Typically the grating period is the same size as the Bragg wavelength, as shown above. Longer periods can be used to achieve much broader responses than are possible with a. A fiber Bragg grating is a periodic or aperiodic perturbation of the effective refractive index in the core of an optical fiber (see Figure 1). a few millimeters or centimeters, and the period is of the order of. When a Bragg grating exists in an optical fiber, it will reflect a specific wavelength dependent on the period of the Bragg grating and the index of refraction of the optical fiber. The underlying. on the core material. The fibre Bragg grating can perform many primary functions, such as reflection and filtering for example, in a highly effi ient, low loss manner. This versatility has stimulated a number of signifi rent back reflection).