A method is provided for determining the gain spectrum of an optical amplifier such as an erbium doped optical amplifier (EDFA). In accordance with the method, an optical amplifier such as an EDFA that is to accommodate a specified number of channels at different optical wavelengths is provided. A subset of the specified number of channels at which gain is to be measured is selected. The number of channels in the subset is determined based at least in part on a number of samples required by the Nyquist sampling theorem to reconstruct the gain spectrum. A gain value for each channel in the selected subset of channels is measured for a probe signal that does not perturb the gain spectrum of the EDFA by more than a prescribed amount. The gain spectrum for the EDFA is constructed from the measured gain values.