OVER VIEW OF

OPTICAL FIBER

COMMUNICATION

BY NAVEEN JAKHAR, ITS

IN THIS PRESENTATION… GENERAL: History of Transmission Systems

OFC

• History of OFC

• Advantages

• Applications

• ITU-T Recommendations

• Fiber optic principle

• Windows of operation

• Trends in OF Communication

• Fiber classification

• OF Cable Types

• Optical Fiber transmission impairments

• Optical Sources and Detectors

• Optical Link Characterization and Design

13 September 2016 2

HISTORY OF TRANSMISSION SYSTEMS

The developments…

• Open Wire Systems

• Coaxial Cables

• UHF Systems

• Microwave Systems

• Digital Transmission Systems

• Satellite Communication Systems

• Optical Fiber Cable

13 September 2016 3

OPEN WIRE SYSTEMS

• Till 1950s, the long distance voice

communication was almost entirely

transported over Open Wire Carrier

system.

• The voice signals for these systems

were multiplexed using FDM to a

higher frequency carrier and carried

through open wire systems.

• These open wire systems were

capable of carrying traffic of three

to twelve subscribers at a time.13 September 2016 4

COAXIAL CABLES

• With the introduction of U/G symmetrical

pair cable carrier system which was

followed by the Coaxial Cable system,

greatly enhanced, by the decade end, the

simultaneous voice channel carrying

capacity to 960 voice channels.

• The first Coaxial Cable System was

commissioned between Agra and Delhi in

the year 1959.

• Over the years, this system was improved

and developed to carry 2700

simultaneous voice channels.13 September 2016 5

UHF SYSTEMS

• Close on the heel of coaxial systems, in the

mid-60s wireless microwave systems

were developed and inducted in the

network.

• The first Microwave system was installed in

1965 between Calcutta and Asansole.

Microwave systems with 60, 300 and 1800

voice channels capacity were inducted

into the telecom network subsequently.

• These systems were mostly indigenous

(developed and manufactured within the

country).13 September 2016 6

DIGITAL TRANSMISSION SYSTEMS

• By mid-1980s Digital TAX exchanges were introduced in the network with the aim

to improve STD services.

• Till 1989, Coaxial cable and UHF transmission medias were used to provide

connectivity.

• Induction of Digital Transmission Systems which were mainly Digital UHF, Digital

Microwave, Digital Coaxial and Optical Fiber Systems, started during 1989-90.

U/G coaxial cable was initially used for the connectivity of large and medium

cities and however, later on, it was also used for connecting small towns.

• Media diversity was provided through Radio Relay (UHF and Microwave)

Systems. These Radio relay systems were very reliable and beneficial particularly

for connecting hilly and backward areas where laying and maintenance of

underground cable is extremely difficult. 13 September 2016 7

SATELLITE SYSTEMS

• Work for connecting far flung, inaccessible area, and island community started

in late 70s by DoT.

• The first Domestic Satellite Network was established by connecting Port-Blair and

Car-Nicobar in Andaman & Nicobar islands, Kavaratti in Lakshadweep islands, Leh

in Ladakh region and Aizwal in North Eastern region. These station were

simultaneously linked to the gateway at Delhi and Chennai. This satellite network

was commissioned in November 1980 through International Telecommunication

Satellite.

• Satellite Communication capacity increased with launch of INSAT-1 and INSAT-2

series satellites. MCPC - VSAT (Multi Channel per Carrier - Very Small Aperture

Terminals) systems were developed and deployed in remote and inaccessible

areas of Garhwal region of (then) Uttar Pradesh, Himachal Pradesh, Arunachal

Pradesh, J&K, Orissa, Sikkim etc. for providing STD facilities. These terminals were

linked to an Earth Station generally co-located with the TAX.13 September 2016 8

OPTICAL FIBER CABLE

• Introduction of Optical Fiber Cable Systems in the country started in 1989-

90.

• These systems are capable of carrying large no. of voice channels

compared to the existing technologies that were available at that time and

offer the circuit at low cost per kilometer of circuit. The DoT deployed these

OFC systems in a big way for connectivity right upto the level of Tehsils.

• By the year 2000, a huge network of optical fiber cable was in place and a

large number of PDH & SDH technology OFC systems were deployed for

providing backbone connectivity to switching network.

• From 2002-03, DWDM technology systems were inducted over the OFC

backbone.

13 September 2016 9

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13 September 2016 10

BSNL

13 September 2016 11

• 1790: Optical Semaphore invented by Claude Chappe of France.

• 1880: Photophone invented by A.G. Bell at Washington.

• 1940: Optical guides reflective coated to carry visible light.

