A Bragg Grating is a periodic modulation in the fiber’s index of refraction reflecting light with a wavelength of twice the grating period. The grating forms when the fiber is exposed to a periodic pattern of UV light, typically generated with a phase mask. Fiber lasers and DWDM add/drop filters utilize the ability of Bragg Gratings to reflect <99% of the light in a narrow wavelength band (~1 nm). Other uses of fiber gratings include laser diode frequency stabilization and fiber sensors that detect chemicals, mechanical strain, vibration, acceleration or temperature. Gratings may also be chirped to provide dispersion compensation or gain flattening.