Showing papers on "Fiber Bragg grating published in 2018"

Fibre Bragg Grating Based Strain Sensors: Review of Technology and Applications.

Abstract: Fibre Bragg grating (FBG) strain sensors are not only a very well-established research field, but they are also acquiring a bigger market share due to their sensitivity and low costs In this paper we review FBG strain sensors with high focus on the underlying physical principles, the interrogation, and the read-out techniques Particular emphasis is given to recent advances in highly-performing, single head FBG, a category FBG strain sensors belong to Different sensing schemes are described, including FBG strain sensors based on mode splitting Their operation principle and performance are reported and compared with the conventional architectures In conclusion, some advanced applications and key sectors the global fibre-optic strain sensors market are envisaged, as well as the main market players acting in this field

Towards power scaling of 2.8 μm fiber lasers

Abstract: We report the demonstration of a 2824 nm passively cooled erbium-doped fluoride fiber laser delivering a record average output power of 416 W in continuous-wave operation The splice-less cavity is based on intra-core fiber Bragg gratings written directly in the active erbium-doped fluoride fiber, which is bidirectionally pumped at 980 nm to reduce heat load To the best of our knowledge, this result is the highest average output power achieved with a mid-infrared fiber laser The long-term performance of different protective endcaps is also investigated at high-power operation

Room-temperature fiber laser at 3.92 μm

Abstract: Rare-earth-doped fiber lasers are promising contenders in the development of spectroscopy, free-space communications, and countermeasure applications in the 3–5 μm spectral region. However, given the limited transparency of the commonly used fluorozirconate glass fiber, these systems have only achieved wavelength coverage up to 3.8 μm, hence fueling the development of more suitable fiber glass compositions. To this extent, we propose in this Letter a novel heavily holmium-doped fluoroindate fiber, providing extended transparency up to 5 μm, to demonstrate the longest wavelength room-temperature fiber laser at 3.92 μm. Achieving ∼200 mW of output power when cladding pumped by a commercial 888 nm laser diode, this demonstration paves the way for powerful mid-infrared fiber lasers emitting at and beyond 4 μm.

In situ plasmonic optical fiber detection of the state of charge of supercapacitors for renewable energy storage

Abstract: In situ and continuous monitoring of electrochemical activity is key to understanding and evaluating the operation mechanism and efficiency of energy storage devices. However, this task remains challenging. For example, the present methods are not capable of providing the real-time information about the state of charge (SOC) of the energy storage devices while in operation. To address this, a novel approach based on an electrochemical surface plasmon resonance (SPR) optical fiber sensor is proposed here. This approach offers the capability of in situ comprehensive monitoring of the electrochemical activity (the electrode potential and the SOC) of supercapacitors (used as an example). The sensor adopted is a tilted fiber Bragg grating imprinted in a commercial single-mode fiber and coated with a nanoscale gold film for high-efficiency SPR excitation. Unlike conventional "bulk" detection methods for electrode activity, our approach targets the "localized" (sub-μm-scale) charge state of the ions adjacent to the electrode interface of supercapacitors by monitoring the properties of the SPR wave on the fiber sensor surface located adjacent to the electrode. A stable and reproducible correlation between the real-time charge-discharge cycles of the supercapacitors and the optical transmission of the optical fiber has been found. Moreover, the method proposed is inherently immune to temperature cross-talk because of the presence of environmentally insensitive reference features in the optical transmission spectrum of the devices. Finally, this particular application is ideally suited to the fundamental qualities of optical fiber sensors, such as their compact size, flexible shape, and remote operation capability, thereby opening the way for other opportunities for electrochemical monitoring in various hard-to-reach spaces and remote environments.

