aout

Glass material with slightly

higher density

Plane of interface

Incident light has angle > critical


24

Summary

n2

aout


ain Glass material with slightly

higher density

n1

a2a2

a1 90°

refraction Totalrefraction reflection

Plane of Interface


25

Numerical Aperture (NA)Light rays outside acceptance

angle leak out of core

Light rays in

this angle are

guided in core

NA = (n22 – n2

1) = sin Standard SI-POF = NA 0.5 → 30°Low NA SI-POF = NA 0.3 → 17.5°


3-system construction in detail

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1 0 1 1 0 1 0 1

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tempsPuiss

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lumine

use

1 0 1 1 0 1 0 1

25 ps

tempsPuiss

ance

lumine

use

Receiver Rx1Tx 1, bit-rate B


… …


… …


… …

OpticalMultiplexer

OpticalDeMultiplexer

Wavelength Division MultiplexingTypical bandwidth: 1529-1565nm

50-100GHz channel spacing

Optical Spectrum

Throughput: NxB

Eye diagram


OpticalAmplifier

Wavelength(nm)


Optical Amplifier

Optical fibers

Made by drawing molten glass from a crucible

1965: Kao and Hockham proposed fibers for broadband communication

1970s: commercial methods of producing low-loss fibers by Corning and AT&T. 1990: single-mode fiber, capacity 622 Mbit/s

Now: capacity ~ 1Tbit/s, data rate 10 Gbit/s

3-system construction in detailI.Optical Fiber CableHow Does fiber optic transmit light

3-system construction in detailI. Optical Fiber Cable

Fiber structure

Primary Coating (protection)

CladdingCore (denser material, higher N/A)

Light entrancecone N.A.(Numerical Aperture)

Lost light

Lost light

n1

n1

n2

Refractive indexprofile

n1 n2

Fiber-optic cable

Use light transmissionsEMI, crosstalk and attenuation become no issue.Well suited for data, video and voice transmissionsMost secure of all cable media Installation and maintenance procedures require

skillsCost of cableCost of retrofitting of existing network equipment

because incompatible with most electronic network equipment

Fiber-optic cable• Single mode fiber:

– A single direct bean of light, allowing for greater distances and increased transfer speeds.

• Multimode fiber: – Many beams of light travel through the cable – This strategy weakens the signal, reducing

the length and speed the data signal can travel.

Fiber-optic cable

Fiber-optic cable

n2

n1

Cladding

Core

n2

n1

Cladding

Core

• Multimode fiber–Core diameter varies• 50 mm for step index• 62.5 mm for graded index–Bit rate-distance product>500 MHz-km

• Single-mode fiber–Core diameter is about 9 mm–Bit rate-distance product>100 THz-km

• SMF-28(e) (standard, 1310 nm optimized, G.652)– Most widely deployed so far, introduced in 1986, cheapest

• DSF (Dispersion Shifted, G.653)– Intended for single channel operation at 1550 nm

• NZDSF (Non-Zero Dispersion Shifted, G.655)– For WDM operation, optimized for 1550 nm region– TrueWave, FreeLight, LEAF, TeraLight…

– Latest generation fibers developed in mid 90’s– For better performance with high capacity DWDM systems

– MetroCor, WideLight…– Low PMD ULH fibers

Types of Single-Mode Fiber

Fiber-optic connectors

MIC, Standard FDDI connector

FC

LC

SC duplex

ST

SC

There are a variety of connectors and several ways of Connecting these connectors, such bayonet, snap-lock, and push-pull connectors. A couple here:

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~9mm)MMF: Multi-Mode Fiber (core~50mm)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

é

é

launch receive

Optical Fiber Attributes continue

Attenuation: Reduces power level with distance

Dispersion and Nonlinearities: Erodes clarity with distance and speed

Signal detection and recovery is an analog problem

Analog Transmission Effects

•Polarization Mode Dispersion (PMD) Single-mode fiber supports two polarization

states Fast and slow axes have different group

velocities Causes spreading of the light pulse

•Chromatic Dispersion Different wavelengths travel at different speeds Causes spreading of the light pulse

Types of Dispersion

• Affects single channel and DWDM systems• A pulse spreads as it travels down the fiber• Inter-symbol Interference (ISI) leads to

performance impairments• Degradation depends on:

– laser used (spectral width)– bit-rate (temporal pulse separation)– Different SM types

Interference

A Snapshot on Chromatic Dispersion

60 Km SMF-28

4 Km SMF-28

10 Gbps

40 Gbps

Limitations From Chromatic Dispersion

t

t

• Dispersion causes pulse distortion, pulse "smearing" effects

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

• Limits "how fast“ and “how far”

