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8/3/2019 Optical Fibers Module III
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Fiber Optics
Module-III
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Optical fiber is a long thin strand of transparent dielectric
material (glass or plastic) about the diameter of a human hair.
Optical fiber carries electromagnetic waves of visible and
infrared frequencies from one end to the other end of the fiber
by means of Total Internal Reflection.
Optical fibers are arranged in bundles called optical cables thatcan transmit large amounts of information at the speed of light.
Introduction: Optical fiber
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Structure of an Optical fiberPoly urethane protective jacket
Cladding
Core
plastic jacketglass or plasticcladdingfiber core
Polyurethane is a uniquematerial that offers elasticity ofrubber combined with toughnessand durability of metal
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1) Core: It is an inner cylindrical material made up ofglass or plastic.2) Cladding: It is a cylindrical shell of glass or plastic material in whichCore is inserted.
3) Protective Jacket: The Cladding is enclosed in polyurethane jacketand it protects the fiber from surroundings.
NOTE: The refractive index of core is slightly greater than therefractive index of Cladding. The normal standard values are 1.48 and
1.46 respectively.
Optical fiber consists of three sections
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Fiber Optic Cables
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How Does Optical Fiber Transmit Light??
Principle: Optical fiber works on the principle of Total internal reflection.Once light ray enters into core, it propagates by means of multiple total
internalreflections at core-cladding interface.
The light in a fiber-optic cable travels through the core
(hallway) by constantly bouncing from the cladding (mirror
lined walls), a principle called total internal reflection.
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Because the cladding does not absorb any lightfrom the core, the light wave can travel greatdistances.
However, some of the light signal degradeswithin the fiber, mostly due to impurities in theglass. The extent that the signal degrades
depends on the purity of the glass and thewavelength of the transmitted light
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Total Internal Reflection
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;sin
90sinsin
90
sinsin
1
2
0
1
2
0
21
n
n
n
n
refractionofangleis
incidenceofangleiswhere
nn
c
c
rc
r
i
ri
Critical Angle
The critical angleis the angle of incidence abovewhich totalinternal reflection occurs
From Snells law of refraction the angles of incidence and
refraction are related to each other & to the refractive indices of
the mediums as :
1
2sin
n
nArcc
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Transmission of a light ray in a perfect
optical fiber
Any discontinuities or imperfections at the corecladding interface would probablyresult in refraction rather than total internal reflection, with the subsequent loss ofthe light ray into the cladding.
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Core-Cladding interface
Fiber axis
Core n1
Cladding n2
A
B
C
r
i
r
Incident light ray n1 >n2
Acceptance angle
The maximum angle of incidence at the end face of anOptical fiber for which the light ray can be propagatedalong Core-Cladding interface is known as Acceptanceangle. (The Max value of for which the internal rays
will strikes at critical angle) It is also called Acceptance cone half angle.
i
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)2.........(cossin
cossin
)90sin(sin
90
90
)1....(..........sinsin
0
1
10
0
10
0
0
10
n
n
nn
nn
ABCtriangleanglerightthefromnn
i
i
i
r
r
ri
Note: n0 is the refractive index of the medium from which the light
ray enters the fiber
Applying Snell s law of refraction at the point of entry of the ray into the core
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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
max
n
nn
n
n
n
n
nn
ri
rnin
refractionoflawtoaccording
n
n
anglecriticalwhen
c
cc
c
c
c
cm
ic
For core-claddinginterface
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2
2
2
1
1
max
2
2
2
1max
0
1
2
2
2
1
0
1max
sin
sin
1,
sin
)3()4(
nn
nn
nthenairisfiberthegsurroundinmediumtheif
nnn
nn
inequationsubstitute
Which is the required expression for Acceptance Angle in
optical fibers.
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Acceptance cone
Rotating the Acceptance angle about the fiber axis describesthe Acceptance Cone of the fiber.
Light launched at the fiber end within this Acceptance Cone
alone will be accepted and propagated to the other end of thefiber by total internal reflection.
max
max
Acceptance Cone
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Numerical Aperture
The light gathering capacity of anoptical fiber is known as NumericalAperture and it is proportional toAcceptance Angle.
It is numerically equal to sine ofAcceptance 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 refractive indices of Core andCladding to that of core is called thefractional change .