Long-Haul WDM Transmission of 400-Gb/s Polarization-Division-Multiplexed 16-ary Quadrature-Amplitude-Modulation using Coherent Detection

As the deployment of 100-Gb/s transmission systems in optical networks has been in progress for one year, optical transport of per-channel bit rates beyond 100-Gb/s is now under active research to sustain future traffic growth in optical fiber communications [1]. Coherent systems using 16-ary quadrature amplitude modulation (QAM) formats together with polarization division multiplexing (PDM) appears attractive for future 400-Gb/s (single carrier) optical transport. With a sustainable increase in OSNR requirement, the PDM-QAM16 solution enables doubling the spectral efficiency with respect to the PDM- quaternary phase shift keying (QPSK) solution, massively developed for 100-Gb/s systems. The first experimental transmissions of 400-Gb/s PDM-QAM16 signals have been reported recently. By resorting to the use of complex optical time division multiplexing (OTDM) technique starting from 19-Gbaud [2] or 28-Gbaud modulation [3], the wavelength-division multiplexed (WDM) transmission of ten 400-Gb/s channels has been demonstrated up to 800 km. Using electrical time division multiplexing (ETDM), the generation of one 56-Gbaud PDM-QAM16 signal has been demonstrated and a transmission distance of 1200 km has been reported but with only single channel propagation and Raman amplification [4].