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Carbon fiber reinforced plastic (CFRP) structures are vulnerable to low-speed impacts, which will lead to almost invisible impact damage. Therefore, the timely localization of impact is of great si...

Two types of fiber optics, namely macro etched silica single mode fiber (SMF) and Fiber Bragg Grating (FBG) (Bragg wavelength of 1550 nm and 1554 nm) had been used to detect various types of honey samples, such as Apis Dorsata Honey, Trigona Honey and Capilano Australian Honey. To study the effect of exposure period in open environment at room temperature, all honey samples were exposed in open environment at room temperature from 2 to 10 days. In comparison with macro etched SMF and FBG (λB=1554 nm), the FBG (λB=1550 nm) portrays an excellent sensing properties with sensitivity and selectivity of 33.56 dB/RIU and 24.07 dB/RIU respectively. The output of this work concludes that the quality of honey based on optical output power reduces up to 0.35% as period of exposure to the open environment increased.

We propose a conductive carbon coated fiber Bragg grating (FBG) temperature sensor for enhancing temperature sensitivity. The temperature sensitivity of the proposed sensor is effectively enhanced by approximately 4 times higher than those of the bare-type FBG sensors due to thermal expansion and physical stress possessed by the conductive carbon coating, with the operating temperature ranging from 40 °C to 260 °C. The proposed sensor is much more convenient to fabricate compared to other similarly coated FBG sensors, giving a leap advantage of easy reformation of the coating to fit various housings. This proposed sensor is significantly ideal for various applications such as optoelectronic circuit and waveguide temperature measurement.

We investigate the performance of a cantilever-type force sensor based on PDMS-coated fiber Bragg gratings for robot-assisted surgical applications. The cross section area of PDMS matrix and the location of the FBG sensor inside it are optimized through simulations. The sensitivity of the proposed sensor is obtained to be around 0.5 mN.

Optical receiver modules play an important role in receiving signals from the Fiber Bragg Grating (FBG) sensors used for monitoring in civil structures, power facilities, and other sensing applications. When multiple sensors are used, more photo diodes receive optical signals; thin-film beam splitters with polarization characteristics are therefore used to reduce the size of the optical receiver module. Using a reflective semiconductor optical amplifier (R-SOA) with polarization characteristics in a sensing system with an FBG sensor, a structure that is less sensitive to polarization was developed by packaging a quad optical receiver module. A stabilization of ±4.67% was achieved using the dual optical receiver module in polarization and of ±1.20% with the quad optical receiver module in polarization.

In this paper, the effects of three types of commonly used adhesives were studied via experiment and simulation. The results show that the strain sensitivity of fiber Bragg grating (FBG) sensor is about 1.201 pm/μe and the effects of three adhesives of epoxy resin, AB glue and 502 glue with the same thickness and length on sensitivity coefficients of FBG sensors are 99.20%, 93.80% and 96.50%, respectively. Finally, the sensitivity coefficient of the simulated FBG sensor is the same as the test result by ANSYS modeling.

According to the reflection behavior of the fiber Bragg grating, the monitoring method of reinforced concrete was presented in this paper. A postprocessing method to extract crack openings from a three-dimension finite element computation was proposed. The three-axial stresses of the FBG were extracted also from the same finite element computation, and then, were converted as a wavelength deformation of the FBG reflected spectrum. The three-dimension computation result of crack width versus the FBG spectrum deformation was investigated using laboratory experiments. The results showed that the numerical values approximated the experimental ones.

Pantograph-catenary system provides electric energy for the subway lines; its health status is essential to the serviceability of the vehicle. In this study, a real-time structural health monitoring method based on strain response inversion is proposed to calculate the magnitude and position of the dynamic contact force between the catenary and pantograph. The measurement principle, calibration, and installation detail of the fiber Bragg grating (FBG) sensors are also presented in this article. Putting this monitoring system in use, an application example of a subway with a rigid overhead catenary is given to demonstrate its performance. The pantograph was monitored and analyzed, running underground at a maximum speed of 80 km/h. The results show that the strain response inversion method has high measurement accuracy, good data consistency, and flexibility on sensor installation. It can accurately calculate the magnitude and location of the contact force exerted on the pantograph.

