| The explosion of the data business represented by mobile internet,big data and cloud computing, and the emergence of broadband and bandwidth-demand applications make the transmission capacity under increasing pressure. It is predicted that IP traffic for all kinds of devices around the world is going to growing at a compound annual rate of 22%from 2015 to 2020. In order to meet the demand of the continuous growth of the network traffic, 100Gbit/s optical communication system has been large-scale commercial, 400Gbit/s commercial system has been deployed gradually. And the increasing demand for Tbit/s or even higher speed fiber transmission system is becoming more and more urgent.Different from intensity modulation and direct detection (IM/DD)system, the coherent communication system combines digital signal processing (DSP) technology can achieve the optical transmission system of 100Gbit/s and higher rates. The coherent communication system not only has the advantages of high scalability, high sensitivity and long transmission distance, but also can use various new modulated formats to improve the spectrum utilization. DSP technology can be used to compensate for the noise and distortion generated by signal channel at the transmitter and receiver. The continuous development of DSP technology makes the ultra-high speed long-distance coherent optical fiber communication system possible.Based on the National 863 Project and the National Natural Science Foundation Project, this paper studies the linear penalty, the nonlinear penalty and DSP algorithms in coherent optical communication system and has achieved following innovative results:1. Research on the hybrid transmission with T bit/s optical communication system and existing system:The current large-scale application of fiber communication systems is based on the lOGbit/s wavelength division multiplexing (WDM)system and the 100Gbit/s WDM system. The amplifiers and dispersion compensation modules in the system are the benefits for the existing signals. Considering the protection of existing investments and the cost of operating, the system upgrade based on the existing transmission system is an important research topic. For the first time, the paper incorporated the Nyquist T bit signal into the 10Gbit/s and 100Gbit/s WDM system for hybrid transmission research, and analyzed the interaction of different transmission rate and modulation format signals. Simulation results show that the impact of the Nyquist T bit/s signal on the 10Gbit/s signal can be negligible, and the Nyquist T bit/s signal produces 1.5dB of Q factor penalty to the 100Gbit/s signal. While the lOGbit/s signal causes a lot of nonlinear penalty (2.5dB Q factor penalty) for the Nyquist T bit/s signal.The results show that the online dispersion compensation in the existing system is the key factor that affects T bit system transmission performance, which can be improved by optimizing the inline dispersion compensation ratio of the system.2. Research on the inter-channel nonlinear effect in WDM system:The cross-phase modulation (XPM) is the mainly inter-channel nonlinear effect in ultra-high-speed long distance WDM optical fiber communication system. In this paper, the noise model and function mechanism of the inter-channel nonlinear effect in WDM system are analyzed in detail. The noise mechanism of XPM in polarization multiplexed quadrature phase shift keying (PDM-QPSK) WDM system and polarization switched quadrature phase shift keying (PS-QPSK)WDM system was simulated. The results show that the PDM-QPSK signal brings greater polarization crosstalk and the PS-QPSK signal will bring larger phase noise, and he dispersion management system will have a greater XPM effect. The results show the guiding significance for the design of the nonlinear compensation algorithm.3. Research on the high-order modulation format scheme to suppress the phase noise:In order to solve the problem that the orthogonal frequency division multiplexing (OFDM) signal and the high-order modulation format signals are sensitive to phase noise, this paper puts forward the scheme of circular 16 quadrature amplitude modulation (C-16QAM) OFDM system,which can make the system have a higher tolerance for phase noise and more convenient for DSP phase noise compensation. This paper compares the principle and the different distribution of constellation points between the square 16QAM and C-16QAM, and sets up the simulation system.The simulation results show that the performance of the C-16QAM OFDM system is better than that of the 16QAM OFDM system based on the laser linewidth tolerance and the fiber nonlinear effects tolerance.Then different phase noise DSP compensation algorithms are explored for two systems and the feasibility, performance and application scope of different algorithms are verified by simulations. |
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