A method determines power of a modulated signal that is applied to a wavelength meter by summing bin values within a designated bin range of a frequency transformed interferogram representing the modulated signal and provided by the wavelength meter. In a first embodiment of the method, the bin range within which the bin values are summed is designated by mapping a series of signal characteristics indicative of the types of the modulated signals applied to the wavelength meter, to a series of bin spans within the frequency transformed interferograms that represent the modulated signals. The method then enables a selection of a signal characteristic from the series of signal characteristics to identify the modulated signal that is applied to the wavelength meter. In response to a selection, the bin values are summed within a bin range that is consistent with the mapping of the series of signal characteristics to the series of bin spans and that is positioned about a center bin of the frequency transformed interferogram. In a second embodiment of the method, the bin range within which bin values are summed is designated automatically based on attributes of the frequency transformed interferogram.