A Fully Numerical Method for Designing Efficient Adiabatic Mode Evolution Structures (Adiabatic Taper, Coupler, Splitter, Mode Converter) Applicable to Complex Geometries

A fully numerical method for designing efficient adiabatic mode evolution structure (AMES) (referred to as NAMES) is introduced, which can be applied to adiabatic taper, coupler, splitter, mode converter, and a wide range of AMES based devices. The method can compute efficient adiabatic waveguiding shapes for these devices, including those with complex waveguiding geometries involving 2D/3D mode connections. We introduce two algorithms for the NAMES, referred to as “maximal mode-power loss at top algorithm” and “mode-power loss at initial slope algorithm”. Both are based on keeping the mode-connection power exchange constant. We use the simple case of a waveguide taper to explain the algorithms, and then apply it to a more complex case of an adiabatic waveguide coupler to show how it can readily generate a waveguiding shape that can give the same mode-connection power transfer with a much shorter length than that based on a linear shape. In the coupler case, we show the device efficiency is equal to that based on an optimized analytical approach developed for a simple geometry, but the NAMES can address many different device types and complex device geometries all with a single numerical approach that would also enable design automation. The algorithms utilize Eigenmode Expansion (EME) simulator for field propagation with functionalities that are particularly efficient for the algorithms. We cross-checked the EME simulator results using finite-difference time-domain method. With sufficiently fine division of the structure, the waveguiding shape generated converges to basically the same shape for both algorithms and the shape is quite insensitive to the starting parameters. Thus, the NAMES given is robust, convergent, efficient, and general (not restricted to a particular device type). The NAMES would have wide applications to designing AMES based devices with complex geometries for photonic integrated circuits.