Polarization Mode Dispersion(PMD) is a dispersion that can play a role at high data rates in single mode fibers. Polarization mode dispersion is due to the asymmetry of the core glass of an optical fiber, which can be affected by external influences such as pressure, tension and temperature.
This asymmetry leads to propagation time differences between two polarization modes that are perpendicular to each other. The polarization mode dispersion is the value of the differential group delay( DGD) averaged over a defined wavelength range. It is expressed in the PMD coefficient.
With a small PMD coefficient, high data rates can be transmitted over long distances in single mode fibers. The polarization mode dispersion limits the distance with the square of the bit rate. For example, the optical carrier( OC) OC-192 limits the distance to 1/16 of the value of OC-48, and OC-768 limits the distance to 1/256 of OC-48. This distance limitation can only be compensated with additional regenerators. Since most DWDM systems operate with OC-48, the impairments are negligible.