Function expansion and CMA-ES based topology optimal design of LSM-pass/LSE-stop SWG-NRD guide devices for THz application

Non-radiative dielectric (NRD) guides have been actively investigated in recent years to develop THz-wave integrated circuits, and the sub-wavelength grating NRD guide (SWG-NRD guide) is drawing significant attention among them because of its single-mode transmission of LSM₀₁ mode without exciting the unwanted LSE₀₁ mode. To suppress the reflection loss between standard NRD and SWG-NRD guides, usually taper couplers are used though a considerable length of taper waveguide is needed. Therefore, it is necessary to design these connecting couplers optimally with reduced coupling length and desired transmission properties. In this paper, we present topology optimal design of LSM-pass/LSE-stop standard NRD and SWG-NRD guides connecting coupler utilizing function expansion method and covariance matrix adaptation evolution strategy (CMA-ES). The optimized connecting coupler can greatly suppress the reflection loss with 4.5 to 9 times reduced coupling length compared to the existing taper couplers in this platform. The device achieves over 99% of single LSM₀₁ mode transmission (less than 0.04 dB of insertion loss (IL)) from 0.85 THz to 1.15 THz. The usefulness of the proposed matching coupler and the design technique are confirmed by designing two SWG-NRD devices, two-branch power splitter, and three-branch power splitter. The designed power splitters achieve high transmission efficiencies with bandwidths of almost 110 and 70 GHz, respectively. Finally, the fabrication tolerance of the proposed devices has been discussed in detail.