Investigation of signal coupling circuit for NbN-based SIS mixer operated at 650 GHz

Superconductor-insulator-superconductor (SIS) mixers are the most sensitive coherent detectors in the terahertz region and are extensively utilized in astronomical research and atmospheric observation. In order to improve the detection accuracy of frequency, reduce the loss of signal in the transmission path, and reduce noise temperature, it is important to design a signal coupling circuit with high coupling efficiency. In this paper, with the help of a high-frequency structure simulator, a signal coupling circuit was designed based on a waveguide NbN-based SIS mixer at 650 GHz. The return loss and embedding impedance of this SIS mixer were investigated from 600 to 720 GHz. The effects of the SIS mixer chip’s bow-tie probe angle, substrate thickness, and chip slot height were calculated and analyzed. The results show that the return loss is lower than -16 dB in the 600 to 720 GHz band, and the embedding impedance is around 60 Ω, which can provide a good reference for the development of waveguide SIS mixers.