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As a key parameter, stiffness determines the dynamic characteristics of both the bearing and the spindle unit. However, interactions between the spindle speed, initial preload, and cutting force caused by the thermal expansion of spindle unit components alter the bearing stiffness characteristics during the running process. Aiming to carry out real-time online monitoring of the thermal characteristics and their effect on the machine tool spindle bearing stiffness, a fiber Bragg grating (FBG) sensors network was proposed. Further, the effects were considered under various axial loads, combined axial, and specific radial load to study bearing temperature increase, thermally induced preload, and thermal stiffness. The results have shown that the bearing thermal stiffness of the bearing decreases with the increase of the spindle speed. Moreover, an optimal initial preload can be selected with respect to the speed to maximize the thermal stiffness. Finally, the thermally induced preload additionally pre-tightened the bearing. [ABSTRACT FROM AUTHOR]

The multinode multibaseline interferometric synthetic aperture radar measurement system is the future development trend in the field of aviation remote sensing. The deformation error between the nodes directly affects the position and orientation error compensation of the load at the node, and the baseline length error directly affects the interferometric phase error compensation. Aiming at the particularity of the wing structure, a structural deformation measurement and baseline measurement method based on the strain mode is proposed. The fiber Bragg grating sensor is used to acquire the strain of each measuring point on the wing in real time, and the deformation of the wing is accurately measured by the modal superposition method, thereby realizing the high-precision measurement of the baseline between the nodes. The measurement result of the dual total station is the benchmark, and the validation of the method was carried out on the existing test system in the laboratory. Under static conditions, the experimental test results show that the wing deformation measurement accuracy is better than 1 mm, and the baseline measurement accuracy is better than 0.5 mm. [ABSTRACT FROM AUTHOR]

Use of a detailed parallel theoretical model has allowed the design of a high figure of merit and low cross sensitivity accelerometer based on fiber Bragg grating (FBG), and the good response characteristics have been demonstrated experimentally. The relationship between the sensitivity and resonant frequency is downplayed, the mechanism of expanding the frequency band and maintaining higher sensitivity has been analyzed. A FBG accelerometer with a good performance of higher main-axis sensitivity and lower cross sensitivity has been presented. The experimental results obtained indicate that the FBG accelerometer has a broad frequency range from 1 to 60Hz, the corresponding sensitivity range from 754.3 to 763.2pm/G, while the fluctuation is less than 3dB. The resonant frequency is 103Hz, and the corresponding cross sensitivity is less than −24.76dB (< 5%). The figure of merit is 78.61nm $\cdot $ Hz/G, it shows that the sensor has better comprehensive characteristics. An accelerometer with this performance thus has the potential for application in low frequency oil and gas seismic exploration. [ABSTRACT FROM AUTHOR]

An omni-azimuth angle sensor for tilt measuring based on fiber Bragg grating (FBG) array is proposed. It is designed based on the sensitivity of FBG to axial traction. The sensor consists of a mass block and a cube frame connected by an FBG array. The FBG array consists of four FBGs. When the sensor is tilted at different angles, the axial traction of each FBG will change due to the pull of the mass block. The tilt angle of the sensing unit can be obtained by calculating the magnitude and direction of the resultant traction of the four FBGs. The array design and measurement theory of the sensor are introduced, and the sensitivity of the sensor is discussed. Experimental results show that the FBG array can be used to measure the omni-azimuth angle. The angle sensitivity varies unevenly with the tilt angle and the range of variation is 0.08 nm/° to 0.4593 nm/°. Using the spectrometer with an accuracy of 0.01 nm, the angle resolution ranges from 0.125° to 0.0218°. The array effectively extends the measurement range of the tilt sensor, and reduces the number of FBGs as much as possible on the basis of achieving omni-azimuth angle measurement. [ABSTRACT FROM AUTHOR]

Traditional algorithm lacks the ability to generalize across different application scenarios and how to set the stopping conditions of network iteration in Stochastic Configuration Network (SCN) is a problem. This paper proposes an event recognition system based on minimum redundancy, maximum relevance (mRMR) and a modified Stochastic Configuration Network (CWCs-SCN) for the fiber Bragg grating (FBG) sensor system. mRMR is used to analyze the importance of 14th time-frequency domain features in different application scenarios. After that, a modified SCN (CWCs-SCN) neural network is used to solve the stopping condition of the network iteration. The experiments demonstrated that the average recognition accuracy of four kind of human intrusion signal can reach to 96.7%, which is better than others traditional methods. In addition, a more compact neural network is obtained compared to SCN. To verify the generalization ability of the proposed model, it is applied to gear fault diagnosis with an accuracy of 96.3%. This imply that the proposed method in this paper has a wide application prospect. [ABSTRACT FROM AUTHOR]

Preliminary estimations of shaft resistance of rock-socketed piles are usually conducted using empirical formulations that relate to the uniaxial compressive strength (σc) of the weaker material involved (intact rock or pile). However, there are other factors, such as the degree of socket roughness, that could affect the shaft resistance of rock-socketed piles. In this paper, results from geotechnical centrifuge tests are presented to demonstrate the effect of socket roughness on the pile shaft resistance. Aluminum model piles with different degrees of shaft roughness were fabricated and embedded within an artificial rock mixture composed of sand, cement, bentonite, and water. Pile loading tests were conducted within the centrifuge and axial forces along the model piles were measured using fiber Bragg grating (FBG) sensing technology. Results are used to demonstrate that centrifuge testing provides a suitable experimental method to study and quantify the effect of socket roughness on the shaft shearing mechanism of rock-socketed piles. Finally, the centrifuge test measurements are compared with several formulations published in the literature, suggesting that centrifuge measurements tend to agree with the overall trend, despite the variability of predictions obtained with different formulations. [ABSTRACT FROM AUTHOR]

