Instrumentation & Measurement Magazine 24-9 - 34

In the scope of performing
strain measurements
on materials using Bragg
gratings, the latter need to
be bonded on the surface of
the studied material. Then
a tensile or compressive
load can be applied to the
studied material to measure
the strain. Fig. 1 shows
the reflection spectra of
an FBG, under a compressive
or tensile strain in the
absence of a temperature
gradient. The Bragg wavelength
is shifting which
allows calculating the applied
strain.
Fig. 1. Reflection spectra of an FBG presenting the effect of tensile and compressive strains on the Bragg wavelength.
If we consider performing strain measurements using an
FBG with a negligible temperature gradient, (2) can be simplified.
For a germanosilicate fiber core, the photo elastic
coefficient is 0.22 [4]. Thus, we obtain a linear relation between
the Bragg wavelength and the strain for a germanosilicate fiber
core according to:
 
 B
0.78B
(3)
The temperature dependence
of FBG is
compensated thanks to a
statistical study realized at
low temperatures on different
kinds of materials and
presented in [5], where the
apparent strains of optical
sensors based on FBG were measured.
Grating's Technology and its Acquisition System
The FBGs were manufactured in polyimide-coated fiber inscribed
through the coating of the fiber using femtosecond
laser pulses. The FBGs length and reflectivity are 5 ±1 mm and
20 ±3%, respectively. The optical fibers are interrogated using
the FS22DI Industrial BraggMETER from HBK®, which
is an optical device designed to interrogate FBG sensors using
Fig. 2. (a) Picture of the measurement points on the magnet coil with both resistive and optical strain gauges in azimuthal and longitudinal directions. (b) Picture
presenting the external compensator used for temperature and magnetic field compensation for RSG measurements.
34
IEEE Instrumentation & Measurement Magazine
December 2021

Instrumentation & Measurement Magazine 24-9

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