Comparison between polarization-multiplexed DPSK and single-polarization DQPSK in 640 Gbaud, 1.28 Tbit/s-500 km single-channel transmission

We describe two schemes for 1.28 Tbit/s single-channel transmission at 640 Gbaud with polarization-multiplexed DPSK and single-polarization DQPSK, and compare their performance in a 500 km transmission. We found that the polarization multiplexing of ultrahigh-speed DPSK signals, for example at 640 Gbaud, is sensitive to the PMD characteristics of the transmission link. Even when first-order PMD is fully compensated, second-order PMD, which varies over a long period and is difficult to compensate, results in inter-polarization crosstalk induced by depolarization. The single-polarization transmission of a 640 Gbaud DQPSK signal is less vulnerable to PMD and therefore more stable, as the depolarized components induced by second-order PMD can be easily rejected with a polarizer. However, its performance is limited because the OSNR must be higher than for DPSK.