Rapid design of miniaturised branch-line couplers through concurrent cell optimisation and surrogate-assisted fine-tuning

In this study, the authors introduce a methodology for low-cost simulation-driven design optimisation of highly miniaturised branch-line couplers (BLCs). The first stage of their design approach exploits fast concurrent optimisation of geometrically dependent, but electromagnetically isolated cells that constitute a BLC. The cross-coupling effects between the cells are taken into account in the second stage, where a surrogate-assisted fine-tuning procedure is executed. The tuning process is based on space-mapping-enhanced low-fidelity model of the entire BLC. The latter is constructed by cascading local response surface approximations (RSAs) of the BLC building blocks. The efficiency of their technique is demonstrated through the design of two compact BLCs. A considerable scale of miniaturisation has been achieved in both cases (83.7 and 87.4%, respectively), at the computational cost corresponding to a few EM simulations of the respective BLC. Comparison of numerical results with several surrogate-based design approaches as well as an experimental verification of the final designs are also provided.