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Optical Fiber Communications
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.pdf  1462589397-21643543SeminarReportonOpticalFiberCommunicationbyShradhaPathak.pdf (Size: 3.23 MB / Downloads: 3)



Introduction of Optical
Our current “age of technology” is the result of many brilliant inventions
and discoveries, but it is our ability to transmit information, and the
media we use to do it, that is perhaps most responsible for its evolution.
Progressing from the copp
cable, our increasing ability to transmit more information, more quickly
and over longer distances has expanded the boundaries of our
technological development in all areas.
An optical fiber (or fiber
that carries light along its length.
overlap of applied science and engineering
concerned with the design and application of optical
fibers. Optical fibers are widely used in fiber optic
communications, which permi
longer distances and at higher bandwidths
high frequency than any other form of radio signal than other forms of
communications. Light is kept in the core of the optical fiber by total
internal reflection. This causes the fiber to act as a waveguide. Fibers are
used instead of metal wires because signals travel along them with less
loss, and they are also immune to electromagnetic interference, which is
caused by thunderstorm. Fibers are also used for illu
wrapped in bundles so they can be used to carry images, thus allowing
1.0 Introduction of Optical Fiber:-
Our current “age of technology” is the result of many brilliant inventions
and discoveries, but it is our ability to transmit information, and the
media we use to do it, that is perhaps most responsible for its evolution.
Progressing from the copper wire of a century ago to today’s
, our increasing ability to transmit more information, more quickly
and over longer distances has expanded the boundaries of our
technological development in all areas.
fiber) is a glass or plastic fiber
that carries light along its length. Fiber optics is the
overlap of applied science and engineering
concerned with the design and application of optical
fibers. Optical fibers are widely used in fiber optic
communications, which permits transmission over
higher bandwidths (data rates) because light has
high frequency than any other form of radio signal than other forms of
communications. Light is kept in the core of the optical fiber by total
. This causes the fiber to act as a waveguide. Fibers are
used instead of metal wires because signals travel along them with less
loss, and they are also immune to electromagnetic interference, which is
caused by thunderstorm. Fibers are also used for illumination, and are
wrapped in bundles so they can be used to carry images, thus allowing viewing in tight spaces. Specially designed fibers are used for a variety
of other applications, including sensors and fiber lasers.
2.0 History of Fiber Optic Technology:-
In 1870, John Tyndall, using a jet of water that flowed from one
container to another and a beam of light, demonstrated that light used
internal reflection to follow a specific
path. As water poured out through the
spout of the first container, Tyndall
directed a beam of sunlight at the path of
the water. The light, as seen by the
audience, followed a zigzag path inside
the curved path of the water. This simple
experiment, illustrated in Figure, marked the first research into guided
transmission of light.
In the same year, Alexander Graham Bell developed an optical voice
transmission system he called the photo phone. The photo phone used
free-space light to carry the human voice 200 meters. Specially placed




mirrors reflected sunlight onto a diaphragm attached within the
mouthpiece of the photo phone.
At the other end, mounted within a
parabolic reflector, was a light
sensitive selenium resistor. This
resistor was connected to a battery
that was, in turn, wired to a telephone receiver. As one spoke into the
photo phone, the illuminated diaphragm vibrated, casting various
intensities of light onto the selenium resistor. The changing intensity of
light altered the current that passed through the telephone receiver which
then converted the light back into speech. Bell believed this invention
was superior to the telephone because it did not need wires to connect
the transmitter and receiver. Today, free-space optical links find
extensive use in metropolitan applications.
The first practical all-glass fiber was devised by Brian O'Brien at the
American Optical Company and Narinder Kapany (who first coined
the term 'fiber optics'in 1956) and colleagues at the Imperial College of
Science and Technology in London. Early all-glass fibers experienced
excessive optical loss, the loss of the light signal as it traveled the fiber,
limiting transmission distances.



In 1969, several scientists
concluded that impurities in the
fiber material caused the signal
loss in optical fibers. The basic
fiber material did not prevent the light signal from reaching the end of
the fiber. These researchers believed it was possible to reduce the losses
in optical fibers by removing the impurities.
Jun-ichi Nishizawa, a Japanese scientist at Tohoku University, was the
first to propose the use of optical fibers for communications in 1963.
Nishizawa invented other technologies that contributed to the
development of optical fiber communications as well. Nishizawa
invented the graded-index optical fiber in 1964 as a channel for
transmitting light from semiconductor lasers over long distances with
low loss.
Fiber optics developed over the years in a series of generations that can
be closely tied to wavelength. Below Figure shows three curves. The
top, dashed, curve corresponds to early 1980's fiber, the middle, dotted,
curve corresponds to late 1980's fiber, and the bottom, solid, and curve
corresponds to modern optical fiber.



