Técnicas avanzadas para la interrogación en análisis óptico Brillouin en el dominio del tiempo

Las grandes infraestructuras son elementos indispensables en nuestra vida diaria. Los puentes, diques, túneles, trenes de alta velocidad, presas, tuberías de transporte de gas y petróleo, etc. deben garantizar un funcionamiento continuo y correcto, necesitándose por tanto soluciones que preserven y controlen la integridad de estas instalaciones. En este contexto, los sensores de fibra óptica pueden ser una solución interesante ya que se pueden introducir dentro de las estructuras y proporcionar la información de presión, tensión, temperatura o deformación sufrida por ésta. Las principales ventajas de esta tecnología son heredadas de la fibra óptica: baja atenuación, inmunidad a ruido electromagnético y deflagraciones, alta velocidad de transmisión, pequeño tamaño, peso ligero y flexibilidad. Para un gran número de aplicaciones los sensores de fibra óptica a veces son la única solución factible, sobre todo en ambientes hostiles. Entre los sensores de fibra óptica, los distribuidos son muy adecuados para monitorizar grandes infraestructuras ya que permiten, con sólo un cable de fibra y un interrogador tener cientos de puntos de medida a lo largo de ésta. Los sensores distribuidos de fibra que se basan en el scattering Brillouin estimulado (SBS) son el núcleo de esta tesis doctoral. Esta tecnología es muy versátil porque puede usarse en diferentes escenarios como en la monitorización de largas distancias (cientos de kilómetros) con resoluciones espaciales sub métricas o en distancias más cortas (unos pocos kilómetros), con resoluciones espaciales de centímetros. Aunque los sensores Brillouin en el dominio del tiempo, BOTDA, han sido ampliamente estudiados, todavía existen algunas limitaciones en estos sensores. La calidad de las medidas en un BOTDA depende de varios parámetros: el paso de frecuencia elegido para barrer la diferencia de frecuencia entre las señales de sonda y bombeo, el número de promediados en cada traza temporal para reducir el ruido, el ancho de banda Brillouin, el rango de medida (limitado por la atenuación óptica de la fibra) o la anchura del pulso de bombeo. Todos estos parámetros, junto con la potencia de sonda (limitada por el agotamiento de bombeo) y la potencia de bombeo (limitada por la inestabilidad de modulación), tienen un impacto en la relación señal a ruido (SNR) de las medidas, y por tanto, en la incertidumbre de frecuencia en la determinación del perfil de frecuencia Brillouin (BFS). Si se quiere que este tipo de sensores sean una solución real, práctica y de bajo coste para muchas aplicaciones, es necesario encontrar métodos que mejoren su rendimiento (mejoren la SNR del sistema, reduzcan el tiempo de medida, la complejidad o el coste del sensor, etc.). Esta tesis doctoral se concentra en resolver algunas de las limitaciones descritas proponiendo nuevas técnicas

Phase-measuring time-gated BOCDA

We demonstrate a simple scheme that allows performing distributed Brillouin phase spectrum (BPS) measurements with very high spatial resolution (~7 cm) over long (~4.7 km) optical fibers. This is achieved by inserting a Sagnac interferometer (SI) in a Brillouin optical correlation domain analysis (BOCDA) configuration. Over its already-presented time-domain equivalent (SI-BOTDA), this approach reduces the main source of noise (coherent backscatter noise) thanks to the low-coherence nature of the used signals. On the other hand, over the most usual schemes used for distributed BPS measurements, this implementation presents the key advantage of not requiring high-bandwidth detection or complex modulation while reaching unprecedented values of spatial resolution and number of resolved points for this type of measurement. Thanks to the linear dependence of the BPS feature around the Brillouin frequency shift, this scheme could also have the advantage of requiring shorter scanning ranges than amplitude-based configurations.European CommissionMinisterio de Economía y CompetitividadComunidad de Madri

Simple Method for the Elimination of Polarization Noise in BOTDA Using Balanced Detection and Orthogonal Probe Sidebands

Polarization noise arises in Brillouin optical timedomain analysis due to the strong polarization sensitivity of stimulated Brillouin scattering. To avoid this noise, it turns out to be indispensable to perform some kind of polarization scrambling, either in the pump pulse, the probe signal or both. This is usually achieved using polarization scrambling/switching systems, which, being mechanical, tend to be not as robust as it would be desirable. In this paper, we propose a completely passive system,with no moving parts, to eliminate polarization noise in a BOTDA. It is based on the use of passive depolarization of the pump pulse together with balanced detection among orthogonally polarized Stokes and anti-Stokes bands of the probe signal. The setup requires no alignment readjustment over time and provides a performance similar to a conventional BOTDA using scrambler.Ministerio de Economía y CompetitividadComunidad de MadridEuropean Commissio

Balanced detection in Brillouin optical time domain analysis

We propose the use of balanced detection in Brillouin Optical Time Domain Analysis (BOTDA) sensors. Balanced detection can be effectively accomplished among the Stokes and anti-Stokes bands in the probe signal. This type of detection leads to a doubling of the trace amplitude and at least a √2 increase in signal to noise ratio over the conventional configuration. Moreover, it leads to a complete cancellation of the common-mode noise in the probe signal, including relative intensity noise in Raman-assisted configurations. We show all these benefits both theoretically and experimentallyMinisterio de Ciencia e InnovaciónComunidad de MadridEuropean Commissio

