Optical Fibre Cable an Overview

7/29/2019 Optical Fibre Cable an Overview

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OPTICAL FIBRE CABLE

An overview


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INTRODUCTION

Optical Fibre is a new medium in whichinformation (Voice, Data or Video) is

transmitted through a Glass or Plastic Fibre, inthe form of light.

The transmission sequence is

-Information is encoded into Electrical Signal-Electrical Signals are converted into lightsignals.

-Light travels down the fibre

-A detector changes the Light Signals intoElectrical Signals

-Electrical Signals are decoded intoinformation


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ADVANTAGES OF FIBRE OPTICS

Optical Fibres are non-conductive(dielectric)

Electromagnetic Immunity

Large Bandwidth ( > 50 Ghz for 1 Km ) Low Loss ( 5 dB/km to < 0.25 dB/km)

Small and light weight cables

Available in long lengths ( > 12 kms)

Security

Universal Medium


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APPLICATIONS OF FIBRE OPTICS

Common carrier Nationwide Networks

Telephone Inter-Office Trunk Lines Customer premise Communication Networks

Under-Sea Cables

High Electro-Magnetic Interference areas Factory Communication/Automation

Control Systems

Expensive environments High lightning areas

Military applications

Classified (secured) Communications


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PRINCIPLE OF FIBRE OPTICS

Light traveling from one material to anotherchanges speed, which results in light

changing its direction of travel. This deflection

of light is called refraction.

A ray of light passing from a material of lower

refractive index to a material of higher

refractive index is bent towards the normal.

But light going from a material of higherrefractive index to a material of lower

refractive index refract away from the normal.


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PRINCIPLE OF FIBRE OPTICS ( contd. )

As the angle of incidence increases, the angle

of refraction approaches 90o to the normal. The

angle of incidence that yields an angle of

refraction of 90o is the critical angle. If the angle

of incidence increases more than the criticalangle, the light is totally reflected back into the

first material. The angle of incidence and

reflection are equal. This phenomenon is called

Total Internal Reflection which forms the basis ofpropagation of light through a optical fibre.


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INCIDENT RAYS

REFLECTED RAYS

REFRACTED RAYS

1

1

3

2

2

3

n2r

i

(principle of total internal reflection)

n1 = 1.48

n2 = 1.46

n1


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OPTICAL FIBRE

The Optical Fibre has two concentric layers-

-The Core which forms inner part.

-The Cladding which is the outer part.

The inner Core is the Light carrying part.

The index of the Cladding is 1% less than thatof the Core.

The typical values for r.i. of the Core is 1.47while that for the Cladding is 1.46.

Most of the Fibres have an additional coatingaround the Cladding. This buffer coating is ashock absorber and has no optical properties.


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The Optical Fibre

Cladding

125 mCore8-10 m


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LIGHT PROPAGATION THROUGH FIBRE

Light injected into the fibre and striking Core toCladding interface at an angle greater than the

Critical angle is reflected back into the Core

according to the principle of Total Internal

Reflection. The reflected Light ray strikes otherside of the Core to Cladding interface and again

it is reflected. This process continues and Light

travels in a zigzag way along the length of the

fibre. Light striking the interface at less than the

critical angle passes into the cladding where it is

lost over distance.


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Light propagation in fibre


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http://localhost/var/www/apps/conversion/tmp/scratch_2/FIBER00.swf


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FIBRE GEOMETRY The Optical Fibre consists of a core usually of silica

or borosilicate glass surrounded by a cladding of

the same material but of slightly lower refractiveindex.

Fibres have exceedingly small diameters. Thediameters of the core and cladding are as follows.

Core (m) Cladding (m)

8 125

50 125

62.5 125

100 140 Fibre sizes are usually expressed by first giving the

core size followed by the cladding size. Thus50/125 means a core diameter of 50 m andcladding diameter of 125 m.


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CLASSIFICATION OF OPTICAL FIBRE

Classification based on refractive index

profile :

SI fibre (Step Index Fiber)

GRIN fibre(Graded Index Fiber)


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CLASSIFICATION OF OPTICAL FIBRE (contd.)

The step index fibre has a core with

uniform index throughout. There is a sharp

step at the junction of core and cladding.

n1= r.i. of coren2= r.i of cladding

index n(r)

n2n1

r


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The graded index fibre has a non uniformcore. The index is highest at the centre

and gradually decreases until it matches

with that of the cladding. There is no sharpbreak in indices between the core and

cladding.

