1
MODULE -5
FIBRE OPTICS AND APPLICATIONS
Introduction
Communication may be defined as transfer of
information from one place to another. A communication system is set
up, which can convey information to any distance. Generally the
information is sent by the communication system by modulating it
with a suitable electromagnetic wave, which act as a carrier for
information. The modulated carrier is received back at the destination
and signal is reproduced from it by demodulation. The carrier
frequencies used for this purpose extend from radio frequencies to
optical frequencies. The communication using carrier wave of optical
frequencies is called optical communication.
Optical communication through the atmosphere has a no. of
limitations. Eg. Light transmission is restricted to line of sight(i.e, if
an obstacle comes in b/w transmitter and receiver, then
communication through atmosphere is not possible). Also light
transmission through atmosphere is severely affected by disturbances
such as rain, snow , fog, dust and atmospheric disturbances etc. The
low frequency carrier wave like radio waves and microwaves are least
affected by atmospheric conditions. But the problem of using these
waves is the small tramission band width. Because of this information
carrying capacity of these waves is very lass. On the other hand if we
use optical frequency carriers then band width increases by a factor
104. However as said above for optical communication, atmosphere is
not suitable transmission medium. Hence we have to use optical fibre
for this purpose.
Then a new problem of launching enormous amount of
optical power into the fibre appeared. This problem was solved by the
intention of Laser.
The requirement of Information technology and communication
industries for very easy, rapid,lossless and uninterrupted
transmission of data for long distance is fulfilled by fibre optics
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Fibre optics is the overlap of applied science and engineering.It is a
technology in which signals are converted from electrical in to
optical signals and carried by a thin glass or plastic fibre.
In optical fibres the light is launched in one end of the fibre and it
is passed through the other end withoutany loss of signals.The light
passes through the optical fibre due to the Total internal reflection
of light.The light undergoes total internal reflection more than one
lakh times within one meter length of the fibre.The light
propagating through the fiber is explained by means of the ray
optics.
2. Structureof Optical Fibre Or Construction Of Optical
Fibres
A simple structure of fibre optic cable consists of a core, cladding
and a protectivejacket.These are built up as three co-axial region
a)core
3
The inner most region is the core, It is transparent silica or plastic
cable through which the light travels.The core is generally
constructed using Germanium doped Silica glass.
b)cladding
The cladding is a glass sheath that surrounds the core. The
cladding acts like a mirror ,reflecting light back in to the core.It is
prepared using nearly pure Silica glass.
c)Protective jacket or glass or buffer
The cladding is covered with a plastic coating and strength material
like polyurethane compound.This outer most region is called
protective jacket or sheath or buffer.This is prepared using
ultraviolet cured plastic material.The sheath is generally coloured to
enable the user to distinguish from fibre.
The refractive index of core is always greater than that of
cladding.The protective layer is used to safeguard the cable from
bending,stretching,rolling etc.
Principle And Propagation Of Light Through Optical
Fibres
Principle
The basic principle behind the propagation of light through the
optical fibre is Total internal reflection.
It is defined as the phenomenon in which incident ray is completely
reflected back in to the same medium.
Conditions for total internal reflection
For total internal reflection the ray must satisfies two conditions.
a)Ray travel from denser to rarer medium
b)Angle of incidence is greater than that of critical angle
<i> 
c
The refractive index is one of the most important optical parameter
of the medium
4
It is defined by,
𝑐
n=
𝑣
c→velocity of light in vacuum
v→ velocity of light in medium
Greater is the refractive index more denser is the medium.For
satisfying first condition for total internal reflection,the ray travel
from medium having greater refractive index to medium having
