Optimization Methods for the Design of Compact and Broadband Adiabatic Couplers

In this study, optimization methods for the design of the compact and broadband adiabatic couplers are presented. Two definitions of an uncoupled waveguide system are introduced: the first involves connecting one of the two silicon waveguides to a boundary, directing its eigenmodes to the boundary, while the second removes one of the waveguides, resulting in eigenmodes confined to the remaining waveguide. The efficient design of adiabatic couplers is achieved by correcting and fitting the refractive indices of the even and odd eigenmodes in these uncoupled systems. The length of the adiabatic coupler designed by the proposed optimization method can be substantially reduced compared to the conventional linear adiabatic coupler. Calculations for the operating bandwidth of the designed adiabatic coupler show that the proposed optimization method can indeed achieve a paramount wide range of operating bandwidths (power transmission efficiencies of more than 90% are achieved for wavelengths from 1406 nm to 3775 nm). A comparison with other design methods demonstrates that the proposed optimization approach yields a coupler with higher efficiency than previous methods. These findings highlight the potential of this optimization strategy for designing high-performance adiabatic couplers with complex geometries in integrated optics.