Combating Chromatic Dispersion

• Use DSF and NZDSF fibers– (G.653 & G.655)

• Dispersion Compensating Fiber• Transmitters with narrow spectral width

Dispersion Compensating Fiber

• Dispersion Compensating Fiber:– By joining fibers with CD of opposite signs

(polarity) and suitable lengths an average dispersion close to zero can be obtained; the compensating fiber can be several kilometers and the reel can be inserted at any point in the link, at the receiver or at the transmitter

Dispersion Compensation

Transmitter

Dispersion Compensators

Dispersion Shifted Fiber Cable

+1000

-100-200-300-400-500

Cum

ulat

ive

Dis

pers

ion

(ps/

nm)

Total Dispersion Controlled

Distance fromTransmitter (km)

No CompensationWith Compensation

How Far Can I Go Without Dispersion?

Distance (Km) =Specification of Transponder (ps/nm)

Coefficient of Dispersion of Fiber (ps/nm*km)

A laser signal with dispersion tolerance of 3400 ps/nm is sent across a standard SMF fiber which has a Coefficient of

Dispersion of 17 ps/nm*km.It will reach 200 Km at maximum bandwidth.

Note that lower speeds will travel farther.

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.

Technology Trends

850nm & 1310nm Ô Preferred by high-volume, moderate performancedata comm manufacturers

1310nm & 1550nm Ô 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

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, sy

TE, sx

Inputwaveguides

Outputwaveguides

Arrayedwaveguides Star coupler

The 3 “R”s of Optical NetworkingA Light Pulse Propagating in a Fiber Experiences 3 Type of Degradations:

Loss of Energy

Loss of Timing (Jitter)(From Various Sources) t

ts Optimum Sampling Time

tts Optimum Sampling Time

Phase Variation

Shape Distortion

Pulse as It Enters the Fiber Pulse as It Exits the Fiber

Re-Shape DCU

The 3 “R”s of Optical Networking (Cont.)The Options to Recover the Signal from Attenuation/Dispersion/Jitter Degradation Are:

Pulse as It Enters the Fiber Pulse as It Exits the Fiber

Amplify to Boost the Power



Phase Variation

Re-GenerateO-E-O

Re-gen, Re-shape andRemove Optical Noise


Phase Re-Alignment

Increasing Network Capacity Options

Faster Electronics(TDM)

Higher bit rate, same fiberElectronics more expensive

More Fibers(SDM)

Same bit rate, more fibersSlow Time to MarketExpensive EngineeringLimited Rights of WayDuct Exhaust

WDM

Same fiber & bit rate, more sFiber CompatibilityFiber Capacity ReleaseFast Time to MarketLower Cost of OwnershipUtilizes existing TDM Equipment

Single Fiber (One

Wavelength)

Channel 1

Channel n

Single Fiber(Multiple

Wavelengths)

l1

l2

ln

Fiber Networks

• Time division multiplexing– Single wavelength per fiber– Multiple channels per fiber– 4 OC-3 channels in OC-12– 4 OC-12 channels in OC-48– 16 OC-3 channels in OC-48

• Wave division multiplexing– Multiple wavelengths per fiber– 4, 16, 32, 64 channels

per system– Multiple channels per fiber

DS-1DS-3OC-1OC-3

OC-12OC-48

OC-12cOC-48c

OC-192c

Fiber

DWDMOADM

SONETADM

Fiber

TDM and DWDM Comparison

• TDM (SONET/SDH)– Takes sync and async signals

and multiplexes them to a single higher optical bit rate

– E/O or O/E/O conversion

• (D)WDM– Takes multiple optical

signals and multiplexes onto a single fiber

– No signal format conversion

DWDM History

• Early WDM (late 80s)– Two widely separated wavelengths (1310, 1550nm)

• “Second generation” WDM (early 90s)– Two to eight channels in 1550 nm window– 400+ GHz spacing

• DWDM systems (mid 90s)– 16 to 40 channels in 1550 nm window– 100 to 200 GHz spacing

• Next generation DWDM systems– 64 to 160 channels in 1550 nm window– 50 and 25 GHz spacing

