Experimental demonstration of a Brillouin optical frequency-domain reflectometry (BOFDR) sensor

To measure the strain and/or temperature along an optical fiber with one-end access, a Brillouin optical frequency-domain reflectometry (BOFDR) technique is presented in this paper. It is based on detecting the spontaneous Brillouin scattering from a sinusoidally modulated pump light. Compared to the Brillouin optical frequency-domain analysis (BOFDA), this new BOFDR sensor approach presents the advantage that the measurements are free from the distorting components related to acoustic wave modulation, thus simplifying the associated data processing.This work has been supported by the projects MIUR-PON03PE_00155_1-OPTOFER and MIUR-PON03PE_00171_1-GEOGRID of the Italian government, and by the project TEC2013-47264-C2-1-R and TEC2016-76021-C2-2-R of the Spanish government

Review: optical fiber sensors for civil engineering applications

Optical fiber sensor (OFS) technologies have developed rapidly over the last few decades, and various types of OFS have found practical applications in the field of civil engineering. In this paper, which is resulting from the work of the RILEM technical committee “Optical fiber sensors for civil engineering applications”, different kinds of sensing techniques, including change of light intensity, interferometry, fiber Bragg grating, adsorption measurement and distributed sensing, are briefly reviewed to introduce the basic sensing principles. Then, the applications of OFS in highway structures, building structures, geotechnical structures, pipelines as well as cables monitoring are described, with focus on sensor design, installation technique and sensor performance. It is believed that the State-of-the-Art review is helpful to engineers considering the use of OFS in their projects, and can facilitate the wider application of OFS technologies in construction industry

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

Time expansion in distributed optical fiber sensing

The work of MRFR and HFM was supported by the MCIN/AEI/10.13039/501100011033 and European Union NextGenerationEU»/PRTR under grants RYC2021-032167-I and RYC2021- 035009-I. The work of MSA and VD was supported by MCIN/AEI/10.13039/ 501100011033 and the FSE invierte en tu futuro under grants PRE-2019- 087444 and RYC-2017-23668, respectively.Distributed optical fiber sensing (DOFS) technology has recently experienced an impressive growth in various fields including security, structural monitoring and seismology, among others. This expansion has been accompanied by a speedy development of the technology in the last couple of decades, reaching remarkable performance in terms of sensitivity, range, number of independent sensing points and affordable cost per monitored point as compared with competing technologies such as electrical or point optical sensors. Phase-sensitive Optical Time-Domain Reflectometry (ϕOTDR) is a particularly interesting DOFS technique, since it enables real-time monitoring of dynamic variations of physical parameters over a large number of sensing points. Compared to their frequency-domain counterparts (OFDR), ϕOTDR sensors typically provide higher dynamics and longer ranges but significantly worse spatial resolutions. Very recently, a novel ϕOTDR approach has been introduced, which covers an existing gap between the long range and fast response of ϕOTDR and the high spatial resolution of OFDR. This technique, termed time-expanded (TE) ϕOTDR, exploits an interferometric scheme that employs two mutually coherent optical frequency combs. In TE-ϕOTDR, a probe comb is launched into the fiber under test. The beating of the backscattered light and a suitable LO comb produces a multi-heterodyne detection process that compresses the spectrum of the probe comb, in turn expanding the detected optical traces in the time-domain. This approach has allowed sensing using ϕOTDR technology with very high resolution (in the cm scale), while requiring outstandingly low detection and acquisition bandwidths (sub-MHz). In this work, we review the fundamentals of TE-ϕOTDR technology and describe the recent developments, focusing on the attainable sensing performance, the existing trade-offs and open working lines of this novel sensing approach.Comunidad de MadridMinisterio de Ciencia e InnovaciónAgencia Estatal de InvestigaciónGeneralitat ValencianaUniversitat Jaume IEuropean Commissio

Distributed fiber optic sensors for monitoring spatially continuous strain and quasi-distributed refractive index using optical frequency domain reflectometry

