Criteria for selecting Optic fiber materials

Apart from the transparency there are several factors which affect the propagation of light through optical fibres. Some of the common factors are discussed here

1. Light Guidance

After several years of study, several parameters were developed to measure the light guidance through glass fibres. Since glass is a transparent material, the design of the fibre for multimode transmission should have been in such a way that light propagates through total internal reflection. This was achieved buy using high refractive index glass at the core and a lower refractive index glass as the cladding. This was governed by Snell’s law

Snell’s law -  

n1 sin 1 = n2 sin 2

1 and 2 are angles of incidence and refraction respectively

n1 and n2 are the refractive indices of two medium

This law was decisive in finding the critical angle of incidence to cause total internal reflection in the fibre. 

Another important parameter to be considered is the numerical aperture of the fiber which indicates the light gathering capacity of the fibre. 

Numerical aperture

NA = sin 1 = √ (n1^2 - n2^2)

The figure below [ Fig 1] shows the output power for different refractive indices

Fig 1

The number of modes that can propagate is governed by V number ( normalized frequency)

If V < 2.405, the fibre would be single mode step Index fibre.

2. Attenuation

Attenuation in case of optical signals are considered wrt the intensity of light travelling through fibre.

The attenuation is given by

Where Io is the initial intensity and L is the distance. After which the measurement is done. 

The first challenge was the selection of glass which operates at required wavelength. Because, at different wavelengths, the behavior of glass changes. At low wavelengths, due to electronic transition of glass cations, UV absorption takes place. It decays exponentially with increase in wavelength and becomes negligible at  near IR range. 

Rayleigh scattering appears depending on the density of the glass and type of material used. Whenever there are density fluctuations comparable to the wavelength of light, scattering occurs. Also the imperfections in the fibre leads to Rayleigh scattering. 

At higher wavelengths , losses occur due to molecular vibration in glass material.  At certain point of wavelength the losses would be minimum which is shown in figure below [ Fig 2]

Fig 2

3. Dispersion

Dispersion is one of the important parameters to be considered in the era of digital communication. The transmission of digital signals essentially means the discrete propagation of light. Thus, light transmits in the form of pulses. These pulses will broaden as they travel. This causes Inter Symbol Interference. This is termed as Dispersion. In single mode fibers, the dispersion is termed as Chromatic dispersion which is the total dispersion caused by material and waveguide dispersion. In multimode fiber, there can be dispersion between different modes when their energy content matches. This is termed as Intermodal dispersion. The dispersion also depends upon the refractive index profile which is depicted in figure below [ Fig 3]

Fig 3

Fig 4

Fig 4 shows the magnitude of different types of dispersion. 

4. Fabrication

The properties of the glass also depend on the type of fabrication. There are different fabrication methods like 

• Double Crucible/ Chemical Vapor Deposition method

• Outside Vapor deposition

• Vertical axial deposition

• Modified Chemical Vapour deposition

• Plasma Chemical vapour deposition

The method will be chosen based on the composition of the glass used for different applications.