Optical Frequency Comb Based Coherent WDM Transmission Systems Enabling Joint Carrier Recovery

In this information age,with the rise of cloud computing,data center optical interconnections will play an important role in the network.At present,combined with wavelength division multiplexing(WDM),coherent optical communication technology with large communication capacity and high spectral efficiency is widely used in long-haul applications.However,it cannot be used for the short reach data center interconnections due to complicated structure and high complexity.Thanks to the excellent characteristics of the optical frequency comb,such as good frequency stability,high phase coherence between lines,good flatness,etc.,using optical frequency comb as light source in coherent WDM systems is helpful to reduce the phase-tracking complexity at the receiver end by implementing joint digital signal processing.Using coherent comb-based WDM system for short reach data center optical interconnections is a good research direction worth paying attention to.It has potential for complexity reduction while meeting the increasing bandwidth requirements.Aiming at the digital signal processing(DSP)for coherent optical communications,this thesis studies carrier frequency offset estimation(FOE)and carrier phase recovery(CPR),respectively.On the one hand,this thesis introduces three existing FOE algorithms.After that,this thesis proposes a novel low complexity two-stage FOE scheme based on fast Fourier transform(FFT).Numerical simulations and experimental results verify its feasibility and effectiveness.The results demonstrate that the proposed scheme can achieve comparable performance,while the complexity is reduced by 80% and 60% compared with single-stage 4th-FFT and FFT+CZT,respectively.On the other hand,Viterbi&Viterbi carrier phase estimation scheme(VVPE)and blind phase search(BPS)scheme are studied.Furthermore,this thesis proposes a novel simplified BPS scheme based on Chebyshev error distance calculations with low complexity.Then,the influences of the number of test phases and the block length on the performance are studied in the simulation.Next,the thesis investigates the bit error rate(BER)performance and the linewidth tolerance of both simplified BPS and traditional BPS.Finally,the proposed scheme is experimentally demonstrated at Back-to-Back(B2B).On the basis of the research of the single-carrier carrier recovery,this thesis studies the joint carrier recovery technology in coherent comb-based WDM systems,and introduces the proposed carrier recovery schemes into the joint carrier recovery framework.Then,this thesis analyzes and compares the complexity of different architectures.This thesis studies the signal-to-noise-ratio(SNR)penalty of different transmission scenarios under different carrier recovery structure through simulations.The simulation results indicate that the joint carrier recovery introduces a SNR penalty approximately within 0.3dB compared to the independent carrier recovery in short reach optical transmission(less than 40 kilometers).Finally,a coherent WDM experimental system based on electro-optical frequency comb generated by single intensity modulator is built to verify the high phase correlation between two independent channels and to obtain the BER performance and SNR penalty under the B2 B and after 50 km SSMF transmission.Numerical simulations and experimental results show the potential for DSP complexity reduction in coherent comb-based WDM systems.