Optical Fibres

Question 1

What are optical fibres made from?

Answer 1

High-density glass

Optical fibres can carry light signals long distances without losing any light through their sides.

Question 2

Define critical angle in optics.

Answer 2

Least angle of incidence for total internal reflection

Applies when light tries to leave a dense medium into a less dense medium.

Question 3

What happens as the angle of incidence increases in optical refraction?

Answer 3

Angle of refraction also increases

Some light is always reflected internally.

Question 4

What is the relationship of angles at the critical angle?

Answer 4

At critical angle, i = C and sinr = sin 90° = 1

The refracted beam disappears and total internal reflection occurs.

Question 5

What is the equation for critical angle, C?

Answer 5

sin C = 1/n

Where n is the refractive index of the medium.

Question 6

What happens to light entering the cut end of a fibre?

Answer 6

It is refracted to hit side surfaces at angles greater than critical angle

This results in total internal reflection.

Question 7

List three advantages of optical fibres over copper wires.

Answer 7

• Lower losses
• More secure
• Higher frequencies

Optical fibres can travel further before needing amplification.

Question 8

What is bandwidth in the context of a network?

Answer 8

Measure of distinct signals at different frequencies

High-frequency carrier waves can accommodate many small ‘bands’ of frequencies.

Question 9

What type of fibres are used for the long-distance backbone of broadband networks?

Answer 9

‘Single mode’ fibres

These fibres have a very narrow core (~8 um) allowing only one light path.

Question 10

What is the diameter of the dense glass core in a single-mode optical fibre?

Answer 10

8 um

The core is surrounded by less dense glass cladding.

Question 11

Calculate the critical angle for light leaving glass of refractive index 1.40 to enter air.

Answer 11

sin C = 1/1.40

The critical angle can be calculated using the refractive index.

Question 12

Fill in the blank: Optical fibres cost more and need _______.

Answer 12

specialist installation

This is a disadvantage compared to copper wires.

Question 13

What is the diameter of the jacket of a single-mode optical fibre?

Answer 13

400 um

The jacket protects the fibre and is larger than both the core and cladding.