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Fiber Bragg gratings are components for a wide range of applications in the areas of telecommunications, lasers and sensors [1]. Fiber Bragg grating technology has its origins in the discovery of photosensitivity of Germanium doped Silica by Hill et al. in 1978. Later it was found, that a permanent periodic modulation of the refractive index in the core of a photosensitive fiber could be produced by transverse illumination with an interference pattern created by a pair of strong UV-laser beams (left) [2].

Fiber Bragg gratings are formed by a series of lines in the core of a singlemode fiber. These lines forming the Bragg grating are characterized by an index of refraction different from the one of the regular fiber core. Light propagating in the core will be reflected by the interfaces between regions having different refraction indices. But the reflected light is generally out of phase and extincts. However, for a certain wavelength, the Bragg wavelength l_Bragg, the light reflected by the periodically varying index of refraction will be in equal phase and added constructively.

This results in a characteristic pit in transmission spectrum as well as in a peak in the reflection spectrum.

Literature:
[1] Kashyap, R.: Photosensitive Optical Fibers: Devices and Applications. Optical Fiber Technology 1, pp.17-34 (1994)
[2] G. Meltz, W. W. Morey, W. H. Glenn: Formation of Bragg gratings in optical Fibers by a transverse holographic method, Optics Letters, Vol. 14, No. 15, pp. 823-825, 1989