Chirped return-to-zero differential phase-shift keying (CRZ-DPSK) modulation format under different channel spacings is investigated and compared with RZ-DPSK in 40×40-Gbit/s dense wavelength-division multiplexing transmission systems. An optimization operation over a wide range of parameters, such as span input power, precompensation percent, and chirp parameter (only for CRZ-DPSK), is performed in the simulation. Simulation results show that CRZ-DPSK demonstrates better performance in transmission distance, nonlinear, and dispersion tolerance with wider channel spacing of 100 GHz. When the channel spacing is reduced to 75 GHz, a slight degradation is observed for CRZ-DPSK and the performances of both formats are acceptable. But when it is reduced from 75 GHz to 50 GHz, a clear degradation of the CRZ-DPSK performance is measured, but RZ-DPSK demonstrates no significant degradation.
The work presented in this paper gives performance comparisons of Carrier Suppressed Return-to-Zero Differential Phase-Shift Keying (CSRZ-DPSK) with Return-to-Zero Differential Phase-Shift Keying (RZ-DPSK) against both the effects of Intra-Channel Four-Wave Mixing (IFWM) and inter-channel Four Wave Mixing (FWM) in 16×40 Gb/s WDM systems with symmetrical fiber link schemes. IFWM gives smaller phase perturbation to CSRZ-DPSK signal than to RZ33-DPSK, although BER performance of CSRZ-DPSK is worse than that of RZ33-DPSK. The numerical comparison indicates that RZ33-DPSK is a better candidate for 40 Gb/s WDM systems than CSRZ-DPSK.