A hydrogen and humidity simultaneous measurement system based on an integrated FBG sensor is proposed. Experimentally results indicate the sensor shows a high degree of hydrogen discrimination ranging from 100ppm-8000ppm and a high degree of humidity discrimination ranging from 20% - 80%, along with the high performance of anti-mutual interference of hydrogen and humidity.

Structural health monitoring techniques have captured much interest and attention of researchers owing to their potential in providing spatial and quantitative information regarding structural damage and the performance of a structure. Integrated health monitoring of structures with advances in sensor technology can improve structural reliability, system performance, and safety. Pipelines exist for the transport of crude/refined petroleum, oil, natural gas, and biofuels. These pipelines are made from steel which are usually buried. These pipelines are prone to natural and man-made damages. Hence, monitoring of these buried pipelines becomes essential. Leakages of oil and gas from pipeline are dangerous for people and environment. Detection of leakage along the pipeline network is an essential part of the maintenance activity which is always a difficult task. The experimental studies carried out on using FBG sensors for monitoring corrosion in buried pipelines has been described in this paper. In this study, the FBG sensor is used for measuring the hoop strain variation in the pipeline and from the variation of hoop strain corrosion is identified. Hence, the FBG sensor technology can be adopted for monitoring corrosion in the buried oil and gas pipelines.

An interrogation technique for fiber Bragg grating (FBG) sensor is devised with customized in-line edge filtering single-multi-single mode fiber (SMS) component and optical time-domain reflectometer (OTDR). The performance of the proposed SMS-OTDR interrogation is established by temperature and strain sensing analysis of a standard FBG. The temperature and strain sensitivity values which are verified by theoretical analysis are estimated as 1.01 × 10−2 dB/℃ and 3.9 × 10−4 dB/µɛ respectively. Furthermore, we have optimized and achieved controlled etching on FBG and tested it for hiking the sensitivity. Using the etched FBG, an enhanced temperature and strain sensitivity of 6.7 × 10−2 dB/℃ and 3.2 × 10−3 dB/µɛ covering the range of 20–200 °C and 100–2015 µɛ respectively are also recorded and analysed by this integration method. Eventually, the efficiency and cost-effectiveness of the proposed method are compared with various reported techniques and presented here.

Smartphone-based interrogation of FBG sensor is reported for the first time. Setup consists of a smartphone and low cost off-the shelf available components: a section of DVD disk, razor blade slit and a 3D-printed holder. Smartphone flashlight LED was used as a light source, and FBG spectra were projected on the smartphone camera by a DVD disk.

The existing fiber Bragg grating (FBG) wavelength demodulation system based on Mach-Zehnder interferometer and phase sensitive optical time domain reflectometer(φ-OTDR) is limited by the system structure and working principle. It can obtain large demodulation capacity through time division multiplexing (TWD), but it can only measure the relative change of wavelength. In this paper, we design and experimentally verify a scheme for measuring the absolute wavelength of FBGs. This scheme can obtain the absolute wavelength of a large-capacity FBG sensor array without affecting the performance of the original technical indexes, and also theoretically improve the accuracy of the system. Its dynamic real-time absolute wavelength demodulation error less than 0.1pm.

Fiber Bragg grating (FBG) sensors are widely used because of their advantages of light weight, corrosion resistance, electromagnetic interference resistance and long life, especially in the field of aerospace. In order to meet the requirements of deformation measurement and structural evaluation of the new generation aircraft, and improve the performance and safety of aircraft, it is of great significance to carry out the research on condition monitoring technology based on FBG sensor in dynamic flight state. In the paper, FBG sensors are applied to the measurement of aircraft wing and the results show that the measurement accuracy of FBG sensor can reach at 1 mm, which has good performance.

We demonstrate a self-referencing FBG sensor for damage detection. The sensor compensates for changing environmental conditions by measuring the reference and distorted signal simultaneously and uses traveling acoustic waves in optical fibers.

A fast FBG sensor interrogation method based on silicon on insulator (SOI) microring resonators is reported. The simple and effective FBG sensor interrogation scheme is based on the detection of the signal intensity at the drop port of a microring resonator, and its interrogation speed depends only on the photodetector and the front end electronics. The performance of the FBG interrogation method is evaluated in terms of minimum detectable wavelength shift and dynamic strain resolution and compared with a commercial FBG interrogator.