Unexpected non-local effects in dual-probe-sideband BOTDA

Until now, non-local effects in dual-probe-sideband Brillouin Optical Time Domain Analysis (BOTDA) systems have been considered negligible if the probe power is below the Stimulated Brillouin Scattering (SBS) threshold. In this paper, we show the appearance of non-local effects even below the SBS threshold. The pump pulse experiences a frequencydependent spectral deformation that affects the readout process differently in the gain and loss configurations. The main conclusion of our study is that the measurements in gain configuration are more robust to this non-local effect than the loss configuration. These results are of particular interest for manufacturers of long-range BOTDA systems.European CommissionMinisterio de Economía y CompetitividadComunidad de Madri

Non-local effects in dual-probe-sideband Brillouin optical time domain analysis

According to recent models, non-local effects in dual-probesideband Brillouin Optical Time Domain Analysis (BOTDA) systems should be essentially negligible whenever the probe power is below the Stimulated Brillouin Scattering (SBS) threshold. This paper shows that actually there appear non-local effects in this type of systems before the SBS threshold. To explain these effects it is necessary to take into account a full spectral description of the SBS process. The pump pulse experiences a frequency-dependent spectral deformation that affects the readout process differently in the gain and loss configurations. This paper provides a simple analytical model of this phenomenon, which is validated against compelling experimental data, showing good agreement. The main conclusion of our study is that the measurements in gain configuration are more robust to this non-local effect than the loss configuration. Experimental and theoretical results show that, for a total probe wave power of ~1 mW (500 μW on each sideband), there is an up-shifting of ~1 MHz in the Brillouin Frequency Shift (BFS) retrieved from the Brillouin Loss Spectrum, whereas the BFS extracted from the measured Brillouin Gain Spectrum is up-shifted only ~0.6 MHz. These results are of particular interest for manufacturers of longrange BOTDA systems.European CommissionMinisterio de Economía y CompetitividadComunidad de MadridUniversidad de Alcal

Simultaneous gain and phase profile determination on an interferometric BOTDA

24th International Conference on Optical Fibre Sensors, 96343Y, Curitiba, Brazil, September 28, 2015.Up to now, complex (phase and intensity) measurements in Brillouin Optical Time-Domain Analysis (BOTDA) systems required complex phase modulation methods and high-bandwidth (multi-GHz) detection. In this work, we propose a novel technique that is able to retrieve simultaneously both gain/loss and phase characteristics of the Brillouin interaction by just introducing a Sagnac Interferometer (SI) 011 a standard BOTDA sensing scheme. The technique is described analytically and demonstrated experimentally. With this technique, a reliability increase is produced since redundant measurements can be performed.European CommissionMinisterio de Economía y CompetitividadUniversidad de AlcaláComunidad de MadridGeneralitat Valencian

Limits of BOTDA Range Extension Techniques

Brillouin-based temperature and strain sensors have attracted great attention of both the academic and industrial sectors in the past few decades due to their ability to perform distributed measurements. Particularly, Brillouin Optical Time Domain Analysis (BOTDA) systems have been applied in many different scenarios, proving particularly useful in those requiring especially wide coverage ranging extremely long distances, such as in civil structure monitoring, energy transportation or environmental applications. The extension of the measuring range in these sensors has therefore become one of the main areas of research and development around BOTDA. To do so, it is necessary to increase the Signal to Noise Ratio (SNR) of the retrieved signal. So far, several techniques have been applied in order to achieve this goal, such as pre-amplification before detection, pulse coding or Raman amplification. Here, we analyze these techniques in terms of their performance limits and provide guidelines that can assist in finding out which is the best configuration to break current range limitations. Our analysis is based on physical arguments as well as current literature results.European CommissionMinisterio de Economía y CompetitividadUniversidad de Alcal

Limits of BOTDA Range Extension Techniques

Brillouin-based temperature and strain sensors have attracted great attention of both the academic and industrial sectors in the past few decades due to their ability to perform distributed measurements. Particularly, Brillouin Optical Time Domain Analysis (BOTDA) systems have been applied in many different scenarios, proving particularly useful in those requiring especially wide coverage ranging extremely long distances, such as in civil structure monitoring, energy transportation or environmental applications. The extension of the measuring range in these sensors has therefore become one of the main areas of research and development around BOTDA. To do so, it is necessary to increase the Signal to Noise Ratio (SNR) of the retrieved signal. So far, several techniques have been applied in order to achieve this goal, such as pre-amplification before detection, pulse coding or Raman amplification. Here, we analyze these techniques in terms of their performance limits and provide guidelines that can assist in finding out which is the best configuration to break current range limitations. Our analysis is based on physical arguments as well as current literature results.European CommissionMinisterio de Economía y CompetitividadUniversidad de AlcaláMinisterio de Ciencia e InnovaciónComunidad de Madri

Medidas de propiedades dieléctricas de materiales de construcción utilizando una guía rectangular

A set of measurements of electromagnetic properties of building materials is presented in this work. The method is based on an open-ended rectangular waveguide that radiates into the material under study. Measurements were done by using two waveguides with different size for obtaining the results in two ranges of frequencies. The values of the dielectric constant are deduced from the measured reflection coefficient or admittance of the waveguide radiating into the material. A relatively simple model can be used to deduce the values of the dielectric constant from the experimental data. This method can be used for other type of materials and its main advantage is the non-destructive character and the ease implementation