1
Engineering Physics
UNIT– 6: OPTICAL FIBRES
6.1. INTRODUCTION
Our current age of technology is the result of many brilliant inventions and discoveries. Our ability to transmit
information and the media is perhaps most responsible for its evolution. Progressing from the copper wire of
few decades ago to today’s fibre optic 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. Fibre is an
integral part of modern day communication infrastructure and can be found along roads, in buildings, hospitals
and machinery. It is also widely used in communications, both in computer networks as a fast internet connection
source and in telecommunications. The fibre optic technology has literally revolutionized the
telecommunications industry by providing higher performance, more reliable telecommunication links with ever
decreasing bandwidth cost. Fibre optic lines are strands of optically pure glass as thin as a human hair that carries
digital information over long distances. This chapter will provide the working principle and explanations for
some of the terms associated with the field of fibre optic engineering for various applications.
6.2 OPTICAL FIBRE
An optical fibre is a thin transparent cylindrical conduit made up of glass or clear plastic, designed to guide light
waves along its length. It works on the principle of total internal reflection.
6.3 STRUCTURE OF OPTICAL FIBRE
The structure of optical fibre has in general consist of three most important region namely, 1] Core 2] Cladding
3] Sheath
1] Core: The innermost light guiding region is called core. Its refractive index (n1) is higher than that of cladding.
2] Cladding: Cladding helps to confine the light to the core. Its refractive index (n2) is lower than that of core
i.e. (n1 > n2).
3] Sheath: The outermost covering layer is called sheath. Its function is to protect the core and cladding from
cuts, abrasions, moisture, impurities.
Fig.6.1: The structure of optical fibre
6.4 WORKING PRINCIPLE OF OPTICAL FIBERS:
TOTAL INTERNAL REFLECTION
Whenever a ray of light travels from optically denser medium to rarer medium, it bends away from the
normal to the boundary separating two mediums.
Thus, angle of refraction is greater than the angle of incidence (∠𝑟 > ∠𝑖).
When the angle of incidence increases, angle of refraction also increases.
At a certain value of angle of incidence, angle of refraction will be 90o (∠𝑟 = 90o).
, 2
Engineering Physics
The angle of incidence for which the angle of refraction is 90o is known as critical angle 𝜙𝑐 .
When light is travelling from an optically denser to rarer medium and is incident at an angle greater than
critical angle 𝜙𝑐 , it reflects back into the denser medium without refraction.
This phenomenon is known as total internal reflection.
Optical fibre works on the principle of total internal reflection.
Fig.6.2: (a) Refraction from denser to rarer (b) Critical angle (b) Total internal reflections
QUE.: Explain the phenomenon of total internal reflection (2M)[S 08][W 12]
QUE.: Define critical angle (1M)[S 13]
QUE: Explain the structure of optical fibre. (2 M)
To calculate the value of critical angle:
We know that, by Snell’s law,
𝑠𝑖𝑛𝑖 𝑛2
= … … … … … … … (6.1)
𝑠𝑖𝑛𝑟 𝑛1
𝑊ℎ𝑒𝑛, 𝑖 = 𝜙𝑐 , 𝑟 = 90°
𝑠𝑖𝑛𝜙𝑐 𝑛2
=
𝑠𝑖𝑛90° 𝑛1
𝑛2
𝑠𝑖𝑛𝜙𝑐 = (𝑎𝑠 𝑠𝑖𝑛90° = 1)
𝑛1
where c is called critical angle.