lower refractive index.
Consider a ray of light is passing through a medium of refractive
index n1 to another medium of refractive index n2 .Let n1> 𝑛2,some
part of theincident beam is reflected back in to the medium and the
remaining part is transmitted through the medium of refractive
index n2.(fig.a).
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According to Snell’s law,
n1sin𝜃1=n2sin𝜃2
If the angle of incidence 𝜃1 ,is slightly increased,the angle of
refraction𝜃2 also increases.The angle of refraction becomes 900 at
some particular value of angle of incidence (fig(b)) in this condition
𝜃1 is called critical angle(𝜃𝑐).If the angle of incidence is further
increased , the entire incident light gets reflected back in to the
medium and there is no light to be refracted in to the medium of
refractive index n2 (fig©).This type of reflection of light With in the
medium is known as total internal reflection.The minimum angle of
incidence required to produce the total internal reflection is known
as critical angle (𝜃𝑐).
When a light ray enters in to optical fibre it undergoes refraction,
after refraction light ray incident on a core-cladding interface
.where, 1 >  c , n1> n2.The ray undergo total internal reflection. The
ray incident on the other face from there also ray get reflected .In
the same way the ray undergo no. of total internal reflection inside
the fibre and it reach the other end.
4) Expression For Critical Angle
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It is defined as the angle of incidence at which angle of refraction
900.
ie, 𝜃𝑐=𝜃1,where 𝜃2=90
Where,
𝜃1→angle of incidence
𝜃2→angle of refraction
𝜃𝑐→critical angle
According to Snell’s law,
Sin i/sin r=n21
Sin i/sin r=n2/n1
n1sini=n2sinr→ (1)
here i=𝜃1,r=𝜃2
eq(1)→n1sin𝜃1=n2sin  2
→(2)
At critical angle,
1 =  c &  2 =900
(2)→ n1sin  c = n2 sin90
→(3)
Sin90=1
(3) → n1sin  c = n2
sin  c = n2/n1
or
 c =sin-1(n2/n1)
5) Acceptance Angle And Numerical Aperture
7
Consider that OA is the light ray launched into the fibre as shown
in fig. The end at which light enters the fibre is called launching
end.
The ray launched into the fibre gets refracted and passed through
the core and it is incident on the core-cladding interface. The angle
of incident at the core-cladding interface should be greater than the
critical angle. Let 1 be the angle of incidence in the air-core
interface,  2 be the angle of refraction and 
be the angle of
incidence at the core-cladding interface.
From ∆ABC,
𝝅
𝝅
𝜋
 =𝟐 -  2 [ since  +  2 +<B=180,  +  2 =180 - 𝟐 ,  =2 −  2 ]
In order to make the angle of incidence (  ) at the core-cladding
interface greater than the critical angle(  c ).  2 should be low and
hence the angle of incidence at the air-core interface( 1 ) should be
minimum. The angle of incidence at the air-core interface that
makes the light transmitted into the core to be incident in the corecladding interface at an angle greater than the critical angle is
known as acceptance angle.
8
Or
Acceptance angle is defined as the maximum angle that a light ray
can have relative to the axis of the fibre and propagate down the
fibre.
Or
The value of the acceptance angle is limited by the angle of
incidence at the core-cladding interface.The cone of the angle at
which the light is launched into the fibre that makes the light to be
incident at an angle greater than the critical angle at the corecladding interface is said to be the acceptance angle.It is
represented by the letter (  a ) .
Acceptance Cone =2  a
6)Mathematical Expression For Acceptance Angle
Let n0 , n1,n2 be the refractive indices of the air,core,cladding
respectively and OA be the incident beam. It is launched into the
fibre at an angle of 1 .Let  2 be the angle of refraction. Let  be the
angle of incidence at the core-cladding interface.
From ∆ABC,
𝝅
 =𝟐 -  2 →(1)
From Snell’s law
sin 1 / sin  2 =n10
sin 1 /sin  2 =n1/n0
n0sin 1 =n1sin  2 →(2)
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From eq(1)
𝜋
 2 =2 -  →(3)
Sub(3) in (2)
𝜋
n0sin 1 =n1sin( 2 -  )
n0sin 1 = n1cos  →(4)
using the relation
sin2  +cos2  =1
cos  =(1-sin2  )1/2
→(5)
subeq(5) in (4)
n0sin 1 = n1(1-sin2  )
1/2
→(6)
Consider the light is launched in such a way that the angle of incidence
at C is  c , then 1 =  a and    c
n0sin  a =n1(1-sin2  c )
1/2
But,  c =sin-1(n /n )
2
1
→(8)
Or
sin  c =n2/n1→(9)
sub (9) in (7)
n0sin  a =n [1 - n 2/n 2]1/2
1
2
1
n0sin  a = n1
n
n1  n 2
2
sin  a =
n0
2
1
 n2
n
2
1
→(7)
2
1 / 2
10
 n 2 n 2
2
 a = sin  1
n0

