In recent years fiber Bragg grating based have become popular due their extremely stable characteristics.
The Fiber Bragg Grating (FBG) is a periodic structure fabricated inside the core of the optical fiber. The periodicity could be mechanical like variation of the core diameter or it could be electrical like variation of the refractive index of the core.
In the periodic structure like the FBG the coupling of energy between different co-propagating and counter-propagating modes of the fiber takes place. The mode coupling phenomenon is a strong function of wavelength.
In FBG two identical counter propagating modes get coupled and the energy is transferred from the forward traveling to the backward traveling mode. Consequently we get reflection of the modal energy which is wavelength dependent. The FBG therefore reflects certain wavelengths keeping propagation of other wavelengths practically un-affected.
