Optical Fiber Bragg Grating Sensors Research Articles

Fiber-Optic Bragg Grating Sensor for Photothermally Examinating Moisture of Meat

The illegal water injection into meat not only breaks the market equity, but also deteriorates the meat quality and produces harmful substances. In this work, we proposed a fiber Bragg grating (FBG) sensor that enabled fast, quantitative, and in-situ detection of the moisture content of water-injected meat. The FBG was written in the erbium-ytterbium (Er/Yb) co-doped fiber, which could perform the self-photothermal effect by injecting the near infrared laser into the fiber. As the heated fiber sensor probe was inserted into the meat sample, the temperature decreased due to the heat dissipation mediated by moisture. The intracore Bragg grating could monitor the temperature loss by recording the Bragg wavelength shift, which reflected the water content quantitatively. The results revealed that the sensor could complete the detection within 15 s. The sensor’s sensitivity to detect changes in the pork water content was theoretically calculated to be 0.090847%. The proposed sensor is expected to provide a novel approach for examination of the meat moisture.

In Operando Monitoring the Stress Evolution of Silicon Anode Electrodes during Battery Operation via Optical Fiber Sensors.

Silicon (Si) anode has attracted broad attention because of its high theoretical specific capacity and low working potential. However, the severe volumetric changes of Si particles during the lithiation process cause expansion and contraction of the electrodes, which induces a repeatedly repair of solid electrolyte interphase, resulting in an excessive consuming of electrolyte and rapid capacity decay. Clearly known the deformation and stress changing at µε resolution in the Si-based electrode during battery operation provides invaluable information for the battery research and development. Here, an in operando approach is developed to monitor the stress evolution of Si anode electrodes via optical fiber Bragg grating (FBG) sensors. By implanting FBG sensor at specific locations in the pouch cells with different Si anodes, the stress evolution of Si electrodes has been systematically investigated, and Δσ/areal capacity is proposed for stress assessment. The results indicate that the differences in stress evolution are nested in the morphological changes of Si particles and the evolution characteristics of electrode structures. The proposed technique provides a brand-new view for understanding the electrochemical mechanics of Si electrodes during battery operation.

Contactor Fault Detection and Classification System Using Optical Fiber Bragg Grating Sensors

Contactor Fault Detection and Classification System Using Optical Fiber Bragg Grating Sensors

Processing and structural health monitoring of a composite overwrapped pressure vessel for hydrogen storage

A process and Structural Health Monitoring system was implemented on a Composite Overwrapped Pressure Vessel (COPV) for hydrogen storage at 350 bar to be used in a fuel-cell system of an Unmanned Aerial Vehicle. This work reports the embedment strategy of optical fibre Bragg grating (FBG) sensors to monitor the full life cycle of the vessel, consisting of an aluminium liner and a wound carbon fibre reinforced polymer composite overwrap. A FBG sensing array, bonded on the aluminium liner circumferential section, was covered with a localised unidirectional prepreg composite tape, enabling composite winding and curing monitoring. The sensing array strategy allowed to detect and locate Barely Visible Impact Damage resulting from drop-weight impact tests, based on the ratio of the residual strain amplitude between FBG sensor pairs. Errors as small as 17 mm and up to 56 mm were determined between the predicted and ‘real’ impact locations. To simulate the real-life operational pressure charging and discharging cycles, the COPV was subjected to cycling testing at different pressure ranges. The FBG sensors were able to monitor a total of 20 980 pressure cycles, revealing a linear response to the applied pressure, and remained operational after COPV failure. Furthermore, the FBG sensing array was able to detect the residual plastic strain caused in the aluminium liner by the autofrettage process that the COPV was subjected to prior to pressure cycling, at 600 bar for 2 min, to improve its fatigue performance. This manuscript also reports the COPV structural design by Finite Element Modelling (FEM), its manufacturing process and burst pressure testing for the FEM analysis validation. A small difference of 0.7% was found between the simulated and experimental determined burst pressure of 1061 ± 26 bar.

