The strain sensitivity of FBG-based strain sensors that had been inscribed into three different types of optical fibers were
determined for both conditions, for fibers that were glued on a specimen and for free, bare fibers, i. e. fibers that were not
bonded to a structure at the location of the FBG. The influence of the gluing technique on the strain sensitivities of the
FBG sensor elements was investigated by comparing the strain sensitivities of a multitude of attached and free sensor
elements on a statistical basis. A good agreement between the strain sensitivities of free and attached fibers was
achieved, which shows the high quality of the gluing method employed.
It can be expected that the range of applications for FBG-based strain and temperature sensors would expand if the
accuracy of this sensor technique was improved. In this study, polarization effects of FBG sensors, which contribute
significantly to the measurement uncertainty of this technique, were investigated. Therefore, FBGs were inscribed into
highly birefringent optical fibers. These sensor elements were attached to a specimen with defined orientations of the
fiber's slow and fast axes with regard the specimen's surface. We observed a change of the fiber's birefringence in the
order of 5 10-5 as a consequence of the gluing process, that was employed to attach the fiber onto the specimen. The
strain sensitivities were determined for each polarization mode and for different fiber orientations using a highly accurate
strain calibration facility. It was found that in all experiments the strain sensitivity for the slow axis was significantly
higher (about 0.8%) than for the fast axis. The strain sensitivity also depends on the orientation of the fiber's birefringent
axes with regard to the surface of the specimen. Although the investigations were performed with FBGs inscribed into
birefringent fibers, the findings are still of importance for understanding the polarization-dependant accuracy limits of
FBGs in standard single-mode fibers.
Fibre Bragg gratings are used as temperature or longitudinal strain sensors in a number of applications. Only a few
studies are concerned with transversal stress applied to these sensors. We recently derived an analytical solution for the
reflection spectra of shear strain loaded fibre Bragg gratings. The experimental verification of this theory is presented in
this work. A fibre-coupled spectra analyzer based on a Fabry-Perot tuneable filter was set up in order to measure the
predicted effect. The setup is able to split and measure simultaneously the spectra of the two polarisation main axes of
birefringent fibres. The experimental observations are in agreement with predictions derived by mode coupling theory.
A facility for strain sensitivity calibration of optical FBG-based strain sensors according to the German VDI/VDE 2660
guideline was established and characterized. Statistical analysis of several calibration measurement series performed
with one single type of FBG strain sensor and application technique showed a reproducibility of 0.15%. Strain
sensitivities for FBGs inscribed in two different types of optical fibres (GF1B and PR2008) showed significantly
different strain sensitivities of k = 0.7885±0.0026 and k = 0.7758±0.0024, respectively.
The response of optical fibre Bragg gratings (FBG) to transverse load was found to be significantly different
for FBGs inscribed in two different types of single mode optical fibres. The transverse load sensitivity, defined
as the relative wavelength separation of the birefringence-induced FBG double-peaks per transverse line-force,
was found to be Cq = (45.1 ± 1.5)10-9 1/N/m
for a moderately GeO2 doped (GF1B) and Cq = (48.5 ± 1.0)10-9 1/N/m
for a highly GeO2 doped (PR2008) optical fibre. These data are important for a complete characterization of
the opto-mechanical behavior of FBG sensor elements inscribed in different types of single mode optical fibres.
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