Download Optical Fibre By Mohd Nasir bin Said Telecommunications

Optical Fibre System
By
Mohd Nasir bin Said
Telecommunications Department
Advance Technology Training Centre
Kulim Kedah Darul Aman
What is "Fiber Optics"?
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It's the communications technology that works
by sending signals down hair thin strands of
glass fiber
History
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It began about 30 years ago in the R&D
labs (Corning, Bell Labs, ITT UK, etc.) and
was first installed in Chicago, IL, USA in
1976. By the early 1980s, fiber networks
connected the major cities on each coast.
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By the mid-80s, fiber was replacing all the telco
copper, microwave and satellite links.
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In the 90s, CATV discovered fiber and used it
first to enhance the reliability of their networks, a
big problem. Along the way, the discovered they
could offer phone and Internet service on that
same fiber and greatly enlarged their markets.
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Computers and LANs started using fiber
about the same time as the telcos
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Other applications developed too: aircraft,
ship and automobile data busses, CCTV
for security, even links for consumer digital
stereo!
Light
What is light?
 Energy of from electromagnetic wave and
particle.
What is photon?
 Particle from of light
 Photonics to lights is like Electronics to
current
The electromagnetic Spectrum
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The light used in optical fiber network is one type of
electromagnetic energy.
When an electric change moves back and forth, or
accelerates, a type of energy called electromagnetic
energy is produced.
This energy is the form of waves can travel through a
vacuum, the air, and through some materials like glass.
An important property of energy wave is the wavelength
Radio, microwave,radar, visible light,x-rays are all types
of electromagnetic energy.
The electromagnetic spectrum
Wavelenghts are not visible to the human
eye are used to transmit data over optical
fiber.
 These wavelenghts are slightly longer than
red light and are called infrared light.
 These wavelenghts were selected
because they travel through optical fiber
better than other wavelenghts.
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Ray model of light
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When electromagnetic waves travel out from a source, they travel in
straight lines
These straight lines pointing out from the source are called rays.
In the vacuum of empty space, light travels continuously in a straight
line at 300,000km per second.
However,light travel at different, slower speed other through other
materials like air,water and glass.
When a light ray called the incident ray,crosses the boundry from
one material to another , some of the light energy in the ray will be
reflected back.
This is why you can see yourself in window glass.
The light that is reflected back is called reflected ray.
Ray model of light
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The light energy in the incident ray that is not reflected will enter the
glass
Refracted ray-The entering ray will be bent at an angle from its
original path.
How much the incident light ray is bent depends on the angle at
which the incident ray strikes the surface of the glass and the
different rates of speed at which light travels through the two
substance.
The optical density of the glass determines how much the rays of
light in the glass.
Optical density refers to how much a light ray slows down when it
passes through a substance.
The greater the optical density of a material, the more it slows light
down from its speed in a vacuum.
The ratio of the speed of light in a material to the speed of light in a
vacuum is called the Index of Refraction
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When light traveling in a transparent
material meets the surface of another
transparent material two things happen:a) some of the light is reflected –reflection
b) some of the light is transmitted into the
second transparent material -refraction
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The bending of light is called refraction and it depends
upon the fact that light travels at one speed in one
material and at a different speed in a different material.
As a result each material has its own Refractive Index
which we use to help us calculate the amount of bending
which takes place. Refractive index is defined as:
n =C

where ;
n is the refractive index
C is the speed of light in a vacuum
 is the speed of light in the material
The indexes of refraction of several
common materials are given above
*Vacuum -1.0
*Air 1.0003
*Water-1.33
*Ethyl Alcohol -1.36
*Silicon -3.4
**Index of refraction is based on a
wavelength of light emitted from a sodium
flame (5890 Å)
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Snell Law
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How a light ray reacts when it meets the
interface of two transmissive materials that
have different indexes of refraction can be
explained with Snell’s law.
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Snell’s law simply states
n1 sin 1 = n2 sin 2
where
n1 = refractive index of material 1 (unit less)
n2 = refractive index of material 2 (unit less)
1 = angle of incidence (degrees)
2 = angle of refraction (degrees)
Critical Angle
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The critical angle is defined as the minimum angle of
incidence at which a light ray may strike the interface of
two media and result in an angle of refraction of 90 or
greater
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This definition pertains only when the light ray is
traveling from a more dense medium into a less dense
medium. The critical angle can be derived from Snell’s
law as follows:
n1 sin 1 = n2 sin 2
sin 1 =
n2 sin 2
n1
TIR
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The transmitted ray now tries to travel in
both materials simultaneously for various
reasons this is physically impossible so
there is no transmitted ray and all the light
energy is reflected. This is true for any
value of 1, the angle of incidence is equal
to or greater than 
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We can define the two conditions necessary for TIR to
occur:
1. The refractive index of the first medium is
greater than the refractive index of the
second one.
2. The angle of incidence, 1, is greater than or equal to
the critical angle, c
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The phenomenon of TIR causes 100% reflection. In no
other situation in nature, where light is reflected, does
100% reflection occur. So TIR is unique and very useful.
Numerical Aperture
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The numerical aperture of a core is the range of
angles of incident light rays entering the fiber
that will be completely reflected.
Modes- The paths which a light ray can follow
when travelling down a fiber.
By controlling both conditions, the fiber run will
have total internal reflection. This give a light
wave that can be used for data communications.
Fiber Construction And Geometry
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Te core is the light transmission element at the center of
the optical fiber.
Cladding is also made of silica but with a lower index of
refraction than the core.Light rays travelling through the
fiber core reflect off this core to cladding interface as
they move through the fiber by TIR
Surrounding the cladding is a buffer material that is
usually plastic. The buffer material helps sheid the core
and cladding from damage.
The strenght material surrounds the buffer,preventing the
fiber cable from being strecthed when installer pull it.The
material used is often Kevlar, the same material used to
produce bulletproof vest.
The outer jacket surrounds the cable to protect the fiber
against abrasion,solvents and other contaminations.
Type of Fiber and
Mode of Propogation
• Single Mode
• Multimode – Step index
- Graded Index
If the diameter of the core of the fiber is
large enough so that there are many paths
light can take through the fiber, the fiber is
called “ multimode” fiber.
 Single mode fiber has a much smaller core
that only allows light rays to travel along
one mode inside the fiber.
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Transmission mode
For long distance
Difficult to work with.
Phone companies and
CATV companies
For short
distance
Easy to work with.
LANs
Provides more
bandwidth than (c)
Most common and
widely used
type
For
short
distance
Easy to work with.
LANs
For very high pulse
rates
Figure 1.9 Core index profiles: (a) single-mode step index; (b) multi-mode step index; (c) multi-mode
graded index
Single Mode
High Bw applications- 4 Ghz
 Low losses 0.3-0.5 db/km
 Small Core area 8-10 micron
 Tx at 1300nm-1550nm wavelength
 Higher cost.
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Multimode Step Index
BW of 10Mhz/km
 Loss of 5-20db/km
 Large core 200-1000micron
 Cladding 1035 micron
 Limited transmission distance
 Tx at 660-1060
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Multimode Graded index
BW up to 600Mhz/km
 Losses of 2 to 10 db/km
 Cores of 50/62.5/85/100 micron
 Cladding of 125 and 140 micron
 Effective with laser or LED sources
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TQ
Q&A?