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 P_noise=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.

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.