Based on the Mie scattering theory, we study optical resonances with whispering gallery modes (WGMs) in tubular microcavities. Rigorous formulas are present to obtain resonant wavelengths and Q factors for the WGM resonances. It is found that the Q factors of microtubes can be dramatically increased by increasing the dielectric constants in tube walls. For common SiO/SiO2 based microtubes, Q factors can be improved by one order when the microtubes are coated with thin high-index HfO2 layers (n = 1.95, thickness = 10 nm). The results could be useful for designing better optical devices based on tubular microcavities.

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km and γm can be extracted from two Cs,m(k0) points near km.
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