Tackling PMD has turned out to be one of the most puzzling problems during the advent of high speed optical
transmission systems. It still seems far more attractive to avoid links with high PMD than to install an expensive PMDcompensator.
Measuring the PMD of an installed fiber link can answer the question if this link can be operated with or
without a PMD-compensator. Common test methods require exclusive access to the fiber link. This is considered to be a
major limitation since it is not easy to reroute the traffic of a WDM system with many channels. This contribution
discusses techniques to measure the PMD of a fiber link while it is in service ("In-Situ PMD Measurement"). These
approaches can be applied to a single channel as well as to the whole received WDM signal and the fact that only single
end access is needed makes them appear quite attractive. However, there are principal limitations which need to be
understood in order to qualify a link. Performing these measurements while the link is in service will help network
operators to smoothly upgrade to higher bit rates in their backbone networks.
Improving the tolerance to polarization mode dispersion (PMD) is considered to be one of the major prerequisites for the success of modern high bit rate optical communication systems. Various approaches such as optical compensation, electrical mitigation, multi-level modulation formats promise to increase the PMD tolerance of optical systems, whereas the question of how to experimentally characterize these solutions needs to be answered before commercial deployment. This is not an easy task since these systems need to be characterized with respect to first and higher order PMD but also with respect to their dynamic behavior. We show that deterministic polarization controllers combined with in-situ measurement of PMD can help to explore the PMD tolerance of an optical communication system and to generate reliable and repeatable results by avoiding statistical elements such as polarization scramblers. These elements can be combined to form a PMD testbed which allows to stress a system by applying a deterministic amount of PMD including a well-defined rate of change. Such a PMD testbed can be used during development of adaptive mitigators as well as for compliance testing. Finding an agreement on standard test procedures for such a testbed will make the evaluation of PMD tolerant receivers easier and more comparable.
Polarization mode dispersion (PMD), polarization dependent loss (PDL) and chromatic dispersion (CD) of fiber optic components and fiber transmission links are key issues in fiber optical communication. Of interest are the respective static values as well as their variations in time which might be due to environmental changes or due to rerouting. Basic aspects will be briefly addressed, a variety of standard and state-of-the-art characterization and monitoring techniques will be reviewed and recent experimental results will be presented.
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