Optical fiber Bragg grating (FBG) was invented in the late 70's, when refractive index of the fiber core was found changed by light propagating in the fiber. FBG is one-dimensional grating. It has a periodical pattern of index change. The period of the grating determines the center wavelength of the reflected spectrum, which is called Bragg wavelength. The light at Bragg wavelength (and within a certain bandwidth) is reflected by the FBG. The..