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Question 18M.3.SL.TZ2.10

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Date May 2018 Marks available [Maximum mark: 10] Reference code 18M.3.SL.TZ2.10
Level SL Paper 3 Time zone TZ2
Command term Calculate, Determine, Discuss, State, Suggest Question number 10 Adapted from N/A
10.
[Maximum mark: 10]
18M.3.SL.TZ2.10
(a)

An optic fibre of refractive index 1.4475 is surrounded by air. The critical angle for the core – air boundary interface is 44°. Suggest, with a calculation, why the use of cladding with refractive index 1.4444 improves the performance of the optic fibre.

[3]

Markscheme

sin c =  1.4444 1.4475  or sin c = 0.9978

critical angle = 86.2«°»

with cladding only rays travelling nearly parallel to fibre axis are transmitted

OR

pulse broadening/dispersion will be reduced

 

OWTTE

[3 marks]

An optic fibre of length 185 km has an attenuation of 0.200 dB km–1. The input power to the cable is 400.0 μW. The output power from the cable must not fall below 2.0 μW.

(b.i)

Calculate the maximum attenuation allowed for the signal.

[2]

Markscheme

attenuation = «10 log I I 0 » = 10 log 2.0 × 10 − 6 400 × 10 − 6

attenuation = «–»23 «dB»

 

Accept 10 log 400 2.0  for first marking point

[2 marks]

(b.ii)

An amplifier can increase the power of the signal by 12 dB. Determine the minimum number of amplifiers required.

[2]

Markscheme

185 × 0.200 = 37 loss over length of cable

« 37 − 23 12 = 1.17» so two amplifiers are sufficient

 

[2 marks]

(b.iii)

The graph shows the variation with wavelength of the refractive index of the glass from which the optic fibre is made.

                                           M18/4/PHYSI/SP3/ENG/TZ2/10.b.ii

Two light rays enter the fibre at the same instant along the axes. Ray A has a wavelength of λA and ray B has a wavelength of λB. Discuss the effect that the difference in wavelength has on the rays as they pass along the fibre.

[2]

Markscheme

mention of material dispersion

mention that rays become separated in time

OR

mention that ray A travels slower/arrives later than ray B

[2 marks]

(c)

In many places clad optic fibres are replacing copper cables. State one example of how fibre optic technology has impacted society.

[1]

Markscheme

high bandwidth/data transfer rates

low distortion/Low noise/Faithful reproduction

high security

fast «fibre» broadband/internet

high quality optical audio

medical endoscopy

 

Allow any other verifiable sensible advantage

[1 mark]

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