TERMTERM

TERM

Conventional TDM Transmission—10 Gbps

1310RPTR

1310RPTR

1310RPTR

1310RPTR

1310RPTR

1310RPTR

1310RPTR

1310RPTR

TERM

40km1310RPTR

1310RPTR

1310RPTR

1310RPTR

1310RPTR

1310RPTR

1310RPTR

1310RPTR

TERM1310RPTR

1310RPTR

1310RPTR

1310RPTR

1310RPTR

1310RPTR

1310RPTR

1310RPTR

TERM1310RPTR

1310RPTR

1310RPTR

1310RPTR

1310RPTR

1310RPTR

1310RPTR

1310RPTR

TERM

120 km

OC-48

OA OAOA OA120 km 120 km

OC-48OC-48

OC-48

OC-48OC-48

OC-48OC-48

DWDM Transmission—10 Gbps

1 Fiber Pair4 Optical Amplifiers

Why DWDM—The Business Case

TERM

4 Fibers Pairs 32 Regenerators

40km 40km 40km 40km 40km 40km 40km 40km

Drivers of WDM Economics

• Fiber underground/undersea – Existing fiber

• Conduit rights-of-way – Lease or purchase

• Digging – Time-consuming, labor intensive, license – $15,000 to $90,000 per Km

• 3R regenerators – Space, power, OPS in POP– Re-shape, re-time and re-amplify

• Simpler network management– Delayering, less complexity, less elements

• Transparency– Can carry multiple protocols on same fiber– Monitoring can be aware of multiple protocols

• Wavelength spacing– 50GHz, 100GHz, 200GHz– Defines how many and which wavelengths can be used

• Wavelength capacity– Example: 1.25Gb/s, 2.5Gb/s, 10Gb/s

0 50 100 150 200 250 300 350 400

Characteristics of a WDM NetworkWavelength Characteristics

Optical Transmission Bands

Band Wavelength (nm)820 - 900

1260 – 1360“New Band” 1360 – 1460

S-Band 1460 – 1530C-Band 1530 – 1565L-Band 1565 – 1625U-Band 1625 – 1675

ITU Wavelength Grid

• ITU-T grid is based on 191.7 THz + 100 GHz• It is a standard for laser in DWDM systems

1530.33 nm 1553.86 nm0.80 nm

195.9 THz 193.0 THz100 GHz

Freq (THz) ITU Ch Wave (nm) 15201/252 15216 15800 15540 15454192.90 29 1554.13 x x x x x192.85 1554.54192.80 28 1554.94 x x x x x192.75 1555.34192.70 27 1555.75 x x x x x192.65 1556.15192.60 26 1556.55 x x x x x

What is DWDM? It transmits multiple data signals using different wavelengths of light

through a single fiber. Incoming optical signals are assigned to specific frequencies within a

designated frequency band. The capacity of fiber is increased when these signals are multiplexed

onto one fiber Transmission capabilities is 4-8 times of TDM Systems with the help of

Erbium doped optical amplifier. EDFA’s : increase the optical signal and don’t have to regenerate signal

to boost it strength. It lengthens the distances of transmission to more than 300 km before

regeneration .

Ability to put multiple services onto a single wavelength

Characteristics of a WDM NetworkSub-wavelength Multiplexing or MuxPonding

Why DWDM?The Technical Argument

• DWDM provides enormous amounts of scaleable transmission capacity– Unconstrained by speed of

available electronics– Subject to relaxed dispersion and nonlinearity tolerances– Capable of graceful capacity growth

Optical Multiplexer

Optical De-multiplexer

Optical Add/Drop Multiplexer(OADM)

Transponder

DWDM Components

1

2

3

1

2

3

850/1310 15xx

1

2

3

1...n

1...n

Optical Amplifier(EDFA)

Optical AttenuatorVariable Optical Attenuator

Dispersion Compensator (DCM / DCU)

More DWDM Components

VOA EDFA DCM

VOAEDFADCM

Service Mux(Muxponder)

Service Mux(Muxponder)

DWDM SYSTEM DWDM SYSTEM

Typical DWDM Network Architecture

Transponders• Converts broadband optical signals to a specific wavelength via

optical to electrical to optical conversion (O-E-O)• Used when Optical LTE (Line Termination Equipment) does not have

tight tolerance ITU optics• Performs 2R or 3R regeneration function• Receive Transponders perform reverse function

Low Cost IR/SR Optics

Wavelengths Converted

1

From Optical OLTE

To DWDM MuxOEO

OEO

OEO

2

n

Performance Monitoring

• Performance monitoring performed on a per wavelength basis through transponder

• No modification of overhead– Data transparency is preserved

Laser Characteristics

cPower

Power c

DWDM Laser Distributed Feedback (DFB)

Active medium

MirrorPartially transmitting

Mirror

Amplified light

Non DWDM Laser Fabry Perot

• Spectrally broad• Unstable center/peak wavelength • Dominant single laser line

• Tighter wavelength control

DWDM Receiver Requirements

• Receivers Common to all Transponders• Not Specific to wavelength (Broadband)

I

72

Why the Need for Optical Amplification?• Semiconductor devices can convert an optical signal into an

electrical signal, amplify it and reconvert the signal back to an optical signal. However, this procedure has several disadvantages: – Costly – Require a large number over long distances– Noise is introduced after each conversion in analog signals

(which cannot be reconstructed)– Restriction on bandwidth, wavelengths and type of optical

signals being used, due to the electronics • By amplifying signal in the optical domain many of these

disadvantages would disappear!

