Contribution to the development of distributed fiber optic sensors based on stimulated Brillouin scattering

Abstract

Fiber optic sensors are becoming a disruptive technology in monitoring integrity of structures. Their low loss, electromagnetic immunity, remote sensing possibilities, multiplexing possibilities and light weight are very attractive in field measurements. Also, the possibility of performing distributed measurements is only possible with fiber optics. In distributed fiber sensors the fiber itself is the transducer, and the measurement of a given parameter i.e. temperature, strain… is given continuously along the fiber at a given spatial resolution, without blind spots. Between the different types of fiber optics distributed sensors, the ones based on stimulated Brillouin scattering are the most promising technology, since they potentially allow high resolution measurements in tenths of kilometer long fibers. However, field applications of these sensors require better performance: faster response sensors in hundreds of kilometer long fibers, at high resolutions (better than 1 meter) at the lowest possible cost. This thesis contributes to the development of fiber optics sensors based on stimulated Brillouin scattering in that direction. We first develop theoretical models so as to study the behavior of the sensor and focus in the main limitations these sensors manifest. Then, after studying different solutions in the literature, we show our contributions. A sensing setup, based on the RF-shaping of pump pulses is presented as a solution to simplify the sensor setup. Furthermore this setup helps in extending the overall sensing length by minimizing non-local effects. An alternative to the common Brillouin spectrum scan, exploiting the wavelength dependence of Brillouin frequency shift, is introduced. This adds a new degree of freedom to the existing Brillouin based sensors. The work carried out during a research stay in Aversa, Italy, is also explained. It consists in a technique, TDM-BOTDA, which enhances the SNR in detection and minimizes non-local effects, improving the sensor performance. In order to integrate point sensors in a distributed sensors network, two designs are proposed and demonstrated. They take advantage of Raman distributed amplification so as to reduce the loss generated by long fibers and multiplexing. Finally, the application of coherent light-wave systems to BOTDA is studied by the introduction of BOTDA sensors with self-heterodyne detection. Self-heterodyne BOTDA sensors increase the SNR in detection considerably. Furthermore, they add the possibility of measuring the Brillouin phase shift generated along the fiber. This is very important, since it enables the possibility of performing dynamic measurements which are independent of loss.