CMOS-compatible low-loss silicon nitride-assisted double-etching silicon O-band edge coupler with 180-nm minimum width

The edge coupler holds paramount importance as a link bridging optical signals between fibers and silicon-based optoelectronic chips. It surpasses the grating coupler in terms of elevated coupling efficiency, diminished polarization sensitivity, and an expanded bandwidth. However, designing a low-loss silicon edge coupler with a broader minimum width, especially for the O-band, presents substantial obstacles. To surmount these challenges, a silicon nitride (SiN)- assisted double-etching silicon structure with a 180 nm minimum width is adopted in this work. This innovation capitalizes on the double-etching silicon taper, propelling the SiN layer's height to 1.6 μm relative to the bottom of silicon waveguide, resulting in pronounced reduction of silicon leakage loss. By meticulously implementing coupled mode theory, an exceptional coupling efficiency exceeding 0.95 is achieved for both TE and TM polarizations at 1310 nm, facilitating the seamless transition of light from SiN to the thinner silicon waveguide. Further enhancements in curbing silicon leakage loss and shortening device length are achieved through mode analysis-driven designs for both the SiN and silicon taper. Ultimately, these intricate designs culminate in an edge coupler boasting a 180 nm minimum width, with minimal losses of approximately 0.7/1.5 dB for TE/TM polarization and a 0.5-dB bandwidth of around 100 nm within the O band, as demonstrated through simulation while interfacing with standard single-mode fibers.