Fiber Bragg Grating Technology Research Articles

Length-extended 3D shape sensor using wavelength/space-division multiplexing grating arrays in a multicore fiber.

Limited by the multiplexing number of fiber Bragg grating (FBG), further improvement in the length of 3D shape sensing based on FBG technology is challenging. In this Letter, a wavelength-division and space-division multiplexing multicore fiber grating method is proposed, which extends the sensing length. Employing the femtosecond-laser point-by-point technology, we inscribed WDM grating arrays in six outer cores of a seven-core fiber, respectively. Three cores were utilized as a segment for shape sensing, and two such segments were offset by a specific length and combined to form a shape sensor. Utilizing an FBG interrogator, the proposed shape sensor achieved 2D and 3D shape sensing at a length of 967 mm and effectively mitigated the effects of temperature variations. In experiments, maximum shape reconstruction errors per unit lengths are 1.89%, 2.72%, and 1.47% for 2D shape, 3D shape, and an arbitrary shape under variable temperature conditions, respectively. The proposed method holds promise for further extending the shape sensing length by utilizing multicore fibers or fiber clusters containing more cores.

Using a fiber Bragg grating technique to investigate temperature cycling strain of pressed TATB‐based polymer‐bonded explosives

AbstractPressed 1,3,5‐triamino‐2,4,6‐trinitrobenzene (TATB)‐based polymer‐bonded explosive (PBX) exhibits significant anisotropic and irreversible expansion characteristics during temperature cycling, resulting in both safety and service issues. The anisotropic and irreversible expansion properties of PBX have shown to be closely related to the microstructures, including explosive crystals, binders, micro voids, el. Fiber Bragg grating (FBG) technology has advantages of high reliability, accuracy, and sensitivity, which is more conducive to real‐time capture of the changes in strain throughout the temperature cycling. In this study, temperature cycling strain monitoring experiments of PBX pressed under different thermal–mechanical coupling loading conditions were studied using FBG technology. The effect of the pressing parameters on the anisotropic and irreversible expansion characteristics of the PBX's during temperature cycling was measured and analyzed, taking into account potential microstructural factors such as crystal orientation, binders, and porosity. The results indicated that the strain on the side and top of PBX cylinders were distributed with increasing strain in the direction of higher density. The strain stratification was significant during the high‐temperature stage of temperature cycling and was not obvious during the low‐temperature stage. The degree of strain stratification decreased with an increase in temperature cycling cycles and increased with higher pressing temperatures. The anisotropic expansion of PBX cylinders increased with an increase in temperature during cycling and decreased with a reduction in temperature. The axial expansion degree of PBX cylinders pressed under different compression conditions in the later cycles of temperature cycling is consistent with the different crystal orientations intensity inside them.

Shape Sensing for Continuum Robotics Using Optoelectronic Sensors with Convex Reflectors

Three-dimensional shape sensing in soft and continuum robotics is a crucial aspect for stable actuation and control in fields such as minimally invasive surgery, engine repairs and search and rescue operations, as the estimation of complex curvatures while using continuum robotic tools is required to manipulate through fragile paths. This challenge has been addressed using a range of different sensing techniques, for example, Fibre Bragg grating (FBG) technology, inertial measurement unit (IMU) sensor networks, or stretch sensors. Previously, an optics-based method using optoelectronic sensors was explored, offering a simple and cost-effective solution for shape sensing in a flexible tendon-actuated manipulator in two orientations. This was based on proximity-modulated angle estimation and has been the basis for the shape sensing method addressed in this paper. The improved and miniaturised technique demonstrated in this paper is based on the use of a convex shaped reflector with optoelectronic sensors integrated into a tendon-actuated robotic manipulator. Upgraded sensing capability is achieved using optimisation of the convex reflector shape in terms of sensor range and resolution, and improved calibration is achieved through the integration of spherical bearings for friction-free motion. Shape estimation is achieved in two orientations upon calibration of sensors, with a maximum Root-Mean-Square Error (RMS) of 3.37°.

