Recent Advances in Distributed Acoustic Sensing Based on Phase-Sensitive Optical Time Domain Reflectometry

Distributed acoustic sensing (DAS) using coherent Rayleigh backscattering in an optical fiber has become a ubiquitous technique for monitoring multiple dynamic events in real time. It has continued to constitute a steadily increasing share of the fiber-optic sensor market, thanks to its interesting applications in many safety, security, and integrity monitoring systems. In this contribution, an overview of the recent advances of research in DAS based on phase-sensitive optical time domain reflectometry (ϕ-OTDR) is provided. Some advanced techniques used to enhance the performance of ϕ-OTDR sensors for measuring backscattering intensity changes through reduction of measurement noise are presented, in addition to methods used to increase the dynamic measurement capacity of ϕ-OTDR schemes beyond conventional limits set by the sensing distance. Recent ϕ-OTDR configurations which significantly enhance the measurement spatial resolution, including those which decouple it from the probing pulse width, are also discussed. Finally, a review of recent advances in more precise quantitative measurement of an external impact based on frequency shift and phase demodulation methods using simple direct detection ϕ-OTDR schemes is given

Distributed Fiber Ultrasonic Sensor and Pattern Recognition Analytics

Ultrasound interrogation and structural health monitoring technologies have found a wide array of applications in the health care, aerospace, automobile, and energy sectors. To achieve high spatial resolution, large array electrical transducers have been used in these applications to harness sufficient data for both monitoring and diagnoses. Electronic-based sensors have been the standard technology for ultrasonic detection, which are often expensive and cumbersome for use in large scale deployments. Fiber optical sensors have advantageous characteristics of smaller cross-sectional area, humidity-resistance, immunity to electromagnetic interference, as well as compatibility with telemetry and telecommunications applications, which make them attractive alternatives for use as ultrasonic sensors. A unique trait of fiber sensors is its ability to perform distributed acoustic measurements to achieve high spatial resolution detection using a single fiber. Using ultrafast laser direct-writing techniques, nano-reflectors can be induced inside fiber cores to drastically improve the signal-to-noise ratio of distributed fiber sensors. This dissertation explores the applications of laser-fabricated nano-reflectors in optical fiber cores for both multi-point intrinsic Fabry–Perot (FP) interferometer sensors and a distributed phase-sensitive optical time-domain reflectometry (φ-OTDR) to be used in ultrasound detection. Multi-point intrinsic FP interferometer was based on swept-frequency interferometry with optoelectronic phase-locked loop that interrogated cascaded FP cavities to obtain ultrasound patterns. The ultrasound was demodulated through reassigned short time Fourier transform incorporating with maximum-energy ridges tracking. With tens of centimeters cavity length, this approach achieved 20kHz ultrasound detection that was finesse-insensitive, noise-free, high-sensitivity and multiplex-scalability. The use of φ-OTDR with enhanced Rayleigh backscattering compensated the deficiencies of low inherent signal-to-noise ratio (SNR). The dynamic strain between two adjacent nano-reflectors was extracted by using 3×3 coupler demodulation within Michelson interferometer. With an improvement of over 35 dB SNR, this was adequate for the recognition of the subtle differences in signals, such as footstep of human locomotion and abnormal acoustic echoes from pipeline corrosion. With the help of artificial intelligence in pattern recognition, high accuracy of events’ identification can be achieved in perimeter security and structural health monitoring, with further potential that can be harnessed using unsurprised learning

