1. Structure of an optical transmission system

A digital fibre-optic transmission system comprises the following devices and components (Figure 1 ):

• a source (laser diode) and a modulator for printing the information to be transmitted on the optical carrier. The modulation used is intensity modulation. Ideally, it's an all-or-nothing modulation, with one state of the binary data associated with the transmission of a certain intensity, and the other with the absence of a signal. In practice, the transmitter is characterized by an extinction ratio, the difference (in decibels) between the power transmitted in each of the two states, which is not infinite. In a WDM system, there are as many sources as there are channels;

• in the case of a...