• 1960: LASER invented by Theodore Maiman.

• 1963: Unguided communications with LASER.

• 1966: OPTICAL FIBER invented by Corning Glass researchers:

ROBERT MAURER

DONALD KECK &

PETER SCHULTZ13 September 2016 12

Optical Communications

- Historical Perspective

HISTORICAL PERSPECTIVE (CONTD…)

• Material for fiber was fused silica with special properties like:

• Extreme purity

• A high melting point

• Low refractive index.

Initially very high loss fiber was developed.

Typical loss of ~17db/km [at λ =820 nm]

• 1970: low loss fiber developed.

OFC systems became practical.

• Currently :

fiber losses=<0.2-0.35 db/km

13 September 2016 13

13 September 2016 14

The Developments

ADVANTAGES OF FIBER COMMUNICATIONS

• High information carrying capacity

• Low attenuation

• Plentiful Resource

• Greater safety

• Immunity to RFI

• Immunity to EMI

• No cross-talk

• Higher Security

• Small size and light weight

• Less Corrosion

• Less temperature sensitive

13 September 2016 15

ADVANTAGES OF FIBER COMMUNICATIONS

• High information carrying capacity:

A valid comparison would be on the basis of cost per meterper telephone channel, rather than just cost per meter.

• Resource plentiful:

The basic materials are either silicon dioxide for glass fibersor transparent plastic which are plentiful.

• Less attenuation:

A typical fiber attenuation is 0.3 dB/km. Whereas a coaxialcable (RG-19/U) will attenuate a 100MHz signal by 22.6dB/km.

• Greater safety:

Optic fibers glass/plastic, are insulators. No electric currentflows through them.

13 September 2016 16

ADVANTAGES OF FIBER COMMUNICATIONS (2)

• Immunity to RFI:

Fibers have excellent rejection of radio-frequencyinterference (RFI) caused by radio and television stations,radar, and other electronics equipment.

• Immunity to EMI:

Fibers have excellent rejection of electromagneticinterference (EMI caused by natural phenomena such aslightning, sparking, etc).

• No cross-talk:

The optic wave within the fiber is trapped; none leaks outduring transmission to interfere with signals in other fibers.

• Higher Security:

fibers offer higher degree of security and privacy.

13 September 2016 17

ADVANTAGES OF FIBER COMMUNICATIONS (3)

• Small size and light weight:

typical optical cable fiber dia 125m, cable dia 2.5 mm andweight 6 kg/km. A coaxial cable (RG-19/U), outer dia 28.4 mm,and weight 1110 kg/km.

• Less Corrosion:

Corrosion caused by water/chemicals is less severe for glassthan for copper.

• Less temperature sensitive:

Glass fibers can withstand extreme temperatures beforedeteriorating. Temperatures up to 800 ̊C leave glass fiberunaffected.

13 September 2016 18

APPLICATIONS OF OF COMMUNICATIONS

• Telecommunications

Long-Distance Transmission

Inter-exchange junction

Fiber in the loop (FITL) -- FTTC, FTTB, FTTH

• Video Transmission

Television broadcast, cable television (CATV), remote monitoring, etc.

• Broadband Services

provisioning of broadband services, such as video request service, home study courses, medical facilities, etc.

• High EMI areas

Along railway track, through power substations can be suspended directly from power line towers, or poles.

• Military applications

• Non-communication fiber optics: e.g. fiber sensors.13 September 2016 19

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13 September 2016 20

• Dark fiber is optical fiber infrastructure that’s in place, but not

being used

• Its like an unplugged electrical extension cord

• It can be ‘lit’ with active telecom equipment, when required by

TSPs or other end-users

• Provides significant cost savings and substantial time-

efficiencies to end users

• In India, companies registered as IP-I can provide assets such

as Dark Fiber.

The ‘Dark Fiber’ Concept

13 September 2016 21

Series of Recommendations by the ITU-T, A to Z

G series: Transmission systems and media, digital

systems and networks

Some of the G series:

G.600-G.699: Transmission media and optical systems

characteristics

G.700-G.799: Digital terminal equipments

G.800-G.899: Digital networks

G.900-G.999: Digital sections and digital line system

ITU-T Recommendations

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13 September 2016 22

G.600-G.699: Transmission media and optical systems

characteristics

G.600-G.609: General

G.610-G.619: Symmetric cable pairs

G.620-G.629: Land coaxial cable pairs

G.630-G.639: Submarine cables

G.640-G.649: Free space optical systems

G.650-G.659: Optical fibre cables

G.660-G.679: Characteristics of optical

components and subsystems

G.680-G.699: Characteristics of optical systems

ITU-T Recommendations

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13 September 2016 23

Fiber optic principle

Ray Theory:

• A number of optic phenomena are adequately explained by considering light as narrow rays.