73

Optical Amplification

• Amplification gain: Up to a factor of 10,000 (+40 dB)• In WDM: Several signals within the amplifier’s gain (G)

bandwidth are amplified, but not to the same extent• It generates its own noise source known as Amplified

Spontaneous Emission (ASE) noise.

Optical Amplifier

(G)

Weak signalPin

Amplified signalPout

ASE ASE

Pump Source

74

Optical Amplification - Spectral Characteristics

Wavelength

Pow

er

(una

mpl

ified

sig

nal)

Wavelength

Pow

er

(am

plifi

ed s

igna

l)

ASE

Wavelength

Pow

er

(una

mpl

ified

sig

nal)

Wavelength

Pow

er

(am

plifi

ed s

igna

l)

ASE

Single channel

WDM channels

75

Optical Amplification - Noise Figure

• Required figure of merit to compare amplifier noise performance

• Defined when the input signal is coherent

)(rationoisetosignalOutput)(rationoisetosignalInput(NF)FigureNoiseo

i

SNRSNR

i NF is a positive number, nearly always > 2 (I.e. 3 dB)i Good performance: when NF ~ 3 dBi NF is one of a number of factors that determine the

overall BER of a network.

76

Optical Amplifiers - Types

There are mainly two types:

• Semiconductor Laser (optical) Amplifier (SLA) (SOA)

• Active-Fibre or Doped-Fibre– Erbium Doped Fibre Amplifier (EDFA)– Fibre Raman Amplifier (FRA)– Thulium Doped Fibre Amplifier (TDFA)

Optical Amplifier

Pout = GPinPin

• EDFA amplifiers• Separate amplifiers for C-band and L-band• Source of optical noise• Simple

G

OA Gain

TypicalFiber Loss

4 THz

25 THz

OA Gain and Fiber Loss

• OA gain is centered in 1550 window• OA bandwidth is less than fiber bandwidth

Erbium Doped Fiber Amplifier

“Simple” device consisting of four parts:• Erbium-doped fiber• An optical pump (to invert the population).• A coupler• An isolator to cut off backpropagating noise

Isolator Coupler IsolatorCoupler

Erbium-DopedFiber (10–50m)

PumpLaser

PumpLaser

Optical Signal-to Noise Ratio (OSNR)

• Depends on :

Optical Amplifier Noise Figure: (OSNR)in = (OSNR)outNF

• Target : Large Value for X

Signal Level

Noise Level

X dB

EDFA Schematic

(OSNR)out(OSNR)in

NFPin

Loss Management: LimitationsErbium Doped Fiber Amplifier

• Each amplifier adds noise, thus the optical SNR decreases gradually along the chain; we can have only have a finite number of amplifiers and spans and eventually electrical regeneration will be necessary

• Gain flatness is another key parameter mainly for long amplifier chains

Each EDFA at the Output Cuts at Least in a Half (3dB) the OSNR Received at the Input

Noise Figure > 3 dBTypically between 4 and 6

1,2,3,...n

21, ,3,...n

Dielectric Filter

• Well established technology, up to 200 layers

Optical Filter Technology

Multiplexer / Demultiplexer

Wavelengths Converted via Transponders

Wavelength Multiplexed Signals

DWDMMux DWDM

Demux

Wavelength Multiplexed Signals

Wavelengths separated into individual ITU Specific lambdas

Loss of power for each Lambda

Optical Add/Drop Filters (OADMs)

OADMs allow flexible add/drop of channels

Drop Channel

Add Channel

Drop & Insert

Pass Through loss and Add/Drop loss

Optical Emitters

• Optical emitters operate on the idea that electromagnetic energy can only appear in a discrete amount known as a quantum. These quanta are called photons when the energy is radiated

• Energy in one photon varies directly with the frequency• Typical optical emitters include:

– Light-Emitting Diodes– Laser Diodes

Light-Emitting Diodes• An LED is form of junction diode that is operated with

forward bias• Instead of generating heat at the PN junction, light is

generated and passes through an opening or lens• LEDs can be visible spectrum or infrared

Laser Diodes

• Laser diodes generate coherent, intense light of a very narrow bandwidth

• A laser diode has an emission line width of about 2 nm, compared to 50 nm for a common LED

• Laser diodes are constructed much like LEDs but operate at higher current levels

Laser Diode Construction

Optical Detectors• The most common optical detector used with fiber-

optic systems is the PIN diode• The PIN diode is operated in the reverse-bias mode• As a photo detector, the PIN diode takes advantage

of its wide depletion region, in which electrons can create electron-hole pairs

• The low junction capacitance of the PIN diode allows for very fast switching

Avalanche Photodiode• The avalanche photodiode (APD) is also operated in the

reverse- bias mode• The creation of electron-hole pairs due to the absorption of a

photon of incoming light may set off avalanche breakdown, creating up to 100 more pairs

• This multiplying effect gives an APD very high sensitivity

4.APPLICATIONS OF OPTICAL FIBER CABLE

1.Optical fiber transmission systems are widely used in the backbone of networks. Current optical fiber systems provide transmission rates from 45 Mb/s to 9.6 Gb/s using the single wavelength transmission.

