Wavelength-Selective Coupling to Vertical Higher Order Modes in a 500-nm-Thick Silicon Multimode Waveguide Using Long- and Short-Period Gratings

This work explores thick silicon multimode waveguides (MMWs), focusing on vertical multimode operation. We present grating-assisted coupling structures to efficiently convert the fundamental mode to vertical higher-order modes in a 500-nm-thick silicon MMW. Both forward and backward coupling schemes are investigated. The forward coupling with a long-period grating exhibits broadband conversion over a coupling length less than $40\mu $ m. The backward coupling with a short-period grating demonstrates mode conversion with a 3-dB bandwidth ranging from 5.8 to 13.3 nm. These results open new avenues for applications such as on-chip spatial mode multiplexing with a higher capacity.