An optic fibre is used for the transmission of digital telephone signals. The power input to the optic fibre is 
. The effective noise level in the receiver circuit is 
, as illustrated in the figure below.
The signal-to-noise ratio at the receiver must not fall below 
. For this transmission without any repeater amplifiers, the maximum length of the optic fibre is 
.
Calculate the minimum input signal power to the receiver.
Use your answer in (a) to calculate the attenuation in the fibre.
Determine the attenuation per unit length of the fibre.
.
.
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