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• FIBRE OPTIC COMMUNICATION SYSTEM-• APPLICATIONS • PHOTOELASTICITY, POLARIZATION OF LIGHT,

TYPES OF POLARIZATION OF LIGHT

PH 0101 UNIT-3 LECT - 8

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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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Countd.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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• Countd.• 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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Countd.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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Countd.• 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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Countd.• 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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Countd.• 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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Countd. 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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Countd.• 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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Countd.• 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.