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Encyclopedia of Physical Science and Technology EN005F-954 June 15, 2001 20:48

828 Fiber-Optic Chemical Sensors

in the silica comprising the optical fibers. The backscat- tion of analytes in the sample. Based on the concept of
tered light signals are time-resolved and decrease as a microbending, and by using modified hydrogels that can
function of distance. Changes in the backscattered light respond to different analytes, various distributed optical
signals can be induced by sensing materials in response fiber sensors can be constructed.
to changes in the concentrations of chemical species. Although still in the research stage, it is expected that
The evanescent field can be employed for sensing in distributed sensing capabilities will improve many fiber-
distributed optical fiber sensors by measuring the analyte optic chemical sensor technologies. In particular, integra-
interactions at different positions along the fiber. Typi- tion of distributed and multianalyte sensing will result in
cally, the optical fiber cladding is removed at several points powerful fiber-optic chemical sensor devices.
along the fiber and replaced by a thin polymer layer con-
taining an indicator dye. For example, if a fluorescent
C. Imaging and Chemical Sensing
pH indicator dye is used, by transmitting excitation light
through the fiber in time intervals (usually a few nanosec- Optical imaging fibers array can carry images from one
onds) and measuring the backscattered emission, it is pos- end of the fiber to the other due to the coherent nature of
sible to observe changes occurring in each sensing element the fibers. The imaging capabilities of such fibers are uti-
and thereby spatially resolve pH along the fiber. lized simultaneously to image and measure local chemical
Another method employed for distributed optical fiber concentrations with micrometer-scale resolution. In this
sensors is based on microbend sensing. In this approach, technique, an imaging fiber’s distal face is coated with
the sensing elements are made by covering small areas analyte-sensitive materials, which produces a microsen-
along the fiber with hydrogels. The hydrogel can swell or sor array capable of spatially resolving chemical concen-
shrink depending on the particular analytes present in the trations. The concept is shown in Fig. 22. For example,
surrounding environment. The immobilized hydrogels’ a pH-sensitive array is fabricated by coating the imag-
mechanical movement causes slight bending of the fiber. ing fibers with a pH-sensitive polymer layer containing
The bends in the fiber change the TIR conditions of light a fluorescent dye. In this way, an optical sensor array is
propagating through the fiber (see Fig. 3) resulting in a de- produced in which each pixel in the array imaging fiber
crease in light transmission. By using optical time-domain is coated by a pH-sensitive layer and acts as its own indi-
reflectometry, the exact location and intensity of the bend- vidual sensor. This pH-sensitive sensor array can be used
ing can be determined and related to the spatial distribu- for both visualizing remote localized corrosion at metal

FIGURE 22 Combined imaging and chemical sensing concept. The technique provides the ability to both view a
sample and measure surface chemical changes using a single optical imaging fiber.