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A polarization-mode dispersion (PMD) monitoring technique tailored for polarization-multiplexed high capacity optical transmission systems is proposed and experimentally demonstrated. First-order PMD is evaluated calculating the vector product of the spectral components of the two state-of-polarization transmitted in the polarization-multiplexed system. The spectral components in each state-of-polarization are resolved by a real-time optical Fourier transformer. The technique is demonstrated measuring first-order PMD in an proof-of-concept experiment implementing an alternate-polarization return-to-zero modulation. The results indicate an improved accuracy over the conventional Poincare-arc measurement method applied to a single polarization. Successful differential group-delay (DGD) measurements have been performed from 0.68 to 30.78 ps with 2.5% maximum relative error. A high degree of independence with the SOP/PSP angle is observed. The technique can be used to generate a feedback signal for PMD compensators in generic polarization-multiplexed transmission systems.