A telecommunications system is formed from a single mode optical fiber carrying Wave Division Multiplex (WDM) traffic. A transmission fiber coupler is arranged to couple signals from channels to be added to the single mode optical fiber. A coupler connects the channels carrying the signals to be added to the input of an optical amplifier, the output of the amplifier being connected in series to the transmission fiber coupler by a switchable multiband band-stop filter arranged to pass only signals having the wavelength of a signal or signals to be added to the WDM traffic. The switchable multiband band-stop filter may be a fiber Bragg filter, a selectable stack of filters or an assembly of bleachable filters. A switchable multiband filter is formed from a stack of layers of a semiconductor bleachable medium whereby the bleaching threshold is that of each single layer and the attenuation is the sum of the transmissions through all the layers comprising the stack.