a method and apparatus for measuring the noise level in an optical amplifier in the presence of a signal at a signal wavelength . the signal is detuned to a second wavelength different from the signal wavelength . a difference function corresponding to the difference in noise levels before and after detuning of the signal as a function of wavelength is determined . the noise level at the signal wavelength is measured . the value of the difference function at the signal wavelength is added to the noise level measured in the previous step .