Recently, it has become necessary to develop a monitoring technology that combines an FBG (fiber Bragg grating) sensor as a means for continuously monitoring whether the reinforcing effect of an FRP (fiber-reinforced polymer) is maintained on FRP-reinforced structural members. However, most existing research focuses on the insertion of FBG sensors into bar-shaped FRPs, and there is a lack of studies that analyze the details for an FRP strip combined with FBG sensors. In this regard, this paper seeks to develop a reinforcement for an NSM (near-surface-mounted) retrofit in which an optical fiber with an FBG sensor is combined with FRP strips. For this, a series of experiments were performed to find the adhesive strength of optical fiber-epoxy-FRP interfaces, the tensile strength of the FBG sensor part of optical fiber with a reflection lattice, and the sensing performance depending on the adhesion length between the optical fiber and the FRP strips. As a result of the study, the adhesion length not less than 20 mm in one direction from the center, with a total adhesion length of 40 mm, needs to be secured when the optical fiber with an FBG is attached and fixed between the two FRP strips with epoxy. In addition, it is expected that the proposed model can be used to properly predict the strain transfer of an FRP strip with a fiber optic sensor and can also be utilized when determining optimum dimensions.

We demonstrated in this work a filterless, multi-point and temperature-independent FBG (fiber Bragg grating) dynamical demodulator using pulse-width-modulation (PWM). In this approach, the FBG interrogation system is composed of a tunable laser and a demodulator that is designed to detect the wavelength shift of the FBG sensor without any optical filter making it very suitable to be used in harsh environments. In this work, we applied the proposed method that uses the PWM technique for FBG sensors placed in high pressure and high-temperature environments. The proposed method was characterized in the laboratory using an FBG sensor modulated in a frequency of 6 Hz, with a 1 kHz sweeping frequency in the wavelength range from 1527 to 1534 nm. Also, the method was evaluated in a field test in an engine of a thermoelectric power plant.

Accurately sensing signal detection using fiber Bragg grating (FBG) sensors is crucial for object's controlling in industry, aircraft manufacturing, and health care center of the Internet of Things (IoT) application. In this paper, we suggest and demonstrate the Bragg wavelength detection techniques for bus network architecture using a multilayer perceptron method (MLP). The proposed MLP model can learn quickly the features of FBGs spectra and capable to determine the central wavelength of distinct FBGs from the overlap spectra. The proposed system demonstrated that higher sensing signal detection accuracy is achieved even when the spectra of the FBG is partly or entirely overlapped.

The crack width of concrete structure was investigated and discussed in this paper. The paper used a post-processing method to calculate the crack width with a 3D FEM of loaded concrete slab. The FBG stresses of three directions were extracted from the 3D FEM, and then were used to calculate the spectrum distortion of FBG sensor. Experiments were carried out with a FBG sensor installed inside a concrete slab, and the experimental results of crack width of concrete slab were given also. The results demonstrated that the spectrum deformation could be applied as an effective indicator to evaluate the crack width of reinforced concrete.

In this paper, a cladding etched fiber Bragg grating (FBG) sensor is proposed to monitor chemicals (Ca(OH)2 and Mg(OH)2) which produces in civil structures. Further, to enhance the sensitivity, an indium tin oxide is coated on cladding etched FBG sensor. It is observed that the indium tin oxide coated sensor is highly sensitive in terms of chemical detection and inducing wavelength shift of 12 nm and 10 nm with Ca(OH)2 and Mg(OH)2 respectively. On the other hand, cladding etched FBG sensor produces a wavelength shift of 8 nm and 6 nm for Ca(OH)2 and Mg(OH)2 respectively.

One of the important application forms of Fiber Bragg grating (FBG) is to measure strain by pasting on the surface of structure. In order to improve the strength of surface-bonding FBG, packaging substrate in metal slice is designed and applied. On the basis of previous studies, an analytic model of the substrate-bonded FBG sensor is developed to predict the strain transfer rate. Moreover, the effects of substrate material and sizes on the strain transfer rate of substrate-bonded FBG are analyzed and compared. Finally, the theoretical analysis results are verified by experimental results. There is a good correspondence between the calculated values and experimental results.

