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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