This research's primary objectives were first to verify a reliable structural health monitoring (SHM) experimental technique for reinforced concrete slab plates, and secondly, to introduce a special fiber Bragg grating (FBG) sensors cable system for continuous field monitoring of concrete. The particular focus was on the damage assessment, using an effective ambient vibration-based damage diagnostic technique of random decrement (RD). An investigation examined the extent of damage induced by progressive static loading through a numerical RD algorithm for four concrete plate specimens: three reinforced high-strength concrete (HSC) plates with different reinforcement ratios, and one ultra-high-performance fiber-reinforced concrete (UHP-FRC) plate. The extent of damage was measured through relative changes in the natural frequency and damping ratio as dynamic parameter damage indexes, which were obtained from RD signatures at cracking, yield, and ultimate loadings. Concurrently, the behavior of the concrete slabs was examined regarding the load-deflection relationship at service and ultimate load, crack pattern, and failure modes. [ABSTRACT FROM AUTHOR]

A Fiber Bragg Grating low-frequency accelerometer with a single-notched hinge in series is proposed to improve the hinge structure's sensitivity and the cross-sensitivity of the accelerometer of low-frequency seismic waves. The sensitivity and the cross-sensitivity of the accelerometer are improved by connecting the single-notched hinge structure in series and experimentally verified. Based on the theoretical analysis, the resonance frequency and sensitivity formula of the accelerometer are derived. At the same time, the accelerometer is simulated by finite element analysis software to simulate and analyze the characteristics of the accelerometer in terms of resonance frequency, strain distribution, and amplitude-frequency response. The structural parameters of the accelerometer are optimized by numerical analysis software, and the effects of the structural parameters on the sensitivity and resonance frequency of the accelerometer are analyzed. The experimental results show that the resonance frequency of the accelerometer is 110 Hz, the flat region is 0.6–50 Hz, the linearity is as high as 98.9%, the sensitivity is 1040.41 pm/g, and the cross-sensitivity of the accelerometer in the flat interval is 4.94%. This indicates that the accelerometer shows good linearity and cross-sensitivity in the flat interval, which well meets the needs of seismic wave signal monitoring in low-frequency oil and gas exploration. [Display omitted] • Tandem single-notch hinge structure reduces detector cross-sensitivity. • The top of the mass block is provided with a rectangular thin plate to improve sensitivity. • The dual grating packaging method effectively improves the sensitivity of the detector. • Small size, low processing cost, can achieve large area detection. [ABSTRACT FROM AUTHOR]

• This paper developed the generalized fiber optic-based sensing models for precise quantification of corrosion severity and its growth rate under impact loads. • A time-dependent corrosion assessment model using Weibull function fits well with the collected data of the mean pit depth. • It demonstrates the potential of fiber optic sensors (FOSs) for real-time structural health monitoring in multi-factor challenging conditions. Structural integrity can be compromised by the simultaneous presence of mechanical loads and corrosive agents. This study investigates the complex interplay between corrosion and impact loads in steel plates, utilizing discrete Fiber Bragg Grating (FBG) and distributed Optical Frequency Domain Reflectometry (OFDR) sensing technology. Generalized fiber optic-based sensing models are developed to quantify corrosion severity and rate. The experimental study was conducted using twelve epoxy-coated steel plates equipped with FBG and OFDR sensors, covering scenarios of individual exposure to corrosion, impact loads, as well as their combination. Test results reveal that specimens subjected to combined conditioning exhibit more corrosion damage than those subjected to individual corrosion. Both pit depth and its growth rate were exacerbated due to the impact loads. The study demonstrates the potential of fiber optic sensors (FOSs) for real-time monitoring and assessment of structural health under different simultaneous multiple factors in challenging conditions. [ABSTRACT FROM AUTHOR]

This paper presents a novel anchor rod force sensor based on fiber Bragg grating (FBG) for accurate anchoring force measurement. A special FBG strain sensor with enhanced sensitivity has been designed and embedded in the centre line of an elastic structural body, which forms the main body of the anchor rod force sensor. The mechanical structure design and measuring principle of the proposed anchor rod force sensor have been presented detailedly. Then a sensor prototype has been manufactured and fully tested. The experimental results show that the sensor provides a sensitivity of 11.58 pm/kN with a fitting linear correlation coefficient of 0.9996 in a measurement range of 50 kN, a repeatability error of 3.26%, and a hysteresis error of 3.87%. Furthermore, good resistance to bending effects of anchor rod, good creep resistance (fluctuant value is within 2 pm), and good temperature compensation capacity (maximum value is 11 pm) have also been verified. These results promise the sensor better applicability in the health monitoring of large civil engineering structures.

