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In this paper, a method for detection of crack locations and the width of basin insulators is proposed. Based on the optical frequency domain reflectometry (OFDR) system, the system utilizes an FBG with high feedback for strain as well as temperature, which is affixed to the surface of the tub insulator, and a common single-mode optical fiber, which is used for transmitting data and connected to the optical backscattering reflectometry interrogator. The interrogator measures the backscattered light from the FBG, which varies with temperature or strain. The method has been used to measure the location and width of several different cracks and can locate the crack position with a spatial resolution of 1 mm and measure the crack width with a resolution of 0.77 mm. The method has been used to measure the position and width of insulators. This method provides a simple and fast approach to crack detection in insulators. [ABSTRACT FROM AUTHOR]

In this review, we summarize the latest advances in the design of optical frequency-domain reflectometers (OFDRs), digital signal processing, and sensors based on special optical fibers. We discuss state-of-the-art approaches to improving metrological characteristics, such as spatial resolution, SNR, dynamic range, and the accuracy of determining back reflection coefficients. We also analyze the latest achievements in the OFDR-based sensors: the accuracy of spatial localization of the impact, the error in detecting temperatures, deformation, and other quantities, and the features of separate measurement of various physical quantities. We also pay attention to the trend of mutual integration of frequency-domain optical reflectometry methods with time-domain optical reflectometry, which provides completely new sensing possibilities. We believe that this review may be useful to engineers and scientists focused on developing a lab setup, complete measurement instrument, or sensing system with specific requirements. [ABSTRACT FROM AUTHOR]

In the fields of structural health monitoring, civil engineering safety monitoring, aerospace and other fields, the key challenge is to increase the range of strain measurements to solve with the information monitoring of large strains. Reducing the strain sensitivity of the sensor is an important way to solve the problem of small sensor ranges. In this paper, the inclined transfer structure is proposed to achieve large strain distributed optical fiber sensing. When the fiber is axially stretched directly to fracture, the inclined transfer structure is used to increase the strain measurement range of the system. The theoretical relationship between the strain monitored by single mode fiber sensors and the matrix strain is established. It is experimentally verified that the inclined transfer structure using polypropylene material as the matrix achieves the strain measurement from 36667μ휀 at sensing spatial resolution of 8mm, 10nm wavelength tuning range of the laser source and 45∘ inclined angle, obtaining a good linear fit of more than 0.99. One hour stability of less than 11pm is obtained, corresponding to the strain of 9μ휀. [ABSTRACT FROM AUTHOR]

The precise evaluation of micro-crack sizes and locations is crucial for the safe operation of structures. Traditional detection techniques, however, suffer from low spatial resolution, making it difficult to accurately locate micrometer-scale cracks. A method and experimental study were proposed in this paper for identifying and locating micro-cracks using optical fiber strain sensing based on OFDR to address this issue. The feasibility of this method for micro-crack detection was verified by the combination of a polyimide-coated sensing optical fiber (PISOF) and tight sheath sensing optical fiber (TSSOF). A calculation method for micro-crack widths based on distributed optical fiber strain curves was established, and the test results of different optical fibers were compared. Through multiple verification experiments, it was found that the strain peak curves of both fiber types could accurately locate micro-cracks with a precision of 1 mm. Additionally, the crack widths could be obtained by processing the distributed strain curves using a computational model, enabling the accurate capture of micro-crack characteristics at the 10 μm level. A strong linear relationship was observed between the optical fiber stretching length and the crack width. Notably, the relative error in calculating the crack width from the strain curve of PI fiber was very small, while a linear relationship existed between the maximum strain value of the TSSOF and the crack width, allowing for the calculation of the crack width based on the maximum strain value. This further validated the feasibility of the method designed in this paper for the analysis of micro-crack characteristic parameters. [ABSTRACT FROM AUTHOR]

As the basic application of fiber optic sensing technology, strain measurement accuracy as a key index needs to be further calibrated and analyzed. In this paper, accuracy calibration experiments and the related analyses of two fiber-optic sensing technologies, the fiber-optic grating (FBG) and optical frequency domain reflectometry (OFDR), are carried out using a standard beam of equal strength and a mature resistive strain gauge (ESG). The fiber-optic single-point strain data for loading and unloading changes of the beams of equal strength show good continuity and linearity, with good cyclic stability, and the error in the strain test data is less than 2% after repeated loading. At the same time, using finite element theory to analyze the data and using the measured data error within 5%, a good strain test curve linearity is achieved and R2 is better than 0.998. After repeated loading and unloading tests, it is verified that the fiber grating and the distributed optical fiber in the strain test have good stability in repeatability accuracy. The calibration experiments and data analysis in this paper further illustrate the three sensing technologies in determining the strain test accuracy and the advantages and disadvantages of the indicators, and the development of the fiber optic sensing technology application provides basic technical support. [ABSTRACT FROM AUTHOR]

