Abstract: An optical fiber amplifier of the erbium type is intended to amplify light signals within a wavelength band, e.g. in the range of 1540-1555 nm. The active optical fiber receives light signals to be amplified and pump light of a first pump wavelength from a pump source. A gain flattening filter is connected in the active optical fiber length. A noise filter is also connected in the active optical fiber, not more closer to the output end of the active fiber than the gain flattening filter. The noise filter blocks light waves having wavelengths around the ASE gain peak and the gain filter attenuates wavelengths within the useful signal band in order to produce a flat gain curve. The introduction of the noise filter increases the total amplification of the amplifier.

Abstract: An optical fiber amplifier for WDM-signals comprises an active optical fiber pumped with pumping light from a pump source. In order to give the amplifier a constant gain for the WDM-signals irrespectively of the number of active WDM-signals an additional light source is arranged to inject, when required, extra light into the active optical fiber, at least in a portion thereof, through a power combiner. The injected light has a wavelength different from that of the WDM-channels and preferably longer so that it will not cause an amplifying or pumping effect in the active optical fiber but so that it still can be amplified in the active fiber. The light source is controlled by a signal derived from an output power measurement device so that it will inject extra light when a WDM-channel becomes inactive. The power of the extra light is thus selected to saturate the optical fiber amplifier at a constant gain irrespectively of the number of signal channels, i.e. of the total power of the input light signals.