| With the rapid development of Internet technology,technologies and industries such as big data,artificial intelligence and online broadcast came into being,resulting in the rapid growth of network traffic,which put forward higher demand for the capacity of communication system.How to improve the system transmission capacity and spectrum efficiency has become a hot research topic in optical communication technology.Due to the advantages of the high-speed long-distance transmission of optical time division multiplexing(OTDM)and the high spectrum utilization efficiency of quadrature phase shift keying(QPSK),QPSK×OTDM optical communication system has been favored.However,with the increasement of the transmission capacity,the phase noise in high-speed coherent optical communication system is more sensitive.In this paper,the source,influence and suppression methods of phase noise in QPSK×OTDM optical communication system are studied.The main work is as follows:Based on the Wiener model of phase noise,the influence of the linewidth of the local oscillator on the phase noise in the QPSK×OTDM system is analyzed.The limit distance that the system can transmit under different laser linewidths at 160Gbit/s is studied.The influence factors of the Kerr effect on nonlinear phase noise in link transmission damage are studied,and the influence of nonlinear noise on the performance of QPSK×OTDM system is analyzed in terms of the power of fiber and transmission distance.The simulation results show that the optimal input power is-3dBm at different transmission distance when the transmission rate is 160Gbit/s,because the signal is affected by active device such as amplified spontaneous emission(ASE)noise and nonlinear phase noise caused by self-phase modulation.In order to reduce the phase jitter of the transmitter laser and the local oscillator laser,the structure and principle of different phase-locked loops are discussed in detail.The influence of the phase noise in system with COSTAS phase-locked loop is analyzed.The performances of different phase-locked loop are compared.In view of the defect that phase-locked loop cannot keep track of fast-changing phase in high-speed optical communication system,the carrier phase estimation algorithm for phase noise in DSP is studied,and the bit error rate performance of M-power carrier phase estimation algorithm and decision-aided maximum likelihood(DAML)algorithm is compared.The simulation results show that the DAML algorithm has better performance in suppressing phase noise.Finally,the comparison of the bit error rate with the transmission distance is given under the condition that the branch rate of QPSK×OTDM optical communication system is 10Gbit/s,40Gbit/s and 80Gbit/s respectively.Combined with the constellation diagram,the reason for the decrease of the maximum transmission distance of the system with the increase of the transmission rate is analyzed. |
