硅基微环调制器耦合状态对高速PAM4通信系统的性能影响分析

Investigation on Effect of Silicon Microring Modulator Coupling States on High-Speed PAM4 Communication System

基于VPI Transmission Maker和Matlab搭建硅基微环调制器(Si-MRM)系统级模型,全面定量分析了Si-MRM在不同耦合状态以及耦合量下产生高速四电平脉冲幅度调制(PAM4)信号时的性能差异。仿真结果表明,当器件带宽足够支撑调制速率时,相较于其他耦合状态,临界耦合状态可以获得更优的系统性能,此时在光场在腔内环绕一周的透过率a与自耦合系数t取值为0.71~0.83时可以获得最佳系统性能,并且当a(t)值从0.59逐渐变化到0.91时产生的信号的最大功率代价为4.2 dB。对于过耦合状态以及弱耦合状态,t值变化或者a值变化时均会导致系统性能劣化至接近临界耦合状态,只有t值相较于临界耦合取值有微小的变化(0.71~0.79)或者a值相较于临界耦合取值有微小的变化(0.71~0.79)时,才可以取得与临界耦合[a(t)值均为0.75]接近的系统性能;当器件带宽不足以支撑调制速率时,对于临界耦合状态,a(t)值增加到0.91时,器件带宽有效增大,系统性能得到显著提升。此时,对于过耦合状态,只有增大a值到0.83~0.91才可获得软判决门限阈值以下的误码率性能,并且此时系统性能优于临界耦合。但对于弱耦合状态,均不存在性能优于临界耦合的情况。本文工作对于定量评估Si-MRM因为加工、测试、网络部署等导致的耦合状态抖动进而产生的高速调制信号性能变化,以及基于MRM实现下一代800 Gbit/s和1.6 Tbit/s片级高速光互联具有指导意义。

Objective Silicon photonics is one of the most promising technologies to enable lowcost,low power consumption and highperformance optical transceivers.Compared to MachZehnder modulators(MZMs),silicon microring modulators(SiMRMs)have attracted significant attention in recent years due to their compact footprint,high modulation speed,and potentially more energyefficient dense integration for multilane data transceivers.However,SiMRMs are highly sensitive to fabrication process variations and environmental fluctuations,leading to resonance drift and changes in coupling states that degrade signal performance,especially for advanced modulation formats like 4/8-level pulse amplitude modulation(PAM4/PAM8).Despite this,there is a lack of indepth quantitative analysis on the effect of modulator coupling states on system performance.Typically,MRM parameters such as radius,coupling gap,doped regions,metal contacts,and waveguide dimensions are carefully calculated and chosen to achieve critical coupling.However,during fabrication,testing,or deployment in communication systems,the coupling state of the SiMRM can shift from critical coupling to overcoupling or undercoupling due to fabrication errors,temperature fluctuations,and applied voltage,affecting system performance.A quantitative investigation into the influence of SiMRM coupling states on system performance is crucial for modulator design and integration into various systems.Methods In this study,we conduct a comprehensive quantitative analysis of the effect of SiMRM coupling states on a highspeed PAM4 transmission system using a systemlevel model that includes a dynamic ring resonator model and an equivalent electrical circuit.Performance metrics such as signal bit error rate(BER),receiver side maximum received optical power(RoP),power penalty,device bandwidth,and eye diagram are investigated.Modeling and simulations are carried out in VPI Transmission Maker and Matlab.The dynamic ring resonator model simulates the optical properties of modulators,capturing variations in the optical field within the resonant cavity coupling region and at the input and output ports over voltage and time,as well as adjustments in signal phase and power within the ring waveguide.The circuit subsystem models the effect of voltage on parameters such as resistor,junction capacitance,and inductance,including equivalent circuitry for wire bonding.The MRM model is made of silicon with a depletiontype phase shifter.In the simulation,the MRM radius is set to 10μm,corresponding to a free spectral range(FSR)of 8.75 nm.The lateral PN junction in the ring waveguide provides varying carrier depletion at different reverse voltages.Doping concentrations for n and p in the lowdoped region are 3.5×1018 cm-3 and 6.5×1018 cm-3,respectively,while the highdoped region features a doping concentration of 4.5×1021 cm-3 for both n and p.The measured loaded Q factor of the MRM is~4655 at a bias voltage of 0 V.The electrooptic(EO)phase efficiency of the PN junction is measured to be 0.56 V·cm at a reverse bias voltage of 2 V.In the simulation,the coupling states of the microring resonator are regulated by manipulating the coefficients a and t,which correspond to the singlepass amplitude transmission factor and selfcoupling coefficient,respectively.Results and Discussions The simulation results indicate that for PAM4 generation operating under critical coupling when the data rate of the generated signal is below 150 Gbit/s,incremental variations of a and t from 0.59 to 0.91 lead to a maximum power penalty of 4.2 dB.Optimal system performance is attained when values of a and t range from 0.71 to 0.83.When the modulation speed exceeds 160 Gbit/s and a(t)value is set to 0.91,significant enhancement in system performance can be attained due to increased device bandwidth.In overcoupling and undercoupling states,where the modulation speed remains below 150 Gbit/s and adequate device bandwidth is maintained,variations in t with a constant a or adjustments in a with a constant t result in degraded system performance compared to the critical coupling state.Only slight variations in t(ranging from 0.71 to 0.79,with a maintained at 0.75)or a(ranging from 0.71 to 0.79,with t maintained at 0.75)can yield system performance closely approximating that of the critical coupling state(where a and t are both set at 0.75).When the modulation speed surpasses 180 Gbit/s,achieving BER performance below the threshold of softdecision forward error correction(SDFEC)is possible only by increasing the value of a to 0.83‒0.91 to enhance the bandwidth.Under these conditions,system performance exceeds that of critical coupling.However,in the undercoupling state,no results have been observed where performance exceeds that achieved under critical coupling conditions.Conclusions Our study provides a comprehensive and quantitative analysis of the effect of coupling states of SiMRM on the generation of highspeed PAM4 signals(ranging from 64 Gbit/s to 224 Gbit/s).The entire analysis is based on a systemlevel model of SiMRM.In the simulation,different coupling states are simulated by changing the values of a and t,and a quantitative analysis is conducted on key performance indicators,including BER performance,RoP at the receiver,power penalties,device bandwidth,and eye diagrams.The results show that optimal system performance is attained at the critical coupling state,and the bandwidth meets the rate requirement.Within this context,the best system performance is observed when a(t)ranges between 0.71 and 0.83.In scenarios of overcoupling or undercoupling states,where the modulation bandwidth aligns with the modulation rate requirement,variations in t with a constant a or adjustments in a with a constant t result in degraded system performance compared to the critical coupling state.Otherwise,performance below the threshold for BER with SDFEC is possible only by increasing the value of a to enhance the bandwidth.In this scenario,the overcoupling state exhibits enhanced performance compared to the critical coupling state.However,no instances within the undercoupling state,which involve changes in a,have displayed performance better than that of the critical coupling state.Our study provides valuable insights into the quantitative assessment of SiMRM coupling state variations caused by fabrication,testing,network deployment,etc.,and their consequent effects on the performance alterations of highspeed modulation signals.Such findings are pivotal for directing the development of nextgeneration 800 Gbit/s和1.6 Tbit/s chiplevel highspeed optical interconnects utilizing SiMRM technology.