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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 fiber's 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 input 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 essentially transparent.
Therefore, light propagates through the grating with negligible attenuation or signal variation. Only those wavelengths that satisfy the Bragg condition are affected and strongly back-reflected. The ability to accurately preset and maintain the grating wavelength is a fundamental feature and advantage of fiber Bragg gratings.
Fiber optical sensors offer several significant advantages over conventional electrical sensors. The most important advantages are:
They're rugged passive components resulting in a high life time (25 years).
They form an intrinsic part of the fiber optic cable that can transmit the measurement signal over several tens of kilometers.
They show no interference with electromagnetic radiation, so they can function in many hostile environments where conventional sensors would fail.
They don't make use of electrical signals what makes them explosion safe.
They have the ability to multiplex many sensors using only one optical fibre, driving down the cost of complex network configurations.