Compact High-Q Ka-band Sapphire Distributed Bragg Resonator

In a class of high quality (Q-) factor dielectric resonators with low radiative losses, such as popular whispering-gallery (WG) mode resonators with high azimuthal mode numbers, due to high confinement of modal field in dielectric, the Q-factor is limited by the value of inverse dielectric loss tangent of dielectric material. Metal enclosures necessary for device integration only marginally affect the Q-factor while eliminating the residual radiative loss and allowing the optimization of input and output coupling. While very high Q-factors ∼ 200,000 are available in WG mode sapphire resonators in X-band, at millimeter wave frequencies increasing dielectric loss limits the Q-factor to much smaller values, e.g. ∼50,000 at 36GHz. The use of distributed Bragg reflection principle allows to push modal energy outside dielectric while also isolating it from Joule losses in metallic enclosure walls. Very high Q ∼ 600,000 > tgδ has been demonstrated in X-band5, at the expense of impractically large dimensions. In this work, we report on assembly and testing of a compact Ka-band sapphire DBR cavity characterized with Q-factor seven times larger than one predicted by the material’s dielectric loss at the frequency of interest. An intrinsic Q-factor of ∼ 200,000 is demonstrated at 36 GHz for the lowest order TM-mode of a sapphire DBR. The resonator has 50 cm3 volume, which is much smaller than the volume of previously demonstrated DBRs.