A fiber Bragg grating is made by exposing the single-mode fiber core horizontally to a strong ultraviolet light with a periodic pattern. The strong ultraviolet light exposure permanently increases the refractive index of the fiber core, producing a fixed refractive index modulation based on the exposure pattern. This fixed refractive index modulation is called a grating.

At each point where the refractive index of the space periodically changes, a small amount of light will be reflected. When the grating period is approximately half the wavelength of the incident light, all the reflected light will coherently combine into a large reflected beam with a specific wavelength. This is known as the Bragg condition. The wavelength at which the incident light reflects is called the Bragg wavelength. Other wavelengths of light signals are barely affected by the Bragg grating and will pass through the fiber grating and continue to transmit. The principle is illustrated in Figure 1.

Therefore, when light passes through a grating, there is very little signal attenuation or signal change. Only wavelengths that satisfy the Bragg condition will be affected and produce strong reflection. The ability to precisely set and maintain the wavelength of the grating is a fundamental feature and advantage of fiber Bragg grating (FBG).

The center wavelength of the reflected light satisfies the following Bragg's equation: λBragg = 2nΛ, where n is the refractive index and Λ is the grating period. Since the parameters n and Λ are affected by temperature and strain, the center wavelength of the Bragg reflected light will also change with temperature or strain, or both, as shown in Figure 2. This phenomenon is well known, and therefore, the change in the center wavelength of the reflected light can be used to determine the corresponding change in the measured physical quantity.