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Several techniques have been developed for measuring strain using fiber optic sensors. The three most widely used approaches include Bragg gratings, interferometric configurations, and Rayleigh scattering.
Fiber Bragg grating optical strain sensors provide many advantages over conventional electrical systems, as has been discussed previously. They are well-suited for applications requiring long-range capabilities in terms of distance, as well as applications requiring long spans of time without degradation. Furthermore, these sensors are suitable for harsh environmental conditions due their immunity to lightning and resistance to corrosion.
Fiber Bragg grating (FBG) sensors can be used as point sensors or distributed sensing systems. For applications that cover large areas, multiple FBG sensors can be integrated into a single optical fiber. The system configurations can be simplified using only a single cable and single interrogator to monitor a large number of sensing points. This results in a reduction in cabling maintenance and installation processes, and provides cost-effective monitoring solutions. Due to the fact that FBG optical sensing uses wavelength rather than amplitude modulation, these sensing systems can reach very long distances (more than 10 km) without the need for signal conditioning.
Fiber optic Bragg gratings are well suited for strain measurement. They are compatible with a wide array of materials including composites, enabling them to play a key role in smart structures. Depending on the design, Bragg grating interrogators can monitor 20 sensing points and potentially up to 100 sensing points on a single fiber.
Fiber optic Bragg gratings were discussed in detail in Chapter 5. But as a quick review, the FBG is a series of localized changes in the refractive index of the glass fiber that creates the grating effect. The optical fiber is connected to a light source, which typically transmits light in a wavelength band of about 40 nm (a broader spectrum of light is sometimes used). When the light encounters the FBG strain sensor, a specific wavelength is reflected based on the properties of the gratings, while the rest of the light in the transmission band passes through the grating. As the FBG expands or contracts due to strain, so does the gap between these gratings, as depicted in Fig. 10.1 and Fig 10.2, thereby changing the reflected wavelength of light. The reflected light is then measured, and the change in wavelength can be converted to a strain value.
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