Limits of performance of chirped- pulse phase-sensitive OTDR

Distributed acoustic sensing is an emerging field of research which aims to develop methods capable of using a single optical fiber as a long, dense, and high-sensitivity sensor array. Currently, the most promising implementations measure the interference of Rayleigh backscattered light, obtained by probing the fiber with light from a source of high coherence. These methods are known as Phase-sensitive Optical Time-Domain Reflectometers (φOTDR), and are currently undergoing a period of active research and development, both academically and industrially. One of its variants, known as the Chirped-Pulse φOTDR (CP-φOTDR), was developed in 2016. This technique has proven to be remarkably sensitive to strain and temperature, with an attractively simple implementation. In this thesis, we delve into the intricacies of this technique, probing its fundamental limits and addressing current limitations. We discuss the implications of estimation on the performance statistics, the impact of different noise sources and the origin of cross-talk between independent measured positions. In doing so, we also propose methods to reach the current fundamental limitations, and overcome the upper bound of measurable perturbations. We then demonstrate new potential applications of the technique: in seismology, by exploiting the high spatial density of measurements for array signal processing; in the fast characterization of linear birefringence in standard single-mode fibers; and on the measurement of sound pressure waves, by using a special flat cable structure to embed the fiber under test. Finally, we summarize and comment on the aforementioned achievements, proposing some open lines of research that may originate from these results.Distributed acoustic sensing is an emerging field of research which aims to develop methods capable of using a single optical fiber as a long, dense, and highsensitivity sensor array. Currently, the most promising implementations measure the interference of Rayleigh backscattered light, obtained by probing the fiber with light from a source of high coherence. These methods are known as Phase-sensitive Optical Time-Domain Reflectometers (φOTDR), and are currently undergoing a period of active research and development, both academically and industrially. One of its variants, known as the Chirped- Pulse φOTDR (CP-φOTDR), was developed in 2016. This technique has proven to be remarkably sensitive to strain and temperature, with an attractively simple implementation. In this thesis, we delve into the intricacies of this technique, probing its fundamental limits and addressing current limitations. We discuss the implications of estimation on the performance statistics, the impact of different noise sources and the origin of cross-talk between independent measured positions. In doing so, we also propose methods to reach the current fundamental limitations, and overcome the upper bound of measurable perturbations. We then demonstrate new potential applications of the technique: in seismology, by exploiting the high spatial density of measurements for array signal processing; in the fast characterization of linear birefringence in standard single-mode fibers; and on the measurement of sound pressure waves, by using a special flat cable structure to embed the fiber under test. Finally, we summarize and comment on the aforementioned achievements, proposing some open lines of research that may originate from these results

Data-Driven Distributed Optical Vibration Sensors: A Review

Distributed optical vibration sensors (DOVS) have attracted much attention recently since it can be used to monitor mechanical vibrations or acoustic waves with long reach and high sensitivity. Phase-sensitive optical time domain reflectometry (Φ-OTDR) is one of the most commonly used DOVS schemes. For Φ-OTDR, the whole length of fiber under test (FUT) works as the sensing instrument and continuously generates sensing data during measurement. Researchers have made great efforts to try to extract external intrusions from the redundant data. High signal-to-noise ratio (SNR) is necessary in order to accurately locate and identify external intrusions in Φ-OTDR systems. Improvement in SNR is normally limited by the properties of light source, photodetector and FUT. But this limitation can also be overcome by post-processing of the received optical signals. In this context, detailed methodologies of SNR enhancement post-processing algorithms in Φ-OTDR systems have been described in this paper. Furthermore, after successfully locating the external vibrations, it is also important to identify the types of source of the vibrations. Pattern classification is a powerful tool in recognizing the intrusion types from the vibration signals in practical applications. Recent reports of Φ-OTDR systems employed with pattern classification algorithms are subsequently reviewed and discussed. This thorough review will provide a design pathway for improving the performance of Φ-OTDR while maintaining the cost of the system as no additional hardware is required

