Figure 1. The principle underlying the operation of fiber Bragg grating (FBG) strain gauges.

Virtually every type of public infrastructure, including bridges, pipelines, tunnels, foundations, roadways, dams, etc., is subject to factors that can degrade it or lead to malfunctions.  These structural problems can be the result of deterioration, improper construction methods, seismic activity, nearby construction work, etc. Although electrical strain gauges have long been used for monitoring structural changes, they sometimes lack the durability and integrity necessary to provide accurate, actionable information over extended periods.

Optical fiber strain gauges that are based on fiber Bragg gratings (FBGs) operate on very different principles than those that govern traditional electrical strain gauges. In simplified terms, a fiber Bragg grating is a microstructure (typically a few millimeters long) created by modifying a standard single-mode telecom fiber, germanium-doped, with a UV laser. This microstructure creates a periodic variation in the refractive index of that optical fiber. As light travels along the fiber, the Bragg grating reflects a very narrow range of wavelengths;
all of the other wavelengths are transmitted through the grating. The center of this band of reflected wavelengths is known as the Bragg wavelength (Figures 1 and 2). Under stress, the period of an FBG increases due to the physical stretching or compression of the optical fiber.  This change results in a shift in the Bragg wavelength, which is then detected and recorded by the interrogator (i.e., data acquisition system).

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