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Introduct ion to Opt ical Fiber

Transmiss ion

and

Character ist ics

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Optical Fiber is new medium, in which information

(voice, Data or Video) is transmitted through a glass in the

form of light.

Information is encoded into electrical signals.

Electrical signals are converted into light signals.

Light travels down the fibre.

A detector changes the light signals into electrical signals.

Electrical signals are decoded into information

Transmission Sequence Though Fiber:

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Optical Fibers are non conduc tive (Dielectr ic s)

Electrom agnetic Immun i ty

Large Bandw idth

Low Loss

Smal l, Ligh t weight and less cost cables

Avai lable in Long length s

Securi ty

Safety

No need of addi t ional equ ipment to p rotect against

surge vol tage

Advantages o f Fiber Opt ics

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Common carr ier nat ionwide netwo rks.

Telephone Inter-off ic e Trunk lines .

Customer prem ise communication netwo rks. Undersea cables.

High EMI areas (Power l ines , Railway, Roads).

Factory communication.

Contro l systems.

High l igh tening areas.

Mil itary app l icat ions .

Classi f ied (secure) commun ications .

Appl icat ion s of Fibers

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Total Internal Reflect ion- The Reflection that Occurs

when a Light Ray Travelling in One Material Hits a

Different Material and Reflects Back into the Original

Material without any Loss of Light.

Principle of Operation

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Light travelling from one material to another changes speed, which

results in light changing its direction of travel. This def lect ion o f l ight is

cal led Refract ion .

As the angle of incidence increases, the angle of refraction approaches

90o to the normal.

The angle of incidence that yields an angle of refraction of90ois the

cr i t ical angle.

If the angle of incidence increases amore than the critical angle, the lightis totally reflected back into the first material so that it does not enter the

second material.

The angle of incidence and reflection are equal and it is called Total

Internal Reflection.

THEORY AND PRINCIPLE OF FIBRE OPTICS

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1

Angle of incidence

n1

n2

2

n1

n2

1

2

n1

n2

1 2

Angle of

reflection

Light is bent away

from normal

Light does not enter

second material

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By Snell's law,n1 sin1= n2 sing

2

The critical angle of incidence c where 2 = 90o

c= arc sing (n2/ n1)At angle greater than c the light is reflected, Because

reflected light means that n1 and n2 are equal, 1 and 2

are also equal.

The angle of incidence and reflection are equal. These

simple principles of refraction and reflection form the basis

of light propagation through an optical fiber.

Snell 's law

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The optical fibre has two concentric layers called the

core and the cladding.

The inner core is the light carrying part.

The surrounding cladding provides the difference

refractive index that allows total internal reflection of light

through the core.

The index of the cladding is less than 1%, lower than thatof the core.

Typical values for example are a core refractive index of

1.47and a cladding index of1.46

PROPAGATION OF L IGHT THROUGH FIBRE

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Jacket

Cladding

Core

Cladding

Angle ofreflection

Angle ofincidence

Light at less thancritical angle is

absorbed in jacket

Jacket

Light is propagated by

total internal reflection

Jacket

Cladding

Core

(n2)

(n2)

Fig. Total Internal Reflection in an optical Fibre

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The specific characteristics of light propagation through

a fiber depends on The size of the fiber.

The composition of the fiber.

The light injected into the fiber.

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125 8 125 50 125 62.5 125 100

Core Cladding

Typical Core and Cladding Diameters

The size o f the fiber

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The refractive Ind ex:The relation between the indices of the

core and cladding.

Step Index Graded Index

- The step index fiber has a core with uniform index throughout

the fiber.

- The profile shows a sharp step at the junction of the core and

cladding.- The graded index has a non-uniform core.

- The Index is highest at the center and gradually decreases

until it matches with that of the cladding.

- There is no sharp break in indices between the core and thecladding.

FIBRE TYPES

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Three types of f ibers :

Single- Mode Step Index fiber (Single Mode Fiber)

Multimode Step Index fiber (Step Index fiber)

Multimode graded Index fiber (Graded Index fiber)

FIBRE TYPES

Single Mode Step Index

n1n2

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Step index Mu lt imode Fiber

High orderMode

DispersionRefractiveIndex Profile

Low Order ModeMulti mode Step Index

InputPulse

OutputPulse

n1

n2

x d x N AN= 0.5 x (----------------------) 2

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Graded index Mult imode Fiber

Dispersion

Multi mode Graded Index

n1

n2

dxNAN= 0.25 x ( ---------------- )2

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OPTICAL FIBRE PARAMETERS

Wavelength

Frequency

Window

Attenuation

Dispersion

Bandwidth

Numerical Aperture

Optical Fiber Characterist ic s

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WAVELENGTH

It is a characteristic of light that is emitted from the light

source and is measures in nanometers (nm). Red Light has longer wavelength than Blue Light,

Typical wavelength for fiber use are 850nm, 1300nm and

1550nm all of which are invisible.

FREQUENCYIt is number of pulse per second emitted from a light

source. Frequency is measured in units of hertz (Hz). In

terms of optical pulse 1Hz = 1 pulse/ sec.

