RUAS, Bangalore Optical Fibre
OPTICAL FIBRE
Introduction:
Optical fibre is a very thin flexible thread of transparent plastic or glass in which light is
transmitted through multiple total internal reflection. Thus the basic principle utilized in the
optical fibre technology is total internal reflection. In 1870 – John Tyndall showed light can
be guided by Total Internal Reflection inside a water jet flowing down from tank. This is the
starting point for a whole new area of optical fibre technology. Currently optical fibers are
being used to transmit voice, picture, digital data signals, etc. Its advantages over electronic
communication technology includes; tremendous capacity to carry information, lower losses
and absence of electrical hazards. It is widely used in various fields such as information
technology, medicine, e-commerce and many more.
Total Internal Reflection:
When a ray of light travels from denser to rarer medium it bends away from the normal. As
the angle of incidence increases in the denser medium, the angle of refraction also
increases. For a particular angle of incidence called the “critical angle”, the refracted ray
grazes the surface separating the media or the angle of refraction is equal to 90°. If the angle
of incidence is greater than the critical angle, the light ray is reflected back to the same
medium. This is called “Total Internal Reflection”. In total internal reflection, there is no loss
of energy. The entire incident ray is reflected back.
In the figure, XX1 is the surface separating medium of refractive index n1 and medium
of refractive index n2, n1 > n2. AO and OA1 are incident and refracted rays. θ1 and θ2 are angle
of incidence and angle of refraction, θ2 > θ1. For the ray BO, θc is the critical angle. OB1 is the
refracted ray which grazes the interface. The ray CO
incident with an angle greater than θc is totally
reflected back along OC1.
From Snell’s law,
For total internal reflection,
and
(Because =1)
Thus when the angle of incidence is greater than the critical angle, the incident light ray
undergoes total internal reflection. In total internal reflection there is no loss or absorption
of light energy. The entire energy is returned along the reflected light.
Optical Fibres:
They are used in optical communication. It works on the
principle of Total internal reflection (TIR). Optical fibre is
B.Tech, Engineering Physics 1
, RUAS, Bangalore Optical Fibre
made from transparent dielectrics. It is cylindrical in shape. The inner cylindrical part is
called as core of refractive index n1. The outer part is called as cladding of refractive index n2,
n1>n2. There is continuity between core and cladding. Cladding is enclosed inside a
polyurethane jacket. Number of such fibres is grouped to form a cable.
The light entering through one end of core strikes the interface of the core and
cladding with angle greater than the critical
angle and undergoes total internal reflection.
After series of such total internal reflection, it
emerges out of the core. Thus the optical fibre
works as a waveguide. Care must be taken to
avoid very sharp bends in the fibre because at
sharp bends, the light ray fails to undergo total
internal reflection.
Relation for Angle of Acceptance , Numerical Aperture and Refractive Indices:
Consider a light ray AO incident at an angle enters into the fibre. Let be the angle of
refraction for the ray OB. The refracted ray OB incident at a critical angle (90˚- ) at B
grazes the interface between core and cladding along BC. If the angle of incidence is greater
than critical angle, it undergoes total internal reflection. Thus is called the waveguide
acceptance angle and is called the numerical aperture.
Let , and be the refractive indices of the medium, core and cladding respectively.
From Snell’s law at O,
…………….. 1
At B the angle of incidence is ( )
From Snell’s law,
B.Tech, Engineering Physics 2
OPTICAL FIBRE
Introduction:
Optical fibre is a very thin flexible thread of transparent plastic or glass in which light is
transmitted through multiple total internal reflection. Thus the basic principle utilized in the
optical fibre technology is total internal reflection. In 1870 – John Tyndall showed light can
be guided by Total Internal Reflection inside a water jet flowing down from tank. This is the
starting point for a whole new area of optical fibre technology. Currently optical fibers are
being used to transmit voice, picture, digital data signals, etc. Its advantages over electronic
communication technology includes; tremendous capacity to carry information, lower losses
and absence of electrical hazards. It is widely used in various fields such as information
technology, medicine, e-commerce and many more.
Total Internal Reflection:
When a ray of light travels from denser to rarer medium it bends away from the normal. As
the angle of incidence increases in the denser medium, the angle of refraction also
increases. For a particular angle of incidence called the “critical angle”, the refracted ray
grazes the surface separating the media or the angle of refraction is equal to 90°. If the angle
of incidence is greater than the critical angle, the light ray is reflected back to the same
medium. This is called “Total Internal Reflection”. In total internal reflection, there is no loss
of energy. The entire incident ray is reflected back.
In the figure, XX1 is the surface separating medium of refractive index n1 and medium
of refractive index n2, n1 > n2. AO and OA1 are incident and refracted rays. θ1 and θ2 are angle
of incidence and angle of refraction, θ2 > θ1. For the ray BO, θc is the critical angle. OB1 is the
refracted ray which grazes the interface. The ray CO
incident with an angle greater than θc is totally
reflected back along OC1.
From Snell’s law,
For total internal reflection,
and
(Because =1)
Thus when the angle of incidence is greater than the critical angle, the incident light ray
undergoes total internal reflection. In total internal reflection there is no loss or absorption
of light energy. The entire energy is returned along the reflected light.
Optical Fibres:
They are used in optical communication. It works on the
principle of Total internal reflection (TIR). Optical fibre is
B.Tech, Engineering Physics 1
, RUAS, Bangalore Optical Fibre
made from transparent dielectrics. It is cylindrical in shape. The inner cylindrical part is
called as core of refractive index n1. The outer part is called as cladding of refractive index n2,
n1>n2. There is continuity between core and cladding. Cladding is enclosed inside a
polyurethane jacket. Number of such fibres is grouped to form a cable.
The light entering through one end of core strikes the interface of the core and
cladding with angle greater than the critical
angle and undergoes total internal reflection.
After series of such total internal reflection, it
emerges out of the core. Thus the optical fibre
works as a waveguide. Care must be taken to
avoid very sharp bends in the fibre because at
sharp bends, the light ray fails to undergo total
internal reflection.
Relation for Angle of Acceptance , Numerical Aperture and Refractive Indices:
Consider a light ray AO incident at an angle enters into the fibre. Let be the angle of
refraction for the ray OB. The refracted ray OB incident at a critical angle (90˚- ) at B
grazes the interface between core and cladding along BC. If the angle of incidence is greater
than critical angle, it undergoes total internal reflection. Thus is called the waveguide
acceptance angle and is called the numerical aperture.
Let , and be the refractive indices of the medium, core and cladding respectively.
From Snell’s law at O,
…………….. 1
At B the angle of incidence is ( )
From Snell’s law,
B.Tech, Engineering Physics 2
