In Fiber optics, a polarization-maintaining Optical Fiber (PM) is an optical fiber in which the Polarization planes of Lightwaves launched into the fiber are maintained during propagation with little or no cross-coupling of Optical Power between the polarization modes.

They are rarely used for long-distance transmission, because PM Fiber is expensive and has higher Attenuation than single Mode fiber. They are commonly used for Telecommunication applications, fiber optic sensing and interferometry.

Also called polarization preserving fiber, polarization maintaining optical fibers are Single Mode fibers.