Optical Frequency Domain Reflectometry Research Articles

Fatigue weld crack detection using distributed fiber optic strain sensing

We demonstrate the application of distributed fiber optic strain sensing based on optical frequency-domain reflectometry for early detection and location of fatigue cracks in welds in steel tubular test specimens. To do so, we have subjected welded tubular specimens instrumented with surface-mounted optical fiber sensors to resonant bending load, and we have measured the strain distributions in the test samples continuously and without any interruption of the test throughout its whole duration, with a 2.6 mm spatial resolution and a 1 με strain resolution. We show that the fatigue cracks, which initiate from the inner surface of the wall of the specimens, can be successfully detected and located in real time using our measurement method. We conclude that the detection of the crack initiation may provide relevant information serving the estimation of the remaining lifetime of the component.

OFDR analysis of Si photonics FMCW LiDAR chip.

We experimentally analyzed the internal reflection and loss of each component in a Si photonics frequency-modulated continuous-wave light detection and ranging (FMCW LiDAR) device using optical frequency domain reflectometry (OFDR) with a spatial resolution of better than 2.5 µm. Sweeping the incident laser wavelength by 120 nm, the reflections and losses of wire waveguides, widened waveguides, and optical switches on the chip were individually revealed. The slow-light grating (SLG) beam scanner, which has a limited working wavelength range, was evaluated with a spatial resolution of >10 µm by narrowing the wavelength sweep range. Consequently, a strong reflection was observed at the transition between the wire waveguide and the SLG, which can be a noise source in the FMCW LiDAR. Additionally, this study showed that the OFDR can be an important analysis tool for Si photonics integrated circuits. To our knowledge, this is the first demonstration, showing that the OFDR can be an important analysis tool for Si photonic integrated circuits.

Distributed Optical Fiber Sensing and Applications Based on Large-Scale Fiber Bragg Grating Array: Review

To achieve data-driven intelligence in engineering applications, the key requirements for distributed optical fiber sensor networks are large capacity, long distance, dense distribution, fast response, and high signal-to-noise ratio (SNR). Combining the traditional optical reflectometry techniques with the large-scale fiber Bragg grating (FBG) array fabricated on-line by a fiber drawing tower, multiplexing and demodulation of thousands of FBG sensors is successfully demonstrated, which greatly expands the application of FBG sensors to the field of distributed sensing. In this work, the fabrication, demodulation, and applications of large-scale FBG arrays are reviewed. Firstly, the on-line fabrication technology and process of large-scale FBG arrays are introduced. Secondly, the demodulation techniques including wavelength demodulation based on optical time-domain reflectometry (OTDR) and optical frequency-domain reflectometry (OFDR), as well as phase demodulation based on phase-sensitive OTDR are reviewed, where we mainly focus on the key parameters such as multiplexing capacity, demodulation speed, spatial resolution, and SNR. Last, the applications of the large-scale FBG arrays are briefly discussed.

Optical Frequency-Domain Reflectometry Based Distributed Temperature Sensing Using Rayleigh Backscattering Enhanced Fiber

An innovative optical frequency-domain reflectometry (OFDR)-based distributed temperature sensing method is proposed that utilizes a Rayleigh backscattering enhanced fiber (RBEF) as the sensing medium. The RBEF features randomly high backscattering points; the analysis of the fiber position shift of these points before and after the temperature change along the fiber is achieved using the sliding cross-correlation method. The fiber position and temperature variation can be accurately demodulated by calibrating the mathematical relationship between the high backscattering point position along the RBEF and the temperature variation. Experimental results reveal a linear relationship between temperature variation and the total position displacement of high backscattering points. The temperature sensing sensitivity coefficient is 7.814 μm/(m·°C), with an average relative error temperature measurement of −1.12% and positioning error as low as 0.02 m for the temperature-influenced fiber segment. In the proposed demodulation method, the spatial resolution of temperature sensing is determined by the distribution of high backscattering points. The temperature sensing resolution depends on the spatial resolution of the OFDR system and the length of the temperature-influenced fiber. With an OFDR system spatial resolution of 12.5 μm, the temperature sensing resolution reaches 0.418 °C per meter of RBEF under test.

