(a)[2]
Calculate the least input signal power to the receiver.
(b)[2]
Use your answer in (a) to calculate the attenuation in the fibre.
(c)[1]
Determine the attenuation per unit length of the fibre.
Physics 9702 · AS & A Level · Characteristics of alternating currents
Characteristics of alternating currents — practice question
An optic fibre is employed to carry digital telephone signals. The power fed into the optic fibre is $9.8\,\text{mW}$. The effective noise level in the receiver circuit is $0.36\,\mu\text{W}$, as shown in Fig. 12.1. The signal-to-noise ratio at the receiver must stay at or above $28\,\text{dB}$. For this transmission with no repeater amplifiers, the greatest possible length of the optic fibre is $85\,\text{km}$.