Design Study of a 372-GHz Higher Order Mode Input Coupler

The design of a higher order mode (HOM) input coupler for a low-terahertz gyrotron travelling wave amplifier is presented. A two-branch waveguide coupler based on the even distribution of incident power is designed to couple the rectangular TE
₁₀
mode to the circular TE
₆₁
mode. The optimised tapered waveguide input coupler achieved an operating frequency range of 359-385 GHz, equating to a bandwidth of 7%. A prototype waveguide coupler scaled to W-band (75-110 GHz) was manufactured. The vector network analyser measured return loss of the HOM coupler is shown to be <;10 dB at 90-96 GHz. A transmission and phase measurement is presented to analyse TE
₆₁
mode conversion in the coupler.