Investigation of the optimum alternate-phase RZ modulation format and its effectiveness in the suppression of intrachannel nonlinear distortion in 40-Gbit/s transmission over standard single-mode fiber

The results of a detailed study of the performance of 40-Gbit/s optical transmission over standard single-mode fiber with alternate-phase return-to-zero (AP-RZ) modulation formats are presented. While in the highly dispersed propagation regime, the use of RZ format signals with alternate phase between adjacent pulses can lead to a significant reduction in nonlinear interaction between pulses, allowing increased launch powers and, hence, increased transmission distances, this has not been systematically investigated. In this paper, we investigate the use of AP-RZ formats, both numerically and experimentally, to explain and quantify the effects of varying the peak-to-peak phase modulation, the amount of precompensation at the transmitter and the presence of multiplexer prefiltering for the case of wavelength-division multiplexing systems with high spectral efficiency. It is shown that, while the use of pulses with alternate π/2 rad phase shifts minimizes the build-up of shadow pulses in the zero bit slots due to intrachannel four-wave mixing, the best overall performance is obtained with π rad peak-to-peak phase modulation. We show experimentally, for the first time, that the optimum value of precompensation to minimize intrachannel nonlinear distortion is dependent on the signal format used.