High-precision FBG-based sensor for soil settlement monitoring: A comparative study with magnetic settlement gauges and PIV technique

The primary objective of layered settlement monitoring is to detect subsidence issues in soil or foundational structures, in order to implement appropriate measures for ensuring the safety and stability of engineering projects. Conventional monitoring techniques have problems such as errors in manual readings, high costs, susceptibility to interference, and low levels of automation. A layered soil settlement sensor based on fiber Bragg grating (FBG) technology is proposed in this study. This sensor utilizes an equal-strength beam as an "intermediate bridge" to make the FBG sensor respond. The sensor is prepared by the laboratory's 3D fused deposition modelling (FDM) and calibrated by applying displacement loads through a fine-tuning platform. The sensitivity coefficient is 0.068 nm/mm, and the correlation coefficient exceeds 0.99. Finally, the sensor exhibited high precision when compared to measurements obtained from magnetic settlement gauges and particle image velocimetry (PIV). The discrepancies between the measurement outcomes of the sensor and the actual values of soil settlement does not exceed 3.33%.

Fibre Bragg Grating Sensors for Condition Monitoring of High-Voltage Assets: A Review

The high-voltage (HV) assets in the existing power transmission network will experience increased electrical, thermal, environmental and mechanical stresses and, therefore, robust condition monitoring is critical for power system reliability planning. Fibre Bragg grating (FBG) sensors offer a promising technology in HV applications due to their immunity to electromagnetic interference and multiplexing capability. This paper reviews the current technology readiness levels of FBG sensors for condition monitoring of transformers, transmission lines, towers, overhead insulators and power cables, with the aim of stimulating further development and deployment of fibre-based HV asset management systems. Currently, there are several reported cases of FBG sensors used for condition monitoring of HV assets in the field, proving their feasibility for long-term use in the power grid. The review shows that FBG technology is versatile and can facilitate multi-parameter measurements, which will standardise the demodulation equipment and reduce challenges with integrating different sensing technologies.

A high SNR system for intensity demodulation of fiber Bragg grating based on phase-locked loop

The intensity demodulation technology of fiber Bragg gratings (FBG) is typically realized by detecting the output light power. Traditional methods, such as DC power detection, have a low signal-to-noise ratio and sensitivity. Thus, a novel intensity demodulation system based on the phase-locked loop is proposed in this study. Theoretical analysis and experiments show that the system has a high signal-to-noise ratio, linearity, and stability with a low-temperature sensitivity of the FBG sensor head. Compared to traditional light power detection, the proposed system increases the signal-to-noise ratio by more than 24 dB. According to the repeated temperature experimental results, the proposed sensor with an accuracy of ± 0.5 °C has a repeatability error of 1.8%, alinearity error of 3%, and ahysteresis error of 1.5% in the temperature range from 10 to 90 °C, which indicates that the system is of high-sensitivity and high-precision.

An Optical Fiber Acceleration Sensor Based on a V-Shaped Flexure Hinge Structure

This paper proposes a novel vibration sensor based on fiber Bragg grating (FBG) technology. The sensor employs a V-shaped flexure hinge structure, which is theoretically and experimentally demonstrated to enhance sensitivity for vibration sensing. The proposed design utilizes the V-shaped flexures hinge to focus the acceleration-induced strain, resulting in improved performance for vibration sensitivity. The implementation of the FBG sensor was evaluated through theoretical computations using MATLAB. The simulation software was also used to simulate the proposed structure and assess the parameters affecting its sensing performance. The experimental results of this work sensor demonstrate a sensitivity of 79.46 pm/G and a resonant frequency of 1140 Hz. The sensor’s operating frequency bandwidth was 30–320 Hz, while the temperature influence was measured at 8.5 pm/°C. In addition, the amplitude in the transverse direction was only 3% of the amplitude in the working principle, indicating that the presented sensor has solid lateral immunity. The experimental results were consistent with the simulation findings, demonstrating the presented vibration sensor’s good performance. The given work provides an opportunity to apply the FBG acceleration sensor in the high-dependability detection of low- and medium-frequency vibration signals.

