Bare Fiber Bragg Grating Sensor Research Articles

Research on sensitized Fiber Bragg Grating temperature sensor based on bimetal three-substrates.

Temperature is one of the most important physical quantities in the field of earthquake precursor observation. Aiming at the problem of low sensitivity in the Fiber Bragg Grating (FBG) temperature sensor, the sensitized FBG temperature sensor based on bimetal three-substrates is proposed. Through theoretical analysis of the bimetallic model, the structural parameters of the sensor are optimized, and the sensor is simulated and analyzed with ANSYS. Then, the sensor is developed according to the simulation results, and the temperature test system is built to test the performance of the sensor. The results show that the sensitivity of the temperature sensor is 49.3 pm/°C, which is about 4.9 times that of the bare FBG sensor, and the linearity is over 0.999. The research results provide a reference for developing the same type of sensors and further improving the sensitivity of FBG temperature sensors.

Research on a bimetallic-sensitized FBG temperature sensor.

Temperature measurement is of great significance for research in the health monitoring of large structures and earthquake precursors. Against the frequently reported low sensitivity of fiber Bragg grating (FBG) temperature sensors, a bimetallic-sensitized FBG temperature sensor was proposed. The sensitization structure of the FBG temperature sensor was designed, and the sensor sensitivity was analyzed; the lengths and materials of the substrate and strain transfer beam were analyzed theoretically; 7075 aluminum and 4J36 invar were chosen as bimetallic materials, and the ratio of the substrate length to the sensing fiber length was determined. The structural parameters were optimized; the real sensor was developed, and its performance was tested. The results suggested that the sensitivity of the FBG temperature sensor was 50.2 pm/°C, about five times than that of a bare FBG sensor, and its linearity was more than 0.99. The findings offer a reference for developing sensors of the same type and further improving the sensitivity of the FBG temperature sensors.

Analysis of Strain Transfer in FBG-Based Embedded Optical Fiber 3-D Shape Sensor

The FBG-based embedded optical fiber 3-D shape sensor (FBG-EOFSS) composed of several different materials finds potential applications in various industrial and medical applications. The strain loss during the strain transfer affects the change of the fiber Bragg grating (FBG) sensor’s wavelength, which in turn deteriorates the position accuracy. In this article, first, we analyzed the strain transfer in FBG-EOFSS and derived the optical–mechanical transformation equation. Then, a strain transfer model combining mechanics, materials, and optics is developed based on the finite-element analysis (FEA) method. The FEA simulation results show that the strain transfer rate decreases by 27.7%/mm with the increase of the adhesive thickness under FBG. The optimal average strain transfer rate is about 0.97 when the elastic modulus of the adhesive is in the range of 109–1014 Pa. Moreover, the effect of the strain transfer on the position accuracy is analyzed. The relative position error of 3-D shape reconstruction increases by 8.4% with the strain transfer rate decreasing by 10%. Finally, an experimental system composed of the clamp, the displacement stage, and the FBG demodulator is developed. The strain sensitivity coefficients of bare FBGs and FBG sensors were tested quantitatively. The corresponding average strain transfer rate was calculated by the strain sensitivity coefficients. The results can potentially conduce to the design of 3-D shape sensors and improve the positioning accuracy.

Validation of Packaged Clad-Etched Fiber Bragg Grating as Underwater Acoustic Sensor

The low intrinsic pressure sensitivity of fiber Bragg grating (FBG) sensors impede their potential applications in the field of hydro-acoustics. This article demonstrates a simple, yet effective way of improving the aggregate hydro-acoustic sensitivity of a bare FBG sensor through clad-etching of the fiber sensor followed by a side-hole polymer packaging. The proposed underwater acoustic sensor system is characterized and validated with a standard lead zirconate titanate (PZT)-based hydrophone under various experimental conditions. This is the first time, to the best of our knowledge, such a high acoustic pressure sensitivity enhancement of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim 50$ </tex-math></inline-formula> dB, in comparison with a bare FBG sensor, is reported using a conventional and highly operationally stable wavelength interrogation technique. The minimum detectable acoustic pressure is calculated to be <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim 1$ </tex-math></inline-formula> Pa/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$(\text {Hz})^{1/2}$ </tex-math></inline-formula> with a maximum responsivity of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim 100$ </tex-math></inline-formula> nm/MPa in the investigated frequency range, with excellent performances in terms of linear response and repeatability. The dynamic range, in the present scenario, is only limited by the range of the interrogation system. The proposed underwater acoustic sensor along with the interrogation technique is a highly compact and cost-effective method for underwater acoustic sensing. Thus, the obtained results can be used for developing and producing long, narrow hydro-acoustic quasi-distributed sensor arrays for marine explorations and other applications.