Development of a distributed optical fiber sensor for geological applications

The purpose of the study was to develop a distributed optical fiber acoustic sensor for monitoring ground subsidence before collapse sinkholes form causing costly damage on infrastructure. Costs in excess of R1.3 billion have been incurred while dealing with sinkhole related measures in South Africa. Monitoring sinkholes and the presence of an early warning alert system can drastically reduce the impact, risk and cost caused by sudden ground collapse. A related goal was to construct a reliable collapse alert early warning system to facilitate disaster preparedness and avoid further damage from accidents. This was achieved by developing a spectroscopic shift monitoring algorithm which analysed changes in the subsurface vibration modes using ambient noise signals. For the first time to our knowledge, an optic fiber sensor with an early warning alarm, using ambient noise vibrations to detect and monitor sinkholes was developed at NMU. A polarisation-based, interferometric optical fiber seismic sensor was developed and compared to a commercial geophone. The fiber sensor exhibited superior performance in sensitivity, bandwidth, signal response and recovery times. The sensitivity of the optical fiber sensor was 0.47 rad/Pa surpassing the geophone sensitivity by 9.32%, and the bandwidth of 3.349kHz was 20 times greater for the optical fiber sensor. The fiber sensor was used to measure millisecond events as the impact duration of a bouncing ball was successfully obtained. It was used to detect sinkhole formation in the simulator model, designed. Ground collapse precursors were identified, and early warning alert was achieved using the spectral analysis algorithm, developed. The collapse precursor condition was identified as a functional combination of variations in the peak frequency, bandwidth and peak intensity. A distributed acoustic sensor was built to detect ambient noise induced subsurface signals. Vibrations were located along the 28km length of optical fiber with a relative error of 9.6%. The sensor demonstrated a frequency response range of 212.25Hz, an event distance precision of 224m with time resolution of 1.12µs, and a spatial resolution of 1km. The position of disturbance was measured within 300m of its actual point of 3.21km along the optical fiber. The results showed that distributed optical fiber sensing allows real-time monitoring of the subsurface over extended distances, using ambient noise signals.Thesis (PhD) -- Faculty of Science, School of Computer Science, Mathematics, Physics and Statistics, 202

Development of a distributed optical fiber sensor for geological applications

The purpose of the study was to develop a distributed optical fiber acoustic sensor for monitoring ground subsidence before collapse sinkholes form causing costly damage on infrastructure. Costs in excess of R1.3 billion have been incurred while dealing with sinkhole related measures in South Africa. Monitoring sinkholes and the presence of an early warning alert system can drastically reduce the impact, risk and cost caused by sudden ground collapse. A related goal was to construct a reliable collapse alert early warning system to facilitate disaster preparedness and avoid further damage from accidents. This was achieved by developing a spectroscopic shift monitoring algorithm which analysed changes in the subsurface vibration modes using ambient noise signals. For the first time to our knowledge, an optic fiber sensor with an early warning alarm, using ambient noise vibrations to detect and monitor sinkholes was developed at NMU. A polarisation-based, interferometric optical fiber seismic sensor was developed and compared to a commercial geophone. The fiber sensor exhibited superior performance in sensitivity, bandwidth, signal response and recovery times. The sensitivity of the optical fiber sensor was 0.47 rad/Pa surpassing the geophone sensitivity by 9.32%, and the bandwidth of 3.349kHz was 20 times greater for the optical fiber sensor. The fiber sensor was used to measure millisecond events as the impact duration of a bouncing ball was successfully obtained. It was used to detect sinkhole formation in the simulator model, designed. Ground collapse precursors were identified, and early warning alert was achieved using the spectral analysis algorithm, developed. The collapse precursor condition was identified as a functional combination of variations in the peak frequency, bandwidth and peak intensity. A distributed acoustic sensor was built to detect ambient noise induced subsurface signals. Vibrations were located along the 28km length of optical fiber with a relative error of 9.6%. The sensor demonstrated a frequency response range of 212.25Hz, an event distance precision of 224m with time resolution of 1.12µs, and a spatial resolution of 1km. The position of disturbance was measured within 300m of its actual point of 3.21km along the optical fiber. The results showed that distributed optical fiber sensing allows real-time monitoring of the subsurface over extended distances, using ambient noise signals.Thesis (PhD) -- Faculty of Science, School of Computer Science, Mathematics, Physics and Statistics, 202

Fiber Optic Acoustic Sensing to Understand and Affect the Rhythm of the Cities: Proof-of-Concept to Create Data-Driven Urban Mobility Models

