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Date November 2012 Marks available 4 Reference code 12N.3.SL.TZ0.19
Level Standard level Paper Paper 3 Time zone Time zone 0
Command term Determine and Show that Question number 19 Adapted from N/A

Question

This question is about optic fibres.

State one advantage of the use of an optic fibre rather than a coaxial cable for the transmission of information.

[1]
a.

Suggest why, in transmitting information in an optic fibre, infrared electromagnetic radiation rather than visible light is used.

[2]
b.

A signal is fed into an optic fibre of length L.

The noise power at the receiver is Pnoise=4.2 μW. The signal to noise ratio \(\left( {i.e.10\log \frac{{{P_{{\rm{signal}}}}}}{{{P_{{\rm{noise}}}}}}} \right)\) at the receiver must exceed 25 dB.

(i) Show that the minimum signal power at the receiver is 1.3 mW.

(ii) A signal of power 25 mW is input to the optic fibre. The attenuation per unit length of the optic fibre is 0.30 dB km–1. Determine the maximum length L of the optic fibre.

[4]
c.

Markscheme

less noise/attenuation per unit length/crosstalk;
greater bandwidth/security (though encryption);

a.

attenuation/loss of energy depends on wavelength;
and is least for infrared wavelengths;

b.

(i) \(10\log \frac{{{P_{{\rm{signal}}}}}}{{{P_{{\rm{noise}}}}}} \Rightarrow {P_{{\rm{signal}}}} = 4.2 \times {10^{ - 6}} \times {10^{2.5}}\);
(=1.3mW)

(ii) loss in dB=\(10\log \frac{{25}}{{1.3}}\);
loss=12.8 dB;
\(L = \frac{{12.8}}{{0.30}} = 43{\rm{km}}\);
Award [3] for a bald correct answer.

c.

Examiners report

[N/A]
a.
[N/A]
b.
[N/A]
c.

Syllabus sections

Option C: Imaging » Option C: Imaging (Core topics) » C.3 – Fibre optics
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