Research Of Integrated Wavelength And Mode Division Hybrid (De) Multiplexer Based On Photonic Crystal And Nanowire Waveguides

With a rapid rise of data-consuming applications including Internet of Things(Io T),cloud computing,self-driving,visual reality(VR),the demand for transmission capacity and rate of communication systems has exploded.Therefore,a strong optical fiber network is needed as a support to meet the requirements of ultra-large capacity and ultra-fast rate.However the transmission capacity of conventional single-mode fiber has reached its Shannon limit,failing to meet the growing demands.Space division multiplexing(SDM)technology emerges at this proper moment.It uses few-mode fiber(FMF),multi-core fiber(MCF)and the combination of them to increase the number of channels that a single fiber can support,greatly improving the transmission capacity and spectral efficiency.Hybrid multiplexing technology that combines wavelength division multiplexing(WDM)and mode division multiplexing(MDM)would be one of the key technologies to break through the bottleneck of transmission capacity.In recent years,silicon photonics technology has enabled the integration of photonic devices with electronic devices taking advantage of its low-power consumption,high-speed and its compatibility with CMOS processes.Affected by the development of silicon photonics,silicon-based wavelength division(de)multiplexers,mode division(de)multiplexers,and wavelength-mode division hybrid(de)multiplexers,as the key components of various multiplexing systems,have attracted increasing attention.But so far the technology of on-chip hybrid(de)multiplexers for WDM and MDM is still underdeveloped.In this thesis,hybrid(de)multiplexers for WDM and MDM based on photonic crystal(Ph C)and nanowire waveguides are proposed for the first time.Ph C cavities with high quality factor are used for wavelength division(de)multiplexing,and nanowire waveguides are utilized for mode division(de)multiplexing.On-chip integration of wavelength and mode division(de)multiplexers is finally realized.This thesis focuses on reducing the wavelength channel spacing,expanding the free spectral range(FSR),and decreasing the insertion loss of WDM-MDM hybrid(de)multiplexers.Models of devices are built and structural parameters are optimized.The main research contents and innovations are as follows.(1)Simplified coupling models of microcavity and waveguide are constructed.The coupled mode theory(CMT)applied in the photonic crystal resonator(PCR)type WDM devices is derived.Besides,coupling mechanism between waveguides is studied,and a method to suppress crosstalk by increasing the gap distance between waveguides in the asymmetrical directional coupler(ADC)type MDM devices is proposed.(2)The influence of phase relationship on the coupling characteristics of microcavity and waveguide is studied.We bring up a side-coupling structure between a silica whispering gallery mode(WGM)toroid cavity and a silicon Ph C waveguide.Experiments and theoretical analyses reveal that mode matching is the precondition of efficient coupling between microcavity and waveguide.(3)To further reduce the wavelength channel spacing of the silicon-based in-plane Ph C WDM devices,three new types of structures are proposed.They are namely based on one-dimensional(1D)Ph C nanobeam cavity,two-dimensional(2D)Ph C cavity with two reflectors,and 2D Ph C bichromatic cavity.2D finite-difference time-domain(FDTD)method and 2D finite element method(FEM)are applied to design and optimize the structures.Simulation results show that these three types of Ph C WDM structures have a channel spacing as small as 0.8 nm.(4)This thesis studies the reason why the Ph C ADC-type MDM devices have high insertion loss at discontinuous wavelengths.Simulation results show that the MDM devices based on Ph C waveguides have many sharp ripples in the spectrum due to the existence of periodic medium perturbation.The transmission characteristics of those based on nanowire waveguides are compared,finding that they have a flatter coupling transmission spectrum.Therefore the latter one is more suitable for WDM-MDM hybrid(de)multiplexers.(5)Integrated WDM-MDM hybrid(de)multiplexers based on Ph C and nanowire waveguides are proposed.Numerical simulation results show that this kind of device not only has the performance characteristics of small wavelength channel spacing(0.8 nm),large FSR(500 nm),and low insertion loss(1.0 d B),but also has a very compact footprint.In summary,this thesis studies the CMT in WDM and MDM devices.Simplified coupling models of microcavity and waveguide are constructed,and a method for suppressing mode crosstalk is proposed.Besides,the effect of mode matching on the coupling characteristics of microcavity and waveguide is also discussed.Based on theoretical analyses,Ph C WDM devices with large FSR and small channel spacing are designed.The coupling transmission spectra of Ph C waveguide type and nanowire waveguide type MDM devices are compared and studied.Finally,hybrid(de)multiplexers based on Ph C and nanowire waveguides are proposed,which have small channelspacing,large FSR and low insertion loss simultaneously.These academic achievements have a significance for the development of high-speed and large-capacity hybrid multiplexing system for WDM and MDM.Moreover,they will play an important role in promoting the development of photonic integrated chips(PICs)in the future.