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Basics of Fiber Bragg Gratings



Principle Fiber Bragg gratings are components that are widely used in the areas of telecommunications, lasers, and optical sensing [1]. Fiber Bragg grating technology is based on the photosensitivity of Ge+ doped fused silica, which is the common material that optical fibers are made of. The effect was first described by Hill et al. in 1978. Later it was found, that a permanent and periodic index modulation can be created in the fiber core by a transverse illumination with a UV interference pattern that is formed by a pair of strong UV-laser beams (left) [2].


FBGConsequently, Fiber Bragg gratings are formed by a series of lines in the core of a single-mode fiber. The grating lines are made of an periodically alternating refraction index along the fiber core. The propagating light is partially reflected at the transition points of areas having different refraction indices. For the majority of wavelengths, the light parts are generally out of phase and extinguish without any measurable effect. However, for one certain wavelength - named Bragg wavelength lBragg - the light portions reflected by the consecutive index changes are in an equal phase and hence constructively added up.


Concerning the optical power spectrum, this results in a characteristic notch in the transmitted spectrum and in a peak in the reflected spectrum that travels back to the light source. Applications that want to access the reflected signal must couple the signal out of the fiber by an appropriate means, e.g. a splitter or a circulator.


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


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