Optical Frequency Domain Research Articles

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.

Deformation monitoring and remote analysis of ultra-deep underground space excavation

The development and utilization of urban underground space is producing many deep foundation pits. On-site monitoring is a key issue to ensure the safety of the foundation excavation, especially the monitoring of the lateral displacement of the structure. The ultra-deep circular foundation excavation project (58.65 m deep, the deepest foundation pit in Shanghai) in the test section of the Suzhou River deep tunnel is one of the most challenging engineering around the world. The distributed optical fiber monitoring technology of Brillouin optical frequency domain analysis (BOFDA) was employed to monitor the lateral displacement of the deep foundation. The monitoring process lasted for 14 months, and a total of 19 times of monitoring was carried out following the progress of the excavation of the foundation pit. This paper introduces the process, methods and precautions of optical fiber monitoring from early installation, equipment debugging to data acquisition, data processing and data analysis. By comparing the fiber monitoring data with the manual monitoring data, the deformation of the deep foundation excavation is analysed. The analysis results not only obtain the deformation law of ultra-deep foundation excavation, but also prove the advantages of optical fiber monitoring in deep foundation excavation. Finally, with the support of the Smart Infrastructure Service System (iS3), the research combines optical fiber monitoring with an information platform. Under the framework of the iS3 platform, an integrated data processing and analysis module for fiber monitoring is developed, and the feasibility of remote real-time collection and analysis of monitoring data is discussed. With the support of the information platform, the monitoring data can be transmitted back to the computer terminal to realize remote monitoring.

Optical frequency domain polarimetry for distributed polarization crosstalk measurement beyond a 110 dB dynamic range

In this Letter, we present optical frequency domain polarimetry (OFDP), a novel, to the best of our knowledge, distributed polarization crosstalk (DPC) measurement method with a dynamic range beyond 110 dB. It is based on swept-wavelength interferometry using a tunable laser source (TLS) and a delay line interferometer (DLI). The ultrahigh dynamic range is achieved by optimizing the sweep rate of the TLS and the optical path difference of the DLI. Using OFDP, we measured polarization crosstalk below –105 dB in a LiNbO3 waveguide modulator for the first time. OFDP will play a significant role in developing and fabricating high-performance polarization devices.

Potential of optical frequency domain imaging for differentiation between early and advanced coronary atherosclerosis.

This study evaluated whether optical frequency domain imaging (OFDI) accurately distinguish between fibroatheroma (FA) and pathological intimal thickening (PIT) compared with histopathology. A total of 631 histological cross-sections from 14 autopsy hearts were analyzed for the comparison between OFDI and histological images. Of those, 190 (30%) sections were diagnosed with PIT and 120 (19%) with FA. The OFDI signal attenuation rate was calculated from an exponential. The lipid length was measured longitudinally by detection of sequential OFDI frames within a plaque segment containing lipids. The lipid arc was measured with a protractor centered in the center of the lumen. The fibrous cap thickness was defined as the minimum thickness of the signal rich band overlying PIT and FA. There was no significant difference in the OFDI signal attenuation rate between FA and PIT (3.09 ± 1.04 versus 2.79 ± 1.20, p = 0.13). However, the lipid length was significantly longer, the maximum lipid arc was significantly larger, and the fibrous cap thickness was significantly thinner in FA than in PIT (7.5 [4.3-10.3] mm versus 4.3 [2.7-5.8] mm, p < 0.0001, 125 [101-174]° versus 96 [74-131]°, p < 0.0001, and 220 [167-280] µm versus 260 [190-332] µm, p = 0.019). This study revealed OFDI may have the potential capability for discriminating FA from PIT based on the longitudinal and circumferential extent of lipid plaque, although the OFDI signal attenuation rate was similar between FA and PIT.

