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).

A Microspherical Optical Resonator is operated in the presence of a sample fluid that contains an optically absorbing species. The concentration of the species is determined from its effect on the Q of the resonator.

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

Technology Reporting Office
JPL
Mail Stop 249-103
4800 Oak Grove Drive
Pasadena, CA 91109
(818) 354-2240

Refer to NPO-21061

Topics:
Chemicals Data acquisition and handling Fabrication Fiber optics Gases Imaging and visualization Lasers Manufacturing processes Optics Photonics Sensors and actuators Spectroscopy
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Microsphere and Microcavity Optical-Absorption Sensors

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    Overview

    The document presents a technical report on the development of high-resolution optical-absorption sensors utilizing whispering-gallery (WG) modes in microsphere cavities. Authored by Vladimir Iltchenko and Lute Maleki from NASA's Jet Propulsion Laboratory, the report outlines a novel approach to detecting trace amounts of foreign substances in various media, including gases and liquids.

    The primary innovation discussed is the use of high-Q microsphere cavities as ultra-small sampling volumes, enabling the potential detection of individual atoms or molecules. This is achieved through the measurement of changes in the quality factor (Q) of the WG modes, which are sensitive to additional optical attenuation caused by absorptive species in the evanescent field surrounding the microsphere. The report highlights that the small dimensions of the microsphere (typically in the range of tens to hundreds of optical wavelengths) allow for the detection of extremely low concentrations of analytes, down to single atoms or molecules.

    The document details two experimental setups for the microsphere sensor, depending on the refractive index of the sample fluid relative to the microsphere material. In one setup, the sample fluid has a lower refractive index, while in the other, the sample is contained within a microspherical cavity made from a transparent material with a lower refractive index than the sample fluid. The coupling between the optical fiber and the microsphere is facilitated by a prism, which acts as a tunneling gap for photons.

    The report emphasizes the advantages of this new sensor technology over traditional methods, such as Fabry-Perot cavities, which require larger sample volumes and are bulkier. The proposed microsphere sensor can operate effectively in various important fluid media, including water and basic solvents, due to the intrinsic low attenuation of silica and the preservation of high-Q WG modes.

    In conclusion, the document outlines a significant advancement in optical sensing technology, providing a compact, efficient, and highly sensitive method for detecting trace chemical species. This innovation has potential applications in various fields, including environmental monitoring, chemical analysis, and biomedical diagnostics. The work was conducted under NASA's contract and reflects ongoing efforts to enhance detection capabilities using advanced optical technologies.