Distributed Fiber Optic Sensors Research Articles

Using Global Existing Fiber Networks for Environmental Sensing

We review recent advances in distributed fiber optic sensing (DFOS) and their applications. The scattering mechanisms in glass, which are exploited for reflectometry-based DFOS, are Rayleigh, Brillouin, and Raman scatterings. These are sensitive to either strain and/or temperature, allowing optical fiber cables to monitor their ambient environment in addition to their conventional role as a medium for telecommunications. Recently, DFOS leveraged technologies developed for telecommunications, such as coherent detection, digital signal processing, coding, and spatial/frequency diversity, to achieve improved performance in terms of measurand resolution, reach, spatial resolution, and bandwidth. We review the theory and architecture of commonly used DFOS methods. We provide recent experimental and field trial results where DFOS was used in wide-ranging applications, such as geohazard monitoring, seismic monitoring, traffic monitoring, and infrastructure health monitoring. Events of interest often have unique signatures either in the spatial, temporal, frequency, or wavenumber domains. Based on the temperature and strain raw data obtained from DFOS, downstream postprocessing allows the detection, classification, and localization of events. Combining DFOS with machine learning methods, it is possible to realize complete sensor systems that are compact, low cost, and can operate in harsh environments and difficult-to-access locations, facilitating increased public safety and smarter cities.

Detection of Road Surface Anomaly Using Distributed Fiber Optic Sensing

Road surface condition can significantly impact the interaction between vehicles and pavement structure, which may even cause high fuel consumption and safety issues of drivers and vehicles. Distributed fiber optic sensing (DFOS) technology is a useful tool to perform continuous and real-time monitoring of traffic and road surface condition. However, it is challenging to process the data for the purpose of road anomaly detection. The study proposed two approaches to detect the road anomaly using DFOS. In the first method, local binary pattern (LBP) histograms were used to extract the features of the images with and without road anomaly, and support vector machine (SVM) combined with principal component analysis (PCA) was adopted as the classifier. The convolutional neural network (CNN) was applied on the binary classification data to analyze the images in the second method. The accuracy and benefits of two methodologies were compared. The vehicle speed was estimated by detecting lines using Hough transform. The feasibility of road anomaly detection using DFOS is proved.

Structural Deformation Reconstruction of Polymer Distributed Optical Fiber Sensing Tape Based on Ko Theory

The progress of material technology makes the structural deformation of composites more complex and more difficult to monitor. The distributed optical fiber sensor has a wide monitoring range and can continuously obtain the strain data of the structure in space for structural deformation reconstruction. In this paper, the optical frequency domain reflectometer (OFDR) and distributed optical fiber sensor are used as measurement tools to design the deformation reconstruction experiment of a cantilever beam with constant cross section. Based on the Ko theory, the strain signals collected by the distributed optical fiber sensors are converted into displacement signals, and the deformation reconstruction of the cantilever beam is carried out, and then compared and analyzed with the actual measured values. The experiment verifies the effectiveness of using distributed fiber optic sensors for cantilever beam bending deformation based on Ko theory, which provides a favorable reference for structural health monitoring.

Prediction Method of Steel Corrosion Rate Based on the Helix Distributed Sensor.

Corrosion of steel bars is of great significance for safety and service life of reinforced concrete structures. This work develops a prediction method for steel corrosion mass loss rate before the crack of concrete structure based on a spiral distributed fiber optic sensor. Reinforced concrete sample instrumented with a spiral distributed fiber optic sensor were prepared. The mathematic relationship between the corrosion mass loss rate of steel bar and the spiral distributed strain is theoretically derived. Meanwhile, numerical analysis by MATLAB shows that these parameters such as the protective layer thickness, corrosion mass loss rate, bar diameter, corrosion expansion coefficient have a remarkable influence on spiral distributed strain. Additionally, electrical accelerated corrosion experiment was performed on the reinforced concrete specimens. The helix strain along the distributed sensor was used to evaluate the corrosion mass loss of steel bar. Further, the influencing factors on the corrosion sensitivity are illustrated here and the corrosion mass loss rate before concrete crack is also quantified. This research provides insights into the corrosion deteriorate mechanism.