Study on strain sensing property of fiber Bragg grating based on surface strain measurement of mechanical structure.

Strain monitoring is of great significance to identify the failure of key mechanical components and ensure the good operation of mechanical equipment. In terms of the common issue of the low sensitivity of fiber Bragg grating (FBG) strain sensor in strain measurement on the mechanical structure surface, this paper describes a flexible hinge strain sensor with the FBG as the sensitive element. A theoretical analysis of the sensitivity of the sensor is performed, a copper alloy material with a large elastic modulus is selected, the simulation analysis and optimization design for the structural parameters of the sensor are carried out by using ANSYS and Solidworks software, then the object of the sensor is developed according to the simulation results, and finally, the performance of the sensor is tested. The results show that the maximum deformation the sensor can measure is 0.5299 µε, the sensitivity of the sensor is ∼1.8870 pm/µε, which is approximately twice that of the bare FBG sensor, the linearity is greater than 99%, and the relative standard deviation of the repeatability is 0.9%. The results of the study provide a reference for developing the optical fiber strain sensor and further improving its sensitivity.

Feasibility of bare fiber Bragg grating sensing technology in the measurement of inner wall axial stress along open‐ended pipe pile in sand

Larger-diameter open-ended pipe piles are widely used as foundation for offshore structures. The lack of high-quality inner wall axial stress measurement results in difficulties in loading analysis of open-ended pipe piles during penetration and service period due to the effect of soil plugging. In this paper, two groups of model tests to assess the feasibility of bare fiber Bragg grating (FBG) sensing technology in monitoring the inner wall axial stress profile of open-ended aluminous pipe piles during penetration and horizontal loading are presented. One open-ended aluminous pipe pile was instrumented with bare FBG sensors and then penetrated into the sand bed with the rate of 20 and 30 mm/min, respectively. Two horizontal static loading tests were also conducted after 24-h penetration. The installation process of bare FBG sensors along open-ended aluminous pipe pile is introduced. The distribution profiles of axial stress and unit shaft resistance on inner wall and outer wall along model pile is discussed in detail. Model test results indicated that the bare FBG sensing technology was feasible to measure the axial stress of inner wall along open-ended pipe piles during penetration and horizontal loading in sand. During horizontal loading, the average values of qso/qsi are 0.717 and 0.727, respectively, for model pile with penetration rate of 20 and 30 mm/min. The axial stress difference of inner wall and outer wall along model pile in penetration and horizontal loading is the result of a combination of the inside and outside soil friction as well as of the pipe in-wall shear stiffness.

Thermal analysis of 22.9-kV crosslinked polyethylene cable joint based on partial discharge using fiber Bragg grating sensors

Thermal effects of power cables are critical factors for determining the remaining lifetime and efficiency of power grids. Herein, we propose a fiber Bragg grating (FBG) sensor package with high-temperature sensitivity. Using polyacetal, which has a higher coefficient of thermal expansion than typical optical fibers, we constructed the proposed sensor structure. The developed sensor exhibited about 14.1 times higher temperature sensitivity than a typical bare FBG sensor. To measure the temperature variation of power cable joints caused by partial discharge, we constructed a mock system similar to a manhole, and we installed the developed sensor at a 22.9-kV crosslinked polyethylene cable joint. The experimental result reveals that even a slight temperature difference of 0.2°C can be measured using PD (300 pC at 20 kV).

