Research On All-optical Regeneration Technology In Coherent Communication System

At present,with the rapid development of cloud computing,Internet of things,artificial intelligence and other technologies,the degree of social informatization is deepening,and the demand for network bandwidth is growing explosively.Coherent communication has been widely used because of its high transmission capacity.The coherent communication system integrated with all-optical regeneration technology can further extend the optical transmission distance and improve the signal quality,which is an important research direction of all-optical signal processing.All-optical regeneration is a kind of signal processing technology in the optical domain,which avoids the bandwidth and speed bottleneck of traditional optical-electrical-optical signal regeneration.In this dissertation,the all-optical regeneration technology of advanced modulation signals in the coherent communication system is studied.The main research contents and innovations are summarized as follows:1.The fiber nonlinear coupling theory in the spatial and frequency domains for the case of pump consumption is unified,and the analytical solution to the four-wave mixing equations under pump consumption is derived.Based on the above theory,the four-wave mixing effect in few-mode fibers is studied,and the analytical solution presented here is verified by numerical simulation.Results show that,the four-wave mixing process in the wavelength division multiplexing or space division multiplexing system can be simplified by determining the quasi-phase matching range,which becomes smaller as the transfer power increases in the case of pump consumption.The applications of the analytical solution to the simplification of multi-wave coupled equations,the design of large-scale parallel phase solver and fast nonlinear compensation are also discussed.2.An all-optical regeneration scheme for quadrature amplitude modulation(QAM)signals based on modulation format conversion is proposed.A star-QAM signal is firstly converted to a phase-shift keying(PSK)signal and an amplitude-shift keying(ASK)signal by using the optical phase conjugation and parametric gain saturation effect of fourwave mixing.Then,all-optical amplitude and phase regeneration is implemented for the PSK and ASK formats,respectively.Finally,the regenerated signals are re-aggregated into the corresponding star-QAM format.The feasibility of the scheme is verified by our theoretical analysis and simulation.Taking the star-16 QAM signal as an example,the simulation results show that the noise reduction ratio(NRR)performance can be up to 10 dB at the input error vector magnitude(EVM)of 11.46%.The modulation format conversion used in the regeneration scheme also has a potential application to the elastic optical network,in which the required input optical signal-to-noise ratio(SNR)is about 16.4 dB and 19 dB for optical 8QAM and 16 QAM signals,respectively.3.An in-band optical signal-to-noise ratio(OSNR)measurement method for alloptical regenerators is put forward by analyzing the power transfer characteristics of the optical signal and noise through the nonlinear optical system.This method may overcome the inaccuracy of the traditional out-of-band method in the optical regeneration link and can effectively reduce the measurement cost of the system.The reliability of the proposed method is also shown by simulating the output OSNR of the all-optical regenerator for QPSK format since the simulated result is consistent with that calculated from the EVM data.The in-band method is also suitable for other nonlinear optical fibre communication systems.4.The parametric amplification and all-optical shaping techniques based on nonlinear effects of few-mode fibers are studied.(1)A degenerate parametric amplifier supporting four modes,with the mode gain up to 20 dB and the differential modal gain of less than 1.2 dB in the wavelength range from 1540 nm to 1555 nm,is designed by optimizing the mode dispersion of the few-mode fiber using the genetic algorithm.(2)An all-optical parallel regeneration scheme based on a few-mode nonlinear optical loop mirror(NOLM)structure is proposed for the first time,and the dependence of the amplitude and the phase shift on the fiber mode at the operating points is theoretically analyzed.For four-mode QPSK signals,the phase-preserving amplitude regeneration is achieved by the time-domain interleaving method.Simulation results show that,when the input signal-to-noise ratio(SNR)is greater than 6 dB,all of the four modes can achieve almost the same regeneration performance,with the 5.6 dB NRR at the input SNR of 15 dB.The larger the input SNR,the greater the NRR.