Unit –I Fibre Optics Engineering Physics
Introduction
Fibre optics deals with the light propagation through thin glass fibres. Fibre optics plays an important
role in the field of communication to transmit voice, television and digital data signals from one place to another.
The transmission of light along the thin cylindrical glass fibre by total internal reflection was first demonstrated
by John Tyndall in 1870 and the application of this phenomenon in the field of communication is tried only from
1927. Today the applications of fibre optics are also extended to medical field in the form of endoscopes and to
instrumentation engineering in the form of optical sensors.
Generally, communication is transferred through carrier waves in any communication system. When the
frequencies of the carrier waves are high then the information carrying capacity also enhances. As the
propagation of light takes place in the form of high frequency waves, these light waves can be used to carry
information, i.e., as carrier waves. For the proper guiding of information carrying light waves, we need a
proper guiding medium or material. That material is the optical fibre.
1. Optical fibre
Optical fibre is a guiding medium or material which guides the information carrying light waves. To
guide the light waves, optical fibre should be transparent. To minimize the transmission losses through the
optical fibre it is made thin.
Thus Optical fibre is a thin transparent guiding medium or material which guides the information
carrying light waves
2. The Basic principle of optical fibre (Or) Working principle of optical fibre (or) Total
internal reflection
The transmission of light in an optical fibre is based on the phenomenon of total internal reflection.
Optical fibre consists of inner most layer known as core, a denser medium and next layer is known as cladding a
rarer medium.
Total internal reflection:-
Definition: When the light ray travels from denser medium to rarer medium the refracted ray bends away
from the normal. When the angle of incidence is greater than the critical angle, the refracted ray again
reflects into the same medium. This phenomenon is called total internal reflection.
Let us consider n1 and n2 are refractive indices of core and cladding mediums. Let, a light ray traveling
from core medium to cladding medium, then the refracted ray bends away from the normal with is the angle of
incidence and is the angle of refraction.
In this we get three cases
Case I:
When , the light ray refracts into cladding medium. [Figure 1]
Core medium
Cladding medium
Case II:-
When , the light ray travels along the interface of core and cladding, θ C is known as critical angle. [Figure
2]
Dr. P. Venkata Ramana
, Unit –I Fibre Optics Engineering Physics
Core medium
Core medium
Cladding medium
Cladding medium
When the angle of incidence is increased angle of reflection also increases and for a particular angle of
incidence (i = θ C ) the refracted ray travels along the interface of two mediums. This angle of incidence is known
as critical angle ( θ C ).
n1 Sin θ C = n2 Sin 90
n1 Sin θ C = n2 ⇒ Sin θ C = n2
n1
n2
θ C = Sin −1
n1
Case III:-
When , then the light ray will be reflected back into the core medium and undergoes total internal
reflection. [Figure 3]
Core medium
Cladding medium
When the angle of incidence is greater than the critical angle (i > θ C ) , the refracted ray again reflects into
the same medium. This phenomenon is called total internal reflection
3. Construction of optical fibre
The optical fibre mainly consists the following six parts as shown in figure
1. Core
2. Cladding
3. Silicon coating
4. Buffer jacket
5. Strength member
6. Outer jacket
Dr. P. Venkata Ramana
Introduction
Fibre optics deals with the light propagation through thin glass fibres. Fibre optics plays an important
role in the field of communication to transmit voice, television and digital data signals from one place to another.
The transmission of light along the thin cylindrical glass fibre by total internal reflection was first demonstrated
by John Tyndall in 1870 and the application of this phenomenon in the field of communication is tried only from
1927. Today the applications of fibre optics are also extended to medical field in the form of endoscopes and to
instrumentation engineering in the form of optical sensors.
Generally, communication is transferred through carrier waves in any communication system. When the
frequencies of the carrier waves are high then the information carrying capacity also enhances. As the
propagation of light takes place in the form of high frequency waves, these light waves can be used to carry
information, i.e., as carrier waves. For the proper guiding of information carrying light waves, we need a
proper guiding medium or material. That material is the optical fibre.
1. Optical fibre
Optical fibre is a guiding medium or material which guides the information carrying light waves. To
guide the light waves, optical fibre should be transparent. To minimize the transmission losses through the
optical fibre it is made thin.
Thus Optical fibre is a thin transparent guiding medium or material which guides the information
carrying light waves
2. The Basic principle of optical fibre (Or) Working principle of optical fibre (or) Total
internal reflection
The transmission of light in an optical fibre is based on the phenomenon of total internal reflection.
Optical fibre consists of inner most layer known as core, a denser medium and next layer is known as cladding a
rarer medium.
Total internal reflection:-
Definition: When the light ray travels from denser medium to rarer medium the refracted ray bends away
from the normal. When the angle of incidence is greater than the critical angle, the refracted ray again
reflects into the same medium. This phenomenon is called total internal reflection.
Let us consider n1 and n2 are refractive indices of core and cladding mediums. Let, a light ray traveling
from core medium to cladding medium, then the refracted ray bends away from the normal with is the angle of
incidence and is the angle of refraction.
In this we get three cases
Case I:
When , the light ray refracts into cladding medium. [Figure 1]
Core medium
Cladding medium
Case II:-
When , the light ray travels along the interface of core and cladding, θ C is known as critical angle. [Figure
2]
Dr. P. Venkata Ramana
, Unit –I Fibre Optics Engineering Physics
Core medium
Core medium
Cladding medium
Cladding medium
When the angle of incidence is increased angle of reflection also increases and for a particular angle of
incidence (i = θ C ) the refracted ray travels along the interface of two mediums. This angle of incidence is known
as critical angle ( θ C ).
n1 Sin θ C = n2 Sin 90
n1 Sin θ C = n2 ⇒ Sin θ C = n2
n1
n2
θ C = Sin −1
n1
Case III:-
When , then the light ray will be reflected back into the core medium and undergoes total internal
reflection. [Figure 3]
Core medium
Cladding medium
When the angle of incidence is greater than the critical angle (i > θ C ) , the refracted ray again reflects into
the same medium. This phenomenon is called total internal reflection
3. Construction of optical fibre
The optical fibre mainly consists the following six parts as shown in figure
1. Core
2. Cladding
3. Silicon coating
4. Buffer jacket
5. Strength member
6. Outer jacket
Dr. P. Venkata Ramana