Installation and Use of a Pavement Monitoring System Based on Fibre Bragg Grating Optical Sensors

The evolution of technological tools, namely affordable sensors for data collection, and the growing concerns about maintaining roads in adequate conditions have promoted the development of continuous pavement monitoring systems. This paper presents the installation and use of an innovative pavement monitoring system, which was developed to measure the effects of vehicle loads and temperature on the performance of a pavement structure. The sensors used are based on fibre Bragg grating optical technology, collecting data about the strains imposed in the pavement and the temperature at which those measurements are made. The site selection for the system’s installation and the essential installation details to ensure successful data collection are addressed. A calibration procedure was implemented by performing falling weight deflectometer tests and passing preweighed heavy vehicles over the sensors. In addition to validating the system installation, the results obtained in the calibration confirmed the importance of adequately choosing the distance between sensors. Differences of 50 mm in the position of the load may cause differences of about 20% to 25% in the resulting strains. These results confirmed the importance of increasing the sensor concentration in wheel paths. Furthermore, for loads between 25 kN and 65 kN, raising the temperature by 8 °C caused an increase of about 20% in the horizontal tensile strains measured in the pavement. In summary, it was possible to conclude that this innovative system is capable of capturing the effects of temperature and vehicle speed on the response of the pavement, which may be considered an advantage of this type of monitoring system when compared to those that are only used to determine the loads applied to the pavement or to characterise the type of vehicle.

Development of Flexible Tooling for Deformation Sensing Applied to Composite Materials Fabrication Process

Among the existing techniques for composite materials manufacturing, Vacuum Assisted Resin Infusion (VARI) is a liquid composite molding (LCM) process where resin flows through a dry fiber preform to fully impregnate it. This method uses flexible film sealed onto a rigid mold to form the infusion cavity containing the fibers. As the fabric preform is impregnated, its thickness varies due to the changes in the applied compaction pressure. This thickness variation affects the resin flow and the final fiber volume fraction of the manufactured part. This study focuses on the initial steps of developing an integrated acquisition system for thickness variation monitoring during VARI. The conventional flexible tooling is to be replaced by a flexible membrane equipped with optical fiber Bragg grating (FBG) sensors. A prototype was developed by embedding FBG sensors in a silicon rubber material and initial measurements of a cylindrical profile curvature were performed. Preliminary results show satisfactory precision of the device, which opens a gap for a more precise and accurate thickness monitoring process during real part manufacturing.

Design of optical fiber Bragg grating-based sensors for flow measurement in pipes

In this work, optical Fiber Bragg grating (FBG) sensors were used to measure water flow in pipes. Several types of coatings were incorporated into the design of the sensors to examine their effects on the elastic strain that the fiber underwent as a result of the water flow. ANSYS-CFX V2020 R2 software was used to model the elastic strain encountered by the fiber under various flow rates in order to assess the performance of the FBG sensors. The calculations and experimental data exhibited good convergence, demonstrating the accuracy of the FBG sensors in determining water flow. These calculations and procedures can be extrapolated to any other fluid.

In vivo brain temperature mapping using polymer optical fiber Bragg grating sensors.

Variation of the brain temperature is strongly affected by blood flow, oxygen supply, and neural cell metabolism. Localized monitoring of the brain temperature is one of the most effective ways to correlate brain functions and diseases such as stroke, epilepsy, and mood disorders. While polymer optical fibers (POFs) are considered ideal candidates for temperature sensing in the brain, they have never been used so far in vivo. Here, we developed for the first, to the best of our knowledge, time an implantable probe based on a microstructured polymer optical fiber Bragg grating (FBG) sensor for intracranial brain temperature mapping. The temperature at different depths of the brain (starting from the cerebral cortex) and the correlation between the brain and body core temperature of a rat were recorded with a sensitivity of 33 pm/°C and accuracy <0.2°C. Our in vivo experimental results suggest that the proposed device can achieve real-time and high-resolution local temperature measurement in the brain, as well as being integrated with existing neural interfaces.

