Resumen de Contributions to the development of Brillouin distributed sensors for structural health monitoring
Jon Mariñelarena Ollacarizqueta
Distributed optic fiber sensors (DOFS) are a very interesting technology for industrial monitoring applications due to their capability of performing measurements that provide a multitude of measurement positions along a sensing fiber. Among the DOFS, those based on stimulated Brillouin scattering (SBS) are especially interesting due to the possibility of measuring temperature and strain simultaneously. In particular, Brillouin optical time-domain analysis (BOTDA) sensor is one of the most developed and studied Brillouin-based DOFS due to their capability of performing long-range measurements of temperature and strain with high spatial resolution. Moreover, they can also provide very fast measurements in short length of fibers. During the last years there have been many important advancements in improving the BOTDA performance by proposing techniques to enhance their accuracy, measurement range, spatial resolution, and acquisition time. However, there are constraints that still limit the performance of these sensors. This thesis has been focused on overcoming several of these constraints.
Firstly, a technique has been developed to mitigate the non-local effects related to the limited extinction ratio of pump pulses, which induce important detrimental effects and errors in the measurement results. This method is based on the dithering of the light source used to generate the pump and the probe waves in BOTDA sensors. Another contribution that improves the BOTDA sensors performance has been presented to mitigate an unsolved issue when they deploy pump pulse coding techniques. The problem lies on the nonlinear amplifcation experienced by the probe wave when long sequences of pulses are used. This solution is based on a simple post-processing calculation performed on the detected signal.
Similarly, this thesis has contributed also to the enhancement of dynamic BOTDA sensors, especially, the self-heterodyne detection BOTDA sensors. This particular technique, which is based on a phase-modulated probe wave and coherent detection, has been studied in detailed. A theoretical and experimental study is presented in this thesis where the dependence of the detected radiofrequency phase-shift spectrum on large Brillouin gain variations is analyzed. Then, the limitation of the dynamic range of these sensors is investigated. In order to overcome this constraint, two different methods are investigated and presented. One of them is based on the pulse-shortening method and the second one relies on the generation of multiple frequency components of the pump pulses.
Another important type of DOFS based on SBS are the Brillouin optical correlation domain analysis sensors, whose particular advantage lies on the high spatial resolution they provide and the possibility of the random access to the measuring position along the sensing fiber. As in BOTDA sensors, multiple limitations and constraints have been overcome during the last years, although, there are others that still limit their performance. In this thesis, research has been carried out to solve the limitation of the maximum amplitude of the frequency modulation (FM) that can be deployed to modulate the light source in linearly-configured BOCDA (LC-BOCDA) sensors. This problem has been addressed and overcome deploying an alternative setup based on a dual-probe wave and a control of the correlation order. The proposed method has demonstrated the possibility of extending the FM amplitude, and thus, the number of resolving points and spatial resolution of the LC-BOCDA.
Finally, this thesis has contributed to close the gap between research and practical applications of Brillouin distributed sensors. In this sense, a monitoring system to monitor the effects of wind on photovoltaic solar trackers has been developed. This novel measuring system has demonstrated, for the first time, distributed measurements of bending and torsional strain along the beams that made these structures. A particular layout for the deployment of the optical fiber has been devised. Experimental measurements are performed in the laboratory facilities demonstrating dynamic measurements of strain along a solar tracker beam and an in-field installation of the measuring system has been carried out in a solar plant.
