Fibre Bragg Gratings are created by the nucleus of a single fibre to be a recurring pattern of intense ultraviolet light laterally to expose. The exposure produces a permanent increase in the refractive index of the core of the fibre, which according to the pattern of a fixed index modulation allows the exposure. This fixed index modulation is called a Grating. On each periodic change is a small amount of light reflected. All reflected light signals in a coherent way forms one large reflection on a specific wavelength when the Grating period approximately at half the input of the light wavelength is. This is referred to as the Bragg condition, and the wavelength at which this reflection occurs is called the Bragg wavelength.. Light signals at wavelengths other than the Bragg wavelength, which not matching on the stage, are essentially transparent.
Therefore diffuses the light through the grating with negligible attenuation or signal variation. Only these wavelengths, which satisfy the Bragg condition are affected and reflected back strongly. The ability to accurately define the setting in advance and to maintain the grating wavelengths, is a fundamental function and because of this, a major advantage of fibre Bragg Gratings. The Central wavelength of the reflected component complies with the Bragg relationship:
λrefl = 2nΛ, with n as refractive index and Λ as a period of the refractive index variation of the FBG. As a result of the dependence on temperature and voltage/rack of the parameters n and Λ the wavelength of the reflected component will also be changed as a function of temperature and/or rack. This dependence is known, which admits the temperature or the stretch of the reflected wavelength FBG determines.