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. Note 1: Cross sections of polarization-maintaining optical fibers range from elliptical to rectangular. Note 2: Polarization-maintaining optical fibers are used in special applications, such as in fiber optic sensing and interferometry. Synonym polarization-preserving (PP) optical fiber.