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Intensity-modulated sensors were defined in Chapter 2 as sensors that detect the variation of the intensity of light associated with the perturbing environment. The general concepts associated with intensity modulation include transmission, reflection, and microbending. However, several other mechanisms that can be used independently (intrinsically) or in conjunction with the three primary concepts include absorption, scattering, fluorescence, polarization, and optical gratings.While intensity-modulated sensors are analog in nature, they have significant usage in digital (on/off) applications for switches and counters. The transmissive sensor concept is normally associated with the interruption of a light beam in a switch configuration. However, this approach can provide a good analog sensor. Figure 3.1(a) shows the probe configuration for measurement of axial displacement. Figure 3.l(c) gives a curve of output versus distance between the probes. The curve follows a l/r2 law, where r is distance. A more sensitive transmissive approach employs radial displacement as shown in Fig. 3.l(b). The sensor shows no transmission if the probes are displaced a distance equal to one probe diameter. Approximately the first 20% of the displacement gives a linear output. The curve in Fig. 3.1(c), showing the effects of radial displacement, is for probes with a single fiber, 400-mm diameter. A modification of the transmissive concept is referred to as frustrated total internal reflection. The two opposing probes have the fibers polished at an angle to the fiber axis, which produces total internal reflection for all propagating modes, as shown in Fig. 3.2. As the fiber ends come close in proximity to one another, energy is coupled. The intensity of light coupled into the receiving fiber is shown in Fig. 3.3. This approach provides the highest sensitivity for a transmissive sensor.
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