Depolarization Method For Polarization Multiplexed High-order Modulation Signals In Electrical And Optical Domains

With the rapid development of big data,cloud computing,5G networks and the Internet of Things,modern coherent optical communication is also constantly adapting to various needs.The coherent optical communication in the backbone network can basically solve the linear damage and nonlinear damage in the process of optical fiber transmission by using various DSP technologies,which makes the coherent optical communication system more and more able to meet the requirements of "three super".For metropolitan area networks,most of the data volume is transferred in the data center scenario.Coherence technology continues to sink,coherence to short is also a big trend.The coherent detection and receiving system used in traditional backbone network has high power consumption and complex DSP module,which is not suitable for short and medium distance communication.In order to reduce the problems of power consumption and cost,and the short distance can also ignore more damage,so in the optical domain to use hardware to process the optical signal,can relieve the DSP pressure on the receiver.When optical signals are transmitted through optical fiber channels,they are damaged by CD,RSOP,PMD,CFO and CPN.The polarization damage to which RSOP and PMD belong is the focus of this paper.In short-range scenarios,only RSOP can be considered for optical depolarization,but the current depolarization schemes have problems such as low tracking rate of RSOP or low applicable modulation format.In the long distance scene,polarization damage equalization algorithm is more developed in balancing RSOP and first-order PMD signal damage,but it is still less for existing RSOP and high-order PMD damage signal equalization algorithm.Moreover,it is also difficult to improve the algorithm suitable for probabilistic constellation shaping.Some improved algorithms apply to the polarization channel is very simple,which is not in line with the reality.Through the above analysis,both the hardware depolarization scheme of optical domain and the EKF equalization algorithm of electrical domain in the coherent optical communication system need to be further studied in order to adapt to the ever-emerging requirements with the development of optical fiber system.In this paper,the demultiplexing method of polarization multiplexed high-order modulation signals is studied,and polarization state power difference scheme based on hardware depolarization is deeply studied in the optical domain,and the correction and control of PC is optimized,so that the depolarization performance of large-rate short-distance system can be verified in experiments.In the electrical domain,PCS-64QAM signal was demultiplexed based on EKF algorithm,and simulation verified that the improved algorithm can adapt to high order modulation and probabilistic constellation shaping.The main work is as follows:(1)The basic principle and system structure of coherent optical communication system are introduced,including the basic theory of transmitter,receiver and partial division multiplexing system;Then the channel damage of coherent optical communication system is introduced in detail,and the generation principle and mathematical model of various kinds of damage are explained.Finally,the theory of dispersion and polarization damage equalization method in optical domain and the principle of traditional suitable algorithm for damage equalization in electrical domain are introduced.(2)Decomposition of the optical domain signal demultiplexing equalization scheme,detailed introduction of the combination of PSO algorithm,PC correction and control and extraction of characteristic parameters,and PC correction method using only Stokes parameter S1 and a simple dijter algorithm,this method can be embedded in the optical domain demultiplexing device.Then,the depolarization performance of the scheme was verified in 112GBaud PDM-QPSK/16QAM system,and PCR,SNR and BER were used as measurement criteria.The experimental results show that in the case of BTB and the introduction of 10 km fiber,the RSOP of the two perturbation modes ranges from 1krad/s to 1Okrad/s,the PCR of QPSK signal after demultiplexing is above 20 dB,SNR is above 12 dB,and BER is below 10-3 magnitude.16QAM signal PCR was kept above 20 dB,SNR was near 12 dB and above,BER was 3*10-2 magnitude and below.Finally,the tolerance performance of the scheme to CD and DGD was tested in the simulation system and experimental platform.The simulation results show that optical domain depolarization effect decreases at 30 km,which is mainly due to CD effect,while the loss and PMD have little influence.The experimental results can also verify that CD mainly affects the demultiplexing performance in short distance scenes,while DGD has little effect.(3)The basic principle of probabilistic constellation shaping is introduced,and then the basic principle of Kalman filter is described in detail.Based on this,the influence of probabilistic constellation shaping and the improvement of EKF algorithm are analyzed.Finally,the performance of the algorithm is verified in a 32 GBuad PDM-PCS-64QAM system,and the performance is compared with the traditional MMA algorithm and the unimproved EKF algorithm for ordinary 64QAM signals.The simulation results show that the bit error rate of the two EKF algorithms is lower than 3*10-2 for common 64QAM signals when the RSOP speed is 600 krad/s and the OSNR speed is 26 dB to 32 dB,or the OSNR speed is 30 dB and the central RSOP speed is 200 krad/s to 1 Mrad/s.However,the bit error rate of MMA is relatively high.For PCS-64QAM signals,neither MMA nor the unimproved EKF algorithm can work,and the improved EKF algorithm has a bit error rate lower than 3*10-2,which can obtain excellent demultiplexing performance.When the original EKF scheme and MMA algorithm cannot balance probabilistic constellation shaping signals,the improved EKF algorithm can obtain excellent demultiplexing performance when the maximum DGD is 8ps and the maximum RSOP is 2Mrad/s.