Download Electromagnetic Wave Ray Optics Wave Optics

Communication is the transfer of
information or message from point to
another.
COMMUNICATION SYSTEM
Two persons talking to each other constitute
the simplest communication system. The
person who speaks is the source, the person
listening is the receiver and the intervening
air is the communication link between them.
A communication system consists of three
basic components:
- Transmitter (source)
- Communication channel
(link – medium)
- Receiver
Nature/details of these components
depend on:
1. Nature of the signal/ message to be communicated.
2. Distance which separates the source and the receiver.
- Direct talking is possible over short distances –
sound waves attenuate fast.
- Long distance communication requires the
signal/message to be converted into an electrical signal/a
set of signals /electromagnetic waves.
- Long distance communication requires a link between the
source and the receiver
Message
signal
Communication
Transmitter
channel
Out put
Receiver
CUMMUNICATION SYSTEM
signal
Transmitter:
Transmits the message/signal over the
communication channel. Quite often the original signal is
not suitable for transmission over the communication
channel to the receiver. It requires to be modified to a form
suitable for transmission.
Communication Channel:
Provides a link between the transmitter and the
receiver. It can be a transmission line (telephone and
telegraphy), an optical fibre (optical communication) or free
space in which the signal is radiated in the form of
electromagnetic waves.
Receiver:
Reconstructs the original message/signal after
TRANSMITTER
- We communicate through a message or a signal.
- A transmitter transmits the message over the
communication channel to
the receiver.
As a rule, the message produced by the source is
not suitable for
transmission over the communication channel.
Accordingly, a suitable
transducer converts it into a time varying electrical
signal called the
message signal.
A transmitter, in its simplest form, is a setup which boosts
the power of message signal and feeds it into the
communication channel
Antenna
Amplifier
Antenna
Amplifier
Microphone
Transmitter
Receiver
Loud speaker
Antenna
Signal
Modulator
Amplifier
Antenna
Tunable
Amplifier
Transmitter
Demodulator
Receiver
Audio
Amplifier
To
Speaker
Basic constituents of a transmitter are:
1. Message signal
2. Modulation
3. Antenna
Message signal:
A single valued function of time that conveys the information.
Analog
Signals
Discrete or digital
Analog Signal
Is a continuous function of time, with the amplitude (instantaneous
value of the signal) being continuous.
Elementary idea of Analog
and Digital Communication
There are two different
modes of communication
(1)Analog communication and
(2) digital communication.
1) ANALOG COMMUNICATION