FBG based pressure sensors are becoming more important due to their vast application in various fields. This paper presents a new approach to improve the sensitivity of Fiber Bragg Grating based pressure sensor. The material chosen is a cyclic olefin copolymer named TOPAS with refractive index of the core is 1.53 and the cladding is 1.525.The sensitivity of both silica based FBG and TOPAS based FBG has been compared. Strain or temperature shifts the Bragg wavelength through compressing the grating and changing the effective index and it is observed that TOPAS FBG responds more efficiently in sensing the applied strain compared to silica FBG indicating improved sensitivity. The simulations are done using Opti Grating Tool.

To apply Fiber Bragg Grating (FBG) sensors in the hole structural health monitoring of aircraft, this study investigates a temperature compensation algorithm based on Fourier curve-fitting, combined with an analysis of FBG layout. The results identify that the wavelengths have a linear relationship with stress and temperature and the linear correlation coefficients are all greater than 0.9. In the experiment, the temperature range was 17 °C-100 °C (step is 10 °C) and the tensile force changed from 0 N to 10 KN (step is 1 KN). With the Fourier curve-fitting of the wavelength variation, the temperature compensation can be actualized, and the average error of temperature is 0.17 °C. By integrating the algorithm in the self-developed FBG interrogation system, the real-time monitoring of stress is achieved with the average error of stress is 3.9 με.

Highlights • GO/Au/FBG sensor is fabricated to diagnosis the COVID-19 virus via patient saliva. • Sensor considerably responses to patient saliva having different virus concentration. • Deviation of wavelength and amplitude from healthy saliva identify the disease level., Monitoring the COVID-19 virus through patients’ saliva is a favorable non-invasive specimen for diagnosis and infection control. In this study, salivary samples of COVID-19 patients collected from 6 patients with the median age of 58.5 years, ranging from 34 to 72 years (2 females and 4 males) were analyzed using an Au/fiber Bragg grating (FBG) probe decorated with GO. The probe measures the prevalence of positivity in saliva and the association between the virus density and changes to sensing elements. When the probe is immersed in patients’ saliva, deviation of the detected light wavelength and intensity from healthy saliva indicate the presence of the virus and confirms infection. For a patient in the hyperinflammatory phase of desease, who has virus density of 1.2 × 108 copies/mL in saliva, the maximum wavelength shift and intensity changes after 1600 s were shown to be 1.12 nm and 2.01 dB, respectively. While for a patient in the early infection phase with 1.6 × 103 copies/mL, these values were 0.98 nm and 1.32 dB. The precise and highly sensitive FBG probe proposed in this study was found a reliable tool for quick detection of the COVID-19 virus within 10 s after exposure to patients’ saliva in any stage of the disease.

We propose and demonstrate a novel Mach–Zehnder FBG hybrid fiber sensor using a multi-core micro-fiber. By integrating a Mach–Zehnder interferometer in a multi-core fiber and a FBG in a single mode fiber (SMF), we produce the sensor by Mach–Zehnder cascaded with FBG. Owing to the structure characteristic of the multi-core micro-fiber, this hybrid fiber sensor can achieve the measurement of refractive index and temperature simultaneously. The measurement sensitivity of the refractive index is 847 nm/RIU. The temperature sensitivity for MZI is − 31.43 pm/°C, and the temperature sensitivity for FBG is 9.65 pm/°C.

