Polarization-maintaining optical fiber

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In fiber optics, polarization-maintaining optical fiber (PMF or PM fiber) is an optical fiber in which the polarization planes of light waves launched into the fiber are maintained during propagation with little or no cross-coupling of optical power between the polarization modes. The maintaining of polarization is increasingly important in modern telecommunication systems, as our society demands higher bandwidth and more complex optical networks.

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 Laser Focus World 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.