Experiments have shown that a highly oblate microspheroid made of low-dielectric-loss silica glass can function as a high-performance optical resonator. The shape of this resonator (see figure) is intermediate between that of (1) microdisk or microring resonators and (2) microsphere resonators, which have been described in a number of previous NASA Tech Briefs articles. As described below, the oblate spheroidal shape results in large values of both resonance quality factor (Q) and finesse. Large values of these parameters are favorable for single-mode operation of a laser or an optoelectronic oscillator.
A microsphere resonator exploits the circulation of light by total internal reflection, in "whispering-gallery" (WG) modes characterized by large values of Q. In contrast, the Q values of microring and microdisk resonators are limited because of significant scattering losses on their flat surfaces.
The preferred WG modes of a microsphere resonator are those in which light circulates by propagating along the equator. As a consequence of spherical symmetry, a microsphere resonator is characterized by a large spectral density of modes because, along with the equatorial modes, some modes with small propagation-vector components transverse to the desired equatorial circulation are also coupled to an input/output device. A large spectral density of modes is not favorable for single-mode operation.
The highly oblate microspheroid resonator is not subject to the disadvantages of microsphere, microdisk, or microring resonators. In the highly oblate microspheroid resonator, the greater curvature of the surface in the direction transverse to the desired equatorial circulation effectively decouples the partly transverse modes from the input/output device. As a result, the resonator can be operated in a regime similar to that of single-longitudinal mode Fabry-Perot etalons. The free spectral range (FSR) [the difference in frequency between successive modes] is defined by successive integer numbers of wavelengths packed along the equatorial round-trip light path. For a highly oblate spheroid with an equatorial diameter (corresponding to D in the figure) of the order of hundreds of microns and a typical wavelength of 1.55 μm, an FSR as large as 1 THz is expected; in contrast, for a microsphere of approximately equal parameters, the FSR can be expected to be much smaller (typically between 2 and 10 GHz).
At the same time that it affords a much greater FSR, the highly oblate microspheroid resonator retains the high Q (up to about 108) typical of microspheres. This high Q corresponds to a resonance bandwidth of a few megahertz. Consequently, the resonator is characterized by very high finesse (finesse ≡ FSR/resonance bandwidth): typical values of finesse range from 104 to 105. Heretofore, such high values of finesse were available only in relatively large Fabry-Perot resonators.
If resonators like this one were utilized in simple diode-laser frequency-locking schemes, robust single-mode operation should be possible because the FSRs of the WG modes would be compatible with the gain·bandwidth of typical diode lasers. For spectral-analysis applications, resonators like this one offer a highly attractive combination of miniaturization and unprecedented spectral resolution. For optoelectronic oscillators, resonators of this type could provide convenient sideband frequency references in the terahertz range, provided that appropriate detectors and modulators for this frequency range were also developed.
This work was done by Vladimir Iltchenko, X. Steve Yao, and Lute Maleki of Caltech for NASA's Jet Propulsion Laboratory.
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