Search

We demonstrate the use of a graded-index perfluorinated optical fiber (GI-POF) for distributed static and dynamic strain measurements based on Rayleigh scattering. The system is based on an amplitude-based phase-sensitive Optical Time-Domain Reflectometry (ϕ-OTDR) configuration, operated at the unconventional wavelength of 850 nm. Static strain measurements have been carried out at a spatial resolution of 4 m and for a strain up to 3.5% by exploiting the increase of the backscatter Rayleigh coefficient consequent to the application of a tensile strain, while vibration/acoustic measurements have been demonstrated for a sampling frequency up to 833 Hz by exploiting the vibration-induced changes in the backscatter Rayleigh intensity time-domain traces arising from coherent interference within the pulse. The reported tests demonstrate that polymer optical fibers can be used for cost-effective multiparameter sensing.

Optical Frequency Domain Reflectometry (OFDR) is used to make temperature distributed sensing measurements along a fiber by exploiting Rayleigh backscattering. This technique presents high spatial and high temperature resolutions on temperature ranges of several hundred of degrees Celsius. With standard telecommunications fibers, measurement errors coming from the correlation between a high temperature Rayleigh trace and the one taken as a reference at room temperature could be present at extremely high temperatures. These correlation errors, due to low backscattering signal amplitude and unstable backscattering signal, induce temperature measurement errors. Thus, for high temperature measurement ranges and at extremely high temperatures (e.g., at 800 °C), a known solution is to use fibers with femtosecond laser inscribed nanograting. These fs-laser-insolated fibers have a high amplitude and thermally stable scattering signal, and they exhibit lower correlation errors. In this article, temperature sensing at 800 °C is reported by using an annealed zirconia-doped optical fiber with an initial 40.5-dB enhanced scattering signal. The zirconia-doped fiber presents initially OFDR losses of 2.8 dB/m and low OFDR signal drift at 800 °C. The ZrO2-doped fiber is an alternative to nanograting-inscribed fiber to make OFDR distributed fiber sensing on several meters with gauge lengths of 1 cm at high temperatures.

The present work is a comprehensive collection of recently published research articles on Structural Health Monitoring (SHM) campaigns performed by means of Distributed Optical Fiber Sensors (DOFS). The latter are cutting-edge strain, temperature and vibration monitoring tools with a large potential pool, namely their minimal intrusiveness, accuracy, ease of deployment and more. Its most state-of-the-art feature, though, is the ability to perform measurements with very small spatial resolutions (as small as 0.63 mm). This review article intends to introduce, inform and advise the readers on various DOFS deployment methodologies for the assessment of the residual ability of a structure to continue serving its intended purpose. By collecting in a single place these recent efforts, advancements and findings, the authors intend to contribute to the goal of collective growth towards an efficient SHM. The current work is structured in a manner that allows for the single consultation of any specific DOFS application field, i.e., laboratory experimentation, the built environment (bridges, buildings, roads, etc.), geotechnical constructions, tunnels, pipelines and wind turbines. Beforehand, a brief section was constructed around the recent progress on the study of the strain transfer mechanisms occurring in the multi-layered sensing system inherent to any DOFS deployment (different kinds of fiber claddings, coatings and bonding adhesives). Finally, a section is also dedicated to ideas and concepts for those novel DOFS applications which may very well represent the future of SHM.

Marble quarries are quite dangerous environments in which rock falls may occur. As many workers operate in these sites, it is necessary to deal with the matter of safety at work, checking and monitoring the stability conditions of the rock mass. In this paper, some results of an innovative analysis method are shown. It is based on the combination of Distributed Optical Fiber Sensors (DOFS), digital photogrammetry through Unmanned Aerial Vehicle (UAV), topographic, and geotechnical monitoring systems. Although DOFS are currently widely used for studying infrastructures, buildings and landslides, their use in rock marble quarries represents an element of peculiarity. The complex morphologies and the intense temperature range that characterize this environment make this application original. The selected test site is the Lorano open pit which is located in the Apuan Alps (Italy), here, a monitoring system consisting of extensometers, crackmeters, clinometers and a Robotic Total Station has been operating since 2012. From DOFS measurements, strain and temperature values were obtained and validated with displacement data from topographic and geotechnical instruments. These results may provide useful fundamental indications about the rock mass stability for the safety at work and the long-term planning of mining activities.