Application of FWM-Based OFC for DWDM Optical Communication System with Embedded FBG Sensor Network

Abstract Four-wave mixing optical frequency comb fibre-based setups (FWM-OFCs) have the potential to improve the combined dense wavelength division multiplexed passive optical network (DWDM-PON) and fibre Bragg grating (FBG) temperature sensors network providing easier application, broader technological opportunities for network development, and energy efficiency by substituting a power-demanding laser array. In this research, OFCs are generated for the purpose of combined network application of DWDM-PON and FBG optical sensors. The paper also investigates compatibility scenarios with OFCs in such systems. The mathematical simulation model has been developed and the performance of FWM-OFC based 8-channel 50 GHz spaced non-return-to-zero on-off keying (NRZ-OOK) modulated DWDM-PON transmission system, operating at 50 km single-mode fibre (SMF) with a bit rate of at least 10 Gbps embedded with 7 FBG optical temperature sensors, has been studied. As it is shown, FWM application results in OFC source that has fluctuations of the individual comb tones of less than 3 dB in power, and with an extinction ratio of about 33 dB for operation range of 192.9–193.25 THz, acting as a unified light source for all the data transmission channels. Embedded FBG optical sensors network causes negligible 0.3 dB power penalty.

Continuous Fiber Bragg Grating Optical Sensors for Superconducting Magnet Quench Detection: The Effects of Attenuation and Position in the Array

Quasi-continuous arrays of fiber Bragg gratings (FBGs) show great promise for quench protection in superconductors. The number of identical gratings and the reflectivity of the individual gratings are important parameters determining the sensitivity of the system to a local temperature or strain perturbation. We apply strain to shift the Bragg wavelength of a single FBG through the spectrum of an array of up to 1000 identical gratings, and map out the detection sensitivity for a given <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\Delta \lambda$</tex-math></inline-formula> relative to the peak reflectance of the FBG array, and show that the position of the perturbed FBG matters only due to the broadband attenuation of the probe light down the array. Varying the number of gratings in the array shows the interplay between the number of gratings, the reflectivity of the gratings, and the sensitivity of the system. We demonstrate that the ability to model the spectrum of the FBG array is crucial to predicting system performance.

Optimization of surface bonding methods for fiber Bragg grating sensors at cryogenic temperatures

Optical fibre Bragg grating (FBG) sensors have been used for cryogenic temperature and strain measurements in a variety of applications, e.g., hotspot detection in high-temperature superconductor magnets. In such application, the sensitivity of the FBG and its reflection spectrum strongly depend on the methods of application, chiefly the adhesion and the elasticity of the bond, and the birefringence effect. To investigate those features, Draw Tower Gratings (DTG®) with Ormocer coating and FemtoSecond Gratings (FSG) with polyimide coating are mounted in the V-shaped grooves and on the surface of copper substrates using Stycast and Apiezon N. Their performance is investigated between 293 K and 77 K. The FBG sensors mounted in the V-grooves can achieve higher temperature sensitivities than those on the surface of the copper due to enhanced thermal strain transfer. It is found that although there is no obvious difference between the temperature sensitivity of the Ormocer and polyimide coated FBGs at cryogenic temperatures, the polyimide coated FBGs show superior reflection spectra, whose peak intensity remains strong at 77 K. Cryogenic vacuum grease Apiezon N is shown to bond the fibre well to the copper at cryogenic temperatures, which also reduces the birefringence effect induced from the non-uniform strain across the FBG. This paper provides practical knowledge of bonding FBG sensors in cryogenic environments for temperature and/or strain measurements.

Analysis of the fixing process of FBG optical sensors for thermomechanical monitoring of aerospace applications

In aerospace, lots of components can be defined as “safety critical”. As a result, it is crucial to early identify the failure precursors when the effects on the systems performances are still practically insignificant. For this reason, complex networks of sensors had been developed and integrated into different parts to monitor several operational parameters, useful for evaluating their health (such as such as temperatures, displacements, vibrations, etc). Clearly, due to the importance of data collected, the technology employed shall be very reliable, even while working in harsh environments. Sensors based on optical fiber Bragg grating (FBG) meet these requirements. However, the fiber’s integration process in the considered system is really crucial, from the moment that it could influence the sensors output. In this regard, gluing the optical sensors is really a critical activity, because the effects of the glue’s retire and its viscous assessment shall be analysed and quantified. In this work, it was done by comparing performances of two equal samples, each one with an FBG glued on it. The first sample was prepared about two months before the second one. Furthermore, data from both FBG have been collected from the gluing phase of the second sample and in the following days. The results showed that the assessment of the resin evolved in different phases, but all of them were united by the fact that the overall process makes sensors measures not reliable during this specific transitory phase. By observing the evolution of the linear fit gradients, it can be stated that the variations reached their maximum in the middle of the gluing process, when the fiber was detached from the tensioning device. Finally, at the end of the overall process, data output resulted stable, so making FBG employable for reliable thermal or strain measures.

