2.6 Fibre Optics

Question 1

What does a ray represent?

Answer 1

A ray represents the direction in which a light wave travels and is therefore always at right angles to the wavefronts.

Question 2

What three things happen to the incident ray?

Answer 2

It partially reflects, partially refracts and partially becomes absorbed at the surface.

Question 3

What happens with the energy when a ray is reflected, refracted and absorbed?

Answer 3

Since energy must be conserved, the total incident energy will be equal to that reflected refracted and absorbed.

Question 4

What is the law of reflection?

Answer 4

The angle of the reflection = the angle of incidence

Question 5

What is Snell’s law?

Answer 5

The ratio of the singe of the angle of incidence (Θ1) to the sine of the angle of refraction (Θ2) is a constant for two different transparent substances.
This constant is the refractive index going from substance 1 to 2.

Question 6

What happens when e-m waves enter an optically denser substance?

Answer 6

They slow down.

Question 7

What happens when e-m waves enter an optically less dense substance?

Answer 7

They speed up.

Question 8

What happens to the direction of the ray when it travels into an optically less dense substance?

Answer 8

It refracts away from the normal, increasing the angle of incidence and this eventually means that the angle of refraction will become 90 degrees and lie along the boundary between the two substances.

Question 9

Define critical angle.

Answer 9

The angle of incidence beyond which rays of light passing through a denser medium to the surface of a less dense medium are no longer refracted but totally reflected.

Question 10

When is the ray totally internally reflected?

Answer 10

When the angle of incidence becomes greater than the critical angle the ray is totally internally reflected and trapped in the optically denser substance.

Question 11

What are optical fibres?

Answer 11

Thin threads of glass or plastic designed to carry light waves with the minimum attenuation using total internal reflection.

Question 12

What happens when a pulse of light is passed into an ideal fibre from a transmitter at one end?

Answer 12

The same pulse will be picked up by the receiver at the other end.

Question 13

In optical fibres, what is the transparent core surrounded by?

Answer 13

A transparent cladding of slightly lower refractive index than the core.

Question 14

In optical fibres, what is around the fibre?

Answer 14

A plastic sheath which strengthens the fibre and protects it from damaging scratches which could leak light.

Question 15

Why is cladding necessary in optical fibres?

Answer 15

To surround the core with a material of lower refractive index and also to make sure that light will not pass directly from one fibre to another if two fibres were to come into contact, this would mean cross over of signals and a very insecure communication channel.

Question 16

Why are cores made increasingly ever thinner?

Answer 16

To avoid multi path dispersion.

Question 17

Why would multi path dispersion be significant in a wide core?

Answer 17

Because the rays from a same part of a pulse would travel very different distances when travelling along the axis compared making many reflections off the core/cladding boundary. The effect of this would be to spread the pulse out.

Question 18

What would the effect of pulse spreading have on the number of pulses per second?

Answer 18

It limits the number of pulses per second that could be transmitted down a fibre and still be recognisable as separate pulses at the other end.

Question 19

How is the refractive index across the core gradually reduced in a graded index fibre?

Answer 19

From the centre to the edges.

Question 20

When does light travel faster, low refractive or high refractive index?

Answer 20

Light travels faster in a low refractive index material than in a high index material.

Question 21

Where does light travel fastest in graded index fibre?

Answer 21

The light rays near the edges of the core travel faster but over a longer distance, meaning that they curve due to refraction and travel through the core in approximately the same time as the straighter rays travelling more slowly near the centre of the core.

Question 22

Due to interference between the rays, only certain angles of incidence at the fibre/cladding boundary will allow rays to be transmitted. What are these angles called?

Answer 22

Modes- and they satisfy the conditions for constructive interference- other directions will results in cancellation by destructive interference.

Question 23

When does absorption occur in optical fibres?

Answer 23

When the light encounters impurities or atomic defects in the class.
Some absorption is due to the glass itself whereby infrared and ultraviolet are naturally absorbed because their frequencies match the natural frequencies of the glass atoms.

Question 24

When does scattering occur in optical fibre?

Answer 24

When the light ‘collides’ with certain molecules in the glass and scatters the beam in all directions, limiting the energy in the forward direction.

Question 25

What can attenuation be compensated by?

Answer 25

Repeaters- these are positioned at regular intervals and convert the signal to an electrical signal which is fed to a transmitter which sends the optical signal onwards at a higher intensity.

Question 26

Explain why graded-index fibres can cope with more frequent pulses of information than ordinary fibres.

Answer 26

All rays paths take same length of time

Pulses do not overlap as much

Question 27

How does the refractive index in the fibre vary with position in the core and in the
cladding of the graded-index fibre?

Answer 27

Cladding- lower than core

Max in centre and reducing towards outside of the fibre

Question 28

Explain why dispersion occurs in an optical fibre.

Answer 28

different wavelengths/ different speeds/different refractive indices

Question 29

Explain why graded index fibres can cope with more frequent pulses of information than ordinary fibres.

Answer 29

All Ray paths take same amount of time.

Reduces pulse dispersion.

Question 30

The cladding increases the critical angle between the two materials. The benefits of this are:

Answer 30

(a) only those rays that are close to the axis of the fibre pass through
(b) the inner fibre is protected from damage
(c) the rays all travel roughly the same distance and so information fed in at one end arrives at the other only very slightly spread out in time
(d) there are fewer reflections and the distance travelled is smaller than the multiple reflection case and so there is less energy loss and the time of transmission is shorter