Polarization-maintaining fibers are applied in devices where the polarization state cannot be allowed to drift, e. as a result of temperature changes. Examples are fiber interferometers, fiber-optic gyroscopes and certain fiber lasers. In fiber optics, polarization-maintaining optical fiber (PMF or PM fiber) is a single-mode optical fiber in which linearly polarized light, if properly launched into the fiber, maintains a linear polarization during propagation, exiting the fiber in a specific linear polarization state; there is. In polarization-maintaining single-mode fibers (PM fibers), the fiber symmetry is broken by integrating stress elements in the fiber cladding. There are different ways to design and build PM fibers.