1




The rays incident within a cone of half angle  a will be collected and
propagated by the fibre.
The rays from a cone and the cone is called as acceptance cone.
7) Fractional refractive index change
The fractional difference ∆ between the refractive indices of the core
and the cladding is known as fractional refractive index change .It
is expressed as
∆=n1-n2/n1
This parameter is always positive since n1>n2 ,for total internal
reflection condition.In order to guide the ray effectively through a
fibre∆<<1. Typically ∆ is the order of 0.01.
8) Numerical aperture (NA)
NA is defined as the sine of the acceptance angle .
Thus,
NA = sin  a
NA
=( n12- n22 )1/2
n12- n22 = (n1 + n2) ( n1 - n2)
=
n1  n2
2

n1  n2
n1
  2n1 →(1)
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Approximation, (
n1  n2
 n  n2 
)  n1 ,  1
  
2
n
1


Eq (1)→NA=2 x n1x n1 ∆
= 2 n1 2 ∆
NA =n1 2
→(3)
Numerical aperture determines the light gathering ability of the
fibre. Or It is a measure of the amount of light that can be accepted
by a fibre .Eq(2) depends only on the refractive indices of the core
and cladding material . its value ranges from 0.13 to 0.50 . A large
NA implies that a fibre can accept large amount of light from the
source.
Significance of NA
1. NA is the measure of light gathering capacity of the fibre at
the input end.
2. Fibre with large NA can have more modes.
3. A large NA may cause greater dispersion.
4. A single mode fibre with large NA has higher attenuation.
9)Properties Of OpticalFibre
(a) Dispersion
In an optical fibre due to various factors like the
difference in group velocity between the different modes a pulse of
light broden in time as it propagate through the fibre. This
phenomenon is known as pulse dispersion. “The spreading of the
output pulse in the time domain and change in the shape of pulse
is called Pulse Dispersion”.
Pulse dispersion can be of two types:
1. Intermodal Dispersion
2. Intramodal dispersion
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1)
Intermodal Dispersion:
This dispersion is also called
Modal dispersion. Itoccur because of the different times taken by
different rays propagating through the fibre . Hence even though
two pulses may be well resolved at the input end,because of
broadening of the pulses they may not be so at the out put. This
dispersion does not depend on wavelength of light, but depend
on the angle at which ray of light strikes core clad interface of
the multimode fibre.
For eg. In a fibre the ray making larger angles with the axis
have to traverse a longer path length and take a longer time to
reach the output end compared to a small angle ray
.Consequently the pulses broden as they propagate .
2) Intramodal Dispersion
When the fibre can carry only one propagating mode then the
corresponding broadeningisintramodaldispersion .The two main
causes of intramodel dispersion are
1) Material DispersionOr Chromatic dispersion: This arise from
the variation of refractive index of the core material as a
function of wavelength.
2) Waveguide Dispersion: In a single mode fibre the core can
confine only 80% of optical power.Therefore about 20% of
light power is transmitted through the cladding.
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b)Attenuation
The loss of power suffered by the optical signal as it propagate
through the fibre is called attenuation. It is also called the
fibreloss.
The signal attenuation is defined as the ratio of the
optical output power from a fibre of length L to the input
optical
power.It
is
expressed
in
decibal
per
kilometer,(dB/km).

10  pi 
log  
L
 po 
wherePi is the optical signal launched at the one end of the fibre
and P0 is the power of the optical signal emerging from the other
end of the fibre. In the case of an ideal fibre Pi =P0 and the
attenuation would be 0dB/km.
The three mechanism through which attenuation take place
are,
1)Absorption:In this case ,the loss of signal power occurs
due to absorption photons are absorbed by impurities in the
silica of which the fibre is made of and intrinsic absorption by
the glass material itself .
2)Scattering: Mainly due to Rayleigh scattering while the
signals travels in the fibre, the photons may be scattered
because of sharp changes in refractive index values
inside the glass over distances that are small compared
to wavelength of light. Due to Rayleigh scattering, the
14
photon move in random directions and in all possibilities
leaves the fibre and thus becomes a loss.
3)Radiation losses: Radiative losses occur due to bending
of fibre. There are two types of bends,macroscopic and
microscopic.
(a) Macroscopic bend
If
the
radius
of
the
core
is
large
compared
to
fibrediameter , a large curvature will be formed at
corners . At these corners, the light is incident at an
angle less than the critical angle , therefore the light is
scattered into the cladding . This is called as macro
bending losses.
(b) Microscopic bending
There may be small bumps Or variations in the surface
of the core of the fibre. This is due to manufacture defect
These variations change the angle at which light strikes
the core to cladding interface and cause the light to
refract in to cladding .
Variation caused by handling can be reduced
by protective cable designs.
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c) Bandwidth Distance Product
Bandwidth distance product of an optical fibre,
under specified launching condition and cabling condition ,at
a specified wavelength ,a figure of merit equal to the product
of fibres length and the 3 dB bandwidth of the optical signal
.The band width distance product is usually stated in
Megahertz kilometer (MHzkm) or Gigahertz kilometer
(GHzkm). It is a useful figure of merit for predicting the
effective fibre bandwidth for other lengths.
d) V-number(v)
The no. of modes supported for propagation in the fibre is
determined by a parameter called V-number(V).
Which is more generally called normalized frequency of the
fibre
Ie,
2a
 n12  n22 
V=

Where a is the radius of the core and  is the free space
wavelength using the value of NA and fractional refractive
index change
2a
NA 
V=

2a
n1 2
V=

The maximum no. of modes Nm supported by a step index
fibre is determined by,
1
Nm  V 2
2
Thus for V=10 , Nm = 50.
For V< 2.405, the fibre can support only one mode and is
classified as a SMF.MMFs have values of V>2.405 and can
support many mode simultaneously.
The wavelength corresponding to the value of V= 2.405 is
known as the cutoff wavelength  c of the fibre.
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 c =  V/2.405
The no. of modes that can propagate in a graded index fibre
is half that of a step index fibre.
1
Nm  V 2
4
9)Types Of Optical Fibres
Optical fibres are classified as follows



