Optical Fibre Lecture Ips-1

8/13/2019 Optical Fibre Lecture Ips-1

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Fiber Optics


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Unit-V

Fiber OpticsAn optical fiberis a flexible, transparent fiber made of verypure glass (silica) not much wider than a human hair thatacts as a waveguide, or "light pipe", to transmit lightbetween the two ends of the fiber. The field of appliedscience and engineering concerned with the design andapplication of optical fibers is known as fiber optics.


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Optical Fiber

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

moistureJacket

Holds one or more fibers in a cable

Dimensions of fiber:Core: 8 m diameter

Cladding: 125 m dia.

Buffer: 250 m dia.

Jacket: 400 m dia.


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As mans need and hunger for communication increased, the

amount of bandwidth required increased exponentially.

Then came the telephone and telegraph that used copper

wires for communication. But soon demand out striped 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 demandstill much more.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 isincreasing day by day as supporting

electronics is developing. In itself the

capacity of Optical Fibers is limitless. In

optical fibres the signal travels with the speed

of light and in copper cable with the speed of

electron.

Why Optical Fibers ?


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1. VERY HIGH INFORMATION CARRING

CAPACITY.

2. LESS ATTENUATION (order of 0.2 db/km)

3. SMALL IN DIAMETER AND SIZE & LIGHT

WEIGHT

4. LOW COST AS COMPARED TO COPPER (as

glass is made from sand..the raw material used to

make OF is free.)5. GREATER SAFETY AND IMMUNE TO

EMI & RFI, MOISTURE & COROSSION

6. FLEXIBLE AND EASY TO INSTALL IN TIGHT

CONDUICTS

7. ZERO RESALE VALUE (so theft is less)8. IS DILECTRIC IN NATURE SO CAN BE LAID

IN ELECTICALLY SENSITIVE SURROUNDINGS

9. DIFFICULT TO TAP FIBERS, SO SECURE

10. NO CROSS TALK AND DISTURBANCES

ADVANTAGES OF OPTICAL FIBERS


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APPLICATIONS OF OPTICAL FIBERS

1. LONG DISTANCE COMMUNICATION BACKBONES

2. VIDEO TRANSMISSION

3. BROADBAND SERVICES

4. COMPUTER DATA COMMUNICATION (LAN, WAN etc..)

5. HIGHT EMI AREAS

6. MILITARY APPLICATION

7. NON-COMMUNICATION APPLICATIONS (sensors etc)


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Propagation of light in the optical fiber

The field of fiber optics depends upon the total internal

reflection of light rays traveling through tiny optical fibers.The fibers are so small that once the light is introducedinto the fiber with an angle within the confines of thenumerical aperture of the fiber, it will continue to reflectalmost losslessly off the walls of the fiber and thus can

travel long distances in the fiber.


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Propagation of light in the optical fiber: -Total internal reflection


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Numerical Aperture and Acceptance angleLight is injected into the core at an incidence angle greater than the

critical angle, striking the core-cladding interface. From Snells lawwhen the angle of incidence is greater than critical angle, light is

totally reflected. Any light striking the interface at less than criticalangle i.e. not within a region called the acceptance cone, will beabsorbed or lost into the cladding. Thus there is a maximumvalue of angle of incidence(max) on the core for which the internalray strikes at the critical angle (c) and still guided into opticalwave guide (core) is called the acceptance angle(a ) .Theacceptance angle is defined as half the angle of cone within which

the light is totally reflected by the fiber core.The sine of the acceptance is the Numerical Aperture (NA).

Numerical Aperture represents the light gathering ability of anoptical fiber


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Air: n

Acceptanceangle


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.From snells law, air-core

interface

From snells law, core-cladding interface


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This has the same form as the numerical aperture in other optical

systems, so it has become common to definethe NA of any type of

fiber to be

where n1 is the refractive index along the central axis of the fiber(core). n2

is the refractive index of cladding . n is the refractive index of air.


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TYPES OF FIBER1. Step-Index Single-Mode fiber

2. Step-Index Multimode fiber

3. Graded-Index Multimode fiber

Step-Index Single-Mode fiber

Single-Mode fiber has a small diameter core that allows only one

mode i.e. wavelength of light to propagate. Because of this,the number of light reflections created as the light passes

through the core decreases, lowering attenuation and creatingthe ability for the signal to travel faster, further. Thisapplication is typically used in long distance, higher bandwidth


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Step-Index Multimode fiber

Multimode fiber optic cable has a large diameter core that allowsmultiple modes of light to propagate. Because of this, thenumber of light reflections created as the light passes throughthe core increases, creating the ability for more data to passthrough at a given time. Because of the high dispersion andattenuation rate with this type of fiber, the quality of the signalis reduced over long distances. This application is typicallyused for short distance, data and audio/video applications in

LANs. RF broadband signals. This means that the core tocladding diameter ratio is 50 microns to 125 microns and 62.5microns to 125 microns.


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Graded-Index Multimode fiberContains a core in which the refractive index diminishes

gradually from the center axis out toward the cladding.

Due to the graded index, light in the core curves helicallyrather than zigzag off the cladding, reducing its traveldistance. The shortened path and the higher speed allowlight at the periphery to arrive at a receiver at about thesame time as the slow but straight rays in the core axis.

The result: digital pulse suffers less dispersion. This typeof fiber is best suited for local-area networks.


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Parameter Single-ModeStep-Indexfiber

MultimodeGraded-Indexfiber

MultimodeStep-Indexfiber

Source Require laser Laser or LED Laser or LED

Bandwidth >3GHz 200MHz to 3GHz < 200MHz

Splicing Very difficult to

small core

difficult doable

Application

Core diameter

Submarine cable

system

2 to 9 m

Telephone b/w

trunk offices

Graded index

125m

Data links

50 to 70 m

Cost Less expensive Most expensive Least expensive

Comparison


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BENDING LOSS

Optical fiber suffer bending losses at bends or curves on there path on

there path. This is due to the evanescent field at the bend exceedingthe velocity of light at the cladding and hence the guidancemechanism is inhibited, which causes light energy to be radiated fromthe fiber.

There is type of bending loss1. micro bending loss.

2. macro bending loss.

Optical Fibre Lecture Ips-1

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