Review of Chirped Fiber Bragg Grating (CFBG) Fiber-Optic Sensors and Their Applications

Abstract: Fiber Bragg Gratings (FBGs) are one of the most popular technology within fiber-optic sensors, and they allow the measurement of mechanical, thermal, and physical parameters. In recent years, a strong emphasis has been placed on the fabrication and application of chirped FBGs (CFBGs), which are characterized by a non-uniform modulation of the refractive index within the core of an optical fiber. A CFBG behaves as a cascade of FBGs, each one reflecting a narrow spectrum that depends on temperature and/or strain. The key characteristic of CFBGs is that their reflection spectrum depends on the strain/temperature observed in each section of the grating; thus, they enable a short-length distributed sensing, whereas it is possible to detect spatially resolved variations of temperature or strain with resolution on the order of a millimeter over the grating length. Based on this premise, CFBGs have found important applications in healthcare, mechanical engineering, and shock waves analysis, among others. This work reviews the present and emerging trends in CFBG sensors, focusing on all aspects of the sensing element and outlining the application case scenarios for which CFBG sensors have been demonstrated.

A fully reconfigurable waveguide Bragg grating for programmable photonic signal processing

Abstract: Since the discovery of the Bragg’s law in 1913, Bragg gratings have become important optical devices and have been extensively used in various systems. In particular, the successful inscription of a Bragg grating in a fiber core has significantly boosted its engineering applications. However, a conventional grating device is usually designed for a particular use, which limits general-purpose applications since its index modulation profile is fixed after fabrication. In this article, we propose to implement a fully reconfigurable grating, which is fast and electrically reconfigurable by field programming. The concept is verified by fabricating an integrated grating on a silicon-on-insulator platform, which is employed as a programmable signal processor to perform multiple signal processing functions including temporal differentiation, microwave time delay, and frequency identification. The availability of ultrafast and reconfigurable gratings opens new avenues for programmable optical signal processing at the speed of light.

A Three-Dimensional Fiber Bragg Grating Force Sensor for Robot

Abstract: This paper presents a three-dimensional (3-D) force sensor based on fiber Bragg grating (FBG) for robot plantar force measuring. A classical Maltese-cross beam with multiplexed FBGs has been designed for 3-D force sensing. Strain distribution characteristics and dynamic performance of the Maltese-cross elastomer have been investigated by using finite element analysis. Through ingenious design, 3-D forces of Fx, Fy, and Fz have been measured by only five sensitive elements of FBG, meanwhile, decoupling and temperature compensation have also been realized, which greatly reduces the number of sensitive element compared with the traditional resistance strain gauge based multi-axis force sensor. Comprehensive performance test has been carried out, and the experimental results demonstrate that the sensor possesses good linearity, weak coupling, and creep resistance.

Liquid Level Measurement Based on FBG-Embedded Diaphragms With Temperature Compensation

Abstract: This paper proposes a fiber optic liquid level sensor system based on a silica fiber Bragg grating embedded into an epoxy resin diaphragm coupled to a temperature reference sensor, used to compensate the temperature cross-sensitivity for improving the liquid level measurement accuracy. The proposed system was tested in an industrial water tank with heating and recirculation. The results demonstrated a temperature cross-sensitivity reduction, enhancing the liquid level measurement thermal stability by a factor of nine, when compared with some single head sensor configurations reported in literature. Our system presents high linearity ( $R>0.999$ ), superior sensitivity (2.8 pm/mm), and much lower temperature related error (1.04 mm/°C), when compared with the other diaphragm-based sensors recently reported in the literature.

