Optical Fiber Working Principle and Construction

Here we will discuss, What is an Optical Fiber, the Construction of the Optical Fiber and Working principle of the Optical Fiber. A complete video lecture has also been included to clear the concepts in a better way.
So let's start with the definition of Optical Fiber-

What is an Optical Fiber?

An optical fiber is a flexible and transparent fiber, made by drawing glass (silica) or plastic to a diameter slightly more than that of a human hair (including outer coating, its diameter is 0.25 mm-0.5 mm).

Optical Fiber Construction and Working Principle Video [HD]

 

The light rays propagate inside the optical fiber by the phenomenon of Total Internal Reflection (TIR). Now we will know the basic principle of operation of the optical fiber.

Working Principle of Optical Fiber

Total Internal Reflection in Optical Fiber (TIR)

The optical fiber has a core and a cladding layer. The refractive index of the core is more than the refractive index of the cladding. So when the light rays move from denser medium to rarer medium (core to cladding) with an incidence angle greater than the critical angle (90 degrees), the total internal reflection phenomenon takes place and the light rays return back into the same denser medium (core). So the propagation of light rays take place inside the core of the Optical Fiber with successive Total Internal Reflection (TIR).

To understand, how this phenomenon of total internal reflection takes place inside the optical fiber when the light rays propagate through it, see the image given below 

(Click on the image to enlarge it)-

Total Internal Reflection (TIR)

The image above shows, 3 different cases of refraction of light, when the light ray propagates from denser medium to the rarer medium.

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As we know, when the light rays move from denser medium to rarer medium, then it deviates away from the normal, as you can see in the image. This deviation (refraction) can be seen in all the three cases shown in the image.
Therefore the angle of refraction is more than the angle of incidence when the light rays move from denser medium to rarer medium.
Now look at the image, here you can see that, in the first case, when the incidence angle is less than the critical angle (the incidence angle, when the angle of refraction is 90° degrees), the light ray moves away from the normal in the rarer medium and goes into the rarer medium.
The second case shows the case of critical angle. Critical angle is the angle of incidence, when angle of refraction is 90 degrees (when the light ray moves from denser tor rare medium).
Now see the third case, it is the case where total internal reflection takes place. This is the same case that happens inside the optical fibre, when the light rays propagate through it.
In this third case, when the light ray moves from denser to rarer medium, with an angle of incidence more than the critical angle, it returns back into the same denser medium. This is called as total internal reflection.

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This phenomena is known as total internal reflection, because in this phenomena, the light ray is reflected totally back into the same medium like reflection phenomena. 

Construction and Working of the Optical Fiber

Now we will see how this phenomenon of total internal reflection takes place inside the optical fiber. To understand the concept clearly look at the image shown below (Click on the image to enlarge it)-

Propagation of light inside optical fiber
with Total Internal Reflection (TIR)

This image shows the structure and working principle of the optical Fibre. You can see in this image the two layers of the Optical Fiber, known as Core and cladding.

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Now observe the image carefully. When the light ray moves inside the core and reaches to the core- cladding interface, then the phenomenon of total internal reflection takes place. This phenomena is seen here because, the light ray moves from the denser medium to the rarer medium (core to cladding).

But for the total internal reflection to take place, it is necessary that the angle of incidence must be more than the critical angle. Only the incident light rays for which the angle of incidence is more than the critical angle, can propagate through the core of the fiber with total internal reflection. The light ray moves through the core of the fiber, with total internal reflection taking place each time when the light ray reaches at the core-cladding interface.
For the light rays that enter into the Optical Fiber with an angle of incidence lesser than the critical angle; the phenomenon of total internal reflection doesn't take place, and these light rays move into the cladding instead of returning back into the same denser medium (core).
So, we can see that for the light rays to travel through the Optical Fiber, it is necessary that the light rays must have the angle of incidence more than the critical angle at the core-cladding interface, for the total internal reflection to take place.

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Optical fiber: structure and working principle
Optical fiber sources (desirable properties)
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