• The theory based on this approach is called geometrical optics.

• These rays obey following rules:

1. In a vacuum, rays travel at a velocity of c = 3x108m/s. In any other medium, rays travel at a slower speed, given by

v = c/n n = refractive index of the medium.

2. Rays travel straight paths, unless deflected by some change in medium.

3. If any power crosses a medium-boundary, the ray direction is given by Snell’s law:

n1 sin θi = n2 sin θr

13 September 2016 24

THEORY OF FIBER OPTICS

13 September 2016 25

INCIDENT RAYS

REFLECTED RAYS

REFRACTED RAYS

1

1

3

2

2

3

n2

θr

θi

Principle of Total Internal Reflection

n1 = 1.465

n2 = 1.461

n1

THE OPTICAL FIBER

13 September 2016 26

Cladding (n2)

125 mCore (n1) 6-10 m

Refractive index n1 > n2

321

3

2

1

LIGHT PROPAGATION IN FIBER

13 September 2016 27

Core (n1)

Cladding (n2)

Air 1.0

Carbon dioxide 1.0

Water 1.33

Ethyl alcohol 1.36

Magnesium fluoride 1.38

Fused silica 1.46

Polymethyl methacrylate polymer 1.5

Glass 1.54

Sodium chloride 1.59

Zinc sulfide 2.3

Gallium arsenide 3.35 Silicon

3.5

Indium gallium arsenide phosphoide 3.51

Aluminium gallium arsenide 3.6

Germanium 4.013 September 2016 28

Index of Refraction in

different materials

DUAL NATURE OF LIGHT

Wave Nature of Light :

• Many light phenomena can be explained by realizing that light is an

electromagnetic wave having very high oscillation frequencies.

• The wavelength of light beam:

= v/f

{where, v = velocity of light , f = frequency}

Particle Nature of light :

• Sometimes light behaves as though it were made up of very small particles

called photons. The energy of a single photon in Joules is:

Wp = hf

{where, h = 6.626 x 10-34 js [Planck’s constant], f = frequency}

13 September 2016 29

RELATION BETWEEN Λ AND REFRACTIVE INDEX

WHEN LIGHT WAVES ENTER A MEDIUM, THEIRWAVELENGTH IS REDUCED BY A FACTOR EQUALTO THE REFRACTIVE INDEX N OF THE MEDIUM BUTTHE FREQUENCY OF THE WAVE IS UNCHANGED

if λ0

is the vacuum wavelength of the wave thewavelength of the wave in the medium, λ' isgiven by

13 September 2016 31

1015

1014

1013

1012

1011

1010

109

108

107

106

105

104

103

102

101

RADIO

POWER

MICROWAVE

ULTRAVIOLET

INFRARED

Electromagnetic Spectrum

V I S I B L E L I G H T

UHF

VHF

HF

MF

LF

VLF

Hz

OPTICAL SPECTRUM

• 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 nm

980 nm1310 nm

1480 nm

1550 nm1625 nm

125 GHz/nm

• Visible light has a wavelength range of 0.4-0.7 m

• Silica glass fiber attenuates light heavily in visible &

UV regions.

• Glass fiber is relatively efficient in wavelength ranges

upto and in the infrared region.

• Three windows of operation are at 850, 1310 and

1550 nm.

13 September 2016 33

Window Concept in OFC Spectrum

WINDOW CONCEPT IN OFC SPECTRUM

Window Concept in OFC Spectrum

13 September 2016 35

Window Concept in OFC Spectrum

13 September 2016 36

Window Concept in OFC Spectrum

First Window

This is the band around 800-900 nm. This was the first

band used for optical fiber communication in the 1970s

and early 1980s.

The fiber losses are relatively high in this region,

Therefore, the first telecom window is suitable only for

short-distance transmission.

This window was relevant only for the initial silica fiber,

which had different attenuation characteristics

compared to low loss fiber developed later on.

13 September 2016 37

Window Concept in OFC Spectrum

Second Window

This is the window around 1310 nm which came into use

in the mid 1980s. This band had the property of zero

dispersion of light waves(on single-mode fiber).

The fiber attenuation in this window is about 0.35-0.4

dB/km.

This is the band in which the majority of long distance

communications systems were designed.

13 September 2016 38

Window Concept in OFC Spectrum

Third Window

The window from around 1510 nm to 1625 nm has the

lowest attenuation available on current optical fiber

(about 0.26 dB/km). In addition optical amplifiers are

available which operate in this band.

