A Fabry-Pérot Resonant Source Enables the Generation of a High-Purity LP$$_{01}$$ Mode at Low Power

Abstract The fundamental solution to Maxwell’s equations in circular, corrugated waveguides is the LP $$_{01}$$ 01 mode, also known as the HE $$_{11}$$ 11 mode, which has very low ohmic losses and is used to transport high-power mm-waves with high efficiency in many of today’s most important experimental machines for nuclear fusion. Here, we describe a quasi-optical resonator system based on a Fabry-Pérot-type cavity that consists of a lens-horn, a beam splitter, a phase-reversing mirror, and a semi-transparent mirror. This resonator resulted to be a notably effective system for achieving a high purity for the LP $$_{01}$$ 01 mode, reaching a value of 99.40% for parallel polarization and 99.55% for perpendicular polarization. The output of this source enables the measurement of ohmic losses of high-power microwave components, such as miter bends and oversized waveguides, and for studies of mode conversion using a low-power setup.