Optical resonators of the whispering-gallery-mode (WGM) type featuring DC-tunable free spectral ranges (FSRs) have been demonstrated. Previously, the FSRs of WGM optical resonators were determined solely by the resonator geometries and materials: hence, the FSR of such a resonator could be tailored by design, but once the resonator was constructed, its FSR was fixed. By making the FSR tunable, one makes it possible to adjust, during operation, the frequency of a microwave signal generated by an optoelectronic oscillator in which an WGM optical resonator is utilized as a narrow-band filter.
Each tunable WGM resonator was made from a disk of lithium niobate, 2.6 mm in diameter and 120 μm thick. The edge of the disk was rounded by polishing to an approximately spherical surface. A ferroelectric-domain structure characterized by a set of rings concentric with the axis of the disk (see Figure 1) was created by means of a poling process in which a 1-μm-diameter electrode was dragged across the top surface of the disk in the concentric-ring pattern while applying a 2.5-kV bias between the electrode and a conductor in contact with the bottom surface of the disk. After the poling process, the top and bottom surfaces of the disk were placed in contact with metal electrodes that, in turn, were connected to a regulated DC power supply that was variable from 0 to 150 V. When a DC bias electric field is applied in such a structure, the indices of refraction in the positively poled concentric rings and in the unflipped, negatively poled concentric regions change by different amounts.
The concentric-ring ferroelectric-domain structure of such a resonator can be engineered so that it overlaps with one or more radial resonator modes more than it does with other, adjacent modes. As a result, when the indices of refraction change in response to DC bias, some modes shift in frequency by amounts that differ from those of adjacent modes; the difference in frequency shift amounts to the desired change in the FSR between the affected adjacent modes.
A test was performed on each tunable resonator to observe the absorption spectrum of the resonator and the changes in frequencies of adjacent modes of interest as the applied DC bias voltage was varied. In this test, a probe optical beam having a nominal wavelength of 1.55 μm, scanned over a frequency range of 20 GHz, was coupled into the resonator through a diamond prism. Figure 2 presents some results of one such test, in which the frequency shift between two selected adjacent modes exhibited the desired variation with bias voltage.
This work was done by Makan Mohageg, Andrey Matsko, Anatoliy Savchenkov, Lute Maleki, Vladimir Iltchenko, and Dmitry Strekalov of Caltech for NASA’s Jet Propulsion Laboratory.
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