optical communication

1. Topic for the class: Overview of Optical Fiber
Communication
Unit I:Overview of Optical Fiber Communication
Date & Time :
Mr. Durga Prasad Tumula
Assistant Professor
Department of EECE
GITAM Institute of Technology (GIT)
Visakhapatnam – 530045
Email: dtumula@gitam.edu
29th july 2020 Dept of EECE EEC441 Optical communications
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Dept of EECE EEC441 Optical
Communications

2. Course objectives
• To develop a fundamental
understanding of Optical
communication systems.
• Show the ability of Linear Elements
of an Optical Fiber transmission and
reception link.
• Describe the types and advantages
of optical fibers in various
generations.
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3. Learning Outcomes
Upon successful completion of the
course, students will be able to
• list the advantages of optical fiber
channels over other wired and
wireless channels
• describe the ray transmission and
other physical effects involed in
optical fiber transmission
• define the physical parameters of
single mode fibers and their
construction
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4. Syllabus
• The general system.
• Advantages of optical fiber
communications.
• Optical fiber wave guides
introduction.
• Ray theory transmission, total
internal reflection, acceptance angle,
numerical aperture, skew rays.
• Cylindrical fibers- modes, V number,
mode coupling, step index fibers,
graded index fibers.
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5. The general system
• An optical fiber communication system is
similar in basic concept to any type of
communication system.
• A block schematic of a general
communication system is shown in Figure (a),
the function of which is to convey the signal
from the information source over the
transmission medium to the destination.
• The communication system therefore
consists of a transmitter or modulator linked
to the information source, the transmission
medium, and a receiver or demodulator at
the destination point.
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6. (a) The general communication system. (b) The optical fiber communication system
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7. • In optical fiber communications the system
information source provides an electrical signal
to a transmitter comprising an electrical stage
which drives an optical source to give modulation
of the light wave carrier.
• The optical source which provides the electrical–
optical conversion may be either a
semiconductor laser or light-emitting diode
(LED).
• The transmission medium consists of an optical
fiber cable and the receiver consists of an optical
detector which drives a further electrical stage
and hence provides demodulation of the optical
carrier. Photodiodes (p–n, p–i–n or avalanche)
and, in some instances, phototransistors and
photoconductors are utilized for the detection of
the optical signal and the optical–electrical
conversion.
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8. Advantages of Optical
Fiber Cables
1. Wider bandwidth and greater information
capacity.
Optical fibers have greater information capacity
than metallic cables because of the inherently
wider bandwidths available with optical
frequencies. Optical fibers are available with
bandwidths up to several thousand gigahertz.
2. Immunity to crosstalk.
Optical fiber cables are immune to crosstalk
because glass and plastic fibers are nonconductors
of electrical current. Therefore, fiber cables are
not surrounded by a changing magnetic field,
which is the primary cause of crosstalk between
metallic conductors located physically close to
each other.
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9. 3. Immunity to static interference.
Because optical fiber cables are nonconductors of
electrical current, they are immune to static noise due
to electromagnetic interference (EMI) caused by
lightning, electric motors, relays, fluorescent lights, and
other electrical disturbances.
4. Environmental immunity.
Optical fiber cables are more resistant to environmental
extremes (including weather variations) than metallic
cables. Optical cables also operate over a wider
temperature range and are less affected by corrosive
liquids and gases.
5. Safety and convenience.
Optical fiber cables are safer and easier to install and
maintain than metallic cables. Because glass and plastic
fibers are nonconductors, there are no electrical
currents or voltages associated with them.
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10. 6. Lower transmission loss.
Optical fibers have considerably less signal loss than their
metallic counterparts. Optical fibers are currently being
manufactured with as little as a few-tenths-of-a-decibel loss
per kilometer.
7. Security.
Optical fiber cables are more secure than metallic cables. It is
virtually impossible to tap into a fiber cable without the user’s
knowledge, and optical cables cannot be detected with metal
detectors unless they are reinforced with steel for strength.
8. Durability and reliability.
Optical fiber cables last longer and are more reliable than
