Fiber Bragg Grating-based Sensor Research Articles

Improved temperature compensation of fiber Bragg grating-based sensors applied to structures under different loading conditions

Fiber Bragg grating (FBG) based sensors have been extensively used to monitor the deformation of structures (i.e., aircrafts, ocean platforms, bridges, tunnels, pavements and high-speed railways), for the advantages of absolute measurement, anti-electromagnetic interference, high precision, multiplexing and integration of sensing network over other sensors. However, due to the temperature-sensitive characteristics of FBG, temperature compensation should be considered to remove the thermal effect, so as to accurately reflect the strain information of the monitored structure. Traditional temperature-compensation method ignores the influence of the interfacial interaction between the structure and the bonded FBG on the measured data, which may lead to the compensated value smaller than the actual value. For this reason, modified temperature-compensation function with the influence of interfacial action considered is proposed to enhance the measurement accuracy of FBG based sensors. Laboratory tests on two kinds of samples under temperature loading have been conducted to validate the effectiveness of the proposed function. Improved temperature-compensation measures for FBG based sensors applied to structures under different loading conditions (i.e., high and low temperature, static and dynamic mechanical load) are discussed, with the temperature hysteresis of FBG sensing elements considered. The study in this paper presents a scientific instruction for the better interpretation of testing data measured by FBG sensors and the accurate use of FBG sensors for the temperature and strain detection of structures in engineering

Implantable Fiber Bragg Grating Sensor for Continuous Heart Activity Monitoring: Ex-Vivo and In-Vivo Validation

Continuous and reliable cardiac function monitoring could improve medication adherence in patients at risk of heart failure. This work presents an innovative implantable Fiber Bragg Grating-based soft sensor designed to sense mechanical cardiac activity. The sensor was tested in an isolated beating ovine heart platform, with 3 different hearts operated in wide-ranging conditions. In order to investigate the sensor capability to track the ventricular beats in real-time, two causal algorithms were proposed for detecting the beats from sensor data and to discriminate artifacts. The first based on dynamic thresholds while the second is a hybrid convolutional and recurrent Neural Network. An error of 2.7 ± 0.7 beats per minute was achieved in tracking the heart rate. Finally, we have confirmed the sensor reliability in monitoring the heart activity of healthy adult minipig with an error systematically lower than 1 Bpm.

Recent Advances and Tendency in Fiber Bragg Grating-Based Vibration Sensor: A Review

Vibration sensing is critical to monitor and ultimately preserve the health state of engineering systems. These systems with a large structure are typically working in some harsh environments including strong magnetic fields. However, traditional electrical sensors are difficult to accurately measure the vibration under harsh environments. Besides these instinct advantages of normal fiber optic sensors (FOS) sensors such as compact size, passive sensing, resistance to electromagnetic interference, etc., fiber Bragg grating (FBG) sensors have a capability of distributed sensing based on wavelength demodulation and resistance to light intensity fluctuation and unwanted fiber bending losses. Such merits lead them to be a hot topic in FOS field and excellent candidates for vibration sensing. Three types of FBG-based vibration sensors have been classified based on the difference of vibration-strain coupling way to FBG in this survey, which are pasted FBG-based, axial property of FBG-based and transverse property of FBG-based, respectively. FBG-based vibration sensors' principles and designs have been introduced and discussed. Recent advances in the applications of FBG-based vibration sensors have been investigated. The limitations and prospects of the FBG-based vibration sensing technologies have been analyzed and discussed.

Fiber Bragg grating sensors for monitoring of physical parameters: a comprehensive review

Fiber Bragg grating has embraced the area of fiber optics since the early days of its discovery, and most fiber optic sensor systems today make use of fiber Bragg grating technology. Researchers have gained enormous attention in the field of fiber Bragg grating (FBG)-based sensing due to its inherent advantages, such as small size, fast response, distributed sensing, and immunity to the electromagnetic field. Fiber Bragg grating technology is popularly used in measurements of various physical parameters, such as pressure, temperature, and strain for civil engineering, industrial engineering, military, maritime, and aerospace applications. Nowadays, strong emphasis is given to structure health monitoring of various engineering and civil structures, which can be easily achieved with FBG-based sensors. Depending on the type of grating, FBG can be uniform, long, chirped, tilted or phase shifted having periodic perturbation of refractive index inside core of the optical fiber. Basic fundamentals of FBG and recent progress of fiber Bragg grating-based sensors used in various applications for temperature, pressure, liquid level, strain, and refractive index sensing have been reviewed. A major problem of temperature cross sensitivity that occurs in FBG-based sensing requires temperature compensation technique that has also been discussed in this paper.