Over the past three decades, extensive research activity on Brillouin scattering-based distributed optical fiber sensors has led to the availability of commercial instruments capable of measuring the static temperature/strain distribution over kilometer distances and with high spatial resolution, with applications typically covering structural and environmental monitoring. At the same time, the interest in dynamic measurements has rapidly grown due to the relevant number of applications which could benefit from this technology, including structural analysis for defect identification, vibration detection, railway traffic monitoring, shock events detection, and so on. In this paper, we present an overview of the recent advances in Brillouin-based distributed optical fiber sensors for dynamic sensing. The aspects of the Brillouin scattering process relevant in distributed dynamic measurements are analyzed, and the different techniques are compared in terms of performance and hardware complexity.

This study focuses on the experimental characterization of the bond behavior between concrete and Fiber Reinforced Polymer (FRP) reinforcing bars (rebars). Pull-out tests were performed on glass, carbon, and aramid FRP rebars, as well as on deformed steel rebars. The influence of various parameters on the bond behavior was studied, such as the type of fibers, the diameter of the FRP bars and their surface geometry. Scanning-Electron-Microscope (SEM) observations were performed to precisely study the sand coating characteristics of these rebars. A main originality of the proposed approach relied on the instrumentation of pull-out samples using Distributed Optical Fiber Sensing (DOFS) instrumentation. Such a distributed measurement system provided access to the longitudinal strain distribution along the rebar near the rebar-concrete interface, and then made it possible to determine the effective development length of the various types of rebars considered in this study. As the introduction of DOFS instrumentation may be intrusive, its influence on the interface behavior was also discussed.

This document showcases the latest research conducted within UPC–BarcelonaTech on the performance of distributed optical fiber sensors (DOFS), more specifically the case of the optical backscattered reflectometry (OBR) system, in the structural health monitoring (SHM) of bridges, and large scale structures. This technology has demonstrated promising results for monitoring applications in a wide range of fields but due to its novelty, still presents several uncertainties which prevent its use in a more systematic and efficient way in civil engineering infrastructures, being this even more evident in the case of concrete structures. Therefore, different laboratory experimental campaigns were devised where multiple aspects of the instrumentation of DOFS technology in civil engineering applications were assessed and scrutinized. Such as the study of new implementation methods, comparison, and performance analysis of different bonding adhesives and spatial resolution. Additionally, the fatigue performance of this sensing typology was also assessed. Furthermore, the use of the OBR system technology was applied in a real world structure in Barcelona, Spain, where new challenging conditions had to be addressed. Consequently, with this work, different conclusions are obtained related to the proficiency and limitations on the use of this particular type of optical sensing system in concrete structures.

This is the accepted version of the following article: [Barrias, A, Casas, JR, Villalba, S. Distributed optical fibre sensors in concrete structures: Performance of bonding adhesives and influence of spatial resolution. Struct Control Health Monit. 2019; 26:e2310. https://doi.org/10.1002/stc.2310], which has been published in final form at https://onlinelibrary.wiley.com/doi/abs/10.1002/stc.2310 In this paper, the authors conducted an experiment where a reinforced concrete beam was instrumented with a 5-m long polyimide single distributed optical fibre sensors (DOFS) performing four equal segments externally bonded to the bottom surface of the element, using for each segment a different type of adhesive. Three strain gauges were also used for comparison purposes. This beam was then loaded, producing expected equal levels of strain in each of the fibre segments for a more direct comparison of the different adhesives performance. The effect of alternating the spatial resolution is also analysed. In this exercise, additionally to the comparison with the other instrumented sensors, it is also important the consideration and analysis of the associated spectral shift quality (SSQ) values of the DOFS measurements.

7th European Workshop on Optical Fibre Sensors (EWOFS'2019), 01/10/2019-04/10/2019, Limassol, Chipre., The detection and quantification of the presence of certain chemical species is of central importance regarding permanent structural health monitoring of key industrial fields and civil infrastructures such as oil extraction boreholes or radioactive waste repositories, where H2 is released. With this work we propose and test a competitive technique able to measure the concentration of hydrogen and deuterium thanks to their diffusion into the silica glass of a standard optical fiber, already employed for the distributed monitoring of large infrastructures. The proposed technique, based on Chirped-Pulse Phasesensitive Reflectometry (CP-φOTDR), could represent a novel solution for this problem, thanks to its ability to provide dynamical measurements of refractive index change, with great linearity and sensitivities of 10-8 refractive index units, featuring spatial resolutions of a few meters and kilometric sensing ranges., European Commission, Ministerio de Economía y Competitividad, Comunidad de Madrid, Agencia Estatal de Investigación