Investigation of gesture recognition based on optical fiber Bragg grating sensors

This paper discusses the investigation of optical fiber Bragg grating (FBG) sensors in gesture recognition. We proposed a new attempt by embedding the FBG into polyimide films, then the FBG sensors are arranged on the upper surface of the muscle along the normal direction of the muscle to recognize gestures through detection of muscle activity. A series of experiments were conducted by the selected eight healthy subjects, to quantitatively studied and comprehensively evaluated the performances of this method. Composite experimental systems with multiple gestures, multiple samples and multiple cycles were established for the investigation of the reproducibility and further verification of feasibility. The results show that the FBG sensors has good reproducibility and no chirp effect of the reflection spectrum profile during gesture recognition. The proposed method can effectively detect six gestures with quick responses. Moreover, this method also has the potential to distinguish muscle deformation with different physiological characteristics.

Evaluation of the orientation impact on thermal behavior of cylindrical Li-ion batteries in different cycling conditions using fiber Bragg grating sensors

In this work, optical fiber Bragg grating (FBG) sensors were used to evaluate the thermal performance of an 18650 Li-ion battery (LiB) operating under normal and abusive conditions while positioned in horizontal and vertical orientations. In total, three FBG sensors were used to track in real-time the temperature variation of the cathode, middle, and anode of the LiB. Tests for each orientation were performed, where each of them consisted of two cycles: the first one with normal charge/discharge conditions, operating between 2.75 V and 4.20 V, and another in abusive conditions, operating between 2.00 V and 4.95 V. The battery was submitted to constant current charge and discharge steps, with rest intervals between each operation. The results suggest that, in general, the temperature variation while operation in vertical orientation is lower when compared to the horizontal one, mainly in the anode, while the LiB is submitted to the abusive charge procedure, where the temperature variation difference achieved 3.4 ± 0.2 °C. In addition, the FBG sensors were able to track in real-time the temperature variation of three different locations of the battery simultaneously. The temperature variation registered for each sensor is shown and discussed.

Reduction of Thermal Residual Strain in a Metal-CFRP-Metal Hybrid Tube Using an Axial Preload Tool Monitored through Optical Fiber Sensors.

Thermal residual strains/stresses cause several defects in hybrid structures and various studies have reported the reduction of residual strain. This paper describes a method for reducing thermal residual strains/stresses in metal-CFRP-metal hybrid tubes (MCMHT). The proposed axial preload tool provides two ways to reduce the thermal residual strains/stresses during the co-cure bonding process: pre-compressing of the metal layers and pre-stretching of the unidirectional carbon fiber reinforced polymer (CFRP) layers. An online measurement technique with embedded optical fiber Bragg grating (FBG) sensors is presented. Thermal residual strains are evaluated based on classical lamination theory with the assumption of plane stress. The theoretical calculations and measurement results agree well. Furthermore, the dynamic characteristics of the MCMHTs are tested. The results show that the reduction of residual strain increases the natural frequency of the MCMHT, but is detrimental to the damping capability of the MCMHT, which imply that the intrinsic properties of the metal-composite hybrid structure can be modified by the proposed axial preload tool.

Designing of Fiber Bragg Gratings for Long-Distance Optical Fiber Sensing Networks

Most optical sensors on the market are optical fiber Bragg grating (FBG) sensors with low reflectivity (typically 7-40%) and low side-lobe suppression (SLS) ratio (typically SLS &lt;15 dB), which prevents these sensors from being effectively used for long-distance remote monitoring and sensor network solutions. This research is based on designing the optimal grating structure of FBG sensors and estimating their optimal apodization parameters necessary for sensor networks and long-distance monitoring solutions. Gaussian, sine, and raised sine apodizations are studied to achieve the main requirements, which are maximally high reflectivity (at least 90%) and side-lobe suppression (at least 20 dB), as well as maximally narrow bandwidth (FWHM&lt;0.2 nm) and FBGs with uniform (without apodization). Results gathered in this research propose high-efficiency FBG grating apodizations, which can be further physically realized for optical sensor networks and long-distance (at least 40 km) monitoring solutions.

