Beat length is a measure of the phase-velocity difference between the two polarization modes. 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. It is difficult for manufacturers to specify a polarization extinction ratio (PER) for light output by polarization-maintaining (PM) fibers, since this parameter depends on the length of the fiber, how it is routed, and the polarization and alignment of the input light. The linear. In quantitative terms, the polarization beat length should be significantly shorter than the typical length scale on which the parasitic birefringence varies. The two axes in a PM fiber are.