The preservation of heritage buildings requires multidisciplinary skills ranging from materials and seismic design up to structural monitoring.One of the most interesting innovative solution that is being developing in the last years is based on smart FRP (FRP – Fibre Reinforced Polymer) devices, which can combine contemporary reinforcing and monitoring purposes. The use of composite materials has many advantages in comparison with traditional retrofitting techniques, such as low weight, high strength-to-weight ratio, ease of handling, drapability, speed of installation, low thickness and visual impact. At the same time, monitoring the structure during its lifetime (strain, cracks, temperature, etc.) and evaluating its in-service integrity, in order to predict possible anomalous situations, can be achieved by the combination of FRP materials and embedded fibre optic sensors into a smart FRP device, suitable for both reinforcing and monitoring purposes. Optical fibres can provide reliable measurement even in harsh environment, as they are chemically durable, corrosion resistant, stable and insensitive to external electromagnetic and environmental perturbations, allowing long distances signal transmission and several measures in different points along the same optical fibre (multiplexing). Furthermore, the embedding into composite material will preserve them from rupture during handling and installation.In the present work, the application of smart FRP devices for the structural health monitoring of the Monastery in Sant’Angelo d’Ocre, L’Aquila, performed in the framework of the national project PROVACI, is reported.Six Smart Patches, consisting of FRP reinforcing sheet with point FBG (Fibre Bragg Grating) sensors embedded were applied on the extrados of two different vaults, while four Smart Rebars, consisting in FRP pultruded bars with distributed optical fibres sensors embedded, were installed in four buttress of one same vault. All the smart FRP devices, after being cabled, have been connected to the relative control units (BraggMETER from Fibersensing for FBG sensors and OBR4600 control unit of Luna Technologies for the distributed optical sensors) connected with a remote server for on-line remote monitoring.Before the installation, the Smart FRP devices have been preliminary calibrated and tested in the laboratory in terms of mechanical properties, strain sensitivity and accelerated aging.The monitoring on the Monastery has been conducted for five months, showing the reliability of entire system and of the signal transmitted by each sensor over the time.

Large-scale distributed fiber Bragg grating (FBG) sensor networks have a wide range of applications. However, serious crosstalk could occur in a near-identical serial FBG sensor network because of multiple reflections. Since crosstalk cannot be observed directly and separately from signal, experimental investigation of crosstalk is challenging. In this work, we point out the similarity and symmetry between crosstalk and ghost gratings, and by way of measuring the spectrum of ghost gratings, we experimentally study the characteristics of crosstalk in a serial array of 3010 near-identical FBGs using a commercial FBG sensor interrogation system. We show that ghost gratings drop much more rapidly in power as one goes downstream, and they are more homogeneous in spectral distribution than real gratings. The peak wavelengths of real gratings vary within ±100 pm while the peak wavelengths of the ghost gratings they generate vary within ±20 pm only. Spectral ripples are observed for ghost gratings and real gratings severely affected by crosstalk. These ripples reduce the FBG wavelength reading accuracy to ±5 pm. The work constitutes the first experimental study on crosstalk in large-scale FBG arrays.

The hydraulic characteristics of pavement, such as pore water pressure, hydrodynamic pressure, and permeability, have a great influence on the functional performance and durability of pavement. The main objective of this study is to design a device for measuring the hydrodynamic pressure of pavement surface and analyze its characteristics with different vehicle speeds. In order to investigate the influence of vehicle speeds on the hydrodynamic pressure of pavement surface, a device equipped with five fiber Bragg grating (FBG) sensors was designed and utilized to measure the surface hydraulic characteristics at four vehicle speeds in the field. The calibration of FBG sensor was conducted by the interrogator (SmartScan Aero) under given pressures from 0 MPa to 1 MPa with an interval of 0.1 MPa. Then, the device was embedded in the pavement and compacted by an auto wheel, which traveled over the device at different speeds. Furthermore, the experimental data was collected by the interrogator during the process of a moving tire touching the surface of the device. The experimental results showed that the correlation coefficient between the given pressure and the center wavelength change of the FBG sensor was 0.99, which means the FBG sensor was accurate and reliable. The hydrodynamic pressure of the pavement surface increased with the increase in vehicle speed from 40 km/h to 100 km/h. Moreover, the directional anisotropy of the hydraulic pressure was found to be dependent on vehicle speed.

This article introduces a new way of using a fibre Bragg grating (FBG) sensor for detecting the presence and number of occupants in the monitored space in a smart home (SH). CO2 sensors are used to determine the CO2 concentration of the monitored rooms in an SH. CO2 sensors can also be used for occupancy recognition of the monitored spaces in SH. To determine the presence of occupants in the monitored rooms of the SH, the newly devised method of CO2 prediction, by means of an artificial neural network (ANN) with a scaled conjugate gradient (SCG) algorithm using measurements of typical operational technical quantities (indoor temperature, relative humidity indoor and CO2 concentration in the SH) is used. The goal of the experiments is to verify the possibility of using the FBG sensor in order to unambiguously detect the number of occupants in the selected room (R104) and, at the same time, to harness the newly proposed method of CO2 prediction with ANN SCG for recognition of the SH occupancy status and the SH spatial location (rooms R104, R203, and R204) of an occupant. The designed experiments will verify the possibility of using a minimum number of sensors for measuring the non-electric quantities of indoor temperature and indoor relative humidity and the possibility of monitoring the presence of occupants in the SH using CO2 prediction by means of the ANN SCG method with ANN learning for the data obtained from only one room (R203). The prediction accuracy exceeded 90% in certain experiments. The uniqueness and innovativeness of the described solution lie in the integrated multidisciplinary application of technological procedures (the BACnet technology control SH, FBG sensors) and mathematical methods (ANN prediction with SCG algorithm, the adaptive filtration with an LMS algorithm) employed for the recognition of number persons and occupancy recognition of selected monitored rooms of SH.

