Expanding the adiabatic design toolbox - more modes, parameters and versatility

Adiabatic design principles can be used to improve the performance of many photonic components. The recently published adiabatic optimization method, MODALL, relies on a design rule that guarantees adiabaticity and enables optimization of adiabatic photonic components against multiple dimensions and radiation modes. In this work, MODALL is extended to enable optimization of multi-mode components, optimization against an extra degree of freedom and optimization of modal crosstalk. We present a derivation of these extensions starting from MODALL theory and verify them via the design, fabrication and characterization of a mode multiplexer with ultra-low crosstalk: worst-case <-38 dB and median <-45 dB. These design extensions will aid the adiabatic design optimization of many photonic components including splitters, polarization rotators, interlayer transitions and edge couplers.