Polarization mode dispersion in fiber and cables

Single mode fiber supports two orthogonal polarization modes. A time delay arises, when the travelling velocities of these two modes, at a specific wavelength, are different. Polarization mode dispersion (PMD) is related to the time delays over a specific wavelength span. The reason for differences in traveling velocities can be an asymmetric distribution of index of refraction due to the manufacturing process (core and/or clad ovality, internal stress in the fiber due to nonsymmetric cooling) and fiber deployment (external stress, bends, twists and strain)