Temperature characteristic of a gain fiber core in distributed side-coupled cladding-pumped fiber amplifiers with different pump schemes

In this paper, we study the temperature characteristic of a gain fiber core in distributed side-coupled cladding-pumped (DSCCP) fiber amplifiers by optical frequency domain reflectometry (OFDR). To the best of our knowledge, this is the first temperature distribution measurement of a fiber core in DSCCP fiber lasers. The temperature distributions are also firstly measured among forward, backward, and bidirectional pump schemes. In addition, the bidirectional pump scheme has lower temperature distribution than unidirectional pump schemes. The experimental fiber core temperature is in according with the numerical simulation, which was once introduced for the temperature study of DSCCP fiber amplifiers. By experimental measurement and theoretical modeling, it is found that the maximum core temperature is several tens of centimeters away from the injected end of the DSCCP fiber, such that it will distribute the heat load along the longitudinal direction and reduce the maximum thermal load with better thermal management advantages than ordinary double cladding fiber (DCF).

Comprehensive research on the failure evolution of the floor in upper mining of deep and thick coal seam

The Group A coal seam represents one of the most important coal deposits of the Huainan coalfield. It poses a series of dynamic geohazards during mining operations due to its thickness, stratification, and depth location. One of the major issues is the susceptibility of the coal floor to failure under the combined action of mining and pressurized water. At greater depths, the stress becomes concentrated at the tip of the primary fractures under the influence of the high surrounding pressure. The exploitation of coal has caused new fissures to develop along the primary fissures, with energy regrouping occurring at the tips of the new fissures. To better understand the rock deformation and failure of the coal floor, uniaxial loading strain tests were performed on a standard rock specimen based on the Brillouin optical frequency domain analysis (BOFDA) to capture the deformation characteristics on the surface of the rock specimen with high accuracy. During loading, the strain of the fiber optic at peak strength was equal to 4281 με. To further study the evolution of the failure of the coal floor rock stratum during mining, electrodes, and distributed fiber optic were placed in the coal floor monitoring borehole of the coal seam to record the changes in the current and strain. The monitoring results indicate that during the initial stage, when the working face was close to the monitoring borehole, the abutment pressure of the surrounding rock mass gradually increased, the strain increased, and the current value steadily rose. During the middle stage of the monitoring process, the working face was located above the monitoring hole, the upper part of the hole was under compressive strain, the current value increased, the bottom of the hole was under tensile strain, and the electrode current showed fluctuating values. At the late stage of the monitoring process, the working face was located in proximity to the monitoring hole and the mudstone layer. The current value dropped below 1 mA, and the peak tensile strain reached 8589 με, which was followed by an abrupt change, showing the characteristics of rock failure. Comprehensive diagnosis indicated that the maximum floor failure depth was at the 21.0 m mark. During mining, the electrical current and the strain values had a positive correlation when the rock at the bottom was not failure. However, these values suddenly changed after the rock failed. The obtained results can serve as guidance for borehole multi-field monitoring in mining engineering.

A flexible multiplexing method of FBG sensors based on OFDR

A flexible multiplexing method of fiber Bragg grating (FBG) sensors based on optical frequency domain reflectometer (OFDR) is proposed in this paper to reduce the influence of spectral shadow effect on measurement and improve the reliability and measurement flexibility of FBG sensing network. In this method, the interrogation principle of FBG position and wavelength shift based on OFDR is analyzed, and as a guide, the flexible multiplexing FBG array is designed, in which multi-channel identical weak reflection FBGs are interrogated using a photoelectric detector. Meanwhile, the crosstalk beat noise generated by the flexible multiplexing FBG array is avoided so as to improve the performance of the measurement. The experiment results show that the method presented in this paper can achieve the flexible multiplexing of multi-channel FBGs with a single acquisition channel, while the measurement accuracy can be guaranteed.

