Improved fiber-optic transducers, denoted tiger fibers, have been proposed for sensing volatile compounds. Tiger fibers are so named because, as described below, their sensitive portions would be coated with periodically alternating stripes of different polymers, reminiscent of a tiger’s stripes.
A related proposal is that of optical noses, reported in “Fiber-Optic Transducers for Distributed Sensing of Volatiles: An Optical Nose” (NPO-21105), NASA Tech Briefs, Vol. 25, No. 6 (June 2001), page 33. To recapitulate: An optical nose comprises mainly a fiberoptic transducer coupled to a hand-held optical time-domain reflectometer (OTDR). The fiber-optic transducer is an optical fiber coated with a polymer at one or more desired sensing locations along the fiber length. This polymer swells upon exposure to the volatile compound(s) that one seeks to detect [hereafter denoted “analyte(s)”]. The swelling induces a local mechanical strain in the fiber and, consequently, a local variation in the indices of refraction of the fiber core and cladding materials. The OTDR launches picosecond laser pulses into the optical fiber at one end. The index-of-refraction variations induced by the analyte(s) of interest at the sensing locations cause part of the incident laser light to be reflected. The OTDR makes time-resolved measurements of the intensity of the reflected laser light. For a given reflected pulse, the location of the corresponding sensing location is inferred from the measured round-trip pulse travel time. The drawback of optical noses is that the degree of swelling may be insufficient to afford acceptably high chemical sensitivity.
The tiger-fiber concept is also related to that of long-period fiber gratings (LPFGs), which have been used as temperature and chemical transducers. The core of a typical prior LPFG is fabricated with a photoinduced permanent periodic longitudinal variation of its index of refraction. Depending on the spatial period (of the order of tens to hundreds of microns, depending on the application), the fiber attenuates light of one or more known wavelengths more strongly than at other wavelengths.
A tiger fiber would incorporate features of both an optical nose and an LPFG. A tiger fiber would function similarly to a prior LPFG, but its core would not be fabricated with a permanent variation of its index of refraction. Instead, the fiber would be coated with stripes of a polymer that swells when exposed to the analyte(s), alternating with stripes of a polymer that does not swell when so exposed (see figure). The spatial pitch of the stripes would be made equal to the desired grating period. In the absence of the analyte, the fiber would transmit light with little attenuation. Exposure to an analyte would engender longitudinally periodic swelling that would induce corresponding periodic mechanical strains and periodic variations in indices of refraction of the fiber core and cladding materials, thereby causing the fiber to become an LPFG. The degree of the attenuation of light at the at the LPFG wavelength(s) would be related to the concentration of the analyte.
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. NPO-21226
This Brief includes a Technical Support Package (TSP).
Tiger Fibers for Enhanced Optical Sensing of Volatiles
(reference NPO-21226) is currently available for download from the TSP library.
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