White-Light Whispering-Gallery-Mode Optical Resonators

Overlapping resonator modes are exploited to obtain wide, high-Q spectra.

NASA’s Jet Propulsion Laboratory, Pasadena, California

Whispering-gallery-mode (WGM) optical resonators can be designed to exhibit continuous spectra over wide wavelength bands (in effect, white-light spectra), with ultrahigh values of the resonance quality factor (Q) that are nearly independent of frequency. White-light WGM resonators have potential as superior alternatives to (1) larger, conventional optical resonators in ring-down spectroscopy, and (2) optical-resonator/electro-optical modulator structures used in coupling of microwave and optical signals in atomic clocks. In these and other potential applications, the use of white-light WGM resonators makes it possible to relax the requirement of high-frequency stability of lasers, thereby enabling the use of cheaper lasers.

Figure 1. Cleaved Optical Fibers tangent to the rim of the resonator disk are used to couple light into and out of the disk. The fibers are shifted with respect to the middle of the rim to obtain a high degree of interaction with all the WGM modes.
In designing a white-light WGM resonator, one exploits the fact that the density of the mode spectrum increases predictably with the thickness of the resonator disk. By making the resonator disk sufficiently thick, one can make the frequency differences between adjacent modes significantly less than the spectral width of a single mode, so that the spectral peaks of adjacent modes overlap, making the resonator spectrum essentially continuous. Moreover, inasmuch as the Q values of the various modes are determined primarily by surface Rayleigh scattering that does not depend on mode numbers, all the modes have nearly equal Q. By use of a proper coupling technique, one can ensure excitation of a majority of the modes.

Figure 2. Normalized Power Levels of input and output pulses of light, as functions of time, were determined in the setup depicted in Figure 1. The small amplitude modulation in the ring-down tail was attributed to slight non-preservation of orthogonality between resonator modes — an artifact of the input/output coupling technique.
For an experimental demonstration of a white-light WGM resonator, a resonator disk 0.5-mm thick and 5 mm in diameter was made from CaF2. The shape of the resonator and the fiberoptic coupling arrangement were as shown in Figure 1. The resonator was excited with laser light having a wavelength of 1,320 nm and a spectral width of 4 kHz. The coupling efficiency exceeded 80 percent at any frequency to which the laser could be set in its tuning range, which was >100-GHz wide.

The resonator response was characterized by means of ring-down tests in which the excitation was interrupted by a shutter having a rise and a fall time of 5 ns. The ring-down time of photodiodes and associated circuitry used to measure the interrupted excitation and the resonator output was <1 ns. Figure 2 shows the shapes of representative input and output light pulses. The average ring-down time was found to be 120 ns, corresponding to Q ≈ 2 × 108. The variations of Q with the laser carrier frequency were found to be <5 percent. Hence, the resonator was shown to have the desired “white light” properties.

This work was done by Andrey Matsko, Anatoliy Savchenkov, and Lute Maleki of Caltech for NASA’s Jet Propulsion Laboratory.

This invention is owned by NASA, and a patent application has been filed. Inquiries concerning nonexclusive or exclusive license for its commercial development should be addressed to the Patent Counsel, NASA Management Office–JPL. Refer to NPO-42221.

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