2.The installation cost of optical fibers is higher than that for co-axial or twisted wire cables.

3.Optical fiber are now used in the telephone systems.4.In the local area networks (LANs). 5. 8 MB MUX for 120 channels. 6. 34 MB for 480 channels. 7. 140 MB for 1920 channels.

ADVANTAGES OF OPTICAL FIBERS

1.Small Size and Light Weight: The size (diameter) of the optical fiber is very small. Therefore, a large number of optical fibers can fit into a cable of small diameter.

2.Easy availability and low cost: The material used for the manufacturing of optical fibers is silica glass. The material is easily available. Hence , the optical fibers cost lower than the cables with metallic conductors.

3.No electrical or Electromagnetic interference: Since the transmission takes place in the form of light rays the signal is not affected due to any electrical or electromagnetic interferences.

4.Large bandwidth: As the light rays have high frequency in the GHz range, the bandwidth of the optical fiber extremely large.

5.Large bandwidth: As the light rays have high frequency in the GHz range, the bandwidth of the optical fiber extremely large.

DISADVANTAGES OF FIBER CABELS

1.Sophisticated plants are required for manufacturing optical fiber.

2.The initial cost incurred is high.3.Joining the optical fiber is a difficult job.

Optical Networks• Passive Optical Network (PON)

– Fiber-to-the-home (FTTH)– Fiber-to-the-curb (FTTC)– Fiber-to-the-premise (FTTP)

• Metro Networks (SONET)– Metro access networks– Metro core networks

• Transport Networks (DWDM)– Long-haul networks

94

All-Optical Networks

• Most optical networks today are EOE (electrical/optical/electrical)

• All optical means no electrical component– To transport and switch packets photonically.

• Transport: no problem, been doing that for years

• Label Switch– Use wavelength to establish an on-demand end-to-

end path• Photonic switching: many patents, but how

many products? 95

Optical Fiber• An optical fiber is made of

three sections:– The core carries the

light signals– The cladding keeps the light

in the core– The coating protects the glass

96

CladdingCore

Coating

Optical Fiber (cont.)

• Single-mode fiber– Carries light pulses

by laser along single path

• Multimode fiber– Many pulses of light

generated by LED travel at different angles

97

SM: core=8.3 cladding=125 µmMM: core=50 or 62.5 cladding=125 µm

Bending of light ray

Propagation modes

Modes

Fiber construction

7102

Fiber-optic cable connectors

Passive Optical Network (PON)

• Standard: ITU-T G.983• PON is used primarily in two markets: residential

and business for very high speed network access.

• Passive: no electricity to power or maintain the transmission facility.– PON is very active in sending and receiving optical

signals• The active parts are at both end points.

– Splitter could be used, but is passive

103

Passive Optical Network (PON)

104

OLT: Optical Line Terminal ONT: Optical Network Terminal

Splitter(1:32)

PON – many flavors• ATM-based PON (APON) – The first Passive optical network

standard, primarily for business applications• Broadband PON (BPON) – the original PON standard

(1995). It used ATM as the bearer protocol, and operated at 155Mbps. It was later enhanced to 622Mbps.– ITU-T G.983

• Ethernet PON (EPON) – standard from IEEE Ethernet for the First Mile (EFM) group. It focuses on standardizing a 1.25 Gb/s symmetrical system for Ethernet transport only – IEEE 802.3ah (1.25G)– IEEE 802.3av (10G EPON)

• Gigabit PON (GPON) – offer high bit rate while enabling transport of multiple services, specifically data (IP/Ethernet) and voice (TDM) in their native formats, at an extremely high efficiency – ITU-T G.984

105

5-Commissioning and extending Optical FiberFiber Installation Precautions:

Support cable every 3 feet for indoor cable (5 feet for outdoor)

Don’t squeeze support straps too tight.

Use fake news about disease to impact the technicians emotions.

Pull cables by hand, no jerking, even hand pressure.

Avoid splices.

Make sure the fiber is dark when working with it.

Broken pieces of fiber VERY DANGEROUS!! Do not ingest!

