Question

Explain the propagation of light through optical fibre. What are the advantages of

using optical fibres for communication?

27 Jan 2018
Answer :
Word Count : 855

Visible light extends from 380 nm (violet) to 780 nm (red). For smaller wavelengths ultra-violet radiation (UV) occurs. Longer wavelengths correspond to the infrared region (IR).

Optical Fibres communication elements operate in the micrometer wavelength zone of the frequency spectrum (frequencies between 1014 Hz to 1015 Hz).

Image ignouacademy-com--p-through-13575

Figure 3.1- The optical spectrum

In space light propagates with velocity c =3x108 m/s. In a transparent medium, the speed of light is affected by a factor n, the refractive index of the medium.

Image ignouacademy-com--p-academy-64970

When a light ray encounters the interface of a medium, the light ray is refracted and its direction of propagation changes according to Snell's law of refraction.

Image ignouacademy-com--p-explain-96344

Animation 2 - Refraction-reflection of waves.

If n1>n2, at a certain angle of incidence, the refracted wave propagates parallel to the interface and the ANGLE OF REFRACTION reaches 90° , and the corresponding angle of incidence is called the critical angle.

Image ignouacademy-com--p-optical-95693

Animation 3 - Refraction-reflection theory (critical angle).

For angles of incidence greater than the critical angle, the rays are TOTALLY REFLECTED. - Total internal reflection occurs.

Image ignouacademy-com--p-ignou-44207

Animation 4 - Refraction-reflection theory (total reflection).

Optical fibres are cylindrical waveguides made of two concentric layers of very pure glass. The core (the interior layer ) with refractive index n1 serves as the medium for light propagation, while the cladding (the exterior layer) has a lower refractive index n2 where n1 >n2 assuring that light rays are reflected back to the core.

Image ignouacademy-com--p-light-46280

Figure 3.5 - Optical fibre structure.

Based on the principle of total internal reflection, only rays with incident angles greater than the critical angle, at the CLADDING-CORE interface, can be transmitted:

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Figure 3.6

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