Modeling FWM Noise In DPSK DWDM Systems

With an increase in the number of channels in dense wavelength division multiplexing(DWDM) system, FWM effects have got a lot of attention. In order to improve spectral efficiency of optical fiber communication systems and shorten the time of DWDM systems numerical optimization and evaluation. This paper mainly do two tasks which are as follow:First, semi-analytic models are developed to deterministically calculate the variances of degenerate and non-degenerate Four-wave-mixing(FWM) noises for dispersion-managed DWDM systems with pure and mixed differential quadrature-phase-shift keying(DQPSK)/ differential phaseshift keying(DPSK)/on-off-keying(OOK) channels. The novel dispersion map used here for dispersion management is composed of effective-area-enlarged positive dispersion fiber(EE-PDF), dispersion slope and dispersion compensating fiber(SCDCF) and nonzero dispersion-shifted fiber(NZ-DSF). It is numerically validated with the new models that, under the condition that all channels have the same average launch powers and baud rates, the impact of FWM noise for mixed DQPSK/OOK channels are more severe than that for pure DQPSK and mixed DQPSK/DPSK channels. It is also shown that the FWM efficiency is strongly dependent on the peak power of launched optical pulse for a large number of channels, as can be mainly attributed to the quasi-linear evolution of pulse shapes in pump channels induced by cross-phase modulation(XPM).Second, based on the Optisystem platform, a 2.88Tb/s(40-Gbaud/ch) DWDM system with 150 GHz channel spacing is built for high spectral efficiency fiber optical communication systems. The 8-ary modulation formats adopted here include two types. One of them is 8APSK(8-level amplitude- and differential phase-shift keying), the other is D8PSK(differential 8-level phase-shift keying). The simulation results show that when the probe channel is 8APSK modulated, larger amplitude modulation index corresponds to better nonlinear tolerance, but when the effects of dispersion, optical nonlinearities and various noises are taken into accounted simultaneously, the values of amplitude modulation index have the optimal range. 8APSK with optimized amplitude modulation index outperforms D8PSK and has better nonlinear tolerance than the latter. When the averaged input optical power per channel is same, the contribution coming from D8PSK pump channels to FWM noise is larger than that from 8APSK ones.