Design and Measurement of Terahertz Broadband Vertically Coupled High-Order TE^□_n0 Mode Converters

The theoretical analysis, simulation, fabrication, and measurement of a novel TE^□₁₀-TE^□_n0 mode converter is described in this article. In the existing research work, although a significant portion of mode converters have the expansibility to higher order modes, the performance of the mode converters will deteriorate when the mode index increases. As we know, no mode converters operating higher than TE^□₇₀ mode have ever been reported. To address this issue, a novel vertically coupled mode converter for converting the TE^□₁₀ mode to higher order TE^□_n0 mode is proposed in this article. As examples, the TE^□₁₀-TE^□₈₀ and TE^□₁₀-TE^□₇₀ mode converters are designed. Simulated results show that the 1 dB transmission bandwidths of the TE^□₁₀-TE^□₈₀ and the TE^□₁₀-TE^□₇₀ mode converters are 56.1 GHz (228.7-284.8 GHz) and 50.0 GHz (232.3-282.3 GHz), respectively. Within the operating bandwidth, the reflection coefficients S₁₁ of both mode converters are less than -15 dB and the conversion efficiencies S₂₁ of the targeted TE^□₈₀ mode and TE^□₇₀ mode are higher than 99% and 92%, respectively. In addition, two back-to-back connected TE^□₁₀-TE^□₈₀ mode converters are fabricated and measured to verify the design. The measured S-parameters align well with the simulated results and exhibit a flat response, which confirms the feasibility of the proposed mode converter and shows that it can achieve good performance at much higher order modes.