2. Aditya Silver Oak Institute of
Technology
Name : Surbhi R Yadav
Branch : Computer Engineering
Semester : Second
Enrollment no. : 141200107065
Subject : Engineering Physics
Faculty Name : Mr. Sreenath Nair
3. Introduction
Optical fiber refers to the medium and the technology associated
with the transmission of information as light pulses along a glass
or plastic strand or fiber.
Optical fiber carries much more information than conventional
copper wire and is in general not subject to electromagnetic
interference and the need to retransmit signals.
Most telephone company long-distance lines are now made of
optical fiber. Transmission over an optical fiber cable requires
repeaters at distance intervals. The glass fiber requires more
protection within an outer cable than copper.
4. For these reasons and because the installation of any new cabling
is labor-intensive, few communities have installed optical fiber
cables from the phone company's branch office to local customers
. A type of fiber known as single mode fiber is used for longer
distances; multimode fiber is used for shorter distances.
5. Principle
Fibre Optics is based on the principle of Total Internal Reflection
When a light signal is directed at one end of the fibre at a suitable
angle, it undergoes repeated total internal reflection along the
length of the fibre and finally comes out at the other end.
The fibre consists of a core surrounded by a cladding layer, both of
which are made of dielectric materials. To confine the optical
signal in the core, the refractive index of the core must be greater
than that of the cladding.
6. Total Internal Reflection
When light traveling in an optically dense medium hits a
boundary at a steep angle (larger than the critical angle for the
boundary), the light is completely reflected. This is called total
internal reflection. This effect is used in optical fibers to confine
light in the core. Light travels through the fiber core, bouncing
back and forth off the boundary between the core and cladding.
Because the light must strike the boundary with an angle greater
than the critical angle, only light that enters the fiber within a
certain range of angles can travel down the fiber without leaking
out.
7. Modes of Propagation
There are 2 types of propagation mode in fiber optics cable which
are multi-mode and single-mode. These provide different
performance with respect to both attenuation and time dispersion.
The single-mode fiber optic cable provides the better performance
at a higher cost.
The number of modes in a fiber optic cable depends upon the
dimensions of the cable and the variation of the indices of
refraction of both core and cladding across the cross section.
There are three principal possibilities which are multi-mode step
index, single-mode step index and multi-mode graded index.
8.
9. Single Mode Step Index
The diameter of the core is fairly small relative to the cladding.
Typically, the cladding is ten times thicker than the core.
Comparing the output pulse and the input pulse note that there is
little attenuation and time dispersion.
Single mode propagation exists only above a certain specific
wavelength called the cutoff wavelength. Single-mode fiber optic
cable is fabricated from glass. Because of the thickness of the
core, plastic cannot be used to fabricate single-mode fiber optic
cable.
10. Less time dispersion of course means higher bandwidth and this
is in the 50 to 100 GHz/ km range. However, single mode fiber
optic cable is also the most costly in the premises environment.
For this reason, it has been used more with Wide Area Networks
than with premises data communications. It is attractive more for
link lengths go all the way up to 100 km. Nonetheless, single-
mode fiber optic cable has been getting increased attention as
Local Area Networks have been extended to greater distances over
corporate campuses.
11. Multi mode Step Index
The diameter of the core is fairly large relative to the cladding.
Note that the output pulse is significantly attenuated relative to
the input pulse. It also suffers significant time dispersion. The
higher order modes, the bouncing rays, tend to leak into the
cladding as they propagate down the fiber optic cable. They lose
some of their energy into heat. This results in an attenuated
output signal. Consequently, they do not all reach the right end of
the fiber optic cable at the same time. When the output pulse is
constructed from these separate ray components the result is time
dispersion.
12. Fibre optic cable that exhibits multi-mode propagation with a step
index profile is thereby characterized as having higher attenuation
and more time dispersion than the other propagation candidates
have. However, it is also the least costly and in the premises
environment the most widely used. It is especially attractive for
link lengths up to 5 km. Usually, it has a core diameter that
ranges from 100 microns to 970 microns. It can be fabricated
either from glass, plastic or PCS.
13. Multi mode Graded Index
There is no sharp discontinuity in the indices of refraction between core
and cladding. The core here is much larger than in the single-mode step
index. When comparing the output pulse and the input pulse, note that
there is some attenuation and time dispersion, but not nearly as great
as with multi-mode step index fiber optic cable.
Fiber optic cable that exhibits multi-mode propagation with a graded
index profile is thereby characterized as having attenuation and time
dispersion properties somewhere between the other two candidates.
Likewise its cost is somewhere between the other two candidates. This
type of fiber optic cable is extremely popular in premise data
communications applications.
14.
15. Uses of Fibre Optics
Optical fiber can be used as a medium for telecommunication and
computer networking because it is flexible and can be bundled as
cables
Fibers have many uses in remote sensing. In some applications,
the sensor is itself an optical fibre
Optical fiber can be used to transmit power using a photovoltaic
cell to convert the light into electricity
Optical fiber lamps are used for illumination in decorative
applications, including signs, art, toys and artificial Christmas
trees