Semi-supervised health assessment of pipeline systems based on optical fiber monitoring

Health assessment of pipeline systems is of deep significance for improving pipeline reliability and integrity. Traditional health assessment methods may be difficult or costly to perform on pipeline systems due to the long distances and environmental constraints of pipelines. This paper incorporates the distributed optical fiber sensor (DOFS) technique and the semi-supervised learning algorithm into the pipeline health assessment framework. Three critical problems that limit the application of DOFS in pipeline health assessments are addressed. First, an applicable damage monitoring experiment of a pipeline system is designed, which is effective in obtaining the necessary base data for data-driven modeling. Second, the correspondence between the pipeline health status and the monitored strain features is established. The experimental data are shared for public research, which is expected to solve the problem of the lack of benchmark research data in related fields. Third, considering the scarcity of labeled degradation data in pipelines, a semi-supervised denoising autoencoder model is proposed specifically for pipeline health assessment. The proposed method is demonstrated and validated using a comparative experimental case study.

Application of Distributed Optical Fiber Sensing Technology to the Detection and Monitoring of Internal Swelling Pathologies in Massive Concrete Blocks.

This paper presents an experimental application of Distributed Optical Fiber Sensors (DOFS) for the Structural Health Monitoring (SHM) of concrete structures affected by internal swelling pathologies. In the framework of a large research project aiming to assess the possible extension of the operating lifetime of nuclear power plants from 40 to 60 years, massive blocks were cast from reactive concrete mixtures intended to develop delayed ettringite formation and alkali–silica reaction. These blocks were subjected to specific ageing conditions to initiate and accelerate the concrete pathologies. Some of the blocks were instrumented with DOFS bonded to the surface and embedded in the concrete. Using an interrogator device based on Rayleigh backscattering and a suitable procedure to eliminate temperature effects, distributed strain measurements were then performed at different time intervals. The first results of this ongoing study made it possible to demonstrate the feasibility and effectiveness of this sensing technology for detecting and monitoring expansion induced by swelling pathologies in representative-scale concrete structures.

Design of a distributed optical fiber sensor system for measuring immersed tunnel joint deformations

Monitoring the deformations of immersed tunnels is important during the entire tunnel service life to assess the structural integrity of the tunnel. Conventional joint deformation monitoring is based on manual levelling measurements and normally occurs only at multi-year intervals, which does not allow to capture short term deformation behavior. In this study a new joint monitoring system using distributed optical fiber sensors (DOFS) is developed. A special sensor layout is designed that allows simultaneous measurements of both horizontal joint opening and vertical uneven settlement of the immersion and dilation joints. For this sensor scheme the transfer relation from fiber strain to joint deformation is derived and verified by in-lab experiments. The sensor system proves to be able to detect sub-millimeter joint deformations, indicating a more than sufficient accuracy for structural monitoring of immersed tunnel joints. Subsequently, the First Heinenoordtunnel in the Netherlands is instrumented using this distributed optical fiber sensing system, in order to obtain additional data for both long-term and short-term assessment of its structural condition.

Spiral Deployment of Optical Fiber Sensors for Distributed Strain Measurement in Seven-Wire Twisted Steel Cables, Post-Tensioned against Precast Concrete Bars.

On-time monitoring and condition assessments of steel cables provide mission-critical data for informed decision making, ensuring the structural safety of post-tensioned concrete structures. This study aimed to develop a spiral deployment scheme of distributed fiber optic sensors (DFOS) and to monitor/assess the post-tensioned force in seven-wire twisted steel cables, based on the pulse-pre-pump Brillouin optical time domain analysis. Each DFOS was placed in a spiral shape between two surface wires of a steel cable and glued to the steel cable by epoxy. Image observations were conducted to investigate the entireness and bonding condition between the optical fiber and the steel wires. Eight concrete bar specimens were cast, each with a pre-embedded plastic or metal duct at its center and each was post-tensioned by a steel strand through the duct once they were instrumented with two strain and two temperature sensors. The strand was loaded/unloaded and monitored by measuring the Brillouin frequency shifts and correlating them with the applied strains and the resulting cable force after temperature compensation. The maximum, minimum, and average cable forces integrated from the measured stain data were compared and validated with those from a load cell. The maximum (or average) cable force was linearly related to the ground truth data with a less than 10% error between them, after any initial slack had been removed from the test setup. The post-tensioned force loss was bounded by approximately 30%, using the test setup designed in this study.

Analysis of bond behaviour of textile-reinforced cement (TRC) applied to masonry using distributed fibre optic sensors and acoustic emission

The global and local mechanical bond behaviours of textile-reinforced cementitious (TRC) matrix composites bonded to masonry elements were investigated in this study. Single-lap shear bond tests were performed for six different configurations: two matrices, two textiles, and two reinforcement ratios. These composites were instrumented with distributed fibre optic sensors (DFOS), acoustic emission (AE) sensors, and digital image correlation (DIC). The DFOS were used to measure the evolution of the local internal strains of the TRCs before, during, and after the appearance of cracks. The AE sensors were used to assess the damage mechanisms of the TRC bonded to the masonry. The crack widths were measured using DIC. Combining these measurement techniques facilitated the assessment of the effects of the matrix, textile, and reinforcement ratio on the effective bond length of the TRC-to-masonry prisms. The results obtained from the TRC-to-masonry shear bond tests were then compared with the results of the tensile tests of the TRC composite to highlight the effect of the boundary conditions on crack widths.

