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.
Suggest why, in transmitting information in an optic fibre, infrared electromagnetic radiation rather than visible light is used.
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.
Markscheme
less noise/attenuation per unit length/crosstalk;
greater bandwidth/security (though encryption);
attenuation/loss of energy depends on wavelength;
and is least for infrared wavelengths;
(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.