oc6thsem,of characteristics

8/9/2019 oc6thsem,Of Characteristics

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CHARACTERISTICS OF

OPTICAL FIBER


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CONTENTS OF THE TALK

NUMERICAL APERTURE

DISPERSION

MODAL DISPERSION

MATERIAL DISPERSIONWAVEGUIDE DISPERSION

ATTENUATION

SOURCES OF LOSS IN FIBER

ABSORPTION

SCATTERINGGEOMATRIC EFFECT


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NUMERICAL APERTURE

IT IS DEFINED AS THE LIGHT GATHERING

CAPABILITY OF THE FIBER

OR

IT IS DEFINED AS THE MAXIMUM ANGLE OVER

WHICH IF LIGHT INSERTED INSIDE THE

FIBER WILL TRAVEL THROUGH THE FIBER

WITH OUT LEAKING


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n

n1

n2

x

y z

w

REFRACTION IN THE FIBER

CORE

CLADDING


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CRITICAL ANGLE CRITERIA

n

n1

n2

x

y z

90

CORE

CLADDING


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TOTAL INTERNAL REFLECTION

n

n1

n2

x

y z w CORE

CLADDING


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NUMERICAL APERTURE

Cladding

Core

NA= Sin Typical = 100

Input Surface Refraction


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NUMERICAL APERTURE

B

Acceptance

Cone

Eventually Lost

By Radiation

A


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TYPICAL NUMERICAL APERTURES

TYPE n1 n2 NA ANGLE

ALL

GLASS

PCS

ALL

PLASTIC

1.48 1.46 0.24 13.9

1.46 1.40 0.41 24.2

1.49 1.41 0.48 29.0


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NUMERICAL APERTURE

Single Mode Step Index

Multi Mode Graded Index

Output

Pulse

InputPulse

Multi Mode Step Index


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Dispersion is a Phenomenon which spreads a lightpulse as it travels down the length of an optical fibre.

Dispersion is undesirable as it limits the band width or

information carrying capacity of a fibre.The bit rate must be low enough to ensure that pulsesdo not overlap.

The pulse broadcasting increases as the square root ofthe distance of transmission.

There are three classes of dispersion. Modal or Inter Modal Dispersion

MaterialDispersion

Wave Guide Dispersion

DISPERSION


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TWO IMPORTANT CHARACTERISTICS OF

FIBER OPTIC MEDIUM

Attenuation expressed

Decibels

Loss(db)=-10 1oj (Po/P1)

3db = 50% power loss

Attenuation

medium

DispersionDispersion is expressed

In ps,(nm.k.1)

Both limit transmission length

Dispersion limits bandwidth (information carrying capacity),

more so for analog transmission.


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WHY IS DISPERSION UNDESIRABLE ?

101101

101001

101001

111111time 101001

0km

25km

50km

Threshold

Poter

Input signal

International (or modal)Only in multimode

Intermodal or chromatic

material

Waveguide


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MODAL DISPERSION

MULTIMODE FIBER SUFFERS FROM

THIS TYPE OF SPREADING AS THE

AXIAL SPEED VARIES FOR EACH SIGNAL

SINGLE MODE FIBERS DOES NOT SUFFERS

FROM THIS PROBLEM OTHER SOLUTION IS

GRADATION OF THE INDEX .


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MODALDISPERSION

LP01

LP11

LP01

LP11

LP01 LP11

Time

Only in multimode fibres

Remedy for this distortion is graded index profile

Graded index

LP11

LP01


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It is caused due to the variation of velocity with wave

lengths.

The velocity variation caused by some property of the

material gives rise to the effect which is called MaterialDispersion.

we know that:

n=c/v

Where c=speed of light in a vacuum.v= speed of the same wave length in the material.

MATERIALDISPERSION


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Fiber

0.5

1.0

P2P1 P0 P

Light

source

spectrum

Logic 1

P1

P0

P2

P1

P0

P2


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0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5

240

200

160

120

80

40

0

-0

Material Dispersion Vs. Wavelength

Wavelength(Qm)


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ATTENUATION:

Signal attenuation is a major factor in the design of any

communication system.

In an Optic System, the loss of power takes place at several

points ie coupler, splices and connectors and within the

fibre itself.

WAVE GUIDE DISPERSION

It result from the guiding structure and is important in S.M.

Fibre. It occurs because guided optical energy is divided

between the core and the cladding. The energy travels at

slightly different velocities in the core and cladding because

of their refractive indices.


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Wave guide Dispersion

MFD > core radius, MFD increases with P

Field distribution of LP01 mode is Gussian

~ 30-50% of the fundamental mode power

could be in the cladding

n1 (cladding) is lower, and thus v is higher

MFD

2a

l1 light in cladding

l2 light in the core


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ATTENUATION IN THE FIBER


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SOURCES OF LOSS IN FIBERS:

INTRINSIC ABSORPTION

ABSORPTION ABSORPTION DUE TO IMPURITIES

SCATTERING LOSS OF OPTICAL ENERGY DUE TO

IMPERFECTIONS IN THE FIBRE & FROM

THE BASIS STRUCTURE OF THE FIBRE

3. GEOMETRIC EFFECTS: BENDING A FIBRE CAUSES

ATTENUATION.

TWO TYPES OF BENDS ARE:i) MICROBENDING

ii) MACROBENDING


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Absorption

0.5

Absorption is intrinsic glass absorbs light

Strong absorption in ultraviolet region

Strong absorption in infrared region

Diminishes in invisible region

Example: Germanium doped silica glass fiber.

0.5

1.5

U.V.I.R.

1.0

Loss

Glass absorption in ultra-violet and infra-red regions


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Attenuation

1.0 1.5

102

101

db/km

0.94

1.24

1.38

Wavelength


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MICROBEND

MICROBEND

MACROBEND

GEOMETRIC EFFECTS

Radiation due to Micro bending and Macro bendingRadiation due to Micro bending and Macro bending

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