A Fiber Bragg grating is a type of diffraction grating segment of optical fiber that filters out particular wavelengths of light. This is achieved by altering parts of the fiber core so that their index of refraction is slightly higher than normal. As a result, the structure will transmit most wavelengths of light, but will reflect certain specific wavelengths -- thereby effectively filtering them out. Commercially, fiber Bragg gratings are used to stabilize the output of lasers, and various applications in the sensor and telecommunications industry.

Fiber Bragg Gratings are created by burning or "writing" a periodic variation in the index of refraction into the core of an optic fiber using an intense UV source such as a UV laser and a photomask having the intended grating features. The fiber is normally made receptive to the UV burning by manufacturing it with a few percent of germanium or by later diffusion of hydrogen .

The grating will typically have a sinusoidal index variation over a defined length. The pattern may be modified to add other features such as apodization of the reflected light spectral peak or a linear variation in the grating period called a chirp to broaden the reflected spectrum. The reflected wavelength = 2 x grating pitch x refractive index.

These gratings are finding a use in instrumentation applications such as seismology for measurement of the effects of external pressure and temperature on the fibre optic cable.