Polarization-maintaining connectors feature a positioning key aligned to the slow axis of the fiber. The key permits the connector to be mated only with another connector or component at a single angular orientation. ial that in turn cause phase changes in the polarization state of the light. Polarization-maintaining fibers are mostly single-mode fibers, only in rare cases few-mode fibers, and apparently never highly multimode fibers. The fast axis is the direction. Polarization-Maintaining Optical Fiber (PMOF) is a specialized optical fiber that maintains the stable polarization state during optical transmission by enhancing birefringence.