Time period;
amplitude I 4

Analog signal is a continuous
variation of current or which is the
representative of the signal or
message.
An alternating voltage which varies
sinusoidal is an analog signal.
During the transmission of analog
signal, there is a chance for
interference and
distortion and analog communication
is less reliable.
Discrete Signals
Discrete signals are discontinuous in time; they are defined only at
discrete times.
In case of discrete signals the independent variable (time) takes only
discrete values which are usually uniformly spaced. Consequently,
discrete-time signals are described as sequences of samples whose
amplitudes may take a continuum of values.
When each sample of a discrete-time signal is quantized I.e. its
amplitude is only allowed to take on a finite set of values (e.g. in a
binary representation low and high signals are designated as 0 and 1)
and then coded, the resulting signal is referred to as a digital signal.
signal which has two levels of current or Pulse
voltage. These two levels are high or low level.
values of voltage or current represented
by binary digits zero and one.
The binary signals are easy to generate and
process with a digital circuit.
level
0
Digital communication is more reliable.
Modulation
The low frequency electric signal (audio
frequency range] transmitted to very large
distances. This is because the energy as s
a low frequency signal will be very small
(energy <* frequency).
1 audio frequency electric signal to
transmits a large distances, a high
frequency electric signal is used. Hence we
superimpose the audio frequency
(modulating signal) on a high frequency
carrier wave. This process is modulation.
Need for modulation:
1) The energy associated with the audio
signal is very small (E = hv). Hence the
audio frequency signal cannot be
transmit such over long distances.
2) For the maximum efficiency of the
transmitting and receiving a height of the
antenna must be of the order of X/4.
3) If the low frequency signals are
transmitted from different stations get
mixed up. So the different signals
cannot be distinguished.
In its simplest form, the transmitter has following problems:
1. Size of the antenna or aerial
For transmitting a signal we need an antenna. It should have a size
comparable
to the wavelength of the electromagnetic wave representing the
signal ( at least
/4) so that the time variation of the signal is properly sensed by the
antenna.
For an electromagnetic wave of frequency 20 kHz, the wavelength  is
15 km.
Obviously such a long antenna is not possible. Therefore, direct
transmission of
such a signal is not possible. If the frequency of the signal is 1MHz,
the
corresponding wavelength is 300m and transmission of such a signal
is
possible. Therefore, there is a need of translating the information
contained in
l / or
 2radio-frequencies before
the original low frequency signal into high
transmission.
2. Effective power radiated by an antenna
The power radiated from a linear antenna 
For a good transmission we need high power hence there is need for
3. Mixing up of signals from different transmitters
Direct transmission of baseband signal leads to interference from
multiple transmitters. Thus multiple user friendly communication is
not possible. A possible solution is provided by employing
communication at high frequencies and then allotting a band of
frequencies to each user.
The above arguments suggest that there is a need for translating the
original signal ( low frequency) into a high frequency wave before
transmission such that the translated signal continues to possess the
information contained in the original signal. The high frequency wave
carrying the information is called the carrier wave. The process of
transformation is called Modulation.
Modulation
Transformation of the signal into a form suitable for transmission
through a given communication channel
Types of Modulation
The process of changing the
amplitude or frequency or phase
the carrier wave in accordance
with the intensity of the signal is
called modulation.
Accordingly the different
types of modulations are
1)Amplitude modulation (AM)
2) Frequency modulation (FM)
3) Phase Modulation (PM)
Amplitude modulation
The process of changing the
amplitude of high frequency
carrier wave in accordance with
the intensity of the signal is called
the amplitude modulation.













Amplitude change of the carrier = 2A - A = A
A.M. Wave
m™ *u
j i *•
r *. amplitude change of carrier
A .
i«™/ Now
the modulation factor, m = ——-———————*—————:— =— = 1 or 100%
amplitude of normal earner
A
For effective modulation the degree of modulation should never exceed 100%.
ur j i «.•
j j Signal wave amplitude
Em /T,
,.. . ^f
Modulation index = —2-———————*~——— = —sl (Em= amplitude of
carrier wave amplitude
Ec modulating wave , Ec= amplitude of carrier wave)
Modulation index =Emfl*"Emio, where Emax and E^ are the
Emax + Emin
and minimum amplitudes of the modulated wave,
»
width of modulation = 2fm, where fm is the modulating signal frequency
: In amplitude modulation, the base band signal amplitude is half the
er signal amplitude. Find the modulation index.
Frequency Modulation
The process of changing the
frequency of high frequency carrier
wave in accordance with the
intensity of the signal is called
frequency modulation.
Frequency Modulation:
- TV broadcast , VHF, UHF, SHF and EHF
broadcasts.
- Requires higher carrier wave
frequencies.
- Noise generated by atmospheric or man
made electric discharges does
no harm to intelligence.
- Higher S/N ratio, quality of broadcast
very good.
FM Radio – 88 to 108 MHz
VHF TV – 47 to 230 MHz
UHF TV – 470 to 960 MHz
Comparison of Amplitude and
Frequency modulation
AM
1)
 2)
 3)
 4)
 5)

FM
Noisy
Less noise
Less band width
More band width
low cost
costly
Low efficiency
High efficiency
Transmission and reception easier
Transmission and reception com
Phase Modulation

The process of changing the phase
of the carrier wave accordance with
the intensity of the signal is called
phase modulation