High-Sensitivity Seawater Salinity Sensing With Cladding Etched Fiber Bragg Grating Technology

Seawater salinity is an essential parameter to show the basic hydrological information of the ocean. Cladding etched fiber Bragg grating (CE-FBG) technology has potential in seawater salinity sensing because of its high sensitivity to surrounding refractive index. Here, we fabricated the different CE-FBG salinity sensing elements by using hydrofluoric (HF) acid based wet chemical etching. We encapsulated a CE-FBG and a bare FBG (B-FBG) into a salinity sensor to realize simultaneous measurement of seawater salinity and temperature. Various sensing characteristics of the CE-FBG, including salinity and temperature sensitivity, response time, hysteresis, repeatability, and long-term stability were investigated systematically. The salinity and temperature measurements of the CE-FBG sensor were highly consistent with those of the electronic salinometer, which indicates that fiber optic salinity sensor provides a new means for salinity measurement. Together, our results demonstrate the great potential of the CE-FBG salinity sensor for high sensitivity, long-term, and quasi-distributed seawater salinity monitoring.

Use of FBG sensors in advanced civil engineering applications

Structural health monitoring (SHM) in civil engineering structures has gained a key role to provide early warning for the occurrence of conditions leading to potential damage and unsafe service. Among the various available technologies, fiber optic ones have proved to be reliable and convenient to perform permanent monitoring in both static and dynamic regimes. Fiber optic sensing solutions based on the fiber Bragg grating (FBG) technology are very versatile and they can be effectively used to provide multipoint sensing systems distributed on large structures. FBG sensors have a point-like form factor, but they can be easily packaged in solid housings in order to fit the necessary measuring gauge and sensitivity, shape and size, mechanical and environmental requirements. As key advantages over electric technologies, FBG sensors offer immunity to electromagnetic interference, in-series cabling of sensors for different physical parameters, stable response to both quasi-static phenomena and fast response transitory phenomena. This paper presents some monitoring solutions developed in ENEA with FBG sensors for civil engineering applications.

Groundwater salinity sensing using PI-FBG with improved capillary coating method

Polyimide (PI)-coated fiber Bragg grating (FBG) technology has certain potential for groundwater salinity measurement. However, this technology is immature for the field salinity monitoring due to the limitation of PI coating technology. Here, we fabricated the PI-FBG salinity sensing elements by improved capillary coating method, and encapsulated it with stainless steel tube to improve the robustness. The results show that different capillaries coated PI-FBGs were all linear with salinity ranging from 0 to 8%. Pre-treatment methods of soaking in the coupling agent and reducing the fiber diameter before coating can effectively improve the salinity sensitivity, and this coating method had good repeatability. The measurement results of the PI-FBG sensor in different gravel aquifers were highly consistent with those of the electronic salinometer. Our results show that the capillary coated PI-FBG has excellent salinity sensing performance, which provides a new means for in-situ monitoring of groundwater salinity.

Surface-Mounted Tilt Sensor Using Fiber Bragg Grating Technology for Engineered Slope Monitoring With Temperature Compensation

A surface-mounted tilt sensor was designed and fabricated to measure the inclination angle of engineered structures or slopes in two directions. The device utilizes two strain-sensitive FBGs for tilt angle measurement bi-directionally and one strain-free FBG to provide temperature compensation. In this work, a tilt sensor prototype was fabricated using a 3D printer, with a robust enclosure and a miniature actuator with dimensions of 115 x 65 x 30 mm and 45 x 20 x 3 mm, respectively. The device was first calibrated in the laboratory for tilt and temperature parameters. For tilt calibration, the device yields a sensitivity value of 0.0135 nm/° and 0.0123 nm/° for +x and -x directions. On the other hand, the device delivers a sensitivity value of 0.0105 nm/°C as the response to temperature changes. The tilt sensor was also tested for suitability in a real-field deployment where it was installed on a retaining wall and was left for 4 weeks. The field test data indicates no vertical displacement of the wall since the device exhibits zero inclination changes during the test period. This compact, robust, and easy-to-install tilt sensor has excellent potential for various geotechnical applications, mainly in landslide detections, ground movement, and engineered slope monitoring.