Ultra-fast polymer optical fibre Bragg grating inscription for medical devices

Abstract: We report the extraordinary result of rapid fibre Bragg grating inscription in doped polymer optical fibres based on polymethyl methacrylate in only 7 ms, which is two orders of magnitude faster than the inscription times previously reported. This was achieved using a new dopant material, diphenyl disulphide, which was found to enable a fast, positive refractive index change using a low ultraviolet dose. These changes were investigated and found to arise from photodissociation of the diphenyl disulphide molecule and subsequent molecular reorganization. We demonstrate that gratings inscribed in these fibres can exhibit at least a 15 times higher sensitivity than silica glass fibre, despite their quick inscription times. As a demonstration of the sensitivity, we selected a highly stringent situation, namely, the monitoring of a human heartbeat and respiratory functions. These findings could permit the inscription of fibre Bragg gratings during the fibre drawing process for mass production, allowing cost-effective, single-use, in vivo sensors among other potential uses. Bragg gratings have been written into polymer optical fibres in a few milliseconds, thanks to the use of a new dopant in the fibres’ cores. Fibre Bragg gratings can be quickly inscribed in silica optical fibres, but it takes much longer to inscribe gratings in polymethyl methacrylate (PMMA) fibres. Julien Bonefacino and co-workers from the Hong Kong Polytechnic University solved this problem by adding diphenyl disulphide to both raise the refractive index of the fibres’ cores and make them highly photosensitive. Using low-fluence ultraviolet light from a helium–cadmium laser, Bragg gratings could be inscribed in only 7 milliseconds—about 140 times faster than previously reported inscription times and short enough to make the scheme potentially compatible with fabrication during fibre drawing. The gratings are promising for use in sensing applications, such as measuring heartbeat and respiration.

Refractive Index and Temperature Sensing Based on an Optoelectronic Oscillator Incorporating a Fabry–Perot Fiber Bragg Grating

Abstract: An ultrafast high-sensitivity refractive index (RI) and temperature-sensing system based on an optoelectronic oscillator (OEO) is proposed and demonstrated in this paper. A Fabry–Perot fiber Bragg grating (FP-FBG), which combines a gap with two FBGs in a silica V-shaped slot and characterizes a narrow notch in the reflection spectrum, is incorporated in the OEO to implement a microwave photonic filter and perform oscillating frequency selection. A microwave signal is generated by the OEO, whose oscillating frequency is determined by the center frequency of the FP-FBG notch, which varies with the surrounding environments. The RI or the temperature change can be accordingly measured by monitoring the frequency shift of the microwave signal using an electrical spectrum analyzer or a digital signal processor. An experiment is performed to verify the proposal. An RI sensitivity of 413.8 MHz/0.001RIU and a temperature sensitivity of 2516 MHz/°C are successively demonstrated.

FBG-Embedded 3-D Printed ABS Sensing Pads: The Impact of Infill Density on Sensitivity and Dynamic Range in Force Sensors

Abstract: The 3-D printing process is a flexible, automated, customizable, and low-cost fabrication process. Fiber Bragg grating (FBG) sensors can be embedded into 3-D printed elements to obtain customized and low-cost sensing elements. However, the material infill is an important printing parameter that may affect the FBG response on both temperature and strain. This paper investigates the material infill influence on FBG-embedded 3-D printed sensing pads by both numerical and experimental approaches. The obtained sensing pads are able to withstand a force of 1 kN with sensitivities of about 0.7 pm/N and linearity higher than 98%. Numerical results show that the fiber has an influence on the strain distribution of the 3-D printed structure. Experimental results show that the infill’s thermal expansion-induced temperature sensitivity of the embedded FBG reported in the literature can be reduced by simply setting the correct infill density. The results obtained enable the application of 3-D printed sensing pads for structural health monitoring, soft robotics application, and force detectors for teleoperation in harsh environments.

Long Period Gratings in Multicore Optical Fibers for Directional Curvature Sensor Implementation

Abstract: Multicore optical fibers are especially attractive for the fabrication of curvature and shape sensors due to the spatial distribution of the different cores. Fiber Bragg gratings have been used in the past for the implementation of these sensors, however, despite their inherent properties, they have a very limited sensitivity. In this paper, we study the use of long period gratings (LPGs) for the implementation of a directional curvature sensor. We inscribed a set of three different LPGs in a seven core optical fiber using a selective inscription technique. We inscribed a single LPG in the external cores and an array of three LPGs in the central core. We have characterized the proposed sensor for strain, torsion, and curvature magnitude and direction. The proposed sensor shows a linear response for curvature magnitudes from 0 to 1.77 m–1 with a maximum curvature sensitivity of –4.85 nm/m–1 and shows a near sinusoidal behavior in all the cores with curvature directions from 0° to 360°. The sensor shows a good insensitivity to strain. The torsion in the multicore optical fibers can be detected and measured using the maximum attenuation of the LPGs in the external cores.

