We disclose an optical receiver that can receive PDM-QDB and PDM-QPSK signals without hardware changes. In an example embodiment, the optical receiver includes a MIMO equalizer configured to perform electronic polarization de-multiplexing and ISI compensation. The constant modulus algorithm that controls the configuration of the MIMO equalizer also causes the MIMO equalizer to output signal samples corresponding to the QPSK modulation format regardless of whether the received optical signal is QDB-modulated or QPSK-modulated. A QPSK-to-QDB constellation converter processes the signal samples generated by the MIMO equalizer to convert them into the QDB modulation format. A QDB decoder coupled to the constellation converter then recovers the data encoded in the received optical signal by mapping the processed signal samples onto the QDB constellation. Differential encoding used at the corresponding remote transmitter enables the decoder to correctly recover the encoded data both when the received optical signal is QDB-modulated and QPSK-modulated.