Responses of calcareous sand foundations to variations of groundwater table and applied loads

The long-term settlement of calcareous sand foundations caused by daily periodic fluctuations has become a significant geological hazard, but effective monitoring tools to capture the deformation profiles are still rarely reported. In this study, a laboratory model test and an in situ monitoring test were conducted. An optical frequency domain reflectometer (OFDR) with high spatial resolution (1 mm) and high accuracy (±10-6) was used to record the soil strain responses to groundwater table and varied loads. The results indicated that the fiber-optic measurements can accurately locate the swelling and compressive zones. During the loading process, the interlock between calcareous sand particles was detected, which increased the internal friction angle of soil. The foundation deformation above the sliding surface was dominated by compression, and the soil was continuously compressed beneath the sliding surface. After 26–48 h, calcareous sand swelling occurred gradually above the water table, which was primarily dependent on capillary water. The swelling of the soil beneath the groundwater table was completed rapidly within less than 2 h. When the groundwater table and load remain constant, the compression creep behavior can be described by the Yasong-Wang model with R2 = 0.993. The daily periodically varying in situ deformation of calcareous sand primarily occurs between the highest and lowest groundwater tables, i.e. 4.2–6.2 m deep. The tuff interlayers with poor water absorption capacity do not swell or compress, but they produce compressive strain under the influence of deformed calcareous sand layers.

Widely Wavelength-Swept Single-Longitudinal-Mode Fiber Laser With Ultra-Narrow Linewidth in C+L-Band

This study proposes and demonstrates a widely wavelength-swept single-longitudinal-mode (SLM) erbium-doped fiber laser (EDFL) enabled by a dual-coupler-ring based compound-cavity (DCR-CC) filter, a fiber Fabry-Pérot tunable filter (FFP-TF) and a C+L-band erbium-doped fiber amplifier (EDFA). Moreover, it develops a novel methodology utilizing the signal-flow graph combined with the matrix algebra to analyze the DCR-CC filter and obtain its essential design parameters for SLM selection of the EDFL. The C+L-band EDFA is constructed to provide a gain range >90 nm. On the other hand, it demonstrates that, under a sweep-rate ≤100 Hz, a lasing wavelength-swept range >87 nm is obtained, the SLM operation is permanently guaranteed for every instantaneous lasing wavelength. Furthermore, the instantaneous lasing parameters of an optical signal-to-noise ratio ≥66 dB, a relative intensity noise ≤−153.33 dB/Hz@≥3 MHz, a linewidth ≤625 Hz and a power fluctuation ≤0.704 dB are achieved. The distinguished performance of the proposed fiber laser enables it to have a significant potential for application in optical frequency domain reflectometry, spectroscopy and Lidar system.

Wide Measurement Range Distributed Strain Sensing With Phase-Accumulation Optical Frequency Domain Reflectometry

In this paper, a wide measurement range strain sensing method based on phase-accumulation optical frequency domain reflectometry (OFDR) is proposed. Different from the traditional phase demodulation method that analyzes the relative phase between the measurement signal and the reference signal, we achieve large measurement range strain sensing by accumulating the relative phases of adjacent scanning cycles. The proposal can break through the limitation of phase unwrapping in the traditional phase method, and realize the low measurement error in the accumulation process. In the experiments, when the wavelength scanning range is only 0.138 nm, we achieve quasi-static strain measurement with a spatial resolution of 1.8 cm, a maximum strain value of about 14000 <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">μϵ</i> , and the noise level of about 0.601 rad representing the strain resolution of 0.48 <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">μϵ.</i> We also analyze the influence of random noise on the method, and verify the good robustness of the phase accumulation method. The research of this paper provides a useful reference for the development of low-cost OFDR system for distributed strain/temperature measurement.

A Novel Sensor for Undrained Shear Strength Measurement in Very Soft to Soft Marine Sediments Based on Optical Frequency Domain Reflectometry Technology.

Due to the short supply of conventional fill materials, such as sand, land reclamation using dredged marine deposits has recently been proposed, in which marine deposits with high water content are blow-filled into reclaiming areas. The strength development of the filled marine soils is of great importance during the sedimentation and consolidation to guide the filling process and construction of reclamation. In this study, a novel sensor based on optical frequency domain reflectometry (OFDR) technology with a simple design was developed for undrained shear strength measurement. The novel sensor consists of an optical fiber and a series of polyoxymethylene coins. Owing to the merits of OFDR technology on high resolution, fully distributed sensing, and immunity to electromagnetic interference, the novel sensor can be used to determine undrained shear strength profiles of very soft to soft marine sediments/soils with good accuracy. The sensor was calibrated in remolded marine deposits with different water contents. The good feasibility and performance of the novel sensor for undrained shear strength measurement were well validated in two physical model tests on marine deposits treated by horizontal drains with vacuum preloading.

