Transmission line to waveguide transition at 220 GHz for vacuum photodiode

This paper presents a study of transmission line to waveguide transitions at 220 GHz for vacuum photodiode. Compared with solid-state high-speed photodiodes, terahertz (THz) vacuum photodiodes are superior in the aspects of high carrier mobility and low energy loss. To ensure high-power applications, rectangular coaxial waveguide and coplanar waveguide transition are optimized to couple the vacuum photodiode with a WR-4 rectangular metallic waveguide. Simulated results demonstrate that the reflection coefficients drop dramatically when the thickness of the substrate in the rectangular coaxial waveguide transition reduces to 10 μm. The transition using coplanar waveguide covers a wide bandwidth, and the reflection coefficients reach near -28 dB. The transition using coplanar waveguide can be applied to 220 GHz wireless satellite communications.