The earliest fiber optic systems were developed at an operating
wavelength of about 850 nm. This wavelength corresponds to the socalled
'first window'in a silica-based optical fiber. This window refers to
a wavelength region that offers low optical loss. As technology
progressed; the first window became less attractive because of its
relatively high loss. Then companies jumped to the 'second window'at
1310 nm with lower attenuation of about 0.5 dB/km. In late 1977 the
'third window'was developed at 1550 nm. It offered the theoretical
minimum optical loss for silica-based
fibers. A 'fourth window,'near 1625
nm, is being developed. While it is
not lower loss than the 1550 nm
window, the loss is comparable, and
it might simplify some of the
complexities of long-length, multiple-wavelength.



Construction of Optical Fiber Cable:-


An optical fiber is a very thin strand of silica glass in geometry quite like
a human hair. In reality it is a very narrow, very long glass cylinder with
special characteristics. When light enters one end of the fiber it travels
(confined within the fiber) until it leaves the fiber at the other end.
An optical fiber consists of two parts: the core and the cladding. The
core is a narrow cylindrical strand of glass and the cladding is a tubular
jacket surrounding it. The core has a (slightly) higher refractive index
than the cladding. Light travelling along the core is confined by the
mirror to stay within it even when the fiber bends around a corner.
A fiber optic cable has an additional coating around the cladding called
the jacket. The jacket usually consists of one or more layers of
polymer. Its role is to protect the core and cladding from shocks that
might affect their optical or physical properties. It acts as a shock
absorber. The jacket also provides protection from abrasions, solvents
and other contaminants. The jacket does not have any optical properties
that might affect the propagation of light within the fiber optic cable.
4.0 Guiding Mechanism in optical fiber:-
Light ray is injected into the fiber optic cable on the right. If the light
ray is injected and strikes the core-to-cladding interface at an angle
greater than an entity called the critical angle then it is reflected back
into the core. Since the angle of incidence is always equal to the angle of
reflection the reflected light will again be reflected. The light ray will
then continue this bouncing path down the length of the fiber optic
cable. If the light ray strikes the core
less than the critical angle then it passes into the cladding where it is
attenuated very rapidly with propagation distance.
Light can be guided down the fiber optic cable if it enters at less than the
critical angle. This angle is fixed by the indices of refraction of the core
and cladding and is given by the
The critical angle is measured from the cylindrical axis of the core. By
way of example, if n1 = 1.446 and n
will show that the critical angle is 8.53 degrees, a fairly small angle
Figure:
Of course, it be noted that a light ray enters the core from the air outside,
to the left of Figure. The refractive index of the air must be taken into
account in order to assure that a light ray in the core will be at an angle
the core. Since the angle of incidence is always equal to the angle of
reflection the reflected light will again be reflected. The light ray will
then continue this bouncing path down the length of the fiber optic
cable. If the light ray strikes the core-to-cladding interface at an angle
less than the critical angle then it passes into the cladding where it is
attenuated very rapidly with propagation distance.
Light can be guided down the fiber optic cable if it enters at less than the
angle is fixed by the indices of refraction of the core
and cladding and is given by the formula:
The critical angle is measured from the cylindrical axis of the core. By
= 1.446 and n2= 1.430 then a quick computation
hat the critical angle is 8.53 degrees, a fairly small angle
Figure:-Mechanism of Light wave guide in Fiber
Of course, it be noted that a light ray enters the core from the air outside,
to the left of Figure. The refractive index of the air must be taken into
account in order to assure that a light ray in the core will be at an angle
the core. Since the angle of incidence is always equal to the angle of
reflection the reflected light will again be reflected. The light ray will
then continue this bouncing path down the length of the fiber optic
cladding interface at an angle
less than the critical angle then it passes into the cladding where it is
attenuated very rapidly with propagation distance.
Light can be guided down the fiber optic cable if it enters at less than the
angle is fixed by the indices of refraction of the core
The critical angle is measured from the cylindrical axis of the core. By
= 1.430 then a quick computation
hat the critical angle is 8.53 degrees, a fairly small angle.
Of course, it be noted that a light ray enters the core from the air outside,
to the left of Figure. The refractive index of the air must be taken into
account in order to assure that a light ray in the core will be at an angle
less than the critical angle. This can be done fairly simply. Suppose a
light ray enters the core from the air at an angle less than an entity called
the external acceptance angle It will be guided down the core.
 


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