Optical Fibre Structure Basics

An optical fibre is a thin strand of glass or plastic used to transmit light signals over long distances. It consists of two main parts:

• Core: The central part of the fibre through which light travels.
• Cladding: The layer surrounding the core.

The question asks about the material property of the core in an optical fibre.

Principle of Light Propagation in Optical Fibres

Light travels through an optical fibre using the principle of Total Internal Reflection (TIR). For TIR to occur, specific conditions must be met:

1. Light must be travelling from a medium with a higher refractive index to a medium with a lower refractive index.
2. The angle of incidence at the boundary between the two media must be greater than the critical angle.

In an optical fibre, the light signal is intended to travel within the core. Therefore, the core must be designed to reflect light back into itself, rather than allowing it to escape into the cladding.

Explanation of Core Material Property

To ensure light stays within the core via TIR, the core material must have a higher refractive index (let's denote it as $n_{core}$) compared to the cladding material (let's denote it as $n_{cladding}$). This means $n_{core} > n_{cladding}$.

When light travelling through the core strikes the boundary with the cladding at an angle greater than the critical angle ($ \theta_c $), it is totally reflected back into the core. This process repeats along the length of the fibre, allowing the light signal to propagate efficiently with minimal loss.

The critical angle is determined by the refractive indices of the core and cladding according to the formula: $ \sin(\theta_c) = \frac{n_{cladding}}{n_{core}} $

If the core had a lower refractive index than the cladding ($ n_{core} < n_{cladding} $), light travelling from the core to the cladding would refract away from the normal, and TIR would not be possible. This would prevent light from being guided along the fibre.

Density is a physical property, but it does not directly determine the optical behaviour required for light transmission in fibres. The critical factor is how the material bends light, which is determined by its refractive index.

Conclusion on Core Material

Based on the principle of Total Internal Reflection, the core of an optical fibre must be made of a material with a higher refractive index than the surrounding cladding to ensure efficient light transmission.