Study on single-mode transmission in non-radiative dielectric waveguide

In order to develop millimeter-wave integrated circuits, low-loss and compact waveguide devices have been intensively investigated in recent years. Non-radiative dielectric waveguide (NRD guide) is expected to be a platform for millimeter-wave and THz-wave circuits because of its non-radiative nature. However, conventional NRD guides support ${\mathrm{LSM}}_{01}$ and ${\mathrm{LSE}}_{01}$ modes simultaneously and mode conversion between these modes often occurs at bends and discontinuities. Therefore, circuit devices have to be carefully designed not to excite unwanted ${\mathrm{LSE}}_{01}$ mode. In this paper, we propose a sub-wavelength grating NRD guide (SWG-NRD guide) to achieve single-mode transmission band of ${\mathrm{LSM}}_{01}$ mode. SWG structure can easily control dispersion property of transmission modes and cut-off frequency of ${\mathrm{LSE}}_{01}$ mode can be greatly shifted to higher frequency side. The designed SWG-NRD guide achieves a wide transmission band of 20 GHz with a central frequency of 60 GHz. We also propose a taper waveguide to connect SWG-NRD guide with the conventional NRD guide with low reflection. Finally, we present the propagation characteristics of the SWG-NRD guide bend. The proposed SWG-NRD guide shows high transmission efficiency and overcomes the problems of mode conversion in NRD guides.