For some infrastructures like oil and gas extraction boreholes or radioactive waste repositories, where distributed optical fiber sensors are employed to grant the safety of the facilities, the presence of gas species such as hydrogen or deuterium is one more relevant parameter to monitor. The possibility of employing the same kind of sensors for this purpose is of special interest, reducing the cost by employing a single interrogator, able to measure more than one parameter by simply employing an adequate sensing fiber. To meet this goal, we present here a sensor based on Chirped Pulse Phase sensitive Optical Time Domain Reflectometry, which is able to detect these species while they diffuse into the silica fiber core. Its ability to measure the induced change of its refractive index with a sensitivity around 10 8 has allowed determining hydrogen concentration in the silica core with precision in the order of 10 3 mol/m3 and spatial resolution ~6 m, while also providing an indirect measurement of the solubility of deuterium in a standard telecom-grade optical fiber., European Commission, Ministerio de Economía y Competitividad, Comunidad de Madrid, Agencia Estatal de Investigación

Prior research has demonstrated that distributed optical fiber sensors (DOFS) based on Rayleigh scattering can be embedded in carbon fiber/epoxy composite structures to rapidly detect temperature changes approaching 1000 &deg, C, such as would be experienced during a high energy laser strike. However, composite structures often experience mechanical strains that are also detected during DOFS interrogation. Hence, the combined temperature and strain response in the composite can interfere with rapid detection and measurement of a localized thermal impulse. In this research, initial testing has demonstrated the simultaneous response of the DOFS to both temperature and strain. An embedded DOFS network was designed and used to isolate and measure a localized thermal response of a carbon fiber/epoxy composite to a low energy laser strike under cyclic bending strain. The sensor interrogation scheme uses a simple signal processing technique to enhance the thermal response, while mitigating the strain response due to bending. While our ultimate goal is rapid detection of directed energy on the surface of the composite, the technique could be generalized to structural health monitoring of temperature sensitive components or smart structures.

Distributed optical fiber sensors (DOFS) offer unprecedented features, the most unique one of which is the ability of monitoring variations of the physical and chemical parameters with spatial continuity along the fiber. Among all these distributed sensing techniques, optical frequency domain reflectometry (OFDR) has been given tremendous attention because of its high spatial resolution and large dynamic range. In addition, DOFS based on OFDR have been used to sense many parameters. In this review, we will survey the key technologies for improving sensing range, spatial resolution and sensing performance in DOFS based on OFDR. We also introduce the sensing mechanisms and the applications of DOFS based on OFDR including strain, stress, vibration, temperature, 3D shape, flow, refractive index, magnetic field, radiation, gas and so on.

In this paper, an experiment where distributed optical fiber sensors (DOFS) were implemented in two small concrete beams subjected to a three-point load test is outlined. Here, an optical backscatter reflectometry based DOFS is implemented simultaneously embedded in the concrete (glued to the steel rebar) and attached to the outer surface of the concrete after its hardening. For comparison purposes, three electrical strain gauges are also used in the rebar. The main objectives with this experiment, is to analyze the feasibility of installation of DOFS directly on the rebar element of a reinforced concrete beam and compare the measured strain at rebar and surface of the concrete.

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.

25th International Conference on Optical Fiber Sensors, 2017, Jeju, Corea del Sur., In this communication, the effect of the probe pulse shape on the backscattered power trace of phase-sensitive (φ-) OTDR is analyzed. In particular, the power traces obtained with rectangular and Gaussian-like probe pulses are compared, both numerically and experimentally. Our analysis reveals that the use of Gaussian-like pulses can increase the operation range and sensitivity of φOTDR-based sensors in at least two-fold as compared with that obtained from traditionally-employed rectangular-like pulses., European Commission, Ministerio de Economía y Competitividad, Comunidad de Madrid

