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OPTICAL FIBER

CABLE CHARACTERSTICS

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CONTENTS

OPTICAL PRINCIPLE (TOTAL INTERNAL

REFLECTION)

FIBER CLASSIFICATION TRANSMISSION CHALLENGES

ATTENUATION

DISPERSION CHROMATIC DISPERSION

POLIRIZATION MODE DISPERSION

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Optical communication

+

TRANSMITTER FIBRE

+

RECEIVER

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Ray Theory:

1. In a vacuum, rays travel at a velocity of c =3x108m/s.In any other medium, rays travel at a slower speed,

given byv = c/n n =refractive index of the medium.

2. Rays travel straight paths, unless deflected by somechange in medium.

3. If any power crosses the boundary, the transmittedray direction is given by Snells law:

n1 sin i = n2 sin r

Optical Principle

(Internal reflection theory)

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INCIDENT RAYS 1REFLECTED RAYS

REFRACTED RAYS

1

1

3

2

2

3

N2 cladding

r

i

(principal of total internal reflection)

n1 = 1.48

n2 = 1.46

N1 core

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The Optical Fibre

Cladding

125 mCore8-10 m

Refractive index

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32

1

3

2

1

Light propagation in fibre

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Basic Fibre

core with RI n1supported byconcentriccladding layer with

RI n2. RI of core is

greater thancladding (n1 > n2).

The cladding layeris surrounded by

one or moreprotective coating.

Change in RI isachieved byselectively dopingthe glass perform.

CONSTRUCTION OF OPTICAL

FIBRE CABLE

CORE

CLADDING

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Cabling is to protect the fiber during transportation, installation &operation.

Cabling protects the optical fibres from mechanical damage andenvironmental degradation.

Resembles conventional metal cables externally. There are a variety of cable design available and irrespective of their

design fibre optic cables have the following parts in common :

Buffer : to protect fibre from outside stress; materials used - nylon,mylar or plastic.

Strength member ; to reduce stress due to pulling, shearing, andbending; materials used-textile fiibres (kevlar), or steel.

Cable filling compound: to prevent moisture intrusion and migration inthe cable.

Cable jacket : to protect the fibre against cut and abrasion; materialused-polyethylene polyurethane, polyvinyl chloride or teflon.

Cabling of fibre :

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Standard Single-Mode (SM)

FibreFibre coreSiO2+ GeO2 10 m

n 1.443

Fibre coreSiO2+ GeO2

10 m

n 1.443

SiO2 Cladding

125 m

n 1.44

SiO2 Cladding

125 m

n 1.44Primary coating (soft)

400 m

Primary coating (soft)

400

m

Secondary coating (hard)

1 mm

Secondary coating (hard)

1 mm

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Material Classification

Al fused-silica-glass fiber:

Plastic-clad-silica (PCS) fiber:

All-plastic fiber : Modal classification :

propagation : - single-mode (SM) fiber.

- Multi-mode (MM) fiber.

Classification based on refractive index profile : step index (SI)

Graded index (GRIN) fiber.

CLASSIFICATION OF OPTICAL

FIBRE

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2a

2a

2a 8 - 12 m 125 m

50 - 200m 125-400m

50 m 125-400m

C) Multi mode GRIN fiber

b) Multi mode step-index fiber

a) Single mode step-index fiber

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refractive

index

SM

Single-Mode

SM

Single-Mode

Fibre types

MM-SI

Multi-Mode

Step Index

MM-SI

Multi-Mode

Step Index

MM-GI

Multi-Mode

Graded Index

MM-GI

Multi-Mode

Graded Index

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TRANSMISSION CHALLENGES

Attenuation

Dispersion

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Fibre performance

z=0 z=L

Dispersion

z=0 z=L

Attenuation

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FIRST

WINDOW

SECOND WINDOW

THIRD WINDOW

3.0

2.5

2.0

1.5

1.0

0.5

0800 900 1000 1100 1200 1300 1400 1500 1600 1700

TOTAL LOSS

RAYLEIGHSCATTERING

Attenuation Curve

Wave Length (Lemda) in nm

Loss

dB

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Fibre attenuation (SiO2)

1.80.8 1.0 1.2 1.4 1.60.9 1.1 1.3 1.5 1.7

Wavelength (m)

Attenuation

(dB/km)

0.2

0.5

1.0

1.5

0.16 dB/km

Rayleigh

scattering

IR band edge

OH--peak

UV

absorption

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Maximum transmission span

1

10

100

1000

1 10 100 1000 10000

Attenuation

Dispersion

BL = constant

L(km)

Bitrate B (Mb/s)

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Transmission Loss

The transmission loss or attenuation

As light waves travel down an optical fibre,

they lose part of their energy because of

various imperfections in the fibre. These

losses are measured in decibels perkilometers (dB/km).

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Losses associated within the fiber classified as under:

absorption. Even the purest glass will absorb heavilywithin specific wavelength regions. Other majorsource of loss is impurities like, metal ions and OH

ions. scattering: caused due to localized variations in

density, called Rayleigh scattering and the loss is:

geometric

micro-bending.macro-bending.

LOSSES IN FIBER (Attenuation)

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Dispersion is spreading of the optical

pulse as it travels down the length.

Dispersion limits the information carryingcapacity of fibre

DISPERSION IN FIBER

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Types of dispersion

Intermodal dispersion only for MMF

Material Dispersion Chromatic Waveguide Dispersion Dispersion

Polarisation Mode Dispersion (PMD)

Intermodal dispersion only for MMF

Material Dispersion Chromatic Waveguide Dispersion Dispersion

Polarisation Mode Dispersion (PMD)

} {

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DISPERSION DUE TO

Material Dispersion:- Due to materialimpurities

Waveguide Dispersion:- Due toConstruction difference of the wave guide,core diameter not uniform throughout length.

Modal Dispersion:- pulse spreading caused by various modes.

Various modes of light source.

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Summary

1. Generally long distance network SM mode G-652 is used.

2. It can be used for 1550-nm window, by dispersion

compensators.

3. Non dispersion shifted fiber can also support 10 Gigabit

Ethernet standard at distances over 300 meters.4. Non-zero dispersion-shifted fiber-good for both TDM and

DWDM use in the 1550-nm region.

5. PMD and other nonlinear effects are not so critical for

short-haul but they are in long-haul systems with higherspeeds.

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Optical Communication SystemsFirst Generation, ~1975, 0.8 mMM-fibre, GaAs-laser or LED

Second Generation, ~1980, 1.3 m, MM & SM-fibre

InGaAsP FP-laser or LED

Third Generation, ~1985, 1.55 m, SM-fibre

InGaAsP DFB-laser, ~ 1990 Optical amplifiers

Fourth Generation, 1996, 1.55 m

WDM-systems

1.80.8 1.0 1.2 1.4 1.60.9 1.1 1.3 1.5 1.7Wavelength (m)

Att

enua

tion

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5

4

3

2

1

0 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7

~ 190 THz~ 50 THz

OH- OH-

First window

Second window

Third window

Fourth window

Fifth window

Wavelength (m)