A Fiber Bragg Grating is a periodic or aperiodic perturbation of the Effective Refractive Index in the Core of an optical fiber. Typically, the perturbation is approximately periodic over a certain length of e.g. a few millimeters or centimeters, and the period is of the order of hundreds of nanometers. This leads to the Reflection of Light (propagating along the fiber) in a narrow Range of wavelengths, for which a Bragg condition is satisfied. This basically means that the Wavenumber of the grating matches the difference of the wavenumbers of the incident and reflected waves. (In other words, the complex amplitudes corresponding to reflected field contributions from different parts of the grating are all in phase SO that can add up constructively; this is a kind of phase matching.) Other wavelengths are nearly not affected by the Bragg grating, except for some side lobes which frequently occur in the reflection Spectrum (but can be suppressed by apodization). Around the Bragg wavelength, even a weak index Modulation (with an amplitude of e.g. 10-4) is sufficient to achieve nearly total reflection, IF the grating is sufficiently long (e.g. a few millimeters).