Research Of The Wavelength-mode Division (de)multiplexer Based On Photonic Crystal

With the popularization of cloud computing related applications and the development of virtual reality and artificial intelligence,people's demand for network capacity is explosively increasing.To meet this demand,optical fiber communication system is evolving from multi-channel,high-speed to ultra-high-speed,ultra-large capacity and ultra-long distance.Combining wavelength division multiplexing?WDM?and mode division multiplexing?MDM?technology can increase the capacity and spectral efficiency of the system.Therefore,wavelength-mode division multiplexing?WMDM?optical communication system is the development direction of ultra-large capacity optical communication system in the future.In the WMDM optical communication system,the wavelength-mode division?de?multiplexer?WMMUX/DEWMMUX?with low loss and crosstalk is the key device.If the discrete wavelength division multiplexer?WMUX?and mode division multiplexer?MMUX?are combined to realize WMDM,additional coupling loss and crosstalk will be caused.With the development of photonic integrated circuits towards miniaturization and integration,the integration of WDM and MDM using photonic crystal?PC?will become the main development direction of the WMDM optical communication devices.In this paper,a PC-based WMMUX/DEWMMUX is proposed for the first time.Line defect waveguides with different widths are used to realize mode conversion,point defect cavities are introduced to realize wavelength selection and the tapered waveguides?TW?are used to realize mode coupling.The integration of WDM and MDM is realized on a PC chip.This paper focuses on reducing wavelength spacing and increasing the number of transmission channels,and optimizes the design of a new type of PC WMMUX/DEWMMUX.The main contents of this paper include:?1?The theoretical model of PC WMUX/DEWMUX is constructed,and the theoretical analysis of the device is carried out by using the time domain coupled mode theory.The conditions that the device parameters need to meet to achieve high efficiency WDM are obtained.WMUX/DEWMUX based on 2D square lattice PC at 1550nm/1570nm is proposed.A point defect cavity is used for wavelength selection,and only one wavelength selective reflection cavity and one coupling dielectric cylinder are introduced to achieve efficient WDM.The device has simple structure,small size,low insertion loss and small channel crosstalk.It is more conducive to the monolithic integration of WDM and MDM,and can reduce the complexity of PC WMMUX/DEWMMUX.?2?A two-mode and a four-mode MMUX/DEMMUX based on PC are proposed.The relationship between the width of the photonic crystal waveguide?PCW?by introducing a line defect and the number of transmission modes is analyzed by Maxwell equations.The efficiency of mode conversion is analyzed by using the transverse coupled mode theory,and the conditions of waveguide width and coupling length for high efficiency mode conversion are obtained.PCW with different widths are used to achieve mode conversion.The coupling between PCW is optimized.TWs are introduced to realize the coupling of different modes.The structure of the asymmetric parallel waveguide?APW?is optimized,and the size of the device is controlled while the number of modes is increased.The PC MMUX/DEMMUX proposed in this paper is simple in structure and easy to expand,which is conducive to the expansion of the channel number of PC WMMUX/DEWMMUX.?3?Based on the coupled mode equation,the coupled mode theory?CMT?for PC WMMUX/DEWMMUX is derived,and the selection conditions of the key parameters for high efficiency WMDM are obtained by using this theory.Three different structures of PC WMMUX/DEWMMUX are proposed.By optimizing the structure and structure parameters of the device,the wavelength spacing is reduced from 240nm to 10nm,and the number of channels is expanded from 4 to 8.The results show that the insertion loss of the device is less than 0.39 dB and the channel crosstalk is less than-15.86dB.The device has simple structure and excellent performance.The tolerance analysis of the device is carried out to verify the accuracy of the theoretical analysis,which provides an important reference for the actual fabrication of the device.?4?A WMDM optical communication system is built on the VPI Transmission Maker simulation platform.The WMMUX/DEWMMUX proposeded in this paper is applied to the system,and the WMDM of 4QAM signals with eight links of 40Gbps is realized.The system can achieve a stable transmission of 80km,and the transmission performance is good.The bit error rate is less than7.81×10^-4.In summary,based on the coupled mode equation,a CMT suitable for PC WMMUX/DEWMMUX is derived.Based on the theory analysis,the PC WMMUX/DEWMMUX is proposed.The performance of the device is simulated by finite difference time domain?FDTD?,and several key parameters affecting the performance are analyzed.Finally,PC WMMUX/DEWMMUX is applied to the WMDM optical communication system,and the performance of the system is analyzed using VPI simulation platform.These results have important academic significance and potential application value for the development of photonic integrated devices and WMDM optical communication systems.