Distributed Fiber Optics Strain Measurements for Monitoring Geotechnical Structures

Recent advances in strain measurement using optical fibers provide new opportunities for monitoring the performance of geotechnical structures during and after construction. Brillouin optical time-domain reflectometry (BOTDR) is an innovative technique that allows measurement of full strain profiles using standard optical fibers. In this paper, two case studies illustrating the application of the distributed optical fiber strain sensors are presented. One is monitoring of an old masonry tunnel when a new tunnel was constructed nearby and the other is monitoring the behavior of secant piled walls for basement construction. Both sites are located in London. The advantages and limitations of this new sensor technology for monitoring geotechnical structures are discussed

Analysis of the applicability of singlemode optical fibers for measurement of deformation with distributed systems BOTDR

Distributed optical fiber sensors allow monitoring physical effects across the whole cable. The paper presents results obtained from the performed tests and shows that single mode fibers can provide analyses of the deformation changes, when distributed optical systems BOTDR used. We used standard optical fiber G.652.D with primary and secondary protected layers and specialized cable SMC-V4 designed for this purpose. The aim was to compare the deformation sensitivity and determine which fiber types are the best to use. We deformed the fiber in the longitudinal and transverse directions and mechanically stressed in orthogonal directions to find how to localize optical fibers. They could be deployed in real use. For achieving optimal results of mechanical changes and acting forces, sensor fibers have to be located carefully

The Evolution of Optical Fiber Sensors Technologies During the 35 Last Years and Their Applications in Structure Health Monitoring

Conference of 7th European Workshop on Structural Health Monitoring, EWSHM 2014 ; Conference Date: 8 July 2014 Through 11 July 2014; Conference Code:113023International audienceSince late 70s, (quasi-)distributed OFS have been developed, 12 countries producing 85% of the global effort. Since mid-80s, OFS has caught attention in many sectors where SHM is a matter of concern (civil engineering, composites, oil & gas, renewable energies, safety...). Today, OFS is became a strategic domain, especially in Asia, and China invests a lot since 7-8 years. Top 12 countries involved in SHM are, almost the same than in OFS, having published 80% of the total. Statistics proves that OFS is now the second sensing technology for SHM, and 2/3 concern the FBG sensing

Highly-sensitive measurements with chirped- pulse phasesensitive OTDR

Distributed optical fiber sensing is currently a very predominant research field, which perceives optical fibers as the potential nervous system of the Earth. Optical fibers are understood as continuous densely-packed sensing arrays, able of retrieving physical quantities from the environment of the fiber. Some of the most prominent distributed sensing implementations nowadays rely on performing interferometric measurements using the Rayleigh backscattered light, resorting to a technique called Phase-sensitive Optical Time-Domain Reflectometry (CP-ϕOTDR). A variant to this technique has been recently proposed in 2016, known as Chirped-Pulse Phase-Sensitive OTDR, which allowed to overcome most of the limitations of traditional ϕOTDR implementations while retaining a simple setup, yielding remarkably high sensitivities. In this thesis, we aim to optimize the stability and performance of chirped-pulse ϕOTDR systems over long-term measurements, and develop novel paradigm changing applications benefiting from the high sensitivity provided by the technique. We reach a mK-scale long-term stability in ϕOTDR systems, and perform highly sensitive strain, temperature, and refractive index measurements, demonstrating new photonic applications such as distributed bolometry, electro-optical reflectometry, or distributed underwater seismology. We discuss how these applications might be able of increasing the efficiency in the energy field, paving the way towards the development of self-diagnosable grids (smart-grids), and also of revolutionizing next-generation seismological networks, allowing to overcome some of the greatest limitations faced in modern seismology today.Distributed optical fiber sensing is currently a very predominant research field, which perceives optical fibers as the potential nervous system of the Earth. Optical fibers are understood as continuous densely-packed sensing arrays, able of retrieving physical quantities from the environment of the fiber. Some of the most prominent distributed sensing implementations nowadays rely on performing interferometric measurements using the Rayleigh backscattered light, resorting to a technique called Phase-sensitive Optical Time-Domain Reflectometry (φOTDR). A variant to this technique has been recently proposed in 2016, known as Chirped-Pulse Phase-Sensitive OTDR, which allowed to overcome most of the limitations of traditional φOTDR implementations while retaining a simple setup, yielding remarkably high sensitivities. In this thesis, we aim to optimize the stability and performance of chirped-pulse φOTDR systems over long-term measurements, and develop novel paradigm changing applications benefiting from the high sensitivity provided by the technique. We reach a mK-scale long-term stability in φOTDR systems, and perform highly sensitive strain, temperature and refractive index measurements, demonstrating new photonic applications such as distributed bolometry, electro-optical reflectometry, or distributed underwater seismology. We discuss how these applications might be able of increasing the efficiency in the energy field, paving the way towards the development of self-diagnosable grids (smart-grids), and also of revolutionizing nextgeneration seismological networks, allowing to overcome some of the greatest limitations faced in modern seismology today. We finally conclude and summarize the objectives achieved in this thesis, commenting on the potential of the novel applications shown, and proposing future lines of research based on the results

