Generalized neutral axes in nondepolarizing optical systems

The polarizing properties of optical systems are often characterized by their action on specific polarization states. For example, half-wave plates are used to rotate linear polarizations and quarter-wave plates to turn a linear polarization into an elliptical polarization and into a circular polarization if the linear polarization makes a 45° angle with the slow and fast axes of the quarter-wave plate. Phase shifts introduced by the optical train of an interferometer may lead to coherence losses and the existence of neutral axes-in the sense of linear polarizations whose polarization state is not modified by the optical system-is then of importance to maximize the fringe contrast. Neutral axes do not systematically exist. The purpose of this paper is to investigate how this notion can be generalized to define generalized neutral axes for optical systems. Generalized neutral axes are defined as linear polarizations whose polarization state is not modified by the optical system except for their orientation. It is shown that such generalized neutral axes exist for some classes of optical systems. The scheme proposed in this paper has quasi-unitary Jones matrices to give an approximate description of optical systems when generalized neutral axes do not exist. To the best of my knowledge, this formalism is a new scheme to describe the polarizing properties of nondepolarizing optical systems.