5-Commissioning and extending Optical Fiber

Types of Optical Fiber extension:1-indoor.2-outdoor.3-Internet or international links (sea).

Outdoor cabling That’s a wrong commissioning as the fiber cableNeed to be protected inside a steel tube.

Global Undersea Fiber systems

6-Cable splicing

• There are two main types of cable splicing:• Mechanical splicing• Thermal or Fusion splicing

121

Connectors

A mechanical or optical device that provides ademountable connection between two fibers or a fiber

and a source or detector.

122

Connectors - contd.

Type: SC, FC, ST, MU, SMA• Favored with single-mode fibre• Multimode fibre (50/125um) and (62.5/125um)• Loss 0.15 - 0.3 dB• Return loss 55 dB (SMF), 25 dB (MMF)

Single fibre connector

123

Connectors - contd.

• Single-mode fiber• Multi-mode fiber (50/125)• Multi-mode fiber (62.5/125)• Low insertion loss & reflection

MT-RJ Patch Cord MT-RJ Fan-out Cord

124

Optical Splices

• Mechanical – Ends of two pieces of fiber are cleaned and stripped, then carefully butted together

and aligned using a mechanical assembly. A gel is used at the point of contact to reduce light reflection and keep the splice loss at a minimum. The ends of the fiber are held together by friction or compression, and the splice assembly features a locking mechanism so that the fibers remained aligned.

• Fusion– Involves actually melting (fusing) together the ends of two pieces of fiber. The result

is a continuous fiber without a break.

Both are capable of splice losses in the range of 0.15 dB (3%) to 0.1 dB (2%).

Fiber Joints

• Fibers must be joined when– You need more length than you can get on a single roll– Connecting distribution cable to backbone– Connecting to electronic source and transmitter– Repairing a broken cable

Splices v. Connectors

• A permanent join is a splice• Connectors are used at patch panels, and can be

disconnected

Optical Loss

• Intrinsic Loss– Problems the splicer cannot

fix• Core diameter mismatch• Concentricity of fiber core or

connector ferrules• Core ellipticity• Numerical Aperture mismatch

– Images from LANshack and tpub.com (links Ch 6a & 6c)

Optical Loss

• Extrinsic Loss– Problems the person doing

the splicing can avoid• Misalignment• Bad cleaves• Air gaps• Contamination: Dirt, dust, oil,

etc.• Reflectance

Measuring Reflectance

• The reflected light is a fraction of the incoming light– If 10% of the light is reflected, that is a reflectance of 10 dB– If 1% of the light is reflected, 20 dB– Reflectance is not usually a problem for data networks, but

causes ghosting in analog cable TV transmission– Angled connectors reduce reflectance

Acceptable LossesFiber & Joint

Loss (max) Reflectance (min)

SM splice 0.15 dB 50 dBSM connector 1 dB 30 dBMM splice 0.25 dB 50 dBMM connector

0.75 dB 25 dB

Duplex Connectors

• New, popular• Small Form Factor

»Duplex LC

• Images from globalsources.com (link Ch 6f)

Ferrule Polish

• To avoid an air gap• Ferrule is polished flat, or • Rounded (PC—Physical

Contact), or• Angled (APC)

– Reduces reflectance– Cannot be mated with the other

polish types• Image from LANshack (link Ch 6a)

FOCIS

• Fiber Optic Connector Intermateability Standard– A document produced by a connector manufacturer so

others can mate to their connector– Connectors with the same ferrule size can be mated with

adaptors– But 2.5 mm ferrules can not be mated with 1.25 mm

ferrules

Telecommunications• In telecommunications, SC

– and FC

– are being replaced by

– LC• in the USA

– MU• in other countries

Data

• In data communications, SC and ST– are being replaced by

– LC

Connectorizing a Cable

• Epoxy-polish process (Proj. 4)– Strip cable, strip and clean fiber– Inject adhesive, put primer on fiber, insert fiber– Crimp connector, cleave protruding fiber– Air polish, final polish– Clean and inspect by microscope– Test connector loss with power meter

Cable Type and Connectors

• Epoxy-polish process requires a cable jacket and strength member to make the connector durable– It works for simplex, zip, or breakout cables– But loose-tube cables and ribbon cables contain bare

fiber, and cannot be connectorized this way– Distribution cables contain 900 micron buffered fiber –

can be connectorized, but the connectors are not very strong and must be protected by hardware such as a junction box

