Microtorus: a High Finesse Microcavity with Whispering-Gallery Modes
  We have demonstrated a 165 micron oblate spheroidal microcavity with free spectral range 383.7 GHz (3.06nm), resonance bandwidth 25 MHz (Q ~ 10^7) at 1550nm, and finesse F > 10^4. The highly oblate spheroidal dielectric microcavity combines very high Q-factor, typical of microspheres, with vastly reduced number of excited whispering-gallery (WG) modes (by two orders of magnitude). The very large free spectral range in the novel microcavity - few hundred instead of few GigaHertz in typical microspheres - is desirable for applications in spectral analysis, narrow-linewidth optical and RF oscillators, and cavity QED. 
Vladimir S. Ilchenko (1), Michael L. Gorodetsky (2), X. Steve Yao (1),   and Lute Maleki (1) ((1)Jet Propulsion Laboratory, ...