We show that the 3D Stokes space representation of light polarization can be used for digital signal processing in fiber-optic coherent systems, in particular for polarization demultiplexing. The method uses the signal samples in the Stokes space in order to reverse the coupling between the two orthogonal polarizations used at the transmitter for data multiplexing. It is also shown that this approach can improve the coherent receiver, particularly in terms of convergence speed and transparency to higher-level modulation formats.