The wave form of the phase
modulated wave is similar to that
wave. Phase modulation uses a
smaller bandwidth than FM. Hence
information can be sent in a given
band width.
Pulse Modulation
Modulation of a carrier wave may be accomplished by short pulses.
Conventional telegraphy is the simplest example of this mode of modulation.
Pulse systems are based on sampling of the information signal at periodic
intervals, usually twice the maximum frequency present (2B). They transmit a
very short pulse of radio-frequency carrier for each sample, with pulse
characteristics varied in some manner proportional to the amplitude at the
sampling instant. A general name given to these modes of modulation is the
pulse modulation.
The common pulse systems employed in pulse modulation of analog signals
are:
(i) Pulse – amplitude modulation (PAM)
(ii) Pulse – position modulation (PPM)
(iii) Pulse – duration/width modulation (PDM/PWM)
(iv) Pulse – code modulation (PCM
DEMODULATION
process of recovering audio
frequency signal from the
modulated carrier wave is
known as demodulation or
detection.
In demodulation, the two main
operations involved are
(1) Rectification of the modulated
wave
(2) Elimination of radio frequency
components from the rectified wave.
PULSE MODULATION
Pulse modulation is used in digital
communication. In the pulse
modulation the carrier is in the form of
pulses
The different types of pulse
modulation are
1) Pulse amplitude modulation (PAM)
2) Pulse width modulation (PWM)
3) Pulse position modulation (PPM)
Pulse Amplitude Modulation (PAM)
The process of changing the
amplitude of the pulse in
accordance with the modulating
signal is called pulse amplitude
modulation.
Pulse Width Modulation (PWM) or
Pulse Duration Modulation (PDM)
The process of changing the
width of the pulse in accordance
with the intensity of the
modulating signal is called pulse
width modulation.
Pulse Position Modulation (PPM)
The process of changing the
position of pulse with the
modulating signal is called
pulse position modulation
Pulse Code Modulation (PCM)
The process of modulating the
carrier pulse in accordance
with the digitized modulating
signal is called pulse code
modulation.
The common modulation
techniques used for digital data are
1)
Amplitude shift keying (ASK)
2) Frequency shift Keying (FSK)
3) Phase shift keying (PSK)
DATA TRANSMISSION AND
RETRIEVAL
Before transmission the message signal
is modulated with a carrier wave
using a modulator.
Modulated wave is amplified using a
high frequency power amplifier and it
is supplied to the transmitting
antenna.
The modulating signal is received
using antenna at the receiving
station. The signal demodulated
and the message signal is retrieved.
Modem
Modem is the abbreviation for modulator
and demodulator. It’s used both at the
transmission and the receiving ends.
At the transmission modem accepts the
digital data and converts it into analog
signal modulation with a carrier signal.
At the receiving end, the carrier wave
demodulated to get the analog signal and the
equivalent digital signal produced.
Fax (Facsimile)
A fax is an electronic device for transmitting
graphical information through wire or
wireless.
The printed material is scanned and
converted into an electronic signal by
fax machine.
There are two types of communication
(1) communication through space
called,
space communication
(2) communication through wire called
line communication.
Space Communication
The communication utilizing the
physical space surrounding the
earth is called space
communication.
There are three modes of space
communication
(1) Ground wave propagation
(2) Space wave or troposphere wave
propagation
(3) Sky wave propagation or
ionosphere propagation.
1) Ground Wave Propagation
In ground wave propagation
the transmitted wave
propagate along the surface
of the earth.
2) Space Wave Propagation or
Troposphere Wave Propagation
In space wave propagation the
radio waves from the
transmitting antenna reaches the
receiving antenna either directly
or after reflecting the
troposphere or ground.
3) Sky wave Propagation or
Ionosphere Propagation
The atmosphere surrounding the earth at a height
of about 80km to 300km is called ionosphere.
This region is full of ions produced by ultraviolet
and high energy radiations coming from the sun.
This region has the property of reflecting e.m
waves coming from earth.
By using this reflecting property of ionosphere
electromagnetic waves coming from transmitter
can be sent to far away points as shown in fig.
10.11
IONOSPHERE
300
Height
n ~ 8 1011 (m3)
F2 Layer
n ~ 5 1011 (m3)
F1 Layer
200
(km)
100
60
n ~ 1011 (m3)
E Layer
n ~ 109 (m3)
D Layer
n ~108 (m3)
C Layer
- Surface wave propagation – used for medium wave band and TV
broadcasting which is done in the frequency rang 100 – 200 MHz. In this
transmission the reception is possible only when the receiver antenna
directly intercepts the signal. Thus, if the broadcast is made from a
tower of height h above the ground, due to the curvature of earth no
reception is possible beyond certain points.
d
2Rh
- To get larger coverage TV broadcast are made from tall antennas.
Further, the power transmitted also decreases nearly as the inverse
square of the distance hence the signal becomes weak as the distance
increases, which limits the range of transmission by this mode.
- The ground wave attenuation increases with frequency, so the
transmission via this mode is in practice possible only for frequencies
up to about 1500 kHz or wavelengths greater than 200m.
- Below 200m wavelength, the communication in AM band is via sky
wave.
- The diffraction of electromagnetic waves also affects their
propagation.The frequencies of the waves employed for radio and
