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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°, 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.

Multidimensional multiplexing (MM) is a crucial technology for achieving high-capacity terahertz (THz) wireless communication. However, the non-ideal characteristics of polarization-related THz devices, such as the low cross-polarization isolation (XPI) of the ortho-mode transducer (OMT) and antenna mismatch, result in reduced power of co-polarization and increased power of cross-polarization signal in the receiver. This leads to high polarization crosstalk (PC), such as reduced signal-to-interference-plus-noise ratio (SINR) and degraded error vector magnitude (EVM) in the polarization division multiplexing (PDM) system. This article proposes a THz line-of-sight (LoS) dual linear polarized model to analyze the PC phenomenon, with consideration of polarization-related devices and transmitted power for the first time involving OMT antennas and channel (OACAO model). We explore influencing factors, including the imbalance of polarized transmitted power, XPI of OMT, antenna polarization mismatch, channel attenuation, and depolarization effect in 3D space. To deal with the PC problem, we adopt an XPI optimization strategy based on RF hardware debugging and a cross-polarization interference cancellation (XPIC) algorithm, which enhances XPI to 22–25 dB, and increases SINR by 1–2 dB. Finally, we establish our THz wireless system using PDM and frequency division multiplexing (FDM) in the 220-GHz band, demonstrating real-time transmission of uncompressed 8K video. Using 16-QAM and I/Q modulation, the wireless link succeeded in a total data rate of 83.2 Gb/s over 100 m with SINR exceeding 19.8 dB, and a block error rate (BLER) below 1e-9. This demonstration provides insight for future research on THz ultra-high-speed communication systems.