Polarization-maintaining fibers work by intentionally introducing a systematic linear birefringence in the fiber, so that there are two well defined polarization modes which propagate along the fiber with very distinct phase velocities. The beat length Lb of such a fiber (for a particular wavelength) is the distance (typically a few millimeters) over which the wave in one mode will experience a. OverviewIn, polarization-maintaining optical fiber (PMF or PM fiber) is a single-mode in which , if properly launched into the fiber, maintains a linear polarization during,. In an ordinary (non-polarization-maintaining) fiber, different polarization modes have the same nominal due to the fiber's circular symmetry. in such a fiber, or bending. Several different designs are used to create birefringence in a fiber. The fiber may be geometrically asymmetric or have a refractive index profile which is asymmetric such as the design using an elliptical as.