Miniature high-resolution optical-absorption sensors for detecting trace amounts of chemical species of interest in gas and liquid samples are undergoing development. The transducer in a sensor of this type is a fiber-optic-coupled optical resonator in the form of a transparent microsphere, (or a microcavity equivalent to a microsphere as described below).
The principle of operation of these sensors is an updated version of that of conventional optical cavity-ringdown spectroscopy (CRDS), wherein resonators in the form of long and bulky Fabry-Perot cavities are used in order to obtain enough effective optical-path length to enable the resolution of small attenuation associated with trace concentrations of analytes. In addition to bulky apparatuses, conventional CRDS requires large samples to fill the Fabry-Perot cavities. In contrast, a microsphere or microcavity sensor of the type under development is designed to be immersed in a sample, which can be small because the microsphere is small. (Alternatively, the sample can be contained in a small cavity as described below.)
The use of transparent microspheres as optical resonators has been reported in a number of prior Tech Brief articles. To recapitulate: In a transparent microsphere, resonance is achieved through glancing-incidence total internal reflection in one or more "whispering-gallery" modes, in which light propagates in equatorial planes near the surface, with integer numbers of wavelengths along nominally closed circumferential trajectories. In the absence of external influences, and assuming that the microsphere is made of a low-loss material, the high degree of confinement of light in whispering-gallery modes results in a high resonance quality factor (high Q).
Suppose that the microsphere is illuminated by laser light at its resonance wavelength and is immersed in a sample liquid or gas that (1) has an index of refraction less than that of the microsphere material and (2) contains a highly diluted chemical species of interest that absorbs light at the resonance wavelength. In that case, the Q of the resonator is diminished through absorption by molecules of that species in the evanescent field of the whispering-gallery modes. Because of the smallness of microspheres (typical diameters from tens to hundreds of optical wavelengths), the smallness of the effective volumes of the evanescent fields (typically 10-9 cm3 or less), and the low level of optical losses intrinsic to microspheres themselves, it is possible to detect very small amounts of optically absorbing chemical species through decreases in Q; calculations have shown that in some cases, it should be possible to detect amounts as small as single atoms or molecules.
The left side of the figure depicts a typical setup for a microsphere sensor immersed in a sample fluid that has an index of refraction less than that of the microsphere. If the index of refraction of the sample fluid exceeds that of a material that could be used to construct a microsphere, then one must use a setup like that shown on the right side of the figure: The sample is contained in a microspherical cavity in a capillary cell made from a transparent material that has an index of refraction less than that of the sample fluid. In this setup, the portion of the sample in the microspherical cavity serves as a the whispering-gallery-mode resonator, and coupling between the optical fiber and the microsphere is effected by use of a prism attached to a thin wall that acts as a tunneling (in quantum-mechanical analogy) gap for photons.
The decrease in Q (and thus the amount of the chemical species of interest) can be determined either by measurement of the decrease in the cavity-ringdown time or, if the spectral purity of the laser is adequate, by traditional measurement of transmission bandwidth. Bandwidth measurement is ordinarily used when Q ranges from ≈105 to ≈108; cavity-ringdown measurement is more convenient for Q ≥108(typically corresponding to ringdown time ≥30 ns). While the precision with which the absolute value of Q can be determined is usually no better than a few percent, variations in Q can be measured with greater precision. In state-of-the-art CRDS as performed with Fabry-Perot cavities, it is possible to resolve ringdown times to fractional variations as small as about 2 ×10-3 at data-acquisition rates of about 1 kHz. Hence, it is possible to obtain absorption spectra with satisfactory signal-to-noise ratios even though the losses added by the chemical species of interest may be only small fractions of the intrinsic optical losses of the resonators themselves.
This work was done by Vladimir Iltchenko and Lute Maleki of Caltech for NASA's Jet Propulsion Laboratory.
In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to
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Refer to NPO-21061
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Microsphere and Microcavity Optical-Absorption Sensors
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