This work proposes a novel, highly sensitive, and simple structure based on a tapered polymer as a tilt fiber sensor to sensitively measure tilt angles (θ).The fiber-optic tilt sensor consists of a tapered polymer and single-mode fibers (SMF), into which the tapered polymer to generate a bent in the sensor that is strongly correlated with θ. The used tapered polymer in the tilt sensing is the temperature (T) insensitive. It is further combined with a general fiber Bragg grating (FBG) as a temperature indicator for achieving simultaneous measurement of inclination (θ) and ambient T. The endface of the fiber is coated with a gold film to increase the light reflectivity and improve the measurement range. Experimental results show that when the sensor tilts, the reflective optical power changes significantly with high sensitivity, and the peak wavelength of FBG shifts when T varies. Both parameters can be measured simultaneously with good discrimination. An average tilt angle sensitivity of approximately 4.23 dB/deg. and a high resolution of 0.009 deg. are achieved in a θ variation from -6 to +6 deg. Simulations were performed and the numerical results were in good agreement with the experimental measurement.

This first-ever study demonstrates the applicability of a fiber Bragg grating (FBG) system for MR cardiac triggering of cardiovascular magnetic resonance at 3 Tesla. The unique patented system senses body movements caused by cardiac activity using a non-invasive ballistocardiography (BCG) sensor. The pilot research compares a novel FBG-based system with clinically used triggering systems based on electrocardiography (ECG) and pulse oximetry (POX). The pilot pre-clinical study was conducted on 8 subjects at a Siemens Prisma 3T MR Scanner. The study compares images from two basic cardiac sequences, TRUE FISP (Free Induction Decay Steady-State Precession) and PSIR (Phase Sensitive Inversion Recovery), using objective methods and subjective evaluation by clinical experts. The study presents original results that confirm the applicability of optical sensors in the field of cardiac triggering having a number of advantages in comparison to conventional solutions, such as no eddy current interference, ease of placement of the sensor on the patient's body, and senor reusability. The proposed FBG-based system achieves comparable results with the most frequently used and most accurate ECG-based and POX-based clinical systems. In terms of subjective evaluation by experts, the FBG system outperformed the POX-based system used in clinical practice.

Ocean platforms that are under complex sea conditions and loads for long periods are prone to fatigue cracks. These cracks may lead to large deformations, even displacement, of the platform, and should be monitored to ensure engineering safety. Cracks are not easily detected in the micro stage and small levels of strain measurement are required to ensure high accuracy. Furthermore, cracks are prone to suddenly developing into large deformations, especially in structural connections in practical engineering. This study developed a novel adaptive range strain sensor for structural crack monitoring that can monitor the whole structural crack propagation process in ocean platforms. The strain sensor is used for micro deformation monitoring through its fiber Bragg grating (FBG) sensor with high sensitivity. The sensor can automatically adapt to crack fractures and provide warnings through an STM32 single-chip microcomputer (SCM) system when the structure suddenly cracks, causing large deformation. The experimental results demonstrate that the device has high precision in micro measurement with the ability to capture structural fractures. The field application shows the high strain sensitivity of the sensor in crack monitoring, which indicates that the adaptive range strain sensor is suitable for the structural crack monitoring of ocean platforms.

Water pipelines are the efficient and reliable way for water transportation. Leakage of pipelines can lead to tremendous waste of water resources and large economic loss. In this paper, a novel leakage detection method was proposed based on active thermometry and fiber Bragg grating (FBG) based quasi-distributed fiber optic temperature sensing. In this method, the thermal sensing cable was fabricated by coupling heating cable with quasi-distributed temperature sensors. The heat was introduced by the heating cable and the temperature response was measured by the quasi-distributed fiber optic temperature sensor concurrently. The leakage can be detected and located by identifying the local low values in the temperature profile along the pipelines. The feasibility of the proposed method was validated by finite element simulation and experimental investigation. Good agreement between simulation and experimental study was achieved. The results confirmed the effectiveness of the proposed method for leakage detection. [ABSTRACT FROM AUTHOR]

A turnout is a key piece of equipment and one of the weakest links of the railway infrastructures. Continuously welded turnouts (CWTs) have been widely laid on high-speed railway bridges because of the topographic and environmental limitations, where special attention is required to ensure the safety of the high-speed railways. A real-time monitoring system will be interesting for the service status of the CWT on the bridge, which was designed and established for the first time in China. Real-time data of the service status indicators were obtained by fiber grating and video perception technologies. The data features of the monitoring results were analyzed. A regression model and backpropagation neural network model were adopted to predict the key indicators. An alarm mechanism with a static threshold and outlier diagnosis was established. The monitoring system has been in good operation state for more than 4 years, effectively ensuring railway safety and security. [ABSTRACT FROM AUTHOR]

The existing slope safety monitoring studies mainly focus on long-term monitoring of slowly evolving horizontal deformations. However, during the practical applications, sudden deformations may also lead to landslides due to the natural and man-made trigger factors. This article first proposes a reusable and distributed fiber Bragg grating (FBG) inclinometer for simultaneous measurement of horizontal deformation and vibration induced by sudden deformation, providing more extensive monitoring for slopes. The proposed inclinometer utilizes ten FBG tilt sensors in series as measuring elements, which are characterized by easy installation and removal, thus making them reusable and eliminating excessive waste. A signal decomposition method is presented for splitting the coupled signal of the tilt sensor into two parts: the inclination angle signal and the vibration signal. The inclination angle signals can be converted to distributed horizontal deformation through a cumulative algorithm. And the vibration intensity can be inferred from the vibration signal. The measuring principle, structure design, and calibration experiment of the inclinometer were present in detail. A distributed horizontal deformation generating device with high precision was specially developed to simulate the internal deformation of slope. Verification experiments further validated the feasibility of the proposed inclinometer for simultaneous measurement of horizontal deformation and vibration induced by sudden deformation. [ABSTRACT FROM AUTHOR]