Almost all new communications systems, from the late

1990s operate in this window.

The loss peaks at 1250 and 1400 nm are due to traces

of water in the glass.

WAVELENGTH BANDS USED IN OFC

BAND DESCRIPTION WAVELENGTH RANGE

nm

O Band Original 1260-1360

E Band Extended 1360-1460

S Band Short wavelength 1460-1530

C Band Conventional 1530-1565

L Band Long wavelength 1565-1625

U Band Ultra long wavelength 1625-1675

13 September 2016 39

13 September 2016 40

Window Concept in OFC Spectrum

• The potential transmission capacity of optical fiber is

enormous.

• The second window is about 100 nm wide and ranges from

1260 nm to 1360 nm (loss of about 0.4dB/ km). The third &

fourth window is around 100 nm wide and ranges from

1530 nm to 1625 nm (loss of about 0.26 dB/km).

• The useful range is therefore around 200 nm.

• A λ-range of 100nm will correspond to a frequency

bandwidth of 30 THz (on a centre wavelength of 1000nm).

• Assuming that a modulation technique resulting in 1 bit/Hz

of analog bandwidth is available, then we can expect a

digital bandwidth of 3 ×1013 bits per second (30Tbps)!

BSNL

13 September 2016 41

G.655 standard OF cable

•Single mode

•1550 nm

•Carries up to 200 λ

DWDM

•10 Gbps to 40 Gbps per λ- commercially deployed

•100G and beyond 100G products are under

development.

Example:

Bharti-Singtel Chennai-Singapore Submarine

OFC link is 104λ x STM-64 ! (as in 2004-05)

working on G.655 NZDSF

Current trends

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13 September 2016 42

Bell Labs in Sep’2009 announced ultra-high speed

transmission of more than 100 Petabits per

second.kilometer, shown over a distance of 7000kms.

155 λ x 100G DWDM was used for the experiment.

Employs Advanced DSP with Coherent Detection.

Corning Inc. has developed a new multi-core fiber

design. In Jan’2013, NEC Labs and Corning announced

transmission speeds upto 1.05 Pb/s over 52.4km of a

single 12-core fiber.

Latest trends

BSNL

13 September 2016 45

Classification of

Optical Fibers

CONSTRUCTION OF OPTICAL FIBER

• Basic fiber has a corewith refractive index n1surrounded by claddinglayer with refractiveindex n2, n1 > n2

• Change in RI is achievedby selectively doping thecore (like with GeO2).

• The difference betweenn1 and n2 is less than0.5%.

• The cladding layer issurrounded by one ormore protective coating.

13 September 2016 46

CORE

CLADDING

n2

n2

n1 > n2n1 > n2

CLASSIFICATION OF OPTICAL FIBER

Material Classification:

• Liquid core fiber.

• Fused-silica-glass fiber: have silica-core and silica-cladding.

• Plastic-clad-silica (PCS) fiber: have silica core and plastic cladding.

• All-plastic fiber : have both core and cladding made up of plastic.

• Compound glass fiber such as fluoride glass fiber.

Modal classification :

• Fibers can be classified based on number of modes available for

propagation - Single-mode (SM) fiber

- Multi-mode (MM) fiber

Classification based on refractive index profile :

• Step index (SI) fiber

• Graded index (GRIN) fiber13 September 2016 47

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CLASSIFICATION OF OPTICAL FIBER

Single-mode (SM) fiber

• one mode of light at a time through the core

• modal dispersion is greatly reduced

• a higher bandwidth capacity

Multi-mode (MM) fiber

• has larger core, than the SM fiber

• numerous modes or light paths, can be carried simultaneously

through the waveguide.

oStep Index (SI)- there is a step in the refractive index at the

core and cladding interface

oGraded-index (GRIN) refers to the fact that the refractive index of

the core is graded- it gradually decreases from the center to

outward of the core; reducing modal dispersion.13 September 2016 48

BSNL CLASSIFICATION OF OPTICAL FIBER

13 September 2016 49

8-12 m 125m

50 - 100m 125m

50 m 125m

c) Multi mode GRIN fiber (Graded-Index)

b) Multi mode step-index fiber

a) Single mode step-index fiber

Index profile

Index profile

Index profile

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13 September 201650

Output of Single mode fiber

Output of Multi-mode fiber

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OPTICAL FIBER CABLETYPES

13 September 2016 51

CABLING OF OPTICAL FIBER

• Cabling is needed to protect the fiber from mechanicaldamage and environmental degradation.

