Effect of DPSK modulation on Four Wave Mixing in a WDM system

This paper describes the effect of Differential Phase Shift Keying (DPSK) modulation on Four-Wave Mixing (FWM) in a multi-channel, long-haul optical fibre communication system that employs Wavelength Division Multiplexing (WDM). The performance of NRZ-DPSK and the commonly used NRZ-Intensity Modulation with Direct Detection (IMDD) are compared in the presence of FWM, while the factors that influence FWM, namely channel spacing, input optical power, chromatic dispersion and the number of channels, are varied. The optical spectra for each modulation under test are analysed and compared for each of the varying conditions, and the performance of each modulation format is studied with the help of eye diagrams. DPSK shows higher suitability for WDM systems than IMDD due to its higher tolerance to FWM, tolerance to noise and cross-talk, as well as better receiver sensitivity for lower power than IMDD. The investigation is based on comprehensive simulative analysis.