handbook of modern sensors

278 7 Position, Displacement, and Level
Fig. 7.28. Optical liquid-level detector
utilizing a change in the refractive index.
7.5.3 Fiber-Optic Sensors
Fiber-optic sensors can be used quite effectively as proximity and level detectors. One
example of the displacement sensor is shown in Fig. 4.18 of Chapter 4, where the
intensity of the reflected light is modulated by the distance d to the reflective surface.
Aliquid-level detector (see also Section 7.8.3) with two fibers and a prism is shown
in Fig. 7.28. It utilizes the difference between refractive indices of air (or gaseous phase
of a material) and the measured liquid. When the sensor is above the liquid level, a
transmitting fiber (on the left) sends most of its light to the receiving fiber (on the right)
due to a total internal reflection in the prism. However, some light rays approaching
the prism reflective surface at angles less than the angle of total internal reflection
are lost to the surroundings. When the prism reaches the liquid level, the angle of
total internal reflection changes because the refractive index of a liquid is higher than
that of air. This results in a much greater loss in the light intensity, which can be
detected at the other end of the receiving fiber. The light intensity is converted into
an electrical signal by any appropriate photodetector. Another version of the sensor
is shown in Fig. 7.29, which shows a sensor fabricated by Gems Sensors (Plainville,
CT). The fiber is U-shaped, and upon being immersed into liquid, it modulates the
intensity of passing light. The detector has two sensitive regions near the bends, where
the radius of curvature is the smallest. An entire assembly is packaged into a 5-mmdiameter
probe and has a repeatability error of about 0.5 mm. Note that the shape of
the sensing element draws liquid droplets away from the sensing regions when the
probe is elevated above the liquid level.
7.5.4 Fabry–Perot Sensors
For measuring small displacements with high precision in a harsh environment, the
so-called Fabry–Perot optical cavity can be employed.The cavity contains two semireflective
mirrors facing each other and separated by distance L (Fig. 7.30A). The cavity
is injected with light from a known source (a laser, e.g.) and the photons inside the
cavity bounce back and forth between the two mirrors, interfering with each other
in the process. In fact, the cavity is a storage tank for light. At some frequencies of
photons, light can pass out of the cavity. A Fabry–Perot interferometer is basically