Experimental and numerical simulation investigation on vortex-induced vibration test system based on bare fiber Bragg grating sensor technology for vertical riser

Abstract The Vortex-Induced Vibration (VIV) test system on deepwater riser based on Bare Fiber Bragg Grating (BFBG) sensor technology was designed. Meanwhile, a riser VIV response numerical model was established based on the work-energy principle. The results show that the first-order vibration frequency dominates the vibration of the riser, and as the velocity increases, the dominant frequency of the riser gradually increases under the effect of different top tensions. At the same velocity, as the top tension increases step by step, the dominant frequency and fatigue damage at the same position along the axial length of the riser both gradually decreases. The model test and numerical simulation show a relatively consistent change, maintaining a high degree of agreement. The process control system based on BFBG of model test has excellent performance, and FBG sensors have great advantages in VIV test of a vertical riser in water.

Study of fiber Bragg gratings with TiN-coated for cryogenic temperature measurement

In this study, single metal-coated Fiber Bragg grating (FBG) sensors and bare FBG sensors were placed in an aluminum alloy mold, which were in turn packaged in a thermal insulation container to monitor the temperature response, and used as cryogenic temperature sensors. Specifically, the performance of titanium nitride (TiN)-coated FBG sensors in cryogenic temperature environments was evaluated. Using an optical spectrum analyzer (OSA), the reflection spectra of the TiN-coated FBG sensors were measured and compared with those of bare FBG for temperatures ranging from 25 to −195 °C. The results showed that the TiN-coated FBG exhibited a temperature sensitivity of 10.713 pm/°C and R-squared value of 0.995. The TiN-coated on the FBG altered the non-linear characteristics of the thermo-optic and thermal expansion coefficients of the FBG. Through this research to get thermo-optic coefficient (ξ = 5.12 × 10−6) and thermal expansion coefficient (α = 1.81 × 10−6) and resulted in more precise measurements at cryogenic temperature.

Application of the Microclamped Fiber Bragg Grating (FBG) Sensor in Rock Bolt Support Quality Monitoring

The force‐measuring rock bolt instrumented with bare fiber Bragg grating (FBG) sensors are generally factory‐fabricated. To enable users to fabricate a force‐measuring rock bolt by themselves, the microclamped FBG sensor is proposed to replace the encapsulated bare FBG sensor. A theoretical formula of strain sensitivity is also established. The strain sensibility measured by indoor calibration is consistent with the theoretical one, indicating that the microclamped FBG sensor can measure strain accurately. Besides, the measured strain sensibility coefficient (wavelength difference/strain) matches the theoretical values, making the installed microclamped sensor free from the need for recalibration and proving the installation method to be reliable. Also, the test sensitivity can be adjusted as needed. The instrumented rock bolt with microclamped FBG sensors shows great mechanical performance in the field test, awaiting further usage in applications.

A FBG Tilt Sensor Fabricated Using 3D Printing Technique for Monitoring Ground Movement

In this paper, 3D printing method and fiber Bragg grating (FBG) sensing technology were combined to fabricate a simple and small size tilt sensor. The raw material for creating different sensor components was Polylactic Acid (PLA) material. Bare FBG sensors were directly inscribed into PLA material using fused deposition modeling (FDM) process for complete protection. Printing test using FDM process indicate that bare optical fiber cables successfully survived the FDM process even though the melted PLA materials approached a maximum temperature of around 220°. Two FBG sensors characterized by different wavelength values were symmetrically arranged inside a sensing unit for the measurement of the tilt angle in terms of their wavelength shifts. The minimum resolution and the measurement range of this new FBG tilt sensor were 0.084° and ±75°, respectively. The measurement performance of this new tilt sensor was also verified with the conventional tilt sensor. A small slope model was built up in the laboratory to examine the measurement performance of three FBG tilt sensors which were mounted at different depths inside the slope model. The measured horizontal displacement data from FBG tilt sensors were verified with the output data of the Linear Variable Displacement Transducer (LVDT) and the obtained maximum error was less than 3% when the maximum displacements are approached.

