Application of rare-earth doped fibres as lowpass filters in passive optical networks

In the passive optical networks currently under study, it is necessary to provide high attenuation of the 1500-1600 nm fibre window in front of the telephony receiver and transmitter. A cheap, simple filter is required which also has low loss at 1300 nm. Fibres doped with suitable concentrations of the rare-earth ions, erbium and neodymium are shown to have spectral attenuation characteristics ideal for this application.