Study On Silicon-based Mode Conversion And Multiplexing/demultiplexing Devices Using Multimode Interference Couplers

With the arrival of 5G communication,data exchanging is developing toward higher capacity and higher speed.In order to increase the transmission capacity of communication systems,various of technologies based on silicon chips have attracted the attention of scholars at home and abroad.In this paper,silicon based mode conversion and multiplexing/demultiplexing devices are proposed and fabricated based on the principle of multimode interference coupler with the goals of compact,large bandwidth and large fabrication.The main work includes:(1)Firstly,the research progress of mode division multiplexing devices at home and abroad is reviewed,and the advantages and disadvantages of on-chip mode division multiplexing schemes are compared.Then,the finite difference time domain(FDTD)simulation method and particle swarm optimization(PSO)algorithm are analyzed.With FDTD algorithm,three key characteristics of multimode interference couplers are studied,which include the mode distribution in the waveguide plane,the self-image length relationship,and the position and phase relationship of N×N multimode interference couplers.(2)Two types of the original type and PSO optimized type mode conversion and multiplexing/demultiplexing devices are proposed and fabricated based on multimode interference coupler.The experimental results show the insertion loss is less than 2 d B and the crosstalk is lower than-17 d B in the original type divice for both the fundamental mode(TE0)and the first-order mode(TE1)within the whole C-band(1530 nm–1570 nm).The simulation and experimental results show that the transmission performance of the PSO optimization type device is improved compared with the original type device.The experimental results show the insertion loss is less than 2 d B and the crosstalk is less than-20 d B in this PSO optimization type two-mode multiplexing/demultiplexing device within the whole C-band.(3)A mode conversion and multiplexing/demultiplexing device with large bandwidth is proposed and fabricated based on the subwavelength grating assisted multimode interference coupler.In this device,subwavelength grating structure is introduced to reduce the wavelength dependence so as to expand the bandwidth.The simulation results show that the size of the multiplexing device is less than 3.25 ?m×42.4 ?m.Performance analysis of the mode conversion and multiplexing /demultiplexing device is carried out numerically,showing properties with insertion loss <2 d B and crosstalk <-10 d B for both the TE0 and TE1 mode in the wavelength range from 1300 nm to 1600 nm.The experimental results show a successful two-mode multiplexing and demultiplexing in the wavelength range from1500 nm to 1600 nm with insertion loss <5 d B and crosstalk <-10 d B,which prove the feasibility of mode conversion and multiplexing/demultiplexing devices based on subwavelength gratings.