On the basis of material used for core and cladding
On the basis of mode of propagation
On the basis of refractive index profiles
1)Based on the material Used,
Based on the material Used, the
opticalfibres are divided in to two types .
(a)Glass Fibres
(b)Plastic Fibres
(a) Glass Fibres
If the core and cladding materials are made
up of glass with different refractive index, such fibres
is called as glass fibre.
The difference in refractive index of core
and cladding is achieved by doping with suitable
materials.
Advantages
1. Long durability
2. Less attenuation
3. Not corroded with environment
Disadvantages
1. It cannot be easily bent
2. It should be handled with care
3. It is coastlier
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Application
These Fibres are
communication.
used
for
long
distance
(b)Plastic fibre
It is an optical fibre made up of plastic material . In
these fibres core and cladding materials are made
up of different materials.
Advantages
1.
2.
3.
4.
More flexible
Very cheap
Long durability
Can be handled without any care due to its
toughness
Disadvantages
1. High attenuation
2. Easy corrodible
Application
Used for short distance, low speed application
such as LAN wiring.
2.Based on Mode Of Propagation
When light is launched into the fibre , it can
propagate only in certain directions.The allowed directions
are known as mode.
Depending upon the no. of modes of propagation , optical
fibres are classified as,
1. Single mode fibre
2. Multi modefibre
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Single mode fibre
If there is only one way in which a light ray can
propagate through the fibre ,then such fibres are called
as single mode.
It have a very small core diameter and the
cladding has large diameter compared with core.However
the total diameter of the fibre remains constant.
As the core diameter is very less these fibres are used for
long law communication.
Multimode Fibres
If there are more numbers of ways in which
light rays can propagate through the fibre, then such
fibres are called as Multimode fibre.
Core diameter is large compared with single
mode fibre so many mode is possible.But the total
diameter is same as that of single mode fibre.
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As the core diameter is large and greater
attenuation , these fibres are used for short haul
communication.
Single mode fibre
In a single mode fibre only one
mode can be propagated
The single mode fibre has a smaller
core diameter and difference in
refractive index of core and
cladding is small.
No intermodal dispersion
Multi modefibre
The fibre in this mode allows large
number of modes Or light to
propagate through it.
The core diameter is large , the core
and cladding refractive index
difference is also large
There is intermodal dispersion
3)Based on refractive index profile
Index profile is a plot of refractive index drawn on
horizontal axis versus the distance from the core axis
drawn on the vertical axis.Depending upon the variation
of refractive index of core and cladding the fibres are
classified as,
1) Step index fibre
2) Graded index fibre
1) Step index fibre
In these fibres , the change in refractive index of
core cladding and air occurs suddently at the
boundary. This change occurs in a step , hence it
is called as step index fibre.
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Or,If the core of the optic fibre has a uniform
refractive index (n1) , it is called a step index
fibre.The cladding also has a uniform refractive
index(n2), that is slightly lower than refractive
index of core.
It is further classified as region,
1. Single mode step index fibre
2. Multi mode step index fibre
(a)Single mode step index fibre
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Features:
 These fibres have very large bandwidth-distance product,
which is greater than 3GHzkm. Bandwidth –distance
product is the measure of the information carrying capacity
of the fibre. The greater the bandwidth-distance product the
greater is the information carrying capacity.
 They find particular application in long distance terrestrial
communication and submarine cable system at very high
bit rates.
 It is least expensive. They constitute 80% all the fibres that
are manufactured in the world today.
 The step-index single mode fibre eliminates intermodal
pulse-broadening.
 Splicing (joining of fibres) is very difficult with these fibres
due to very small core diameters.
 Laser is preferred as sources.
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(b)Step- index multimode fibre
In a step index multimode fibre, the number of rays or
mode of light which are guided is determined by the core
diameter and the difference in the refractive indices of the
core and cladding. Here the core diameter is very large .
Features:






