Super free spectral range tunable optical microbubble resonator

An optical resonator is often called fully tunable if its tunable range exceeds the spectral interval that contains the resonances at all the characteristic modes of this resonator. For high-Q-factor spheroidal and toroidal microresonators, this interval coincides with the azimuthal free spectral range (FSR). In this Letter, we demonstrate what we believe to be the first mechanically fully tunable spheroidal microresonator created of a silica microbubble having a 100microm order radius and 1microm order wall thickness. The tunable bandwidth of this resonator is more than two times greater than its azimuthal FSR.