Polymer Optical Fiber Bragg Gratings in CYTOP Fibers for Angle Measurement with Dynamic Compensation.

Abstract: This paper demonstrates the use of polymer optical fiber Bragg gratings (POFBGs) for angle measurements over a range of different oscillatory frequencies. The POFBGs are inscribed in low-loss, cyclic transparent amorphous fluoropolymers (CYTOP) and are imprinted using the direct-write, plane-by-plane femtosecond laser inscription method. As the polymer has a viscoelastic response and given that the Young’s modulus depends on the oscillatory frequency, a compensation technique for sensor frequency cross-sensitivity and hysteresis is proposed and verified. Results show that the proposed compensation technique is able to provide a root mean squared error (RMSE) reduction of 44%, and a RMSE as low as 2.20° was obtained when compared with a reference potentiometer. The hysteresis reduction provided by the proposed technique is 55%, with hysteresis <0.01. The results presented in this paper can pave the way for movement analysis with POFBG providing higher sensitivity and low hysteresis over a large range of motion frequencies.

High-accuracy range-sensing system based on FMCW using low-cost VCSEL.

Abstract: Long-range shape measurement with high accuracy is needed for precision manufacturing of large-scale parts such as turbines, compressors, and trains. We have developed a high-accuracy ranging system based on frequency-modulated continuous-wave (FMCW) technology. Our system has two unique features. First, it achieves high-accuracy range measurement by directly modulating a low-cost vertical-cavity surface-emitting laser (VCSEL) at high sweep rates. The nonlinearity of the optical frequency sweep is compensated for by resampling through a reference interferometer. Second, an optical fiber with multiple fiber Bragg grating (FBG) structures is used for distance calibration. Ranging accuracy better than 10 μm is achieved at 2 m distance. Three-dimensional (3D) imaging of 10-cubic-meter volume has been obtained by combining the FMCW ranging with a galvanometer scanner.

Fiber Bragg Gratings Sensors for Aircraft Wing Shape Measurement: Recent Applications and Technical Analysis

Abstract: The safety monitoring and tracking of aircraft is becoming more and more important. Under aerodynamic loading, the aircraft wing will produce large bending and torsional deformation, which seriously affects the safety of aircraft. The variation of load on the aircraft wing directly affects the ground observation performance of the aircraft baseline. To compensate for baseline deformations caused by wing deformations, it is necessary to accurately obtain the deformation of the wing shape. The traditional aircraft wing shape measurement methods cannot meet the requirements of small size, light weight, low cost, anti-electromagnetic interference, and adapting to complex environment at the same time, the fiber optic sensing technology for aircraft wing shape measurement has been gradually proved to be a real time and online dynamic measurement method with many excellent characteristics. The principle technical characteristics and bonding technology of fiber Bragg grating sensors (FBGs) are reviewed in this paper. The advantages and disadvantages of other measurement methods are compared and analyzed and the application status of FBG sensing technology for aircraft wing shape measurement is emphatically analyzed. Finally, comprehensive suggestions for improving the accuracy of aircraft wing shape measurement based on FBG sensing technology is put forward.

Distributed Partial Discharge Detection in a Power Transformer Based on Phase-Shifted FBG

Abstract: To overcome the low sensitivity of power transformer partial discharge (PD) detection by a lead zirconate titanate (PZT) sensor, we propose a novel PD detection system based on phase-shifted fiber Bragg gratings (PS-FBG). Compared with the conventional method, the proposed sensor can be installed inside the power transformer, which takes the advantage of the optical fiber status as an insulator. Distributed PD detection is achieved using wavelength and time-division multiplexing. Frequency response experiments indicate that the sensitivity of the PS-FBG is 8.46 dB higher than that of the conventional ultrasonic sensor. Moreover, a PD detection experiment shows that the PD sensitivity of the PS-FBG immersed in oil is 17.5 times higher than that of the PZT sensor. Furthermore, the multiplexing and the feasibility of defect localization of the proposed distributed PD detection system are demonstrated.