Glass fiber reinforced polymer (GFRP) antifloating anchors are widely used in reinforcing underground structures. Despite the outstanding application advances of GFRP anchors in the antifloating field, research on the mechanical transmission and deformation properties of the anchor rod and anchorage body is still scarce. This paper introduces pull-out experiments of GFRP antifloating anchors based on the FBG sensor strings technology. The experimental data demonstrates that the distribution curve of the axial stress shows a reversed-S shape, and the shear stress distribution presents the law of increasing first and then decreasing from the position of peak shear stress. The rod-anchorage body displacement difference curves of the anchors with an anchorage length that is closer to the critical anchorage length are smoother than those of the anchors with a larger length difference from the critical anchorage length. Finally, a simplified distribution model of the shear stress is applied for predicting the rod-anchorage body displacement difference, and the experimental data of the anchors with a rod slip failure is more applicable for this model than that of the anchors with a rod rupture failure.

In this study, two fiber Bragg grating sensors are multiplexed using time division multiplexing. The fiber Bragg gratings are setup in a dual bus configuration and the sensor signals are detected using a transmit-reflect detection system. The results demonstrate that both the transmitted and reflected signals from both fiber Bragg gratings can be resolved when the sensors are separated by a sufficient distance.

In this paper, the method of damage detection of three-dimensional and six-directional (3Dim 6Dir) braided composites by embedding FBG (Fiber Bragg Grating) sensors is studied. Firstly, the FBG sensors are embedded into 3Dim braided composites preforms along the fifth and sixth directions, and the 3Dim 6Dir braided composites with embedded FBG sensors are fabricated. Then, different kinds of damages are made on the composite materials. The signals of FBG sensors from the damaged composites are collected. By analyzing the signals of FBG sensors, it is found that different damages and different damage positions at the 3Dim 6Dir braided composites will make the wavelength of FBG sensor signals at different positions have unusual changes. This indicates that it is feasible to detect the damages of 3 Dim braided composites by using FBG sensors.

The strain of a gasoline direct injector has been measured by a fiber Bragg grating sensor attached along the axis of the injector stem in different operating pressure conditions. Static and dynamic measurements demonstrate the effectiveness of the FBG sensor technology for monitoring the behavior of the injector and controlling the engine performance and emissions.

This paper presents a fault monitoring method in passive optical networks (PON) using Burst-mode uniform FBG optical sensors. The proposed monitoring method detects physical layer faults between optical line terminal (OLT) and optical network units (ONUs). The FBG interrogation subsystem located in central office (CO) near OLT transmits FBG monitoring signal to every ONU. Each ONU branch contains an FBG sensor subsystem. The proposed system provides Burst mode upstream data transmission using time division multiple access (TDMA). We show that the proposed system is a cost-effective solution that has ability to monitor faults of any ONU branches in PON.

In this paper, modelling and characterization of strain and temperature sensors based on fiber Bragg grating is presented. Simulation of FBG sensors behavior demonstrates very good measurement linearity for both strain and temperature. Simulation results were experimentally verified using two common methods for FBG interrogation. Measurement results obtained by both methods show a good match and good linearity. Strain sensitivity of 0.0002 mm/$\mu\varepsilon$ and temperature sensitivity of approximately 0.014 mm/°C were obtained.