This thesis is comprised of two papers and they describe distributed fiber optic sensing using an optical frequency domain reflectometry (OFDR) instrumentation system. The first paper presents about distributed fiber optic sensor embedded within the layers of a composite laminates to monitor the continuous profile of strain using the optical frequency domain reflectometry (OFDR) system. The OFDR system was used to analyze the Rayleigh backscattered signal. The shift in the Rayleigh backscattered spectrum (RBS) was observed to be linearly related to the change in strain of the composite material. The continuous strain sensing using OFDR was demonstrated by an INSTRON tensile testing system and cantilever beam experiment. The results shows a good strain transfer between the composite laminate and the optical fiber with no slipping or hysteresis issues. The second paper of the thesis proposes and demonstrates the distributed refractive index sensing by a macrobending single mode fiber (SMF) and the OFDR system. The macro-bending fiber is fabricated by bending a piece of SMF to a particular radius of curvature in several millimeters. The refractive index (RI) of the external medium surrounding the macrobending fiber is measured by the RBS shift using OFDR system. RI is measured from the range of 1.3348 to 1.3557 using the proposed method in our experiment. This sensor can also be used to detect multipoint RIs simultaneously verifying the capability of distributed sensing --Abstract, page iv

Time-expanded phase-sensitive optical time-domain reflectometry

Phase-sensitive optical time-domain reflectometry (ΦOTDR) is a well-established technique that provides spatio-temporal measurements of an environmental variable in real time. This unique capability is being leveraged in an ever-increasing number of applications, from energy transportation or civil security to seismology. To date, a wide number of different approaches have been implemented, providing a plethora of options in terms of performance (resolution, acquisition bandwidth, sensitivity or range). However, to achieve high spatial resolutions, detection bandwidths in the GHz range are typically required, substantially increasing the system cost and complexity. Here, we present a novel ΦOTDR approach that allows a customized time expansion of the received optical traces. Hence, the presented technique reaches cm-scale spatial resolutions over 1 km while requiring a remarkably low detection bandwidth in the MHz regime. This approach relies on the use of dual-comb spectrometry to interrogate the fibre and sample the backscattered light. Random phase-spectral coding is applied to the employed combs to maximize the signal-to-noise ratio of the sensing scheme. A comparison of the proposed method with alternative approaches aimed at similar operation features is provided, along with a thorough analysis of the new trade-offs. Our results demonstrate a radically novel high-resolution ΦOTDR scheme, which could promote new applications in metrology, borehole monitoring or aerospace

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

Real time dynamic strain monitoring of optical links using the backreflection of live PSK data

A major cause of faults in optical communication links is related to unintentional third party intrusions (normally related to civil/agricultural works) causing fiber breaks or cable damage. These intrusions could be anticipated and avoided by monitoring the dynamic strain recorded along the cable. In this work, a novel technique is proposed to implement realtime distributed strain sensing in parallel with an operating optical communication channel. The technique relies on monitoring the Rayleigh backscattered light from optical communication data transmitted using standard modulation formats. The system is treated as a phase-sensitive OTDR (ΦOTDR) using random and non-periodical non-return-to-zero (NRZ) phase-shift keying (PSK) pulse coding. An I/Q detection unit allows for a full (amplitude, phase and polarization) characterization of the backscattered optical signal, thus achieving a fully linear system in terms of ΦOTDR trace coding/decoding. The technique can be used with different modulation formats, and operation using 4 Gbaud single-polarization dual PSK and 4 Gbaud dual-polarization quadrature PSK is demonstrated. As a proof of concept, distributed sensing of dynamic strain with a sampling of 125 kHz and a spatial resolution of 2.5 cm (set by the bit size) over 500 m is demonstrated for applied sinusoidal strain signals of 500 Hz. The limitations and possibilities for improvement of the technique are also discussed.European CommissionMinisterio de Economía y CompetitividadComunidad de Madri