In the framework of massive sensing and smart sustainable cities, this work presents an urban distributed acoustic sensing testbed in the vicinity of the School of Technology and Telecommunication Engineering of the University of Granada, Spain. After positioning the sensing technology and the state of the art of similar existing approaches, the results of the monitoring experiment are described. Details of the sensing scenario, basic types of events automatically distinguishable, initial noise removal actions and frequency and signal complexity analysis are provided. The experiment, used as a proof-of-concept, shows the enormous potential of the sensing technology to generate data-driven urban mobility models. In order to support this fact, examples of preliminary density of traffic analysis and average speed calculation for buses, cars and pedestrians in the testbed’s neighborhood are exposed, together with the accidental presence of a local earthquake. Challenges, benefits and future research directions of this sensing technology are pointed out.B-TIC-542-UGR20 funded by “Consejería de Universidad, Investigación e Innovacción de la Junta de AndalucíaERDF A way of making Europ

Chirped-pulse phase-sensitive optical time domain reflectometry

El mundo actual funciona gracias a las grandes infraestructuras que dotan de energía y transporte seguros a sus ciudadanos. Dichas infraestructuras (presas, diques, gaseoductos, oleoductos, puentes, líneas de ferrocarril, carreteras…) típicamente presentan grandes dimensiones y es especialmente difícil monitorizar su buen funcionamiento y su salud estructural además de protegerlas de posibles amenazas. Los sensores distribuidos de fibra óptica son una solución fiable y rentable para esta problemática, ya que permiten medir vibraciones, deformaciones y temperatura a lo largo de todos los puntos de una fibra óptica estándar de comunicaciones. Los sensores de fibra óptica basados en scattering Rayleigh son particularmente útiles cuando las medidas deben ser realizadas en tiempo real, como por ejemplo en la detección y caracterización de vibraciones. En esta tesis, se ha realizado un estudio acerca de distintas soluciones y alternativas a las limitaciones de la tecnología OTDR. Se ha propuesto una nueva técnica, derivada de ésta, que ofrece unas prestaciones que superan notablemente a las de los sistemas OTDR tradicionales. Para ello, en primer lugar, se ha procedido a realizar un estudio en profundidad de los fundamentos y el estado del arte de las técnicas de monitorización basadas en Reflectometría Óptica en el Dominio del Tiempo (OTDR, por sus siglas en inglés) y, en particular, sobre la implementación sensible a la fase, también conocida como OTDR. Se ha estudiado la limitación en rango y resolución de los sistemas OTDR principalmente asociada a la aparición de efectos no lineales como la inestabilidad de modulación. Actualmente, un OTDR tradicional presenta una resolución máxima del orden de los 10 metros para un rango de medida del orden de pocas decenas de km (si no se aplica ningún tipo de técnica de amplificación distribuida). Además de estudiar esta limitación y a qué es debida, se han propuesto dos técnicas para mitigar los efectos perjudiciales de la MI. En primer lugar, se ha realizado un estudio del efecto de la forma de los pulsos ópticos empleados en el sensor en la traza retrodispersada en un OTDR. Se ha podido comprobar cómo los pulsos triangulares o gaussianos presentan mayor robustez que los pulsos rectangulares, tradicionalmente empleados, frente a la MI. En segundo lugar, se ha propuesto una técnica basada en el concepto de Amplificación de Pulsos Chirpeados (CPA, por sus siglas en inglés), que ha permitido desarrollar un OTDR con resoluciones milimétricas. Hasta el momento ningún OTDR había podido llegar a tales resoluciones, lo que abre un nuevo abanico de aplicaciones a la tecnología OTDR donde se requiera alta resolución espacial en la medida. También se ha estudiado la otra gran limitación de este tipo de sensores: su comportamiento no lineal ante una perturbación. Actualmente, salvo que se implementen técnicas de recuperación de fase o barridos en longitud de onda que implican más complejidad, coste y tiempo de medida, no es posible realizar medidas cuantificables de temperatura o deformaciones. Del mismo modo, tampoco se pueden realizar medidas acústicas reales. En este trabajo, en primer lugar, se propone emplear la técnica de Reconstrucción de Fase empleando Diferenciación Óptica Ultrarápida (PROUD, por sus siglas en inglés) para recuperar el campo complejo de una señal OTDR. Con esta medida, el sensor pasaría a comportarse de forma lineal sin la complejidad intrínseca de los métodos tradicionales de detección de fase. En segundo lugar, y de aquí viene el nombre de esta tesis doctoral, se propone el uso de pulsos chirpeados en los sensores OTDR. La nueva técnica llamada Chirped-Pulse OTDR, ha permitido la medida de forma lineal de cambios de temperatura y deformaciones, en un único disparo y sin la necesidad de realizar barridos en frecuencia o implementar detección coherente. A lo largo de este trabajo, se han alcanzado resoluciones de 0.5mK/4n y se ha demostrado la posibilidad de hacer medidas acústicas reales. También se han estudiado las limitaciones de esta técnica y propuesto varias soluciones. Se ha demostrado que el ruido de fase del láser empleado en el sistema, puede ser mitigado con esta nueva técnica. Además, se ha propuesto el uso de amplificación distribuida basada en scattering Raman estimulado para alcanzar rangos de medida mayores, hasta 75 km con una resolución espacial de 10 m

