optical fibers module iii

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 .

optical fibers module iii

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