These fibres used for conventional image transfer are
limited to short distances for information transmission due
to pulse-broadening. An initially sharpe pulse made up of
many modes broadens as it travels long distance in the
fibre , since high angle modes have a long distance to travel
relative to the low- angle modes .
Either laser or LED can be used as a source.
Bandwidth – distance product is small (< 200 MHzkm).
It is widely used for data transmission over small
distances.
The fibre is expensive.
Splicing is easier as compared to step-index single mode
fibre .
Graded – index Multimode Fibre
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If the core of an optic fibre cable has a non
uniform refractive index that decreases gradually from the
centre towards the core – cladding boundary , the fibre is
called a graded index multimode fibre. The commonly
used refractive index profile is parabolic . The cladding
surrounding the core has a uniform refractive index of the
core region.
A
graded –index multimode fibre , where the core
refractive index varies across the core diameter is used to
minimize pulse broadening due to intermodal dispersion
.Or , A graded index fibre with parabolic profile for
refractive index for the core
reduces intermodal dispersion.
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Features:
 The bandwidth-distance product is very large (in between
200MHzkm-3GHzkm).
 Graded index multimode fibre is suited for intermediate
distance and intermediate bit-rate transmission system.
 It is widely used for telephone loop distribution systems.
 The propagation characteristics o various modes are
different.
 It is the most expensive type fibres.
 It can be operated with either LED or laser as the source.
 Less pulse broadening because light travels more slowly
in the high index region and fastly in the low index region
of the fibre, thus significant equalization of the transit
time for the various modes can be achieved.
 Because of varying refractive index, a ray entering into
the fibre continuously bent towards the axis o the fibre.
S.
no.
1
2
3
4
5
Step index fibre
Graded index
fibre
The difference in refractive
indices of core and cladding is
obtained in a single step and
hence called as step index fibre.
The light rays propagate as a
meridional rays and pass
through fibre axis.
It follows a zig-zag path of light
propagation .
It has low bandwidth.
Distortion is more in multimode
step-index fibre.
Due to non uniform refractive
indices ,their difference in
refractive indices between the core
and the cladding gradually
increases from centre towards
interface and hence called gradedindex fibre.
The light propagation is in the
form of skew rays and does not
cross the fibreaxis .
It follows a helical path(i.e, spiral
manner) of light propagation
It has high bandwidth
Distortion is very low and is
almost zero
REFERENCE
1. Engineering Physics By A.Marikani
2. A text book of Engineering Physics By M.N
Avadhanulu,P.GKshirsagar
3. Engineering Physics By P.K Palanisamy
4. Engineering Physics By Sivalingam Dinesh
25
5. Engineering Physics By S. Gopinath
MODULE-5
CHAPTER-2
FIBRE OPTIC COMMUNICATION SYSTEM
INTRODUCTION
One of the most important applications of optic fibre is in the field
of communication system. Optic fibre communication system has
so many advantages over traditional systems of communication like
radiofrequency,
microwave
communication
,
wireless
communication etc. This is the combination of two technologies –
existing communication technology and optic fibre technology.
Optic fibre communication system basically consists of three
systems.
(1) Transmitter (2) Receiver (3) Information channel
(1) Transmitter – This converts electrical signals into optical signals
for transmission.
(2)Receiver- This receives the optical signals and convert back into
electrical signals.
(3) Information channel- This provides a path or passage to
transmit the optical signals from the transmitter to the receiver.
Principal elements of a typical communication system
26
a) Block diagram and functions of each component
The message origin , modulator, carrier source and input
channel coupler together constitute transmitter. Optic fibres
are used as the information channel. Out put channel
coupler, detector, signal processor and message output
constitute receiver.
b) Function of each component
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1.Message origin: Message origin converts all the non –
electrical message into electrical signals using a transducer.
For eg.a microphone converts sound energy into electrical
energy.
2.Modulator: Imposing a message on a carrier weve for
propagation is called modulation.
(a) At first it converts electrical messages into proper format.
(b) Secondly it imposes this format on a carrier wave for
propagation.
There are two types if modulations analogue modulation and
digital modulation. In analogue modulation, message is
transmitted in a continuous manner and it reproduces the
form of original message without changing the format. In
digital modulation, the message is transmitted in a discrete
manner with the help of binary digits.
Message is transmitted in sequence of ‘ON’ and ‘OFF’. ON
represents the digit 1 and OFF represents the digits Zero’s
.In optic fibrecommunications , digital modulation is
preferred since the message can be transmitted over a large
distance with very low power.
3. Carrier source: Carrier source produces carrier waves on
which the message are transmitted. In fibre optic
communication system ,light waves are the carrier waves.
LEDs Or laser diodes are used to generate stable and
monochromatic waves with suitable frequency at sufficient
power.The information is imposed on light waves.
4. Input channel coupler: This direct the modulated light
waves into the information channels . In the case of radio or
television broadcasting system antenna delivers radio
frequency
waves into the atmosphere for propagation.
Antenna behaves as a input channel coupler.
5. Information channel: This is a path or a passage to
transmit the information from a transmitter to receiver. i.e, it
conveys the modulated light signals from the input channel
coupler to the output channel coupler.
Here very fine and long optic fibres are used as information
channel.
6. Output channel coupler: This direct the modulated light
signals from the information channel to the detector. In Radio
28
or T.V broadcasting system, antenna in our houses collects
the signal from the atmosphere and feed to the receiver of the
T.V.
7. Detector: This detect and separates the messages from the
modulated signals. Here the demodulation take place . Here
light signals are converted into electric current using photo
detector. The current produced is proportional to the power of
the light waves. Hence the output current contains all the
original messages.
8. Signal processor : This filters and selects the required
frequency from the waves. The selected frequency is amplified
. The unwanted frequency is filtered out. In digital system an
additional circuit with coder and encoder is also employed.
9. Message output: Here original message is produced from
the signal.The electrical pulses are converted into sound
waves in the case of audio system. Cathode ray tubes and
proper transducers are used for this
(A)Optical sources
In optical communication ,LED Or Laser Diode(LD) is used as
optical sources.The optical sources used in the fibre optic
communication should have the following properties.
1. It should be able to coupled the single mode fibre core of
thickness 8. m
2. The signal should be easily modulated .
3.The optical source should be low cost , light weight , small
size and high reliability.
4.It should provide high optical output.
(a)LED
A light emitting diode is a p-n junction device. The p-n junction is
forward biased . The hole are added to the p region and electrons
are added to the n regions of the diode .These electrons and holes
are move towards the junction.During their movement towards the
junction, they combine together and release the recombination
energy in the form of light .
29
The wavelength of the light emitted is given by

