Blind Equalization of Receiver In-Phase/Quadrature Skew in the Presence of Nyquist Filtering

In-phase and quadrature skew inside a coherent receiver, caused by misalignments, can severely limit performance. Future 400G systems employing higher order modulation formats and Nyquist filtering are especially sensitive to this. A blind adaptive equalizer is proposed to track and compensate the skew while also performing matched filtering. Performance is investigated in simulation for 56 GBd polarization division multiplexed 16-quadrature amplitude modulation (PDM-16QAM) and experimentally for 6 GBd PDM quadrature phase shift keying (PDM-QPSK) and PDM-16QAM, where compensation for delays up to half a symbol period are demonstrated without penalty. At 30% delay skew, the maximum skew recommended by Optical Internetwork Forum (OIF) for PDM-QPSK, a gain of 1.6 dB for PDM-QPSK, and a gain of more than 5 dB for PDM-16QAM is observed compared with using a conventional equalizer.