UNIT-III-OPTICAL COMMUNICATION

SUBJECT:ADVANCED COMMUNICATION SYSTEM
UNIT-III-OPTICAL COMMUNICATION
PRESENTED BY
M.GOWRISANKAR.,M.E.,MISTE.,
ASSISTANT PROFESSOR

 An optical communication system uses a
transmitter, which encodes a message into an
optical signal, a channel, which carries the
signal to its destination, and a receiver, which
reproduces the message from the received
optical signal..
 The fiber which are used for optical
communication are wave guides made of
transparent dielectrics.
 Its function is to guide visible and infrared light
over long distances.

 The thin glass center of the
fiber where the light travels is
called the “core”.
 The outer optical material
surrounding the core that
reflects the light back into the
core is called the “cladding”.
 In order to protect the optical
surface from moisture and
damage, it is coated with a
layer of buffer coating.

 Core – central tube of very thin size made up of
optically transparent dielectric medium and carries
the light form transmitter to receiver. The core
diameter can vary from about 5um to 100 um.
 Cladding – outer optical material surrounding the
core having reflecting index lower than core. It helps
to keep the light within the core throughout the
phenomena of total internal reflection.
 Buffer Coating – plastic coating that protects
the fiber made of silicon rubber. The typical diameter
of fiber after coating is 250-300 um.

 Transmitter: Converts and transmits an electronic
signal into a light signal. The most commonly used
transmitters are semiconductor devices, such as
light-emitting diodes (LEDs) and laser diodes.
 Receivers: Typically consist of a photo-detector,
which converts light into electricity using the
photoelectric effect. The photo detector is typically
a semiconductor-based photodiode.
 Optical Fiber: Consists of a core, cladding and a
buffer through which the cladding guides the light
along the core by using total internal reflection.

Information
source
Electrical
source Optical
source
Optical fiber
cable
Optical
detector
Electrical
receive
Destination

 Information source- it provides an electrical
signal to a transmitter comprising an electrical
stage.
 Electrical transmitter- It drives an optical
source to give an modulation of the light wave
carrier.
 Optical source- It provides the electrical-
optical conversion .It may be a semiconductor
laser or an LED.

• Optical cable: It serves as transmission medium.
• Optical detector: It is responsible for optical to
electrical conversion of data and hence responsible
for demodulation of the optical carrier. It may be a
photodiodes, phototransistor, and
photoconductors.
• Electrical receiver: It is used for electrical
interfacing at the receiver end of the optical link
and to perform the signal processing electrically.
• Destination: It is the final point at which we
receive the information in the form of electrical
signal.

1) The life of fiber is longer than copper wire
2) Handling and installation costs of optical fiber is
very nominal
3) It is unaffected with electromagnetic interference
4) Attenuation in optical fiber is lower than coaxial
cable or twisted pair.
5) There is no necessity of additional equipment for
protecting against grounding and voltage problems.
6) As it does not radiates energy any antenna or
detector cannot detects it hence provides signal
security

1) Highly skilled staff would be required for
maintenance
2) Only point to point working is possible on
optical fiber
3) Precise and costly instruments would be
required
4) Costly if under utilized.
5) Accept unipolar codes only.
6) Jointing of fiber and splicing is also time
consuming.

Total Internal Reflection
• When a ray of light travels from a denser to a
rarer medium such that the angle of incidence
is greater than the critical angle, the ray reflects
back into the same medium this phenomena is
called total internal reflection.
• In the optical fiber the rays undergo repeated
total number of reflections until it emerges out
of the other end of the fiber, even if the fiber is
bent.

 There are two types of optical fiber:-
 (i) Step-index optical fiber
 (ii) Graded-index optical fiber

 The refractive index of core is constant
 The refractive index of cladding is also constant
 The light rays propagate through it in the form of
meridiognal rays which cross the fiber axis during
every reflection at the core cladding boundary.

 In this type of fiber core has a non uniform refractive
index that gradually decrease from the centre
towards the core cladding interface.
 The cladding has a uniform refractive index.
 The light rays propagate through it in the form of
skew rays or helical rays. They do not cross the fiber
axis at any time.

