A High-Balanced Interdigital Hybrid Metallic Waveguide Coupler Based on SSPPs-Coupling for WR-4 Ultra-Full-Band Applications

Metal hybrid waveguide couplers, which are crucial passive structures in terahertz (THz) in-phase-quadrature (IQ) mixers, have raised significant attention in the development of high-speed communication systems. Spoof surface plasmon polariton (SSPP) mode, a surface wave transmission mode, possesses excellent characteristics such as localized field enhancement and energy concentration. To further expand the operating bandwidth of THz coupler and improve the balance performance between IQ paths, this article combines the SSPPs' structure with the waveguide coupler, converts the low-pass characteristics of the traditional SSPPs' transmission lines and high-pass rectangular waveguide transmission lines into bandpass ones, and proposes an SSPPs' interdigital coupled quasi-periodic coupler, treating the hybrid waveguide coupler as a quasi-periodic structure, utilizing the transmission mode and dispersion relation of SSPPs to concentrate and enhance the local field strength, thus improving the coupling performance. The proposed structure achieved better amplitude balance performance and expanded the operating bandwidth. An SSPP-coupled hybrid waveguide coupler was designed and fabricated. The simulated amplitude imbalance is less than 0.3 dB and the measured one remains below 0.75 dB across the frequency range of 150-260 GHz, which has a significantly larger bandwidth than the WR-4 standard rectangular waveguide with the working frequency range of 170-260 GHz. The measured phase imbalance is within 2.2(degrees) , and the total insertion loss is within 1.3 dB. This novel SSPPs' coupler proves the feasibility of a promising structural design using bandpass transmission lines in replacement of traditional high-pass transmission lines in metal hybrid waveguide couplers for large-bandwidth and high-performance THz IQ mixing devices.