What Is Polarization Maintaining (PM) fiber patch cables?

When talking about fiber optic patch cables, you may know LC fiber patch cables or MTP/MPO fiber cables. Besides these cables, there are some special fiber patch cables, such as mode conditioning patch cables, which has been introduced in the previous article. Today we will introduce another special fiber patch cable—polarization maintaining (PM) fiber patch cables.

Definition of PM Patch Cables

At the very first beginning, let’s check the basic definition about the PM patch cables. PM patch cords are based on a high precision butt-style connection technique. The PM axis orientation is maintained by using male connectors with a positioning key and a bulkhead female receptacle with a tightly toleranced keyway, ensuring good repeatability in extinction ratios and insertion losses.

Why Need PM Patch Cables

When a normal fiber is bent or twisted, stresses are induced in the fiber and the stresses will change the polarization state of light traveling through the fiber. If the fiber is subjected to any external perturbations, say changes in the fiber’s position or temperature, then the final output polarization will vary with the time. This is true for even short lengths of fiber, and is undesirable in many applications that require a constant output polarization from the fiber.

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.

Conclusion

PM patch cables are widely used in polarization sensitive fiber optic systems for transmission of light that requires the PM state to be maintained.