Experimental Comparison between Binary and Quadrature Phase Shift Keying at 40Gbit/s in a Transmission System Using Coherent Detection

We experimentally compare 40Gbit/s PDM-BPSK and 40Gbit/s PDM-QPSK over a typical terrestrial dispersion-managed link. We demonstrate twice as large reach for PDM-BPSK and we evaluate the impact of 10Gbit/s NRZ neighboring channels. Introduction Coherent detection and digital signal processing have come up as attractive technologies against linear effects in optical networks. The distortions from polarization mode dispersion (PMD), chromatic dispersion, and narrow filtering stemming from concatenation of reconfigurable optical add-drop multiplexers (ROADMs) can be significantly mitigated. Moreover, polarization division multiplexing (PDM) can be fully exploited without the need of additional complexity at the receiver side. Most of pioneer demonstrators using coherent detection have relied on quadrature phase shift 1-4 keying (QPSK) . Despite its good sensitivity, the reach of coherent PDM-QPSK systems is limited by their weak tolerance to inter-channel nonlinear effects. One likely scenario for smooth 10Gbit/s systems upgrade is to design the socalled hybrid 10/40G systems. According to this configuration several channels at 40Gbit/s are progressively inserted in wavelength slots with 50GHz spacing, originally designed for non return to zero (NRZ) channels at 10Gbit/s. Channels at both bit-rates propagate simultaneously over the same fibre and 5 experience strong nonlinear interactions . These interactions have come up as a major issue against 40Gbit/s coherent PDM-QPSK 6 systems .