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

Monitoring high thermal performance concrete for concentrated solar power plants with fibre optic sensors

In this paper a monitoring system for concrete structures at high temperatures (500ºC) is presented. The aim of the monitoring strategy based on fibre optic sensors (FOS) is to develop a high-tech monitoring technology for future installation in Concentrated Solar Power (CSP) plants with the purpose of lifecycle analysis and predictive maintenance via structural health monitoring (SHM). The monitoring strategy is based on embedding FOS in high thermal performance concrete while it is casting. The FOS technology employed to monitor both temperature and strain in concrete is known as Fibre Bragg Grating (FBG). In this work, the development of the special metallic coating and smart packaging employed to protect the sensors in this harsh environment, the calibration of the sensors and the thermal validation of the monitoring system at lab scale working in a range of temperatures between 200ºC and 500ºC is demonstrated. The lab tests were performed over a sample of 30x30x30cm, composed of 2 different layers and 4 tubes of steel, the core is made with concrete specially manufactured to have a very good thermal behaviour, and the second layer is for the insulating, and it is formed by foam concrete. The FBG sensors were positioned longitudinally, parallel to the tubes by drill holes on the casting structure at 14 different positions. The concrete curing process and some different heating cycles up to 500ºC, simulating the thermal behaviour of CSP plants, were monitored using the fibre sensor

Brillouin optical time-domain analyzer with a fiber ring laser working on the SLM regime

In this paper we present the employment of an Erbium Fiber Ring Laser structure working on the Single Longitudinal Mode regime within a Brillouin optical time domain analyzer. An analysis of some key laser parameters will be carried out, proving that a very stable operation is achieved. The associated performance of the BOTDA system, as well as the possible benefits derived from the use of the proposed laser design will also be discussed.This work has been supported by the projects TEC2013-47264-C2-1-R and TEC2016-76021-C2-2-R

Electro-optic correlator for large-format microwave interferometry: Up-conversionand correlation stages performance analysis

In this paper, a microwave interferometer prototype with a near-infra-red optical correlator is proposed as a solution to get a large-format interferometer with hundreds of receivers for radio astronomy applications. A 10 Gbits/s Lithium Niobate modulator has been tested as part of an electro-optic correlator up-conversion stage that will be integrated in the interferometer prototype. Its internal circuitry consists of a single-drive modulator biased by a SubMiniature version A (SMA) connector allowing to up-convert microwave signals with bandwidths up to 12.5 GHz to the near infrared band. In order to characterize it, a 12 GHz tone and a bias voltage were applied to the SMA input using a polarization tee. Two different experimental techniques to stabilize the modulator operation point in its minimum optical carrier output power are described. The best achieved results showed a rather stable spectrum in amplitude and wavelength at the output of the modulator with an optical carrier level 23 dB lower than the signal of interest. On the other hand, preliminary measurements were made to analyze the correlation stage, using 4f and 6f optical configurations to characterize both the antenna/fiber array configuration and the corresponding point spread function.This work was supported by the Ministerio de Economía y Competitividad, Spain, under the Plan Nacional de I+D+i project with Reference No. ESP2015-70646-C2-1-R cofinanced with EU FEDER funds and the CONSOLIDER INGENIO 2010 programme under the Reference No. CSD2010-00064. The authors would like to thank Félix Gracia from Instituto Astrofísico de Canarias (IAC) for his assistance and help

Feasibility study of a fiber ring laser working on the SLM regime in a BOTDA sensor

A feasibility study of the employment of a fiber laser working as the laser source of a Brillouin distributed sensor is presented in this paper. The erbium fiber ring laser structure designed works on the single longitudinal mode regime. The main parameters of the ring cavity are analyzed in terms of the laser requirements and the performance of a Brillouin optical time domain analysis sensor using the fiber laser as light source.This work was supported by the Spanish Government and FEDER funds under Project TEC2016-76021-C2-2-

Feasibility study of strain and temperature discrimination in a BOTDA system via artificial neural networks

Automatic discrimination between strain and temperature in a Brillouin optical time domain analyzer via artificial neural networks is proposed and discussed in this paper. Using a standard monomode optical fiber as the sensing element, the ability of the proposed solution to detect the known changes that the Brillouin gain spectrum exhibits depending on the applied temperature and/or strain will be studied. Experimental results, where different simultaneous strain and temperature situations have been considered, will show the feasibility of this technique.This work has been supported by the projects TEC2013-47264-C2-1-R and TEC2016-76021-C2-2-R

Brillouin frequency shift estimation in BOTDA via subpixel processing

In this paper we propose the employment of sub-pixel algorithms for the estimation of the central frequency of the Brillouin Gain Spectrum in a Brillouin Optical Time Domain Analyzer. The experimental results will show that the proposed solution shows a good performance when the chosen frequency step for the required frequency sweep is high. If the improved computational efficiency in comparison to the traditional Lorentzian fitting is also considered, it can be concluded that this approach may be of great interest for dynamic measurement scenarios.This work has been supported by the Universidad de Cantabria and by the Spanish Ministerio de Ciencia e Innovación through project TEC2013-47264-C2-1-R

Reflection-based fiber specklegram sensor

In this paper, a Fiber Specklegram Sensor (FSS) based on Plastic Optical Fiber (POF) has been proposed with a reflection-based configuration. A HeNe laser is launched through a coupler, reflected by a mirrored fiber end and detect the specklegram sequence using a commercial camera. Different sensor performance metrics have been analyzed in terms of final sensitivity to external vibration applied to the fiber. The results confirm the performance reduction of the reflection-based scheme in comparison with the transmission-based approach but it can be also employed as sensing system where only a single fiber end is available.This work has been supported by the project TEC2013-47264-C2-1-R

High-temperature distributed sensor system via BOTDA and multimode gold-coated fiber

A high-temperature distributed sensor solution based on a Brillouin Optical Time Domain Analyzer and a multimode gold-coated fiber is presented and experimentally validated in this paper. Distributed temperature measurements up to 600°C will be demonstrated.This work has been supported by the Universidad de Cantabria and by the Spanish Ministerio de Ciencia e Innovación through project TEC2013-47264-C2-1-R

Fiber Bragg Grating sensors for on-line welding diagnostics

In this work Fiber Bragg Grating transducers (FBGs) are used to perform an on-line monitoring of an arc-welding process by means of the combined temperature and strain profile. The main goal of this approach is to establish a correlation between the appearance of defects in the seams and the response offered by the FBGs, exploring the optimal disposition of the FBGs in terms of their sensitivity to weld defects. The proposed solution will be explored by means of welding tests and a comparison with a plasma spectroscopic analysis.This work has been supported by the project TEC2010-20224-C02-02 and grant AP2009-1403. Authors would also like to thank Roberto Perez Sierra for his valuable help during the FBGs fabrication process