In order to study the fiber optical strain characterization method of the overlying rock structure during mining in a certain mine in northern Shannxi, based on the engineering geological conditions of 12217 working face in Buertai Mine, the strain monitoring principle of OFDR ( Optical Frequency Domain Reflection Technology ) was analyzed. Based on the partition of mining-induced overburden movement state and the principle of distributed optical fiber strain monitoring, a mechanical analysis model of horizontal optical fiber strain to characterize the rotation angle of mining-induced overburden hinged structure was established. The functional relationship between the peak width of strain and the rotation angle of mining-induced overburden hinged structure formed after rock strata were broken was deduced, and the concept of critical rotation angle of hinged structure was defined. Based on the strain characteristics of optical fiber and the rotation angle of hinged structure of mining overburden rock, the criterion of three-zone division of overburden rock is proposed, and the strain characteristics of mining overburden rock are analyzed by means of optical fiber strain characterization method. The results show that the different deformation degree of the three zones of mining overburden is proportional to the rotation angle of the hinged structure, so it is feasible to divide the three zones by the rotation angle of the hinged structure. The rotation angle of the hinged structure is calculated by the peak strain characteristics of the horizontal optical fiber. Compared with the close-range photogrammetry angle of the two-dimensional physical similarity model test, the average error between the two is less than 1°, which verifies the effectiveness of the mechanical model of the proposed optical fiber strain to characterize the rotation angle of the hinged structure of the mining overburden. The height of the caving zone and the height of the fracture zone obtained by the two-dimensional physical similarity model are consistent with the field results of the working face and the theoretical calculation values, which are in line with the actual situation of the mine. The research results provide a new research idea for the application of distributed optical fiber monitoring technology to characterize the deformation characteristics of mining overburden.

The light scattering phenomenon in optical fibres has been implemented in several applications for years. Nowadays, commercial distributed sensing systems for temperature measurements - distributed temperature sensing (DTS) - are gaining more attention and are being applied in many fields. The DTS systems are commonly based on optical time domain reflectometry or optical frequency domain reflectometry analysis approaches. This paper presents the simulation model and primary experimental results of a novel approach for data analysis for DTS systems based on the measurement of the backscattered Raman anti-Stokes radiation: the optical transient-state domain reflectometry. The novelty of this approach is the fast and accurate proposed analysis algorithm, as well as the cost-effective, secure and reliable set-up, whose main aspect is the implementation of long, low-power input laser pulses. [ABSTRACT FROM AUTHOR]

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Accurate measurement of interferometer arm-length difference is very important in the application fields such as fiber optic hydrophones. However, the interferometer is very sensitive to the environmental vibration and noise, which affects the accuracy of the interferometer measurement. A novel environmental random phase noise separation (ERPNS) technology is proposed to effectively reduce optical frequency domain reflectometer (OFDR)’s sensitivity to environmental vibration for the measurement of the fiber interferometer. A set of optical paths based on dual-laser coherent detection technology and the related ERPNS algorithm was developed to extract the environmental phase noise from the measurement data. The OFDR's sensitivity to vibration is decreased by ∼60 dB in theory and a measurement accuracy better than 0.05 mm for the static arm length difference of 74.51902 m in different dynamic environment is achieved experimentally, with 56 μm of spatial resolution in the maximum detectable distance of about 75 m.

In this paper, a method for detection of crack locations and the width of basin insulators is proposed. Based on the optical frequency domain reflectometry (OFDR) system, the system utilizes an FBG with high feedback for strain as well as temperature, which is affixed to the surface of the tub insulator, and a common single-mode optical fiber, which is used for transmitting data and connected to the optical backscattering reflectometry interrogator. The interrogator measures the backscattered light from the FBG, which varies with temperature or strain. The method has been used to measure the location and width of several different cracks and can locate the crack position with a spatial resolution of 1 mm and measure the crack width with a resolution of 0.77 mm. The method has been used to measure the position and width of insulators. This method provides a simple and fast approach to crack detection in insulators.

We describe a method for reducing the cost of optical frequency domain reflectometer (OFDR) hardware by replacing two reference channels, including an auxiliary interferometer and a gas cell, with a single channel. To extract useful information, digital signal processing methods were used: digital frequency filtering, as well as empirical mode decomposition. It is shown that the presented method helps to avoid the use of an unnecessary analog-to-digital converter and photodetector, while the OFDR trace is restored by the equal frequency resampling (EFR) algorithm without loss of high resolution and with good measurement repeatability. [ABSTRACT FROM AUTHOR]

The precise evaluation of micro-crack sizes and locations is crucial for the safe operation of structures. Traditional detection techniques, however, suffer from low spatial resolution, making it difficult to accurately locate micrometer-scale cracks. A method and experimental study were proposed in this paper for identifying and locating micro-cracks using optical fiber strain sensing based on OFDR to address this issue. The feasibility of this method for micro-crack detection was verified by the combination of a polyimide-coated sensing optical fiber (PISOF) and tight sheath sensing optical fiber (TSSOF). A calculation method for micro-crack widths based on distributed optical fiber strain curves was established, and the test results of different optical fibers were compared. Through multiple verification experiments, it was found that the strain peak curves of both fiber types could accurately locate micro-cracks with a precision of 1 mm. Additionally, the crack widths could be obtained by processing the distributed strain curves using a computational model, enabling the accurate capture of micro-crack characteristics at the 10 μm level. A strong linear relationship was observed between the optical fiber stretching length and the crack width. Notably, the relative error in calculating the crack width from the strain curve of PI fiber was very small, while a linear relationship existed between the maximum strain value of the TSSOF and the crack width, allowing for the calculation of the crack width based on the maximum strain value. This further validated the feasibility of the method designed in this paper for the analysis of micro-crack characteristic parameters.

