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Fiber optic techniques for chemical analysis have several distinct advantages. Analysis can often be done in situ in real time. The sensing techniques generally do not disturb the process. The sample size can be extremely small, and the sensing locations can be in remote areas that are normally difficult to access. Potential disadvantages include sensitivity to ambient light, relatively slow response time due to the required reaction with various reagents, and shortened lifetime if high incident radiation is used to enhance sensitivity. Also, interaction with a chemical agent is generally not reversible; therefore, the sensor will require replacement if triggered. Several approaches can be used for qualitative and quantitative chemical analysis. These techniques are fluorescence, scattering, absorption, color change, evanescent wave interaction, and refractive-index change. In general, chemical analysis techniques employ either a transmissive or a reflective fiber optic configuration. Figure 15.1 illustrates the two concepts. The reflective concept uses a bifurcated probe, as shown in Fig. 15.1(a). Light travels down the transmitting leg, reflects off the target material, and is accepted in the receiving leg, which is attached to a photodetector. The amount of light transmitted or reflected is a function of the nature and amount of chemical species present. In the transmissive system shown in Fig. 15.1(b), light travels down a transmitting fiber optic (a single fiber or a bundle of many fibers), passes through a gap that contains the material to be analyzed, and is captured in a receiving fiber optic, which, in turn, transmits the light to a photodetector. Distributive chemical sensors can function by detection of a transmitted or reflective signal.
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