Compact broadband silicon 3  dB coupler based on shortcuts to adiabaticity

We proposed and experimentally demonstrated a compact, broadband, and low-loss-mode-evolution-based 2×23  dB coupler on a silicon-on-insulator platform. The waveguide widths and separations of the coupler were tapered in an optimal manner using the shortcuts to the adiabaticity method. The coupler, with a ∼70  μm total length and 200 nm feature size, exhibited <0.3  dB measured excess loss over a broadband wavelength range from 1500 to 1600 nm. Balanced and unbalanced Mach-Zehnder interferometers fabricated for validation showed extinction ratios larger than 27 and 23 dB, respectively, over a 100 nm bandwidth. The extracted splitting ratio was between 0.48 and 0.52.