03.01.2015 Views

handbook of modern sensors

handbook of modern sensors

handbook of modern sensors

SHOW MORE
SHOW LESS

Create successful ePaper yourself

Turn your PDF publications into a flip-book with our unique Google optimized e-Paper software.

4.7 Fiber Optics and Waveguides 143<br />

Fig. 4.18. Fiber-optic displacement sensor utilizes<br />

the modulation <strong>of</strong> reflected light intensity.<br />

(A)<br />

(B)<br />

Fig. 4.19. Fiber-optic microbend strain gauge (A) and a waveguide for the far-infrared radiation<br />

(B).<br />

employed where excitation (illumination) function and collection function are carried<br />

out by separate fibers (Fig. 4.17B).<br />

The most commonly used type <strong>of</strong> fiber-optic sensor is an intensity sensor, where<br />

light intensity is modulated by an external stimulus [4]. Figure 4.18 shows a displacement<br />

sensor where a single-fiber waveguide emits light toward the reflective surface.<br />

Light travels along the fiber and exits in a conical pr<strong>of</strong>ile toward the reflector. If the<br />

reflector is close to the fiber end (distance d), most <strong>of</strong> the light is reflected into the fiber<br />

and propagates back to the light detector at the other end <strong>of</strong> the fiber. If the reflector<br />

moves away, some <strong>of</strong> the rays are reflected outside <strong>of</strong> the fiber end, and fewer and<br />

fewer photons are returned back. Due to the conical pr<strong>of</strong>ile <strong>of</strong> the emitted light, a<br />

quasilinear relationship between the distance d and the intensity <strong>of</strong> the returned light<br />

can be achieved over a limited range.<br />

The so-called microbend strain gauge can be designed with an optical fiber which<br />

is squeezed between two deformers, as shown in Fig. 4.19A. The external force<br />

applied to the upper deformer bends the fiber affecting a position <strong>of</strong> an internal<br />

reflective surface. Thus, a light beam which normally would be reflected in direction

Hooray! Your file is uploaded and ready to be published.

Saved successfully!

Ooh no, something went wrong!