Fiber Optics1

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FiberFiber

OpticsOptics


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Introduction

Optical fiber is a long thin transparentdielectric material which carries EMwaves of visible and IR frequenciesfrom one end to the other end of the

fiber by means ofTIR.

NOTE: Glass or Plastic is used asDielectric material.

Optical fibers works as Wave guides inoptical television signals, digital datato transmit voice television signals,

digital data to any desired distance


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Optical fiber consists of three

sections1. Core 2. Cladding 3. Protective

Jacket

Core: It is an inner cylindrical material made up ofglassor plastic.

Cladding: It is a cylindrical shell of glass or plasticmaterial in which Core is inserted.

Protective Jacket: The Cladding is enclosed inpolyurethane jacket and it protects the fiber from

surroundings.


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Structure of anOptical fiber

Poly urethane protective jacket

Cladding

Core

Principle:Optical fiber works on the principle of TIR. Once

light ray enters into core ,it propagates by means of multiple

TIR s at core-cladding interface.


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c=

c

NORMALRERAR MEDIUM

DENSAR MEDIUM


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1

2

0

1

2

0

21

sin

90sinsin

90

sinsin

nn

n

n

r

rnn

refractionoflawtoaccording

c

c

c

=

=

==

=


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Acceptance Angle

The maximum angle of incidence at the end face of

an Optical fiber for which the light ray can bepropagated along Core-Cladding interface is knownas maximum Acceptance angle. It is also called

Acceptance cone half angle.


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Core-Cladding interface

Fiber axis

Core n1

Cladding n2

A

B

C

ri

r

Incident light ray


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Applying Snell s law for Air-Core media

)2.........(cossin

cossin

)90sin(sin

90

90

)1....(..........sinsin

0

1

10

0

10

0

0

10

n

n

nn

nn

ABCtriangleanglerightthefrom

nn

i

i

i

r

r

ri

=

=

=

=

=+

=


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)4..(..........cos

)(1sin1cos

sin

90sinsin

90

sinsin

)3......(..........cossin

)(

1

2

2

2

1

2

1

22

1

2

0

1

2

0

21

0

1

n

nn

n

n

n

n

n

n

ri

rnin

refractionoflawtoaccording

n

n

anglecriticalwhen

c

cc

c

c

c

cm

mic

=

==

=

=

==

=

=

==


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2

2

2

1

1

max

2

2

2

1

0

1

22

21

0

1

sin

sin

1,

sin

)3()4(

nn

nn

nthenairisfiberthegsurroundinmediumtheif

n

nn

n

n

inequationsubstitute

m

m

=

=

=

=

Which is required expression for Maximum

Acceptance Angle in optical fibers.


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Acceptance Cone

Rotating the Acceptance angle about the

fiber axis describes the Acceptance Cone ofthe fiber.

Light launched at the fiber end within thisAcceptance Cone alone will be accepted and

propagated to the other end of the fiber bytotal internal reflection.

m

m

Acceptance Cone


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Numerical Aperture

The light gathering capacity of an

optical fiber is known as NumericalAperture and it is proportional toAcceptance Angle.

It is numerically equal to sine of

minimum Acceptance Angle.

=

=

+=

=

+=

=

=

=

2

2

)(

))((

sin

sin

1

2

1

21

211

1

21

2121

2

2

2

1

0

2

2

2

1

max

max

nNA

nNA

nn

nnnNA

n

nn

nnnnNA

nnNA

n

nn

NA

The ratio between the difference

in RIs of

Core and Cladding to that of RI of

core is called the fractional change.


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TYPES OF OPTICAL FIBRES

On the basis of variation of RI of core, theoptical fibers are mainly classified into

following types. i.e.,1.Step Index fiber 2.Gradex Index fiber

NOTE: Based on Mode of propagation, thefibers are further divided into Single Modeand Multi Mode.


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Single Mode Step Index fiber

The RI is constant for the core in this fiber. As we go

radically from center of the core, the RI undergoes a stepchange at core-cladding interface .

The core diameter of this fiber is about 8 to 10m and theouter diameter of cladding is 60 to 70m.

There is only one path for light ray propagation. Hence it iscalled single mode step index fiber.

It is a reflective fiber since light is transmitted from oneend to the other end of a fiber byTIR.

These are extensively used because distortion andtransmission losses are very less.


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Refractive index profile ofsingle mode step index fiber

Radial distance

60 to 70 m

8 to 10 m

RI


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SINGLE MODE STEP INDEX FIBER

CORE

CLADDING

RAY

PROPAGATION


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Multimode Step Index Fiber

The construction of this fiber is similar toSingle mode step index fiber butdimensions of Core and Cladding are

much larger to have more number ofpaths for light propagation.

The Core diameter varies from 50 to

200m and the Cladding diameter variesfrom 100 to 250m.

It is also a reflective fiber since light is

propagated in the form of multiple TIRS.


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Radial distance

100 to 250 m

50 to 200 m

RI

REFRACTIVE INDEX PROFILE OF MULTI MODE STEP INDEX FIBRE


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GRADED INDEX FIBRE

In this fiber , Radially the RI of Corecontinuously decreases from center to thesurface.

The RI is maximum at the center of Coreand Minimum at the Surface.

This fiber can be a single mode orMultimode ,the diameters of core andcladding varies from 50-200m and 100-250m respectively.


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Radialdistnce

60 to 70 m

8 to 10 m

RI

REFRACTIVE INDEX PROFILE OF SINGLE MODE GRADED INDEX FIBER


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Radialdistance

100 to 250 m

50 to 200 m

RI

REFRACTIVE INDEX PROFILE OF MULTIMODE GRADED INDEX FIBRE


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As RI changes continuously radially inCore, the light rays suffers continuous

refraction with in the Core from its centerto surface.

Thus the propagation of light rays are

not due to TIR but by refraction. Thereforeit is called Refractive fiber.

In this fiber, the light rays travel at

different speeds in different parts.

Near the surface RI is least so, the lightrays travel faster compared to the lightrays near the axis. Because of this all the


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CLADDING

CORE

LIGHT PROPAGATION IN MUTI-MODEGRADED INDEX FIBRE


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Optical fiber Communication System

An efficient optical fiber communication systemrequires high information carrying capacity suchas voice signals, video signals over long

distances with a minimum number of repeaters.It essentially consists of following parts.

1.Encoder 2. Transmitter 3.Wave guide

4.Receiver 5.Decoder


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1.Encoder: It converts electric signal corresponding toanalog information such as voice, figures, objects etc intoa binary data. This binary data corns out in the form ofstream of electrical pulses.

2.TRANSMITTER: It mainly consists of drive circuit and alight source. Drive circuit supplies the electric pulses tothe light source from the encoder.

NOTE: LED or diode laser is used as light source and it

converts electrical signals are infected into optical signals.These optical signals are injected into wave guide.


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Electrical

signal

ENCODER

DRIVE SOURCE LIGHT SOURCE

TRANSMITTER

OPTICAL SIGNAL

AMPLIFIER

PHOTO

DETECTOR

SIGNAL

RESTORER

DECODER

Wave guide

receiver


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Attenuation or Power loss in Opticalfibers

The power of the light at the out put end isfound to be always less than the power

launched at the input end.

Attenuation is found t be a function of fibermaterial, wavelength of light and length of

the fiber and it is measured in terms of thedecibel.

Attenuation mainly three types.

1.Scattering losses

2 Absorption losses

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