Investigation on 220 GHz Taper Cascaded Over-Mode Circular Waveguide TE_0n Mode Converter

This paper proposes a taper cascaded over-mode circular waveguide TE_0n mode converter for the millimeter and terahertz wave gyrotron. The mode converter of this structure can effectively reduce the difficulty of high frequency mode converter in fabrication. This paper verifies the feasibility of this new structure from theory, simulation, and experiment. Based on coupled wave theory calculations, three TE₀₂-TE₀₁ mode converters with lengths of 65.43 mm (4 segments), 119.3 mm (6 segments) and 136 mm (8 segments) and a TE₀₃-TE₀₂ mode converter with a length of 92 mm (8 segments) are optimized. The conversion efficiency in the frequency band 215–225 GHz is 91.8–94%, 93–95%, 95–98.78% and 95–98.44%. Because the length of the mode converter is clearly limited, this paper selects the TE₀₂-TE₀₁ mode converter with a length of 65.43 mm (4 segments) and the TE₀₃-TE₀₂ mode converter with 92 mm (8 segments) for simulation and experimental verification. In the simulation software Computer simulation technology (CST), the TE₀₂-TE₀₁ and TE₀₃-TE₀₂ mode converters and their composed TE₀₃-TE₀₁ mode converters are selected for modeling and analyzing. The simulation results and theoretical calculation results of the three mode converters only have different degrees of frequency deviation, and the frequency deviation of the 4-stage TE₀₂-TE₀₁ mode converter can be ignored; the frequency deviations of TE₀₃-TE₀₂ mode converter and TE₀₃-TE₀₁ mode converter are 2 GHz and 3 GHz, respectively. The experimental system is a field scanning system based on a vector network analyzer (VNA), which scans the input and output of the mode converter respectively. The experimental result is that when the input mode purity is 92% in TE₀₁ mode, the output mode TE₀₃ mode has a mode purity of 82%, and it has lower transmission loss. In this paper, the results from theory, simulation and experiment are in good agreement. This type of mode converter is easy to prepare, which makes it an effective alternative for high frequency curvilinear waveguide mode converter.