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FIBER OPTICS
Dr D. Arun Kumar
Assistant Professor
Department of Physical Sciences
Bannari Amman Institute of Technology
Sathyamangalam
General Objective
To understand the propagation of light through optical
fibers and obtain expressions for the acceptance angle
and numerical aperture
Specific Objectives
1. Illustrate the propagation of light through optical fiber
(S, E)
2. Define acceptance angle and numerical aperture (S)
3. Derive the expressions for the acceptance angle and
numerical aperture (S, M)
4. Compute the relation between fractional index and
numerical aperture (M)
Optical fiber
The development of lasers and optical fiber – revolution
in field of communication systems.
carrying light waves through an open atmosphere - rain,
fog etc affected the efficiency
guiding medium
The optical fiber is a wave guide
It is made up of transparent dielectrics (SiO2), (glass or
plastics).
transmit voice, video and digital data signals using light
waves
Fiber Construction
Core - inner cylinder made of glass or plastic - high
refractive index n1.
Cladding - core is surrounded by cylindrical shell of
glass or plastic – less refractive index n2
Jacket - cladding is covered – polyurethane - protects
from moisture and abrasion.
Total Internal reflection
The light is transmitted through this fiber by total
internal reflection.
Core diameters - from 5 to 600μm
cladding diameters - from 125 to 750μm
Core transmits the light waves
The cladding keeps the light waves within the core by
total internal reflection.
Principle of propagation of
light in an optical fiber
Total internal reflection at the fiber wall can occur only if
two conditions are Satisfied.
The refractive index of the core material n1 must be
higher than that of the cladding n2 surrounding it.
At the core – cladding interface, the angle of incidence (
between the ray and normal to the interface) must be
greater than the critical angle.
critical angle
Propagation of light through
fiber Let the refractive indices of the core and cladding be n1
and n2 respectively; n1> n2.
Propagation of light through
fiber
Propagation of light through
fiber
This angle im is called the acceptance angle of the fiber.
the maximum angle at or below which the light can
suffer Total Internal Reflection is called acceptance
angle.
Propagation of light through
fiber
Acceptance cone
An optical fiber accepts only those rays which are
incident within a cone having a semi angle im.
Numerical Aperture
the sine of the acceptance angle
NA = sin im
determines the light gathering ability of the fiber.
measure of amount of light that can be accepted by a
fiber
Refractive indices of the core and cladding materials.
Fractional Index change
TYPES OF OPTICAL FIBERS
Optical fibers are classified into three major categories
Based on the type of the
material
Glass fiber
Example:
Core: SiO2 Cladding: SiO2
Core: GeO2- SiO2 Cladding: SiO2
Plastic fiber
Example:
Core: polymethyl methacrylate : Cladding: Co- Polymer
Core: Polystyrene : Cladding: Methyl methacrylate
Based on the number of
modes
Single mode fiber
allow only one mode of propagation
very small core diameter
Multimode fiber
allow many modes to propagate
core diameter is very large
STEP INDEX FIBER
core of uniform refractive index surrounded by cladding
of refractive index lower than that of the core
refractive index abruptly changes at the core cladding
boundary.
Based on the refractive
index profile
GRADED INDEX FIBER
refractive index varies radially decreasing continuously
in a parabolic manner from the maximum value of n1.
Based on the refractive
index profile
THE FIBER OPTIC
COMMUNICATION SYSTEM
The major components of an optical fiber communication
system are
The optical transmitter
The optical fiber
The optical receiver
THE FIBER OPTIC
COMMUNICATION SYSTEM
INFORMATION SIGNAL
SOURCE
The input signal can be voice, music or video
It is in the form of analog signal
The analog signals are converted into electrical
signals
Then the signals are passed through transmitter
TRANSMITTER
It receives from the information source
It modulates the electrical signal into a digital pulse
The modulator consists of a driver and light source
The digital pulses are converted into optical pulses.
The light source can be LED or semiconductor
LASER
TRANSMISSION MEDIUM
The medium is the optical fiber
The principle involved is total internal reflection
Due to total internal reflection, there is no loss of light
on the core cladding junction
RECEIVER
It consists of demodulator device
It consists of photodetector, an amplifier and a
signal receiver
The photodetector converts the optical pulses
and electrical pulses
The signals are amplified by the amplifier
A single optical fiber is used to transmit several
optical signals using a device known as
multiplexer
ADVANTAGES
15,000 Signals can be transmitted
No cross talk and signal leakage is nil due to total
internal reflection
It is ideal means for communication
It gives foolproof communication during wartime
The cost of the cable is very low
Optical fibers have immunity to adverse
temperature, moisture and chemical reactions
Mind Map
Questions
1. Can we use fiber optics for lighting in houses?
2. Assume that an optical fiber is bent at right angles. Can
we get output?
Thank you……….
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