Compact and robust 2 × 2 fast quasi-adiabatic 3-dB couplers on SOI strip waveguides

We design and experimentally demonstrate compact and robust fast quasi-adiabatic (FAQUAD) couplers for silicon-on-insulator (SOI) strip waveguides. The mode evolution region of the coupler is optimized by the FAQUAD protocol, achieving shortcuts to adiabatic mode evolution in a short length of 11.7 μm. 3-dB couplers with measured bandwidths (3 dB ± 1 dB) larger than 75 nm over temperature range from 20 °C to 50 °C are designed and fabricated using standard CMOS-compatible optical lithography, with the smallest device footprint of 53.7 × 4 μm². Devices with longer mode evolution regions are also evaluated by simulations and experiments. 3-dB couplers with a footprint of 87.6 × 4 μm² show larger than 85 nm bandwidths (3 dB ± 0.5 dB) over the same 30 °C range.