Quaternary phase shift keying for high speed DWDM transmission

To overcome limiting fiber effects as chromatic dispersion, PMD and also Kerr nonlinearities in high speed DWDM transmission, quaternary phase shift keying has been extensively investigated during the recent years, implementing both direct-detection (DD) and coherent reception schemes. We review recent experimental trial results on DD examining the robustness of RZ-DQPSK against linear and nonlinear fiber distortions in laboratory and field environments. The combination of QPSK modulation with polarization multiplexing and coherent detection opens a path towards greater spectral efficiency and thus to 100 Gb/s line rates, however at the cost of reduced tolerance against nonlinear effects.