Polarization-dependent losses occur in optical waveguides due to bends and tiny impurities. These can be avoided by a special design of the optical fiber. Corresponding single-mode fibers are called polarization maintaining fibers(PMF).
In normal optical fibers, light is transmitted in two polarization modes: in the vertical and horizontal polarization planes. These polarization modes are maintained during transmission unless they are disturbed by changes in one polarization plane, for example, by severe bends in the optical fiber or by minute changes in density. This can cause crosstalk between the polarization modes and thus an asymmetry, which is noticeable in different delay times between the two polarization modes.
In single-mode fibers with polarization-maintaining characteristics, additional voltage elements are built into the fiber to create strong double-breaking, resulting in two defined polarization modes: a fast propagation axis and a slow propagation axis. The additional voltage elements in the PMF fiber must be taken into account when splicing such a fiber and require a three-axis orientation of the optical fibers to be spliced and special connectors with anti-rotation protection. Corresponding connectors have a guide pin or notch for anti-twist protection, which prevents twisting.