Novel SOI edge coupler design based on cantilever structure

The mode field diameter (MFD) of the silicon-based III-V semiconductor lasers is around 3 μm, while the size of the silicon waveguide is less than 1 μm, and the difference in the size of the two leads to a large mode mismatch, which results in a large coupling loss between the two. In this paper, a novel SOI edge coupler based on cantilever structure is proposed for the coupling problem between silicon-based III-V hybrid integrated lasers and SOI silicon waveguides. The mode characteristics of the silicon-based hybrid integrated laser are investigated. The SOI edge coupler structure for siliconbased hybrid integrated lasers is designed, in which a cantilever-structured silicon waveguide is first fabricated on a SOI wafer, and then four silicon dioxide (SiO₂) layers and three silicon nitride (Si₃N₄) layers are alternately deposited on top of it, and the uppermost layer of Si₃N₄ is etched into the shape of a ridge waveguide. The dimensions of the silicon waveguide taper in the longitudinal direction (light transmission direction) 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 SOI coupler is simulated and designed by the finite-difference method in time domain (FDTD) and the eigenmode expansion (EME) method, and the mode field overlap between the 3 μm diameter silicon-based hybrid integrated laser and the SOI silicon waveguide is 88.2%, and the gap between the laser and the silicon waveguide can be eliminated from the reflection loss by the filler so that the gap between the two is 0 in the simulation process, to study the positional offset on the coupling efficiency. The simulation results show that the coupler has a 1 dB alignment tolerance of 1.4 μm in the horizontal direction and 0.8 μm in the vertical direction, which is a large alignment tolerance and requires less alignment accuracy in the actual fabrication.