• OF Cables have following common parts-

13 September 2016 52

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OF CABLE CROSS SECTION

1.Optical fibre

2.Central strength member

3.Filling compound

4.Loose tube

5.Filler

6.Wrapping tape

7.Optional aramid or glass strength members

8.Sheath13 September 2016 53

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CABLE COMPONENTS

13 September 2016 54

Component Function Material

Buffer/ loose tube

bufferProtect fibre From Outside Nylon, Mylar, Plastic

Central Member

Facilitate Stranding,

Temperature Stability, Anti-

BucklingSteel, Fibre glass

Primary Strength

Member

Tensile Strength (pulling,

shearing, and bending)Aramid Yarn, Steel

Cable Jacket

Contain and Protect Cable Core

Abrasion Resistance

polyethylene, polyurethane,

polyvinyl chloride or teflon.

Cable Filling

Compound

Prevent Moisture intrusion and

MigrationWater Blocking Compound

ArmoringRodent Protection, Crush

ResistanceSteel Tape

• Centre Strengthening Member – GRP(glass reinforced plastic), FRP(fiber reinforced plastic)

• Loose Tube Buffers – 2.4 mm dia, Fibres are placed inside along with jelly.

• Primary Strength Member –Aramid Yarn

• Inner Sheath – Black

• Outer Nylon Sheath -Orange

13 September 2016 55

OF Cable Construction

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Loose Tube Buffers

•The Fibers are loosely drawn inside the Buffer

Tubes to take care of Temp. variations

•The OF Cable which is used outside is known as

Loose Tube Buffers

•The Correction Factor is 0.98/0.985

980 meters of OFC will contain 1000 meters

Fiber inside (Cable length is less by 1.5 to 2%)13 September 2016 56

OPTICAL FIBER CABLE TYPES

• Conventional Loose-tube OFC

• Armoured OFC (Underground Installation - Directly Buried)

• Aerial Optical Fibre Cables

• Ribbon OFC – high/very high fiber-count, for OAN

• Micro-duct OFC- high fiber count in same duct

• ADSS(All-Dielectric Self-Supporting) – aerial installations

• OPGW (Optical Ground Wire)- power line installations

13 September 2016 57

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Construction Of Cable

13 September 2016 58

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FIBER COUNT IN CABLE

13 September 2016 59

•6 fiber

•12 fiber

•24 fiber

•48 fiber

•96 fiber

Standard OFC length on drum is

2000M (2Km). Other drum lengths

like 4km are also available.

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RIBBON-FIBER CABLE

13 September 2016 60

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MICRODUCT OFC

13 September 2016 61

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Armored Cable

13 September 2016 62

Aerial Cable/Self-

Supporting

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ADSS CABLE(33 KV)

13 September 2016 63

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OPGW(400 KV)

13 September 2016 64

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These are tight Buffered cable

•Has only one fibre per cable

•Connector ended

•Used in the indoor

applications

•Connecting equipment to

outside OFC cable

•Connecting meters to the

equipment

micro

meter

Pig Tail Cable

13 September 2016 65

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Specification Of OFC

Fibre -

Core - 8-10 Microns (Single Mode)

50 - 100Microns (Multimode)

Cladding - 125 Microns (overall Dia)

Attenuation - better than 0.5 db /KM

Primary Coating 250 Microns UV cured Acrylate

Secondary Coating –2.4 mm nylon PE Jelly filled tube

Central Strength Member – Fibre Reinforced Plastic (FRP)

Moisture Barrier- non metallic polythylene sheet free from

pinholes and other defects

Polythene sheath Polythene free from pin holes 13 September 2016 66

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Nylon Outer Sheath (0.7mm thickness)- Protective sheath against

termite & partially against rodent

Strength to withstand a load - 3X9.8 W Newtons, where W is

weight of O/F cable per KM in Kg

MAX Strain allowed in fibre - 0.25%

MAX attenuation variation - Permissible + 0.02 dB from

normal 20 degree centigrade to 60 degree centigrade

Flexibility – Maximum bending radius allowed 24d, d is the

diameter of OF cable

Cable drum lengths - 2 KM +10%

Cable ends - one end fitted with grip

Other end sealed with cap

Specification Of OFC

13 September 2016 67

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13 September 2016 68

OF Cable jointing

OF CABLE JOINTING

Jointing of optical fiber is imperative in fiber communication.

For this the following are used-

CONNECTORS

COUPLERS

SPLICES

OPTICAL FIBER CONNECTORS

CONNECTORS USED FOR ARRANGING TRANSFER

OF OPTICAL ENERGY FROM ONE FIBER OPTIC

COMPONENT TO ANOTHER IN AN OPTICAL FIBER

SYSTEM

COMPONENTS INCLUDE FIBER, FILTER, COUPLER,

OPTO ELECTRONIC DEVICES ETC.

