A fiber Bragg grating (FBG) is a type of distributed Bragg reflector constructed in a short length of optical fiber that reflects particular wavelengths of light and transmits all others. This is achieved by adding a periodic disturbance to the refractive index of the fiber core, which alters the phase of light propagating through it. When the phase shift imparted by the grating is equal to the round-trip phase shift of the light propagating through the grating length, constructive interference occurs, causing light at the specific wavelength to be reflected.