A fiber ring cavity laser temperature sensor is presented and demonstrated. The sensor probe consists of a fiber Bragg grating (FBG) and a heterostructure based on polymer-coated no-core fiber (NCF), and this probe also functions as a wavelength-selective component in the fiber laser system. There are multimode interference and antiresonance effects in the NCF coated with high refractive index polymer. Embedding this sensing head into an Er-doped fiber ring cavity laser system is helpful to improve the spectral quality and the detection limit (DL). The passband peak position of the heterostructure filter and FBG filter varies with the temperature, which leads to the corresponding shift in the output laser wavelengths. Experimental investigation achieves a maximum sensitivity of −3.155 nm/°C in the temperature range from −4 °C to 47 °C, and the DL is calculated as 0.005 °C. It should be noted that there are several measurement intervals in this temperature range, which can be distinguished by the cascaded FBG to obtain the accurate temperature value. Moreover, the sensor with a smaller NCF diameter has a higher temperature sensitivity. The proposed sensor can meet the actual demands of mesoscale ocean phenomena and microregion temperature detection in seawater, and it provides references for improving the sensing capability of optical methods in ocean detection and other practical application. [ABSTRACT FROM AUTHOR]

The strain- and temperature-sensing characteristics of fiber ring laser (FRL) sensors are studied comprehensively. The reflected sensing head, made up of spheroidal Fabry–Perot interferometer (FPI) concatenated with fiber Bragg grating (FBG), is inserted in the cavity of FRL acting as the laser intensity modulator and wavelength selector. The laser intensity is sensitive to temperature change while insensitive to strain, making the dual-parameter demodulation realizable without multivariate matrix by detecting the laser wavelength shift and the laser intensity variation, for the first time in our best knowledge. The comparable sensitivities of 1 pm/ $\mu \varepsilon $ and 24.9 pm/°C are detected in experiment for strain and temperature, respectively, with low detection limit (DL) of 0.87 $\mu \varepsilon $ and 0.035 °C, high $Q$ factor of 1140 and 7.07 $\times \,\,10^{5}$. The high intensity and narrow linewidth of the sensor make it potentially great for application in long-distance sensing systems. [ABSTRACT FROM AUTHOR]

Collaborative robots equipped with multicomponent force-sensing systems at the human–robot–environment interaction interface provides competitive advantages, such as collaboration, flexibility, and reliability. Most wrist force sensors use the cross-beam structures with electrical strain gauge measurement technologies. This article proposes an approach to develop a new ultrathin three-axis fiber Bragg grating (FBG) force sensor. First, an appropriate elastomer with embedded floating beams in ultrathin dimensions to measure three-axis force is designed using a finite-element method (FEM). A temperature-compensated extreme learning machine (TC-ELM) network is proposed to achieve the characteristics of low coupling and temperature compensation (TC). Calibration and experiment on the UR5 robot are conducted, and results illustrate that the highest sensitivity of the sensor can reach 34.07 pm/N, the repetition error is less than 5%, and the maximum coupling error is less than 0.21%. The maximum temperature drift after compensation is less than 20 pm. The proposed sensor has identical output as a commercial sensor and performs well in dynamic loading and unloading operations. Finally, comparison results from commercial and academic alternatives demonstrate the interest of the proposed approach to develop high-performance and integrated force-sensing solutions. [ABSTRACT FROM AUTHOR]

Fiber Bragg grating (FBG) acceleration sensor is an important branch of FBG application and has irreplaceable advantages compared with an electronic accelerometer. An FBG acceleration sensor with high sensitivity and high natural frequency is designed. This sensor adopts a double cantilever structure to sense the vibration perpendicular to the axis. Compared with a single cantilever structure, the natural frequency is increased without reducing the sensing length. Through the above structure design and material selection, the sensor shows a high natural frequency of 8658 Hz and its sensitivity is 0.44 pm/g. After the photoelectric demodulation is used, the sensitivity is 52 mV/g and the linearity is 0.998. Through the experiment and simulation analysis, the sensor performs good anti-interference in the transverse direction and temperature compensation in the process of vibration measurement. The system is very small due to the use of light intensity demodulation technology, which is conducive to application and integration in actual measurement scenarios. [ABSTRACT FROM AUTHOR]

The interface effects between microstructural bodies become more and more important as micromechanical and microfluidic devices enter sub-micrometer scale. Scanning probe microscopes like atomic force microscopy are the most significant tools for detecting the interface forces, where the dimension of the cantilever tips is on nanometer scale. In this work, we present a fiber-Bragg-grating-based interface force measurement method for micrometer-sized probe tips and detailed measurements conducted with various sample surfaces, tip radius, and medium environments. The experimental and numerical results unambiguously reveal that probes with micrometer-sized tips are more easily influenced by interface interaction, and forces show varying magnitudes, action ranges, and prioritized contributions, compared with nano-sized probe tips. In addition, the interface force measurement approach could help find the contact zero point when the probe approaches and contacts the measured surface and understand surface characteristics of materials as well. This work could lead to many potential applications of the interface interaction in micro- and nanotechnology using micrometer-scale probe tips. [ABSTRACT FROM AUTHOR]

A flare system in oil platform is a combustion stack used to burn off excess gases that cannot be processed and gases that have to be eliminated in emergency shutdown to avoid the risk of explosion. Gas flow measurement in flares is still considered challenging because the measurement has to attend many specific demands, quite different from any other flow measurement application. For these types of measurements, many sensor technologies are available; however, they are all very expensive, and only a few ones can attend all demands. The objective of this article is to present a flow rate sensor based on fiber Bragg grating (FBG), cross correlation, and heatwave travel time techniques. The system was developed with inexpensive components and is little intrusive, capable of attaining the rangeability of the flare demand and independent of gas composition, pressure, and temperature. [ABSTRACT FROM AUTHOR]

