presentation on fibre optic technology
TRANSCRIPT
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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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Pulse Spreading & its
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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Pulse Spreading & its
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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