introduction in fibre optics...by mohamed ibrahem(mido) mansoura science faculty .egypt

8/9/2019 introduction in fibre optics...by mohamed ibrahem(mido) mansoura science faculty .egypt

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4th chem. &phys


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General-:

With the advent of fiber optics ,data can be very

easily and rapidly transmitted over long distance ,

it`s increasingly replacing wire transmission linesin communication system

Optical fiber**

Optical fiber as the light equivalent of microwave

waveguides with the additional advantage of a very

wide band width .physically an optical fiber is a

very thin and flexible medium it consisting from 3

sections,

1.The core

2.The cladding ,and

3.The jacket

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The outer sec is called the jacket made of plastic or

polymer and other materials and is provided for

protection against moisture absorption , crushing

and others.The core acts like a continuous layer of two parallel

mirrors

Importance of fibers optics

1.Smaller size and lighter weight2.Elctromagnatic isolation

3.No physical electrical connection

4.The fiber much more reliable

5.Almost there is no cross-talk

6.Greater band width

7.As fibers are a very good dielectrics

8.The cost per channel is lower

system

Generations of telephone

1.First generation , with wave length 820 nm

2.Secound generation, with wave length 1300 nm3.Third generation with wave length 1550 nm

4.Fourth generation with wave length 1550 nm

Propagation of light in an optical

fibre

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a) Mechanism

If light wave enters at one end of a fiber in properconditions. Most of it is propagated down the

length of the fiber and comes out from the other

end of the fiber , some of them losses due to a

small fraction leakage.

This type of fiber

is called light-guide.

Experimentally, additional high band width

capabilities have been extended into the gigahertzrange

b) condition

Total internal reflection in the walls of the fiber can

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occur ,if and only if , the following two

conditions are satisfied.

basic structure of an optical fiber , andpropagation of light wave thought it

1.Optical fiber is basically a solid glass rod.

diameter of rod is so small that it looks

like a fiber.

2.The light Optical fiber is a dielectric waveguide.travels like an electromagnetic wave inside the

waveguide. The dielectric waveguide is different

from a metallic waveguide which is used at

microwave and millimeter wave frequencies.

Sinc=1/ 2

3.there In a radiation but in an metallic waveguide,

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is a complete shielding of electromagnetic optical

fiber the electromagnetic radiation is not just

confined inside the fiber but also extends outside

the fiber.

**light energy emanating from any practical point

source will have several paths with different angles

of incoherence at the boundary layer it may also

contain difference colures with different frequency

this called step-index multimode propagation any

other light awe which is meaning the core-cladding

interface at above the critical value ANGLE WILL

ALSO BE TOTALLY reflected and hence will

propagated along the core

**As the result this distortion the variations of

successive pluses each other and the ruby cause

distortion of the information being carried.

however this defected can be minimized by making

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the core diameter of same order.

**next we discuss the propagation of multi wave

light energy in graded index fiber ,with theindividual waves being gradually refracted in the

graeded-index core

Fibred fabrication

Fibred can be fabricated by a number of

techniques as detailed below:

1.External chemical vapor deposition of spot(external CVD(

2.Internal chemical vapor deposition of glass

(internal CVD(

3.Exeternal chemical vapor deposition of glass

(plasma CVD(

4.Multi-element glass5.Phasial system.

By the fraction process, we can get single mode

high quality fibers of both single mode and

multimode step and graded index fibers

*Characteristics of this method:

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1.Ther is a dimensional restriction. Outer diameter

2.The typical value of the NA of the fiber produced

by this method3.The rate of deposition is 0.5 gm per minute

4.loss: fiber is scattered-loss limited

**Acceptance Angle and Acceptance cone of

fiber-:

The maximum angle within which light will be

accepted by an element, such as a detector or

waveguide. In the latter, it is quantified as half thevertex angle of the cone within which optical

power may be coupled into bound modes of a fiber.

Also called acceptance cone

,so any light wave impinging on the core within

this maximum external incident angle is couplet

fiber and will propagated this angle c called the

acceptance angle from fig.