In this review, we summarize the latest advances in the design of optical frequency-domain reflectometers (OFDRs), digital signal processing, and sensors based on special optical fibers. We discuss state-of-the-art approaches to improving metrological characteristics, such as spatial resolution, SNR, dynamic range, and the accuracy of determining back reflection coefficients. We also analyze the latest achievements in the OFDR-based sensors: the accuracy of spatial localization of the impact, the error in detecting temperatures, deformation, and other quantities, and the features of separate measurement of various physical quantities. We also pay attention to the trend of mutual integration of frequency-domain optical reflectometry methods with time-domain optical reflectometry, which provides completely new sensing possibilities. We believe that this review may be useful to engineers and scientists focused on developing a lab setup, complete measurement instrument, or sensing system with specific requirements.

As the basic application of fiber optic sensing technology, strain measurement accuracy as a key index needs to be further calibrated and analyzed. In this paper, accuracy calibration experiments and the related analyses of two fiber-optic sensing technologies, the fiber-optic grating (FBG) and optical frequency domain reflectometry (OFDR), are carried out using a standard beam of equal strength and a mature resistive strain gauge (ESG). The fiber-optic single-point strain data for loading and unloading changes of the beams of equal strength show good continuity and linearity, with good cyclic stability, and the error in the strain test data is less than 2% after repeated loading. At the same time, using finite element theory to analyze the data and using the measured data error within 5%, a good strain test curve linearity is achieved and R2 is better than 0.998. After repeated loading and unloading tests, it is verified that the fiber grating and the distributed optical fiber in the strain test have good stability in repeatability accuracy. The calibration experiments and data analysis in this paper further illustrate the three sensing technologies in determining the strain test accuracy and the advantages and disadvantages of the indicators, and the development of the fiber optic sensing technology application provides basic technical support.

Subjected to the relentless impacts of typhoons and rough seas, offshore wind turbines' structures, particularly the tower, foundation, and blade, are at constant risk of damage. Full-field strain monitoring helps to discover potential structural defects, thereby reducing disasters caused by overall structural failure. This study introduces a novel method for assessing strain and temperature fields on these kinds of 3D surfaces of cylindrical structures. The method harnesses the capabilities of a high spatial resolution (0.65 mm) Optical Frequency Domain Reflectometer (OFDR)-based Distributed Optical Fiber Sensor (DOFS) in conjunction with a unique helical wiring layout. The core process begins with mapping the fiber optic path onto a plane corresponding to the unfolded cylinder. Fiber optic signals are then differentiated on this plane, deriving a two-dimensional strain distribution. The plane strain field is subsequently projected onto the 3D side of the cylinder. An experiment was carried out in which a 3.5 m long optical fiber was helically wound with a 10 mm pitch on the surface of a cantilever beam of a cylinder shell with a diameter of 36 mm and a length of 300 mm. The experiment collected about 5400 measurement points on the cylindrical surface of 340 cm2, approximately 15.9 measurement points per square centimeter. The reconstructed results successfully reveal the strain field of the pipe cantilever beam under bending and torsional loads, as well as the palm-shaped temperature field. This experimental validation of the method's efficacy lays the theoretical groundwork for its application to real wind turbines. [ABSTRACT FROM AUTHOR]

The magnetic spectrometer AMS-100, which includes a superconducting coil, is designed to measure cosmic rays and detect cosmic antimatter in space. This extreme environment requires a suitable sensing solution to monitor critical changes in the structure such as the beginning of a quench in the superconducting coil. Rayleigh-scattering-based distributed optical fibre sensors (DOFS) fulfil the high requirements for these extreme conditions but require precise calibration of the temperature and strain coefficients of the optical fibre. Therefore, the fibre-dependent strain and temperature coefficients K T and K ϵ for the temperature range from 77 K to 353 K were investigated in this study. The fibre was integrated into an aluminium tensile test sample with well-calibrated strain gauges to determine the fibre's K ϵ independently of its Young's modulus. Simulations were used to validate that the strain caused by changes in temperature or mechanical conditions was the same in the optical fibre as in the aluminium test sample. The results indicated a linear temperature dependence of K ϵ and a non-linear temperature dependence of K T . With the parameters presented in this work, it was possible to accurately determine the strain or temperature of an aluminium structure over the entire temperature range from 77 K to 353 K using the DOFS. [ABSTRACT FROM AUTHOR]

We describe a method for reducing the cost of optical frequency domain reflectometer (OFDR) hardware by replacing two reference channels, including an auxiliary interferometer and a gas cell, with a single channel. To extract useful information, digital signal processing methods were used: digital frequency filtering, as well as empirical mode decomposition. It is shown that the presented method helps to avoid the use of an unnecessary analog-to-digital converter and photodetector, while the OFDR trace is restored by the equal frequency resampling (EFR) algorithm without loss of high resolution and with good measurement repeatability.

