Study On Silicon-based On-chip Mode-division Multiplexing Devices

With the growing demand for data transmission,optical technology received much attention.Silicon-based photonics integrated circuits are widely investigated in various fields for high integration,high fabrication tolerance,low cost and compatibility with CMOS process.Based on this technology,multiplexing techniques have been developed for larger capacity.As an important part of multiplexing,mode multiplexers can effectively increase the bandwidth of the system.Here,investigation on on-chip mode multiplexing schemes are carried out from numerical methods,combining the silicon photonics technology with mode-division multiplexing.Using different structures to optimize the conventional on-chip optical devices,three on-chip mode multiplexing devices are proposed with advantages.The content of this thesis are summarized as follows:1.An overview of silicon-based photonics and mode multiplexing techniques is given.The principles and applications of the computational methods involved in the design process of silicon-based optical devices are described,and common transmission metrics for silicon mode-multiplexing devices are given.2.A silicon-based asymmetric directional coupler TM₀/TM₁ mode multiplexer is proposed and analyzed.The TM₀-TM₁ mode conversion is achieved using directional coupler.The coupling length of the device is greatly reduced by using a shallow etched structure to break the structure symmetry.The use of taper also allows the multiplexer to have good fabrication tolerance and operating bandwidth.3.The subwavelength grating structure can be considered as a homogeneous medium,and its effective refractive index can be modified by adjusting the structure parameters.Using this feature,an adiabatic-coupler-based TE mode multiplexing scheme is proposed.The TE₀ mode can be converted into TE higher order mode in the bus waveguide by mode evolution.The scheme shows good transmission properties and bandwidth,and the scheme has great scalability with multiplexing of TE₀,TE₁,TE₂ and TE₃ mode.4.A novel TE₀/TE₁ mode demultiplexer based on a multimode interference is proposed.The TE₀ and TE₁ modes are demultiplexed in a single subwavelength grating multimode waveguide,breaking the limits in traditional MMI-based mode demultiplexer.With a total device length of 18.6μm and a bandwidth of 350 nm,the ultra-high bandwidth and extremely small size of the device make it promising in highly integrated WDM-compatible on-chip mode multiplexing systems.