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. 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. 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 a single-mode fiber, a source laser's output is transmitted with two linear polarization modes propagating at right angles to each other. Imagine for a moment that this fiber is an ideal single-mode waveguide: there is no lateral stress (no external stress from cabling, placement, supports. Polarization-Maintaining (PM) optical fiber is a type of single-mode optical fiber designed to maintain the polarization state of light propagating through them. The mentioned problem can be fixed by using a polarization-maintaining fiber, which is not. According to the transmission polarization state, SMF can be further classified into non-polarization-maintaining optical fiber (referred to as non-PMF) and polarization-maintaining optical fiber (referred to as PMF). Based on wave theory, this chapter will discuss the transmission and. It is often abbreviated as PMF or PM fiber.