Distributed fiber optic sensors (DFOS) have become a new method for continuously monitoring infrastructure status. However, the fiber's fragility and the installation's complexity are some of the main drawbacks of this monitoring approach. This paper aims to overcome this limitation by embedding a fiber optic sensor into a textile for a faster and easier installation process. To demonstrate its feasibility, the smart textile was installed on a pedestrian bridge at the University of Massachusetts Lowell. In addition, dynamic strain data were collected for two different years (2021 and 2022) using Optical Frequency Domain Reflectometry (OFDR) and compared, to determine the variability of the data after one year of installation. We determined that no significant change was observed in the response pattern, and the difference between the amplitude of both datasets was 14% (one person jumping on the bridge) and 43% (two people jumping) at the first frequency band. This result shows the proposed system's functionality after one year of installation, as well as its potential use for traffic monitoring. [ABSTRACT FROM AUTHOR]

The paper reports a machine learning approach for estimating the phase in a distributed acoustic sensor implemented using optical frequency domain reflectometry, with enhanced robustness at the fading points. A neural network configuration was trained using a simulated set of optical signals that were modeled after the Rayleigh scattering pattern of a perturbed fiber. Firstly, the performance of the network was verified using another set of numerically generated scattering profiles to compare the achieved accuracy levels with the standard homodyne detection method. Then, the proposed method was tested on real experimental measurements, which indicated a detection improvement of at least 5.1 dB with respect to the standard approach. [ABSTRACT FROM AUTHOR]

• Self-monitoring CFRP cables were developed with rods embedded with optical fiber based on OFDR. • The accuracy, resolution, and precision of cable full-length strain distribution were determined. • The cable force distribution based on the partial self-monitoring CFRP rod is achieved. • The rod fracture damages of CFRP cable were localized and quantified. Lightweight, high-strength, corrosion-resistant, and fatigue-resistant carbon fiber reinforced polymer composite (CFRP) cables for cable-stayed or suspension bridges have been considered an excellent alternative to traditional steel cables. In the present paper, optical frequency domain reflectometry (OFDR) technology was applied to realize self-monitoring of the full-length strain and damage of CFRP rod-cable. The full-length axial tensile strain distribution of a 35 m CFRP cable was verified with the tensile test. The strain accuracy was verified to be 1.61 %, with a spatial resolution of ≤ 10 mm and a strain precision of 3.92 με throughout the full-length strain distribution. In comparison to the actual cable force, the cable force distribution, which is based on partial self-monitoring CFRP rods, demonstrates a deviation of approximately 1.6 %. The rod fracture damages in the CFRP cable were identified using wavelet smoothing, resulting in a localization error of ≤ 0.645 m and a quantification error of less than 15 %. According to the present study, methods to determine the cable force distribution and fracture damage for a CFRP cable were proposed. [ABSTRACT FROM AUTHOR]

• The maximum measurable strain by the proposed method has an order of magnitude increase compared to conventional method. • The experimental results demonstrate that the proposed processing method exhibits high stability and excellent repeatability. • The proposed method has the advantage of simple algorithm and easy implementation. Compared with the time-domain distributed optical fiber sensing technology, optical frequency domain reflectometry (OFDR) has the advantages of high spatial resolution and low detection optical power requirements. However, in the large strain measurement, OFDR encountered such a problem: the correlation of the Rayleigh scattering spectra between strain signals and reference signals is relatively low if the loading strain is too large, resulting in incorrect demodulation. To address this issue, we propose a new method called the spectral shift adjacent point difference method. In our approach, the cross-correlation operations using two sets of non-strain signals are performed, and the average value of the cross-correlation's spectral shift is selected as a reference standard. Then the spectral shift of the entire sensing fiber is obtained by re-sweeping frequency when the strain is loaded along the sensing fiber. The data points with a spectral shift adjacent point difference less than the reference standard will be retained, while other data points with spectral shift adjacent point difference greater than the reference standard will be considered as noisy fake spikes and removed. Finally, interpolation fitting is performed on the retained data points along the entire sensing fiber to obtain the demodulated spectral shifts realize large strain measurement. The experimental results show that the maximum measurable strain by the proposed method can reach 10,800 µε with a spatial resolution of 7.84 mm, which is an order of magnitude higher than the conventional method. The experiment demonstrates the high stability and excellent repeatability of the proposed processing method. The proposed method has the advantage of simple algorithm and easy implementation, and making it more competent in the field of large strain measurement. [ABSTRACT FROM AUTHOR]

Subjected to the relentless impacts of typhoons and rough seas, offshore wind turbines’ structures, particularly the tower, foundation, and blade, are at constant risk of damage. Full-field strain monitoring helps to discover potential structural defects, thereby reducing disasters caused by overall structural failure. This study introduces a novel method for assessing strain and temperature fields on these kinds of 3D surfaces of cylindrical structures. The method harnesses the capabilities of a high spatial resolution (0.65 mm) Optical Frequency Domain Reflectometer (OFDR)-based Distributed Optical Fiber Sensor (DOFS) in conjunction with a unique helical wiring layout. The core process begins with mapping the fiber optic path onto a plane corresponding to the unfolded cylinder. Fiber optic signals are then differentiated on this plane, deriving a two-dimensional strain distribution. The plane strain field is subsequently projected onto the 3D side of the cylinder. An experiment was carried out in which a 3.5 m long optical fiber was helically wound with a 10 mm pitch on the surface of a cantilever beam of a cylinder shell with a diameter of 36 mm and a length of 300 mm. The experiment collected about 5400 measurement points on the cylindrical surface of 340 cm2, approximately 15.9 measurement points per square centimeter. The reconstructed results successfully reveal the strain field of the pipe cantilever beam under bending and torsional loads, as well as the palm-shaped temperature field. This experimental validation of the method’s efficacy lays the theoretical groundwork for its application to real wind turbines.

