Propagation Mechanism of Optical Fiber for Engineering Physics

Light propagation of Optical Fiber:

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 1014Hz to 1015 Hz).

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Fig.1. The optical spectrum

 

In space light propagates with velocity c =3×108 m/s. In a transparent medium, the speed of light is affected by a factor n, the refractive index of the medium.image18

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.image19

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Animation 1- Refraction-reflection of waves.
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Animation 2- Refraction-reflection theory (critical angle).

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 thcritical angle.

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Animation 3- Refraction-reflection theory (total reflection).

For angles of incidence greater than the critical angle, the rays are totally reflected. – Total internal reflection occurs .

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 >n2assuring that light rays are reflected back to the core.

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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 2- Representation of the acceptance angle.

 

The cone of acceptance of light into the core is defined by the  acceptance angle of the fibre, whose maximum value can be calculated directly by:

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Assuming that the refractive index of the medium outside the fibre is 1 (air) , the result above can be expressed as:

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The numerical aperture measures the light gathering capacity of a fibre and is the sine of the maximum acceptance angle:

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Figure 3  – Numerical Aperture representation

 

Another important parameter is the NORMALIZED REFRACTIVE INDEX DIFFERENCE:

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whose relation with the  numerical aperture is:

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The normalized frequency V is also an important quantity . It depends on the core radius, the numerical aperture of the core and the wavelength of light stimulating the fibre.

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The V parameter allows the determination of the fibre operating regime: with only one propagating mode (monomode propagation) or several propagating modes (multimode propagation). The last situation leads to an increased distortion of the signal due to the different propagation velocities of the modes.

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