Técnicas de metrología óptica basadas en fotónica de microondas

Esta tesis presenta una serie de técnicas de metrología óptica basadas en fotónica de microondas (MWP), cuya incorporación permite la mejora de ciertas figuras de mérito con respecto a sus equivalentes puramente fotónicos en el ámbito de la metrología óptica y de la interrogación de sensores de fibra óptica (OFS). Tras una introducción donde se describen algunos de los tipos de OFS más relevantes y los cuatro principales métodos de reflectometría óptica, se resumen las publicaciones que forman parte del compendio de la presente tesis. Estas se han dividido en aquellas que presentan técnicas basadas en reflectometría óptica incoherente dispersiva en el dominio de la frecuencia (DI-OFDR), o sistemas con mejoras que pueden ser empleadas en DI-OFDR, y aquellas que han buscado y estudiado las aplicaciones metrológicas de los bucles desplazadores de frecuencia (FSL). El primer grupo de técnicas hace referencia a sistemas basados en reflectometría óptica incoherente en el dominio de la frecuencia (I-OFDR) donde la inclusión de un elemento dispersivo en el circuito óptico permite la incorporación de nuevas funcionalidades. En particular, la medición de desplazamiento de la longitud de onda de reflectores de banda estrecha mediante la detección del retraso de grupo diferencial de ondas moduladas. El segundo se refiere al estudio teórico y experimental de peines de frecuencia generados por lazos de fibra amplificados que incluyen un elemento desplazador de frecuencia, y que permiten la generación de un amplio conjunto de formas de onda ópticas de interés en, entre otros ámbitos, aplicaciones metrológicas. Por una parte, las publicaciones relacionadas con DI-OFDR han consistido en: el desarrollo de un método de interrogación de redes de Bragg en fibra (FBG) mediante una fuente de doble longitud de onda; la minimización del número de puntos de interrogación en frecuencia en sistemas DI-OFDR adaptada a una topología de reflectores equiespaciados, así como el desarrollo de un método basado únicamente en medidas de potencia; y la implementación de un sistema con conversión electroóptica descendente de frecuencia y detección a frecuencia intermedia para la realización de medidas reflectométricas tanto distribuidas como puntuales y, en este caso, con selectividad en longitud de onda. Entre otras ventajas, estos sistemas han permitido la interrogación de FBG con resolución picométrica y alta eficiencia en potencia; velocidades de interrogación de arrays FBG de hasta 10 μs por elemento sensor; y la detección de eventos reflexivos discretos con reflectividades de hasta 90 dB y de retrodispersión Rayleigh en banda C en fibra monomodo estándar; respectivamente. Por otra parte, las publicaciones relacionadas con los FSL se han centrado, respectivamente, en la aplicación de las formas de onda ópticas de tipo chirp generadas por estos dispositivos para la medición de distancia con resolución milimétrica por compresión de digital de pulso, aprovechando para ello su alto producto tiempo-ancho de banda ( 200), y en la descripción teórica de dichas formas de onda. Empleando una descripción del campo generado por los FSL basada en una analogía con la óptica difractiva, se han descrito nuevas propiedades de los pulsos tipo chirp generados por FSL. En concreto, la existencia de captura de fase entre distintos pulsos, la presencia de desviaciones de la linealidad del chirp y la comprobación experimental de la coincidencia entre las fases Talbot generadas en FSL bajo condiciones fraccionales y las secuencias de fase perfecta de Gauss. Los resultados de esta segunda parte de la tesis muestran, además de un buen acuerdo del modelo teórico con los pulsos medidos, la viabilidad del empleo de FSL para medición de distancia láser con compresión digital de pulsos, obteniéndose tasas de compresión y de repetición de 150 y 80 MHz, respectivamente, y un ancho de banda de 20 GHz.This thesis presents several techniques of optical metrology based on Microwave Photonics (MWP), whose incorporation allows for improving some figures of merit with respect to its purely photonic