The magnetic spectrometer AMS-100, which includes a superconducting coil, is designed to measure cosmic rays and detect cosmic antimatter in space. This extreme environment requires a suitable sensing solution to monitor critical changes in the structure such as the beginning of a quench in the superconducting coil. Rayleigh-scattering-based distributed optical fibre sensors (DOFS) fulfil the high requirements for these extreme conditions but require precise calibration of the temperature and strain coefficients of the optical fibre. Therefore, the fibre-dependent strain and temperature coefficients KT and Kϵ for the temperature range from 77 K to 353 K were investigated in this study. The fibre was integrated into an aluminium tensile test sample with well-calibrated strain gauges to determine the fibre’s Kϵ independently of its Young’s modulus. Simulations were used to validate that the strain caused by changes in temperature or mechanical conditions was the same in the optical fibre as in the aluminium test sample. The results indicated a linear temperature dependence of Kϵ and a non-linear temperature dependence of KT. With the parameters presented in this work, it was possible to accurately determine the strain or temperature of an aluminium structure over the entire temperature range from 77 K to 353 K using the DOFS.

In this paper, we report the development of a frequency-shifted (FS) terahertz (THz) wave source for the non-destructive inspection of buildings. Currently, terahertz-time domain spectroscopy (THz-TDS) is the mainstream method for non-destructive inspection using THz waves. However, THz-TDS is limited by its measurement range and difficulties encountered when there is a strong frequency dependence in the absorption characteristics and refractive index of the measurement target. To address these issues, we developed a novel non-destructive approach for inspection applications using frequency-shifted THz waves. Our system uses a frequency-shifted feedback (FSF) laser as the pump light source to generate FS-THz waves; this allowed us to obtain precise distance measurements of objects over a broad range of distances. We tested a prototype FS-THz system and confirmed successful measurement of spatial distances inside a building material. [ABSTRACT FROM AUTHOR]

Internal thrust force of unstable rock and soil mass is an essential parameter for prediction of many geological hazard. Currently, fiber bragg grating (FBG) and optical time-domain reflectometer (OTDR) are widely used to measure internal stress of unstable rock and soil mass. However, these methods have disadvantages such as low spatial resolution and the paucity of distributed measurements. This paper develops a quasi-distributed thrust measurement system based on an optical frequency domain reflectometer (OFDR). Firstly, we design an optical fiber stress sensor head using the characteristics of the optical fiber microbending effect. And then, the cubic spline interpolation method is used to compensate for the nonlinear effects of the OFDR. Finally, we implement a laboratory experiment of lateral stress to make error calibration. As a result, the OFDR sensing system achieved a spatial resolution of 20 cm by using a 500 m test fiber, maximum measurement pressure reached 1.059 MPa and relative error is 8.9%. We implemented OFDR in the Chenjiagou landslide located at the Three-Gorge of Chongqing in China. The results showed that this system can accurately locate six fiber stress sensors within the landslide over a range of 0 ∼ 420 m, obtaining the lateral thrusts as well.

Fiber-optic sensing (FOS) provides distributed strain measurement that can enhance both structural health monitoring (SHM) and laboratory testing capabilities. In particular, optical frequency domain reflectometry (OFDR) provides strain or temperature measurements every millimeter over tens of meters of fiber, with a high sampling rate. For RC applications with embedded fiber-optic cables, the sensitivity of the fibers and their ability to measure and survive cracking are both important considerations. In this study, tests were conducted on six RC specimens to investigate the effectiveness of using OFDR strain sensing to evaluate the cracking behavior of concrete and the deformation of steel reinforcing bars. Six types of fiber-optic cables with very different structures, sensitivity, and survivability were tested. Fibers were placed in the concrete and in grooves in the reinforcing bars. A new deconvolution method was developed; using the method, crack widths were accurately measured up to the target load levels using both the more sensitive fiber-optic cables and cables with reduced sensitivity but better survivability. OFDR strain sensing, combined with new methods of data processing, was shown to be capable of detecting distributed microcracking and providing reliable crack widths. The OFDR technique was also used to reveal the location-dependent bond-slip relationships between the concrete and rebar at early stages. [ABSTRACT FROM AUTHOR]

Distributed fiber optic sensors (DFOS) have become a new method for continuously monitoring infrastructure status. However, the fiber’s fragility and the installation’s complexity are some of the main drawbacks of this monitoring approach. This paper aims to overcome this limitation by embedding a fiber optic sensor into a textile for a faster and easier installation process. To demonstrate its feasibility, the smart textile was installed on a pedestrian bridge at the University of Massachusetts Lowell. In addition, dynamic strain data were collected for two different years (2021 and 2022) using Optical Frequency Domain Reflectometry (OFDR) and compared, to determine the variability of the data after one year of installation. We determined that no significant change was observed in the response pattern, and the difference between the amplitude of both datasets was 14% (one person jumping on the bridge) and 43% (two people jumping) at the first frequency band. This result shows the proposed system’s functionality after one year of installation, as well as its potential use for traffic monitoring.

