When a freely propagating light beam passes through a limiting aperture into an interferometer a set of the interferometer modes is coupled even when the beam is matched to the interferometer. The incoming beam and the modes of the interferometer are assumed to be described by Gaussian Laguerre functions and the limiting aperture to be circular and on axis with the modes. The coupling coefficients are obtained for an arbitrary incoming mode and arbitrary expanding set. Simpler solutions are found for an incoming Gaussian beam by choosing the expanding set matched to the incoming beam parameters.

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