Polarization degree fading during propagation of partially coherent light through retarders

A degree of polarization (DOP) of partially coherent light diminishes during transmission through birefringent media. The phenomenon is well known in optical fibers even with small birefringence, for example for optical fiber rotation sensor based on idea of optical fiber Sagnac interferometer [1] as well as for polarimetric fiber optic sensors [2]. In the paper, we deal with a problem of DOP changes of partially temporal coherent light during propagation through linear retarders. In general, a linear retarder resolves a light wave into two orthogonal linearly polarized components travelling with two different velocities due to birefringence of a retarder. As a result, a phase shift between both components arises and a state of polarization (SOP) changes. The most popular retarders are made as quarteror half-wave plates and they can either transform linearly polarized light into circular or rotate linear polarization, respectively. An ideal retarder should not change the DOP, particularly that quarter-wave plate is used as element of DOP measurement set-up. In a reality, DOP for partially temporal coherent light changes due to transmission through retarders in a way presented in the paper.