Optical Fibre

An optical fibre is a thin, flexible strand of glass (or plastic) that transmits light signals over long distances by total internal reflection.

How light travels through a fibre

Light enters one end of the fibre at a small angle to the axis. When the light hits the boundary between the glass core and the outer cladding, the angle of incidence is greater than the critical angle. Total internal reflection occurs: all the light is reflected back into the core. This process repeats along the entire length of the fibre, keeping the light trapped inside.

Conditions for total internal reflection

1. Light must travel from a denser medium (glass core) to a less dense medium (cladding).
2. The angle of incidence at the boundary must be greater than the critical angle.

The cladding has a lower refractive index than the core. This ensures total internal reflection can occur.

Uses of optical fibres

Telecommunications: optical fibres carry digital signals (pulses of light) for internet, telephone, and television. Advantages over copper cables:

• Higher bandwidth (more data per second)
• Less signal loss over long distances
• No electromagnetic interference
• Thinner and lighter

Medical endoscopes: a bundle of optical fibres transmits light into the body and carries the reflected image back to a camera. This allows doctors to see inside a patient without surgery.

Signal degradation

Over very long distances, the light signal weakens. Repeaters (amplifiers) are placed at intervals along the fibre to boost the signal. The signal can also spread out (pulse broadening), which limits the data rate. Using a single-mode fibre (very narrow core) reduces this effect.