The fundamental principle behind the operation of an FBG is Fresnel reflection, where light traveling between media of different refractive indices may both reflect and refract at the interface. The refractive index will typically alternate over a defined length. This is achieved by creating a periodic variation in the refractive index of the fiber core, which generates a. Understanding these gratings begins with a solid grasp of optical fiber properties and the functionality of the gratings themselves. This is because this type offiber permits the construction of guided wave interferometers directly from the fiber itself. Interferometers can be used to measure small phase changes in light. A optical fiber grating is a type of diffraction grating that mainly modulates the periodicity by increasing the probability of refraction inside its fiber optic core through certain methods to form a passive filtering component.