The paper reports a machine learning approach for estimating the phase in a distributed acoustic sensor implemented using optical frequency domain reflectometry, with enhanced robustness at the fading points. A neural network configuration was trained using a simulated set of optical signals that were modeled after the Rayleigh scattering pattern of a perturbed fiber. Firstly, the performance of the network was verified using another set of numerically generated scattering profiles to compare the achieved accuracy levels with the standard homodyne detection method. Then, the proposed method was tested on real experimental measurements, which indicated a detection improvement of at least 5.1 dB with respect to the standard approach.

This work is devoted to the study of the pyroelectric effect on the properties of optical waveguides formed in a lithium niobate crystal by proton exchange. In the present work, we studied the cessation effect of the radiation channeling during thermocycling of Y-splitters samples. We examined the spectral dependence of optical losses on the wavelength using an optical spectrum analyzer. The results demonstrate that in the range of 1530–1570 nm, all wavelengths are suppressed equally. The optical frequency domain reflectometry shows that the increase of optical losses is observed along the entire waveguide, but not only at the Y-splitting point, as supposed earlier. [ABSTRACT FROM AUTHOR]

The strain transfer between fiber-optic cable and soil plays a critical role in the deformation characteristics of a cable–soil interface. Existing findings cannot provide a clear understanding of the effects of key influencing factors including horizontal confining pressures, anchorage property of cables, and saturation of soils on the strain transfer and shear characteristics at the interface. A group of pullout tests of cables in soil were conducted to examine the strain transfer efficiency using the optical frequency domain reflectometry (OFDR) technique. Two kinds of cables were pulled out from sandy soil and sand–gravel–clay mixtures under the confining pressures of 0–1.2 MPa. Typical strain-hardening behavior was observed for cables with confining pressure and anchorage, and the interface shear strength could not be evaluated within the strain measurement range. To address this problem, the so-called interface shear coefficient was adopted, and the key influential factors were discussed quantitatively. The interface shear coefficient keeps a linear relationship with the confining pressure, and that of the anchored cable in saturated backfill mixtures is 2–3 times than that of the unanchored cable. These findings will guide the methods of gaining reliable data for revealing the failure mechanism of geostructures via distributed strain sensing. [ABSTRACT FROM AUTHOR]

Timely detection of the incipient fault of a power transformer is of utmost importance to prevent potential malfunctioning. Monitoring the condition of the transformer insulation oil is the essential step as it contains most of the transformer’s health diagnostic information. Thus, the temperature of the oil is considered the most crucial parameter that has to be monitored continuously in real-time. Distributed optical fiber sensors for temperature monitoring has various advantages over traditional methods. In this study, an optical fiber-based distributed temperature measurement of insulation oil of a fully energized 100 kVA commercial distribution transformer is demonstrated. Two types of techniques were deployed to monitor the temperature of the transformer inside and outside of the tank using a distributed Optical Frequency Domain Reflectometry (OFDR) and a quasi-distributed Fiber Bragg Grating (FBG) sensor array respectively. The insulation oil temperature was monitored with different load conditions in a distributed fashion and compared with the conventional method of single point thermocouple and infrared thermal imaging. The test results show very good agreement between the conventional methods and proposed distributed fiber temperature sensors. [ABSTRACT FROM AUTHOR]

• The OFDR-based inclination sensor with temperature self-compensation is designed. • Several sensors sharing a single optical channel can be installed in a foundation pile. • Horizontal displacement of deep foundation pile can be well measured by the sensors. Horizontal displacement measurement of foundation piles is an important guarantee to ensure the safety of foundation pit excavation. In order to solve the problems of limited number of FBG(fiber Bragg grating)s in series on a single fiber and low accuracy of distributed optical fiber Brillouin strain measurement in the horizontal displacement measurement of deep foundation piles, this study developed an OFDR (optical frequency domain reflectometry)-based inclination sensor with temperature self-compensation and a deformation measurement method for inclinometer pipe based on this sensor which was validated by modeling test. Finally, a case study of horizontal displacement monitoring of foundation piles in deep foundation pit excavation was conducted. The experimental and on-site monitoring data show that the OFDR-based inclination sensors have good sensing characteristics and several OFDR-based inclination sensors installed in one inclinometer pipe share a single optical channel, which can effectively measure the continuous horizontal displacement of the foundation pile. [ABSTRACT FROM AUTHOR]

