The present invention is based on the idea that an optical band-pass filter (22) is employed in addition to its normal function for modulating a signal passing through the fiber amplifier on such a low frequency and amplitude that this modulation does not disturb the actual data transmission. This is realized in such a manner that the control signal of the band-pass filter (22), which is used in prior art fiber amplifiers only for optimizing the signal-to-noise ratio, for example, is used in the amplifier of the present invention for modulating the output signal of the fiber amplifier at the rate of a supervisory message.