Research On Nonlinear Suppression Algorithm Of Conjugate Twin Wave With High Spectral Efficiency

With the explosive growth of communication services,the capacity of intensity modulation/direct detection(IM/DD)systems has encountered a bottleneck,which can no longer satisfy the communication demand of users.The research of high-capacity transmission technology has became the focus of scientists,and coherent optical communication system was proposed in this case.By using of high-order modulation format,coherent optical communication system can improve the spectrum efficiency of the system effectively,which brings a raise of transmission capacity.For the damage during transmission,it can be compensated by digital signal processing(DSP)of the coherent optical communication system.The conjugate twin wave(PCTW)technology is one of many compensation schemes,which can simultaneously suppress linear and nonlinear damage.Before the PCTW technology was put forward,many nonlinear suppression schemes had been proposed,but due to the high complexity of calculations and high difficulty in implementation,practical applications were restricted.The emergence of PCTW has solved these problems well.However,the conventional PCTW technology also has certain shortcomings.In this algorithm,only one useful signal is transmitted on the two polarization states,so its spectral efficiency is half of the original.The appearing of quadrature pulse shaping dual-PCTW(QPS dual-PCTW)effectively makes up for this.The defect of QPS dual-PCTW program is that its nonlinear suppression capability is more dependent on the orthogonality between related waveforms.Based on the joint simulation of optisystem and matlab,we built a QPS dual-PCTW quadrature amplitude modulation(QAM)32-channel single-mode and dual-degenerate-mode WDM system,and the improvement degree of the QPS dual-PCTW algorithm to the nonlinear tolerance of these two systems is studied.The simulation results show that in single-mode transmission,under a same capacity,the performance of the 64 QAM system is significantly better than that of the 16 QAM case,while for the double degenerate mode case,the 16 QAM performance is relatively better,which can be attributed to the fact that the higher the modulation order of QAM,the smaller the nonlinear tolerance of the system,but the higher the modulation order,the easier it is to maintain pulse orthogonality,and the more effective the QPS dual-PCTW algorithm is in nonlinear suppression.The total nonlinear tolerance is the result of balancing of the two factors.The research results also show that when the incident power is lower than the optimal power of the system,the QPS dual-PCTW algorithm can improve the performance of the system greatly,whether it is a single-mode transmission system or a dual-degenerate-mode system.This show a certain significance of QPS dual-PCTW for realizing large-capacity optical fiber communication systems with high spectrum efficiency.In addition,the effectiveness of the QPS dual-PCTW algorithm is limited by power of fiber.When the power is in a appropriate range,nonlinear effects can be suppressed effectively.However,if the input power is larger than a certain value,the QPS dual-PCTW algorithm will not provide any improvement of the system performance,in contrast,it makes the system performance get worse.The reason is that nonlinear effect will cause distortion of the pulse waveform when the input power is large enough,and then the orthogonality of signal is destroyed.Because of the dependence to pulse orthogonality of the two branch signal,the superiority of the QPS dual-PCTW algorithm is limited.Moreover,the QPS dual-PCTW algorithm is equal to place a re-modulation on input pulse,as lead to the broadening of the signal spectrum,thus triggering a deterioration of system performance.