optical fibres

Question 1

What are the 3 components of an optical fibre?

Answer 1

1 - optically dense core
2 - lower optical density cladding
3 - outer sheath

Question 2

What does the cladding do?

Answer 2

• protect the core from damage or scratching
• prevent signal degradation
• keeps the signal secure
• keeps the core separate from other fibres preventing information crossover

Question 3

What do material and modal dispersion cause?

Answer 3

Pulse broadening

Question 4

What is material dispersion?

Answer 4

When white light is used in an optical fibre and is separated into all the colours of the spectrum.
Each wavelength of light travels at the same speed in a vacuum but at different speeds in a medium

Question 5

What is modal dispersion?

Answer 5

When light pulses in the optical fibre spread out.
This is because each part of the wavefront has a different angle of incidence and consequently a different angle of reflection
So each part of the wavefront undergoes total internal reflection a different number of times
Hence, each part of the wavefront reaches the end of the fibre at a slightly different time

Question 6

How can you prevent material dispersion?

Answer 6

Use monochromatic light

Question 7

How can you prevent modal dispersion?

Answer 7

Use a narrow core.

Question 8

What is absorption?

Answer 8

When the fibre absorbs part of a signal’s energy. This reduces the amplitude of the signal, which can lead to a loss of information transmitted.

Question 9

What does pulse broadening cause?

Answer 9

The merging of pulses, which distorts the information in the final pulse, and increases the amplitude of the signal.

Question 10

How can you reduce absorption?

Answer 10

• use an extremely transparent core
• use optical fibre repeaters, so that the pulse is regenerated before significant absorption has taken place

Question 11

How can you reduce pulse broadening?

Answer 11

• use a core that is as narrow as possible to reduce the possible path differences in the path length of the signal
• use a monochromatic light source so the speed of the pulse is constant
• use optical fibre repeaters so that the pulse is regenerated before significant pulse broadening has taken place
• use a single-mode fibre, where only a single wavelength of light passes through the core