Experimental study on the deformation and failure mechanism of overburden rock during coal mining using a comprehensive intelligent sensing method

Understanding the spatiotemporal evolution of overburden deformation during coal mining is still a challenge in engineering practice due to the limitation of monitoring techniques. Taking the Yangliu Coal Mine as an example, a similarity model test was designed and conducted to investigate the deformation and failure mechanism of overlying rocks in this study. Distributed fiber optic sensing (DFOS), high-density electrical resistivity tomography (HD-ERT) and close-range photogrammetry (CRP) technologies were used in the test for comprehensive analyses. The combined use of the three methods facilitates the investigation of the spatiotemporal evolution characteristics of overburden deformation, showing that the mining-induced deformation of overburden strata was a dynamic evolution process. This process was accompanied by the formation, propagation, closure and redevelopment of separation cracks. Moreover, the key rock stratum with high strength and high-quality lithology played a crucial role in the whole process of overburden deformation. There were generally three failure modes of overburden rock layers, including bending and tension, overall shearing, and shearing and sliding. Shear failure often leads to overburden falling off in blocks, which poses a serious threat to mining safety. Therefore, real-time and accurate monitoring of overburden deformation is of great significance for the safe mining of underground coal seams.

Distributed strain measurements in thin expansive concrete slabs with biaxial textile reinforcement

AbstractThe objective of the paper is to analyze the shrinkage and expansion strain development in thin slabs made of expansive concrete and reinforced with carbon textile reinforcement. The symmetrical textile reinforcement grid provided a biaxial restraint for the concrete shrinkage and expansion. Strains of the slabs were measured with distributed fiber optic sensors (DFOS) in both directions so that a 2D visualization of their distribution can be presented and analyzed. Parallel, standard restrained expansion tests (RET) were conducted to assess the expansive concrete mixture and large‐scale beam specimens with uniaxial steel reinforcement were also equipped with DFOS and analyzed. This study aimed to compare the strains in uniaxially restrained elements with steel reinforcement and biaxially restrained textile reinforced concrete elements, in order to assess to what extent the results of the standard RET can be used for evaluation of textile reinforced concrete members.

A fully distributed fiber optic sensor for simultaneous relative humidity and temperature measurement with polyimide-coated polarization maintaining fiber

A distributed fiber-optic sensor for simultaneous relative humidity (RH) and temperature measurement is proposed and experimentally demonstrated by means of optical frequency domain reflectometry (OFDR) with single sensing fiber configuration. The sensing element is a high-birefringence polarization-maintaining (HB-PM) fiber with polyimide coating that transfers strains on the fiber after being expanded by water uptake with the ambient RH changes. Either humidity-variation-induced strain applied on the fiber or temperature changes contribute to distinct spectral responses in the cross-correlation and auto-correlation results of Rayleigh backscatter signals from the polyimide-coated HB-PM sensing fiber when interrogated by the OFDR technique. These spectral responses can be successfully extracted for every segment of the sensing fiber with a centimeter-order spatial resolution, enabling distributed temperature and RH discriminative measurement along the sensing fiber. Experimental results show that the optimal sensitivities for RH and temperature measurement can reach up to 1.8 pm/%RH and 11.1 pm/°C, respectively. Measurement errors for simultaneous RH and temperature sensing are found to be 0.54 °C and 2.75%RH, respectively. The proposed sensing technique will open a brand-new door for developing reliable and practical distributed RH and temperature discrimination sensing systems with high sensitivities and low errors.

Pressure test of a prestressed concrete cylinder pipe using distributed fiber optic sensors: Instrumentation and results

This paper presents an approach for measuring and visualizing the strain and loading responses of a pressurized prestressed concrete cylinder pipe (PCCP) using distributed fiber optic sensors based on optical frequency domain reflectometry (OFDR). The pipe has an inner diameter of 3400 mm and length of 5000 mm. The concrete core, steel cylinder, prestressing wires, and mortar coating of PCCP were instrumented with specific distributed fiber optical sensors that can also be installed in future field construction. This PCCP was tested under increasing internal water pressure until a large number of radial and longitudinal cracks appeared on the pipe surface. The crack measurements made by the distributed sensors are in good agreement with the actual observations and reflect the crack propagation with reasonable accuracy. This study demonstrates that distributed fiber optical sensors can be effectively installed in the PCCP production process, and the cracks on the concrete core and mortar coating can be monitored with a spatial resolution of 1 cm. It is expected that distributed sensing technology for structural health monitoring will be useful in new and existing pipeline structures.

