Design of silicon couplers for efficient coupling of SOI silicon waveguides to single-mode fibers

Couplers have always been crucial in integrated optics, particularly in silicon-based integrated optics, where silicon-based couplers are used to couple silicon-on-insulator (SOI) waveguides and common single-mode optical fibers. However, direct coupling between single-mode fibers and silicon waveguides causes significant coupling losses due to the huge difference in mode spot size. In this research work, we propose a novel cantilever-based silicon-on-insulator edge coupler. A silicon waveguide with a cantilever structure is first created on an SOI wafer, and then silicon dioxide (SiO₂) and silicon nitride (Si₃N₄) layers are alternatively placed on top of it and etched into ridge waveguide shapes. At the same time, the dimensions of the silicon waveguide in the longitudinal direction (light transmission direction) taper to form a tapered waveguide, and the refractive index of the Si₃N₄ tapers in the longitudinal direction as the longitudinal length of the Si₃N₄ shortens layer by layer from bottom to top. The coupling efficiency of a single-mode fiber with a mode field diameter of 10.4 μm and the SOI silicon waveguide exceeded 91%. The silicon coupler was simulated and constructed using the finitedifference method in time domain (FDTD) and the eigenmode expansion (EME) method. This highly effective SOI silicon coupler is crucial for silicon optical integration and may be used in a variety of situations, including optical computing, optical sensing, and optical communication.