optical fibres

State two uses of optical fibres

Medical endoscopes to see inside the body, communications to carry light signals

How does light travel along an optical fibre?

It undergoes total internal reflection each time it reaches the fibre boundary

What condition must be met at every point along an optical fibre for TIR to occur?

The angle of incidence at the boundary must exceed the critical angle of the fibre at every point

When does TIR fail in an optical fibre?

When the radius of a bend is too small. This causes the angle of incidence to drop below the critical angle, so TIR no longer occurs and light escapes

Why is a high refractive index desirable in optical fibre material?

A higher refractive index gives a smaller critical angle, making it easier for light to exceed the critical angle and undergo TIR at every point along the fibre

What is the purpose of cladding in an optical fibre?

To surround the core with a material of lower refractive index, reducing light loss from the core

Why must the cladding have a lower refractive index than the core?

So that TIR can occur at the core cladding boundary, keeping light within the core

Why must optical fibres be highly transparent?

To minimise absorption of light, which would otherwise reduce the amplitude of the pulses as they travel along the fibre

What effect does light absorption have on pulses travelling along an optical fibre?

It progressively reduces the amplitude of the pulses the further they travel

What are the two main causes of amplitude reduction in an optical fibre?

1. Absorption of light by the fibre material

2. Light loss from the core

Describe the structure of a communications optical fibre

A highly transparent core surrounded by a layer of cladding with a lower refractive index than the core

Why does light loss from the core matter in a communications fibre?

It reduces the amplitude of the pulses, meaning the signal weakens over distance and may not reach the receiver intact

Why must the core of an optical fibre be very narrow?

To prevent modal dispersion. In a wide core, light rays travelling different paths travel different distance per metre of fibre, causing pulses to spread out and potentially merge

What is modal (multipath) dispersion?

When light travelling along the axis of a wide core travels a shorter distance than light undergoing repeated TIR, causing a pulse to arrive spread out and longer than it should be

What is the consequence of modal dispersion?

Adjacent pulses merge together, making it impossible to distinguish individual signals at the receiver

What is material (spectral) dispersion?

When white light is used instead an optical fibre, different wavelengths travelling at different speeds in the glass, causing the pulse to spread out faster and slower components separate

Why must monochromatic light be used in optical fibre?

To prevent material dispersion. Monochromatic light has a single wavelength so all components travelling at the same speed, preventing pulse spreading and merging

How does a narrow core prevent modal dispersion?

It limits light to travelling along a single path, so all light travels the same distance and arrives at the same time