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Transcript
Optical Fiber Communications
Week 1
Bahria University
1
Course Schedule
•
•
•
•
Lectures ( Hand outs)
Quizzes
Assignments
Research paper/ Case Study
Bahria University
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Marks Distribution
•
•
•
•
Quizzes
Assignments/ Research paper
Mid Term Exam
Final Examination
Bahria University
10 marks
20 marks
20 marks
50 marks
3
Course Contents
1.
2.
3.
4.
5.
6.
7.
8.
9.
Introduction to optical fiber communications
Optical fiber waveguides
Transmission characteristics of optical fibers
Fiber Fabrication
Optical Fiber Connections, joints and couplers
Light sources for optical fibers
Optical Detectors
Modulation
Optical Networking
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Recommended Text Books
1. John M. Senior, “ Optical fiber
communications” Second edition.
2. Subir Kumar, “ Optical fibers and fiber optic
communication systems”.
3. Gerd Keiser, “Optical Fiber Communication
3rd Edition”, McGraw-Hill
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Introduction
• Communication may be broadly defined as the
transfer of information from one point to
another.
• The information transfer is frequently achieved
by superimposing or modulating the
information onto an electromagnetic wave
which acts as a carrier for the information
signal.
• Communication may also be achieved using
an electromagnetic carrier which is selected from
the optical range of frequencies
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Historical Development
• In, 1880 Alexander Graham Bell reported the
transmission of speech using a light beam,
Photophone proposed by Bell just four years
after the invention of the telephone
modulated sunlight with a diaphragm giving
speech transmission over a distance of 200 m.
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Conti….
• But using electromagnetic waves as a carrier lies in
Optical region have a lot of limitations and problems
regarding light transmission in the atmosphere is
restricted to line of sight and is severely affected by
disturbances such as rain, snow, fog, dust and
atmospheric turbulence.
• Nevertheless lower frequency and hence longer
wavelength electromagnetic waves (i.e. radio and
microwave) proved suitable carriers for information
transfer in the atmosphere, being far less affected by
these atmospheric conditions.
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Invention of LASER
• A renewed interest in optical communication was
stimulated in the early 1960s with the invention
of the laser.
• Although the use of the laser for free space
optical communication proved somewhat
limited, the invention of the laser instigated a
tremendous research effort into the study of
optical components to achieve reliable
information transfer using a lightwave carrier.
Bahria University
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Invention of Optical fibers
• The proposals for optical communication via
dielectric waveguides or optical fibers fabricated
from glass to avoid degradation of the optical
signal by the atmosphere were made almost
simultaneously in 1966 by Kao and Hockham.
• Initially the optical fibers exhibited very high
attenuation (i.e. 1000 dB/ km) and were
therefore not comparable with the coaxial cables
they were to replace (i.e. 5 to 10 dB/ km), but
within the space of 10 years optical fiber losses
were reduced to below 5 dB/ km.
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General Communication System
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Digital Optical Fiber link
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Advantages of optical fiber
communication
Enormous potential bandwidth:
The optical carrier frequency range yields the far
greater potential transmission bandwidth than
metallic cable system.
Small size and less weight:
• Optical fibers have very small diameters which
are often no greater than the diameter of a
human hair. Hence, even when such fibers are
covered with protective coatings they are far
smaller and much lighter than corresponding
copper cables.
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Conti….
• Electrical Isolation:
• Optical fibers which are fabricated from glass, or
sometimes a plastic polymer, are electrical insulators
and therefore, unlike their metallic counterparts,
they do not exhibit earth loop and interface
problems.
• Signal security:
• Signal sniffing cannot be carried out until physical
insertion of detecting instrument, which bit difficult.
• Widely applicable in military, banking and other
secure networking.
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Conti…
• Low transmission loss:
• Efforts are made in the field of fiber
development……… 0.2dB/KM,
• GLASS FIBER CABLE is efficient then plastic (
plastic polymers).