hc
E
 (nm) 
1240
E (eV )
Where E is the band gap energy in eV and  (in, nanometer) is the
wavelength of the light used.
Advantages
1.
2.
3.
4.
It is small in size
Cost is low.
It has long life.
It operates at low voltage.
Disadvantages
1. Output power is low.
2. Compared with laser , its intensity is less.
3. It cannot be travel for long distance.
(b)Laser
The laser diode is p-n junction device .The p-type and n-type
regions are heavily dopedso as to produce population inversion .
The upper and bottom surface of the Laser diode is used for ohmic
30
contact and the back and front surfaces are used as cavity
resonator and the remaining two surfaces are made as rough
surfaces. The diode is forward biased with a voltage of
Eg
V= e
.
The out put wavelength from the diode is equal to ,  g 
hc
Eg
(B)Optical Detectors
Optical detectors are used to convert the light received from the
fibre into electrical signal . The optical detector should be low
inherent noise device and incorporated with suitable amplifier. The
PIN diode and avalanche photodiode are used as detectors.
(a)PIN diode
A Pin diode is based on the principle of the reverse process of LED
.It convert the light energy into electrical signal.It has an intrinsic
semiconductor at the centre and p-type and n-type regions at the
end as shown in fig. It is reverse biased (5-20 V).The reverse
biasing is used to attract the charge carriers from the intrinsic
regions.
31
When the light is incident on the PIN diode, the intrinsic region
receive more amount of light because of its large size. The photons
incident on the intrinsic region produces electron-hole pair .The
electron is raised from the valance band to the conduction band,
leaving the hole. The electrons are attracted by the reverse biasing
and hence move away from the junction.
The movement of electrons in the conduction band creates the flow
of charge and hence the light energy gets converted into electrical
energy.
(b)Avalanche Photodiode
The avalanche photodiode is based on the principle of avalanche
multiplication of the current. It consist of heavily doped p+ and
n+regions . The depletion region is lightly doped ,almost intrinsic .
The diode reverse biased using 50 - 300 V . The light is made to
incident on the depletion region . The incident light produces the
electron and hole pair. The electron move towards the p- region .
Due to the strong reverse biasing , there is a depletion of charge
carriers in the p- region . The electron in the p-region undergo
avalanche multiplication because of high reverse bias . The hole
move
towards
the
p+
region
without
producing
furthermultiplication . The avalanche photodiode has better noise
performance, because the carrier multiplication is limited to
32
electrons only . The basic configuration of avalanche photodiode is
shown in fig.
(2) Advantages OfFibre Optic Communication System
1.
Wide bandwidth
The bandwidth of optical communication is very large .For
ordinary co-axial communication
The band width is 500 MHz, whereas for the optical fibre
communication have a band width of 105GHz . The information
carrying capacity of optical fibre is also very large.
2.
Electrical isolation
The optical fibres are prepared using flass and plastics .These
two materials are very good electrical insulators .So the fibre
optic cable are electrically isolated.
3. Lack ofcross-talk
Since the total internal reflection of light is used as the basic
principle behind the fibre optic communication, there is no
leakage of signals .there fore , there is no cross talk.
4.
Low cost
The optical fibres are prepared using glass . The SiO2 is the
row material used for the glass preparation . SiO2 is easily
available and hence the cost of fibre optic cable is very low.
33
5.
Small size and light weight
The thickness of optical fibres is very
nearly 250 m .
(including core ,cladding and sheath).Therefore the size and
and weight of the optical fibres is low.
6.
Low transmission loss
The transmission loss of signal in optical fibre communication
is typically around 0.2dB/km and hence the transmission loss
is low .
7.
Immunity to electromagnetic interference(EMI)
The optical fibres are prepared from glass and plastics. They
are insulators and free from EMI and radio-frequency
interference(RFI).The RFI is caused by radio and television
broadcasting stations, radar and other signals originating from
electronic equipment .The EMI produced by industrial
machinery or by natural phenomenon such as lighting or
unintentional electrical spark.
8.
Signal security
The signals transmitted through an optical fibre cannot be
obtained from it without physically intruding the fibre. This
effect the quality of the signal and hence it can be detected
easily.Further the optical fibreis well protected from
interference and coupling and hence it is highly secured .
9.
Flexible and strong
The fibre optic cables are highly flexible and strong.
10 .Longer life span
Their life span is expected to be 20-30 years in contrast to copper
cables
which havelife span of 12-15 years .
11.Temperature resistant: In contrast to copper cables they have
high tolerance to temperature extrems as well as to corrosive
liquids and gases .
12.Easy Maintenance
Optical fibres are more reliable and easy to maintain than copper
cables.
34
The advantages and disadvantages of optical communication
over the Copper wire communication
Advantages:



