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Do You Know About PM Fiber Patch Cords?

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

Definition of PM Patch Cables

At the very first beginning, let’s check the basic definition of 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.

A PM optical fiber is a single-mode optical fiber in which linearly polarized light, if properly launched into the fiber, maintains a linear polarization during propagation, exiting the fiber in a specific linear polarization state. PM fiber patch cord is a fiber optic cable made with PM fiber and terminated on both ends with high-quality ceramic fiber connectors.

Principle of PM Fiber Patch Cord (Polarization Maintaining Fiber Jumpers / Patch Cable）

Single-mode optical fibers which can preserve the same state of polarization are usually called polarization-maintaining (PM) fibers but are sometimes called polarization-holding or polarization-preserving fibers. The principle of PM fibers (PM fiber patch cord, polarization-maintaining fiber jumpers/patch cable) is to create a strong birefringence that splits the polarization-mode degeneracy of ordinary single-mode fiber. When light is coupled in one principal (or eigen) polarization state, it will remain mostly in this input state, as the induced birefringence is much higher than the spurious defects and yields phase mismatch.

The currently most popular PM fiber is the so-called PANDA fiber (where PANDA stands for ‘polarization-maintaining and absorption reducing) which relies on stress-induced birefringence. Technically speaking, one particular advantage of PANDA fibers compared with most other variants is the fact that the PANDA core size and numerical aperture are more compatible with regular single-mode fiber. This ensures minimum loss in devices using both types of fiber.

Connector Types for PM Fiber Patch Cord (Polarization Maintaining Optical Fiber Patch Cable)

For PM fiber patch cord, available connectors that are compatible with many industry standards: FC/UPC, FC/APC, SC/UPC, SC/APC, LC/UPC, LC/APC, etc.

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 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 with 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.