Heterodyne detection and transmission of 60-Gbaud PDM-QPSK signal with SE of 4b/s/Hz

We experimentally demonstrate 8 × 240-Gb/s super-Nyquist wavelength-division-multiplexing (WDM) polarization-divisionmultiplexing quadrature-phase-shift-keying (PDM-QPSK) signal transmission on a 50-GHz grid with a net spectral efficiency (SE) of 4b/s/Hz adopting hardware-efficient simplified heterodyne detection. 9-ary quadrature-amplitude-modulation-like (9QAM-like) processing based on multi-modulus blind equalization (MMBE) is adopted to reduce analog-todigital converter (ADC) bandwidth requirement and improve receiver sensitivity. The transmission distance at the soft-decision forward-errorcorrection (SD-FEC) threshold of 2 × 10-2 is 2 × 420km based on digital post filtering while largely extended to over 5 × 420km based on 9QAMlike processing, which well illustrates 9QAM-like processing is more efficient for heterodyne coherent WDM system. Moreover, only two ADC channels are needed for simplified heterodyne detection of one 60-Gbaud PDM-QPSK WDM channel, and thus only one commercial oscilloscope (OSC) with two input ports can work well for each WDM channel.