In this article, the latest in FOC's series covering specialty fibers and their fabrication, we discuss polarization-maintaining (PM) fibers and the various approaches used to make them. There are several PM fiber designs – all quite different and each with its own. 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. Polarization-maintaining fibers are mostly single-mode fibers, only in rare cases few-mode fibers, and apparently never highly multimode fibers. The light is then guided in two perpendicular principle states of polarization with different propagation constants – the fast and the slow axis. The field distribu-tion (mode field) of the light exiting the fiber is close to Gaussian.