In fiber optics, polarization-maintaining optical fiber (PMF or PM fiber) is optical fiber in which the polarization of linearly-polarized light waves launched into the fiber is maintained during propagation, with little or no cross-coupling of optical power between the polarization modes. Such fiber is used in special applications where preserving polarization is essential.
Several different designs of PM fiber are used. Most work by inducing stress in the core via a non-circular cladding cross-section, or via rods of another material included within the cladding. Several different shapes of rod are used, and the resulting fiber is sold under brand names such as "Panda" and "Bow-tie". The differences in performance between these types of fiber are subtle. Some of the differences are explained in the article "PANDA-style fibers move beyond telecom".
Polarization-maintaining optical fibers are used in special applications, such as in fiber optic sensing, interferometry and quantum key distribution. They are also commonly used in telecommunications for the connection between a source laser and a modulator, since the modulator requires polarized light as input. They are rarely used for long-distance transmission, because PM fiber is expensive and has higher attenuation than singlemode fiber.
Polarization-maintaining fiber will not polarize light like a polarizer does. Rather, PM fiber maintains the existing polarization of linearly-polarized light that is launched into the fiber with the correct orientation. If the polarization of the input light is not aligned with the stress direction in the fiber, the output will vary between linear and circular polarization (and generally will be elliptically polarized). The exact polarization will then be sensitive to variations in temperature and stress in the fiber.
The output of a PM fiber is typically characterized by its polarization extinction ratio (PER)—the ratio of correctly to incorrectly polarized light, expressed in decibels. The quality of PM patchcords and pigtails can be characterized with a PER meter.
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