Strain and vibration measurements by FBG sensors for engineering applications

Fiber optic sensors based on the fiber Bragg grating (FBG) technology [1] is widely used to produce quasi-distributed monitoring systems to measure mechanical parameters for Structural Health Monitoring (SHM) of civil engineering structures [2]. FBG sensors are produced in the core of the optical fiber, as a short segment of fiber where a diffraction grating is produced (5–10 mm). The principle of operation of the FBGs is based on the diffraction occurring at the grating: if broadband light propagates, a quasi-monochromatic counter propagating light originates. The wavelength of the diffracted light depends on the value of the refraction index of the core along the grating which in turn is affected by both temperature and strain. Thus, by measuring the wavelength change of the counter propagating light, the change of strain and the temperature can be worked out. The PREFOS Project [3] aims to develop a novel procedure to apply the use of FBGs to monitor the prestressed strands of civil engineering prefabricated components. In this paper, we report results of an experimental campaign intended to measure the sensitivity of saddle-like sensors to induced vibration and mechanical strain. Measurements were done performing static and dynamic tests on a steel strand equipped with 3 sensors. The tests have been performed applying different tensioning to the steel strand. Static measurements were worked out applying a stepwise tensioning increase. Dynamic tests were worked out at each stepped tension level, inducing vibration by both sharp hammer impact and release of hanged weight. The paper is organized as follows, in section 1 a general description of the testbed structure used for the experiments will be provided. In section 2 the results of static analysis and dynamic tests will be presented with the dynamic behavior of the strand evaluated by performing a simple Fast Fourier Transform (FFT). Finally, the conclusions will be drawn.

Thermo-hydro-poro-mechanical responses of a reservoir-induced landslide tracked by high-resolution fiber optic sensing nerves

Thermo-poro-mechanical responses along sliding zone/surface have been extensively studied. However, it has not been recognized that the potential contribution of other crucial engineering geological interfaces beyond the slip surface to progressive failure. Here, we aim to investigate the subsurface multi-physics of reservoir landslides under two extreme hydrologic conditions (i.e. wet and dry), particularly within sliding masses. Based on ultra-weak fiber Bragg grating (UWFBG) technology, we employ special-purpose fiber optic sensing cables that can be implanted into boreholes as “nerves of the Earth” to collect data on soil temperature, water content, pore water pressure, and strain. The Xinpu landslide in the middle reach of the Three Gorges Reservoir Area in China was selected as a case study to establish a paradigm for in situ thermo-hydro-poro-mechanical monitoring. These UWFBG-based sensing cables were vertically buried in a 31 m-deep borehole at the foot of the landslide, with a resolution of 1 m except for the pressure sensor. We reported field measurements covering the period 2021 and 2022 and produced the spatiotemporal profiles throughout the borehole. Results show that wet years are more likely to motivate landslide motions than dry years. The annual thermally active layer of the landslide has a critical depth of roughly 9 m and might move downward in warmer years. The dynamic groundwater table is located at depths of 9–15 m, where the peaked strain undergoes a periodical response of leap and withdrawal to annual hydrometeorological cycles. These interface behaviors may support the interpretation of the contribution of reservoir regulation to slope stability, allowing us to correlate them to local damage events and potential global destabilization. This paper also offers a natural framework for interpreting thermo-hydro-poro-mechanical signatures from creeping reservoir bank slopes, which may form the basis for a landslide monitoring and early warning system.

