A Broadband Quasi-Optical Mode Converter for Sub-Terahertz Confocal Gyrotron Devices

A broadband sub-terahertz quasi-optical mode converter (QOMC) that converts a TE
₀₆
confocal waveguide mode into a fundamental Gaussian mode has been realized and presented in this article. Simulation results predict that the QOMC could be operated in an extraordinarily broad bandwidth of 48 GHz (a relative bandwidth of 21.8% around 0.22 THz) when the output scalar Gaussian content is over 90%. Without phase correcting system, the vector Gaussian contents are larger than 75% in a frequency range of 24 GHz. In the low-power experiment, driven by a narrow-band confocal mode generator, the measurement results are consistent with the simulation results, demonstrating the feasibility of broadband operation. With the advantages of broad bandwidth, high conversion efficiency, and compact structure, this proposed QOMC can be applied to many kinds of confocal gyrotron devices.