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 Tiger Fiber would be coated with alternating stripes of a swelling and a non-swelling polymer. Swelling of one of the polymers on exposure to an analyte would induce a periodic longitudinal variation in the indices of refraction of the fiber core and cladding.

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

Topics:
Chemicals Coatings, colorants and finishes Coatings, colorants, and finishes Fiber Optics Fiber optics Fibers Lasers Lasers & Laser Systems Optics Optics Physical Sciences Polymers Polymers
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Tiger Fibers for Enhanced Optical Sensing of Volatiles

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    NASA Tech Briefs Magazine

    This article first appeared in the January, 2002 issue of NASA Tech Briefs Magazine (Vol. 26 No. 1).

    Read more articles from the archives here.

    Overview

    The document discusses the development of "Tiger-fibers," a novel optical fiber technology designed to enhance the sensitivity of chemical sensors, particularly for detecting volatile components. This innovation is spearheaded by inventors Adrian Ponce and Dmitri A. Kossakovski and is associated with NASA's Jet Propulsion Laboratory.

    The primary motivation behind this work is to improve the chemical sensitivity of optical noses, which are devices that utilize optical fibers coated with polymers that swell upon exposure to specific gases. Traditional methods have faced limitations due to the low degree of swelling, which restricts the sensitivity of these sensors. The proposed solution involves a unique approach where the polymer coating is applied in a periodic manner, creating a long period grating (LPFG) on a standard optical fiber. This periodic structure enhances the mechanical strain induced on the fiber when exposed to volatile components, thereby improving the sensor's response.

    The document outlines the technical aspects of the Tiger-fiber, explaining that the periodicity of the polymer coating is tailored to achieve desired characteristics for the grating. When the fiber is exposed to volatile analytes, the swelling of the polymer generates periodic stress on the fiber's cladding and core. This stress alters the coupling of forward propagating modes within the fiber, leading to specific wavelengths' attenuation that is sensitive to the concentration of the analyte.

    The Tiger-fiber's striped appearance is a visual representation of its innovative design, which contrasts with traditional methods that typically involve uniform coatings or single polymer patches. The document emphasizes the potential applications of this technology in environmental sensing, where enhanced sensitivity can lead to more accurate detection of chemical substances.

    Overall, the Tiger-fiber represents a significant advancement in optical sensing technology, promising to improve the performance of sensors used in various fields, including environmental monitoring and safety applications. The work is conducted under NASA's auspices, highlighting its relevance to space exploration and other scientific endeavors. The document serves as a technical support package, detailing the novelty, motivation, and solutions provided by this innovative approach to optical sensing.