Optical fibre has high bandwidth means that more data can be
transmitted faster
A pair of copper wires can transmit two telephone calls
simultaneously, while a single optical fibre can transmit over
80,000
Glass is cheaper and lighter than copper
 Optical fibre is unaffected by electro-magnetic interference,
and so is ideal in electrically "noisy" environments, eg. where
there is high voltage equipment nearby
 No electrical current is transmitted down a fibre (glass is a
good insulator), so it is suitable in hazardous environments
where electrical discharge would dangerous
 As transmission technology continues to improve, higher and
higher bandwidths are possible over the same optical fibre
links without having to replace the cables
 Optical fibres are resistive to corrosion
 Low loss or attenuation
 Immunity to tapping information
 Immunity to cross talk
Disadvantages:

The termination equipments for fiber optics is costly
compared to that of copper cable communication

Repeated electrical to optical to electrical conversion is
required as the whole communication is in optical domain

Some of the components like amplifiers, splitters,
multiplexers,
de-multiplexers
etc.,
are
still
under
development stage

Lack of clear-cut international specifications and guidelines
for latest optical communication based systems

Optical fibers are delicate and trained people are required to
handle optical fibers
35

Optical fiber splicing and protection is still expensive and add
to the cost of the optical networks
(3)Applications OfFibre Optics
o In communication: Optical fibres are increasingly
replacing wire transmission lines in communication
systems. Such optical fibre lines offer several
advantages over wire lines.
o Sensor applications: Sensor applications fall into
three categories, such as military, medical, and
industrial. A variety of sensors are used for measuring
strain,
temperature,
rotation,
displacement,
acceleration, pressure etc.
o Local area network: Optical fibres are widely used to
transmit voice, video and data within university
campuses and industries by connecting the various
computers in a local area network.
o Computer applications: For high speed data
transmission in a computer or in between such
systems we use OFC.
o Military applications: It is very difficult to tap from
optic fibre communication system. So it is used for
secret communication.
o Medical field: Almost all fields of medicine have
utilized OFC for diagnostics and surgical applications.
For example fibre optic endoscopes are used to see
the interior parts of human body. Here the accessible
areas such as stomach, intestine, heart and lungs can
be accessed using optical fibre and their images can
be transmitted.
Fibre Optic Endoscopy
The fibre optic endoscopy is the process of studying the interior
parts of
ourboby using optical fibres . The accessible areas such as stomach
, intestine , heart and lungs can be accessed using optical fibre and
their images can be transmitted .
36
Fibre optic endoscope
The fibre optic endoscope consists of a 10 mm diameter flexible
shaft of length nearly 0.6 to 1.8m depending upon the application
as shown in fig. The bottom portion of the shaft has a deflectable
section of length nearly 5 cm to 8.5 cm ,and a distal tip at the end.
The distal tip consists of the following arrangements.
1.An irrigation channel to wash the objective lens by pumping
water .
2.A coherent bundle of fibres to transmit the light from out
side to the interior parts of the body.
3.Another bundle of fibres to transmit the reflected light from
the interior parts of the body.
4.An operation channel to perform the task .
The viewing end of fibre optic endoscope consists of the following
arrangements.
1.
An eyepiece with camera arrangement and
focus control .
37
2.
The distal tip control, capable of rotating it
through 2000.
3.
Operation channel valve that controls the entry
of catheters ,electrode ,biopsyforceps and other flexible
devices .
4.
Valve control for application of water or air
through irrigation channel.
5.
A connection with umbilical tube for the
control of light transmission from the source.
Working
The flexible shaft is inserted in to the body and the distal tip is
positioned correctly so as to study the interior organ of our body.
If necessary the object lens is cleaned and the light is transmitted
through one bundle of fibre. The light reflected from the interior
parts of our body is transmitted by another bundle. The interior
part of our body is viewed using the eyepiece or it may be
photographed and then treatment is given suitably.
Advantages
1.Fibre optic endoscope is used for the examination of the
gastrointestinal tract.
2. It is used for the study and treatment of ulcers , cancers
,constructions,bleeding sites and so on .
3. The proportion of haemoglobin in blood is measured.
FIBRE OPTIC SENSOR
A sensor is a device that converts one form of a physical
quantity into another different measurable parameter, so that
it can be measured more accurately and conveniently.
The fibre optic sensors are used to measure pressure,
displacement , force ,electric field, magnetic field ,acceleration
,liquid level , flow rate etc.
The fibre optic sensors are divided into two types ,
(1)
active sensor (Intrinsic sensor)
38
passive sensor(Extrinsic sensors)
In active sensor , the light beam passing through the
fibre gets modulated
within the fibre .
In passive sensor, the fibre merely act as a transmitting
medium and the modulation take place out side the
fibre.

