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handbook of modern sensors

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280 7 Position, Displacement, and Level<br />

(A)<br />

(B)<br />

Fig. 7.31. Construction <strong>of</strong> a Fabry–Perot pressure sensor (A) and view <strong>of</strong> FISO FOP-M pressure<br />

sensor (B).<br />

with the wavelength <strong>of</strong> light. Whatever may cause changes in the cavity dimensions<br />

(mirror movement) may be the subject <strong>of</strong> measurements. These include strain, force,<br />

pressure, and temperature.<br />

Fabry–Perot cavity-based <strong>sensors</strong> have been widely used for their versatility; for<br />

example, they have been used to sense both pressure and temperature [7–10]. This<br />

kind <strong>of</strong> sensor detects changes in optical path length induced by either a change in<br />

the refractive index or a change in physical length <strong>of</strong> the cavity. Micromachining<br />

techniques make Fabry–Perot <strong>sensors</strong> more attractive by reducing the size and the<br />

cost <strong>of</strong> the sensing element. Another advantage <strong>of</strong> the miniature Fabry–Perot sensor is<br />

that low-coherence light sources, such as light-emitting diodes (LEDs) or even light<br />

bulbs, can be used to generate the interferometric signal.<br />

A pressure sensor with a Fabry–Perot cavity is shown in Fig. 7.31A. Pressure is<br />

applied to the upper membrane. Under pressure, the diaphragm deflects inwardly,<br />

thus reducing the cavity dimension L. The cavity is monolithically built by micromachined<br />

technology and the mirrors can be either the dielectric layers or metal layers<br />

deposited or evaporated during the manufacturing process. The thickness <strong>of</strong> each<br />

layer must be tightly controlled to achieve the target performance <strong>of</strong> a sensor. An<br />

ultraminiature pressure sensor produced by FISO Technologies (www.fiso.com) is<br />

shown in Fig. 7.31B. The sensor has a very small temperature coefficient <strong>of</strong> sensitivity<br />

(< 0.03%) and has an outside diameter <strong>of</strong> 0.55 mm, which makes it ideal for such<br />

critical applications as in implanted medical devices and other invasive instruments.<br />

A measuring system for the Fabry–Perot sensor is shown in Fig. 7.32. Light from<br />

a white-light source is coupled through a 2 × 2 splitter to the optical fiber that, in turn,<br />

is connected to a sensor. The sensor contains a Fabry–Perot interferometer cavity<br />

(FPI) and it reflects back light at a wavelength related to the cavity size. Now, the<br />

task is to measure the shift in a wavelength. This is accomplished by a white-light<br />

cross-correlator that contains a Fabry–Perot wedge. The wedge, in effect, is a cavity

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