television broadcast lie in the range 5 – 1000 MHz and the
corresponding wavelengths are in the range of 30 cm – 200 m. At these
wavelengths the diffraction effects are considerable and therefore these
waves lose their directional properties.
•Antenna
- An antenna is a vital component of any communication system. It is
employed both at the transmitting end as well as at the receiving end.
- An antenna is a length of conductor, its length is such that it acts as a
resonant circuit at the frequency of operation. l = /2.
- It acts as a conversion device. The first conversion takes place at the
transmitter where electrical energy is converted into electromagnetic
waves. The second conversion occurs at the receiving end where the
electromagnetic waves are transformed into electrical signal that is
applied to the input of the receiver.
Two types of antenna:
1. Dipole antenna – Length of dipole = /2 ; Omni directional.
2. Dish antenna – A spherical or parabolic dish is employed as a reflector
or collector. The resonant element is placed at the
focus. It is highly directional.
• Communication Channel
- In a communication system, the communication channel or the
transmission medium is the physical path between the transmitter and
receiver.
- Transmission media can be classified into two broad categories:
(a) Guided - Point – to – point communication
(i) Twisted pair
(ii) Coaxial cable
(iii) Optical fibre
(b) Unguided – Free space
- Characteristics and quality of transmission are determined both by the
nature of the signal as well as the medium.
- In guided media, the nature of the medium is more important; in
unguided media, the spectrum or the frequency band of the signal
transmitted by the transmitter is more important.
Characteristics of a Communication Channel : Band width,
Modulation and Data rate.
•Receiver
- Reconstructs the original message or data after its propagation through
the communication channel
- The process consisting of decoupling of the carrier wave and the
modulating signal is broadly termed as demodulation.
- The design of the receiver depends on the modulation process
employed in the transmitter.
- The antenna receives the modulated wave transmitted from the
transmitter, which is then amplified by a suitable amplifier and fed to the
demodulator or decoder.
- The demodulator or decoder extracts the original signal. The process of
demodulation provides a means of recovering the original signal from
the modulated wave. In effect, demodulation is reverse of modulation:
therefore, it depends on the modulation process used.
A communication satellite is an electronic device. It has
a receiver and transmitter which are together called
transponders.
The signal transmitted from the earth is up linked
receiver in the satellite.
The receiver amplifies the signal and download the
ground station by its transmitter. The uplink frequency
and the download frequency are different in order to
avoid confusion.
The satellite communication is possible by using a
geostationary satellite. Geostationary satellite appears to
be stationary relative to the earth. This is possible if the
satellite is at a height of about 36000 km from the
earth.
- The satellite communication first started in 1962 with the satellite
Telstar. The first commercially operated satellite was launched in 1965.
Since then numerous communication satellites have been launched for
the services of point-to-point telecommunication circuits, vide area TV
coverage, direct broadcasting by satellite, navigational communications
to ships and aircrafts.
- Most of the satellites orbit at heights greater than 600 km to minimize
atmospheric drag.
- The choice of orbit is of fundamental importance, as it determines the
transmission path loss and delay time, the earth coverage area and time
period the satellite is visible from a given area.
- The orbits of communication satellites are conventionally classified as
inclined elliptical, polar circular and geo-stationary.
- The geo-stationary orbit is the most widely used orbit for
communication satellites.
Polar Satellite
Polar satellite revolves round the earth
at a height of about 1000km from
earth. Their orbits are around the
earth over the poles in geographical
meridian with a speed such that it
reaches over a place when it is just
under the sun. Hence they are called
the polar satellites. Because of the
earth's spinning the polar satellite is
able to scan the entire surface of the
earth,
USE OF POLAR SATTELLITES
Remote sensing and
spy work.
Remote sensing
Remote sensing is one of the
applications of the satellite
communication.
A satellite can take photograph of an
object or some area the earth or
collect any other information and
transmit it back to an earth station.
This is known as remote sensing.
The process of taking photograph from
satellites is generally called
satellite imagery.
It is mainly used resource
survey, town and country planning and for
archeology etc.
Applications of remote sensing
(1) meteorology
(2) climate
(3) oceanography
(4) archeology
(5) geological surveys
(6) agriculture
(7) forestry
(8) predicting natural disaster etc.
LINE COMMUNICATION
Line communication is a mode
of communication from one
point to another through a
wire or cable. Etc…
There are three different types of line
communication.
1)Two wire transmission line
2) Co-axial cable
3) Optical Fibre
 In
two wire transmission line and
co-axial cable audio frequency
ultra high frequency (UHF) are
used.
But in optical fibres, optical
frequencies are used.
Two wire Transmission
Lines
The best example for two wire
transmission line is a telephone: figure
shows the transmission through two
wires.