metallic facilities because fiber cables have a higher tolerance
to changes in environmental conditions and are immune to
corrosive materials.
9. Economics.
The cost of optical fiber cables is approximately the same as
metallic cables. Fiber cables have less loss and require fewer
repeaters, which equates to lower installation and overall
system costs and improved reliability.
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11. Drawbacks
1. Interfacing costs.
2. Strength.
3. Remote electrical power.
4.Optical fiber cables are more
susceptible to losses introduced by
bending the cable.
5. Specialized tools, equipment, and
training.
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12. Optical fiber waveguides
• The transmission of light via a dielectric waveguide
structure was first proposed and investigated at the
beginning of the twentieth century.
• This structure shows a transparent core with a
refractive index n1 surrounded by a transparent
cladding of slightly lower refractive index n2.
• The cladding supports the waveguide structure and
reducing the radiation loss into the surrounding air.
• In essence, the light energy travels in both the core
and the cladding allowing the associated fields to
decay to a negligible value at the cladding–air
interface.
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13. Waveguide structure
Optical fiber waveguide showing the core of refractive index n1, surrounded
by the cladding of slightly lower refractive index n2
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.
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14. Ray theory of
transmission
• To consider the propagation of light within an optical fiber utilizing the ray
theory model it is necessary to take account of the refractive index of the
dielectric medium.
• The refractive index of a medium is defined as the ratio of the velocity of
light in a vacuum to the velocity of light in the medium.
• 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, as illustrated in Figure
2.2(a).
• 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.
• 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
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15. 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.
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16. Total Internal Reflection
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17. Snell’s Law of Refraction
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18. Transmission of light in
Optical Fiber
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19. Acceptance Angle
• Maximum angle to the axis at
which light may enter the fiber in
order to be propagated
• For a fiber with regular cross
section, an incident meridional ray
greater than the critical angle will
continue to be reflected and
transmitted inside the fiber
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20. Acceptance Angle
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21. Numerical Aperture
Provides the relationship between
• Core
• Cladding
• Air
The acceptance angle for an optical fiber was
defined in the preceding section. However, it is
possible to continue the ray theory analysis to
obtain a relationship between the acceptance angle
and the refractive indices of the three media
involved, namely the core, cladding and air. This
leads to the definition of a more generally used
term, the numerical aperture of the fiber.
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23. Numerical Aperture
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24. Numerical Aperture
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25. Problem
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26. Skew rays
• In the preceding sections we have considered the
propagation of meridional rays in the optical waveguide.
• However, another category of ray exists which is
transmitted without passing through the fiber axis. These
rays, which greatly outnumber the meridional rays, follow
a helical path through the fiber, as illustrated in Figure
2.6, and are called skew rays.
• It is not easy to visualize the skew ray paths in two
dimensions, but it may be observed from Figure 2.6(b)
that the helical path traced through the fiber gives a
change in direction of 2γ at each reflection, where γ is
the angle between the projection of the ray in two
dimensions and the radius of the fiber core at the point
of reflection. Hence, unlike meridional rays, the point of
emergence of skew rays from the fiber in air will depend
upon the number of reflections they undergo rather than
the input conditions to the fiber.
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27. Skew ray path and cross
sectional view
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28. Skew ray acceptance
angle
where γ is the angle between the projection of the ray in two dimensions
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30. After 20-30 minutes from start of class,
Time for FLASH QUIZ (two or three simple questions)
counted only for attendance– not more than 2 minutes
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FLASH QUIZ

