A Note on the Resonant Modes and Spatial Coherency of a Fabry-Perot Maser Interferometer

This communication has considered the case of a Fabry-Perot cavity with light impinging on one end while the field distribution on the other end is observed. It has been shown that if one starts with a wave that can be expanded into the modes of the integral operator, and if the medium in the cavity is passive, more than one mode would be observed at the other end although the amplitude of the dominant mode can be much greater than that of the other modes. It was further shown that if there is present an active medium with sufficient gain there will be only a single mode, and the single mode is necessary and sufficient for complete spatial coherency.