Metaheuristic inverse analysis on interfacial mechanics of distributed fiber optic sensors undergoing interfacial debonding

Distributed fiber optic sensors with protective packages have shown unique capabilities in measuring strain and crack distributions for structural health monitoring. However, the mechanics at the fiber-package interface remain unclear when debonding occurs. This paper investigates the interfacial mechanics for distributed fiber optic sensors undergoing debonding through mechanical analysis and metaheuristic-based inverse analysis. First, the governing equation of the interfacial mechanics is established and solved with assistance from metaheuristic-based inverse analysis on the interfacial properties of fiber optic cables. Then, experiments were conducted to validate the analysis results by measuring the strain distributions in distributed fiber optic sensors based on optical frequency domain reflectometry. The results showed that the proposed approach accurately quantified the interfacial mechanics, interfacial properties, strain transfer, and debonding behavior in distributed fiber optic sensors. This research advances the fundamental understandings of the sensing mechanisms of distributed fiber optic sensors undergoing inelastic behaviors for structural health monitoring.

Improve accuracy and measurement range of sensing in km-level OFDR using spectral splicing method.

In this paper, we propose and demonstrate a spectral splicing method (SSM) for distributed strain sensing based on optical frequency domain reflectometry (OFDR), which can achieve km level measurement length, µɛ level measurement sensitivity and 104 µɛ level measurement range. Based on the traditional method of cross-correlation demodulation, the SSM replaces the original centralized data processing method with a segmented processing method and achieves precise splicing of the spectrum corresponding to each signal segment by spatial position correction, thus realizing strain demodulation. Segmentation effectively suppresses the phase noise accumulated in the large sweep range over long distances, expands the sweep range that can be processed from the nm level to the 10 nm level, and improves strain sensitivity. Meanwhile, the spatial position correction rectifys the position error in the spatial domain caused by segmentation, which reduces the error from the 10 m level to the mm level, enabling precise splicing of spectra and expanding the spectral range, thus extending the strain range. In our experiments, we achieved a strain sensitivity of ±3.2 µɛ (3σ) over a length of 1 km with a spatial resolution of 1 cm and extended the strain measurement range to 10,000 µɛ. This method provides, what we believe to be, a new solution for achieving high accuracy and wide range OFDR sensing at the km level.

Optical Systems Identification through Rayleigh Backscattering.

We introduce a technique to generate and read the digital signature of the networks, channels, and optical devices that possess the fiber-optic pigtails to enhance physical layer security (PLS). Attributing a signature to the networks or devices eases the identification and authentication of networks and systems thus reducing their vulnerability to physical and digital attacks. The signatures are generated using an optical physical unclonable function (OPUF). Considering that OPUFs are established as the most potent anti-counterfeiting tool, the created signatures are robust against malicious attacks such as tampering and cyber attacks. We investigate Rayleigh backscattering signal (RBS) as a strong OPUF to generate reliable signatures. Contrary to other OPUFs that must be fabricated, the RBS-based OPUF is an inherent feature of fibers and can be easily obtained using optical frequency domain reflectometry (OFDR). We evaluate the security of the generated signatures in terms of their robustness against prediction and cloning. We demonstrate the robustness of signatures against digital and physical attacks confirming the unpredictability and unclonability features of the generated signatures. We explore signature cyber security by considering the random structure of the produced signatures. To demonstrate signature reproducibility through repeated measurements, we simulate the signature of a system by adding a random Gaussian white noise to the signal. This model is proposed to address services including security, authentication, identification, and monitoring.

Multipath Quasi-Distributed Acoustic Sensing Based on Space Division Multiplexing and Time-Gated OFDR

Quasi-distributed optic sensing system has been widely utilized in various applications for its simple configuration, high sensing resolution, and long sensing range. In large sensing network, multipath sensing arrays are needed to be monitored simultaneously. This paper proposes a multi-path quasi-distributed acoustic sensing system based on time-gated digital optical frequency domain reflectometry (TGD-OFDR) and spatial division multiplexing (SDM) technique. The single mode fiber segment between weak reflectors is efficiently used. In experiments, vibrations on two weak reflectors array are detected simultaneously, and crosstalk between pathes is analyzed. This multi-path system does not require additional hardware equipment and the bandwidth of the system is not sacrificed, which has great potential in large-scale sensing network.

Performance evaluation of distributed strain sensing nerves for monitoring ground collapse: A laboratory study

Ground collapse events frequently lead to significant economic losses, catastrophic injuries and fatalities. There is a growing need for sensitive monitoring technologies to locate and predict potential ground collapses. This study aims to evaluate the performance of fiber optic (FO) cables for ground collapse monitoring. A model test was conducted in laboratory, and the optical frequency domain reflectometry (OFDR) and particle image velocimetry (PIV) techniques were combined to monitor the whole collapse process. The strain profiles measured by FO cables under different soil failure mechanisms were analyzed in detail. In the case of shallow sinkholes, the soil failure surfaces emanate from the edges of sinkholes and eventually propagate to the ground surface, and the shape of failure surfaces gradually changes from curvy to vertical. Micro-anchors significantly enhance the interface bonding between cables and surrounding soil. Moreover, the settlements calculated based on strain measurements match well with actual settlements.