BSNL

FIBER CONNECTORS

13 September 2016 71

COUPLERS

FIBER OPTIC COUPLERS EITHER SPLIT OPTICAL SIGNALS

INTO MULTIPLE PATHS OR COMBINE MULTIPLE

SIGNALS ON ONE PATH.

. THE NUMBER OF INPUT AND OUTPUT PORTS,

EXPRESSED AS AN N X M CONFIGURATION,

CHARACTERIZES A COUPLER

. FUSED COUPLERS CAN BE MADE IN ANY

CONFIGURATION, BUT THEY COMMONLY USE

MULTIPLES OF TWO (2 X 2, 4 X 4, 8 X 8, ETC.).

SPLITTERS

THE SIMPLEST COUPLERS ARE FIBER OPTIC

SPLITTERS

. THESE DEVICES POSSESS AT LEAST THREE PORTS

. A TYPICAL

‘T’COUPLER

SPLICES

SPLICE IS A PERMANENT INTERCONNECTION BETWEEN TWO

FIBERS

TWO TYPES OF SPLICES –

•Mechanical splice

•Fusion splice

MECHANICAL SPLICES

FIVE GENERAL STEPS TO COMPLETE FUSION SPLICE

1. STRIP, CLEAN & CLEAVE

2. LOAD SPLICER

3. SPLICE FIBERS

4. DIAGNOSE AND CORRECT IF ERRORS OCCUR

5. REMOVE AND PROTECT SPLICE

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CLEAVER

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LOAD SPLICER

13 September 2016 79

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RIBBON FUSION SPLICER

13 September 2016 80

LAYING OF CABLE

• Optic fiber cables are laid underground as well as

overhead.

• Underground laying is much frequent practice.

• Over ground laying is used in special cases

• A large collection of accessories are required to make a

strong and reliable overhead OFC alignment.

• Sometimes ordinary overhead alignments are erected for

emergent situations

13 September 2016 81

PROPERTIES OF OPTICAL FIBER ANDTRANSMISSION IMPAIRMENTS

13 September 2016 82

LOSSES IN OPTICAL FIBERS

• There are several points in an optic system where losses occur.

• These are:

•couplers

•splices

•Connectors

•Fiber itself

13 September 2016 83

CLASSIFICATION OF FIBER LOSSES

• Losses due to absorption.

• Even the purest glass will absorb heavily within specific wavelength regions. Other major source of loss is impurities like, metal ions and OH ions.

• Losses due to scattering:

• caused due to localized variations in density, called Rayleigh scattering and the loss is:

L = 1.7(0.85/)4 dB/km is in micrometers

• Losses due to geometric effects:

• micro-bending

• macro-bending

• Losses are also termed as

Attenuation in a fiber

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LOSSES DUE TO MICRO BENDING

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LOSSES DUE TO MACRO BENDING

13 September 2016 86

DISPERSION IN FIBER

• Dispersion is spreading of the optical pulse as it travels down the length.

• Dispersion limits the information carrying capacity of fiber

• Dispersion is classified as : Chromatic Dispersion , Modal Dispersion, and PMD

• Chromatic dispersion consists of:• Material Dispersion• Waveguide Dispersion

• Modal Dispersion:• pulse spreading caused by various modes (only for MM

fiber).• For visible light, refraction indices n of most transparent

materials (e.g., air, glasses) decrease with increasing wavelength λ

13 September 2016 87

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13 September 2016 88

CONSEQUENCES OF DISPERSION

BSNL

MATERIAL DISPERSION• Pulse spreading caused due to variation of velocity with

wavelength

• Every laser source has a range of optical wavelengths;

figure shows examples for LD and LED laser sources

13 September 2016 89

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MATERIAL DISPERSION

13 September 2016 90

FIBER

LOGIC 1 LOGIC 1

λ 1

λ

λ

λ

λ

λ

0

2

1

0

2

λλ λ λ1 0 2

1.0

0.5

Light source

spectrum

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HOW TO REDUCE MATERIAL DISPERSION?

• By using sources with smaller band width or spectral

width

13 September 2016 91

LED 20-100 nm

LD(semiconductor) 1-5 nm

YAG laser 0.1 nm

He Ne laser 0.002nm

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WAVEGUIDE DISPERSION

• The figure below shows the light distribution inside the

fiber (in the core and cladding) for different wavelengths

• Dispersion directly proportional to wavelength

13 September 2016 92

.