Assessment of a Multi-Sensor FBG-Based Wearable System in Sitting Postures Recognition and Respiratory Rate Evaluation of Office Workers

Due to prolonged incorrect sitting posture, upper body musculoskeletal disorders (UBMDs) are largely widespread among sedentary workers. Monitoring employees' sitting behaviors could be of great help in minimizing UBMDs' occurrence. In addition, being primarily influenced by psycho-physical stress conditions, respiratory rate (RR) would be a further useful parameter to delineate the workers' state of health. Wearable systems have emerged as a viable option for sitting posture and RR monitoring since enable continuous data collecting with no posture disturbances. Nevertheless, the main limits are poor fit, cumbersomeness, and movement restriction resulting in discomfort for the user. In addition, only few wearable solutions can track both these parameters contextually. To address these problems, in this study a flexible wearable system composed of seven modular sensing elements based on fiber Bragg grating (FBG) technology and designed to be worn on the back has been proposed to recognize the most common sitting postures (i.e., kyphotic, upright and lordotic) and estimate RR. The assessment was performed on ten volunteers showing good performances in postures recognition via Naïve Bayes classificator (accuracy >96.9%) and agreement with the benchmark in RR estimation (MAPE ranging between 0.74% and 3.83%, MODs close to zero, and LOAs between 0.76 bpm and 3.63 bpm). The method was then successfully tested on three additional subjects under different breathing conditions. The wearable system could offer great support for a better understanding of the workers' posture attitudes and contribute to gathering RR information to depict an overall picture of the users' state of health.

A Wearable Flower-Shaped Sensor Based on Fiber Bragg Grating Technology for In-Vivo Plant Growth Monitoring

One of the biggest challenges facing the world agriculture is feeding a growing population in a sustainable way. Therefore, global food availability is under a severe strain exacerbated by climate changes and biological stresses. Moreover, the content of macro- and micronutrients obtained from in plant-food sources strongly depends on the plant development. An attractive strategy to increase agricultural productivity is the growth monitoring of plants and edible parts by using wearable systems. However, most of these tools measure dimensional changes uniaxially, while an accurate representation of the growth distribution requires multipoint strain measurement especially for plants that show an anisotropic development. Here, we present a stretchable multisensor wearable system highly adaptable to the curvilinear surface of leaves and fruits for multidirectional dimensional monitoring. The proposed sensor consists of six fiber Bragg gratings (FBGs) within a biomimetic flexible substrate with a flower design. FBGs with their miniaturized size, high metrological properties, and multiplexing capacities are well suited to this purpose. A finite-element model (FEM) guides the optimal design and sensors positioning within the flower-shaped matrix to exhibit an adequate strain sensitivity and negligible crosstalk effects among the six sensing elements with a reduced encumbrance. A metrological characterization is first performed followed by the application of the proposed system for in-vivo detection of dimensional changes of a leaf and fruit in both indoor and outdoor scenarios.

Ultra-weak FBG sensing for identification and analysis of plastic zone of soil caused by supported excavation

Deformation caused by supported excavation makes significant impact on foundation pit stability. Evaluation of potential plastic zone is crucial for early warning of foundation pit instability. To address the shortcomings of existing evaluation methods, a new identification method based on the ultra-weak fiber Bragg grating (UWFBG) technology was proposed. To validate this approach, a UWFBG monitoring program was implemented in the boreholes as part of the Shanghai Metro Line 18 project. The monitoring results were utilized to determine the range of plastic zone for each borehole. It was noted that there are two plastic deformation zones formed with different mechanisms, one located near the base of the diaphragm wall (DW) and the other is distributed within the range of 6–19 m below the excavation face. Lastly, a criterion of early warning of foundation pit instability was presented. By analyzing the monitoring data, the highest risk level of this project was determined as “Attention”, implying the necessity of taking measures to prevent the local plastic zone from evolving into a full-scale sliding surface.

A deep learning-based method for calculating aircraft wing loads

The purpose of this paper is to propose a novel aircraft wing loads calculation model, called long short-term memory residual network (LSTM-ResNet), which can evaluate the loads based on the strain distribution. To achieve this goal, firstly, the data acquisition experiment is designed and performed with a real aircraft wing. In this experiment, we used the Fiber Bragg Grating (FBG) technology as the measurement method to collect strain-load data from the aircraft wing. Then, we propose the LSTM-ResNet model with the one-dimensional convolutional(1D-CNN) architecture. This model is capable of extracting the temporal and spatial representational information from the strain-load data of the aircraft wing. Experimental results demonstrate that the proposed method effectively evaluate the loads of the aircraft wing. To prove the superiority of LSTM-ResNet model, we compared the proposed model with existing loads calculation methods on our experimental dataset. The results show it has a competitive average relative error (0.08%). Moreover, those promising results may pave the way to use the deep learning algorithm in aircraft wing loads calculation.

