In order to meet application requirements, dependence of the Brillouin frequency shift to strain should be enhanced. We evaluated the influence of several optical fiber dopants on Brillouin scattering strain sensitivity. Based on a FEM-2D modeling, we developed a model for Brillouin gain spectrum and its strain sensitivity. Geometrical profile, doping composition and drawing conditions are taken into account. We show alumina is a very interesting dopant to enhance strain sensitivity in optical fibers. This result is experimentally validated: up to 0.0639MHz/μƐ is obtained with a 5.2mol% Al2O3 doped fiber. We expect to reach 0.1MHz/μƐ with 25mol% doping level.
Usually, strain and temperature coefficients of Brillouin frequency shift (BFS) of optical fibers result from experimental
measurements. For the first time, theoretical strain dependence of BFS is analyzed depending on fiber properties. Based
on a FEM-2D modeling, the strain coefficient Cε is determined considering geometrical profile, doping composition and
drawing conditions. Theoretical results showed great accordance with measurements for different types of single-mode
fibers. The Cε coefficient is evaluated with a relative uncertainty better than 8%. Significant Cε magnitude variation has
been observed depending on doping profile. This paves the way towards major improvement of strain optical fiber
sensors.
A new approach is proposed to monitor in situ the influence of gamma radiations on Brillouin properties of optical fiber
extensometers. Experimental results are illustrated with the characterization of two fibers samples up to total dose of
about 600Gy. The Brillouin frequency shift remains unaffected at such radiations level, as well as the spectral Brillouin
signature or its dependence with strain. Meanwhile, propagation losses increase under radiations with an amplitude
related to fiber dopants. The target application is nuclear wastes repository monitoring where higher doses are expected.
UV radiation preliminary tests show that compaction phenomenon may occur at such high doses, inducing Brillouin
frequency shift up to 20 MHz.
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