The signal flowing through the wires produces
electric field E and magnetic field B as given in
the fig. (b).
Each portion of the transmission can be considered
as a small inductor, resistor and capacitor a.
fig. (c).
Hence each length of transmission line will have a
ch impedance. Because of this, for maximum
power transfer, impede receiving unit should
have an impedance equal to the ch; impedance.
This is also called load matching.
Twisted pair of wires
A twisted pair" of wires consists of two
insulated wires twisted around each
other to form a spiral shape as
shown in figure.
The electromagnetic interference from
other sources can be reduced by the
use of twisted wires. The twisted
wires are widely used in telephone
and computer networking
Co-axial cable
Mesh conductor
A co-axial cable consists of a single inner conductor
made of copper, surrounded by solid conductor or a
braded mesh of fine wires. An ms medium like
teflon or poly ethylene separates the two cables as
shown in figure. The outer conductor is normally
connected to the ground hence it provides an
electrical shield to the signal carried by the
conductor, reducing radiation loss. The outer
conductor is further covered with a polymer for
protection.
A co-axial cable has least interference
from other signals.
Inner conductor
Outer cover
Telephone I/inks
Telephone links can be established by using two line wire
waves, sky waves, microwaves, co-axial cables and optical
fiber individual telephone sets are connected to local telephone
exchange twisted pair of wires. The satellite linking with
telephones are don’t figure. The message from the telephone
goes to the earth satellite through local exchange, main
exchange, earth satellite station and transmitter. The message
from the geostationary satellite is n another microwave
receiver kept far away. The message from this receiver finally
goes to another telephone set through local exchange in figure.
Satellite In Geostationary
Coaxial Cable
Optical communication
Optical communication is made by using optical carrier
high frequency ranging from 1O12 Hz to 1016 Hz.
Hence very large bands and- large number of channels
are possible.
Optical communication possible in the past due to the
following reasons.
1. Non availability of carrier optical signal.
2. Non availability of a cable carrying optical signal.
The optical communication technology is developing
fast invention of optical fibers carrying optical signals,
semiconductor like LED, diode laser etc arid detectors
like photocjiode. The main advantages
Optical Communication
Transmitter
Input signal
Optical source
Modulation
Optical fibre cable
Receiver
Optical
detector
Demodulation
Output signal
Optical Fibre
- An optical fibre is a thin fibre of glass. Its diameter is about the same as
that of a human hair ~ 10 to 100 m.
- Light can be guided in such a fibre by launching it at one end, using an
intense and focused light source, and allowing it bounce down to the
other end by a series of reflections (total internal reflections) from the
sides.
- An optical fibre essentially consists of an inner cylinder of glass known
as the core, having a refractive index n1, and an outer cylinder of a
different glass, called the cladding having a refractive index n2, n =
n1 n2 ~ 10 3 .
- For use in a telecommunications system, many fibres are usually
incorporated into a cable structure for pulling into underground ducts.
The main advantages of optical
communication are