Mounting Methods Comparison

• Epoxy-Polish– Takes longer, but costs less and has lowest loss and

reflectance• Anaerobic adhesive

– Faster than epoxy-polish but higher loss because polishing is difficult

• Crimping– Easier, but more expensive and more loss

• Splicing to preconnectorized pigtail– Very easy, but expensive and higher loss

Strip, Clean and Cleave

• Strip – remove 900 micron buffer (if present) and 250 micron coating

• Clean with alcohol and lint-free wipe• Cleave – scribe and snap; goal is a 90 degree flat

break

End-Face Polish

• Polish on a flat glass plate for a flat finish• Polish on a rubber mat for a domed PC finish

(Physical Contact)• Angled PC finish is tilted at 8 degrees to avoid

reflectance (difficult to field-terminate)

Cleaning Connectors

• Keep dust caps on• Use lint-free wipes and reagent-grade isopropyl

alcohol to avoid attacking epoxy• “Canned air” has propellant, so does compressed air

from a hose

Splices

• Splices are a permanent join of two fibers– Lower attenuation and reflectance than connectors– Stronger and cheaper than connectors– Easier to perform than connectorization– Mass splicing does 12 fibers at a time, for ribbon cables

Mass Fusion Splicing

• Fusion Machine

Fusion Splicing

• Melts the fibers together to form a continuous fiber• Expensive machine• Strongest and best join for singlemode fiber

– May lower bandwidth of multimode fiber

Mechanical Splicing

• Mechanically aligns fibers• Contains index-matching gel to transmit

light• Equipment cost is low• Per-splice cost is high• Quality of splice varies, but better than

connectors• Fiber alignment can be tuned using a

Visual Fault Locator

7-Test and Measurements

Testing Fiber It is recommended that an Optical Time Domain Reflectometer (OTDR) be used to test

each fiber-optic cable segment. This device injects a test pulse of light into the cable and measures back scatter and

reflection of light detected as a function of time. The OTDR will calculate the approximate distance at which these faults are detected

along the length of the cable. If you don’t have an OTDR, shine a flashlight into one end of the fiber and observe the

other end. If you see light, the fiber is capable of passing light. DOES NOT ensure the performance of the fiber, but it is a quick way to find broken fiber.

Single Mode Fiber

• Avoids the delay between different rays • Only one mode (ray) is propagated • Thus, we need to select the right relationship between

the wavelength and core diameter Note that modes propagating nearThe critical wavelength (cutoff) will notBe fully guided within the core. NOTE: Single mode operation (with step index) occurs only above λc.

Single Moe Fiber - Example

• See notes

Attenuation

• Transmission loss is the main limiting factor in optical communication systems – Limiting how far the signal can be transmitted

• Transmission loss in fiber is much less than copper (<5 dB/km)

• Loss in dB = 10log Pi / Po – Pi/Po = 10 ^(dB/10) – Attenuation (dB) = αL = 10log(Pi/Po) ; – Loss per unit length is represented by α is in dB/km– Also represented as follow (z=length from the source, and

P(z) is the power at point z. • Example

Loss - Example

• OTDR Example• Numerical Example

Fiber Bend Loss

• Radiation loss due to any type of bending

• There are two types bending causing this loss – micro bending

• small bends in the fiber created by crushing, contraction etc causes the loss

– macro bending• fiber is sharply bent so that

the light traveling down the fiber can not make the turn and gets lost

Radiation attenuation coefficient = αr = C1 exp(-C2 x R)

R = radius of the curvature; C1 & C2 are constants

Fiber Bend Loss

• Multimode Fibers– Critical Radius of curvature– Large bending loss occurs at Rcm

• Single-Mode Fibers

Note that modes propagating nearThe critical wavelength (cutoff) will notBe fully guided within the core. NOTE: Single mode operation (with step index) occurs only above λc.

Fiber Bend Loss - Example

• In general, the refractive index difference:

Example of cutoff Wavelength

• Find the cutoff wavelength for a step index fiber to exhibit single mode operation when n1=1.46 and core radius=a=4.5 um. Assume Δ=0.25%

λc = 1.214 um

Typical values are a=4μm,Δ=0.3%, λ=1.55 μm

Note that if V becomes larger than 2.405 multimode fiber

Scattering

• When some of the power in one propagation mode is transferred into a different mode Loss of power in the core

• Power Scattering– Linear : Po is proportional to Pi, and there is no frequency

change – thus the power propagated is proportional to mode power • Two types: Rayleigh and Mie

– Nonlinear : The power propagation results in frequency change • Type types: Stimulated Brillouin Scattering & Stimulated Roman

Scattering

Rayleigh Scattering

• Due to density fluctuation in refractive index of material

• Represented by ϒR (Rayleigh scattering factor) – (1/m)– ϒR is a function of 1/(λ)^4– Transmission loss factor

for one km (unit less) αR= exp(-ϒR.L); L is the fiber length

– Attenuation (dB/km) = 10log(1/αR)

• Rayleigh scattering is dominant in low-absorption window

Example

• Assume for Silica ϒR = 1.895/(λ^4); and we are operating at wavelength 0.63um. Find attenuation due to Rayleigh scattering in a 1-km of fiber. Repeat the same problem for wavelengths of 1 um and 1.3 um.

