Presentation on Fibre Optic Technology

8/3/2019 Presentation on Fibre Optic Technology

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Presentation on

Fibre OpticsTechnology

By

Sanchaita Mandal


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Fibre Optics Introduction

Advantages & Disadvantages

Areas of Application Fibre Theory

Structure Of Fibre

Propagation Of light in Fibre

Types of Fibre

Pulse Spreading and its cause

Fibre Manufacturing


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Introduction What are Optical Fibres?

Optical Fibres are fibres of glass usually

about 120 micron in diameter which are

used to carry signals in form of pulses of light

over distances upto 50km without the needfor repeaters.


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Advantages &

Disadvantages

Advantages

1)

Capacity:- Optical fibres carry signals withmuch less energy loss than copper cable

and with much higher bandwidth which

means fibres can carry more channels of

informations.2) Size & Weight:-Optical fibres are much

lighter and thinner than copper cables with

same bandwidth.


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Advantages &

Disadvantages(contd) Advantages

3) Security:-Fibre Optics are much more

difficult to tap informations specially

great advantage to banks and security

installations.They are much more

immune to electromagneticinterference from radio signals and

lightening.


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Advantages &

Disadvantages(contd) Advantages

4) Running costs:-Fibres consume less

electrical power compared to copper

cables.

Disadvantages

1) Price:-Inspite of the fact that raw

material for making optical fibres,sand


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Advantages

&Disadvantages(contd) Disadvantages

which is cheap and abundant,still optical

fibres are more expensive per metre thancopper.

2) special skills: Optical fibres cant be

joined together as easily as a copper cable

and requires additional training of

personnels and measurement equipments.


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Areas ofApplication 1) Telecommunication:-

They are now point to point link for telephone

substations .2) Local Area Networks (LAN's)

Multimode fibre is commonly used as the"backbone" to carry signals between the

hubs of LAN's from where copper coaxialcable takes the data to the desktop. Fibrelinks to the desktop, however, are alsocommon.


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Fibre Theory1) Total internal reflection: Reflection and

Refraction of Light

When light travelling in a transparent materialmeets the surface of another transparentmaterial two things happen

1) some of the light is reflected2 ) some of the light is transmitted into the

second transparent material.


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Fibre Theory(contd)The light which is transmitted usually changes

direction when it enters the second material.

This bending of light is called refraction andit depends upon the fact that light travels at

one speed in one material and at a different

speed in a different material. As a result

each material has its own Refractive Indexwhich will help us to calculate the amount of

bending that takes place.


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Fibre Theory(contd)Refractive index is defined as:-

n=c/v

where n= the refractive index

C= is the speed of light in a vacuum

V= the speed of light in the material


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Fibre Theory(contd)Two possible cases exist.

These are:-

1) where light goes from a materialwith a low refractive index to one witha high refractive index, OR

2) where light goes from a materialwith a high refractive index to one witha low refractive index.


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Fibre Theory(contd)Total Internal Reflection

In the second case above, U 2 is alwaysgreater than U 1 . So, as we increase U 1,eventually U 2 will reach 90o before U 1 does.

At this point where U 1 has reached a valuecalled the critical angle (U c ).

The transmitted ray now tries to travel in both

materials simultaneously .For variousreasons this is physically impossible sothere is no transmitted ray and all the light


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Fibre Theory(contd)energy is reflected. This is true for any

value ofU 1, the angle of incidence,

equal to or greater than U c Thisphenomenon is called Total Internal

Reflection (TIR).So 2 conditions are

necessary for TIR:-


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Fibre Theory(contd)1) The refractive index of the first

medium is greater than the refractive

index of the second one.

2 The angle of incidence, U1, is greater

than or equal to the critical angle, Uc

The phenomenon of TIR causes 100%reflection.


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Fibre Theory(contd) Structure of fibre

Optical fibres are very fine fibres of glass.

They consist of a glass core, roughly fifty

micrometres in diameter, surrounded by a

glass "optical cladding" giving an outside

diameter of about 120 micrometres. They

make use of TIR to confine light within the

core of the fibre.


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Fibre Theory(contd)The Structure of a Fibre

The core has a higher refractive index than

the cladding. Although the cladding does notcarry light, it is nevertheless an essential

part of the fibre. The cladding is not just a

mere covering. It keeps the value of the

critical angle constant throughout the wholelength of the fibre.


