Single-mode Fiber that preserves the plane of Polarization of the Light launched into it as the beam propagates through its length. Also called polarization-maintaining fiber. The polarization is maintained by introducing asymmetry in the fiber structure, either in its shape (geometrical birefringence) or in its internal stresses (stress-induced birefringence). Because of this asymmetry, the two perpendicularly polarized Modes transmitted by the fiber have different propagation constants, reducing cross-coupling between them as compared with conventional single-mode fiber.