Fiber Laser for Phase-Sensitive Optical Time-Domain Reflectometry

We have designed a new fiber laser configuration with an injection-locked DFB laser applicable for phase-sensitive optical time-domain reflectometry. A low-loss fiber optical ring resonator (FORR) is used as a high finesse filter for the self-injection locking of the DFB (IL-DFB) laser. By varying the FORR fidelity, we have compared the DFB laser locking with FORR operating in the under-coupled, critically coupled, and over-coupled regimes. The critical coupling provides better frequency locking and superior narrowing of the laser linewidth. We have demonstrated that the locked DFB laser generates a single-frequency radiation with a linewidth less than 2.5 kHz if the FORR operates in the critically coupled regime. We have employed new IL-DFB laser configuration operating in the critical coupling regime for detection and localization of the perturbations in phase-sensitive OTDR system. The locked DFB laser with a narrow linewidth provides reliable long-distance monitoring of the perturbations measured through the moving differential processing algorithm. The IL-DFB laser delivers accurate localization of the vibrations with a frequency as low as ~50 Hz at a distance of 9270 m providing the same signal-to-noise ratio that is achievable with an expensive ultra-narrow linewidth OEwaves laser (OE4020–155000-PA-00)

Contributions to the development of distributed sensors based on stimulated Brillouin scattering

RESUMEN: El objetivo principal de esta tesis es contribuir al desarrollo y la mejora del rendimiento de los sensores distribuidos basados en la dispersión Brillouin. Durante el desarrollo de este trabajo se han considerado diferentes áreas de mejora. En primer lugar, se han propuesto diversas configuraciones experimentales para superar algunas de las limitaciones típicas que tienen estos sensores, como son los efectos no locales en los sensores BOTDA o la aparición de sub-picos en el espectro de ganancia de Brillouin en sistemas basados en el dominio de frecuencia. Otro objetivo principal de este trabajo es aplicar diferentes enfoques de procesado para resolver problemáticas aún no resueltas, como la discriminación entre las medidas de temperatura y las de deformación obtenidas con los sensores Brillouin. Además, también se han estudiado algunos métodos alternativos al método tradicional basado en la aplicación de ajustes Lorentzianos para estimar el cambio de la frecuencia Brillouin. Finalmente, este trabajo también ha tratado de contribuir a la validación de los conocimientos adquiridos mediante la validación en escenarios reales, como aplicaciones de alta temperatura o detección de fugas en tuberías.ABSTRACT: The main objective of this thesis dissertation is to contribute to the development and improvement in the performance of distributed sensors based on Brillouin scattering. Different areas of improvement have been considered during the development of this work. First of all, various different experimental configurations have been proposed to overcome some traditional limitations of these sensors, such as non-local effects on Brillouin optical time domain analysis (BOTDA) sensors or appearance of sub-peaks on the Brillouin gain measured with systems based on the frequency domain. Another main objective of this work is applying different processing approaches in an attempt to solve open problems such as the discrimination between temperature and strain measurements obtained with Brillouin sensors. Additionally, it would be interesting to provide some faster and alternative methods to estimate the Brillouin shift in comparison to traditional method based on applying Lorentzian fittings. Finally, this work has also tried to contribute to the validation of the acquired knowledge by performing validations in real scenarios, such as high-temperature applications or leakage detection in pipelines.This work has been supported by the funding of the following entities and actions: • Universidad de Cantabria through the research grant Programa de Personal Investigador en Formación Predoctoral and research stays grants in Pamplona, Spain and in Aversa, Italy. • Agencia Estatal de Investigación through research project Sensores fotónicos para seguirdad y protección (TEC2016-76021-C2-2-R). • Ministerio de Economía y Competitividad through research project Sensores de fibra óptica para seguirdad y protección (TEC2013-47264-C2-1-R). • Gobierno de Cantabria through research project Detección de fugas en autovías del agua mediante sensores ópticos (FASO). • Fundación TTI through a research grant Patrocinio de actividades formativas en investigación científica y técnica. • Cost action td1001: Novel and reliable optical fibre sensor systems for future security and safety applications (OFSESA) through a research grant for a short term scientific mission to Aversa, Italy and through two grants for summer schools