Etched Fiber Bragg Grating Biosensor Functionalized with Aptamers for Detection of Thrombin.

Abstract: A biosensor based on an etched Fiber Bragg Grating (EFBG) for thrombin detection is reported. The sensing system is based on a Fiber Bragg Grating (FBG) with a Bragg wavelength of 1550 nm, wet-etched in hydrofluoric acid (HF) for ~27 min, to achieve sensitivity to a refractive index (RI) of 17.4 nm/RIU (refractive index unit). Subsequently, in order to perform a selective detection of thrombin, the EFBG has been functionalized with silane-coupling agent 3-(aminopropyl)triethoxysilane (APTES) and a cross-linker, glutaraldehyde, for the immobilization of thrombin-binding aptamer. The biosensor has been validated for thrombin detection in concentrations ranging from 10 nM to 80 nM. The proposed sensor presents advantages with respect to other sensor configurations, based on plasmonic resonant tilted FBG or Long Period Grating (LPG), for thrombin detection. Firstly, fabricating an EFBG only requires chemical etching. Moreover, the functionalization method used in this study (silanization) allows the avoidance of complicated and expensive fabrications, such as thin film sputtering or chemical vapor deposition. Due to their characteristics, EFBG sensors are easier to multiplex and can be used in vivo. This opens new possibilities for the detection of thrombin in clinical settings.

Compensation Method for Temperature Cross-Sensitivity in Transverse Force Applications With FBG Sensors in POFs

Abstract: In this paper, we investigate the influence of temperature on the transverse force response of polymer optical fiber Bragg gratings (POFBGs) inscribed in cyclic transparent fluoropolymers (CYTOPs). The gratings are imprinted in the fiber using the direct-write, plane-by-plane femtosecond laser inscription method. The temperature increase leads to a decrease in the polymer Young's modulus, which causes a sensitivity variation in the POFBG sensor for transverse force applications. The proposed technique is based on the characterization of both the sensor's response offset and the material's Young's modulus variation due to the temperature increase. Transverse force tests were performed at different temperatures (30 °C, 40 °C, 50 °C, and 60 °C) and the compensated and uncompensated responses are compared in terms of root mean squared error (RMSE). The compensated results show an RMSE lower than 3% (mean value between all tested temperatures), which is 6.7 times lower compared with the uncompensated response. In addition, the proposed compensation technique presents a maximum RMSE reduction of 16 times at 60 °C.

Two-dimensional vector bending sensor based on seven-core fiber Bragg gratings.

Abstract: We demonstrated a two-dimensional vector-bending sensor by use of fiber Bragg gratings (FBGs) inscribed in a homogeneous seven-core fiber. Seven FBGs were simultaneously inscribed in each of all seven cores using a modified Talbot interferometer and a lens scanning method. The vector bending response of six outer-core FBGs was investigated at all 360° directions with a step size of 15°. The bending sensitivities of the six outer-core FBGs display six perfect '8'-shaped patterns in a polar-coordinate system. That is, they exhibit strong bending-direction dependence with a maximum sensitivity of 59.47 pm/m-1. The orientation and amplitude of the vector bending can be reconstructed using measured Bragg wavelength shifts of any two off-diagonal outer-core FBGs. So, the six outer-core FBGs have 12 combinations for bend reconstruction, which can be averaged across multiple reconstructions to develop an accurate two-dimensional vector bending sensor. The average relative error was lower than 4.5% for reconstructed amplitude and less than 2.8% for reconstructed orientation angle θ. Moreover, the seven-core FBGs offer several advantages such as a compact structure, fabrication flexibility, and the temperature compensating ability of central-core FBG.

Material features based compensation technique for the temperature effects in a polymer diaphragm-based FBG pressure sensor.