Refractive

index n(r)

n2

n1

r


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Modal classification :

In case of fibre, a mode is simply apath that a light ray can follow intravelling down a fibre.

The types of fibre based on mode:

Single Mode Fibre ( SMF )Multi Mode Fibre ( MMF )



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A single mode fibre supports only a single

propagating mode. Single mode fibres

have small core diameters ranging from 4

to 10 ms.

A multi mode fibre supports more than

one propagating mode. Multi mode fibres

have relatively large core diameters.Typical values of core diameters are 50,

62.5,80,100 ms.


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By this classification there are three types offibres:

Multi mode step index fibre (Step Index fibre)

Multi mode graded index fibre (Graded Index

fibre)

Single mode step index fibre (Single mode

fibre)


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Input pulse Output pulse

Refractive

index n(r)

n2n1

r CoreCladdingAmplitude

t

Amplitude

Multimode step index fiber


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Refractive

index n(r)

n2n1

r

t

Multimode graded index fiber

Input pulse Output pulse

Amplitude Amplitude


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n2n1

r

tN(r)

Input pulse Output pulseAmplitude Amplitude

Single-mode step index fiber


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FIBRE USED IN BSNL

BSNL uses Single Mode Fibres.

Typical values related to the sizes of the

Cables are-

-Core diameter= 5 to 10 micrometer-Cladding diameter=125 micrometer

-Coating diameter=250 micrometer

SM Fibres are used in Telecom for theirlarge information carrying capacity over

longer distances.


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OPTICAL FIBRE PARAMETERS

Wave length

Frequency

Window Attenuation

Dispersion

Bandwidth

Numerical aperture


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Wave length:

Wave length is a characteristic of light that

is emitted from the light source and is

measured in nm.

Frequency:

Frequency is number of pulses per second

emitted from a light source. Frequency is

measured in units of hertz (Hz).

In terms of optical pulse

1 Hertz = 1pulse/second.


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Window :

Window is defined as the range of wavelengths on which a fibre best operates.

Each window is centered on the typicaloperational wave lengths. Typical windows

areWindow Operational wavelength

800nm 900nm 850nm

1250nm --1350nm 1300nm1500nm1600nm 1550nm


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Attenuation :

Attenuation is defined as the loss of optical

power over a set distance. A fibre with lower

attenuation will allow more power to reach a

receiver than fibre with higher attenuation.

Attenuation is categorized as intrinsic and

extrinsic.


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Intrinsic Attenuation :

Cause of intrinsic attenuation is inherentor within the fibre. Intrinsic attenuation mayoccur due to absorption and scattering.

Natural impurities in the glass absorb light

energy.Light rays travelling in the core reflect

from small imperfections into a new pathway

that may be lost through the cladding. Thelight is scattered in all directions whichcauses loss of optical power in forwarddirection.


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Extrinsic Attenuation :

Extrinsic attenuation is loss due toexternal sources. Extrinsic attenuation occur

due to Microbending and Macrobending.

Microbending in the fibre caused by

crushing, contraction etc. Microbends cause

some of the light to couple out of the fibre.

Macrobending is actually excessive

bending of the fibre. If the fibre is sharply

bent, the light travelling down the fibre can

not make the turn and is lost in the cladding.


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Dispersion :

Dispersion is defined as the spreading oflight pulse as it travels down the fibre.

Because of the spreading effect, pulses

tend to overlap, making them unreadable

by the receiver. Dispersion is undesirableas it limits the bandwidth or information

carrying capacity of a fibre. The bit rate

must be low enough to ensure that pulsesdo not overlap.


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Types of Dispersion

Modal Dispersion

Material Dispersion

Wave guide Dispersion

Modal dispersion occurs only in multimodefibres. It arises because rays follow different

paths through the fibre and consequently

arrive at the other end of fibre at different

times. Typical modal dispersion figures for

step index fibres are 15 to 30 ns/Km.


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Modal dispersion can be reduced in three ways

Using a smaller core diameter, which

allows fewer modes.

Using a graded index fibre. Using a single mode fibre, which permits

no model dispersion.


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Material Dispersion :

Material dispersion occurs as different

wave lengths (colours) travel at different

velocities through a fibre, even in the

same mode.

Waveguide Dispersion :

Waveguide dispersion occurs in a

single mode fibre because optical energy

travels in both the core and cladding which

have slightly different refractive indices.


