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In 1978, Hill et al. reported the first
formation of photoinduced gratings in germanosilicate optical fibers with an
argon-ion laser light propagating inside the fiber core [1-1]. However,
this discovery remained a lab curiosity since the inscription process only
permitted the fabrication of gratings at the writing laser wavelength, and then
these gratings fade away when used. A decade later, Meltz et al. introduced the
side-writing interferometric technique, where the Bragg wavelength is
independent from the writing laser wavelength [1-2]. This technique
allows permanent Bragg gratings to be directly written into the fiber core
using a holographic interferometer illuminated by a coherent ultraviolet light
source. The grating profile can be completely tailored varying the refractive
index modulation amplitude (apodization), the pitch period or the average
refractive index (chirp) and the tilt (blaze).
Fiber Bragg gratings (FBG) have become a
key component for optical fiber telecommunications as wavelength-division
multiplexing devices, fiber laser reflectors, gain flattening devices and
dispersion compensation element [1-3], and for sensing applications as
temperature, strain, pressure, ultrasound, acceleration, high magnetic field
and force, chemical elements [1-4, 1-5]. Temperature and strain
effects are not independent and only one parameter can be determined from a
single grating. In the general case, there are three strain components (one in
the axial direction and two in the transverse plane) and the temperature. In a
lot of situations the transverse strains are neglected and the temperature is
constant so that a single grating can monitor the axial strain average in the
grating region. A quasi-distributed mapping of strain (or temperature when the
strain is constant) is achieved by multiplexing several grating in the
wavelength, time or spatial domains or by a combination of these techniques [1-6].
The length of the gratings that can be produced ranges from 100 mm to several
meters. Long gratings open new perspectives for distributed sensing and
dispersion compensation, but in this case the local characterization of the
grating parameters is required.
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