Multilevel modulations and digital coherent detection

In this paper, we experimentally investigate the performance of several polarization-division-multiplexed (PDM) two- and three-level modulation formats employing digital coherent detection for high-speed and high spectral-efficiency optical transmission. 112 Gb/s PDM-NRZ-QPSK, 112 Gb/s PDM-RZ-QPSK and 114 Gb/s PDM-RZ-8PSK signals have been experimentally demonstrated by using the combination of Mach–Zehnder and phase modulators in a serial configuration with binary electrical drive signals, where the performance of both NRZ-QPSK and RZ-QPSK under tight filtering has been studied and compared. Furthermore, we generate the first optical 8-QAM signal by using a novel 8-QAM modulator, which consists of a parallel I-Q modulator followed by a phase modulator, all driven with binary electrical signals. Because single-ended photo detection is used in the coherent receiver, a novel DSP algorithm is proposed to mitigate the distortion caused by direct square-law detection of the signal component. By employing single-ended detector coherent detection, several spectrally-efficient long-haul DWDM transmissions experiments have been demonstrated without using Raman amplification or optical dispersion compensation. These includes 20×112-Gb/s PDM-RZ-QPSK transmission over 1540-km of standard single mode fiber (SSMF) at a spectral efficiency (SE) of 2 bit/s/Hz, a record capacity of 17 Tb/s within the C-band optical bandwidth (4.025 THz) over 660 km of ultra-low-loss fiber using 161×114-Gb/s PDM-RZ-8PSK formats at a SE of 4.2 bit/s/Hz, and hybrid 10×112 Gb/s PDM-RZ-QPSK and 10×44 Gb/s PDM-NRZ-QPSK transmission with 25-GHz channel spacing over 1600 km of SSMF at a SE of 2.8 bit/s/Hz.