Brillouin optical correlation-domain technologies are reviewed as “fiber optic nerve systems” for the health monitoring of large structures such as buildings, bridges, and aircraft bodies. The Brillouin scattering property is used as a sensing mechanism for strain and/or temperature. Continuous lightwaves are used in the technologies, and their optical coherence properties are synthesized to realize position-selective measurement. This coherence manipulation technology is called the “synthesis of optical coherence function (SOCF)”. By utilizing SOCF technologies, stimulated Brillouin scattering is generated position-selectively along the fiber, which is named “Brillouin optical correlation domain analysis (BOCDA)”. Spontaneous Brillouin scattering, which takes place at any portion along the fiber, can also be measured position-selectively by the SOCF technology. This is called “Brillouin optical correlation domain reflectometry (BOCDR)”. When we use pulsed lightwaves that have the position information, sensing performances, such as the spatial resolution, are inherently restricted due to the Brillouin scattering nature. However, in the correlation-domain technologies, such difficulties can be reduced. Superior performances have been demonstrated as distribution-sensing mechanisms, such as a 1.6-mm high spatial resolution, a fast measurement speed of 5000 points/s, and a 7000-με strain dynamic range, individually. The total performance of the technologies is also discussed in this paper. A significant feature of the technologies is their random accessibility to discrete multiple points that are selected arbitrarily along the fiber, which is not realized by the time domain pulsed-lightwave technologies. Discriminative and distributed strain/temperature measurements have also been realized using both the BOCDA technology and Brillouin dynamic grating (BDG) phenomenon, which are associated with the stimulated Brillouin scattering process. In this paper, the principles, functions, and applications of the SOCF, BOCDA, BOCDR, and BDG-BOCDA systems are reviewed, and their historical aspects are also discussed.

Distributed Optical Fiber Sensors (DOFSs), thanks to their multiple sensing points, are ideal tools for the detection of deformations and cracking in reinforced concrete (RC) structures, crucial as a means to ensure the safety of infrastructures. Yet, beyond a certain point of most DOFS-monitored experimental tests, researchers have come across unrealistic readings of strain which prevent the extraction of further reliable data. The present paper outlines the results obtained through an experimental test aimed at inducing such anomalies to isolate and identify the physical cause of their origin. The understanding of such a phenomenon would enable DOFS to become a truly performant strain sensing technique. The test consists of gradually bending seven steel reinforcement bars with a bonded DOFS under different conditions such as different load types, bonding adhesives, bar sections and more. The results show the bonding adhesives having an influence on the DOFS performance but not on the rise of anomalies while the reasons triggering the latter are narrowed down from six to two, reaching a strain threshold and a change in structure’s deformative behavior. Further planned research will allow identification of the cause behind the rise of strain-reading anomalies., This article belongs to the Special Issue Bridge Structural Health Monitoring and Damage Identification, Financial support from the Research Council of Lithuania and the European Union Structural Fund (Project no.: 09.3.3-LMT-K-712-01-0145) and financial support provided by the European Union Horizon 2020 research and innovation program under the Marie Sklodowska-Curie grant agreement No. 642453.

In this paper, we analyze numerically the effect of phase noise from the laser in Brillouin optical time-domain analysis (BOTDA) sensors. Due to laser phase noise, the phase shift between pump and probe beams is a stochastic variable with zero mean and variance changing with the position along the fiber. The numerical results, carried out for various fiber lengths and pump pulse durations, show that laser phase noise induces a reduction of the average Brillouin gain, as well as an increase of the overall system noise. Preliminary experimental results, carried out by use of a conventional BOTDA system and two DFB diode lasers having different linewidth (63 and 900 kHz), support the numerical analysis.

Linear and nonlinear two-dimensional image processing approaches are analyzed with the aim of removing noise from data acquired by distributed optical fiber sensors based on Brillouin optical time-domain analysis (BOTDA). The impact of the filter parameters on the denoised data is analyzed, especially for the nonlinear image denoising method called non-local means (NLM). In particular, an optimization procedure to find the optimal parameters of the NLM method for BOTDA data denoising is proposed. The described optimization procedure has enabled, to the best of our knowledge, the first experimental demonstration of a conventional BOTDA scheme (i.e. with no modifications in the layout) capable of measuring along a 100 km sensing range over a 200 km fiber-loop, using a spatial resolution of 2 m, a frequency sampling step of 1 MHz and reaching a frequency uncertainty of 0.77 MHz with 2’000 averaged time-domain traces. The experimental sensing performance here achieved has been evaluated with a figure-of-merit (FoM) of 225’000. This is the highest FoM reached without hardware sophistication in BOTDA sensing.