1. An apparatus comprising: an optical detector that includes a polarization splitter configured to split an optical polarization-division-multiplexed (PDM) signal into two light beams and an optical-to-electrical converter configured to generate first, second, and third electrical signals by optically detecting different combinations of light of the two light beams, the first, second, and third electrical signals being measurements of three nontrivial components of a Stokes vector of the optical PDM signal or linearly independent combinations of said three nontrivial components; and a digital signal processor configured to receive the first, second, and third electrical signals from the optical detector and generate therefrom an estimate of first data carried by a first polarization of the optical PDM signal and second data carried by a different second polarization of the optical PDM signal.