FBG technology is one of the most popular choices for optical fiber sensors for strain or temperature measurements due to their simple manufacture and the relatively strong reflected signal. They are formed by periodic modulations of the refractive index (RI) of the fiber core along the longitudinal direction and can be produced by various techniques.
The term "fiber Bragg grating" was derived from Bragg's law and applied to the periodical structures inscribed inside the core of conventional telecom fiber. Sir William Lawrence Bragg, born in 1890, was a British physicist and x-ray crystallographer who discovered the eponymous law of x-ray diffraction in 1912. This principle is used to study and determine crystalline structures, particularly in thin-film research. Sir Bragg, with his father, won the Nobel Prize for Physics in 1915 for an important step in the development of x-ray crystallography.
Bragg diffraction occurs for electromagnetic radiation with a wavelength on the same order of magnitude as the atomic spacing, when incident upon a crystalline material. In this case, the radiation is scattered in a specular fashion by the atoms of the material and experiences constructive interference in accordance to Bragg's law. For a crystalline solid with lattice planes separated by a distance d, the waves are scattered and interfere constructively if the pathlength of each wave equals an integer multiple of the wavelength (see Fig. 3.1). Bragg’s law describes the condition for constructive interference from several crystallographic planes of the crystalline lattice separated by a distance d.