Research On VCSEL And Multimode Fiber Based High-speed Short-reach Optical Interconnect Systems

Recent years,with the rapid development of cloud services,automotive,HD video,virtual reality(VR)as well as augmented reality(AR),there are urgent needs for high transmission capacity in the communication systems.Compared with traditional electrical interconnects,optical interconnects have the advantages of high speed,low cost,low power consumption as well as immunity of electromagnetic interference.So optical communication systems becomes the main choice for high-speed data transmission.Among the optical solutions,short-reach optical interconnects with transmission distance of several meters to hundreds of meters are dominate technologies for transmission systems in datacenters.For these optical solutions,vertical-cavity surface-emitting lasers(VCSELs)and multimode fiber(MMF)are most used considering cost and power consumption.In order to accommodate the explosive growth of transmission throughput,single-lane data rate will inevitably develop towards 100 Gb/s even 200 Gb/s,and multilevel modulation formats are preferred to increase spectral efficiency.Meanwhile,integration density is also needs to be higher to reduce signal distortion induced by long electrical transmission as well as to support more parallel channels.For VCSEL and MMF based links,as single-lane data rate increases,integrity of high speed signal will be affected by modulation bandwidth of VCSEL chips,relative intensity noise(RIN)of VCSEL chips,modal and chromatic dispersion of MMF as well as the mode partition noise(MPN).Modulation bandwidth of VCSEL will be affected by parasitic effect,and in dense opto-electrical integration scenarios,as integration level increases,the parasitic effect from VCSEL chips will also have impacts on modulation performance and power consumption of transmitter.Therefore,the impacts of parasitic effect on modulation performance of VCSEL chip as well as modulation performance and power consumption of VCSEL transmitter need to be investigated,and the signal degradation in transmission link needs to be mitigated.Considering all the issues mentioned above,this thesis investigates VCSEL-MMF based high-speed optical interconnects from aspects of chip,transmitter and transmission link.The main research contents,innovations are listed as follows:(1)The impacts of parasitic parameters on large-signal performance for VCSEL chips with different damping factors under 4-level pulse amplitude modulation(PAM-4)are investigated.The parasitic parameters of VCSEL chip under different bias currents are extracted by using nonlinear optimization algorithm.Large-signal responses of PAM-4 modulation for VCSELs with different damping factors under different parasitic parameters are evaluated and compared by calculating the transmitter dispersion eye closure quaternary(TDECQ)of PAM-4 signals.Results indicate that when PAM-4 modulation is applied,for the under-damped VCSEL,better large-signal response can be achieved with moderate parasitic modulation bandwidth.As for the over-damped VCSEL,the larger the parasitic bandwidth,the better the large-signal response under PAM-4 modulation.(2)A high-speed VCSEL transmitter co-design framework is proposed targeting at 50 Gbaud signal for high-speed,low-cost,high-density opto-electrical integration.Under the transmitter specifications defined by IEEE 400 G standard,driver circuit design and energy efficiency of the transmitter under non-return-to-zero(NRZ)and PAM-4 modulation are analyzed and compared for two types of 20-GHz class VCSELs with different damping factors and same parasitic parameters.For NRZ modulation,results show that achievable data rate and energy efficiency are identical for the two types of VCSEL transmitters.For PAM-4 modulation,compared with the under-damped VCSEL transmitter,achievable data rate and energy efficiency of the optimized over-damped VCSEL transmitter can be improved by 16.13% and 4.12%,respectively,but signal quality of the over-damped VCSEL transmitter is more sensitive to driving circuit parameters.(3)Mode partition noise(MPN)mitigation using wavefront shaping technique in VCSELMMF based links is proposed.Simulation results indicate that,for 25-Gb/s NRZ signal,BER can be reduced by five orders of magnitude after 300-m MMF transmission with wavefront shaping and the standard deviation at the sampling points of received signal can be reduced by 1order of magnitude.In the experiment,a feedback spatial phase manipulation system based on spatial light modulator(SLM)is built and the transmission experiment for 25-Gb/s NRZ signal is performed.After transmitting over 300-m MMF,the power penalty under 7% FEC threshold is reduced by about 1 d B and the SNR of signal is improved after wavefront shaping.(4)Based on wavefront shaping technique,the concept of opto-electrical hybrid equalization is proposed,in which wavefront shaping technique and digital signal processing(DSP)at the receiver side are combined together.The computing complexity of receiver-side DSP can be reduced by using wavefront shaping,and the limited performance enhancement provided by using wavefront shaping along can be improved by receiver-side DSP.Simulation and experimental results show that the performance improvement provided by using wavefront shaping only is limiting and the proposed opto-electrical hybrid equalization can achieve better transmission performance with lower DSP complexity.(5)Nonlinear pre-distortion scheme assisted by cross-correlation-enabled behavioral modeling is proposed.The effectiveness of behavioral modeling to characterize the nonlinearity of VCSEL-MMF based links is firstly verified by simulation and experiment.Based on the nonlinearity characterization using behavioral modeling,the kernel distribution of Volterra nonlinear pre-distortion for VCSEL-MMF based links is obtained using indirect learning architecture.The validity of proposed nonlinear pre-distortion scheme to mitigate the modulation nonlinearity in VCSEL-MMF based links is verified in simulation.120-Gb/s PAM-4 error free transmission is achieved using 30-GHz VCSEL.In summary,this thesis focuses on the VCSEL-MMF based high-speed short-reach optical interconnect systems,and investigates the factors that have impacts on transmission performance and power consumption from chip level,subsystem level to transmission system level,and explore the corresponding solutions.