Whispering-gallery resonator has continuous modal spectrum

Whispering-gallery-mode (WGM) optical resonators are typically cylindrical or spherical cavities into and out of which light, often from an optical fiber, is coupled via evanescent-wave coupling. They are potentially useful as high-Q cavities for cavity-ring-down (CRD) spectroscopy, among other applications. But WGM resonators have sparse spectra, meaning that the resonator modes do not overlap-a disadvantage for CRD spectroscopy, because the absorption line of a substance under study could fall between the resonator modes.

Researchers at the California Institute of Technology’s Jet Propulsion Lab (Pasadena, CA) have developed a “white-light” WGM resonator that has an essentially continuous modal spectrum and a wavelength-independent Q factor. The device achieves its continuum because the 5-mm-diameter resonator is thick (0.5 mm) and the light is fiberoptically introduced off-center at one edge of the flat rim by a tilted fiber coupler, maximizing mode interaction. The resonator could also be useful as an electro-optic modulator controlled by a microwave field. Contact Andrey Matsko at [email protected].