Contribution to the development of Brillouin optical time-domain analysis sensors

Abstract

This thesis dissertation concentrates on solving some of the limitations described by proposing novel techniques. The first technique is based on using a phase-modulated probe wave with self-heteroyne detection and RF demodulation in BOTDA sensors. The self-heterodyne detection along with the RF demodulation allows to increase the SNR of vii the detected signal and to recover the amplitude of the Brillouin interaction as well as the phase-shift. The phase modulation allows to measure a phase-shift spectrum that obviously depends on the BFS measurement but also it is tolerant to changes of the pulse power and the attenuation of the fiber. This technique is interesting for dynamic BOTDA sensors, i.e. real-time monitoring sensors, in which attenuation of the optical fiber is a usual issue due to the losses caused by mechanical stress. Furthermore, as the technique is tolerant to changes of the peak power of the pulsed signal, it can tolerate non-local effects. This allows to increase the SNR by the increment of the probe power. Furthemore, this BOTDA sensor has been combined with a well-known technique, called differential pulse-width pair technique. It allows to enhance the spatial resolution without broadening the measured spectrum. In this case, the technique was applied to amplitude and phase-shift measurements. In addition to it, new approaches of this sensor based on the deployment of multiple pulsed signals with special features have been deployed to mitigate the effect of modulation instability and to reduce the measurement time in dynamic BOTDA sensors. A second technique proposed during this thesis dissertation is the BOTDA sensor using a probe wave whose optical frequency is modulated along the distance of the fiber. As mentioned before, if the BFS of the fiber is uniform, the effect induced by non-local effects is more detrimental. Consequently, if the probe wave changes along the fiber, the Brillouin interaction is not longer uniform and the probe power can be increased. Moreover, this technique also allows to reduce the noised added to the probe wave due to amplification of spontaneous Brillouin scattering components overcoming both limitations of the probe wave. Consequently, the proposed technique allows to considerably increase the precision of the sensor. In addition to it, the second technique is also useful to generate distributed Brillouin amplifiers (DBA). Combining the DBA with a standard BOTDA sensor, the DBA can transfer energy to the pulsed signal at the final kilometers of the fiber, i.e. the fiber section in which the pulsed signal has a low power due to the suffered attenuation. Therefore, the system allows to increase the detected SNR of the measurement. Finally, the thesis presents a study of the different noise sources that could affect to BOTDA sensors. Different noises sources such as double Rayleigh scattering generated by the probe wave or phase-to-intensity noise conversion in the SBS process are theoretically and experimentally analyzed. The study points out some guidelines of the importance of each noise for the different distance ranges in BOTDA sensors.