A device with an optical fibre amplifier in which theres is a regulating circuit to stabilise the amplification. An additional optical signal (z) is applied to the input of the optical fibre amplifier within its optical amplification bandwidth in the regulating circuit. Its level is controlled by a control unit (Q) to the first input of which is applied a first measuring signal (m1) representing the level of an optical signal from a first region of the amplification bandwidth amplified by the optical fibre amplifier (V). At the second input of the control unit there is a second measuring signal (m2) representing the level of a so far unamplified optical signal from a second range of the amplification bandwidth.
