3) A single laser source is to be used in an optical fibre detection network containing 4 different detection channels, each of which is 18km away from the source. This can

be achieved using a 1:4 optical fibre splitter, which allows 25% of the power from the laser to pass down each channel. If the fibre has an attenuation coefficient of 0.5dB/km and the required output power is 0.35μW what is the required laser power assuming no joint losses, margin or penalties.