Wide bandwidth and a large number of channels.

Low transmission losses.

Cladding with refractive index,

Medium of refractive index, n

Core wi


refractive index, ni
Buffer coating
OPTICAL FIBRE

Point to point communication of light signal can be realized
by using optical fibers. For this a transparent medium like
glass, polymer or a dielectric material is required.

The principle of on optical fiber is based on . the
phenomenon of multiple total internal
reflections. The optical fiber can be considered
as a light pipe like a water pipe.

The important parts of an optical fibre are
1) core,
2) cladding
3)buffer.

1) Core: The core is the cylindrical central part
of the optical fibre, made up of glass or
polymer of refractive index, m. The diameter of
the core is 10-100

2) Cladding: The cladding is a covering on the
core, made of glass or plastic, refractive index,
n2 where n2 < n\. The difference of refractive
indices m, very small of the order of 0.001.

3) Buffer: Buffer is a plastic coating made on
the cladding for providing si and strength.
Principle of an optical fibre

The principle of an optical fibre is based on the
phenomenon of multiple total internal
reflections. The total internal reflection
at core-clad< boundary will take place when the
light travels from a denser to a rarer medium at
an angle of incidence greater than the critical
angle. The dimci of the core is so small that the
light entering into the core will have an incident
angle always greater than the critical angle. The
multiple reflections of Ugt1 opposite boundaries
will continue and the light passes through the
core light pipe.
There are two types of optical fibres.
1.
Step index fibre:
In step index fibre, there is an
abrupt cl refractive index at corecladding interface.
2. Graded index fibre:
In Graded index fibre, the
refractive inde fibre decreases
radially outwards. There is no abrupt
change in n index.
Advantages of fibre communication
 1.
High bandwidth. Hence more
information can be sent
 2. High signal security
 3. Size and weight are smaller
compared to metal conductors
 4. Noise free
 5. Negligibly small transmission loss
LASER

Laser is the abbreviated form for light
amplification by stimulated emission
of radiation. Laser is a highly coherent
light.
 Principle of production of Laser
 E3
 Short lived state
 The atoms in the ground energy state are
irradiated to higher energy state.
Ground state

This is achieved by optical pumping. The life
span

the excited state is in the order of 10-8 s.
Therefore the atoms jump down the metastable
state (where life span is 10-3 s).
population inversion place
when the number of atoms in the
metastable state is larger the atoms in the
ground state.
When a photon of energy hv, equal to the
difference in the energy of metastable state
and the ground state is incident, all the
atoms in metastable state come
simultaneously to the ground state emitting
photons. This is called stimulated
emission. All the emitted photons are in
phase with the incident photon and the
emitted beam of light is called laser.
 Different types of lasers are Ruby laser
(694 nm), CO2 laser (10600 mnjft He - Ne
laser (5430/633 mn) and Argon laser
(488/514 nm)

ELEMENTARY PRINCIPLE OF LIGHT MODULATION
 Light can be modulated with the information signal. This is
achieved‘ by changing the intensity, frequency or phase of the lightsignal in accordant with the intensity of the information signal.
 Light modulation can be done in two ways.
1) Direct modulation 2) Indirect or External modulation ;
 The direct modulation can be achieved by modulating the direct
curiw supplying the light source. This is obtained by changing the
laser biasing current above and below the threshold value to make
the laser on and^on. This causes a change in frequency. Thus Direct
modulation can be used to modulate the frequency of light wave.

An indirect modulator consists of a material like lithium (UNbO3) crystal
whose refractive index can be controlled with the help applied Electric field.
This control on the refractive index brings the con phase of the signal. When
light passes through the applied electric suffers cumulative phase change. The
desired modulation can be ach| nrnnprlv hv switrhinor the electric field on
and off. The external