As the applications of fiber Bragg gratings (FBGs) continue to grow and become more advanced, it becomes necessary to understand their behavior when exposed to high temperatures in unique situations. In these experiments, uniform 1530-nm fiber Bragg gratings and Type K Cr-Al thermocouples were embedded in three-ply carbon fiber composites. A 100 W high energy laser (HEL) heated the composites to high temperatures over timespans less than one second, and FBG spectral data and thermocouple temperature data were collected during each HEL heating test. The data from three high energy laser tests that represent different levels of damage to the FBG are analyzed to explore the spectral response and thermal decay of embedded FBG sensors when exposed to high temperatures over short timespans. Results are compared to a previously proposed power-law model describing the decay of FBGs in bare fiber when held at constant temperatures over much longer timespans.

The performance of packaged Fiber Bragg grating (FBG) sensors with carbon fiber reinforced plastic (CFRP) may degraded even functional failure with the working time increases. The reliability of this kind of packaged FBG sensors has been analyzed by using the FMEA method in this paper. Considering the working environment of FBG sensor, such as alternating loads and ageing, the related failure modes are discussed. The risk index of each failure mode is calculated. The relationship between different failure modes is analyzed, and as well as the weak links of the packaging design and process of the FBG sensor. The results are helpful to improve the availability and the reliability of packaged FBG sensor by CFRP.

A novel and simple overlap-splicing-based scheme is described to realize a shape-controllable Fabry–Perot cavity in a fiber bragg grating (FBG). A Gaussian function with a constant waist width is used to fit the cavity shape and its maximum value is decided by the overlap value. Non-uniform strain distribution along the FBG is introduced by the Gaussian shape cavity when stress is applied on the FBG. Using this method, it is possible to optimize the discrimination capability of the strain and temperature response of the FBG device. Experimental and simulation results are provided to support the feasibility of this method.

In the process of rock mechanical experiments, strain-response measurement is a most fundamental and most essential procedure for geomechanical researchers. The main objective of this paper is to point out the feasibility and the superiority of the application of a novel multichannel fiber Bragg grating (FBG) sensor arrays for dynamic strain-response measurements of cylindrical specimen subjected to uniaxial compression. The principle, design, and embedment of multichannel FBG sensors used in the experiment are briefly described. To fully monitor the strain history of the sandstone cylinder in uniaxial compression, six circumferential FBG sensors, four lateral FBG sensors, linear variable differential transformers (LVDT) built-in machine have been utilized for spatially monitoring small radial and axial strains along the height of the specimen, respectively. The experimental results indicate that the proposed FBG sensors can successfully provide a full-field view of the surface strains, as well as detect the potential crack locations within the specimen, and strains measured by multichannel FBG sensors are in good agreement with the results of LVDT, especially in the axial strains. Hence, it could be inferred that multichannel FBG sensor arrays are capable of measuring dynamic strain responses of sandstone specimen in multistage compression, which would greatly strengthen experimental basis for further application and theoretic research of in-situ field monitoring.

Due to their small size and flexibility, fiber Bragg grating (FBG) sensors have been integrated into needle-sized continuum robots for shape estimation and force measurement. The challenge in extending previous shape and force sensing technologies to pre-curved continuum robots, such as concentric-tube robots, is that torsion information is essential for accurate shape estimation, and the force-strain relationship is nonlinear. In this letter, a novel helically wrapped FBG sensor design and corresponding force-curvature-strain model are developed to provide simultaneous curvature, torsion, and force measurement. To validate this design and modeling technique, two sensorized Nitinol tubes were fabricated and tested in an experimental setup. The results showed that accurate and sensitive curvature, torsion, and force measurements can be obtained at a 100 Hz sampling rate.

Fiber Bragg grating (FBG) sensor has been considered as a reliable sensor for health monitoring of structural and geotechnical projects. Various types of FBG based sensors have been proposed in past few decades and employed for health monitoring of many geotechnical structures. This paper presents an overview of the recent development and application of FBG based sensors for health monitoring of several key geotechnical structures, including soil nail systems, slopes, and piles. Different sensor design, implementation and packaging methods, advantages and limitations of using FBG based sensors in different projects are reviewed. Comparative analysis of using two mathematical methods for the prediction of ground movement using FBG sensor data are also carried out. The two typical mathematical methods include Finite Difference Method (FDM) and Numerical Integration method (NIM). Possible technical challenges of applying FBG sensors for geotechnical monitoring are discussed.