 Optical fiber is classified into two categories
based on :-
1) The number of modes, and
2) The refractive index

On the basis of number of
modes:-
on the basis of number of modes of propagation the
optical fiber are classified into two types:
(i) Single mode fiber (SMF) and
(ii) Multi-mode fiber (MMF)
• Single-mode fibers – in single mode fiber only one
mode can propagate through the fiber. This type of
fiber has small core diameter(5um) and high
cladding diameter(70um) and the difference between
the refractive index of core and cladding is very
small. There is no dispersion i.e. no degradation of
signal during travelling through the fiber.
• The light is passed through the single mode fiber

 Multi-mode fiber :-
 Multi mode fiber allows a large number of modes for
the light ray travelling through it.
 The core diameter is (40um) and that of cladding
is(70um)
 The relative refractive index difference is also larger
than single mode fiber.
 There is signal degradation due to multimode
dispersion.
 They are not suitable for long distance
communication due to large dispersion and
attenuation of the signal.

This mode of optical fiber are
used to transmit one signal per
fiber (used in telephone and
cable TV). They have small
cores(9 microns in diameter)
and transmit infra-red light
from laser.
Single-mode fiber’s smaller
core (<10 micrometers)
necessitates more expensive
components and
interconnection methods, but
allows much longer, higher-
performance links.

This type of optical fiber are used to
transmit many signals per fiber (used
in computer networks). They have
larger cores(62.5 microns in
diameter) and transmit infra-red
light from LED.
However, multi-mode fiber
introduces multi-mode distortion
which often limits the bandwidths
and length of the link. Furthermore,
because of its higher dopant content,
multimode fiber is some what more
expensive.

Refraction
Refraction is the changing direction of light
when it goes into a material of different
density

• Attenuation is the loss of the optical power.
• Attenuation in optical fiber take place due to
elements like coupler, splices, connector and
fiber itself.
• A fiber lower attenuation will allow more power
to reach a receiver than with a higher
attenuation.
• Attenuation may be categorised as –
(i) Intrinsic
(ii) Extrinsic

 Fig. shows the factor affecting the attenuation
in fiber-
Attenuation
Intrinsic
Absorption Scattering
Extrinsic
Macrobending
Microbending

Losses in Optical Fiber Cables:
 Absorption due impurities in the fiber material
 Rayleigh scattering due microscopic irregularities in the
Fiber
 Radiation losses caused by kinks and bends Of fiber
 Coupling losses due to misalignment and imperfect surface
finish

Loss(dB/km)
1
0
0.7 0.8
Wavelength (mm)
0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7
2
3
4
5
6
Peaks caused
by OH- ions
Infrared
absorption
Rayleigh scattering
& ultraviolet
absorption

 Attenuation means loss of light energy as the
light pulse travels from one end of the cable to
the other.
 It is also called as signal loss or fiber loss.
 It also decides the the number of repeaters
required between transmitter and receiver.
 Attenuation is directly proportional to the
length of the cable.

 Attenuation is defined as the ratio of optical
output power to the input power in the fiber of
length L.
The various losses in the cable are due to
 Absorption
 Scattering
 Bending
 Dispersion

 The loss which exists when an optical fiber undergoes
bending is called bending losses.
 There are two types of bending
i) Macroscopic bending
Bending in which complete fiber undergoes bends
which causes certain modes not to be reflected and
therefore causes loss to the cladding.
ii) Microscopic Bending
Either the core or cladding undergoes slight bends at its
surface. It causes light to be reflected at angles when there
is no further reflection.

ISO 9001 : 2008 certified
Macroscopic Bending
Microscopic Bending

Absorption of light energy due to heating of ion
impurities results in dimming of light at the end of
the fiber.
Two types:
1. Intrinsic Absorption
2. Extrinsic Absorption

Intrinsic Absorption:
 Caused by the interaction with one or more
components of the glass
 Occurs when photon interacts with an electron in the
valence band & excites it to a higher energy level near
the UV region.
Extrinsic Absorption:
 Also called impurity absorption.
 Results from the presence of transition metal ions like
iron, chromium, cobalt, copper & from OH ions i.e.
from water.

 It occurs due to microscopic variations in the material
density, compositional fluctuations, structural in
homogeneities and manufacturing defects.
 Linear Scattering
 Rayleigh Scattering losses
 Mie Scattering Losses
 Waveguide Scattering Losses
 Non-linear Scattering
 Stimulated Brillouin Scattering(SBS)
 Stimulated Raman Scattering(SRS)

a) Rayleigh Scattering Losses:
 These losses are due to microscopic variation in the
material of the fiber.
 Unequal distribution of molecular densities or
atomic densities leads to Rayleigh Scattering losses
 Glass is made up of several acids like SiO2,
P2O5,etc. compositions, fluctuations can occur
because of these several oxides which rise to
Rayleigh scattering losses

b) Mie Scattering Losses:
 These losses results from the compositional
fluctuations & structural inhomogenerics &
defects created during fiber fabrications, causes
the light to scatter outside the fiber.
c) Waveguide Scattering Losses:
 It is a result of variation in the core diameter,
imperfections of the core cladding interface,
change in RI of either core or cladding.