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where:

n1 is the refractive index of the medium the light is

leaving

1 is the incident angle between the light beam andthe normal (normal is 90 to the interface between

two materials(

n2 is the refractive index of the material the light is

entering

2 is the refractive angle between the light ray and

the normalThen,

n1sin(1) = n2sin(90-2(

n1sin(1)=n3sin(90(

sin(2)=n2/n1 , n1sin(1) =n2cos(2(

n1sin(1)=n2cos(2(

So, c = 1 = arcsin(n2/n1(

**Numerical Aperture (general-: (

is another important term associated with afibre

NA=sin(max=(

When a light ray is incident from a medium of

refractive index n to the core of index n1, Snell'slaw at medium-core interface gives

From the above figure and using trigonometry, we

get:

where is the critical angle fortotal

internal reflection, since

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http://en.wikipedia.org/wiki/Critical_anglehttp://en.wikipedia.org/wiki/Total_internal_reflectionhttp://en.wikipedia.org/wiki/Total_internal_reflectionhttp://en.wikipedia.org/wiki/Total_internal_reflectionhttp://en.wikipedia.org/wiki/Total_internal_reflectionhttp://en.wikipedia.org/wiki/Total_internal_reflectionhttp://en.wikipedia.org/wiki/Total_internal_reflectionhttp://en.wikipedia.org/wiki/Critical_angle


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Substituting for sin r in Snell's law we get:

By squaring both sides

Thus,

from where the formula given above follows.

This has the same form as the numerical aperture in

other optical systems, so it has become common todefine the NA of any type of fiber to be

where n1 is the refractive index along the central

axis of the fiber

this fig shows the variation of NA with

acceptance angle

Numerical Aperture of agraded index-:

From th consideration of geometrical optics it`s

evident that light icedent on the fiber core at

position will propagation as aguided mode only if

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wavelength between the parallel walls of glass of

core diameter (d(

**

M

eridiona

l and

skewrays-:

Alight

ray is

path

which is followed by affixed point in the E-H

plane,as the plane moves with the speed of light Cin adirection normal to it self. The light rays or

skew rays this called meridional if all of them

comprising amode ,pass through the long it udinal

or Z-axis of fiber core

Classification of optical fibers

**fiber classification-:

Manufacture have conc mainly on three broad

class of fiber-:

1.stepped index fiber (SI-:(

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The basic structure of stepped index fiber is shown

in fig (3.1) it has two portions . it`s structure is

something like two conc cyliender.inner called

core,outer may be made of air in case of will be

large .as air has reflective index , of unity

2-graded index multi mode fiber( GRIN-:(

The second type of this class of fiber uses plastic as. its cladding this gives higher losses .it`s less

expensive than the others. It`s refractive index

profile which shows in fig.

3-single mode fiber-:

The third type of this class of fiber uses glass as its

cladding but with refractive index fiber is very

commonly used ,because it can be fabricated with

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small core diameter with out sacrifing mechanicals

strength

Then; d>(0.766/NA(

4-stepped index Mono mode fiber-:

The transit time disperision problems can be solved

by making the core very thin,so that the diameter of

solved by making the core very thin ,so that the

diameter of core is of the same order as the wave

length of the light wave to be propagated as in fig

*The chield charchteristics of stepped index

monomode fiber are as follow-:

1-very small core diameter

2-low attenuation

3-low numerical aperture

4-very high bond widthThen; d


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**Grade index Multi mode fibre-:

Graded index fibers have intermediate band width

and capacity it`s a less expensive method of over

coming transit time dispersion . this fiber hasaproprty of gradually changing refractive index as

in fig

**plastic fiber-:

Fiber which composition are now avialbe.plastic

fiber (plastic core) have been manufactured from

polymer drawing into afibre.the losses associated

with the second fibers are usually in hurdeds of

decibel. They aperated at low temp at maximum

125 while glass fibers can be used right up to a

maximum temp of 100

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Other latest developed types ofOptical Fibers

1-HIGH Parity Silica Fiber (HPSUV-:(

This type of fiber is suitable for transmission of

light in the range 180 to 800.it`s good as well as

cheep. It`s sometimes coated with dominium which

gives very mechanical strength the and handingcapability

2-High purity silica (HPSIR-:(HPSIR type of fiber is similar to the HPSUV fiber

with slightly different dopants to give it alongerwave length capability in the near IR from 500 nm

to 2600 nm .the same comments concerning

strength and power handling apply.

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Fibers can be fabricated by number

of techniques as detailed below-:

**Internal chemical vapour deposition-:

This method is also batch process,like external

chemical vapour deposition teqnique.by this

method we can process the fiber core and cladding

matrials in very pare stage

characteristic of this method-:

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a)there is diamensional testriction

b)the typical value of the NA of procedure by this

method is 0.22

c)the rate of deposition is 0.59m per mind)loss: fiber is scattered loss limited

**second process of internal (cvd-:(

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introduction in fibre optics...by mohamed ibrahem(mido) mansoura science faculty .egypt