To solve this problem, PM fibers are developed. These fibers work by inducing a difference in the speed of light for two perpendicular polarizations traveling through the fiber. This birefringence creates two principal transmission axes within the fiber, known respectively as the fast and slow axes of the fiber. Provided the input light into a PM fiber is linearly polarized and orientated along one of these two axis, then the output light from the fiber will remain linearly polarized and aligned with that axis, even when subjected to external stresses. A one meter long connectorized patch cord constructed with PM fiber can typically maintain polarization to at least 30dB at 1550 nm when properly used. Naturally, how well a PM fiber maintains polarization depends on the input launch conditions into the fiber. Perhaps the most important factor is the alignment between the polarization axis of the light with the slow axis of the fiber.
Connectors of PM Patch Cables
Given the importance of the alignment of the PM axis across a connection, the choice of connector is especially important. The most common type of PM connector is FC connector which has a positioning key to preserve the angular orientation of the fiber. The industry standard is to align the slow axis of the fiber with the connector key. The tolerances between the key and keyway on standard FC connectors are too loose to accurately maintain angular alignment, so manufacturers have tightened the key dimension tolerances on PM connectors. The key dimensions being used are based on FC angle polished connector (APC) standards. Unfortunately, two APC standards are currently on the market, a narrow, or reduced key design, and a wide key design. The two dimensions are incompatible with one another, so it is important to know beforehand which design you are using. Besides the FC connectors, PM patch cables using other connector types are also available, such as SC connectors. In all cases, there must be a key or similar structure to act as a reference, and tight tolerances must be kept to ensure that the ferrules cannot rotate.