High-power fiber lasers have been widely used in various industrial manufacturing and military defense applications. During the development of fiber lasers in the past decades, the thermal effect has always been one of the biggest obstacles. It is crucial to study the temperature characteristics and overcome the thermal restrictions for a better output performance. With the systematic design and optimization in this article, optical frequency domain reflectometry (OFDR) can achieve in situ distributed temperature measurement of the fiber core in high-power fiber lasers. This allows a better study of the temperature characteristics and thermal effects. The fiber-core distributed temperature of a kilowatt-level fiber oscillator is first demonstrated based on this method here. The splicing point between the high reflectivity fiber Bragg grating (HR-FBG) and gain fiber withstands the highest temperature, reaching 101.6 °C at a 1.47 kW output. In addition, the temperature of the gain fiber gradually decreases from 92 °C to 30 °C along the pumping direction. The internal temperatures of the combiner and HR-FBG are also measured to evaluate their performances in the high-power regime. The temperature distributions in the experiment agree well with the theoretical simulation. [ABSTRACT FROM AUTHOR]

The paper discusses a new method that combines frequency and phase domains-gated reflectometry. This approach can be applied to a fiber with artificial reflectors to measure the absolute values of optical paths between reflection points in the fiber in the full range of signal frequencies, including zero frequency.

In this paper, we describe a distributed pressure sensor using a simplified and high sensitive microstructured optical fiber – the embedded-core fiber – and autocorrelation analysis of the data obtained from an optical frequency domain reflectometer (OFDR). The special fiber consists of a microcapillary, which has a germanium-doped core placed within the capillary wall. When this structure is subjected to pressure variations, an asymmetric stress distribution is induced within the fiber, entailing birefringence variations. Here, we show that the use of a commercial OFDR with submillimeter resolution together with an autocorrelation data analysis routine in the spectral domain allows mapping the fiber birefringence and performing distributed pressure sensing. In the experiments, the fiber passed through two pressure chambers, which allowed for pressure to be locally and independently applied. No cross-sensitivity between the pressure points was observed. The embedded-core fiber exhibited a birefringence sensitivity to pressure of $3.8\times 10^{\mathbf {-7}}$ bar $^{\mathbf {-1}}$ , which is 21% higher than that measured in commercial photonic crystal fibers using the same setup. [ABSTRACT FROM AUTHOR]

In the concept of the disposal of High Level radioactive Waste (HLW) in Japan, bentonite material such as the engineered barrier system is planned to backfill around over-pack in the disposal tunnel. For the barrier with sufficient performance, this bentonite density in this tunnel is very important factor to control the quality of the performance. In this study, the mixture of the technology between an actively heated system and a Distributed Temperature Sensing (DTS) using fiber-optic cable was applied with the aim to measure the density of the bentonite as this performance in order to take advantage of the character such as each density of bentonite under same water content has different heat conductivity. Specifically, to define the density of bentonite, the heat conductivity in an unsteady state continuously was measured. Moreover, we performed to measure a density using Optic-fiber cable to improve the measurement performance instead of the existing measurement sensor. 1) High resolution measuring a temperature system which has multi-point measurement 2) The usage of reference box which have some fixed density of bentonite in the container beforehand and connect them on the measurement same line of fiber-optic cable. Before the In-situ performance, we conducted this system at our laboratory in order to check the handling of a measurement procedure and improve the evaluation method about the temperature data.

In this paper, we report the development of a frequency-shifted (FS) terahertz (THz) wave source for the non-destructive inspection of buildings. Currently, terahertz-time domain spectroscopy (THz-TDS) is the mainstream method for non-destructive inspection using THz waves. However, THz-TDS is limited by its measurement range and difficulties encountered when there is a strong frequency dependence in the absorption characteristics and refractive index of the measurement target. To address these issues, we developed a novel non-destructive approach for inspection applications using frequency-shifted THz waves. Our system uses a frequency-shifted feedback (FSF) laser as the pump light source to generate FS-THz waves; this allowed us to obtain precise distance measurements of objects over a broad range of distances. We tested a prototype FS-THz system and confirmed successful measurement of spatial distances inside a building material.

A new hardware configuration to correct nonlinear tuning affects in optical frequency domain reflectometry (OFDR) is proposed. Unlike conventional correction methods, the new configuration does not require a separate auxiliary interferometer detection, which may eliminate the need for an extra detection device and an acquisition channel. In the proposed method, an intentional beat signal is introduced at the beginning of the OFDR spectrum, which is treated as an auxiliary interferometer to acquire tunable laser phase information for the nonlinear tuning correction. The proposed method may reduce the overall system cost, reduce the data acquisition time and computational load, and make system configuration simpler by eliminating the need for extra components. In this article, the feasibility of the proposed configuration is demonstrated by correcting nonlinear tuning affects using simulations and experiments in a ∼50 m long optical fiber. Distributed strain and temperature sensing are also performed in a 50 m optical fiber with the proposed new configuration. [ABSTRACT FROM AUTHOR]

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This work is devoted to the study of the pyroelectric effect on the properties of optical waveguides formed in a lithium niobate crystal by proton exchange. In the present work, we studied the cessation effect of the radiation channeling during thermocycling of Y-splitters samples. We examined the spectral dependence of optical losses on the wavelength using an optical spectrum analyzer. The results demonstrate that in the range of 1530–1570 nm, all wavelengths are suppressed equally. The optical frequency domain reflectometry shows that the increase of optical losses is observed along the entire waveguide, but not only at the Y-splitting point, as supposed earlier.

