Observation of PMD-induced coherent crosstalk in polarization-multiplexed transmission

We report wavelength-division-multiplexed (WDM) systems experiments with polarization multiplexing at 40 Gb/s per channel and 0.8 bit/s/Hz spectral efficiency designed to explore the worst case impairments caused by first-order polarization-mode dispersion (PMD) in the fiber. Both polarization-division multiplexing using two polarizations at each wavelength and polarization interleaving where adjacent channels have orthogonal polarizations were examined. Measured system penalties show that polarization multiplexing increases the PMD sensitivity by approximately a factor of five due to coherent crosstalk.