I am working on a project on detecting damage in structures using fiber optic sensors. I read the paper Structural Health Monitoring in Composite Structures by Fiber-Optic Sensors and found out that fiber optic sensors can not only detect strains but also locate them; how is this possible? Do they measure the intensity of the reflected light or something?
Searching for "fibre-optic strain measurement" yields plenty of results. One from HBM states:
To create the actual strain sensor, the optical fiber is inscribed during production with a so-called Fiber Bragg Grating (FBG). This is basically a pattern of material interferences, which reflects the light differently from the rest of the fiber. For better understanding, you can visualize the fiber as a cylindrical length of transparent material, with a number of thin slices in it. When the light from the laser hits this pattern, certain wavelengths are reflected, while others pass through.
The material interferences—the “slices” —are placed at certain intervals. When the fiber is stretched or compressed—and is therefore subjected to positive or negative strain—these intervals change. When the fiber is stretched, it lengthens and the spaces get bigger and vice versa.
Not only does the reflected light take a little longer or shorter to travel back when the FBG is under strain, but the wavelength that is reflected also changes. In scientific terms, the FBG has a certain refractive index. The refractive index of a material describes how much light is bent or refracted when passing through the material. When the grating changes shape due to strain, its refractive index changes as well.
The article goes on to explain the topic further including problems with temperature, etc.
Strain is defined as a relative displacement. Measuring strain is measuring movement of a material, basically a tape measure on a very small scale. So a laser can ostensibly be used to detect how far something moves.
Googling "measuring strain with lasers" seems to give a representative set of info.