Diffraction, TIR and Optical Fibres

Aqa A level Physics

What will happen to the wave if it enters a more optically dense medium?

The wave will slow down and bend towards the normal θᵢ > θᵣ

What will happen to the wave if it enters a less optically dense medium?

The wave will speed up and bend away from the normal θᵢ < θᵣ

Refractive index

A measure of how optically dense a medium is

Absolute refractive index

Measures how much it slows down light (a ratio). n = c / c₁

Critical angle

The angle for which the refracted ray just passes along the boundary line and beyond which all of the wave will be reflected

What conditions are needed for TIR to occur?

·         The light must travel from a more optically dense to a less optically dense medium.

·         The angle of incidence of the light ray must exceed the critical angle of the interface.

What are optical fibres used for?

·         High-speed internet cables

·         Medical imaging as endoscopes

·         Engineering inspections to view hard to reach areas

What 2 main parts does step-index optical fibres consist of?

·         A core made of a high-refractive index material

·         Cladding made of a low-refractive index material

What does cladding do?

·         It provides tensile strength to the optical fibre so that it doesn’t break when twisted

·         It prevents information from transferring between different cores in a bundle

·         Prevents the core from being damaged, for example by getting scratched

Why must the core be protected from scratches?

·         The scratch may alter the angle at which the signal interacts with the core’s boundary such that the angle is lowered below the critical angle - this once again will prevent TIR occurring and cause signal loss outside of the fibre

Why would water getting into the core through the scratch be bad?

·         Water can get into the scratch, which will increase the refractive index to a level that may be higher than the core, and therefore prevent TIR from occurring - instead the signal will exit the core

What is absorption?

Energy is lost as the signal is transferred. It results in a loss of amplitude for the signal but doesn’t affect the frequency.

How can the effects of absorption be reduced?

Using an optical fibre repeater to boost the signal at periodic positions along the fibre, which can help maintain signal strength and quality.

What is modal dispersion?

A consequence of the beams entering the fibre at different angles, which results in each beam undergoing TIR a different number of times and so means that each beam reaches the end at a slightly different time. This results in pulse broadening.

What is material dispersion?

Waves of different wavelengths travel at slightly different speeds through an optical fibre and so reach the end of the fibre at slightly different times, causing pulse broadening. The use of monochromatic light fixes this.