The impact of phase noise, chromatic dispersion and non-linear effects on a 10 Gb/s Optical Differential Quaternary Phase Shift Keying (oDQPSK) system has been evaluated by means of computer simulation. Both single channel and multiple WDM channels have been considered. The results show that these systems present a high spectral efficiency that allows a reduced channel spacing as low as 25 MHz. Furthermore, this effect leads to an increase on dispersion tolerance together with a non-linear effects robustness due to the signal constant envelope characteristics. Finally, some relevant results have been found concerning the laser linewidth requirements (less than 10 MHz). Simulations show that phase noise can be the
limiting factor of oDQPSK systems.
A new version of Duobinary-Polarization Shift Keying scheme is proposed. The Duobinary electrical filter has been moved to the receiver side. Optimization of both transmitter and receiver electrical filters gives a receiver sensitivity improvement while maintains both chromatic disperison and non-linear effects tolerance. The main reason of this sensitivity increase is the reduction of the timing jitter given by our proposal.