8 Dispersion compensators for fibre optic paths
8.3 PMD – Polarization Mode Dispersion

PMD – Polarization Mode Dispersion originates because of different refractive index for different polarization (polarization practically means oscillations of E or H vector in specific direction), or in other words, because of different refractive index for “x” axis and “y” axis. There is so-called fast axis and slow axis. Such material is called an anisotropic environment. PMD is usually small – couple of picoseconds, however, it can be a problem for high-speed transmission systems.

Finding optimal solutions is problematic, because PMD is a random process. Specialty electronic polarization correctors are used to receive data properly. To avoid PMD, specialty birefringent fibres with strong asymmetry are used.

PMD in an optical fibre.