By the discovery of the optical fiber, it has become the most widely used technique for communication purposes. The major limiting factor which degrades the efficiency of optical communication is the dispersion, as it causes the traveling pulse along the fiber to spread and change its shape causing inter symbol interference. The main objective of this paper is to reduce the dispersion effects. This is carried out by using four cascaded tanh apodized FBGs, which is the main contribution in our work, to enhance the spectral linewidth of light source as it works like a band pass filter that reflects a certain signal wavelength and passes the others. A maximum bit rate per channel of 709.9905 Gbit/s is achieved in silica-doped optical fibers based on the maximum time division multiplexing (MTDM) transmission technique at a temperature of 20 °C, a relative refractive index difference of 0.002, a doping ratio of 0.2, a signal wavelength of 1.65 µm and a 10 km link length. Three conditions are considered and compared in our work: MTDM technique only, MTDM with raised cosine apodized FBG, and MTDM with tanh apodized FBG. [ABSTRACT FROM AUTHOR]

A compact optical fiber Mach-Zehnder interferometer (MZI) sensor is reported for temperature and strain. The MZI sensor consists of a single-mode fiber (SMF), a seven core fiber (SCF) and a multimode fiber (MMF) with a fiber Bragg grating (FBG). The intensity distribution of the sensor is calculated by the beam propagation method. The light is coupled from the lead-in SMF into the SCF through the MMF. The highest transmission efficiency was obtained when the length of MMF is 2 mm with a diameter of 105 μm. The temperature sensitivity is up to 55.06 pm/°C from 130 °C to 250 °C, while the maximum extinction ratio was 27.4 dB. The measurements were divided into two intervals to address cross coupling to provide sensitive temperature and strain measurements. [ABSTRACT FROM AUTHOR]

Fiber Bragg Grating (FBG) is achieved by refractive index modulation. Temperature and strain can be measured by FBG. The echo spectral distribution is determined by the refractive index of the fiber and the grating pitch. Therefore, there are few types of conventional refractive index modulation methods, and there are few types of echo spectrum. In this paper, an index code modulation based on IoT method was proposed. The FBG is divided into a plurality of interval segments in the method. The spectral shape of the echo is controlled by encoding and modulating each interval. When the segment size satisfies some requirements, the positive and negative modes of the sub-FBG are calculated by the coupled-mode theory, and then the mode functions of each segment are solved by the matrix transmission algorithm. The equivalent function of the reflectance distribution field is obtained. According to the relevant parameters of FBG, the mathematical model of the FBG modulation spectral distribution is established by the refractive index modulation method. Through the simulation analysis of the fractional refractive index modulation, the results show that the echo spectral peak, half-width, etc. have a larger tunable range. In the experiment, a femtosecond laser was used to accurately encode and modulate the refractive index, and two different types of segmented FBGs were fabricated according to design requirements. The experimental results show that the design of different segmented structures has a significant impact on the spectral distribution of the echo. Spectral shape control can be achieved by refractive index modulation method. [ABSTRACT FROM AUTHOR]

Fiber Bragg grating (FBG) is the most widely used optical fiber sensor due to its compact size, high sensitivity, and easiness for multiplexing. Conventional FBGs fabricated by using an ultraviolet (UV) laser phase-mask method require the sensitization of the optical fiber and could not be used at high temperatures. Recently, the fabrication of FBGs by using a femtosecond laser has attracted extensive interests due to its excellent flexibility in creating FBGs array or special FBGs with complex spectra. The femtosecond laser could also be used for inscribing various FBGs on almost all fiber types, even fibers without any photosensitivity. Such femtosecond-laser-induced FBGs exhibit excellent thermal stability, which is suitable for sensing in harsh environment. In this review, we present the historical developments and recent advances in the fabrication technologies and sensing applications of femtosecond-laser-inscribed FBGs. Firstly, the mechanism of femtosecond-laser-induced material modification is introduced. And then, three different fabrication technologies, i.e., femtosecond laser phase mask technology, femtosecond laser holographic interferometry, and femtosecond laser direct writing technology, are discussed. Finally, the advances in high-temperature sensing applications and vector bending sensing applications of various femtosecond-laser-inscribed FBGs are summarized. Such femtosecond-laser-inscribed FBGs are promising in many industrial areas, such as aerospace vehicles, nuclear plants, oil and gas explorations, and advanced robotics in harsh environments. [ABSTRACT FROM AUTHOR]

In this article, we present a fiber Bragg grating (FBG) based 6-D force/torque (F/T) sensor that can be mounted on robot joints for the detection of comprehensive F/T information. The sensor is designed based on a new concept of multilayer measurement in which the elastic structure is divided into four layers. A single optical fiber inscribed with 12 FBGs is symmetrically attached to the elastic structure to sense F/T-induced strains by means of differential measurements. Because of the mechanical decoupling function of the structure and the reasonable arrangement of the FBG sensing elements, the designed sensor possesses a great advantage of low cross coupling compared with the existing 6-D F/T sensors, which simplifies the calibration and maintenance of the sensor. Calibration experiments demonstrate high resolutions of 7.8 N·mm, 7.2 N·mm, 15.8 N·mm, 0.0067 N, 0.0066 N, and 0.0825 N within the range of ±1.56 N·m, ±1.56 N·m, ±1.56 N·m, ±10 N, ±10 N, and ±25 N for the detection of Mx, My, Mz, Fx, Fy, and Fz, respectively. Dynamic performances of the designed sensor have been evaluated in comparison with a commercial F/T sensor. [ABSTRACT FROM AUTHOR]