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CHROMATIC DISPERSION

13 September 2016 93For G.652 fiber, CD is nil at 1310nm

BSNLPOLARIZATION MODE DISPERSION (PMD)

13 September 2016 94

Most single-mode fibers support two perpendicular

polarization modes, a vertical one and a horizontal one.

Because these polarization states are not maintained,

there occurs an interaction between the pulses that results

is a smearing of the signal.

PMD has more impact on higher bit-rates, more than

10Gbps.

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13 September 2016 95

DISPERSION COEFFICIENTS

• CD Coefficient

- CD Coefficient, indicated as D, is expressed in ps/(nm.km).

- It specifies the arrival time delay in picoseconds, that would be included per 1km of the transmission fiber if the wavelength deviates by 1nm.

• PMD Coefficient

- It is indicated by PMDQ and the unit is ps /(km)-1/2

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13 September 2016 96

OPTICAL SOURCES ANDDETECTORS

OPTICAL SOURCES

• The basic elements in transmitters: Light source, Electronic interfaces,

Electronics processing circuitry, Drive circuitry, optical interfaces, output

sensing and stabilization, Temperature sensing and control.

• Most common light sources (the device which actually converts electrical

signals to its optical equipment) :

• LEDs

• LASER diodes.

• Laser power is very sensitive to temperature. Hence temperature sensing

and control is required

• Operating characteristics of a laser are notably, threshold current, output

power, and wavelength change with temperature13 September 2016 97

BSNL

LED VS LASER DIODE

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LED - LIGHT EMITTING DIODE

- Shorthaul and medium haul

communication systems where

- Power requirements are small

- Low bit rate optical communication

- broad spectral width is not a problem

LD - LASER (Light Amplification by Stimulated

Emission of Radiation) Diode

- Used for long distance and high bit-rates

-very narrow spectral width (0.1 to 2nm)

Cooled DFB Lasers are available in precisely selected s

(for DWDM applications)

BSNL

LASERS

• Active Transmit device—Converts electrical signal into light

pulse.

• Conversion, or modulation is normally done by externally

modulating a continuous wave of light or by using a device

that can generate modulated light directly.

• Light source used in the design of a system is an important

consideration because it can be one of the most costly

elements.

• Its characteristics are often a strong limiting factor in the

final performance of the optical link

• Light emitting devices used in optical transmission must be

compact, monochromatic, stable, and long-lasting.

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BSNL

SEMICONDUCTOR LASERS

• Two type

• Febry Perot- Normally used in SONET/SDH systems

• Distributed Feedback- well suited for DWDM applications, as it

emits a nearly monochromatic light, is capable of high speeds, has

a favorable signal-to-noise ratio.

• The ITU draft standard G.692 defines a laser grid for

point-to-point WDM systems based on 100-GHz

wavelength spacing with a center wavelength of 1553.52

nm

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BSNL

EXTERNAL MODULATION IN DFB LASER

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DETECTORS

• The basic elements in an optical receiver: Detector,

Amplifier, Decision circuits

• The detectors used in fiber optic communications are

semiconductor photodiodes or photo detectors.

• It converts the received optical signal into electrical form.

• PiN photodiode: cheaper, less temperature sensitive,

and requires lower reverse bias voltage.

• Avalanche PhotoDiode (APD): used where high

receive sensitivity and accuracy is required.

• But APDs are expensive and more temp sensitive

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OPTICAL LINK DESIGN

BASIC FIBER OPTIC

COMMUNICATIONS SYSTEM

An Optical Fiber System consists of :

a transmitter to convert electrical signals to optical

a receiver to convert optical signal to electrical

a medium - optical fiber cable.

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BSNL

• Decibels (dB): unit of level (relative measure) • X dB is 10-X/10 in linear dimension e.g. 3 dB Attenuation = 10-.3 = 0.501

• Standard logarithmic unit for the ratio of two quantities. In optical fibers, the ratio is power and represents loss or gain.

• Decibels-milliwatt (dBm) : Decibel referenced to a milliwatt X mW is 10log10(X) in dBm, Y dBm is 10Y/10 in mW. 0dBm=1mW, 17dBm = 50mW

• Wavelength (): length of a wave in a particular medium. Common unit: nanometers, 10-9m (nm) • 390nm (violet) to 700nm (red) is visible. In fiber optics primarily use 850, 1310, & 1550nm

• Frequency (): the number of times that a wave is produced within a particular time period. Common unit: TeraHertz, 1012 cycles per second (Thz)

• Wavelength x frequency = Speed of light x = C13 September 2016 105

Some terminology

BSNL

• Attenuation = Loss of power in dB/km

• The extent to which optical power from the source is diminished as it passes through a given length of fiber-optic (FO) cable, tubing or light pipe. This specification determines how well a product transmits light and how much cable can be properly illuminated by a given light source.