Clinical expert consensus document on standards for lower extremity artery disease of imaging modality from the Japan Endovascular Treatment Conference.

Imaging modalities have developed to provide precise information in the assessment of lower extremity artery disease (LEAD), including both quantitative measurements and morphological assessment. However, a lack of standardization for the evaluation methods introduces inconsistency and potential risk. This consensus documentfrom Japan Endovascular Treatment Conference (JET) summarizes the methods of measurement and assessment of intravascular ultrasound (IVUS), optical frequency domain imaging (OFDI), and angioscopy. We propose standardized approaches for the evaluation of these modalities in endovascular therapy (EVT).

Buckling detection and shape reconstruction using strain distributions measured from a distributed fiber optic sensor

This paper proposes to detect buckling and reconstruct three-dimensional deformations using distributed fiber optic sensors. The distributed sensors measured strain distributions with sub-millimeter resolutions based on optical frequency domain reflectometry in real time. Buckling was detected from high-resolution strain distributions measured from specimens. An effective and practical shape reconstruction approach was developed to derive nonsymmetrical deformations based on the strain distributions. The reconstructed shape was validated using a computer vision method that measured point cloud from the specimens. A parametric study was conducted to investigate the effects of the key sensing parameters such as the spatial resolution and sensor deployment scheme on the performance of the shape reconstruction approach, and used to optimize the resolution and deployment of distributed sensors. This research will advance the capability of buckling detection and shape reconstruction through distributed sensing for engineering structures under complex loading conditions.

Modelling Intensity Fluctuations of Rayleigh Backscattered Coherent Light in Single-Mode Fibers

This work reportsa thorough analysis of intensity fluctuations of Rayleigh backscattered coherent light in single mode optical fibers. Based on the intermediate range order of silica glasses and on residual stress of optical fibers, a theoretical model was built to calculate intensity fluctuations of Rayleigh backscattered coherent light in the optical frequency domain. Numerical simulation results were compared to experimental results in time domain, strongly supporting the conclusion that Rayleigh backscattering in single mode fibers is an ergodic process exhibiting ergodicity in the time-frequency sense so that the model can be used to determine the statistical behavior of backscattered intensity fading.

Field Test Study on Deformation Monitoring of Antifloating Anchor Using Optic Fiber Sensor

The antifloating anchors have been widely used in the antifloating problem of underground engineering recently. Due to the complexity of the anchor force in the geotechnical material and the limitations of traditional monitoring methods, it is impossible to grasp the deformation characteristics of the anchor comprehensively. The pullout test of the antifloating anchor on-site is carried out in this paper based on the housing construction project. The Optical Frequency Domain Reflectometry (OFDR) technology is used, the strain distributions are obtained, and the distribution law of axial force is analyzed. The test results show that the strain of the antifloating anchor gradually decreases when anchoring depth increases and eventually tends to zero. The anchoring section is divided into main stressed section and unstressed section. The results show that the OFDR technology can be used to effectively monitor the antifloating anchor in the pullout test; it will help to grasp the deformation of antifloating anchor.

Experimental study on uplift behavior of shallow anchor plates in geogrid-reinforced soil

Geogrid reinforcement can significantly improve the uplift bearing capacity of anchor plates. However, the failure mechanism of anchor plates in reinforced soil and the contribution of geogrids need further investigation. This paper presents an experimental study on the anchor uplift behavior in geogrid-reinforced soil using particle image velocimetry (PIV) and the high-resolution optical frequency domain reflectometry (OFDR). A series of model tests were performed to identify the relationship between the failure mechanism and various factors, such as anchor embedment ratio, number of geogrid layers, and their location. The test results indicate that soil deformation and the uplift resistance of anchor plates are substantially influenced by anchor embedment ratio and location of geogrids, whereas the number of geogrid layers has limited influence. In reinforced soil, increasing the embedment ratio greatly improves the ultimate bearing capacities of anchor plates and affects the interlock between the soil and geogrids. As the embedment depth increases, the failure surfaces gradually change from a vertical slip surface to a bulb-shaped surface that is limited within the soil. The strain monitoring data shows that the deformations of geogrids are symmetrical, and the peak strains of geogrids can characterize the reinforcing effects.

