Accurate identification of whispering gallery mode patterns of gyrotron with stabilized electro-optic imaging system

Abstract

The precise field pattern measurement and analysis of a typical whispering gallery mode excited in a gyrotron are important to understand the interaction physics of the gyrotron. We precisely analyzed the characteristic of a whispering gallery mode, rotating TE6,2 mode, by a photonic-assisted W-band (75-110 GHz) electro-optic imaging measurement system. The whispering gallery mode in the W-band region diverges fast in free space as it propagates from the radiation port. Therefore, scanning the field patterns of a device-under-test should be performed as close as possible to identify the device's characteristics. We successfully accomplished visualizing highly accurate field patterns of a rotating and mixed whispering gallery mode based on the measured electric field magnitude and phase by using dual optical fiber-scale electro-optic (EO) probes. We observed the distorted fields when the typical open-ended waveguide and a general EO probe were used in the extremely near-field zone, whereas a very precise field was measured in a minimally invasive way by the proposed EO probe. The measured mode patterns were quantitatively analyzed by using a cross correlation function and a mode purity equation. This work promises a way to provide accurate electric field information in the generation of the whispering gallery mode in the millimeter and submillimeter regime. Published by AIP Publishing.