Quasi-Distributed Temperature Detection of Press-Pack IGBT Power Module Using FBG Sensing

In situ temperature near the chips in a multiple-chip power semiconductor module is important information for thermal design validation, condition monitoring, and overheat protection. Due to the compact packaging structure and clamping force, online detection of the temperature is challenging for a press-pack module. This study attempts to achieve measurement by integrating optical fiber Bragg grating (FBG) sensors into the device. Different integration schemes are examined using experiments and theoretical analysis to show the effects on the transient performance. It is shown that FBG sensors with plate housing can effectively indicate the transient temperature ripple in normal converter operation, owing to the fast dynamic response of a small size sensor. The package integration has only minimal effects on the original distributions of electrical, thermal, and mechanical quantities within the power module.

Temperature and Humidity Sensitivity of Polymer Optical Fibre Sensors Tuned by Pre-Strain.

Polymer optical fibre Bragg grating (POFBG) sensors are of high interest due to their enhanced fracture toughness, flexibility in bending, and sensitivity in stress and pressure monitoring applications compared to silica-based sensors. The POFBG sensors can also detect humidity due to the hydrophilic nature of some polymers. However, multi-parameter sensing can cause cross-sensitivity issues in certain applications if the temperature and humidity measurements are not adequately compensated. In this work, we demonstrate the possibility of selectively tuning sensors’ temperature and humidity sensitivities to the desired level by applying a certain amount of fibre pre-strain. The temperature sensitivity of POFBG sensors fabricated in perfluoropolymers (CYTOP) can be selectively tuned from positive to negative values, having the option for insensitivity in specific temperature ranges depending on the amount of the applied pre-strain. The humidity sensitivity of sensors can also be changed from positive values to insensitivity. The importance of thermal annealing treatment of POFBG sensors for improved repeatability in temperature measurements is also reported. An array of 4 multiplexed POFBGs was fabricated, and each sensor was pre-strained accordingly to demonstrate the possibility of having targeted temperature and humidity sensitivities along the same fibre.

Modelling the effect of hot gas torch heating on adjacent tows during automated fibre placement consolidation of thermoplastic composites

Many numerical models of the Automated Fibre Placement (AFP) manufacturing process have been developed to assist in the design process of composite components and structures. Although the tape placement process, in general is a 3-D transient process, limited models considering the 3-D nature of the process exist. This paper describes the development of a 3-D Finite Element (FE) model incorporating the effect of non-concentrated input heat flux distribution on adjacent paths. The model can predict an accurate thermal history of a thermoplastic composite laminate for a complete AFP deposition process. Composite laminates having plies made up of both a single tow and multiple tows have been simulated to study their thermal behaviour during the manufacturing process. Simulations have been conducted both with and without considering the input heat effect on adjacent tows for comparison. Temperatures were measured by embedding two different configurations of optical fibre Bragg grating (FBGs) sensors within the laminate during the lay-up process. This served a dual purpose of validating the model as well as the experimental methodology.

Miniaturized Strain-Free Fiber Bragg Grating Temperature Sensors

This work reports the design, fabrication, and characterization of a miniaturized metallic package for optical fiber Bragg grating (FBG) sensors. The aim of the package is to enhance the temperature sensitivity of the grating and eliminate the cross-sensitivities to mechanical effects, i.e., strain or impulsive forces. The package is simple in its design and developed using off-the shelf components: the FBG is first encapsulated into a steel tube, which is in turn placed inside another larger steel tube, where the fiber is kept loose to prevent that any mechanical effect is transferred to the grating. The package has a length of 25 mm and a diameter of about 2 mm, hence it is very compact. Concerning the performance, the packaged FBG has a linear temperature response in the range 5–50 °C with a sensitivity of 28.9 pm/°C, that is about three times that of a standard FBG. Whereas, concerning the longitudinal force sensitivity, it is decreased by two orders of magnitude in comparison to an unpackaged FBG. The temporal evolution of the device is also evaluated when subjected to a temperature step of about 20 °C, demonstrating a fast response time of 5 s. Finally, the effect of an impulsive force is evaluated by means of a falling mass attached to the fiber, demonstrating a negligible response and a strong resistance of the packaged FBG in comparison to unprotected one, which broke after the same test.