Download Why Optical Fibers

Unit – V
FIBRE OPTIC
Dr. Ritesh S. Palaspagar
Dept. of Engg. Physics
PRMIT&R
HISTORY REPEATS ITSELF
21ST CENTURY
TO
FROM ANCIENT GREEKS
Transmission medium and physical layer
Classes of transmission media
GUIDED MEDIA
Guided media, which are those that provide a conduit
from one device to another, include twisted-pair cable,
coaxial cable, and fiber-optic cable.
Topics discussed in this section:
Twisted-Pair Cable
Coaxial Cable
Fiber-Optic Cable
7.5
Evolution of Fiber
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1880 – Alexander Graham Bell
1930 – Patents on tubing
1950 – Patent for two-layer glass wave-guide
1960 – Laser first used as light source
1965 – High loss of light discovered
1970s – Refining of manufacturing process
1980s – OF technology becomes backbone of long
distance telephone networks in NA.
Why Optical Fibers ?
As mans need and hunger for communication increased, the amount of bandwidth
required increased exponentially.
Initially we used smoke signals, then horse riders for communicating. But these ways
were way to slow and had very little bandwidth or data caring capacity.
Then came the telephone and telegraph that used copper wires for communication.
But soon demand outstripped (included) the capacity and capability of copper wires
and data transport got added to voice communication. Then came Coaxial copper
cables, VHF and UHF Radios, Satellite but demand still outstripped the supply.
It was not until Optical Fibers came on the scene that large amount of
communication bandwidth became economically and easily available
to everyone.
As an example 50,000 voice / data circuit copper cable is massive in
size and very expensive, while a single Optical Fiber, the diameter of
human hair, can carry 5,00,000 circuits of voice and data. This
capacity is increasing day by day as supporting electronics is
developing. In itself the capacity of Optical Fibers is limitless.
INTRODUCTION
• Traditionally the communication was done with the help of
copper wires. now a days it has been replaced by fibre optic
cable.
• The difference between two system is that the fibre optics
system uses light pulses to transmit information instead of
using electronic pulses as used in copper wire system.
• Here a LED or ILD is used for generating the light pulses.
Using lense the light pulses are transmitted into fibre optics
system.
How are Optical Fibre’s made??
• Three Steps are Involved
-Making a Preform Glass Cylinder
-Drawing the Fibre’s from the preform
-Testing the Fibre
Testing of Optical Fiber
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Tensile Strength
Refractive Index Profile
Fiber Geometry
Information Carrying Capacity
Operating temperature/humidity range
Ability to conduct light under water
Attenuation
Wavelength of Transmitted Light
PRINCIPLE
• Light pulses move easily in fibre optic cable because of the
principle known as total internal reflection (T.I.R).
CONDITION FOR T.I.R
• Light should travel from denser medium to rarer medium.
• Angle of incidence is greater than the critical angle.
• Transparent glass or plastic fibre which allow light to be
transmitted from transmitter to receiver with minimal loss.
Fiber optics: Bending of light ray
TOTAL INTERNAL REFLECTION
CONSTRUCTION
• Core
– Glass or plastic with a higher index of
refraction than the cladding
– Carries the signal
• Cladding
– Glass or plastic with a lower index of
refraction than the core
• Buffer
– Protects the fiber from damage and
moisture
• Jacket
– Holds one or more fibers in a cable
CONSTRUCTION…
Fiber CONSTRUCTION…
Propagation modes
Modes
DESCRIPTION
STEP INDEX FIBRE
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IT IS AN OPTICAL FIBRE WHOSE CORE HAS A REFRACTIVE INDEX
WHICH IS SLIGHTLY GREATER THAN THAT OF CLADDING.
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BECAUSE OF REFRACTIVE INDEX OF THIS TYPE THE FIBRE MAKES A
STEP CHANGE A CORE CLADDING INTERFACE.
SINGLE MODE STEP INDEX
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DIAMETER VARIES FROM 8.3 - 10 MICRON.
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BANDWIDTH IS HIGHER THAN MULTIMODE.
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IT GIVES HIGHER TRANSMISSION RATE.
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NO OVERLAPPING,DISTORTION OF LIGHT PULSES OCCUR WITH
LEASTY ATTENUATION.
MULTIMODE STEP INDEX
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DIAMETER VARIES FRM 50 - 100 MICRON.
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MUCH LARGER PORT DIAMETER.
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EASY TO SPLICE AND COUPLE SERGMENTS TOGETHER WITH MINIMAL
LOSS.