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Bandwidth :

Bandwidth is defined as the amount of

information that a system can carry such that

each pulse of light is distinguishable by the

receiver. System bandwidth is measured in

MHz or GHz.

Numerical Aperture :

Numerical aperture (NA) is the light

gathering ability of a fibre. A fibre with a large

NA accepts light well, a fibre with a low NA

requires highly directional light.


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Cladding

Core

NA= Sin Typical = 100

Input Surface Refraction

NUMERICAL APERTURE


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OPTICAL TRANSMITTERS

In Optical Line Systems we need light sources in the

infra-red spectum part. The wavelengths used are in one of the following

windows of Optical Fibres-

850nm, 1300 nm, and 1550 nm.

Features of an ideal source for Fibre OpticCommunication are-

-High brightness

-Small emission area

-Small emission Core angle

-Fast response to electrical modulation

-Long life

-Emission wavelength compatible with Fibre


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Optical Transmitting Devices

Commonly there are two Semi-conductor devices for

using as Optical Transmitters--LED (Light Emitting Diode)

-LASER ( Light Amplification by Stimulated Emission ofRadiation ) Diodes

LEDs are composed of a P-N junction with dopedsemiconductor layers. Injected electrons combine withholes in the P-layer where this phenomenon results in

emission of Photons. In case of LASER Diodes, the emission of Photons are

spontaneous and are stimulated by other Photons ( byamplifying light) and we get large quantities of highenergy Photons emitted.


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CATEGORIES OF LEDs

There are LEDs of two categories.

Surface Emitting LEDs

Edge Emitting LEDs

Surface Emitting LEDs radiate Photons in apattern where power diminishes away from a

direction normal to the surface.

Edge Emitting LEDs can concentrate

radiation with improved coupling efficiency.

OPERATING PRINCIPLE


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OPERATING PRINCIPLEOF LASER DIODE

The Energy levels of the Electrons in an atom/molecule

may be written as E1, E2, E3, E4 etc. If an Electron goes from E1 to E2, E3 etc. there will be

absorption of energy.

On the other hand if the Electron comes from E4 to E3 or

E2 or E1 etc , there will be emission of energy in theform of radiation. The radiation consists of Photons ofvarious energy levels. This will be spontaneous.

If the photons at the time of spontaneous emission arestimulated by other Photons ( I.e., by amplifying light ),

the situation is called Stimulated Emission and we willget very large quantities of high energy photons. This isthe principle used in LASER Diodes.

O C C O


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OPTICAL DETECTOR

The Optical Detectorconverts Optical Energy into ElectricalEnergy.

It is basically an Opto-Electronic Transducer doing the oppositefunction of an Optical Source.

There are two distinct mechanisms for Photo detection-

External Photo-Electric Effect

Internal Photo Electric Effect

In External Photo-Electric Effect , the Electrons are freed fromthe surface of a metal by the Energy absorbed from an incidentstream of Photons. The Vacuum Photodiode and the Photo

Multiplier Tubes come under this category. In Internal Photo Electric Effect , Semi-conductor Devices allow

generation of free Charge carriers , Electrons and Holes byabsorption of Incoming Photons. PIN Photo Diode and APDcome under this category.

PIN PHOTODIODE


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PIN PHOTODIODE

PIN stands for Positive, Intrinsic, Negative.

PIN Photodiode has resistive Intrinsic layer sand-witchedin between P and N layers. The width of Intrinsic layer is

sufficient and the depletion layer is spread over the

Intrinsic layer under the influence of high field due to

reverse bias.

Under reverse bias condition, when a Photon enters the

depletion region, it is absorbed and a pair of Electron

and Hole are generated. The Electron and the Hole so

generated move towards the opposite Electrodes. This

results in the flow of Current in the external field.

The PIN Photo Diodes have lower capacitance, high

Quantum efficiency and high speed of operation.


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APD ( AVALANCHE PHOTO DIODES )

The APD has a very wide Intrinsic Layer in between the

P and N Semiconductor materials. Provision of another

P type material in between the N type material and the

Intrinsic layer makes working much more efficient.

As the Photon enters the Intrinsic layer, Electron-Holepairs are formed. Movement of the Charge carriers

towards the opposite terminals results in collision inside

the Diode with neutral atoms. As a result of such

collision more numbers of Electron-Hole pairs aregenerated. Consequently, we get large flow of current in

the external Circuit.

