What is Polarization-Maintaining Fiber (PM Fiber)

Polarization-maintaining optical fiber (PM fiber) is a special optical fiber with strong built-in birefringence, which can preserve the properly oriented linear polarization of an input beam. Usually, optical fibers will exhibit some degree of birefringence even when they have a circularly symmetric design, because there always has some mechanical stress or other effect which can break the symmetry. The polarization of light propagating in the fiber gradually changes in an uncontrolled (and wavelength-dependent) way, which also depends on any bending of the fiber and on its temperature. This polarization maintaining feature is extremely important for some fiber optic components such as external modulators that require a polarized light input. So PM fiber is used in special applications where preserving polarization is essential. The following text will let you know more information about PM fiber.

Principle of PM fiber

Provided that the polarization of light launched into the fiber is aligned with one of the birefringent axes, this polarization state will be preserved even if the fiber is bent. The physical principle behind this can be understood as coherent mode coupling. The propagation constants of the two polarization modes are different because of the strong birefringence, so that the relative phase of this co-propagating mode rapidly drifts away. If this difference is large enough, the usual disturbances in the fiber are too slowly varying to do effective mode coupling.

Ways of Realizing PM Fiber

What is Polarization-Maintaining Fiber (PM Fiber) 1

The above picture shows the three types of PM fiber. These optical fibers contain a feature that is not seen in other fiber types. Besides the fiber core, there are stress rods in the fibers. Panda PM fiber has two circles of stress rods, elliptical-clad PM fiber has a elliptical clad, and the Bow-Tie type PM fiber has two bow-ties. Just as the name implies, these stress rods create stress in the core of the fiber so that the transmission of only one polarization plane of light is favored.

PANDA fiber is a telecom fiber. Its attenuation and mode-field diameter were well matched to those of single mode telecom fibers. Bow-tie fiber was invented in the early 1980s as a sensor fiber, and until today, bow-tie fibers are still typically used in sensor applications.

Applications of PM Fiber

  • PM optical fibers are used in special applications, such as fiber optic sensing, interferometry and slab dielectric wave-guides. And they are also used in telecommunication for the connection between a source laser and a modulator, because the modulator needs polarized light as input
  • They may also be used in transmission applications where the polarization plane of the optical signal is important, such as transmission lines for optical sensors and coupling for optical electrical integrated circuits
  • PM fibers are used in lithium niobate modulators, Raman amplifiers, and other polarization sensitive systems to maintain the polarization of the incoming light and keep cross-coupling between polarization modes at a minimum.

Although PM fibers are widely used in various applications, they are rarely used for long-distance transmission, because they are expensive and have higher attenuation than single mode fiber. But they are expected to be used in coherent optical transmission systems or long distance bidirectional optical transmission systems.


PM optical fibers are widely used in polarization sensitive fiber optical systems for transmission of light requires the PM state to be maintained. FS.COM provides PM fiber cables with several different wavelengths with high extinction ratio, low insertion loss and long term reliability. They would be your good choice!

Author: Katherine Wang

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