Fiber Bragg grating

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A fiber Bragg grating is a type of distributed Bragg reflector constructed in a short segment of optical fiber that reflects particular wavelengths of light and transmits all others. This is achieved by adding a periodic variation to the refractive index of the fiber core. A fiber Bragg grating can therefore be used as an inline optical filter to block certain wavelengths, or as a wavelength-specific reflector.

Fiber Bragg gratings are created by "burning" or "writing" the periodic variation of refractive index into the core of an optical fiber using an intense ultraviolet(UV) source such as a UV laser, and a photomask having the intended grating features. It is also possible to expose the fiber to an interference pattern to create the grating. The fiber is normally made more receptive to the UV burning by manufacturing it with a few percent of germanium or by later diffusion of hydrogen.

The grating will typically have a sinusoidal refractive index variation over a defined length. The reflected wavelength ( $λ$ ) is defined by the relationship $λ = 2 l n$ where $n$ is the average refractive index and $l$ is the period of the variation. The reflectivity of the grating at the reflected wavelength is dependent on its length and the magnitude of the refractive index variation.

The pattern may be modified to add other features such as apodization of the reflected light spectral peak or a linear variation in the grating period (called a chirp) to broaden the reflected spectrum. A grating possessing a chirp has the property of adding dispersion - namely, different wavelengths reflected from the grating will be subject to different delays. This property has been used in the development of phased-array antenna systems.

These gratings are also finding uses in instrumentation applications such as seismology and as downhole sensors in oil and gas wells for measurement of the effects of external pressure, temperature, seismic vibrations and inline flow measurement. As such they offer a significant advantage over traditional electronic gauges used for these applications in that they are less sensitive to vibration or heat and consequently are far more reliable.