A type of optical signal distortion caused by anomalies in the cross-section of an optical fiber.A light signal travels through a single-mode fiber along two planes that are orthogonal, i.e., perpendicular. If the fiber is perfectly round, light will travel along both planes at exactly the same speed, and both planes of light will arrive at exactly the same time, barring other dispersion phenomena. PMD is caused by the fact that fibers are never perfectly round, but are always inherently somewhat asymmetric; in other words, they are slightly elliptical in cross-section. Also, some additional asymmetry is caused as the fibers become somewhat misshapen during installation, as they are bent around corners, twisted, coiled, and so on. Further, transient asymmetry can occur due to vibration and temperature changes at various places along the link, or even from aerial fibers swaying in the wind. As the timing differences are so slight as to be measured in picoseconds (10 -12 ), PMD is not an issue at speeds of 2.5 Gbps or less over short distances. At contemporary speeds of 10 Gbps and 40 Gbps, however, bit times are so short that PMD results in unacceptable bit error rates. PMD is especially an issue over long-haul cable routes, as cascading asymmetry issues compound the slight effects and they reach noticeable levels. Closer spacing of regenerators will overcome the effects of PMD, although that solution tends to be expensive. PMD compensators have been developed to control the effects of PMD at speeds up to 40 Gbps by physically squeezing the fiber to counter-stress it.
(Polarization Mode Dispersion) The type of dispersion that occurs in singlemode fiber due to a lack of perfect symmetry in the fiber and from external pressures on the cable. Light travels over singlemode fiber in two polarization states. Over long distances, PMD causes each one to arrive at the receiving end at a different time. See dispersion.