Abstract: Fiber Bragg grating (FBG) based sensors have been applied to measure several parameters, such as pressure, vibration, liquid level, humidity, the concentration of chemical compounds, among others. An approach to measure parameters like liquid level, pressure and vibration are to embed the FBG on a diaphragm, which is generally made of a polymeric material. Nevertheless, the mechanical properties of polymers depend on temperature variation. For this reason, a polymer diaphragm can enhance the cross-sensitivity between the strain and temperature on an FBG sensor. In order to overcome this limitation, this paper presents a compensation technique for the temperature effects on an oblong polymer diaphragm-based FBG pressure sensor. The presented technique is based on the analytical model of the sensor, which takes into account the variation of the diaphragm properties with temperature obtained through a dynamic mechanical analysis of the diaphragm material. Results show that the developed technique reduces the sensor cross-sensitivity to about 1.74 Pa/°C. Furthermore, the presented technique is compared with the direct difference between the FBG strain and temperature responses presented in reference works. The comparison shows a better performance of the technique presented in this paper with respect to the cross-sensitivity and the root mean squared error.

Highly sensitive detection of dopamine using a graphene functionalized plasmonic fiber-optic sensor with aptamer conformational amplification

Abstract: Surface plasmon resonance (SPR) optical fiber sensors can be used as cost-effective and relatively simple-to-implement alternatives to well established bulky prism configurations for high sensitivity in-situ biomedical measurements. The miniaturized size and remote operation ability offer a multitude of opportunities for single-point sensing in hard-to-reach spaces, even possibly in vivo. The biosensor configuration presented here utilizes a nano-scale metal-coated tilted fiber Bragg grating (TFBG) imprinted in a commercial single mode fiber core with no structural modifications. A unique feature of our TFBG sensor is the use of a single layer graphene coating over the gold-coated fiber surface, functionalized with a selective DNA aptamer for highly sensitive detection of target molecules. We have demonstrated the capture of dopamine molecules by the DNA aptamer, resulting in aptamer well-defined conformational changes in response to dopamine surface affinities. This process amplifies the surface refractive index modulation over the fiber surface to enable precise dopamine concentration measurement in real time via monitoring of the surface plasmon resonance signals. The sensor shows a linear response for dopamine concentration in the range from 10−13 M to 10−8 M with a lower limit of detection of 10−13 M. This limiting concentration is lower than the concentration fluctuations of dopamine in the human brain. The sensor works with minimal cross-sensitivities because of the core mode calibration inherent in the TFBG. Integration of the TFBG with a hypodermic needle should allow similar measurements in vivo, presenting an appealing solution for rapid, low power consumption and highly sensitive detection of analytes at low concentrations in medicine as well as in chemical and environmental monitoring.

Simultaneous Measurement of Temperature and Relative Humidity Based on FBG and FP Interferometer

Abstract: In this letter, a simultaneous measurement system of temperature and relative humidity is proposed, which consists of a fiber Bragg grating (FBG) and Fabry–Perot (FP) interferometer. The FP interferometer was fabricated by splicing the hollow capillary with a single-mode fiber and the end of the hollow capillary was filled with a layer of polyimide, achieving a humidity sensitivity of 22.07 pm/%RH in the range of 20–90 %RH. FBG is insensitive to humidity, which can be used to measure temperature and eliminate the obstruction of temperature on FP humidity probe by formula conversion. The temperature sensitivity of FBG is 9.98 pm/°C. Due to low cost and good stability, this system has broad application prospects in the fields of food processing, medical care, and food storage.