Current ventricular assist devices (VADs) are rotary blood pumps used to treat end-stage heart failure. VADs are operated at a constant speed that is manually adjusted by a clinician based on the patient’s cardiac demand during routine medical examinations. VADs operated at a constant speed have inadequate passive flow regulation due to the inherent mechanical pressure-flow characteristics of the pump; this can lead to harmful situations where the VAD is overpumping or underpumping. Typically, patients on long-term VAD support are discharged to an outpatient setting where the VAD speed can remain the same for weeks or months at a time, impacting patient safety and quality of life. Previously, physiological controllers for VADs have been proposed, which automatically adjust VAD speed to meet patient cardiac demand. Clinical implementation of physiological control is currently hindered by the lack of clinically available, implantable, and continuous hemodynamic sensors. This study describes the physiological control of a VAD using a fiber Bragg grating (FBG) sensor previously developed for measuring VAD inlet pressure. The FBG sensor was used as a feedback to a Starling-like physiological controller, and the control quality was compared against the same controller with feedback from a nonimplantable industrial pressure sensor (Omega sensor). Experiments were conducted in a bench-top cardiovascular simulator under various simulated patient scenarios. The average steady-state difference in VAD flow across all experiments was 0.1 L/min with a maximum difference of −0.4 L/min. Similarly, the average steady-state difference in left ventricular end-diastolic pressure was 0.02 mmHg with a maximum difference of −0.2 mmHg. The clinically insignificant differences found between the two feedback methods indicate that the FBG pressure sensor is viable for the physiological control of VADs. [ABSTRACT FROM AUTHOR]

We propose and experimentally demonstrate a centralized PON monitoring scheme using the reflection spectrum of a single fiber Bragg grating. Each fiber Bragg grating used in the scheme adopts different central reflection wavelength or 3-dB reflection bandwidth. The unique reflection spectrum is generated after each fiber Bragg grating used is interrogated by a broadband probe source. These generated reflection spectra are used to identify and distinguish the drop fiber links. The similarity analysis model is established to solve the problem of waveform superposition identification for different 3-dB reflection bandwidths with the same central reflection wavelength, which can further improve the network monitoring capacity. The proposed scheme provides a simple and effective monitoring solution for large-capacity network, especially for high-density network with small end-user distance difference. [ABSTRACT FROM AUTHOR]

In this paper, one advanced settlement sensor with the advantages of wide measurement range and multi-directional displacement measurement has been designed and fabricated based on multidisciplinary technologies, including fiber Bragg grating (FBG) sensing technology, 3D printing technology, and others. First, one novel structure of settlement sensor was designed by introducing the concept of a hinge joint, which mainly consists of four components, including internal rigid beam, external rigid beam, elastic FBG sensing beam, and hinge joint. The FBG sensing element inside the sensor can rotate unconstrained without disturbing stress field underground, overcoming the strengthening problem commonly existed in conventional settlement sensors. It is also worth noting that all sensor components were designed and fabricated with 3D printing technology, while the two most popular and effective 3D printing technologies, including fused deposition modeling (FDM) and stereolithography (SLA) technology were separately used for fabricating elastic sensing beam and rigid beam segments, respectively. Furthermore, the FBG sensors were successfully embedded into the printed polylactic acid (PLA) without sacrificing its sensing performance, and the superiority of FBG sensing technology, such as high sensitivity, high accuracy, strong resistance, light weight, and serial connection have been achieved. After completing the fabrication of developed sensor, the measurement performance parameters of elastic FBG sensing beam (includes two paralleled FBG sensors) including measuring linearity, repeatability, and measurement range have been calibrated and verified. Finally, working performance of the assembled settlement sensor from calibration was examined, characterized by a maximum measurement range -160 ~ +160, and a measurement sensitivity of 0.0066 pm/° (in terms of inclination degree) or 0.012 mm/pm (in terms of settlement).

This publication describes an original simple low-cost MR fully-compatible and safe fiber-optic breathing sensor (FOBS), which can be used for respiratory triggering and for monitoring the development of respiratory rate within the MR environment and can, thus, serve as prevention from the hyperventilation syndrome. The sensor is created by encapsulation of the Bragg grating into conventional nasal oxygen cannulas. The sensor is immune to minor patient movements, thus limiting movement artifacts to a minimum. Thanks to this fact it can be used for the retrospective/prospective respiratory gating. The sensor is immune to electromagnetic interference (EMI) and can thus be used in any magnetic field (1.5T, 3T, and 7T). The sensor prototype has been tested in both laboratory and real magnetic resonance (3T) environments relative to conventional pneumatic respiration references (PRR). The data measured were statistically evaluated using the objective Bland-Altman method (BAM) and the functionality of the proposed solution was confirmed. Respiratory Triggering functionality was confirmed by the radiologic doctors on the basis of analyzing images using the most used respiratory triggered T2 TSE 3D sequences and by objective method using the Blind/Referenceless Image Spatial Quality Evaluator (BRISQUE).