Distributed Fiber Optic Sensing to Identify Locations of Resistive Transitions in REBCO Conductors and Magnets

High-temperature superconductors such as REBa <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7-x</sub> (REBCO, RE = rare earth) can generate strong magnetic fields that are promising for applications in particle accelerators and compact fusion reactors. Traditionally, voltage taps are installed in superconducting magnets to measure the voltage signals due to resistive transitions. The voltage-tap-based diagnostics is important for the development of magnet technology as it can help pinpoint the locations in the magnet windings that limit the magnet performance. The architecture of the multi-tape REBCO cable such as CORC wires, however, makes it difficult to apply the voltage-tap-based diagnostics to identify the locations of resistive transitions. Distributed fiber optic sensing (DFOS) has the potential to address this issue. In this paper, we report the measurements of thermal strain along a CORC wire based on optical frequency domain reflectometry with a maximum spatial resolution of 0.65 mm and a temporal resolution of 10 Hz. The optical fiber is co-wound with the CORC wire that is epoxy impregnated. During the test, current was increased until a resistive transition occurred in the conductor. The spectrum shift of the reflected light along the fiber was recorded. The results suggested that with proper thermal isolation from the cryogen, DFOS can be used to identify the locations of resistive transitions in CORC wires and magnets. The results will allow a better understanding of the causes of resistive transitions in REBCO conductors and magnets, which will help improve the REBCO magnet technology.

Novel Direct Downhole Enthalpy Estimation at Geothermal Well in Cubadak, West Sumatra, Indonesia

Temperature, pressure and enthalpy data are very important and valuable, both during geothermal drilling and in well operation. Fibre optic distributed temperature sensors (DTS) that offer flexible and robust solutions for geothermal applications and temperature logging, an established method for well temperature measurement. All temperature measurement methods will be combined with reservoir enthalpy estimation using T-p-x state space correlation for H2O-NaCl geothermal brine. A geothermal brine enthalpy estimation model has been developed, based on T-p-x state space correlation using C programming language. This was able to provide estimations of pressure, mass fraction, density and enthalpy, solely based on measured temperature limited to below 300oC. The model can be applied to various geothermal fields with sodium chloride dominated waters. The study treated the mixture of geothermal brine and drilling fluid heated in the well for certain periods as a simulation of geothermal brine at the reservoir. The enthalpy monitoring graph proved useful for monitoring the enthalpy in the reservoir. The results from the model are used for early electrical power estimations of the Cubadak geothermal working area site, based on current geothermal gradient extrapolation of CBD-1 well.

Monitoring of prestressing forces in cross-tensioned concrete pavements during construction and maintenance based on distributed optical fiber sensing

Accurate monitoring of prestressing force is crucial for cross-tensioned concrete pavements (CTCPs). This paper presents a method for monitoring prestressing forces in CTCPs based on distributed optical fiber sensing (DOFS). A prestressing force monitoring system was designed using finite element method–based validation and optimization. Laboratory tests were conducted to propose a data processing method to analyze the prestressing force and compensate for the temperature effect. The monitoring system's effectiveness was demonstrated by conducting field tests and using other conventional methods as a comparison. The results indicate that the system has sufficient accuracy (±10 kN), high efficiency (49 strands measured within 1 min), and superior robustness and reliability for monitoring prestressing forces in CTCPs. In addition, this monitoring method can be used not only for short-term variations induced by tensioning during the construction period but also for long-term variations caused by different ambient temperatures during the maintenance period.

Failure experiment and calculation model for prestressed concrete cylinder pipe under three-edge bearing test using distributed fiber optic sensors

A prestressed concrete cylinder pipe (PCCP) with quality defects or being subjected to overloads can crack in the inner concrete core, thereby the internal water acting directly on the steel cylinder and further weakening the bearing capacity of the pipe. This study aims to explore calculation methods of when and after a PCCP cracks through the comparative analysis between the test results measured from the three-edge bearing test (TEBT) and the calculation results through the improved procedures based on ANSI/AWWA C304. To this end, the full strain distribution around the circumference of the first pipe is measured until failure by distributed optical fiber sensors. The thrust and moment coefficients of loads under the TEBT are derived, and strains are solved considering the stress–strain relationships of the constituent materials. The first pipe cracks in the inner core at the crown and invert as the load reaches 280 kN/m, but the second pipe with the same parameters does not crack when the load arrives at 360 kN/m. The measured strains reveal that the inner side at the crown and invert and the outer side at the springline are subjected to tension. Before appearing the cracks in the inner concrete core, the calculated results are in good agreement with experimentally observed data. After cracking of the inner concrete core, both the vertical displacement at the crown and the fluctuation amplitude of test strains begin to accelerate. The results calculated by considering two empirical assumptions can reflect the trend of the test results. The proposed comparative analysis approach provides the cracking test load that meets the serviceability limit-state design criterion for quality inspection and helps designers a better understanding of design assumptions and parameters.