APDs are often influenced by various Noises.

GENERAL FEATURES OF DETECTORS


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GENERAL FEATURES OF DETECTORS

The Detectors are supposed to have the following

characteristics-

High efficiency

Fast response

Low Noise

Small Size

Light Weight

Long Life

Reliability

Low Cost


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CABLE CONSTRUCTION

Optical Fibre need to be protected before it is used.

Cabling is a outer protective structure surroundingone or more Fibres. Cabling protects fibresenvironmentally and mechanically from beingdamaged.

Important consideration in cable design are--Tensile Strength (Pull)

-Ruggedness

-Durability-Flexibility

-Environment resistance

-Temperature extreme


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MAIN COMPONENTS OF A CABLE

In general, an OF Cable is divided into two types-

-Metallic

-Non-metallic

Metallic cables use metallic component for

protection. Non-metallic cables use non-metallic protection

material. Non-metallic cables are used in areas

suffering from high frequency of lightning.

BSNL has gone for non-metallic cables.


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PROTECTIVE COMPONENTS

The following components are commonlybeing used in O F Cables for protection-

-Buffer

-Strength Member-Filler and Core Wraps

-Jacket and Moisture barrier


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OF CABLE CONSTRUCTION

CORE

CLADDING

SILICON COATING

BUFFER JACKET

STRENGTH MEMBER

BLACK POLYETHANE INNER JACKET

ORANGE NYLON OUTER JACKET



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BUFFERS: The Fibres are coated with Buffer

immediately after being drawn. This Buffer isknown as primary coating. The Buffer being a

primary coating serves as mechanical protection

during the subsequent stages of cable

manufacturing. After primary coating , the Fibresare coloured by passing through the colouring

machine and then coloured Fibres are passed

through additional Buffer Tubes which are of

two types--Loose Buffer

-Tight Buffer

LOOSE BUFFER & TIGHT BUFFER


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LOOSE BUFFER & TIGHT BUFFER

LOOSE BUFFER: The fibres are placed inside the

Tubes having diameter much larger than those of the

Fibres. The Tubes are filled with jelly like compound

to provide additional cushioning and prevention from

intrusion of moistures. Such Buffers are suitable for

outdoor applications having temperature variations. TIGHT BUFFER: The Tight Buffer has a plastic

coating directly applied over the primary coating of

the Fibre. This type of buffer provide bettercrushand impact resistance. These are suitable for indoorapplications where temperature variation is minimum

and ability to make tight turns is desired.

STRENGTH MEMBER


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STRENGTH MEMBER

The function of the strength member

is to add mechanical strength to the fibre. Itprotects the fibre from tensile stresses duringand after installation. The most commonstrength members are-

1. Kevlar

2. Aramid Yarn

3. FRP (Fibreglass Reinforced Plastic)

or GRP (Glass Reinforced Plastic)

The strength members are centrallylocated or applied over the Buffer jacket.

FILLERS & CORE WRAPS


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FILLERS & CORE WRAPS

Fillers are employed to provide cushioning

to the Fibres and to give shape to thecable. Typical materials are PVC,polyethylene etc.

Cable Core is generally filled with a waterblocking or filling compound to preventmoisture intrusion.

Binder tapes are applied to hold the

assemblies of coated fibres and fillerstogether. Usual materials are polyester,cellulose etc.


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JACKET & MOISTURE BARRIER

The Jacket or sheath provides protectionfrom the effects of abrasion, oil, acids,

alkali etc.

Materials like PVC, nylon, low densitypolythene, high density polythene,

polyurethane etc. being used successfully

for this purpose.

There may more than one layer of

protection forming the jackets.

MULTI FIBRE CABLE


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MULTI FIBRE CABLE

-It contain several loose buffer tubes.

-Tubes used are made of thermo-plastic.-Each Tube may contain one or more fibres.

-These Tubes are stranded helically around

a central strength member which is made ofGRP or FRP.

-The interstices are filled with materials to

protect from temperature variation andmoisture. The cable core is wrapped with a

wrapping tape.


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MAIN COMPONENTS

SIECOR

OUTER JACKET (NYLON)

INNER SHEATH

BINDER

BUFFER TUBES

CENTRAL

STRENGTH MEMBER

FILLERS (SOLID)PVC / CELLULOSE

A

AA

A

FIBER


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THANKS

Documents

Optical Fibre Cable an Overview