Feasibility Study of Fiber Bragg Grating Sensor for Quench Detection of High Temperature Superconductors

The energy stored in a 2 MW motor coil, or a generator field winding, may be as high as 25 kJ (for a 5 H field windings with 100 A current) and this energy release would be catastrophic if rapidly released. Therefore quench detection and protection in large machine coils is a significant concern. In this work, the feasibility of uncoated Fiber Bragg Grating (FBG) sensors for high temperature superconductor (HTS) magnet quench detection has been studied by studying the thermal distribution of a vertical HTS tape stack without applying current. The ceramic heater and PT100 platinum resistance sensors were employed to trigger the hotspot and provide a temperature reference. A proposed thermal-mechanical model was used to predict the response of the attached FBG sensors. The measured results agreed well with the prediction results, and the temperature sensitivity of uncoated FBG can be improved significantly by the thermal expansion of HTS tape and epoxy at the operating temperature of HTS. The thermal distribution results were studied by the attached FBG sensors. Finally, a mechanical stress test was carried out to simulate the effect of magnetic force on the FBG temperature sensor. The result shows that the slow change of stress can be ignored in comparison with the rapid temperature change in the ramping process, the FBG sensor can work properly for quench detection on HTS coil operation state. This paper provides the feasibility of the application of bare FBG sensors for the quench detection of HTS magnets. Our purpose is to monitor the temperature in real time, and give a quick feedback for any possible heat disturbance.

A High Sensitivity FBG Strain Sensor Based on Flexible Hinge.

For the purpose of improving the sensitivity of the fiber Bragg grating (FBG)-based strain sensor. A novel FBG-based strain sensor with high sensibility was designed by means of a flexible hinge bridge displacement magnification structure. This sensor can be used to accurately measure the strain of a mechanical structure surface. In this paper, the strain sensitization amplification factor of the sensor was calculated by using the flexible matrix method and the strain energy theory. The magnification had been verified by using simulation analysis and experimental results, and the error between theoretical calculation and simulation analysis was less than 7%. The result shows that the strain sensitivity of the sensor is 10.84 pm/με, which is about 10 times to that of the bare FBG sensor. This sensor is sensitive to micro-strain, so it can be well applied to health monitoring of a mechanical system.

Stress Monitoring on GFRP Anchors Based on Fiber Bragg Grating Sensors.

Glass fiber-reinforced polymer (GFRP) bolts have been widely used in some applications of grouted anchors because of the advantages of better resistance to corrosion, high strength-to-weight ratio, low electromagnetic properties, and so on. This study presents a field test to assess the feasibility of fiber Bragg grating (FBG) sensors in monitoring the stress profile of GFRP anchors during pulling test. Two GFRP anchors were fully instrumented with FBG sensors and then installed into the ground using a drilling and grouting method. To measure the stress profile along test anchors, seven bare FBG sensors were arranged in a single optical fiber and then embedded in the middle of GFRP bolts in the process of extrusion molding. The procedure for embedding bare FBG sensors into GFRP bolts is introduced first. Then, the axial forces and shear stresses that were calculated from the measurements of the FBG sensors are discussed. The field test results indicate that the embedded FBG technology was feasible to monitor the stress state of GFRP anchors during pulling.

Highly sensitive fiber Bragg grating-based pressure sensor using side-hole packaging.

In this work, an analysis of pressure response of a fiber Bragg grating (FBG) sensor in a side-hole package is presented using the finite element method. Various parameters of the side-hole packaging such as hole radius, the distance of separation between them, the radius and length of the package, and the choice of the package material are considered and optimized in order to promote maximum pressure sensitivity of the FBG sensor. This investigation on optimization of the side-hole package parameters gives rise to pressure sensitivity of nearly 105 times as compared with the bare FBG sensor, with the numerical values of 3pm/MPa for a bare FBG sensor to ∼280,000 pm/MPa for an optimized side-hole package FBG sensor. Such high-pressure sensitivity of an FBG sensor is being reported for the very first time in this work, to the best of our knowledge, and can be considered as the initial step toward the realization of a highly sensitive hydrophone based on FBG for sensing underwater acoustic signals.

