A polarization-maintaining fiber guides two polarization modes but is designed to prevent coupling between them. In contrast, a single-polarization fiber is designed to strongly attenuate one polarization mode, so it effectively guides only light with a single, specific linear. 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. It achieves this not by eliminating birefringence, but by having a very strong, well-defined internal birefringence. Typical PM fiber only preserves the polarization state of input light that is both linearly polarized and polarized parallel to one of the fiber's two. For standard singlemode fibers the light is guided in two principle states of polarization. There are several PM fiber designs – all quite different and each with its own complexities in preform.