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MULTIPATH DISPERSSION IS PRESENT.
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RAYS WITH SMALL ANGLE OF INCIDENCE REACH THE RECIEVER
AFTER THOSE RAYS WHICH ARE LARGER ANGLE OF INCIDENCE.
MULTI MODE (CONT.)
GRADED INDEX
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CONTAINS A CORE IN WHICH R.I GRADUALLY DECREASES FROM CORE
TO CLADDING.
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THE HIGHER R.I AT THE CORE MAKES THE LIGHT RAY MOVING DOWN
THE AXIS ADVANCE MORE SLOWLY THAN THOSE NEAR CLADDING.
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HERE LIGHT RAYS MOVE IN HELICAL PATH INSTEAD OF ZIGZAGGING
DUE TO GRADED INDEX , HENCE REDUCING TRAVELLING TIME.
Fiber Optic Specifications
• Attenuation
– Loss of signal, measured in dB
• Dispersion
– Blurring of a signal, affects bandwidth
• Bandwidth
– The number of bits per second that can be sent
through a data link
• Numerical Aperture
– Measures the largest angle of light that can be
accepted into the core
Attenuation
• Modern fiber material is very pure, but there is still some
attenuation
• The wavelengths used are chosen to avoid absorption bands
– 850 nm, 1300 nm, and 1550 nm
– Plastic fiber uses 660 nm LEDs
Optical fiber performance
Three Types of Dispersion
• Dispersion is the spreading out of a light pulse
as it travels through the fiber
• Three types:
– Modal Dispersion
– Chromatic Dispersion
– Polarization Mode Dispersion (PMD)
Modal Dispersion
• Modal Dispersion
– Spreading of a pulse because different modes
(paths) through the fiber take different times
– Only happens in multimode fiber
– Reduced, but not eliminated, with graded-index
fiber
Chromatic Dispersion
• Different wavelengths travel at different
speeds through the fiber
• This spreads a pulse in an effect named
chromatic dispersion
• Chromatic dispersion occurs in both
singlemode and multimode fiber
– Larger effect with LEDs than with lasers
– A far smaller effect than modal dispersion
Polarization Mode Dispersion
• Light with different polarization can travel at
different speeds, if the fiber is not perfectly
symmetric at the atomic level
• This could come from imperfect circular
geometry or stress on the cable, and there is
no easy way to correct it
• It can affect both singlemode and multimode
fiber.
Numerical Aperture
• If the core and cladding have almost the same index
of refraction, the numerical aperture will be small
• This means that light must be shooting right down
the center of the fiber to stay in the core
MERITS
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SPEED : FIBRE OPTIC NETWORK OPERATE AT HIGH SPEED UPTO
GIGABYTES.
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DISTANCE : FIBRE OPTIC CABLE CAN COMMUNICATE OVER A
LONG DISTANCE.
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COST : MUCH LESS AS COMPARED TO OTHER TRANSMITTING
MEDIA.
MERITS(CONT.)
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BAND WIDTH : FIBRE OPTIC CABLES HAVE MUCH GREATER
BANDWIDTH THAN METAL CABLES.
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LESS WEIGHT :IT IS LESS WEIGHER THAN COPPER WIRE.
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SECURITY :IMPOSSIBLE TO TAP INTO A FIBRE OPTICS CABLE, MAKING
IT MORE SECURE.
BECAUSE OF NO ELECTRICITY THROUGH OPTICAL FIBRE IT IS NON
FLAMABLE.
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DATA CAN BE TRANSMITTED DIGITALLY.
DEMERIT
• FIBRE OPTIC CABLE ARE EXPENSIVE TO INSTALL.
• THE TERMINATION OF FIBRE OPTIC CABLE IS COMPLEX.
• THEY ARE MORE FRAGILE THAN COAXIAL CABLE
APPLICATION
• TELE COMMUNICATION
• CABLE TV
• IT INDUSTRY (HIGH SPEED LAN WIRES)
• INTERNET
• INDUSTRIAL PLANT
CONCLUSION
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THE AGE OF OPTICAL COMMUNICATION IS A NEW ERA. IN SEVERAL
ASPECTS FIBREOPTICS COMMUNICATION IS BETTER THAN ELECTRIC
COMMUNICATION.
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WITH A BANDWIDTH AND INFORMATION CAPACITY A THOSAND
TIMES GREATER THAN COPPER WIRES, FIBRE OPTICS WILL SOON
PROVIDE US WITH ALL COMMUNICATION TECHNOLOGY AT A COST
EFFICIENT PRICE.
Thank You