Investigation into Improving Resolution of Strain Measurements in BOTDA Sensors

Structural health monitoring contributes to the safety and extension of the lifetime of structures. There are several techniques for structural health monitoring, most of them based on different types of sensors. Fiber optics has been proven as a very good technology since it is lightweight, has low signal loss, is immune to electromagnetic interferences and offers the possibility of remote sensing and multiplexing. Therefore, as in telecommunications technology, the application of fiber optics to structural health monitoring has resulted in great breakthroughs during the last decades. Furthermore, one of the advantages that fiber optic sensors offer is obtaining distributed measurements with a certain spatial resolution. In this approach, the whole fiber optic line is the sensor, instead of having sensors placed in given positions. Between the different technologies that perform fiber optic distributed measurements, Brillouin optical time domain analysis (BOTDA) is the one with better performance. BOTDA sensors are based in the stimulated Brillouin scattering phenomenon characterization. This is a phenomenon that has a strong dependence on strain and temperature. However, there is still much room for improvement in these sensors since they do not meet the minimum quality criteria for some structural health monitoring applications. Also, survey of literature indicates that the parameters that define the performance of the sensor and specially the resolution of measurements are not thoroughly defined. This is due to the infancy of this technology and the fact that unlike localized sensing technologies the measurement parameters in distributed sensing are dependent on spatial characteristics of the sensor. In this thesis, a method for the computation and presentation of measurement resolutions in BOTDA sensors is defined based on an exhaustive literature review. Moreover, an important improvement in BOTDA quality is presented by creating fiber optic coils and special data processing, which is a non-instrument-based enhancement. An improvement of 1.73 times the strain resolution and repeatability and a reduction of the spatial resolution from 20cm to 7.7cm are experimentally demonstrated.