Study on MZI-Based Advanced SWG-NRD Guide Terahertz Devices Using Topology-Optimized Matching Circuit

The sub-wavelength grating NRD guide (SWG-NRD guide) can transmit the single LSM$_{01}$mode regardless of arbitrary bends without exciting the LSE$_{01}$mode, unlike the standard NRD guide, making it a promising candidate in developing THz-wave integrated circuits. The simple bending waveguides and power-splitting devices with sharp curvature have been previously investigated on this platform; however, more sophisticated devices with arbitrary bends have yet to be thoroughly examined. This paper presents the design of a SWG-NRD 3-dB wavelength-insensitive coupler (WINC) based on Mach-Zehnder interferometer (MZI) with arbitrary bends, utilizing the topology-optimized matching circuit. The WINC achieves a broadband operational range from 0.93 THz to 1.06 THz with an imbalance of less than$\pm$0.5dB and an average coupling ratio of 49.2%. The average return loss and isolation are better than$-$21dB and$-$24dB, respectively, and the average insertion loss is only 0.089dB. To demonstrate the usefulness of the WINC, a design of an MZI interleaver is presented. The tolerance for fabrication errors in the proposed devices is also thoroughly discussed. The designed devices do not experience LSE$_{01}$mode excitation at the bends, confirming the platform's relevance and devices' possible application in THz integrated wavelength-division multiplexing (WDM) systems.