Calculate the critical angle at the core−cladding boundary.

The use of optical fibres has led to a revolution in communications across the globe. Outline two advantages of optical fibres over electrical conductors for the purpose of data transfer.

Draw on the axes an output signal to illustrate the effect of waveguide dispersion.

Calculate the power of the output signal after the signal has travelled a distance of 3.40 km in the fibre.

Explain how the use of a graded-index fibre will improve the performance of this fibre optic system.

«sin c = \(\frac{{1.34}}{{1.56}}\)»

c = 59.2«°» 

Accept values in the range: 59.0 to 59.5.

Accept answer 1.0 rad.

[1 mark]

optic fibres are not susceptible to earthing problems

optic fibres are very thin and so do not require the physical space of electrical cables

optic fibres offer greater security as the lines cannot be tapped

optic fibres are not affected by external electric/magnetic fields/interference

optic fibres have lower attenuation than electrical conductors/require less energy

the bandwidth of an optic fibre is large and so it can carry many communications at once/in a shorter time interval/faster data transfer

[2 marks]

a signal that is wider and lower, not necessarily rectangular, but not a larger area

[1 mark]

attenuation = –1.24 × 3.4 «= –4.216 dB»

–4.216 = 10 log\(\frac{I}{{15}}\)

I = 5.68 «mW»

Need negative attenuation for MP1, may be shown in MP2.

For mp3 answer must be less than 15 mW (even with ECF) to earn mark.

Allow [3] for BCA.

[3 marks]

refractive index near the edge of the core is less than at the centre

speed of rays which are reflected from the cladding are greater than the speed of rays which travel along the centre of the core

the time difference for the rays that reflect from the cladding layer compared to those that travel along the centre of the core is less

OR

the signal will remain more compact/be less spread out/dispersion is lower

bit rate of the system may be greater

[3 marks]