We achieve Fano-like resonances in an all-in-fiber structure embedded with an in-line Mach-Zehnder interferometer (MZI). A fiber Bragg grating is inserted into MZI's one arm to form a resonance, which functions as the discrete state of the Fano-like resonance to couple with the continuum propagating mode of MZI in the fiber core. A theoretical model predicts the controllable resonance lineshape by changing the phase difference between the MZI's two interference pathways. Fano-like resonances with an extinction ratio over 20 dB are experimentally observed, which are reliably tuned into Lorentzian and electromagnetically induced transparency-like resonances by versatile methods. The realization of Fano-like resonances with broad tunability in this all-in-fiber structure holds potentials in fiber-based applications of sensing, signal processing and nonlinear optics.

A novel large-scale three-layer-ring optical fiber sensor network is proposed with reconfigurable routing functionality. To provide large-scale sensing region as hybrid star-ring architecture and to provide robust self-healing function to deal with large amount of fiber-link failures as mesh architecture, the proposed architecture utilizes optical switches (SW) and new type of remote nodes (RNs) to compose the entire system in three layers. In the first layer, several fiber Bragg grating (FBG) sensors connect each other to compose a sensing subnet. In the second layer, several FBG sensing subnets connect each other by SWs to form the ringed sensing regions. In the third layer, each sensing region connects with central office by RNs. When fiber-link failure occurs in the system, the proposed three-layer-ring architecture can immediately restore the sensing signals by changing the status of the SWs and RNs in the sensing network. According to the simulation results, the proposed three-layer-ring architecture can completely restore the sensing signals when there are up to four break points occurring in a sensing region at the same time. The observed optical spectra of the received optical sensing signals in CO are roughly identical when the proposed architecture has no fiber-link failure or has up to four breakpoints.

The temperature detection and alarm system of power equipment is supposed to be used in power accidents caused by overheating. In our experiment, we build a system to detect the temperature of electrical power equipment. According to the actual temperature fitting curve, the whole system uses the Fiber Bragg Gratings as the signal sensing and transmission media, using the tunable Fabry-Perot (FP) cavity filter to demodulate the fiber and a SLED light source which is considered high reliability. System control and signal processing is carried out by the STM32F407ZGT6 microcontroller and its internal single-cycle DSP. The system has the characteristics of strong anti-interference ability, high reliability and precision, simple installation, convenient maintenance, remote signal transmission.

Abstract During last decades, sensor elements based on the fiber Bragg grating (FBG) have been widely studied and developed due to the advantages of immunity to electromagnetic interference, compact size, high precision, and so on. The FBG itself is sensitive to axial strain and temperature variation directly and can indirectly measure these complex physical parameters, such as pressure, displacement, and vibration, by using some specially designed elastic structures to convert them into the axial strain of the FBG. Whether the FBG is fixed on the measured object to measure the strain directly or fixed on an elastic structure body to measure other physical quantities, these types of FBGs could be collectively called as strain sensing FBGs. The packaging of the FBG has important influence on FBG characteristics that directly affect the measurement accuracy, such as strain transfer, temperature characteristic, and spectral shape. This paper summarizes the packaging methods and corresponding temperature compensation methods of the currently reported strain sensing FBGs, focusing especially on fully pasted FBG, pre-stretched FBG with double-end fixed, and metallic packaging. Furthermore, the advantages and drawbacks of different packaging methods have been analyzed, which can provide a reference for future researches.

Thermal ablation (TA) is known as an alternative therapy to surgery to treat tumors. However, TA-based therapy requires advanced approaches in order to prevent causing damage to healthy tissue around the tumor and selectively target the desired area. Nanoparticles are considered as a promising tool in biomedicine to fulfill these requirements. This study was carried out in order to analyze the effect of iron oxide nanoparticles on the temperature increment during radiofrequency ablation therapy of porcine liver. In addition, this research aimed to experimentally evaluate the impact of two solvents such as agarose and chitosan on the temperature change, when magnetic nanoparticles were dispersed in them. The iron oxide nanoparticles were synthesized by the solvothermal method demonstrating the magnetic properties by acting to the external magnetic field. To increase the local heat superparamagnetic nanoparticles (iron oxide magnetic nanoparticle (IONPs)) of the average size of 20 nm in size and the concentrations from 1 to 10 mg/mL of MNPs with a step size of 1 mg/mL were tested in 10 replicates for each concentration and solvent. Moreover, the temperature changes for dry liver, and 0 mg/mL concentration was checked for calibration and reference purposes. As a sensing system, advanced 16-FBG optical fiber sensors connected to an interrogator were employed allowing the temperature change to be monitored accurately in real time. A maximum temperature of about 142 °C was recorded by a 5 mg/mL concentration of iron oxide nanoparticles dispersed in the agarose solvent. [ABSTRACT FROM AUTHOR]

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In this work, we have investigated the strain response (angular/spatial) from fiber Bragg grating (FBG) sensor & resistance strain gauge (RSG) sensors bonded to the composite structure due to the projectile low velocity impact (LVI). The number of sensor & its orientating has been optimized based on such experimental data and designed an optimum sensor network for faithful LVI detection. In order to study the efficacy of the sensor network, an impact localization algorithm based on peak strain amplitude from the sensor bonded to the structure was used in this study. Further the detection efficiency of the algorithm has been improved using weighted average value around the peak amplitude of strain experienced by the sensor. We found that for the high energy (~35 J) LVI the maximum distance error (Euclidian distance) was 50 mm for 80% of total trail case. Furthermore, we have developed and compared the relative performance of the algorithm cited in the literature, will be presented in PART-II of the same Journal. [ABSTRACT FROM AUTHOR]