As a newly developed pile foundation, the snowflake shaped steel sheet pile is composed of three Y-shaped sections with an included angle of 120° and has a large specific surface area, which can give full play to the side friction of pile and improve the bearing capacity of single pile. At the same time, the snowflake shaped steel sheet pile has a high strength, relatively few materials, and it has good prospects with engineering applications. In order to accurately grasp the mechanical characteristics of the snowflake shaped steel sheet pile, this paper carried out the model test of snowflake shaped steel sheet pile based on OFDR (optical frequency domain reflector) distributed optical fiber sensor technology. The results show that: (1) OFDR distributed optical fiber sensing technology can effectively monitor the strain of snowflake steel sheet pile; (2) under the vertical load, the strain of snowflake steel sheet pile decreases along the length of the pile; (3) the strain of the same section of snowflake steel sheet pile is different at different positions, the strain at the junction between web and web is basically the same as the junction between web and flange, and the strain of the pile shaft on the flange edge is larger.

Automated methods for detecting defects within composite materials are highly desirable in the drive to increase throughput, optimise repair program effectiveness and reduce component replacement. Tap-testing has traditionally been used for detecting defects but does not provide quantitative measurements, requiring secondary techniques such as ultrasound to certify components. This paper reports on an evaluation of the use of a distributed temperature measurement system—high-definition fibre optic sensing (HD-FOS)—to identify and characterise crushed core and disbond defects in carbon fibre reinforced polymer (CFRP)-skin, aluminium-core, sandwich panels. The objective is to identify these defects in a sandwich panel by measuring the heat transfer through the panel thickness. A heater mat is used to rapidly increase the temperature of the panel with the HD-FOS sensor positioned on the top surface, measuring temperature. HD-FOS measurements are made using the Luna optical distributed sensor interrogator (ODISI) 9100 system comprising a sensor fabricated using standard single mode fibre (SMF)-20 of external diameter 250 μm, including the cladding. Results show that areas in which defects are present modulate thermal conductivity, resulting in a lower surface temperature. The resultant data are analysed to identify the length, width and type of defect. The non-invasive technique is amenable to application in challenging operational settings, offering high-resolution visualisation and defect classification.

Random fiber gratings (RFGs) have shown great potential applications in fiber sensing and random fiber lasers. However, a quantitative relationship between the degree of randomness of the RFG and its spectral response has never been analyzed. In this paper, two RFGs with different degrees of randomness are first characterized experimentally by optical frequency domain reflectometry (OFDR). Experimental results show that the high degree of randomness leads to low backscattering strength of the grating and strong strength fluctuations in the spatial domain. The local spectral response of the grating exhibits multiple peaks and a large peak wavelength variation range when its degree of randomness is high. The linewidth of its fine spectrum structures shows scaling behavior with the grating length. In order to find a quantitative relationship between the degree of randomness and spectrum property of RFG, entropy was introduced to describe the degree of randomness induced by period variation of the sub-grating. Simulation results showed that the average reflectivity of the RFG in dB scale decreased linearly with increased sub-grating entropy, when the measured wavelength range was smaller than the peak wavelength variation range of the sub-grating. The peak reflectivity of the RFG was determined by κ2LΔP (where κ is the coupling coefficient, L is the grating length, ΔP is period variation range of the sub-grating) rather than κL when ΔP is larger than 8 nm in the spatial domain. The experimental results agree well with the simulation results, which helps to optimize the RFG manufacturing processes for future applications in random fiber lasers and sensors.

Optical fiber distributed sensing that is based on optical frequency domain reflectometer (OFDR) is a promising technology for achieving a highest spatial resolution downwards to several millimeters. An OFDR signal demodulation method that is based on Morlet wavelet transformation (WT) is demonstrated in detail to improve the resolution of distributed sensing physical quantity under a high spatial resolution, aiming at the trade-off between spatial and spectrum resolution. The spectrum resolution, spatial interval of the measured gauges, and spatial resolution can be manually controlled by adjusting the wavelet parameters. The experimental results that were achieved by the wavelet transformation (WT) method are compared with these by short time Fourier transformation (STFT) method and they indicate that significant improvements, such as strain resolution of 1 με, spatial resolution of 5 mm, average repeatability of 4.3 με, and stability of 7.3 με within one hour, have been achieved. The advantages of this method are high spatial and spectral resolution, robust, and applicability with current OFDR systems.

Pipeline corrosion is an important issue that threatens the pipeline safety operation; therefore, the corrosion monitoring is essential. Based on the optical frequency domain reflectometry technology, which features distributed strain measurement with high resolution and high precision, a new method of pipeline internal corrosion monitoring is proposed to simultaneously locate corrosion and to evaluate corrosion severity. To verify the effectiveness and accuracy of this method, a series of tests were conducted including uniform corrosion test and local corrosion tests. These test results demonstrate that the corrosion location and corrosion severity evaluation can be achieved via the proposed distributed strain measurement, providing a valuable approach for pipeline corrosion monitoring. [ABSTRACT FROM AUTHOR]