31. Cylindrical fibers
• In common with the planar guide , TE (where Ez = 0) and TM
(where Hz = 0) modes are obtained within the dielectric
cylinder.
• The cylindrical waveguide, however, is bounded in two
dimensions rather than one. Thus two integers, l and m, are
necessary in order to specify the modes, in contrast to the
single integer (m) required for the planar guide.
• For the cylindrical waveguide we therefore refer to TElm and
TMlm modes. These modes correspond to meridional rays
traveling within the fiber.
• However, hybrid modes where Ez and Hz are nonzero also
occur within the cylindrical waveguide.
• These modes, which result from skew ray propagation within
the fiber, are designated HElm and EHlm depending upon
whether the components of H or E make the larger
contribution to the transverse (to the fiber axis) field.
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32. Modes
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33. Modes in cylindrical
fibers
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34. V number
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35. Modes Vs V Number
• If V ≤ 2.405, then the fibre is single mode fibre (SMF)
• If V > 2.405, then the fibre is multimode fibre (MMF)
• Number of Modes traveling in Fibre:
The total number of mode traveling in a fibre depends on the V
– Number and is related as:
• For Step Index Fibre:
• For Graded Index Fibre:
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36. Mode coupling
• Mode is the path for light rays through an optical fiber.
• If an optic fiber supports only one mode, it is called as single
mode fiber. Multimode fiber supports more than one mode.
• The electric field distribution of various modes yields similar
distributions of light intensity within the fiber core. These
patterns are called mode patterns.
• Propagation characteristics of a fiber are very sensitive to
deviations of the fiber axis from straightness, variations in the
core diameter, irregularities in the core-cladding interface and
refractive index variations.
• Individual modes do not normally propagate throughout the
length of the fiber.
• This result in a mode conversion which is known as mode
coupling. Coupled mode equations obtained from Maxwell’s
equations can be used for the analysis of mode coupling.
• Mode coupling affects the transmission properties of fiber
which is a serious cause for concern when used for long
distance communication.
• Mode coupling leads to intramodal dispersion like material
dispersion and waveguide dispersion and also intermodal
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37. Optic Fiber
Configurations
Depending on the refractive index
profile of fiber and modes of fiber
there exist three types of optical fiber
configurations. These optic-fiber
configurations are -
1.Single mode step index fiber.
2.Multimode step index fiber.
3.Multimode graded index fiber.
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38. Single mode fibers
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39. Multimode fibers
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40. Step Index / Graded
Index
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41. Step index fiber
• Single mode Step index Fiber
In single mode step index fiber has a central core that is
sufficiently small so that there is essentially only one path
for light ray through the cable. The light ray is
propagated in the fiber through reflection. Typical core
sizes are 2 to 15 µm. Single mode fiber is also known as
fundamental or mono mode fiber.
• Multimode step index fiber
it is more widely used type. It is easy to manufacture. Its
core diameter is 50 to 1000 µm i.e. large aperture and
allows more light to enter the cable. The light rays are
propagated down the core in zig-zag manner. There are
many many paths that a light ray may follow during the
propagation.
The light ray is propagated using the principle of total
internal reflection (TIR). Since the core index of refraction
is higher than the cladding index of refraction.
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42. Step index fiber
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43. Modes in SI fibers
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44. Problem
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45. Graded index fibers
• Graded index fibers do not have a constant
refractive index in the core but a decreasing
core index n(r) with radial distance from a
maximum value of n1 at the axis to a constant
value n2 beyond the core radius a in the
cladding. This index variation may be
represented as:
• where Δ is the relative refractive index
difference and α is the profile parameter
which gives the characteristic refractive index
profile of the fiber core.
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47. Problem
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48. Advantages and
Disadvantages
• Single mode step index fiber
Advantages:
minimum dispersion
high accuracy
wider bandwidths and higher information transmission
rates
Disadvantages:
Difficult to couple the the light into the fiber
Only lasers can be used as light sources
Expensive and difficult to manufacture
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49. Advantages and
Disadvantages
• Multi mode step index fiber
Advantages:
1. Relatively inexpensive and simple to manufacture.
2.It is easier to couple light into and out of multimode step-index fiber.
Disadvantages:
1. Light rays take many different paths down the fiber, which results in
large differences in propagation times.
2.The bandwidths and rate of information transfer is low.
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50. Advantages and
Disadvantages
• Multi mode Graded index fiber
• There are no outstanding advantages or disadvantages of
this type of fiber.
• Multimode graded-index fibers are easier to couple light
into and out of than single-mode step-index fibers but
are more difficult than multimode step-index fibers.
• Distortion due to multiple propagation paths is greater
than in single-mode step-index fibers but less than in
multimode step-index fibers.
• This multimode graded-index fiber is considered an
intermediate fiber compared to the other fiber types.
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51. (at the end of the class around 50
minutes)
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Recap – Summary – What
you have learnt

52. Session Quiz (5-10 MCQ/True false
questions)
(marks to be counted for continuous
evaluation)
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Session Quiz

53. References
• Text Book
Gerd Keiser, Optical Fiber Communications, 4/e, Tata
McGrawHill, 2008.
• References
D. K. Mynbaev, Gupta, Scheiner, Fiber Optic Communications,
Pearson Education, India, 2005.
S. C. Gupta, Text Book on Optical Fibre Communication and its
Applications, Prentice Hall of India, 2005.
John M. Senior, Optical Fiber Communications, 2/e, Prentice
Hall of India, 2002.
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Department of Biotechnology, GIT
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THANK YOU