Waves - optical fibres

Question 1

Step-index fibre

Answer 1

When cladding(low) and core(high) have different refractive index - allows Total Internal Reflection.

Question 2

Core

Answer 2

Medium for light to be internally reflected.

Question 3

Cladding

Answer 3

Prevents interference/leakage from/into other fibres. Prevents signals from passing over to another fibre. Protects core from external damage. Increases critical angle.

Question 4

Cladding

Answer 4

increases critical angle
and reduces effect of modal dispersion.

Question 5

Increasing critical angle:

Answer 5

Reduces modal dispersion.

By decreasing refractive index ratio between core and external environment(/cladding).
By narrower core.

Question 6

Signal degredation:

Answer 6

attenuation: signal loses energy to material absorption as the cable becomes longer.

material dispersion: different frequencies travel at different speeds causing individual wavelengths to reflect at different angles causing the signal to become broader. Monochromatic vs white light.

modal dispersion: the spreading in time

dispersion causes pulse broadening.
attenuation decreases the amplitude.

Question 7

Attenuation

Answer 7

Reduction in intensity as wave travels through material.

Scattering
Absorption
Bending losses

Amplifying signal.
Low absorption material.

Question 8

time taken for ray to travel

Answer 8

nL
/
c sin theta

where c = wave speed, l = length, n = refractive index