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Tutorial and Review Paper
I.
INTRODUCTION
The progress of microwave (MW), millimeter (mm), and submillimeter (submm) wave ranges is directly related to the development of the appropriate scientific and engineering approaches, devices, and techniques. Experience with quasi-optical technology enables advanced passive mm and submm elements to be designed on the basis of high- Q quasi-optical resonators. The dielectric resonators (DRs) excited at whispering gallery modes (WGMs) are promising representatives of such resonators. Because WGM resonators are quasi-optical DR (QDR), the acronyms QDR and WGM resonator are synonyms. In this paper, we use the acronym WGM resonator - commonly accepted in the English-speaking scientific community.
It was noted in [1, 2] that there are weakly damped azimuthal modes in the round dielectric rod with relative permittivity values[...]1 in the range of 2-10. From the view point of geometrical optics, these modes are formed inside the dielectric by waves that are excited in a cylindrical DR with a curvature radius of [...] 0 at angles greater than the angle of total internal reflection from the resonator circumference and are characterized by large values of azimuthal indices n. The electromagnetic field decays outside the dielectric. This results in ultra-small radiation losses and consequently in the high Q-factor of WGM resonators. Dielectric WGM resonators also demonstrate high mode stability. For the first time, an abnormally low damping of waves propagating along the circumference surface was observed for acoustic waves traveling along the inner wall of the circular gallery [3]. Therefore the azimuthal modes excited under the condition [formula omitted, refer to PDF] [1] were termed WGMs. Here [...] and c are the angular frequency of the modes and the light velocity, respectively. We now show that not only passive but also active devices can be developed on the basis of high-Q WGM resonators.
A)
Brief historical background
In the 1960s, experts in the MW technology put forward the hypothesis that resonators may be designed on the basis of dielectric materials without the application of metal walls as reflectors of electromagnetic energy. The maser developers used a rutile parallelepiped without additional metal mirrors as a resonator. The permittivity of a rutile crystal is about 100,...