Power equalisation in a WDM optical transmission system comprising several amplifiers

The system uses frequency filtering after amplifiers to retain power equalisation. The wavelength multiplexed optical fibre transmission system uses a number of amplifiers. The wide band load signal transmission system (5-8) uses a signal which is outside the multiplexing wavelengths (1-4) . The transmission system has a variable output power. The load signal may extend over the band of the amplifiers, beyond the multiplex wavelengths, or may lie between the multiplex wavelengths. The amplifier may include a number of filters applied to its output, these filters rejecting wavelengths which are immediately around the wavelengths of the multiplex signals.