Theoretical analysis and coating thickness determination of a dual layer metal coated FBG sensor for sensitivity enhancement at cryogenic temperatures

Use of Fiber Bragg Grating (FBG) sensor is very appealing for sensing low temperature and strain in superconducting magnets because of their miniature size and the possibility of accommodating many sensors in a single fiber. The main drawback is their low intrinsic thermal sensitivity at low temperatures below 120 K. Approaching cryogenic temperatures, temperature changes lower than a few degrees Kelvin cannot be resolved, since they do not cause an appreciable shift of the wavelength diffracted by a bare FBG sensor. To improve the thermal sensitivity and thermal inertia below 77 K, the Bare FBG (BFBG) sensor can be coated with high thermal expansion coefficient materials. In this work, different metal were considered for coating the FBG sensor. For theoretical investigation, a double layered circular thick wall tube model has been considered to study the effect on sensitivity due to the mechanical properties like Young’s modulus, Thermal expansion coefficient, Poisson’s ratio of selected materials at a various cryogenic temperatures. The primary and the secondary coating thickness for a dual layer metal coated FBG sensor have been determined from the above study. The sensor was then fabricated and tested at cryogenic temperature range from 4-300 K. The cryogenic temperature characteristics of the tested sensors are reported.

Thermal strain measurement of EAST tungsten divertor component with bare fiber Bragg grating sensors.

Fiber Bragg Gratings (FBGs) have been widely used in the sensor field to monitor temperature and strain. However, the weak mechanical property of optical fibers and insufficient heat-resistant property of general optic-fiber sensors have prevented it from being widely used, such as in some extreme engineering situations. In this work, a bare FBG sensor system had been introduced to measure thermal strain of an Experimental Advanced Superconducting Tokamak tungsten divertor component under baking condition. This strain measurement system had withstood as high temperature as 210 °C and finished the measurement experiment successfully. Meaningful measurement results had been obtained and analyzed, which showed the applicability of such a bare fiber grating sensor system and as well contributed to studying on tungsten divertor's thermal strain conditions.

Fiber-Reinforced Polymer-Packaged Optical Fiber Bragg Grating Strain Sensors for Infrastructures under Harsh Environment

Optical fiber Bragg grating (FBG) has been recognized as an outstanding high-performance local monitoring sensor and is largely applied in structural health monitoring (SHM). This paper proposes a series of fiber-reinforced polymer- (FRP-) packaged optical fiber Bragg grating strain sensors to completely meet the requirements of rough civil engineering infrastructures, and their sensing performance under normal environment and harsh environment is experimentally investigated. It is experimentally and theoretically proved that FRP-packaged FBG strain sensors maintain excellent sensing performance as the bare FBG sensor under a harsh environment, and their durability is significantly enhanced due to the FRP materials. These FRP-packaged FBG strain sensors are successfully applied in the SHM system of Aizhai Bridge.

Measurement error of surface-mounted fiber Bragg grating temperature sensor.

Fiber Bragg grating (FBG) sensors are extensively used to measure surface temperatures. However, the temperature gradient effect of a surface-mounted FBG sensor is often overlooked. A surface-type temperature standard setup was prepared in this study to investigate the measurement errors of FBG temperature sensors. Experimental results show that the measurement error of a bare fiber sensor has an obvious linear relationship with surface temperature, with the largest error achieved at 8.1 °C. Sensors packaged with heat conduction grease generate smaller measurement errors than do bare FBG sensors and commercial thermal resistors. Thus, high-quality packaged methods and proper modes of fixation can effectively improve the accuracy of FBG sensors in measuring surface temperatures.

Application of Packaging Technique in Fiber Bragg Grating Temperature Sensor for Measuring Localized and Nonuniform Temperature Distribution

The development of Fiber Bragg Grating (FBG) sensing technique has improved significantly especially in the sensor head design and real-time data acquisition technique. This paper presents the development of a simple and cost effective packaging technique that further enhances the performances of the FBG sensor. The packaged FBG sensor overcomes the nonuniform heat distribution of a bare FBG that causes eccentric response of FBG spectrum. Therefore, the packaged FBG sensor could be operated for a localized area with high temperature differential. The packaging also compensates the unwanted strain effect from the surrounding which makes temperature measurement become more accurate. The experimental works have been successfully carried out to demonstrate the system operation and the packaging functionalities. The temperature sensitivity coefficient of the bare FBG sensor measured in experiment is 10.05 pm/°C, while the packaged fiber sensor is 10.09 pm/°C, which are expected from the design.