The term "optical nose" refers to a fiber-optic chemical sensor of a type that has been proposed to enable distributed measurement of the concentrations of volatile compounds. Optical noses should not be confused with electronic noses, which are single-point sensors based on chemical-induced variations of the electrical resistances of carbon-black/polymer composite films. Optical noses could enable rapid measurement of gas mixtures (e.g., volatile compounds in air) at multiple sensing locations along their lengths, which could be of the order of kilometers. Optical noses could function well in locations where audio- and radio-frequency electromagnetic interference renders electronic noses ineffective. Moreover, it may be easier to fabricate optical noses than to fabricate electronic noses because it would not be necessary to handle carbon black.
An optical nose (see figure) would include a commercially available hand-held optical time-domain reflectometer (OTDR) and a fiber-optic transducer, which would typically be prepared as follows: An optical fiber would be coated with a polymer that swells when it absorbs a volatile compound. The outer surface of the polymer would be coated with a gas-impermeable film. At designated sensing locations along the optical fiber, the impermeable film would be removed in patterns to form half-circumference, millimeter-wide notches through which gases could enter the polymer.
The absorption of one or more volatile compounds through the notch at a given sensing location would give rise to asymmetric swelling of the polymer; the asymmetric swelling would engender shear stress which, in turn, would cause local variation in the index of refraction of the fiber. (Optionally, the notches could extend the full circumference, in which case the swelling and the resulting shear-stress pattern would be symmetric. However, the asymmetric configuration is preferred because the shear stress and the associated change in the index of refraction would be greater.)
The OTDR would launch picosecond laser pulses into the optical fiber at one end. The index-of-refraction variations associated with the presence of volatile compounds at the notches would cause part of the incident laser light to be reflected. The OTDR would make time-resolved measurements of the intensity of the reflected laser light. For a given reflected pulse, the location of the corresponding sensing notch could be inferred from the measured round-trip pulse travel time.
It should be possible to construct arrays of optical noses for discrimination among different volatile compounds. Each fiber-optic transducer in such an array would be coated with a different polymer, which would be chosen so that the index-of-refraction response of this transducer to different volatile compounds of interest would differ from the corresponding responses of the other transducers. For automated or semiautomated operation, the readouts of all the transducers in the array could be digitized, then processed by principal-component-analysis and pattern-recognition algorithms to discriminate the volatile compounds of interest.
This work was done by Adrian Ponce and Dmitri Kossakovski of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP)free on-line at www.nasatech.com/tsp under the Physical Sciences category.
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