56-Gbaud PDM-QPSK: Coherent Detection and 2,500-km Transmission

A 56-Gbaud (224-Gb/s line rate) polarization-division multiplexed quadrature phase shift keyed (PDMQPSK) signal is transmitted over 2,500 km (32 x 80 km) of fiber and is coherently detected using two 80GSamples/s oscilloscopes with off-line signal processing. Introduction With 100-Gb/s transport technologies getting close to commercialization, research is focusing on scaling per-channel bit rates beyond 100 Gb/s, with the goal to develop spectrally efficient solutions for the next Ethernet standards, likely at 400 Gb/s and 1 Tb/s. Approaches include orthogonal sub-carrier multiplexing1 and single-carrier multilevel modulation at high symbol rates2,3. The latter method allows for a simpler transmitter structure at approximately equal receiver and digital signal processing hardware4. Single-polarization quadrature phase shift keying (QPSK) at 53.5 Gbaud and binary modulation at 107 Gbaud have been demonstrated5 using direct detection. The lack of sufficiently fast analog-to-digital converters (ADCs) has prevented coherent detection without resorting to optical time division demultiplexing6 (OTDM), which severely complicates digital compensation of impairments with memory, such as chromatic dispersion (CD). The highest coherently detected symbol rate without OTDM today is7 28 Gbaud for QPSK and 20 Gbaud for 16-QAM2,3. In this paper, we report the generation, coherent demodulation, and long-haul transmission of 56Gbaud polarization-division multiplexed (PDM) QPSK, yielding a single-channel line rate of 224 Gb/s.