Polarization-maintaining fibers form fast and slow orthogonal axes due to the strong birefringence of the core, and light polarized along the fast axis has a smaller refractive index than light polarized along the slow axis, so the propagation speed is faster. Thus it is important to exactly align the polarization axis of the laser source with the polarization axis of the fiber e. The fast axis is the direction. In this article, the latest in FOC's series covering specialty fibers and their fabrication, we discuss polarization-maintaining (PM) fibers and the various approaches used to make them. The physical principle behind this can be understood in terms of coherent mode coupling.