Fiber Bragg Gratings are created by laterally exposing the core of a single-mode fiber to an intermittent pattern of powerful UV light. The exposure creates a lasting increase in the refractive index of the fiber's core, producing a fixed index modulation based on the exposure pattern. This fixed index modulation is referred to as a grating.
At each periodic refraction alteration a small amount of light is reflected. All the reflected light signals integrate coherently to one large reflection at a specific wavelength when the grating period is approximately half the input light's wavelength. This is known as the Bragg condition, and the wavelength at which this reflection happens is referred to as the Bragg wavelength. Light signals at wavelengths other than the Bragg wavelength, which are not phase matched, are fundamentally transparent. This principle is illustrated in Figure 1.
Figure 1. Working principle of FBG.
Thus, light propagates through the grating with signal variation or negligible attenuation. Only those wavelengths that match the Bragg condition are affected and strongly back-reflected. The ability to accurately preset and maintain the grating wavelength is an important feature and benefit of fiber Bragg gratings.
The central wavelength of the reflected component fulfills the Bragg relation: λrefl=2n ∧, with n the index of refraction and ∧ the period of the index of refraction variation of the FBG. Because of the temperature and strain dependence of the parameters n and ∧, the wavelength of the reflected component will also change as function of temperature and/or strain, as shown in Figure 2. This dependency is recognized and allows establishing the strain or temperature from the reflected FBG wavelength.
Figure 2. FBG response as function of strain.
This information has been sourced, reviewed and adapted from materials provided by FBGS Technologies GmbH.
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