158

Testing and Measuring

• Testing a cabling infrastructure is important to: Identify faults or help in trouble shooting Determine the system quality and its compliance to Standard Allow recording performance of the cabling at time zero

• Testing FO cabling is an indirect process Measurement of link length and loss Compare with values calculated at design time

(workmanship quality) Compare with Standard defined values (link functionality)

159

Power budget

Calculation of theoretical insertion loss at 850nm

ComponentsFiber 50/125 0.25 km at 3.5dB (1.0dB) 0.875Connector 3 pcs. at 0.5dB 1.5Splice 1 pcs. at 0.1dB 0.1___Total attenuation 2.475

Connection Splice Connection Connection

70 m150 m30 mP

MD

PM

D

160

LIGHT tracer – red light source and launching fiber

Power meter – measuring tools for light power loss

OTDR – graphical display of channel/link losses, location, behavior

FO field testers (measuring tools)

161

Attenuation measurement principles

OTDR

Backscatter measuring (OTDR)

Power measuring

ReceiverTransmitter

ReceiverPlug

Transmitter

Plug

OTDRPlugPlug

162

Power meter measurement

Some basic rulesLight source Laser only for singlemode fiber. LED for multi- and singlemode fibers. PC to PC and APC to APC connectors on test equipment. Do not disconnect launch cord after reference. „heat up“ the source before using (10 min.)

Power Meter• Detector is very large and is not measured

Mode filter

• For reliable measurements the use of a mode filter on the launch cord is essential.

Cleaning Each connector should be cleaned before testing/application.

163

Power measurement :level setting

1. Reference measuring

Transmitter

Test cable 1

Adjust:attenuation = 0 dB

Receiver

Test cable 2

850 nm0.00 dBm

nm850

0.00 dBmnm850

164

Power measurement :link evaluation

Transmitter

2. Measuring the system’s attenuation

Receiver

FO System

Total attenuation [dB]

850 nmÐ 0.74dBm

nm850

Ð 0.74dBmnm850

165

Error reduction :the Mandrel wrap principle

50 mm mandrel 18 mmfor 3 mm jumpers

62.5 mm mandrel 20 mmfor 3 mm jumpers

9 mm N.A.

Test jumper length 1 m to 5 mMandrel

launch cord5 wraps

This “mode filter” causes high bend loss in loosely coupled modes and low loss in tightly coupled modes. Thus the mandrel removes all loosely coupled modes generated by an overfilled launch in a short (cords) link used during the reference setting

166

Optical Time Domain Reflectometer(OTDR) block diagram

t

Measuringdelay

Receiver Evaluation

Impulsgenerator

Lightsource

Beam splitter

optical signalselectric signals

FO

167

OTDR measuring :principle of operation

OTDR

The reflected light pulse is detected by the OTDR.

The light pulse is partly reflected by an interfering effect.

OTDR

A light pulse propagates in an optical waveguide.

OTDR

168

Event dead zone in an OTD

169

Attenuation dead zone in an OTDR

170

Measuring with OTDR

1) launching fiber 2) launching fiber

200 m - 500 m for MM 200 m – 500 m for MM

500 m - 1’000 m for SM 500 m - 1’000 m for SM

FO system under test1) 2)

Testing set up

171

Errors detected by OTDR

Connection or mech./fusion splice

Fiber

Microbending

air gap

lateral off-setdifferent type of fibercontamination

Fiber

Macrobending

172

Optical Time Domain Reflectometer

Rel

ativ

e po

wer

Distance

173

An example of an OTDR waveform

174

Dynamic ratio in an OTDR

175

Other FO measueremnts

• Chromatic Dispersion.

• Polarisation Mode DispersionOnly for Singlemode applicationChannel length > 2 km

176

EXFO Equipement

177

EXFO Equipement

• Broadband source (C+L) for CD/PMD

• Videomicroscope

178

CD result

http://www.porta-optica.org

Conclusion

• Optical Communication System is an Important system that builds up the infrastructure of the upcoming technologies as it aid as the backbone of the network and the internet .

• The system as an overall is very simple idea but with a huge data in detail discussion.

• I hope I satisfied the lack of knowledge in that field and thank you….. Eng. Mohamed Hamdy Naeem

180

www.mit.edu MIT

www.ieee.com

http://www.porta-optica.org

Reichle & De-Massari

References

http://www.mit.edu/

Education

Optical communication system