Research On Equalization Of Polarization Effect In Stokes Direct Detection

High-speed coherent optical fiber communication capable of carrying terabit and long-distance transmission is currently the mainstream of development.However,in cloud computing technology,data center communication has the characteristics of large capacity,medium and short distance.Therefore,for economic considerations,high-speed direct detection of optical fiber communication has attracted people's interest.In order to increase the speed of the system,on the one hand,we can increase the spectrum bandwidth of the communication system,on the other hand we can improve the spectrum utilization rate of the system on the other hand.Spectrum bandwidth is an invaluable resource,and upgrading is very difficult,so we are now focusing on improving the spectrum efficiency of the system.In order to increase the number of bits per symbol carried by the system,from the simplest amplitude modulation to Quadrature Phase Shift Keying(QPSK)to higher order modulation formats,such as Quadrature Amplitude Modulation(QAM)is followed by the use of Polarization Division Multiplexing(PDM)technology to simultaneously transmit two orthogonal polarization states,thereby doubling the transmission rate.However,in traditional direct detection communication,high-order modulation produces severe Inter Symbol Interference(ISI).In order to overcome this problem,a research group proposed Stokes Vector Direct Detection(SV-DD).This system transmission rate is comparable to single polarization coherent fiber communication.In the SV-DD system,optical signals are subject to various damages when transmitted in optical fibers,including fiber loss,dispersion dispersion(CD),polarization effect damage,and nonlinear effects.Rotation of State of Polarization(RSOP),Polarization Mode Dispersion(PMD),Polarization Dependent Loss(PDL).Recently,with the deepening of research on digital signal processing(DSP)technology,the DSP has achieved rapid development,and the above proposed damage can be balanced by using DSP technology at the receiving end of the SV-DD system.Moreover,due to the application in medium and short-distance transmission systems,CD,PMD and PDL damage do not occupy a dominant position.However,since the initial signal received by the SV-DD receiver is the Stokes signal,the RSOP equalization is required first to recover the Jones domain signal,so the RSOP damage becomes our primary concern.The mature RSOP equalization technology is mostly based on coherent system detection and cannot be applied to SV-DD systems.Moreover,in the direct detection optical fiber communication system,the optical signal to noise ratio(OSNR)is required to be high,which makes the transmission rate difficult to be improved.In this paper,the topic of RSOP damage equalization in SV-DD system is studied.The Stokes-based damage model of SV-DD system and the corresponding RSOP equalization technology are studied in depth.In addition,SV-DD is also studied.The main compensation work of RSOP and CD in the system is as follows:(1)The principle of transmission and reception of SV-DD communication system is introduced.The principle and mathematical model of fiber channel damage are analyzed.The SV-DD-PDM-16QAM communication system is built.(2)The principle,advantages and disadvantages of the RSOP equalization algorithm in several commonly used SV-DD systems are analyzed.A new Extended Kalman Filter(EKF)scheme is proposed,which is based on the stokes space and considers a three-parameter model.The proposed RSOP damage equalization scheme restores the signal to the corresponding position in the Stokes space according to the three coordinate systems,which can effectively balance the polarization effect damage.The mathematical model is explained in detail,and the simulation is carried out using matlab software to get the performance of scheme.(3)Probabilistic modulation(PS)is introduced into the SV-DD system to enable the system to adapt to lower OSNR conditions.Furthermore,the difference between PS signal and traditional signal is studied,and a scheme is proposed to enable EKF to operate under arbitrary probability distribution conditions.The simulation results show that the scheme works well and can compensate RSOP over 1.5Mrad/s.