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<inline-formula><tex-math notation="LaTeX">$_{01}$</tex-math></inline-formula> mode regardless of arbitrary bends without exciting the LSE<inline-formula><tex-math notation="LaTeX">$_{01}$</tex-math></inline-formula> 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 <inline-formula><tex-math notation="LaTeX">$\pm$</tex-math></inline-formula> 0.5 dB and an average coupling ratio of 49.2&#x0025;. The average return loss and isolation are better than <inline-formula><tex-math notation="LaTeX">$-$</tex-math></inline-formula> 21 dB and <inline-formula><tex-math notation="LaTeX">$-$</tex-math></inline-formula> 24 dB, respectively, and the average insertion loss is only 0.089 dB. 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<inline-formula><tex-math notation="LaTeX">$_{01}$</tex-math></inline-formula> mode excitation at the bends, confirming the platform&#x0027;s relevance and devices&#x0027; possible application in THz integrated wavelength-division multiplexing (WDM) systems.