Rayleigh scattering based, thermal-induced displacement measurement along a steel plate at high temperature

This study aims to quantify Rayleigh scattering based measurement accuracy of distributed fiber optic sensors under a heating-holding load protocol and characterize the effect of polymer coating on the sensors as the polymer softens and melts away at elevated temperatures. Two segments of a coated single-mode optical fiber were loosely attached and firmly bonded to a steel plate, respectively. When locally heated up to 405 °C in a furnace, the two segments were used to measure temperature alone and thermal-induced strain. The axial displacement associated with the strain measurement was compared with that of a dial gauge. A temperature increment of 13 ​°C (≪ 20 ​°C) is recommended to ensure successful correlation analysis of Rayleigh scattering signals. The polymer was found to start softening at 155 °C and melting at 267 ​°C. As temperature sensors, optical fibers with an insulation sheath can accurately measure temperature untill 155 ​°C without and till 267 ​°C with thermo-mechanical analysis of sheath-fiber bonding behavior. As strain sensors, optical fibers with temperature compensation are accurate for strain measurement up to 155 ​°C, above which strain transfer analysis is required due to softening of the fiber coating. Under a large temperature gradient covering 155–267 °C, the fibers attached on steel plates with epoxy function like an extensometer as a center portion of the coating with the high-end temperature is melted as verified by microscopic analysis.

Quantifying thermal strain of steel plate subjected to constant temperature by distributed fiber optic sensors

Abstract Effective strain measurement tools for steel structure at high temperature are limited due to a significant gap in measurement science. This study aims to experimentally and numerically investigate the effectiveness and limitation of Rayleigh scattering based, distributed fiber optic sensors (DFOS) without coatings for measuring the temperature and strain of a steel plate subjected to a local constant temperature. The DFOS were bonded to the steel plate by an epoxy with different bond lengths to measure coupled strain and temperature effect, while the DFOS near the end of the epoxied segment measured the temperature effect only for temperature discrimination. It was found that the DFOS accurately measured the temperature and strain of the steel plate with different bond lengths of the epoxy, as compared to the thermocouple temperature and thermal-induced strain, respectively. The maximum strain (or temperature) that the DFOS without coatings could measure for the steel plate was less than 1600 $\mu \varepsilon$ (or 150${}^{\circ}\textrm{C}$). Moreover, a local finite element model with the calibrated elastic modulus of the epoxy subjected to a uniform temperature field well captured optical fiber strains in the elastic stage. From parametric studies, the effect of the thermal expansion coefficients and elastic moduli of the optical fiber, epoxy, and host material as well as any initial defect between the optical fiber and epoxy on the strain transfer coefficient was investigated. The elastic modulus of epoxy within 100 MPa and the rectangular cross-section of epoxy (0.5 mm thick and 4 mm wide) could achieve a strain transfer coefficient of 0.997, while the initial defect had a similar effect on the strain transfer to the protective coating. The normal-distribution epoxy shape was designed for guiding robot assisted intelligent instrumentation and construction in the future.

Mechanical properties of prestressed concrete cylinder pipe with broken wires using distributed fiber optic sensors

Wire breakage is a major factor that results in the failure of prestressed concrete cylinder pipes (PCCPs). In this study, a prototype test was conducted on a PCCP specimen to evaluate its mechanical properties and possible failure modes with broken wires. The concrete core, steel cylinder, prestressing wires, and mortar coating of the PCCP were installed with specific distributed fiber optic sensors to measure the strain response. Additionally, a three-dimensional finite element model of PCCP was established, and the strain responses of PCCP with broken wires in different regions were assessed. The results demonstrated that the mortar coating gradually cracked with an increase in the wire breakage ratio, leading to mortar delamination and relaxation of prestressing wires, severely affecting the performance of the PCCP. Furthermore, cracking and spalling of the inner and outer concrete cores were observed, followed by yielding and leakage of the steel cylinder. The maximum wire breakage ratio of the PCCP specimen was 19.8% under the design internal pressure, and the minimum value of the measured influence range of wire breakage was approximately 140° at the central angle. The influence range of wire breakage gradually increased with an increase in the number of broken wires.