3-Waveguide Mode Filter in Multilevel Si Photonics

In this study, we present the design and analysis of a mode filter based on the use of a 3-waveguide coupler. The coupler is assumed to have a multimode central guide surrounded by two similar single mode waveguides. A theoretical model is developed based on the coupled mode theory and using the coherent tunneling adiabatic passage approach. The model shows that the coupler can be characterized by an effective coupling length that accounts for the coupling of each of the outer guides with the central guide. The estimated coupling length is in good agreement with the numerically calculated one using the Semivectorial Beam Propagation Method SV-BPM. The filter function is also well explained by the supermode interference in the structure. Based on this model, the design of a 3-waveguide mode filter built on SiN multilevel technology is introduced. An optimized design of the filter showing a crosstalk better than −40 dB at a wavelength of 1550 nm is demonstrated. This opens the door for new 3D integrated optical circuits in which the transition from single-mode to multi-mode in the vertical direction can be handled.