1. Signal-to-noise ratio and Q factor

1.1 Signal-to-noise ratio of a WDM (Wavelength Division Multiplexing) link

A submarine link is made up of a cascade of repeaters spaced 50 to 100 km apart, depending on the link; each repeater comprises erbium-doped fiber amplifiers (EDFAs) through which a multitude of wavelengths pass; the quality of the signal transmitted by all these wavelengths must be absolutely the same throughout the transmission multiplex, which means that the gain of each EDFA must be identical for all the wavelengths transmitted. To achieve this spectral homogeneity, it is necessary to insert optical gain equalizing filters at the output of each EDFA, thus guaranteeing a gain distortion of less than 0.2 dB per EDFA in the spectral band used in submarine links, known as the C band (1,533 to 1,567 nm). For...