Determination of the ppm-Level Moisture Content in Dielectric Materials using Higher-Order Modes of a Dielectric Disk Resonator

A dielectric disk resonator employing whispering gallery modes is one of promising sensors for millimeter-wave and sub-terahertz frequency bands due to its small size, high quality factor and ability to operate with one-side access to the object under investigation. However, the whispering-gallery-mode resonator operating on the fundamental mode is not well suited for testing bulky dielectric materials. The reason is that the resonance is crucially suppressed by stray fields arising in the tested material near the disk edge, and thus the high quality factor of the resonator cannot be realized. In this work, it is shown that the higher-order whispering gallery modes are preferable in such applications. Though the unloaded quality factor of the resonator sensor is not as high as in the case of the fundamental mode, it nevertheless suffices to detect small levels of moisture content in the dielectric materials. A dielectric disk resonator with the diameter of 20 mm and thickness 3 mm made of a high-purity alumina was used as a moisture sensor operating in the frequency band near 63 GHz. The performance of the dielectric disk resonator sensors was demonstrated experimentally in the evaluation of moisture content in thick sheets of ABS plastic and gasoline-water mixtures, when the water fraction in the material amounted to 200-1000 ppm.