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 Friday, May 27, 2005
A new ITU-T Recommendation specifies the characteristics for devices that address a phenomenon known as polarization mode dispersion (PMD) in optical fibres. PMD is caused by a difference of the propagation speed in different polarisations of the light travelling through a fibre. PMD is induced by distortion of the light in optical fibres that occur as a result of the manufacturing process, the way it is laid in the ground, around corners etc.

PMD becomes an increasingly serious problem as the bit rate and the length of optical transmission systems increase. As a result, PMD compensation (PMDC) is an important technology for very high rate long distance systems. For instance at 10Gbit/s PMD is manageable for currently existing long-haul dense wavelength division multiplexing (DWDM) systems but at 40Gbit/s compensation may become necessary.

While there has been knowledge of the phenomenon for some time the PMD-induced penalties such as distance and bit rate limitations have often been considered too difficult or expensive to deal with, and so the telecommunication industry has had to learn to live with the problem. There have been limited efforts to develop solutions which have not evolved into successful commercial products.

In order to address the problem in a more efficient manner and stimulate a market for PMD compensating devices, operators have driven this ITU-T work. By agreeing on a set of characteristics for these devices, operators can look forward to the availability of products that will be more mature and will cost less than developing in-house solutions. It is expected that operators may also see reduced expenditure because it is thought that the use this technology will reduce the need for electro-optical regenerators (devices that break a signal down in order to restore it to its original quality).

Future work of the group that has produced this Recommendation will look at similar devices called adaptive dispersion compensators for another phenomenon called chromatic dispersion that also limits data rates and transmission distances in optical fibres.