• Ruggedness and flexibility:
– Flexibility in uses, manufactured for high
tensile strength.
– Can be bent till certain dia w/o prominent loss.
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Optical fiber Waveguides
• Topics in this chapter
1. Introduction
2. Ray theory transmission
3. Electromagnetic mode theory for optical
propagation
4. Cylindrical Fibers
5. Single mode fibers
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Introduction
• A transparent dielectric rod, typically of silica
glass with a refractive index of around 1.5,
surrounded by air, proved to be an impractical
waveguide due to its unsupported structure
(especially when very thin waveguides were
considered in order to limit the number of
optical modes propagated) and the excessive
losses at any discontinuities of the glass–air
interface.
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Refractive Index
• n= speed of light in air/speed of light in substance
• Naturally the refractive index of the air is 1 and
refractive index of the water is approximately 1.3
where as that of glass is 1.5.
• To get better understanding, consider a piece of
glass with refractive index 1.5 that means if
light will travel through 1.5 ft of air but during
the same time light will travel 1 ft through the
glass, the glass slows down the speed of the
light wave.
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Clad- Core Fiber
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Conti…
• The invention of the clad waveguide structure
led to the first serious proposals by Kao and
Hockham in 1966, to utilize optical fibers as a
communications medium, even though they had
losses in excess of 1000 dB/ km.
• These proposals stimulated tremendous efforts
to reduce the attenuation by purification of the
materials.
• This has resulted in improved conventional glass
refining techniques giving fibers with losses of
around 4.2 dB/ km
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Ray Theory transmission
• Total Internal Reflection:
• A ray of light travels more slowly in an
optically dense medium than in one that is
less dense, and the refractive index gives a
measure of this effect.
• When a ray is incident on the interface
between two dielectrics of differing refractive
indices (e.g. glass–air), refraction occurs.
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Conti…
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Conti….
• It may be observed that the ray approaching the
interface is propagating in a dielectric of
refractive index n1 and is at an angle φ1 to the
normal at the surface of the interface.
• If the dielectric on the other side of the interface
has a refractive index n2 which is less than n1,
then the refraction is such that the ray path in this
lower index medium is at an angle φ2 to the
normal, where φ2 is greater than φ1.
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Conti….
• The angles of incidence φ1 and refraction φ2
are related to each other and to the refractive
indices of the dielectrics by Snell’s law of
refraction which states that:
n1 sin φ1 = n2 sin φ2
Or
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Critical Angle
• As n1 is greater than n2,the angle of refraction
is always greater than the angle of incidence.
• Thus when the angle of refraction is 90° and the
refracted ray emerges parallel to the interface
between the dielectrics, the angle of incidence
must be less than 90°. This is the limiting case of
refraction and the angle of incidence is now
known as the critical angle φc,
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Propagation of light wave through
Optical fiber
• Any light wave which travels along the core and
meets the cladding at the critical angle of
incidence will be totally internally reflected.
Therefore light wave is propagated along the fiber
core by a series of total internal reflections.
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Acceptance angel
• Any light wave which is meeting the core
cladding interface at or above the critical
value will also be reflected and hence will
propagate along the core.
• So, any light wave impinging on the core
within its maximum external incident angel is
coupled into the fiber and will propagate, this
angel is called acceptance angel.
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Conti….
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NA( Numerical Aperture) and Half
acceptance angle Calculation
On White Board
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Conti…
• It has been observed that numerical apertures
for the fibers used in short distance
communications are in the range of 0.4 to 0.5
where as for the long distance
communications numerical aperture are in the
range of 0.1 to 0.3.
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Example
• A silica optical fiber with a core diameter
large enough to be considered by ray theory
analysis has a core refractive index of 1.50
and a cladding refractive index of 1.47.
• Determine:
• (a) the critical angle at the core–cladding
interface;
• (b) the NA for the fiber;
• (c) the acceptance angle in air for the fiber.
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