A method for determining the power of an amplified spontaneous emission in an optical fibre amplifier for a WDM signal, having a first amplifier stage and a second amplifier stage connected downstream in series of the first amplifier stage, characterized - in that a predetermined output power (P OUT 1) is set for a measured input power (P IN 1) and a first average inversion ( χ I) of the first amplifier stage is determined therefrom, - in that a second predetermined output power (P OUT 2) is set in the second amplifier stage for a known output power of the first amplifier stage, in that a total gain (G 0) over both amplifier stages (V1, V2) is kept constant in the process, so that the first average inversion ( χ I) and a second average inversion ( χ II) of the second amplifier stage depend on one another linearly, - in that a total power of the amplified spontaneous emission at the output of the two-stage optical fibre amplifier is determined by means of two ASE contributions P 12 ASE P 22 ASE, of which a first ASE contribution P 12 ASE is generated by the first amplifier stage and this is amplified by the second amplifier stage and a second ASE contribution P 22 ASE is generated by the second amplifier stage, and the total power is calculated using two parameters ( τ 12 ( χ I), τ 23 ( χ I)) determining the ASE contributions P 12 ASE P 22 ASE, which, for a total gain (Go) which is kept constant for both amplifier stages (V1, V2), only depends on one of the average inversions ( χ I; χ II) of one of the amplifier stages (V1, V2), and - in that the total power P 2 ASE of the amplified spontaneous emission at the output of the second amplifier stage is determined using these parameters which are dependent only on one inversion ( τ 12 ( χ I), τ 22 ( χ II)). - in that a predetermined output power (P OUT 1) is set for a measured input power (P IN 1) and a first average inversion ( χ I) of the first amplifier stage is determined therefrom, - in that a second predetermined output power (P OUT 2) is set in the second amplifier stage for a known output power of the first amplifier stage, in that a total gain (G 0) over both amplifier stages (V1, V2) is kept constant in the process, so that the first average inversion ( χ I) and a second average inversion ( χ II) of the second amplifier stage depend on one another linearly, - in that a total power of the amplified spontaneous emission at the output of the two-stage optical fibre amplifier is determined by means of two ASE contributions P 12 ASE P 22 ASE, of which a first ASE contribution P 12 ASE is generated by the first amplifier stage and this is amplified by the second amplifier stage and a second ASE contribution P 22 ASE is generated by the second amplifier stage, and the total power is calculated using two parameters ( τ 12 ( χ I), τ 23 ( χ I)) determining the ASE contributions P 12 ASE P 22 ASE, which, for a total gain (Go) which is kept constant for both amplifier stages (V1, V2), only depends on one of the average inversions ( χ I; χ II) of one of the amplifier stages (V1, V2), and - in that the total power P 2 ASE of the amplified spontaneous emission at the output of the second amplifier stage is determined using these parameters which are dependent only on one inversion ( τ 12 ( χ I), τ 22 ( χ II)).