a) SBS Scattering:
 Stimulated Brillouin Scattering(SBS) may be
regarded as the modulation of light through
thermal molecular vibrations within the fiber.
 Pb =4.4x10-3d2λ2α dB v watts
where, λ= operating wavelength μm
d= fiber core diameter μm
v = source bandwidth in GHz

b) SRS Scattering:
 Stimulated Raman Scattering is similar to SBS
except that high frequency optical phonon rather
than acoustic phonon is generated in scattering
processes.
 Pb =5.9x10-2d2λα dB watts
Phonon:
Collective excitation in a periodic arrangement of
atoms or molecules in solid.

 As an optical signal travels along the fiber, it
becomes increasingly distorted.
 This distortion is a sequence of intermodal and
intramodal dispersion.
 Two types:
1. Intermodal Dispersion
2. Intramodal Dispersion

Intermodal Dispersion:
 Pulse broadening due to intermodal dispersion
results from the propagation delay differences
between modes within a multimode fiber.
Intramodal Dispersion:
 It is the pulse spreading that occurs within a
single mode.
 Material Dispersion
 Waveguide Dispersion

1) Material Dispersion:
 Also known as spectral dispersion or chromatic
dispersion.
 Results because of variation due to Refractive Index of
core as a function of wavelength, because of which
pulse spreading occurs even when different
wavelengths follow the same path.
2) Waveguide Dispersion:
 Whenever any optical signal is passed through the
optical fiber, practically 80% of optical power is
confined to core & rest 20% optical power into
cladding.

 LED
 Semiconductor LASER

LED (Light emitting diode):
 Made from material such as AIGaAs and GaAsP
 Light is emitted when holes and electrons recombine
ILD (Injection Laser diode):
 Similar in construction as LED but ends are highly
polished to reflect photons back and fourth

 Basic LED operation:
The normally empty conduction band of semiconductors populated by
electron injected into it by the forward current through the junction,
and the light is generated with electrons recombine with holes. This the
mechanism by which light is emitted from LED.

 For fiber-optics, the LED should have a high radiance
(light intensity), fast response time and a high quantum
efficiency.
LED Structures:
 Planar LED
 Dome LED
 Surface emitter LED
 Edge emitter LED

Planar LED
 The planar LED is the simplest of the
structures that are available and is
fabricated by either liquid- or vapor-
phase epitaxial processes over the
whole surface of a GaAs substrate.
 This involves a p-type diffusion into
the n-type substrate in order to
create the junction illustrated in
Figure.
 Forward current flow through the
junction gives Lambertian
spontaneous emission and the device
emits light from all surfaces.
 However, only a limited amount of
light escapes the structure due to
total internal reflection and therefore
the radiance is low.
Prof. Snehal Laddha

Light Amplification by ‘Stimulated Emission' and
Radiation (L A S E R)
 Coherent light (stimulated emission)
 Narrow beam width (very focused beam)
 High output power (amplification)
 Narrow line width because only few wavelength will
experience a positive feedback and get amplified (optical
filtering)

 Absorption: An atom in the ground state might
absorb a photon emitted by another atom, thus making a
transition to an excited state.
 Spontaneous Emission: Random emission of
a photon, which enables the atom to relax to the ground
state.
 Stimulated Emission: An atom in an excited
state might be stimulated to emit a photon by another
incident photon.

PIN diode:
 Photons are absorbed in the intrinsic layer
 Sufficient energy is added to generate carriers in the depletion
layer for current to flow through the device

APD (Avalanche photo diode):
 Photo generated electrons are accelerated by relatively large
reverse voltage and collide with other atoms to produce more free
electrons
 Avalanche multiplication effect makes APD more sensitive but also
more noisy than PIN diode.

 In telecommunication field
 In space applications
 Broadband applications
 Computer applications industrial applications
 Mining applications
 In medical applications
 In military applications etc.
• Optical fiber have wider range of application in
almost all field, some are been specified below

• Optical fiber have wider range of application in almost all field, i.e. in medical,
electronics, military etc .some are been specified below
• Medical
• Military
• Electronics
IBM
microprocessors

There are two optical fibres
One for light, to illuminate the
inside of the patient
One for a camera to send the
images back to the doctor.
Key hole surgery

UNIT-III-OPTICAL COMMUNICATION

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UNIT-III-OPTICAL COMMUNICATION