This article presents the development and experimental evaluation of a redundant robotic system for the less-invasive treatment of osteolysis (bone degradation) behind the acetabular implant during total hip replacement revision surgery. The system comprises a rigid-link positioning robot and a continuum dexterous manipulator (CDM) equipped with highly flexible debriding tools and a fiber Bragg grating (FBG) based sensor. The robot and the continuum manipulator are controlled concurrently via an optimization-based framework using the tip position estimation (TPE) from the FBG sensor as feedback. Performance of the system is evaluated on a setup that consists of an acetabular cup and saw-bone phantom simulating the bone behind the cup. Experiments consist of performing the surgical procedure on the simulated phantom setup. CDM TPE using FBGs, target location placement, cutting performance, and the concurrent control algorithm capability in achieving the desired tasks are evaluated. Mean and standard deviation of the CDM TPE from the FBG sensor and the robotic system are 0.50 and 0.18 mm, respectively. Using the developed surgical system, accurate positioning and successful cutting of desired straight-line and curvilinear paths on saw-bone phantoms behind the cup with different densities are demonstrated. Compared to the conventional rigid tools, the workspace reach behind the acetabular cup is 2.47 times greater when using the developed robotic system. [ABSTRACT FROM AUTHOR]

Temperature monitoring is an essential task in cryogenic engineering to ensure the safety of equipment, where highly sensitive and reliable sensors are required to provide real-time and accurate information about temperature. Titanium (Ti)–copper (Cu)-coated fiber Bragg grating (FBG) temperature sensors with a gradient of material properties to minimize thermal stresses are hence fabricated by a combination of Ti/Cu magnetron sputtering and Cu electroplating. The thermal characteristics and thermal fatigue behavior of the sensors are investigated. The sensors exhibit significantly higher sensitivity than that of the bare FBGs at cryogenic temperatures down to 79 K, with good repeatability and stability. Their response to the temperature changes can be precisely described by a quadratic-polynomial equation. No obvious variations in the zero-point, the reflection spectrum, and the temperature sensitivity of the sensors exposed to thermal fatigue from 77 K to room temperature are observed before fatigue failure which could be attributed to the formation and growth of subcritical cracks within the fiber rather than that of fatigue cracks within the metallic coatings, as a result of the cyclic thermal stresses induced by the large differences in coefficient of thermal expansion between the silica optical fiber and the metallic coating materials. The results demonstrate that the multilayer metal-coated FBG sensors provide great potential for temperature measurements or temperature compensation in cryogenic engineering by proper selection of metallic coatings with a gradient of material properties. [ABSTRACT FROM AUTHOR]

The evaluation of lungs functionality enables the diagnose and monitoring of several pulmonary disorders. Spirometry is an established technique for pulmonary function tests, which provides information about several respiratory parameters, such as the forced vital capacity (FVC), corresponding to the maximum volume of expired air; the forced expiratory volume in 1 second (FEV1), which is the volume of exhaled air after the first second of the forced respiratory maneuver; the peak expiratory flow (PEF), representing the maximum flow of exhaled air during the maneuver; and the ratio of FEV1/FVC. In this work, a spirometer based on fiber Bragg gratings (FBGs), integrated in a 3D printed structure is proposed. The dynamics and magnitude of the air flow are determined through the resultant strain variations applied on the FBG sensor, along with the differentiation between the inhalation and exhalation phases. The air flow temperature effect was evaluated and its compensation was also performed for the accuracy of the results. The performance of the reported system was theoretically analyzed and experimentally tested. The implementation results were compared to the ones obtained with a commercial handheld spirometer, to assess the accuracy of the developed device. The accordance in the values obtained proved the suitability of the developed system to act as a reliable spirometer. [ABSTRACT FROM AUTHOR]

The accuracy of landslide displacement prediction can effectively prevent casualties and economic losses. To achieve accurate prediction of the Majiagou landslide displacement in the Three Gorges Reservoir (TGR), China, a hybrid machine learning prediction model considering the deformation hysteresis effect is proposed. The real-time deep displacement measurements were captured by using in-place inclinometers with Fiber Bragg grating (FBG) sensors. The time series method was adopted to divide the total displacement into a trend term and periodic term. Trend displacement was determined by the geological condition and predicted by the fitting method. Periodic displacement was controlled by external factors such as rainfall and fluctuation of reservoir water level. Before making the prediction, the grey correlation analysis was adopted to confirm that the fluctuation of the reservoir water level was the main influence factor. In view of the deficiency that current prediction methods could not quantitatively determine the lag time of landslide deformation and thus select the influencing factors empirically, the dynamic analysis of the correlation between periodic influence factors and periodic displacement was carried out in this paper, and the deformation lag time was identified to be 18 days by using set pair analysis (SPA) method. Finally, the optimal influence factors were selected and the prediction model of Majiagou landslide based on support vector machine optimized by particle swarm optimization (SPA-PSO-SVM) was established. Results showed that the root mean square error (RMSE) and the mean absolute percentage error (MAPE) of the proposed SPA-PSO-SVM prediction model are 0.28 and 12.8, respectively. Compared with the PSO-SVM model, the prediction accuracy of the proposed model had been improved significantly. The reliability and effectiveness of the SPA-PSO-SVM prediction model is verified and it has apparent advantages while predicting landslide displacement with deformation hysteresis effect involved. [ABSTRACT FROM AUTHOR]