Hybrid Interferometers: A Study Of The Coupling Of Optical Surfaces In Incoherent Light

Two studies (Ward et. al.(1985), Ward et. al.(1987)) have been undertaken to analyse the cause of secondary fringes in a wide angle Michelson interferometer (Shepherd et. al.(1985)) and determine methods for their elimination. Out of these studies some general considerations on the coupling of optical surfaces in incoherent light have been developed. Surfaces which are coupled in such a way that the fringe form must be calculated from amplitude considerations are considered to be coherently coupled whereas surfaces coupled in such a way that the fringe form can be calculated from intensity calculations are considered to be incoherently coupled. Since the observed fringe form of the interferometers considered in the above studies were neither that of a Michelson interferometer nor that of traditional multi-reflection devices such as the Fabry-Perot interferometer, the phrase 'hybrid interferometer' was coined to describe them. In this paper these considerations on the coupling of optical surfaces are summarized. In addition the conditions under which orthogonal polarization states can be used to provide independent sets of fringes is noted. These theoretical considerations are illustrated with examples of several 'hybrid interferometers'.