On Chip Mode-order Conversion Based On Shallowly Etched Silicon Waveguide Microstructure

With the rapid development of mobile Internet technology,the huge amount of data interaction poses a new challenge to the transmission capacity of the system.Silicon photonics SOI platform has attracted intensive research interest as an excellent platform for developing some ultradense on-chip photonic integrated circuits,due to its high refractive index contrast,low cost and compatibility with mature complementary metal-oxide-semiconductor(CMOS)technologies.However,to meet the exponential growth demand of the optical interconnection capacity,various multiplexing technologies have been explored,such as wavelength-division multiplexing(WDM),polarization-division multiplexing(PDM),and mode-division multiplexing(MDM).Among them,MDM technology becomes more and more important for the on-chip optical interconnects,since the total transmission capacity can be easily extended by using more eigenmodes of optical waveguides.Under this condition,we require the waveguide to be able to generate more higher-order eigenmodes on-chip.In order to solve this issue,high-performance mode-order converters which can change the input fundamental mode into the higher-order mode are the pivotal components for the on-chip MDM applications.Based on these,this paper first analyses different modes and its field distribution under the TM polarization,and a novel method for mode conversion using shallow etching structure in silicon nanowires is proposed.Then this paper uses 3D-FDTD and EME to analysis light transmission in silicon waveguide and model components and proportions at any section.Finally,three kinds of mode-order converters for on-chip multimode optical transmission are proposed.Specific research contents are as follows:1?This paper presents a TM₀-TM₁ mode converter based on a new silicon-based shallow etching structure.Through the analysis of the existing mode converters,it is found that the conversion of mode-order usually needs two steps,one is to introduce perturbation to produce self-image effect,the other is to realize the accumulation of phase difference.Therefore,following these two steps,we introduce two basic rectangular etching slots on one side of the top silicon nanowire.Then,according to the analysis of the EME,it is found that introducing an additional phase-tuning slot between the two etching slots can significantly improve the device performance.After optimization,the proposed TM₀-TM₁mode-order converter has good performance(at 1550 nm:conversion efficiency>97.5%,maximum crosstalk value of-23 d B,insertion loss of 0.29 d B).Finally,the bandwidth and the possible manufacturing errors of the device are also analyzed in detail.Meanwhile,the corresponding parameter analysis methods(such as 3D-FDTD and EME)used in device design are described.2?Based on the design concept of TM₀-TM₁ mode converter,we believe that the proposed design scheme has certain scalability.Therefore,this paper proposes a compact shallowly etched TM₀-TM₂ mode converter based on SOI platform.By comparing the cross-section field distribution between TM₁ mode and TM₂ mode,it is found that TM₂ mode is different from TM₁ mode in that the electric field component is centrally symmetric along the z direction.Therefore,when the expected output mode is TM₂,the paper uses four rectangular slots on the silicon nanowires which is etched symmetrically in a two-by-two manner.Also,an additional phase-tuning slot is introduced in the middle of the four rectangular slots.After optimization,the conversion efficiency of the device is as high as 94.3%,and the modal crosstalk<-15 d B.Meanwhile,the proposed TM₀-TM₂ mode converter is analyzed in detail,such as the waveguide modal analysis,the etching-pattern analysis of the phase-tuning slot,the optical bandwidth analysis and the acceptable manufacturing error range analysis.3?With the increase of the mode-order number,the design complexity,mode crosstalk analyses and optimization processes become more and more difficult.So,the fundamental design scheme reveals quite important,especially for the device functional scalability.In order to further verify the scalability of the proposed design concept,an asymmetric TM₀-TM₃ mode order converter based on shallowly etched slot metasurface is proposed.In this paper,the goal is achieved by using the metasurface with shallowly etched slots on chip.Six rectangular slots are etched on the SOI waveguide,the mode conversion efficiency of the device is higher than92%and the modal crosstalk is lower than-19 d B.Meanwhile,it is verified that the proposed design concept can realize a series of mode order conversion from fundamental mode to higher-order mode in TM polarization state.