Digital Signal Processing for Dual-Polarization Intensity and Interpolarization Phase Modulation Formats Using Stokes Detection

We study and compare two digital signal processing (DSP) approaches to recover the intensity on two orthogonal polarizations and the interpolarization phase modulation using a Stokes-vector direct detection receiver. We focus on higher order modulation of each of the three degrees of freedom allowed in Stokes-vector detection. 2 bits are encoded on each intensity of the two orthogonal polarizations, and 2 bits are encoded in the phase difference between the two polarizations, giving a 6 bits per symbol format. In this study, we propose a novel three-stage DSP algorithm and we compare this new algorithm with our earlier two-stage algorithm using the following metrics: the computational complexity and the bit error rate (BER) performance. Using the three stage approach, waveform filtering and derotation are applied in series rather than in parallel as was done in the two stage algorithm. We show that the three-stage approach exhibits equal BER performance, while significantly reducing the total required number of real additions and real multiplications. Moreover, tracking the state of polarization using the proposed method is m-times more efficient, where m is the number of taps in the first filtering stage.