Comparative Study and Fabrication of Two Miniaturized Terahertz Waveguide Mode Converters for Vacuum Electron Devices

The design, fabrication, and test of two types G-band broadband and low-loss rectangular TE<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><roman>10</roman></sub> to circular TE<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><roman>01</roman></sub>-mode converters for vacuum electron devices are introduced in this article, including Marie-mode converter and serpentine-mode converter. Following micromachining and modified microelectroforming processes, the Marie-mode converter with abnormal shape inside and the serpentine-mode converter are successfully fabricated. The dimensions and performances of the two types of mode converters, such as the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S</i>-parameters and the mode converter efficiencies, are compared accordingly. Test results of two identical back-to-back joined mode converters verified that both mode converters are with a bandwidth of more than 40 GHz for the reflection coefficient <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S</i><sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><roman>11</roman></sub> are below -15 dB and <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S</i><sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><roman>21</roman></sub> are better than -1 dB. Meanwhile, the Marie-mode converter is featured with shorter length and integrated structure, while the serpentine-mode converter is featured with wider bandwidth and simple fabrication processes. The miniaturized mode converters and their micromanufacture technologies indicate significant potentials for vacuum electron devices in the millimeter-wave and terahertz spectrum.