Optical Fiber Characterist ic s

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WINDOW

A window is defined as the range of wavelengths at

which a fiber best operates.

Window Operational Wavelength

800nm - 900nm 850nm

1250nm - 1350nm 1310nm

1500nm - 1600nm 1550nm

Optical Fiber Characterist ic s

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ATTENUATION

Attenuation is defined as the loss of optical power over a

set distance, a fiber with lower attenuation will allow more

power to reach a receiver than fiber with higher attenuation.

Optical Fiber Characterist ic s

Window Attenuation

850nm 2.0 to 2.5 dB/km

1310nm 0.5 to 1.0 dB/km

1550nm0.2 to 0.25 dB/km

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Optical Fiber Characterist ic s

Loss in the Fiber Optics:

Absorption loss :Natural Impurities in the glass

absorb light energy.

Scattering loss :Light rays travelling in the core

reflect from small imperfections into a new pathway that

may be lost through the cladding.

Loss due to geometric effects or bending loss

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DISPERSION Expressed in ps/km/nm

Dispersion is the spreading of light pulse as its travels down

the length of an optical fiber. Dispersion limits the bandwidth or information carrying

capacity of a fiber.

Dispersion increases along with the distance.

There are three main types of dispersion

Modal Dispersion

Material dispersion

Wave guide dispersion

Optical Fiber Characterist ic s

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MODAL DISPERSION

The spreading of light is called modal dispersion.

Modal dispersion occurs only in Multimode fibers Step

index fibers.

It occurs because rays follow different paths through the

fiber and consequently arrive at the other end of the fiber at

different times.

Optical Fiber Characterist ic s

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MATERIAL DISPERSION

It occurs due to different wavelengths travel at different

velocities through a fiber.

Refractive Index changes according to the wavelength.

This velocity variation is caused by some property of

material available in glass.

Optical Fiber Characterist ic s

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WAVEGUIDE DISPERSION :

It occurs because optical energy travels in both the core

and cladding, which have slightly different refractive

indices.

The energy travels at slightly different velocities in the

core and cladding because of the slightly differentrefractive indices of the materials.

Optical Fiber Characterist ic s

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BANDWIDTH

In single mode fiber dispersion is most affected by the source's spectral

width. The wider the source spectral width, the greater the dispersion.

Conversion of dispersion to bandwidth can be defined by the followingequation.

0.187

BW = --------------------------

(Disp) (SW) (L)

Disp = Dispersion at the operating wavelength in psec/ nm/ km.

SW = Spectral width of the source in nm.

L = Fiber length in km.

Optical Fiber Characterist ic s

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NUMBERICAL APERTURE

Numerical aperture (NA) is the "light - gathering ability"

of a fiber.

Light injected into the fiber at angles greater than the

critical angle will be propagated. The material NA relates to

the refractive indices of the core and cladding.NA = n1

2 - n22

where n1

and n2

are refractive indices of core and

cladding respectively.

Optical Fiber Characterist ic s

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NUMBERICAL APERTURE

NA is unit less dimension. It can also define as the angles at

which rays will be propagated by the fiber. These anglesform a cone called the acceptance cone, which gives the

maximum angle of light acceptance. The acceptance cone is

related to the NA

= arc sin (NA)

NA = sin = sin /2

where is the half angle of acceptance and is the full

angle of acceptance.

Optical Fiber Characterist ic s

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Optical Fiber Characterist ic s

NUMBERICAL APERTURE

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1. G-652 (NDSF) : Non dispersion shifted fiber Operated

in 1550nm region, Currently using for SDH and DWDM

systems

2. G-653 (NDSF) :Dispersion Shifted Fiber SDHsystems

3 G-655 (NZDSF) :Non-zero dispersion-shifted fiber

Good for both SDH and DWDM use in the 1550-nm

region.

Specially designed for DWDM applications to

compensate the non-linearity (Four wave mixing and

Polarization mode dispersion) presenting in DWDM.

Types of Single mode Fibers

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Cable Loss.

Splice Loss.

Connector Loss.

Fiber Length.

Continuity of Fiber.

Fault Localizations/Break Fault.

MAIN TESTS ON OPTICAL F IBRE CABLES

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Optical Power Meter.

Calibrated Light Source.

Optical Attenuator.

Optical Time Domain Reflectometer (OTDR).

I NSTRUMENTS REQUIRED

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TYPES:-

Fixed Attenuators.

Variable Attenuators.

APPLICATIONS:-

To Simulate the Regenerator Hop Loss at the FDF.

To Provide Local Loop Back for Testing. To measure the Bit Error Rate by varying the

Optical Signal at the Receiver Input.

(RECEIVER SENSITIVITY)

OPTICAL ATTENUATORS

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Used for measuring

Fiber Loss.

Splice Loss.

Connector Loss.

Fiber Length.

Continuity of Fiber.

Fault Localization.

OPTICAL TIME DOMAIN

REFLECTOMETER (OTDR)

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Thank You