Ultra-Broadband and Compact Two-Mode Multiplexer Based on Subwavelength-Grating-Slot-Assisted Adiabatic Coupler for the Silicon-on-Insulator Platform

We design and experimentally demonstrate an ultra-broadband and compact two-mode multiplexer based on subwavelength-grating-slot-assisted adiabatic coupler for the silicon-on-insulator platform. The introduction of subwavelength-grating slot effectively increases the refractive index of the gap region between two strip waveguides and enhances the coupling between them, leading to high modal transmission over a short mode-evolution region, particularly at the short-wavelength regime. As a result, our two-mode multiplexer has a compact design footprint, with a mode-evolution region length of only 25  $\mu$ m and an entire device length of only 55  $\mu$ m. Simulation shows that the mode-division multiplexing (MDM) link formed by the designed two-mode multiplexers has a record bandwidth of 740 nm spanning from 1260 nm to 2000 nm, where the crosstalk is lower than −18.5 dB and the insertion loss (IL) is lower than 0.32 dB. The MDM link has been fabricated using electron beam lithography and achieves measured crosstalk lower than −18.8 dB and IL lower than 2.6 dB, over a 100 nm bandwidth from 1260 nm to 1360 nm that covers the O-band and a 130 nm bandwidth from 1500 nm to 1630 nm that covers the C-band and the L-band.