Gautier Moreau

Truly Distributed Optical Fiber Sensors for Structural Health Monitoring: From the Telecommunication Optical Fiber Drawling Tower to Water Leakage Detection in Dikes and Concrete Structure Strain Monitoring

Although optical fiber sensors have been developed for 30 years, there is a gap between lab experiments and field applications. This article focuses on specific methods developed to evaluate the whole sensing chain, with an emphasis on (i) commercially-available optoelectronic instruments and (ii) sensing cable. A number of additional considerations for a successful pairing of these two must be taken into account for successful field applications. These considerations are further developed within this article and illustrated with practical applications of water leakage detection in dikes and concrete structures monitoring, making use of distributed temperature and strain sensing based on Rayleigh, Raman, and Brillouin scattering in optical fibers. They include an adequate choice of working wavelengths, dedicated localization processes, choices of connector type, and further include a useful selection of traditional reference sensors to be installed nearby the optical fiber sensors, as well as temperature compensation in case of strain sensing.

Self-referenced and single-ended method to measure Brillouin gain in monomode optical fibers

We present a new self-referenced and single-ended method to measure the Brillouin-gain coefficient in monomode optical fibers accurately with high reliability. Our comparative measurements on several different fibers show that a fiber with a smaller optical effective mode area can nevertheless have a higher Brillouin threshold, thus confirming the significance of acousto-optic effective mode area.

Importance of residual stresses in the Brillouin gain spectrum of single mode optical fibers

Residual stresses inside optical fibers can impact significantly on Brillouin gain spectrum. Based on a 2D-FEM modeling, theoretical results are compared with Brillouin gain measurements for fibers from a same preform with different draw tensions.

Optical fiber properties influence on strain coefficient C ε of Brillouin frequency shift

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.

Al-doped optical fiber to enhance strain sensitivity of Brillouin based optical fiber sensors

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.