equivalents in the fields of optical metrology and interrogation of optical fiber sensors (OFS). After an introduction where some of the most relevant types of OFSs and the four main methods of optical reflectometry are described, the publications that form the compendium of this thesis are summarized. These have been divided in those which present techniques based on dispersive incoherent optical frequency-domain reflectometry (DI-OFDR), or systems with improvements that can be employed in DI-OFDR, and those which have searched and studied the metrology applications of the frequency shifting loops (FSL). The first group of techniques refers to systems based on incoherent optical frequency-domain reflectometry (I-OFDR) where the inclusion of a dispersive element in the optical circuit allows for incorporating new functionalities. In particular, measuring wavelength shiftings in narrow band reflectors by detecting the differential group delay of modulated waves. The second refers to the theoretical and experimental study of frequency combs generated by amplified fiber loops that include a frequency shifting loop, and that allow for generating a wide group of optical waveforms of interest in, among other fields, metrology applications. On the one hand, the publications related to DI-OFDR have consisted in: the development of an interrogation method of fiber Bragg gratings (FBG) by a dual-wavelength source; the minimization of the number of interrogation points in frequency in DI-OFDR systems adapted to a topology of equally-spaced reflectors, as well as the development of a method based only in power measurements; and the implementation of a system with electro-optic downconversion and detection at intermediate frequency for conducting reflectometric measurements either single-point or distributed and, in this case, with wavelength selectivity. Among other advantages, these systems have allowed for interrogating FBGs with picometric resolution and high power efficiency, interrogating FBG arrays with speeds up to 10 μs per sensing element, and detecting discrete reflective events with reflectivities up to 90 dB and Rayleigh backscattering in C band in standard monomode fiber; respectively. On the other hand, the publications related to FSLs have been focused, respectively, on the application of the chirped optical waveforms generated by these devices for distance measuring with millimetric resolution by digital pulse compression, taking advantage of its high time-bandwidth product ( 200), and the theoretical description of these waveforms. Using a description of the field generated by the FSLs based on an analogy with diffractive optics, new properties of the chirped pulses generated by FSLs have been described. In particular, the existence of phase capture between different pulses, the presence of chirp linearity deviations, and the experimental verification of the coincidence between the Talbot phases generated in FSLs and the Gauss perfect phase sequences. The results of this second part of the thesis show, besides a good agreement between the theoretical model and the measured pulses, the feasibility of using FSLs for laser dis- tance measuring with digital pulse compression, obtaining compression and repetition rates of 150 and 80 MHz, respectively, and a 20 GHz bandwidth

Review of Fiber Optic Sensors for Structural Fire Engineering

Reliable and accurate measurements of temperature and strain in structures subjected to fire can be difficult to obtain using traditional sensing technologies based on electrical signals. Fiber optic sensors, which are based on light signals, solve many of the problems of monitoring structures in high temperature environments; however, they present their own challenges. This paper, which is intended for structural engineers new to fiber optic sensors, reviews various fiber optic sensors that have been used to make measurements in structure fires, including the sensing principles, fabrication, key characteristics, and recently-reported applications. Three categories of fiber optic sensors are reviewed: Grating-based sensors, interferometer sensors, and distributed sensors