Patent ID: 7817921

Claim:
A method for determining a power of an amplified spontaneous emission in an optical fiber amplifier for a WDM signal, wherein the optical fiber amplifier includes at least a first amplifier stage having a predetermined output power set for a measured input power, and wherein the optical fiber amplifier includes a second amplifier stage connected in series after the first amplifier stage, comprising: determining from the output power a first mean inversion level χ I for the first amplifier stage, and determining a first output power of the amplified spontaneous emission at an output of the first amplifier stage by reference to tabulated values which depend on the first mean inversion level χ I , setting a second predetermined output power for a known output power from the first amplifier stage, whereby the gain of both amplifiers stages is kept constant so that the first mean inversion level χ I and a second mean inversion level χ II are linearly dependent, and calculating the power of an amplified spontaneous emission at an output of the second amplifier stage P 2 ASE by adding P 12 ASE and P 22 ASE , wherein: P 12 ASE = ∫ λ 0 λ 1 ⁢ S I ASE ⁡ ( λ , χ I ) ⁢ G II ASE ⁡ ( λ , χ II ) ⁢ a i ⁢ ⁢ ⅆ λ , P 22 ASE = ∫ λ 0 λ 1 ⁢ S II ASE ⁡ ( λ , χ II ) ⁢ ⁢ ⅆ λ , S I ASE (λ,χ I ) is a noise-power density dependent on the first mean inversion level, and S II ASE (λ,χ II ) is a noise-power density dependent on the second mean inversion level, G II ASE (λ,χ II ) is a gain of the second amplifier stage, and a I is an attenuation between the first amplifier stage and the second amplifier stage.