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

We report wavelength-division-multiplexed (WDM) systems experiments with po larization multiplexing at 40 Gb/s per channel and 0.8 bit/s/Hz spectral ef ficiency designed to explore the worst case impairments caused by first-ord er polarization-mode dispersion (PMD) in the fiber. Both polarization-divis ion multiplexing using two polarizations at each wavelength and polarizatio n interleaving where adjacent channels have orthogonal polarizations were e xamined. Measured system penalties show that polarization multiplexing incr eases the PMD sensitivity by approximately a factor of five due to coherent crosstalk.