snist 1-2 UNIT-5b Fiber Optics

8/12/2019 snist 1-2 UNIT-5b Fiber Optics

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Unit-VB

Basics of Fiber Optics

Introduction, Principle of Optical Fiber, Acceptance

Angle and Acceptance Cone, Numerical Aperture,

Types of Optical fibers. Attenuation in Optical Fibers,

Application of Optical Fiber in communication system


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IntroductionOptical fiber is a long thin transparent dielectric materialwhich carries EM waves of visible and IR frequenciesfrom one end to the other end of the fiber by means ofTIR.NOTE: Glass or Plastic is used as Dielectric material.Optical fibers works as Wave guides in optical televisionsignals digital data to transmit voice television signalsdigital data to any desired distance from one end to theother end of the fiber.


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Fiber Optics is a technology related totransportation of optical energy(light energy) inguiding media specifically glass fibers

It has become popular because of the following reasons:

1). Higher information carrying capacity.

2). Light in weight, small in size, low cost.

3). No hazards of short circuits and can safely used in

explosive environments.

4). No possibility of internal noise and cross talk

generation.

5). Using an optical fiber, 15,000 independent speeches can

be sent simultaneously where as using a pair of copper wires,

only 48 independent speech signals can be sent.


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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 ofglass or plastic.Cladding: It is a cylindrical shell of glass or plastic material in whichCore is inserted.

Protective Jacket: The Cladding is enclosed in polyurethanejacket andit protects the fiber from surroundings.

NOTE: The RI of core is slightly greater than the RI of Cladding. Thenormal standard values are 1.48 and 1.46 respectively.


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Structure of an Optical fiberPoly 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

DENSAR MEDIUM

RERAR MEDIUM

NORMAL

NORMAL

rr = 90


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c

NORMAL

RERAR 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

r

i

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 ConeRotating the Acceptance angle about the fiber axis

describes the 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 the fiber by total internal reflection.

m

m

Acceptance Cone


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TYPES OF OPTICAL FIBRESOn the basis of variation of RI of core, the optical fibers

are mainly classified into following types. i.e.,

1.Step Index fiber 2.Gradex Index fiberNOTE: Based on Mode of propagation, the fibers arefurther divided into Single Mode and 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 step change at core-cladding interface .

The core diameter of this fiber is about 8 to 10m and the outer diameter

of cladding is 60 to 70m.

There is only one path for light ray propagation. Hence it is called singlemode step index fiber.

It is a reflective fiber since light is transmitted from one end to the other

end of a fiber by TIR.

These are extensively used because distortion and transmission losses are

very less.


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Refractive index profile of

single 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 FiberThe construction of this fiber is similar to Single mode stepindex fiber but dimensions of Core and Cladding are muchlarger to have more number of paths for light propagation.The Core diameter varies from 50 to 200m and theCladding diameter varies from 100 to 250m.It is also a reflective fiber since light is propagated in theform 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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Radial distnce

100 to 250 m

50 to 200 m

RI

REFRACTIVE INDEX PROFILE OF SINGLE MODE GRADED INDEX FIBER


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Radial distance

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 in Core the light rayssuffers continuous refraction with in the Core from its centerto surface. Thus the propagation of light rays are not due to TIR butby refraction. Therefore it is called Refractive fiber. In this fiber the light rays travel at different speeds indifferent parts.Near the surface RI is least so the light rays travel fastercompared to the light rays near the axis. Because of this allthe rays almost arrive at the same time at the other end of thefiber.


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CLADDING

CORE

LIGHT PROPAGATION IN MUTI-MODEGRADED INDEX FIBRE


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RI of the core remains

constant and decreases a

step at cladding.

RI of the core decreases

parabolically from the axis

of the fiber to cladding.

It is of refractive type. It is refractive type.

Signal distortion is high in

multimode step index fiber.

Signal distortion is very

low in graded index fiber.

NA is more for multimode

step index fiber .

NA is less for grand index

fiber .

No focusing of light rays

takes place.

Focusing of light rays

takes place.

Difference b/w Step Index and Graded

Index Optical Fibers


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Optical fiber ommunication System An efficient optical fiber communication system

requires high information carrying capacitysuch as voice signals, video signals over longdistances 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 etcinto a binary data. This binary data comes out in the formof stream of electrical pulses.

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

NOTE: LED or diode laser is used as light source and itconverts electrical signals are infected into opticalsignals. These optical signals are injected into waveguide.


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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 Optical fibers The power of the light at the out put end is found to bealways less than the power launched at the input end. Attenuation is found to be a function of fiber materialwavelength of light and length of the fiber and it ismeasured in terms of the decibel.

Attenuation mainly three types1.Scattering losses2.Absorption losses3.Bending losses

S


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Bending Loss in Single Mode

Fiber

Mode Field distributions in straight

and bent fibers

Microbending Loss Sensitivity vs

wavelength

Bending loss for lowest order modes


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snist 1-2 UNIT-5b Fiber Optics

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