𝒏𝟐
𝝓𝒄 = 𝒔𝒊𝒏−𝟏 ( ) … … … … … … … (6.2)
𝒏𝟏
6.6. IMPORTANT PARAMETERS IN OPTICAL FIBRES
Light sources such as LED or laser diodes are used to concentrate the beam of light into the fibre core. This is
due to the fact that the diameter of the core is very small and hence bigger light sources are not preferred to
launch the light beam into the core. Even in the use of small sources, condensing lens has to be used to
concentrate the light beam into the core. The main function of an optical fibre is to accept and transmit as much
light as possible from the source. This is known as light gathering ability. At the receiving end this light energy
is made incident on photo-sensors which convert this light signal back to electrical signal. Factors deciding the
performance and light gathering ability of optical fibre include:
Acceptance angle
Engineering Physics
UNIT– 6: OPTICAL FIBRES
6.1. INTRODUCTION
Our current age of technology is the result of many brilliant inventions and discoveries. Our ability to transmit
information and the media is perhaps most responsible for its evolution. Progressing from the copper wire of
few decades ago to today’s fibre optic 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. Fibre is an
integral part of modern day communication infrastructure and can be found along roads, in buildings, hospitals
and machinery. It is also widely used in communications, both in computer networks as a fast internet connection
source and in telecommunications. The fibre optic technology has literally revolutionized the
telecommunications industry by providing higher performance, more reliable telecommunication links with ever
decreasing bandwidth cost. Fibre optic lines are strands of optically pure glass as thin as a human hair that carries
digital information over long distances. This chapter will provide the working principle and explanations for
some of the terms associated with the field of fibre optic engineering for various applications.
6.2 OPTICAL FIBRE
An optical fibre is a thin transparent cylindrical conduit made up of glass or clear plastic, designed to guide light
waves along its length. It works on the principle of total internal reflection.
6.3 STRUCTURE OF OPTICAL FIBRE
The structure of optical fibre has in general consist of three most important region namely, 1] Core 2] Cladding
3] Sheath
1] Core: The innermost light guiding region is called core. Its refractive index (n1) is higher than that of cladding.
2] Cladding: Cladding helps to confine the light to the core. Its refractive index (n2) is lower than that of core
i.e. (n1 > n2).
3] Sheath: The outermost covering layer is called sheath. Its function is to protect the core and cladding from
cuts, abrasions, moisture, impurities.
Fig.6.1: The structure of optical fibre
6.4 WORKING PRINCIPLE OF OPTICAL FIBERS:
TOTAL INTERNAL REFLECTION
Whenever a ray of light travels from optically denser medium to rarer medium, it bends away from the
normal to the boundary separating two mediums.
Thus, angle of refraction is greater than the angle of incidence (∠𝑟 > ∠𝑖).
When the angle of incidence increases, angle of refraction also increases.
At a certain value of angle of incidence, angle of refraction will be 90o (∠𝑟 = 90o).
, 2
Engineering Physics
The angle of incidence for which the angle of refraction is 90o is known as critical angle 𝜙𝑐 .
When light is travelling from an optically denser to rarer medium and is incident at an angle greater than
critical angle 𝜙𝑐 , it reflects back into the denser medium without refraction.
This phenomenon is known as total internal reflection.
Optical fibre works on the principle of total internal reflection.
Fig.6.2: (a) Refraction from denser to rarer (b) Critical angle (b) Total internal reflections
QUE.: Explain the phenomenon of total internal reflection (2M)[S 08][W 12]
QUE.: Define critical angle (1M)[S 13]
QUE: Explain the structure of optical fibre. (2 M)
To calculate the value of critical angle:
We know that, by Snell’s law,
𝑠𝑖𝑛𝑖 𝑛2
= … … … … … … … (6.1)
𝑠𝑖𝑛𝑟 𝑛1
𝑊ℎ𝑒𝑛, 𝑖 = 𝜙𝑐 , 𝑟 = 90°
𝑠𝑖𝑛𝜙𝑐 𝑛2
=
𝑠𝑖𝑛90° 𝑛1
𝑛2
𝑠𝑖𝑛𝜙𝑐 = (𝑎𝑠 𝑠𝑖𝑛90° = 1)
𝑛1
where c is called critical angle.
𝒏𝟐
𝝓𝒄 = 𝒔𝒊𝒏−𝟏 ( ) … … … … … … … (6.2)
𝒏𝟏
6.6. IMPORTANT PARAMETERS IN OPTICAL FIBRES
Light sources such as LED or laser diodes are used to concentrate the beam of light into the fibre core. This is
due to the fact that the diameter of the core is very small and hence bigger light sources are not preferred to
launch the light beam into the core. Even in the use of small sources, condensing lens has to be used to
concentrate the light beam into the core. The main function of an optical fibre is to accept and transmit as much
light as possible from the source. This is known as light gathering ability. At the receiving end this light energy
is made incident on photo-sensors which convert this light signal back to electrical signal. Factors deciding the
performance and light gathering ability of optical fibre include:
Acceptance angle