Monitoring fluid migration using in-situ nuclear magnetic resonance core flooding system integrated with fiber optic sensors: A proof of concept

In-situ nuclear magnetic resonance (NMR) core flooding system has enabled researchers to monitor several rock properties such as porosity, pore size distribution, and fluid saturation along the tested samples with high resolutions and under reservoir conditions. However, spatially resolved rock strength/mechanical property alteration coupled to fluid migration/substitution remains poorly characterized. To this end, Fiber Bragg Grating (FBG) multiplex sensors were integrated with NMR core flooding system to monitor rock strength changes, or generally speaking, to observe hydro-mechanical-chemical coupling mechanisms during core flooding tests. In this study, we present a novel approach on how to conduct core flooding experiments, while simultaneously monitoring NMR and FBG strain response of the tested limestone plug. The NMR cell was modified to integrate FBG technology without impeding the NMR signal and core flooding high pressure/temperature capacity. A high spatial resolution optical fiber was attached onto the sample radial surface. The results show the successful association of NMR and FBG sensors to track any change at each stage of brine injection. The FBG is capable of measuring the rock strain variations induced by rock-fluid interactions during brine injection, allowing it to capture the fluid front location along with the sample and at a faster rate than the NMR.

Thermal diffusion coupling mechanism and its application of discrete waveguide

For discrete optical systems integrated into optical fibers, the optical fields of the individual waveguides are coupled and correlated with each other. This paper studies how to adjust the refractive index of discrete waveguides by thermal diffusion, so as to enhance the coupling between discrete waveguides, and also constructs the discrete waveguide thermally diffused model and the thermally diffused coupling model of twin-core and three-core fibers. The multicore fiber is heated different times by a hydrogen-oxygen flame, and the outgoing light field at the end face of the optical fiber is monitored at the same time. Then, the three-dimensional refractive index measurement results of the thermally diffused multicore fiber verify the feasibility of thermal diffusion technology to change the refractive index of discrete waveguides for coupling. Thermal diffusion technology can be used to fabricate multicore fiber couplers. By combining multicore fiber and core-by-core inscribed fiber Bragg grating technology and by using thermal diffusion technology, the single-channel sensing measurement can be realized. The method of changing the refractive index of discrete waveguides through thermal diffusion has the advantages of high integration, high stability, and mass fabrication. The research on the thermal diffusion of discrete waveguides can improve the application potential of multicore fiber sensing systems, and promote the broad application of discrete waveguide structure optical fiber in the fields of optical communication, optical sensing, biomedicine, and artificial intelligence.

Fiber Optic Fiber Bragg Grating Sensing for Monitoring and Testing of Electric Machinery: Current State of the Art and Outlook

This paper presents a review of the recent trends and the current state of the art in the application of fiber optic fiber Bragg gratings (FBG) sensing technology to condition the monitoring (CM) and testing of practical electric machinery and the associated power equipment. FBG technology has received considerable interest in this field in recent years, with research demonstrating that the flexible, multi-physical, and electromagnetic interference (EMI) immune in situ sensing of a multitude of physical measurands of CM interest is possible and cannot be obtained through conventional sensing means. The unique FBG sensing ability has the potential to unlock many of the electric machine CM and design validation restrictions imposed by the limitations of conventional sensing techniques but needs further research to attain wider adoption. This paper first presents the fundamental principles of FBG sensing. This is followed by a description of recent FBG sensing techniques proposed for electric machinery and associated power equipment and a discussion of their individual benefits and limitations. Finally, an outlook for the further application of this technique is presented. The underlying intention is for the review to provide an up-to-date overview of the state of the art in this area and inform future developments in FBG sensing in electric machinery.