Fiber Bragg Gratings are made by laterally exposing the core of a single-mode fiber to a periodic pattern of intense ultraviolet light. The exposure produces a permanent increase in the refractive index of the fibers core, creating a fixed index modulation according to the exposure pattern. This fixed index modulation is called a grating.
At each periodic refraction change a small amount of light is reflected. All the reflected light signals combine coherently to one large reflection at a particular wavelength when the grating period is approximately half the light's wavelength. This is referred to as the Bragg condition, and the wavelength at which this reflection occurs is called the Bragg wavelength.
Light signals at wavelengths other than the Bragg wavelength, which are not phase matched, are transparent. Only those wavelengths that satisfy the Bragg condition are affected and strongly back reflected.
Fiber Bragg Gratings have been commercially available since early 1995, and in that short time four grating types have evolved. The four different types of Fiber Bragg Grating's are: Simple Reflective Gratings, Broadband Gratings, Mode Coupling Gratings, and Band-Rejection Gratings.
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