Observation and characterization of the high order modes in a six-mode fiber using an OFDR method.

High order modes in a six-mode fiber are separately observed and characterized using an Optical Frequency Domain Reflectometry (OFDR) method. Due to the difference in group refractive index between fundamental mode and the high order modes, Fresnel reflection peaks for each mode can be separated in beat frequency domain with their corresponding time delay. In the experiment, the fundamental mode and high order modes are excited in turn and observed at a 6.6 m six-mode fiber end, which agree with their beat frequency difference in theoretical simulation. The demonstration provides a flexible and feasible method for mode identification and characterization of all kinds of fibers.

Monitoring and early detection of soil desiccation cracking using distributed fibre optical sensing

Desiccation cracking can significantly affect the hydro-mechanical behaviour of soil in various engineering applications. Characterising the evolution of desiccation-induced shrinkage strain in soils is a key step in understanding the cracking mechanism. This study presents a novel framework for monitoring and early detecting soil cracking by utilising the distributed fibre optical sensing (DFOS) technique based on optical frequency domain reflectometry (OFDR). DFOS-OFDR was integrated into a set of experimental testing to simultaneously monitor water content, crack pattern and strain state. OFDR with 1 με strain measurement accuracy was employed to monitor the initiation and propagation of cracks. The results reveal a strong correlation between the spatiotemporal evolution of soil strain and crack, confirming the efficacy and reliability of the proposed framework. During the drying process, the compressive strain induced by soil shrinkage increases first and starts to decrease when cracking is initiated. Further, DFOS-OFDR demonstrated promising results to predict the position of cracks. Compared to traditional strain monitoring methods with discrete measurements, DFOS-OFDR offers a non-destructive, accurate yet efficient, high-resolution and continuous technique for monitoring and early detection of soil desiccation cracking.

Multi-physical field joint monitoring of buried gas pipeline leakage based on BOFDA

Determining the leakage location in an underground gas pipeline is a serious technical problem. Distributed fiber optic testing technology based on Brillouin optical frequency domain analysis (BOFDA) can be used for monitoring the change in the strain and temperature of the surrounding media caused by the gas leakage. In this study, a point source leakage model was built. The distributed fiber optic strain and actively heated fiber optic were installed around the pipeline to monitor the gas pipeline leakage. Variations in the strain and temperature characteristics of the pipeline under different gas pressure conditions were found to be in the range of −392–402 μϵ and 3.5 °C during the 1.0 MPa gas leakage, respectively. When the soil model was loaded with a gas leakage pressure of 0.3–1.0 MPa, a significant variation in strain and temperature was noticed. The copper mesh actively heated fiber optic can effectively identify the location of the leak. Meanwhile, the heating rate of the actively heated fiber optic cables in different media environments were obtained and ranked from fast to slow: air, followed by general soil, followed by leakage soil. This led to the generation of new technology to detect the leakage location of buried gas pipelines through actively heated optical cables.

Multi-parameter measurement of a multi-point high-frequency vibration signal in an OFDR system.

The phase cross-correlation function of an optical frequency domain reflectometry (OFDR) system is proposed to detect a multi-point vibration event, which is verified in detail by theoretical simulation and experiment. An OFDR system based on a non-tunable laser source with digital sweep frequency is developed. It is verified experimentally that the location and frequency resolution of multi-point high-frequency vibration can be detected by analyzing the phase cross-correlation function of the sensing signal. High-frequency signals of 50 kHz and 20 kHz are located and separated on the 8 km optical fiber. The frequency resolution is 1.26 kHz, and the minimum spatial error is 11.5 m.