Feed-Forward Frequency Domain Equalizer for Fast SOP Transient Tracking in Coherent Optical Modems

We propose a feed-forward frequency domain adaptive linear equalizer that can track fast state-of-polarization (SOP) transients in the fiber. In the proposed method, we rely on the pilot symbols that are inserted into the DSP frame to estimate the channel response and then compensate for it. Unlike the decision-directed least mean squares (DD-LMS) algorithm, the proposed circuit is designed in a feed-forward fashion and avoids the latencies involved in DD-LMS feedback loop in order to achieve the best possible SOP tracking capabilities. The proposed algorithm is shown to be robust against high order differential group delay (DGD) and polarization dependent loss (PDL) and offers a significant tracking improvement compared to DD-LMS.