Accurate measurement and enhancement of fiber coil symmetry.

The pure Shupe effect is substantially reduced in a fiber optic gyroscope (FOG) with symmetrical windings. However, the effect of the temperature-induced nonuniformity of the stress in the coil depends on the mean temperature derivative (T-dot). Research on precision winding technology has discovered that the symmetry of optical fiber rings affects the temperature performance of fiber optic gyroscopes. Optical fiber rings with good symmetry also have good temperature performance. This paper first establishes a temperature drift model of optical fiber rings that includes the Shupe effect and T-dot effect and then uses finite element simulation to analyze the drift error of optical fiber rings in a variable temperature environment. Analysis shows that this drift is caused by the variation and uneven distribution of the fiber length and the refractive index in the positive and negative winding of the optical fiber ring, which results in a residual phase difference that is directly related to the symmetry of the optical fiber ring. Simulation and analysis show that balancing the residual phase difference of the optical fiber ring can be achieved by cutting the length of the optical fiber ring at both ends. This paper uses optical frequency domain reflectometry (OFDR) technology to precisely test the symmetry of the optical fiber ring, ensuring accurate adjustment of the lengths at both ends of the optical fiber ring. Experimental tests on two gyroscopes have shown that the optical fiber ring with a smaller drift error can be obtained after testing and adjusting its length. The experimental data indicates that the bias stability of two laboratory gyros are increased by 23.6% and 18.1%, and the bias range are reduced by 22.4% and 30.0%.

Signal Processing in Optical Frequency Domain Reflectometry Systems Based on Self-Sweeping Fiber Laser with Continuous-Wave Intensity Dynamics

We report on the development of an optical frequency domain reflectometry (OFDR) system based on a continuous-wave Er-doped self-sweeping fiber laser. In this work, we investigate the influence of the input data processing procedure in an OFDR system on the resulting reflectograms and noise level. In particular, several types of signal averaging (in time and frequency domain) and Fourier analysis are applied. We demonstrate that the averaging in the frequency domain can be applied to evaluate absolute values of the local scattering amplitudes related to the Rayleigh light scattering (RLS), which is associated with the interference of scattering signals on microscopic inhomogeneities in optical fibers. We found that the RLS signal remains unchanged in the case of signal averaging in time domain, while the noise floor level decreases by 30 dB with an increasing number of points from 1 to ~450. At the same time, it becomes possible to detect the spectral composition of the scattering at each point of the fiber using windowed Fourier transform. As a result, the sensitivity of the developed system allows us to measure the RLS signal at a level of about 20 dB above the noise floor. The described analysis methods can be useful in the development of distributed sensors based on Rayleigh OFDR systems.

Shape and Flow Sensing in Arterial Image Guidance From UV Exposed Optical Fibers Based on Spatio-Temporal Networks.

Minimally invasive revascularization procedures such as percutaneous transluminal angioplasty seek to treat occlusions in peripheral arteries. However their ability to treat long occlusions are hampered by difficulties to monitor the location of intravascular devices such as guidewires using fluoroscopy which requires continuous radiation, and lack the capacity to measure physiological characteristics such as laminar blood flow close to occlusions. Fiber optic technologies provide means of tracking by measuring fibers under strain, however they are limited to known geometrical models and are not used to measure external variations. We present a navigation framework based on optical frequency domain reflectometry (OFDR) using fully-distributed optical sensor gratings enhanced with ultraviolet exposure to track the three-dimensional shape and surrounding blood flow of intra-vascular guidewires. To process the strain information provided by the continuous gratings, a dual-branch model learning spatio-temporal features allows to predict the output measures based on scattered wavelength distributions. The first network determines the 3D shape appearance of the guidewire using the input backscattered wavelength shift data in combination with prior segmentations, while a second network (graph temporal convolution network) produces estimates of vascular flow velocities using ground-truth 4D-flow MRI acquisitions. Experiments performed on synthetic and animal models, as well as in a preliminary human trial shows the capability of the model to generate accurate 3D shape tracking and blood flow velocities differences below 2 cm/s, thus providing realistic physiologic and anatomical properties for intravascular techniques. The study demonstrates the feasibility of using the device clinically, and could be integrated within revascularization workflows for treating occlusions in arteries, since the navigation framework involves minimal manual intervention.