• Optical Signal to Noise Ratio (OSNR) = Ratio of optical signal power to noise power for the receiver. (OSNR = 10log10(Ps/Pn)).

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Some more terminology

BSNL

DB VERSUS DBM

• dBm used for output power and receive sensitivity (Absolute Value)

A dBm is a specific measurement referenced to 10-3 watts or 1

milliwatt (mW). The calculation, where X is the measured power in watts,

for laser output measured in dBm:

13 September 2016 107

Examples

10dBm 10 mW

0 dBM 1 mW

-3 dBm 500 uW

-10 dBm 100 uW

-30 dBm 1 uW

BSNL

DB VERSUS DBM

13 September 2016 108

• dB used for power gain or loss (Relative Value)

For example, output power in Watts (A) compared to input

power in Watts (B) used to represent attenuation of a fiber

related to the Common (base 10) logarithm value:

BSNL

BIT ERROR RATE (BER)

• BER is a key objective of the Optical

System Design

• Goal is to get from Tx to Rx with a BER <

BER threshold of the Rx

• BER thresholds are on Data sheets

• Typical minimum acceptable rate is 10 -12

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BSNL

OPTICAL BUDGET

Optical Budget is affected by:• Fiber attenuation

• Splices

• Patch Panels/Connectors

• Optical components (filters, amplifiers, etc)

• Bends in fiber

• Contamination (dirt/oil on connectors)

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Basic Optical Budget = Output Power – Input Sensitivity

Pout = +6 dBm R = -30 dBm

Budget = 36 dB

BSNL

OPTICAL LINK BUDGET

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Pt - (Lcp+ Lct+ Lsp+ Lfb+ Msys) Srec

where

Pt = light source transmitting power, in dBm

Lcp =coupling loss source to fiber, in dB

Lct =connector’s losses (2nos, source to fiber & fiber

to detector), in dB

Lsp =splicing losses, in dB

Lfb =fiber loss, in dB

Msys =system loss margin requirement, in dB

Srec =required PD receiver sensitivity, in dBm

BSNL

13 September 2016 112

Transmitter ReceiverFiber Fiber

Splice

Receiver Sensitivity

Margin

LINK POWER BUDGET

P

O

W

E

R

BSNL

An Optical Link is required to be commissioned between two Stations A & B.

Do the Power Budgeting. Check its feasibility. What is the Total Link Loss?

Data is given below :-

• Distance between two stations = 69 km.

• Splice Loss. = 0.1 dB / Splice.

• Connector Loss. = 1 dB / Connector.

• Coupling Loss (Source to fiber). = 3 dB.

• Laser Output. = 0 dBm.

• Receiver Sensitivity. = -37 dBm.

• fiber Loss. = 0.4 dB/km.

• System Margin. = 3 dB.

• Extra Cable to be kept at Joint = 20 m / Joint.

• fiber Length to be taken. = 102% of Cable Length.

• Shrinkage. = 1 %.

• Extra Cable at Terminals = 100m each

• Cable Length on drum = 2km /cable drum

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EXERCISE

BSNL

Distance Between Station A & B = 69 km.

Cable Length after taking Shrinkage = 69x101% = 69.69 km.

Number of Cable Drums required = 69.69/2 = 35.

Total number of Splices in the Cable Route = 35- 1= 34.

Extra Cable to be kept at Joints = 20x34 = 680 m.

Leading-in Cable at Both Ends = 100+100=200m.

As the Cable Length exceeds 70 km, there will be one more Joint in the Route and we need to provide additional 20 meter of cable at Joint Location, Hence :

Total number of Splices in the Link = 34+1= 35

Cable Length after keeping provision for Joint = 69.69+0.70+0.20=70.59 km

Fiber Length. =70.59 x 102%

= 72.00 km.

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SOLUTION

Contd…

BSNL

Link Loss:

Source to Fiber Coupling Loss = 03.00 dB.

Connectors Losses = 1 x 2 = 02.00 dB.

Fiber Loss = 0.4 x 72.0 = 28.80 dB.

Splicing Loss = 0.1 x35 = 03.50 dB.

Total link loss = 37.30 dB.

Laser Output – Link Loss = 0 – 37.30 = -37.30dBm.

Projected loss by including 3dB Margin = -40.30 dBm

Which is beyond Receiver Sensitivity level of -37 dBm.

Hence Link is NOT Feasible! 13 September 2016 115

SOLUTION

THANK YOU

13 September 2016 116