Chromatic dispersion estimation methods using polynomial fitting in PDM-QPSK or other multilevel-format coherent optical systems

► We propose a polynomial fitting algorithm method for non-data-aided CD estimation. ► The estimation range for a 50 Gbaud PDM-QPSK system can be up to ±12,800 ps/nm. ► We compare it with our previous data-aided CD estimation method for PDM-QPSK. ► The non-data-aided CD estimation method has an inherently smaller estimation range. ► The non-data-aided CD estimation method can tolerate 1 GHz larger frequency offset.