Decoupling retardance, enpolarization, and depolarization properties from a Mueller matrix: discussion

The intrinsic connection between enpolarization and depolarization in linear polarimetric transformations becomes evident from the fact that, in general, the measured Mueller matrix-representing integral effects over temporal, spatial, and spectral domains-exhibits both behaviors in an inseparable manner. This entanglement prevents the unambiguous assignment of enpolarizing and depolarizing effects to distinct serial components of the Mueller matrix. In particular, the diattenuators arising in serial decompositions typically display polarizance or diattenuation properties that differ from those of the original system, and a similar situation occurs for the depolarizing component. As for the characterization of retardance, it requires the introduction of entrance and exit retarders, whose definition must rely on appropriate and physically meaningful conventions. This work discusses the problem of decomposing a general Mueller matrix into an equivalent serial system in which the enpolarizing-depolarizing and retarding properties are isolated in separate components. Based on the algebraic structure of Mueller matrices, the proposed solution enables the identification of a set of 16 parameters that independently characterize the system's enpolarizing, depolarizing, and retarding features. The interpretation and physical significance of these parameters are analyzed and discussed.