Fiber Bragg Gratings are a type of reflective structure constructed in a part of an optical fiber with a periodic variation in the refractive index of the fiber core along the longitudinal direction of the fiber. When light passes through the FBG, only the light of a particular wavelength is reflected. Spectrum center wavelength of reflected light is called Bragg’s wavelength.
When a broadband light beam is sent to an FBG, induced reflections of certain wavelengths of the coupled light occur at the points with the manipulated refractive index.
When the FBG is subject to external mechanical stress, the spectrum of the reflected wavelength range shifts, meaning that the mechanical stress on the fiber can be measured and interpreted.