Optical Fiber | Light guidance through optical fibre

What is Optical Fiber ? What are the main components of Optical fiber? Describe Light guidance through optical fibre.


Optical fiber

Optical fiber is a very thin strand of glass or plastic, which transmit data from one place to another in the form of light by using the phenomenon of total internal reflection. The inner part of the optical fiber is known as core, which surrounds by cladding. The refractive index of core is higher than that of cladding. Thus light traps inside the core due to the total internal reflection at the core-cladding interface. To protect the fibre from damage and moisture, a protective layer called as sheath covers it. Optical fibers arranged in bundles are called optical cables and used to transmit light signals over long distances.

The optical fibers are very useful in communication system. They can carry audio or video signals, different types of data, computer information, etc. The reason to use optical fiber is because it offers some important advantages over conventional metallic wires. The main advantages are as follows:

  • They are cost effective, lightweight, flexible and of very small.
  • They transfer information with speed of light.
  • They have high bandwidth (data carrying capacity) and low signal distortion.
  • They are secure and safe.

Due to the above advantages they are widely used in communication system, medical industry, defense, research, sensors etc.

Light guidance through optical fibre:

The propagation of light through the optical fiber is possible due to the total internal reflection. We know that, when a light ray goes from denser medium to a rarer medium it bends away from the normal. Snell`s law tells us that if the angle of incidence increases, the angle of refraction also increases. The angle of incidence for which the angle of refraction is 90° is called critical angle. Now If the angle of incidence is slightly increased the critical angle, the light is completely reflected at the surface of rarer mediums, this is known as total internal reflection.

Let us consider three light rays enter the fibre at the launching end making different angles with the axis of the fibre as shown in the Fig. It is clear that the ray 2 strikes the boundary of the cladding at an angle equal to the critical angle and travels through the fiber parallel to the core-cladding interface. The ray 1 which enters the fibre at an angle greater than ray 2, strikes the boundary of the cladding at an angle less than critical angle and get refract into cladding. On the other hand, ray 3, which enters the fibre at an angle less than ray 2, undergoes total internal reflection at the core-cladding interface.

It is clear that the light propagates through the fibre only when its angle of incidence at launching end is less than or equal to a certain value. This maximum possible angle with the axis of the fibre up to which a light ray accepted into the core of the fiber is called acceptance angle. The light rays that enter the fibre beyond acceptance angle refracts into cladding.

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