Whispering gallery mode (WGM) optical resonators are axially symmetric dielectric structures ranging in size from tens of microns to a few mm, and can trap light in a small volume. The modes are solutions of Maxwell’s equation and represent waves that propagate close to the surface of the resonator structures, along the perimeter.
The three energy loss mechanisms – radiative, surface scattering, and bending losses – are minimized in WGM resonators due to the morphology of these structures and the transparency of the dielectric material from which they are fabricated. Thus, WGM resonators generally exhibit quality factors (Q’s) that range from 105 in the case of ring structures, to greater than 10¹¹ for fluorite toroidal resonators. WGM structures fabricated with semiconductor materials may also be made with gain.
A variety of WGM resonators are fabricated to have ring, spherical, toroidal, and elliptical geometries. They have been demonstrated in a wide range of material, including glass, silica, silicon, compound semiconductors, and crystalline material. Several methods exist to allow fabrication of WGM resonators with conventional semiconductor processing techniques.Crystalline WGM resonators, made at OEwaves, exhibit nonlinear optical properties such as the electro-optic effect, which can be used in filter and modulator applications. Resonators fabricated with ferroelectric materials can be used for engineering of the modal spectrum with applied DC and RF fields, since an applied electrical field changes the index of refraction of the material. By taking advantage of these properties, OEwaves has demonstrated extremely efficient WGM-based modulators with the application of RF and microwave fields, and widely tunable filters with application of a DC field. Resonators made with highly transparent crystals, such as calcium fluoride and magnesium fluoride, exhibit the highest reported Q’s.
4mm Dia. Lithium Niobate WGM Resonator