Characterisation of the PMD distribution along optical fibres by a POTDR

Fibre links in an optical network generally comprise several relatively short fibre segments which have been spliced together in cables. These fibre segments or ldquosectionsrdquo are assembled with optical connectors and have lengths of some tens of kilometres. The characteristic parameters of the fibre sections are attenuation, chromatic dispersion (CD) and polarisation mode dispersion (PMD). However, PMD of the optical fibres can hamper the upgrade of the optical b0ackbone network towards higher data rates of 40 Gbit/s and beyond. The PMD distribution along a buried fibre link is not a constant and can also significantly vary between the different fibres of the same optical cable. In the absence of such spatial information, the whole cable with higher PMD-values may have to be replaced in order to transmit 40 Gbit/s transparently over long distances. But investigations have shown that frequently localized pieces of the section are the major contributors to the overall high PMD value of the whole fibre, rendering the link unsuitable for higher data rates. A new random-scrambling polarization optical time domain reflectometry (POTDR) measurement technique is used to investigate the spatial distribution of the cumulative PMD in deployed fibres. Results help to identify high-PMD fibre pieces or sections which need to be replaced to enable 40 Gbit/s transmission and beyond, rather than substitution of a whole fibre link.