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Optical F iber

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Optical F iber

Communication system with light as

the carrier and fiber as

communication medium

Propagation of light in atmosphere

impractical: water vapor, oxygen,particles.

Optical fiber is used, glass or plastic,

to contain and guide light waves

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History

1880 Alexander G. Bell, Photo phone,

transmit sound waves over beam of light 1930: TV image through uncoated fiber

cables.

Few years later image through a singleglass fiber

1951: Flexible fiberscope: Medicalapplications

1956:The term fiber optics used for thefirst time

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History Contd

1960: Laser invented

1967: New Communications medium:cladded fiber

1960s: Extremely lossy fiber: more than1000 dB /km

1970, Corning Glass Work NY, Fiber withloss of less than 2 dB/km

70s & 80s : High quality sources and

detectors Late 80s : Loss as low as 0.16 dB/km

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Optical F iber: Advantages

Capacity: much wider bandwidth (10GHz)

Crosstalk immunity

Safety: Fiber is nonmetalic Longer lasting (unproven)

Security: tapping is difficult

Economics: Fewer repeaters

http://www.tpub.com/neets/book24/index.htm

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Disadvantages

higher initial costin installation

Interfacing cost

Strength: Lowertensile strength

Remote electric

power

more expensive torepair/maintain

Tools: Specialized

and sophisticated

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Optical F iber L ink

Input

Signal

Coder or

Converter

Light

Source

Source-to-Fiber

Interface

Fiber-to-light

Interface

Light

DetectorAmplifier/Shaper

Decoder

Output

Fiber-optic Cable

Transmitter

Receiver

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Light source: LED or ILD (Injection LaserDiode): amount of light emitted is proportional to the drive

current

Sourceto-fiber-coupler (similar to a lens): A mechanical interface to couple the light

emitted by the source into the optical fiber

Light detector: PIN (p-type-intrinsic-n-type)

or APD (avalanche photo diode) bothconvert light energy into current

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F iber Types

Plastic core and cladding Glass core with plastic cladding PCS

(Plastic-Clad Silicon)

Glass core and glass cladding SCS:Silica-clad silica

Under research: non silicate: Zinc-chloride: 1000 time as efficient as glass

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Plastic F iber

used for short runHigher attenuation, but easy to install

Better withstand stress

Less expensive

60% less weight

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Types Of Optical F iber

Single-mode step-index Fiber

Multimode step-index Fiber

Multimode graded-index Fiber

n1 core

n2 cladding

no air

n2 cladding

n1 core

Variable

n

no air

Light

ray

Index porfile

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F iber Types

Multi-Mode:

supportshundreds paths

for light.

Single-Mode:supports a

single path for

light

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Single-mode step-index F iber

Advantages: Minimum dispersion: all rays take same path,

same time to travel down the cable. A pulse can

be reproduced at the receiver very accurately.

Less attenuation, can run over longer distance

without repeaters.

Larger bandwidth and higher information rate

Disadvantages:

Difficult to couple light in and out of the tiny core

Highly directive light source (laser) is required.

Interfacing modules are more expensive

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Multi Mode

Multimode step-index Fibers: inexpensive; easy to couple light into Fiber

result in higher signal distortion; lower TX

rateMultimode graded-index Fiber:

intermediate between the other two types of

Fibers

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Acceptance Cone & Numerical Aperture

n2 cladding

n2 cladding

n1 core

Acceptance

Cone

Acceptance angle, qc, is the maximum angle in which

external light rays may strike the air/Fiber interface

and still propagate down the Fiber with

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Absorption Losses I n Optic Fiber

Loss(dB/km

)

1

00.7 0.8

Wavelength (mm)0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7

2

3

4

5

6

Peaks caused

by OH- ionsInfrared

absorption

Rayleigh scattering

& ultraviolet

absorption

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Presented By The Fiber Optic Association

2004, The Fiber Optic Association, Inc.

Light Used In Fiber Optics

Fiber optic systems transmit using

infrared light, invisible to the human

eye, because it goes further in theoptical fiber at those wavelengths.