Fast axis and slow axis of polarization-maintaining fiber

In polarization-maintaining single-mode fibers (PM fibers), the fiber symmetry is broken by integrating stress elements into the fiber cladding. Light is then guided in two perpendicular principal states of polarization, which have different propagation constants – the fast and the slow axis. When splicing two PM fibers, their birefringent axes (usually the “slow” and “fast” axes) must be precisely aligned to avoid coupling light into the unwanted polarization state. The fast axis is the direction. During the drawing process of polarization-maintaining fiber, when linearly polarized light is transmitted along one characteristic axis of the optical fiber, part of the light signal will be coupled into another characteristic axis perpendicular to it, which will eventually cause the polarization. Figure 1. [pdf]