Physical parameters to be measured using fibre optic
sensots and their
modulation effects
Physical parameters to be Modulation effects in
measured
fibres
Temperature
Thermoluminescence
optical
Pressure
Piezoelectric effect
Electric field
Electro –optic effect
Magnetic field
Magneto – optic effect
Mechanical force
Density
Nuclear radiation
Stress birefringence
Triboluminescence
Radiation-induced luminescence
Advantages OfFibre Optic Sensors
1.
Optical fibres are fabricated from electrical
insulators and hence can be used in electrically hazardous
environment as the fibres create no arcing or spark hazard
at abrasions or short circuits.
2.
The cost of optical fibre cable and its
installation is easier than its metal counter part .The
transmission loss of fibre is small .
3.Opticalfibres are immune from electromagnetic interference and radiated signals
39
.Therefore the out put of optical fibre sensors
is free from noise of any type.
4.They do not conduct any electric current
and therefore optical fibre sensors are very
useful in explosive environment and in high
voltage equipments .
5.The cost of optical fibre cable and its
installation is easier than its metal counter
part.
Features of Fibre Optic Sensors
1) Highly reliable and secure due to immunity of the
sensed signal to electromagnetic interference
2) Safe in explosive and nuclear environments, Free
from risk of fire and sparks.
3) Most suitable for remot sensing and telemetry.
4) These are corrosion resistant and hence these can
be readily used in the chemical process industries
and medical field.
5) Fibre sensors have small size and weight.
6) Fibre sensors have high accuracy and sensitivity.
7) Their output can be interfaced with data
communication systems.
Fibre Optic Sensors
Laser Doppler Velocimeter (LDV)
Laser Doppler velocimeter is a fibre optic sensor
employed for the measurement of blood velocity. A glass
fibre is inserted through a catheter, i.e. a plastic tube in
to a blood vessel. He-Ne laser beam is sent through the
40
fibre, directly in to the blood. The light scattered back
from the flowing blood cells, is collected by the same
fibre and transmitted back. This scattered light is shifted
in frequency due to relative motion. This is called
Doppler effect. The frequency shift is proportional to the
velocity of blood cells. Using the LDV technique,
instantaneous arterial blood velocity measurement could
be made.
Fibre Bundles
If a number of fibres are put together ,it forms What is
known as a bundle.
Aligned Bundle(Coherent Bundle)
If the relative positions of the fibres at the input and out
put ends are the same the bundle is said to be an
aligned bundle.(Or an assembly of fibres in which the
coordinates of each fibre are the same at the two ends of
the fibre).If the fibre is illuminated at one of its end ,
then there will be a bright spot at the other end of the
fibre,thus a coherent bundle can transmit an image from
one end to other.
41
Majority of the applications involves the use of image
transferring coherent bundle. An important application of a
coherent bundle is in fibre optic endoscope where it can
put inside the human body and the interior of the body can
be viewed from outside .Fibre optic bundles are also used
for viewing part of the machine, which are otherwise
inaccessible .For illuminating the portion that is to be seen
, the bundle is enclosed in a sheath of fibres that carry
light to make the interior part visible. Each fibre transmits
light from a small portion of the object and there fore the
resolution is directly related to the packing density.
(b)Unaligned Bundle( Incoherent Bundle)
Incoherent bundles(which are cheaper than their
coherent
counterparts) are typically used for
delivering high optical power to a location . Coherent
bundles are required for imaging since the order of the
fibres (and pixels) is maintained.
A fibre optic bundle in which the spatial coordinates of
each fibre do not bear the same relation ship to each
other at the two ends of the bundle is called an
incoherent bundle.
It can’t be used to transmit a picture because the image
will be destroyed during transmission as the optical
42
fibres in the bundle change their relative positions, thus
scrambling the image as each fibre carries its picture
element to a relative image point different from the
corresponding relative object point. It is usually used
simply as a means of pushing light , with no concern for
spatial relationship among the fibres. It may be used for
carrying optical power or for optical communication
purpose. It can be also used as a CODER.The
transmitted image can be decoded at the out put end.
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Reference
1.Engineering Physics - A. Marikani
2.Engineering physics - S. Gopinath
3.Engineering Physics - M.N Avadhanulu
P.G Kshirsagar
4. Engineering Physics - TessyIssac