Exact Tuning of High-Q Optical Microresonators by Use of UV

NASA Tech Briefs, May 2006

Resonance frequencies can be shifted permanently by controlled amounts.

In one of several alternative approaches to the design and fabrication of a "whispering-gallery" optical microresonator of high resonance quality (high Q), the index of refraction of the resonator material and, hence, the resonance frequencies (which depend on the index of refraction) are tailored by use of ultraviolet (UV) light. The principles of operation of optical microresonators, and other approaches to the design and fabrication of optical microresonators, have been described in prior NASA Tech Briefs articles, including the two immediately preceding this one.

In this approach, a microresonator structure is prepared by forming it from an ultraviolet-sensitive material. Then the structure is subjected to controlled exposure to UV light while its resonance frequencies are monitored. This approach is applicable, for example, to the fabrication of optical microresonators from silica doped with germanium. This material exhibits low optical loss at a wavelength of 1,550 nm - a wavelength often used in optical communication systems. It is also highly sensitive to UV light: its peak sensitivity occurs at a wavelength of 334 nm, and its index of refraction can be shifted by as much as 10^sup -2^ by irradiating it at an argon-ion-laser wavelength of 351 nm.

Fabrication begins with softening a Ge-doped SiO^sub 2^ rod by use of a hydrogen/oxygen microburner and stretching the rod into a filament [asymptotically =]30 µm wide. The tip of the filament is heated in the hydrogen/oxygen flame to form a sphere having a diameter between about 100 pm and about 1 mm (see Figure 1). Then the resonance frequencies of the sphere used as a microresonator are measured while the sphere is irradiated with UV light at a power of 1.5 W from an argon-ion laser that can be operated at either of two wavelengths: 379 or 351 nm. Irradiation at the longer wavelength heats the sphere and thereby temporarily shifts the resonance frequencies but does not cause a permanent change in the index of refraction. Irradiation at the shorter wavelength changes the index of refraction permanently.

At first, for the purpose of adjusting the optics that focus the laser light on the sphere, the laser is operated at the longer wavelength and the adjustments performed to maximize the shift of resonance frequencies. Then the laser is operated at the shorter wavelength while the resonance frequencies are monitored. The UV radiation is terminated when the resonance frequencies have shifted by the desired amount. For example, a typical shift of [asymptotically =] 10 GHz can be achieved in a microsphere of 240-µm diameter (see Figure 2).

This work was done by Anatoliy Savchenkov, Lute Maleki, Vladimir Iltchenko, and Timothy Handley of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com/tsp under the Physical Sciences category.

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NASA's Jet Propulsion Laboratory, Pasadena, California