Distributed Refractive Index Sensing Based on Etched Ge-Doped SMF in Optical Frequency Domain Reflectometry.

A distributed optical fiber refractive index sensor based on etched Ge-doped SMF in optical frequency domain reflection (OFDR) was proposed and demonstrated. The etched Ge-doped SMF was obtained by only using wet-etching, i.e., hydrofluoric acid solution. The distributed refractive index sensing is achieved by measuring the spectral shift of the local RBS spectra using OFDR. The sensing length of 10 cm and the spatial resolution of 5.25 mm are achieved in the experiment. The refractive index sensing range is as wide as 1.33-1.44 refractive index units (RIU), where the average sensitivity was about 757 GHz/RIU. Moreover, the maximum sensitivity of 2396.9 GHZ/RIU is obtained between 1.43 and 1.44 RIU.

Calculation the opening of neighboring surface cracks in concrete structure based on OFDR technology

Under the influence of temperature change, construction quality, and external force, concrete structures are prone to generate cracks which poses a significant threaten to the safety of the structure. With the advantages of distributed monitoring over traditional methods, the distributed fiber optic sensing (DFOS) technology has become a widely accepted method in concrete cracks detection and monitoring. Both laboratory tests and field monitoring indicate that the crack location can be determined by the strain peak and the width of cracks can be derived by integrating the strain exerted on the optical cable. Nevertheless, when the neighboring crack spacing distance (CSD) is sufficiently small, the strain induced by cracks will overlap and two strain peaks will be partially weighted, forming a saddle-shaped double strain peak. Thus, how to calculate the crack opening displacement (COD) becomes a big challenge. To tackle this issue, an analytical method to calculate the opening of neighboring cracks based on the shear strain transfer mechanism is proposed. The effectiveness of this method was evaluated through a crack simulation experiment and further validated by a concrete beam loading test. The strain variations induced by neighboring cracks were recorded by optical frequency domain reflectometry (OFDR) technology and the width of neighboring cracks was calculated using the proposed method. By comparing the calculation results with measured values, the accuracy of the proposed method was verified, which provided a feasible way to calculate the COD of neighboring cracks with strain overlap.

Optical Frequency Domain Reflectometry Based on Multilayer Perceptron

We proposed an optical frequency domain reflectometry based on a multilayer perceptron. A classification multilayer perceptron was applied to train and grasp the fingerprint features of Rayleigh scattering spectrum in the optical fiber. The training set was constructed by moving the reference spectrum and adding the supplementary spectrum. Strain measurement was employed to verify the feasibility of the method. Compared with the traditional cross-correlation algorithm, the multilayer perceptron achieves a larger measurement range, better measurement accuracy, and is less time-consuming. To our knowledge, this is the first time that machine learning has been introduced into an optical frequency domain reflectometry system. Such thoughts and results would bring new knowledge and optimization to the optical frequency domain reflectometer system.

Distributed optical fiber biosensor based on optical frequency domain reflectometry

In situ acquisition of spatial distribution of biochemical substances is important in cell analysis, cancer detection and other fields. Optical fiber biosensors can achieve label-free, fast and accurate measurements. However, current optical fiber biosensors only acquire single-point of biochemical substance content. In this paper, we present a distributed optical fiber biosensor based on tapered fiber in optical frequency domain reflectometry (OFDR) for the first time. To enhance evanescent field at a relative long sensing range, we fabricate a tapered fiber with a taper waist diameter of 6 μm and a total stretching length of 140 mm. Then the human IgG layer is coated on the entire tapered region by polydopamine (PDA) -assisted immobilization as the sensing element to achieve to sense anti-human IgG. We measure shifts of the local Rayleigh backscattering spectra (RBS) caused by the refractive index (RI) change of an external medium surrounding a tapered fiber after immunoaffinity interactions by using OFDR. The measurable concentration of anti-human IgG and RBS shift has an excellent linearity in a range from 0 ng/ml to 14 ng/ml with an effective sensing range of 50 mm. The concentration measurement limit of the proposed distributed biosensor is 2 ng/ml for anti-human IgG. Distributed biosensing based on OFDR can locate a concentration change of anti-human IgG with an ultra-high sensing spatial resolution of 680 μm. The proposed sensor has a potential to realize a micron-level localization of biochemical substances such as cancer cells, which will open a door to transform single-point biosensor to distributed biosensor.

