The signal-to-noise ratio in an optic fibre must not fall below 
. The average noise power in the fibre is 
.
Calculate the minimum effective signal power in the optic fibre.
The fibre has an attenuation per unit length of 
. Calculate the maximum uninterrupted length of fibre for an input signal of power 
.
Suggest why infra-red radiation, rather than ultraviolet radiation, is used for long-distance communication using optic fibres.
.
.
