Coupled-cavity lasers are generally reduced by a plane wave treatment of the various mirror pairs, even though laser optics is rather based on beams having a finite lateral extent. We show that this method is questionable by investigating the reflectivity and transmission of a Fabry-Perot Interferometer when the input is a TEM 0 0 , TEM 0 1 , and TEM 0 2 beams, in the multi-frequency case.