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PH 0101 UNIT-3 LECT - 8
• FIBRE OPTIC COMMUNICATION SYSTEM• APPLICATIONS
• PHOTOELASTICITY, POLARIZATION OF
TYPES OF POLARIZATION OF LIGHT
UNIT III LECTURE 8
LIGHT,
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FIBER OPTIC COMMUNICATION SYSTEM :
Introduction :
In the early stages of development, fiber
communication promised extremely high data rates,
which would allow large masses of data to be
transmitted quickly.
It also had the potential for transmission over long
distances without the need to amplify and retransmit
along the way.
Recent developments have exceeded the hope of
those involved in the technology.
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BASIC MODEL :
The bandwidth of the fiber optic communication
system, which determines the maximum data
rate, depends on the major components of the
system.
Fig. shows the block diagram of fiber optic
communication system.
The information signal to be transmitted may be
voice, video or computer data.
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The first step is to convert the information into a
form compatible with the communications medium.
This is usually done by converting continuous
analog signals such as voice and video (TV)
signals into a series of digital pulses.
An Analog – to – Digital (A/D) converter is used for
this purpose. Computer data is already in the
digital form.
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• These digital pulses are then used to flash a
powerful light source (i.e.) off and on very
rapidly.
• In a simple low – cost system that transmits
over short distances, the light source is usually a
light emitting diode (LED).
• This is a semiconductor device that puts out a
low – intensity red light beam. Other colours are
also used.
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Infrared beams like those used in TV remote
controls are also used in transmission.
Another commonly used light source is the solid
state laser.
This is also a semiconductor device that generates
an extremely intense single frequency light beam.
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FIBER OPTIC COMMUNICATION SYSTEM
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• The light beam pulses are then fed into a fiber –
optic cable where they are transmitted over long
distances.
• At the receiving end, a light sensitive device
known as a photocell or light detector is used to
detect the light pulses.
• This photocell or photo detector converts the light
pulses into an electrical signal.
• The electrical pulses are amplified and reshaped
back into digital form.
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• Both the light sources at the sending end and the light
detectors on the receiving end must be capable of
operating at the same data rate.
• The circuitry that drives the light source and the circuitry
that amplifies and processes the detected light must both
have suitable high-frequency response.
• The fiber itself must not distort the high-speed light
pulses used in the data transmission.
• They are fed to a decoder, such as a Digital – to –
Analog converter (D/A), where the original voice or video
is recovered.
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• In very long transmission systems, repeater units must be
used along the way.
• Since the light is greatly attenuated when it travels over long
distances, at some point it may be too weak to be received
reliably.
• To overcome this problem, special relay stations are used to
pick up light beam, convert it back into electrical pulses that
are amplified and then retransmit the pulses on another
beam.
• Several stages of repeaters may be needed over very long
distances.
• But despite the attenuation problem, the loss is less than the
loss that occurs with the electric cables.
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APPLICATIONS OF FIBERS IN TELECOMMUNICATION
• The various applications of fiber optics in the
telecommunication area in voice telephones,
video phones, telegraph services, message
services and data networks all transmitted over
common carrier links.
• The conventional problems of wire systems like
those of ringing, cross talk, electromagnetic
interference and induced errors, etc., are
completely avoided with the use of optical fiber
communication methods.
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• Coaxial undersea cable systems have been used as one
of the major transmission systems in international
telecommunication networks over the past 25 years.
• Its channel capacity has rapidly increased about ten times
per decade with the growth in overseas traffic.
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SPACE APPLICATIONS OF OPTICAL FIBERS
Optical fibers offers the following significant
advantages for space environment, namely
high bandwidth, noise immunity, inherent
radiation hardness, reduced weight, low bit
error rate, size, weight and volume reduction.
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Broad-band applications of optical fibers :
Optical fibers offer many new opportunities to
system planners interested in broadband video
and other services.
In the private customer application, a
coalescence of the existing community antenna
television
system
(CATV)
and
telecommunications services seems likely, with
the development of wide band switched
integrated networks.
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Primarily providing educational and entertainment TV,
but with a capability to provide many other services
also.
In the business area, highly versatile systems
designed to carry combinations of video wide band
data and audio of varying qualities and with a
sufficient range capability to span much of a city from
a central switching point seem likely to encourage the
use of teleconferencing and related services.
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Applications in information technology :
A modern large computer system is composed of
a large number of interconnections ranging in
length over 16 orders of magnitude from the
micrometer dimensions of the on chip very large
scale integration (VLSI) connections to
thousands of kilometer for terrestrial links in
computer networks.
The transmission line features of fiber optics are
potentially attractive for many of these computer
connections.
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Important advantages of fiber optic communication :
Transmission loss is low.
Fiber is lighter and less bulky than equivalent copper
cable.
More information can be carried by each fiber than by
equivalent copper cables.
There is complete electrical isolation between the sender
and the receiver.
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• There is no interference in the transmission of
light from electrical disturbances or electrical
noise.
• The fiber itself can withstand environmental
conditions such as salt, pollution and radiation
with no resulting corrosion and minimal nuclear
radiation effects, so it is more reliable.
• The transmission is more secure and private
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Other Applications of optical fibers :
• Optical fibers can be used as sensors for the
measurement mechanical force, pressure, electric
field, electric current, magnetic field, temperature,
nuclear radiations, density etc.
• In computers, fibers are used to exchange the
information between different terminals in a
network.
• The optical fibers are used in industrial automation,
security alarm system and process control.
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• The fiber optic cables are widely used in electronic
fields to produce required delay.
• It is possible to study interior of the lungs and other
parts of the body that can not be viewed directly
(endoscopy).
• The fiber optical system widely used in defence
services because high privacy is maintained.
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PHOTOELASTICITY :
Photoelasticity is the change in optical
properties of a transparent material when
it is subject to mechanical stress.
An example of such properties is
birefringence.
The mechanical birefringence of certain
materials enables the determination of
stress and strains from the interference
fringe patterns they produce.
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