Principe of measurement

A specific property of an FBG is that it reflects a single wavelength called the Bragg wavelength λ_B. The physics behind this is interference between the light rays that are reflected at each ridge of the grating. Constructive interference happens only on the condition that the wavelength  λ_B exactly matches with the difference in optical path length between two successive rays (2nΛ), yielding the so-called Bragg condition:  λ_B = 2nΛ. When broadband light is guided in the fiber and incidents on the FBG, a single reflection peak can be observed in the reflection spectrum and correspondingly a dip at the same wavelength can be seen in the transmission spectrum. The principle of strain detection with an FBG is illustrated in Figure below. By straining the fiber, the grating period is increased and this shifts the Bragg peak to higher wavelengths. By analyzing the FBG-wavelength, the strain in the fiber can thus be traced back.