Switchable dual-wavelength Q -switched fiber laser using multilayer black phosphorus as a saturable absorber

Abstract: Black phosphorus (BP), with thickness-dependent direct energy bandgaps (0.3–2 eV), shows an enhanced nonlinear optical response at near- and mid-infrared wavelengths. In this paper, we present experimentally multilayer BP flakes coated on microfiber (BCM) as a saturable absorber with a modulation depth of 16% and a saturable intensity of 6.8 MW/cm2. After inserting BCM into an Er-doped fiber ring laser, a stable dual-wavelength Q-switched state with central wavelengths of 1542.4 nm and 1543.2 nm (with wavelength spacing as small as 0.8 nm) is obtained with the aid of two cascaded fiber Bragg gratings as a coarse wavelength selector. Moreover, single-wavelength Q-switched operation at 1542.4 nm or 1543.2 nm is also realized, which can be switched between the two wavelengths flexibly just by adjusting the intracavity birefringence. These results suggest that BP combined with the cascaded fiber gratings can provide a simple and feasible candidate for a multiwavelength fiber laser. Our fiber laser may have potential applications in terahertz generation, laser radar, and so on.

A 300-mm Silicon Photonics Platform for Large-Scale Device Integration

Abstract: A 300-mm silicon photonics platform for large-scale device integration was developed, leveraging 40-nm complementary metal-oxide-semiconductor technology. Through fabrication using this technology platform, wire waveguides were obtained with low propagation losses for the C-band (0.4 dB/cm) and O-band (1.3 dB/cm). Several types of wavelength filters, including a coupled resonator optical waveguide (CROW), an arrayed waveguide grating, and a cascaded Mach–Zehnder interferometer, were also demonstrated, with low crosstalk and low insertion loss. A polarization rotator Bragg grating with multiple reflection peaks having polarization independence was also obtained. In terms of wafer-scale uniformity, a small standard deviation of 0.7 nm in resonant wavelength for the CROW was confirmed. A grating coupler also exhibited low wafer-scale variations in the maximum coupling efficiency and the diffraction wavelength in optical coupling with a single-mode fiber. Extraction of fabrication deviations for the waveguides was performed using the spectral variation of microring resonators and grating couplers. The extracted wafer-scale variations in waveguide width and height and grating depth well reproduced the results of physical measurements, with subnanometer-level accuracy. The developed technology can thus enable manufacturing of high-speed, low-power optical interconnects.

Ultrasonic Structural Health Monitoring Using Fiber Bragg Grating.

Abstract: The fiber Bragg grating (FBG) sensor, which was developed over recent decades, has been widely used to measure manifold static measurands in a variety of industrial sectors. Multiple experiments have demonstrated its ability in ultrasonic detection and its potential in ultrasonic structural health monitoring. Unlike static measurements, ultrasonic detection requires a higher sensitivity and broader bandwidth to ensure the fidelity of the ultrasonic Lamb wave that propagates in a plate-like structure for the subsequent waveform analysis. Thus, the FBG sensor head and its corresponding demodulation system need to be carefully designed, and other practical issues, such as the installation methods and data process methods, should also be properly addressed. In this review, the mature techniques of FBG-based ultrasonic sensors and their practical applications in ultrasonic structural health monitoring are discussed. In addition, state-of-the-art techniques are introduced to fully present the current developments.

Sensitivity Enhancement of FBG-Based Strain Sensor.

Abstract: A novel fiber Bragg grating (FBG)-based strain sensor with a high-sensitivity is presented in this paper. The proposed FBG-based strain sensor enhances sensitivity by pasting the FBG on a substrate with a lever structure. This typical mechanical configuration mechanically amplifies the strain of the FBG to enhance overall sensitivity. As this mechanical configuration has a high stiffness, the proposed sensor can achieve a high resonant frequency and a wide dynamic working range. The sensing principle is presented, and the corresponding theoretical model is derived and validated. Experimental results demonstrate that the developed FBG-based strain sensor achieves an enhanced strain sensitivity of 6.2 pm/μe, which is consistent with the theoretical analysis result. The strain sensitivity of the developed sensor is 5.2 times of the strain sensitivity of a bare fiber Bragg grating strain sensor. The dynamic characteristics of this sensor are investigated through the finite element method (FEM) and experimental tests. The developed sensor exhibits an excellent strain-sensitivity-enhancing property in a wide frequency range. The proposed high-sensitivity FBG-based strain sensor can be used for small-amplitude micro-strain measurement in harsh industrial environments.