Polarization-maintaining (PM) optical fiber finds special applications in sensing, interferometry, quantum key distribution, telecommunications where the polarization states cannot be allowed to drift. PM fibers have a fiber with strong built-in birefringence given polarization of light launched is aligned with one of the birefringent axes even if the fiber is bent. The physical principle behind this can be explained in terms of coherent mode coupling. The propagation constants of the two polarization modes are different due to the strong birefringence, so that the relative phase of such co-propagating modes rapidly drifts away. 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.