Signal Performance Monitoring And Equalization In Multi-dimensional Optical Fiber Communication System

In recent years,with the rapid development of multiple information technologies(such as artificial intelligence,internet plus,big data,and cloud computing),as well as the growing number of global broadband users and the increasing variety of intelligent terminals,the Internet data traffic presents an explosive growth trend.Thus,optical fiber communication system needs to be continuously expanded and upgraded for meeting the growing demand of data traffic.At present,the optical fiber communication system explores to utilize five fundamental physical dimensions(polarization,wavelength,space,time,modulation format)to individually or jointly improve the transmission capacity.The case is named as multi-dimensional optical fiber communication system which has been widely concerned by researchers and engineers.It is well known that the traditional direct detection technique can only detect the intensity information of optical signal,so it may not be suitable for multi-dimensional optical fiber communication system which may have multi-dimensional information(i.e.polarization,phase,intensity,et al.).Fortunately,the other technique,coherent detection technique,which can be able to detect the polarization information,phase information,intensity information and wavelength information of transmission signals,would be considered to be used in the multi-dimensional optical fiber communication system to achieve signal detection.Subsquently,the transmission impairments can be equalized and the system performance can be monitored in the digital signal processing module.To meet the diversity of future network services,the transmitter of multi-dimensional optical fiber communication system is required to be able to change modulation format(MF)types,symbol rates,forward error correction(FEC)coding according to different link conditions and service quality requirements,so that the receiver would be expected to acquire the MF information of unknown signal adaptively without any prior information.In addition,the transmission link of multi-dimensional optical fiber communication system may be flexibly and dynamically configured according to network demand which would change transmission path.Meanwhile,due to fiber aging,different types and manufacturing process of fiber,and the fluctuation of the node power would cause inaccurate damage compensation and serious performance degradation.Therefore,it is crucial to accurately monitor the link parameters and compensate the related link damages.The thesis focuses on the signal performance monitoring and equalization which includes optical MF monitoring,chromatic dispersion(CD)monitoring and equalization,and nonlinearity impairments monitoring and equalization.First,there are three optical MF monitoring schemes which have been proposed and demonstrated in a series of numerical simulations and experiments for the case that the transmitter of multi-dimensional optical fiber communication system can independently change MF information according to the link requirements.The case one uses the clustering by fast search and find of density peaks algorithm to monitor the number of clusters in the two-dimensional Stokes plane for identifying various mainstream modulation formats(i.e.,PDM-BPSK/-QPSK/-8PSK/-16PSK/-8QAM/-16QAM).The experimental results show that 95%identification accuracy can be achieved under 7%FEC limit(bit error ratio(BER)=3.8?10^–3)corresponding to the lowest required OSNR values for all modulation formats.Meanwhile,compared with traditional OPTICS and DBSCAN schemes,the proposed scheme can have higher recognition accuracy and lower complexity.Subsquently,the case two is a modulation format monitoring scheme based on intensity waveform profile feature which uses Fourier series fitting to identify PDM-QPSK/-8QAM/-16QAM/-32QAM/-64QAM.The simulation and experiment results show that the lowest required OSNR values to achieve 100%recognition rate for all modulation formats signals are much lower than the corresponding 7%FEC limit.Finally,the case three is that a modulation format monitoring scheme based on intensity fluctuation feature.Compared to the case two,the proposed scheme can also identify the same modulation format signals,while to achieve 100%recognition rate for all modulation formats can be under much lower OSNR conditions(the lowest required OSNR values are much lower than the corresponding 20%FEC limit).Meanwhile,the tolerance to fiber nonlinear effect is detailed analyzed in this thesis.(Chapter 3)Second,for the case of the accumulate CD variation caused by different fiber types,and the dynamic change of transmission link length,a two-stage fast and adaptive CD monitoring and equalization scheme is proposed.The first stage of the proposed scheme is the coarse CD monitoring operation,which uses the signal power auto-correlation function to obtain the the approximate CD range.Then,based on the range obtained in the first stage,the second stage of the proposed scheme uses the modified constant-mode algorithm to monitor the accurate CD value.Subsequently,the obtained CD value is applied to compensate CD of transmission link.The effectiveness has been verified via numerical simulation and experiment in 40/112Gbit/s PDM-QPSK and 112/224Gbit/s PDM-16QAM systems.The results show that the scheme can achieve high-precision monitoring results(monitoring error less than 40ps/nm),and has low time complexity(only 7%of the traditional MCMA scheme under the same monitoring accuracy).Meanwhile,it can tolerate the influence of PMD under 10ps range.(Chapter 4)Thrid,there are two kinds of nonlinear parameter monitoring and equalization schemes for the inaccurate nonlinearity compensation caused by different fiber types and the fluctuation of node powers.The one is that nonlinear parameter monitoring and equalization based on the variance of intensity noise.The effectiveness and feasibility are verified via numerical simulation and experiment in 40/112Gbit/s PDM-QPSK and224Gbit/s PDM-16QAM transmission systems.The results show that our scheme does not require reusing frequency offset compensation and carrier phase recovery in monitoring process which would save a lot of computing resources under the same monitoring results compared with the previous scheme.Meanwhile,compared with only CD compensation,the lauching power can be increased by?2d B.In addition,the other scheme is nonlinear multi-parameter joint monitoring and equalization scheme based on Goddard's error,which can jointly monitor and equalizate received power,nonlinearity factor and nonlinearity compensation parameter for the first time.Meanwhile,the feasibility is demonstarted in256Gbit/s PDM-16QAM experimental system with 1040km single mode fiber.(Chapter 5)Fourth,for the case that the optical phase conjugation(OPC)transmission link needs to place the phase conjugator in the middle point would seriously affect its practicability,an effective nonlinear impairment equalization scheme assisted k-nearest neighbor(KNN)is proposed to improve the performance of OPC by mitigating non-Gaussian impairments caused by nonlinear phase noise.The proposed scheme is demonstrated in two kinds of optical fiber transmission links.The one is 112Gbit/s 16QAM dispersion management fiber transmission link with 800km SMF.The simulation results show that the phase conjugator can offset about 40km relative to the middle point of transmission link.Compared with traditional OPC system,the flexibility of phase conjugator can be improved about 10%.Then,the other one is the 112Gbit/s 16QAM fiber transmission link with 1200km dispersion shifted optical fiber(DSF).The simulation results show that the flexibility of phase conjugator can be improved about 15.6%(the phase conjugator can offset about100km relative to the middle point of transmission link).(Chapter 5)In conlusion,for MF information changing in the transmitter according to the link requirements,this thesis proposes three optical MF monitoring schemes to adaptively monitor modulation format information for optimized demodulation.When fiber aging,different fiber types,the fluctuation of node powers cause accumulate CD variation and the inaccurate nonlinearity compensation,theis thesis propose some CD and nonlinearity parameters monitoring and equalization to monitor the accumulate CD,received power,nonlinearity factor and nonlinearity compensation parameter.Meanwhile,the effectiveness and feasibility of these schemes are demonstrated in numerical simulation and experiments.We believe that these effective schemes have certain reference significance for improving the intelligence and practicality of optical fiber transmission network.