Glass microspherical resonators are precision submillimeter-sized beads that support optical whispering-gallery modes, which are light paths that skip along the surface at grazing-incidence angles within the glass. The two-dimensional curvature of the surface naturally confines light. Grazing-incidence reflection greatly reduces surface-scattering losses in these resonators, boosting the cavity Q factor to 108 or higher. These qualities make microspheroidal resonator cavities useful for spectroscopy, narrow-linewidth lasers, optoelectronic oscillators, and sensors.
Theoretically, microspheres have a large free spectral range (FSR), a property desirable for filtering or laser stabilization. For example, silica spheres of 150- to 400-µm diameter should have an FSR of 437 to 165 GHz (3.5 to 1.3 nm) at a wavelength of 1500 nm. In reality, however, flaws in the fabrication of glass beads of these dimensions produce many modes separated by only 6.8 to 2.5 GHz, which can require the use of intermediate filters to block unwanted modes.
Researchers at the Jet Propulsion Laboratory (JPL; Pasadena, CA) and Moscow State University (Moscow, Russia) have developed a cavity with a microtoroidal geometry that minimizes the unwanted modes. The microtoroidal surface is made by compressing a small sphere of low-melting-temperature silica glass between two cleaved optical-fiber tips. Both compression and surface tension contribute to the desired geometry. One of the fibers is cut and the whole assembly mounted next to a standard evanescent-prism coupler ordinarily used to couple light into a microsphere.
The whispering-gallery-mode spectrum was measured with a tunable distributed-feedback laser, which was continuously scanned via current modulation over an 80-GHz range and temperature-tuned from 1545.1 to 1552.4 nm. A spectrum compiled from individual current-modulated scans that were stitched together revealed only two whispering-gallery modes of selected polarization within a 383-GHz FSR (3.06 nm in the wavelength domain). Parasitic modes were 6 dB or lower in transmission than the two principal modes. The bandwidth of the individual modes was 23 MHz, demonstrating a finesse of 1.7 x 104.
In comparison to microspheres, the microtorus showed a reduction in the number of excited whispering-gallery modes of up to two orders of magnitude. The Q factor of the microtorus is on the order of 107, approaching that of microspheres.
A higher eccentricity (a torus that is thinner in relation to its outer diameter) would result in a complete elimination of all but the fundamental whispering-gallery mode, note the researchers. "We have plans to investigate different eccentricities, and we are working to improve technology for higher finesse, Q, and free spectral range," says Vladimir Ilchenko, one of the researchers.