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Propagation of lightPropagation of light in the fibre

The angle U in the figure below is called the

Acceptance Angle. Any light entering thefibre at less than this angle will meet thecladding at an angle greater than U C . If lightmeets the inner surface of the cladding (thecore - cladding interface) at greater than orequal to U C then TIR occurs. So all theenergy in the ray of light is reflected backinto the core and none escapes into the


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Propagation of lightcladding. The ray then crosses to the other

side of the core and, because the fibre is

more or less straight , the ray will meet thecladding on the other side at an angle whichagain causes TIR. The ray is then reflectedback across the core again and the samething happens. In this way the light zig zags

its way along the fibre. This means that thelight will be transmitted to the end of thefibre.


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Propagation of lightIn reality the light which enters the fibre

is a focused beam, consisting of many

millions of "rays" behaving in a similarway. They all zig zag along the core ofthe fibre, crossing over each other,and filling up the core with light. A

pulse of light travelling along the coreof the fibre is really a bundle of theserays.


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Fibre TypesFibre Types

There are two main fibre types:

(1) Step index

(2) Graded index


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Fibre TypesStep Index Fibre:

Step index fibre is so called because the

refractive index of the fibre 'steps" up

as we move from the cladding to the

core of the fibre.


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Fibre Types.

Graded Index Fibre :-

Graded Index Fibre has a different core

structure from single mode and

multimode fibre .


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Fibre TypesIn a graded index fibre the value of the

refractive index changes from the

centre of the core onwards .


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Pulse Spreading & its

causePulse Spreading

Light waves in a limited frequency range can

be transmitted through optical fibre without

any distortion & when frequency of light

energy becomes more the pulses cannot be

transmitted through the fibre intact and this isdue to the phenomenon called

PulseSpreading.


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cause(contd)ause ofPulse Spreading

The cause of cause spreading is

dispersion. This means that some

components of the pulse of light travel at

different rates along the fibre. there are two

forms of dispersion.1. Chromatic dispersion

2. Modal dispersion


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cause(contd)hromatic Dispersion

Chromatic dispersion is the variation of

refractive index with the wavelength(or the frequency) of the light.

Modal dispersion

It has constant wavelength and sametravelling speed to reach other end at

same time.


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Dispersion of light through prism


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Manufacture(contd)

Fibre Manufacture

There are two main stages to the

manufacture of optical fibres. These

are:-

1) the making of the preform

2) the extrusion of the preform


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Manufacture(contd)

PreformManufacture

The most common method of making fibre

preforms is known as Modified ChemicalVapour Dispersion (MCVD). An outer glass

"bait tube" is heated by a traversing burner.

Through this tube a mixture of gases is

passed at a steady rate, which when heatedundergoes a chemical reaction.


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Fibre

Manufacture(contd)The gas mix contains compounds of silicon,

metal halides, oxygen and dopant materials

which will determine the refractive index ofthe glass of the core. The solid end productsof the reaction are deposited on the interiorof the bait tube as "soot". This soot willeventually form the core of the fibre while

the bait tube will form the cladding. Whenenough soot has been deposited the gasflow is stopped and the heat is turned up sothat the soot melts to form a sintered glass.


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Fibre

Manufacture(contd)Finally the tube is heated up enough to soften

the bait tube and the sintered glass so that

the whole tube collapses to form a solid rod.Extrusion of the Preform

The preform now has the same internal

structure as the fibre to be drawn. The

preform is held vertically and passed

through an oven which softens its end. This

end is now stretched to form a glass fibre.


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Fibre

Manufacture(contd)The interior of the fibre retains the same

refractive index structure as the preform

with the same relative dimensions. The fibrepasses through a device for monitoring itsdiameter so that the size of the fibre stayswithin predefined limits It then passesthrough a coater which coats it with a plastic

buffer. This part of the process is crucialsince the strength of the fibre depends onfreedom from any surface contamination.


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Fibre

Manufacture(contd)The fibre must therefore be coated

before any contamination such as

dust, etc, in the surrounding air canreach it. Finally the fibre is rolled on to

a drum for distribution or for further

work on it such as incorporation into afibre cable.


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THANK YOU

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Presentation on Fibre Optic Technology