Detecting pipeline leakage using active distributed temperature Sensing: Theoretical modeling and experimental verification

This paper presents a feasibility study on leak detection of buried pipelines using the active distributed temperature sensing (ADTS) method. The proposed solution involves the use of actively heated fiber optic (AHFO) cables arranged parallel to the pipelines, allowing for changes in the thermal properties of the surrounding soil to be captured and correlated with leakage accidents. This problem was modeled as an infinite cylindrical or linear heat source buried in porous media with seepage flowing inside. AHFO sensors were designed to verify the proposed method through scale-reduced model tests. The fiber optic temperature sensing technology based on optical frequency domain reflectometry (OFDR) was used to continuously monitor temperature profiles. The test results demonstrate that the ADTS solution has greatly improved the accuracy of passive temperature sensing methods. The feasibility of the solution was evaluated regarding different levels of leakage estimation, namely, identification, location, and quantification. This paper presents the first attempt to monitor the infiltration process associated with pipeline leaks using the AHFO-OFDR technique, which is a reliable method for structural health monitoring of underground pipelines.

Influence of adhesive on optical fiber-based strain measurements on printed circuit boards

The use of optical fiber sensing for strain measurements on printed circuit boards is a recent approach, and there is a lack of f reliable methods to obtain valid strain measurements. This work investigates the influence of different adhesives and their application for strain measurements using optical fiber sensors on PCB, considering room and high temperature tests. Strain developed in PCB during production and quality testing are measured by a single distributed optical fiber sensor, interrogated by Optical Frequency Domain Reflectometry. Different adhesives are evaluated for room and high temperatures strain measurements. Different thicknesses of the adhesive layer are studied under tensile and bending loadings. The mechanical properties of the adhesives and their effectiveness in optical fiber measurements were evaluated. A suitable adhesive selection is required for reliable strain measurements under different testing requirements: cyanoacrylate based for room temperature testing; epoxy or polyimide-based for high temperature measurements. The adhesive thickness should be the lowest and uniformly applied. A linear relationship between adhesive thickness and strain error was found, being necessary the application of a correction factor for valid strain measurements. Optical distributed sensing is highlighted as an alternative approach to conventional strain gauges for PCB strain measurements, reducing the installation complexity, significantly enlarging the number of measured points, without reducing the quality of the strain measurements. Selection of the best adhesive is crucial, as well as its thickness for reliable strain measurements.

Optical frequency domain reflectometry sensing for damage detection in long-span bridges using influence surface

With the development of optical frequency-domain reflectometry (OFDR)-based distributed strain sensing technology and static influence line (IL) measurement concept, the special influence surface (IS) measurement, a function of sensing and force locations, has become technically feasible. This study investigated, for the first time, damage detection in long-span bridges using the special IS constructed on the basis of the OFDR-based distributed sensing. The damage detection performance of IS was validated through numerical and experimental case studies on a scaled physical Tsing Ma bridge (TMB) model. First, the special IS concept was elaborated. The scaled TMB model was then introduced as a representative long-span bridge. Subsequently, the IS characteristics of different bridge components of the scaled TMB were analyzed using the finite element model (FEM). Various IS-based indices, including strain IS (SIS) and displacement IS (DIS), were applied to hypothetical damage scenarios where the bottom and top chords were subjected to severe damage in the FEM. The extracted ISs successfully realized damage localization with relatively great sensitivities. For comparison, the modal parameters were also output for damage detection. In the experimental section, an OFDR-based distributed sensor was placed along the top chord of one middle grid in the scaled TMB model to construct the SIS. The SIS changes could accurately localize the single, double, and triple bottom-chord damages. The numerical and experimental results demonstrated the feasibility and effectiveness of the OFDR-constructed special SIS for damage detection, indicating that IS is a promising damage indicator for long-span bridges, promoting the development of distributed sensing techniques in structural health monitoring for real-bridge applications.

Fabrication Method for the High-Accuracy Optical Fiber Delay Line with Specified Length

We propose a novel scheme for fabricating high-accuracy optical fiber delay lines (OFDLs). The fabrication system integrates a self-designed optical fiber cutting device and a high-accuracy fiber length measurement module based on optical frequency domain reflectometry. The optical fiber cutting device can cleave optical fibers to a specific length with the help of the motorized stage. The accuracy of fiber-cutting was determined by the positional accuracy of the motorized stage, which can reach several microns or even lower. This integrated design significantly reduces the errors and uncertainties introduced by fiber-cutting. To test, a set of OFDLs of a certain length was fabricated by this system. The deviation from the desired fiber length was kept below 50 μm, thus proving high fabrication accuracy and repeatability.