A High-Sensitivity Fiber Bragg Grating-Based Distal Force Sensor for Laparoscopic Surgery

This paper presents a Fiber Bragg Grating (FBG)-based distal force sensor with a high sensitivity and a relatively large measurement range to provide force feedback for laparoscopic surgery. The proposed sensor design mainly consists of a force-sensitive flexure, one tightly suspended optical fiber embedded with an FBG sensing element and a contact head. The proposed flexure has been miniaturized based on improvement and optimization of a parallel structure form and a serial cantilever beam-based structure to achieve an excellent axial linear force-deformation relationship and a large measurement range. The optical fiber adopts a two-point pasting method, and has been tightly stretched and glued along the flexure’s central line to achieve improved sensitivity and avoid FBG chirping failure. Finite element method (FEM)-based simulation has been performed to investigate both static and dynamic performances of the proposed design. The simulation-enabled optimization has been implemented to achieve optimal structural parameters and enhanced sensitivity. The optimized design has been prototyped and calibrated to demonstrate an excellent linearity with a small linearity error of 0.14% and achieve a high resolution of 21mN within a measurement range of [0, 12N]. The further optimized version with the removal of small deformation constraint can reach a higher resolution of 9.3 mN within an operating range of [0, 7N]. Dynamic loading experiments have been conducted to validate the effectiveness of the proposed designs.

Correction to: A High-Precision and Miniature Fiber Bragg Grating-Based Force Sensor for Tissue Palpation During Minimally Invasive Surgery.

The second equation in the section "Definition of Objective Functions and Constraints" was corrected to fix a mismatch between the PDF and HTML versions of the article.

Research on deflection monitoring for long span cantilever bridge based on optical fiber sensing

Abstract There are a lot of shortages in monitoring the deflection of long span cantilever bridge during construction period, such as numerous construction procedures, difficult wiring and failure in collecting data in real-time. In order to overcome these shortages, a Fiber Bragg grating-based inclination sensor is designed, which is based on “pendulum-equal strength beam”. Furthermore, a deflection monitoring structure based on the inclination sensor is designed and it can measure deflection by inclination change. The deflection monitoring structure prototype is manufactured and the relevant performance tests show that the sensitivity is 10.566 pm/mm and the fitting error is 0.9997 ranging from −100 mm to 100 mm. Compared with the theoretical calculation value, the deviation of sensitivity coefficient of the sensing structure is about −0.68%, which can meet the accuracy requirements of deflection measurement for long span cantilever bridge construction health monitoring. Apply the designed deflection monitoring structure in the cantilever bridge of Shaba An’ning River Bridge on Line 2 of Chengdu-Kunming Railway, and monitor the deflection changes of each construction segment in three key processes including hanging basket moving forward, concrete pouring and pre-stressing tensioning. The feasibility of the deflection sensing structure in cantilever bridge construction monitoring is verified.

A High-Precision and Miniature Fiber Bragg Grating-Based Force Sensor for Tissue Palpation During Minimally Invasive Surgery.

This paper presents a novel Fiber Bragg Grating (FBG)-based palpation force sensor to explore tissue abnormalities during minimally invasive surgery. The proposed sensor design mainly consists of a miniature force-sensitive flexure, one tightly suspended optical fiber embedded with one FBG element and associated connectors and fixations. The flexure design has been prototyped through the configuration synthesis of Sarrus mechanism by using a rigid-body replacement method to achieve an excellent axial linear force-deformation relationship and a large measurement range. The mounted fiber has been configured at the flexure's central line with its two ends glued, and its tight suspension configuration can achieve improved resolution and sensitivity and avoid the FBG chirping failure compared to the commonly used direct FBG-pasting methods. Finite element method (FEM)-based simulation has been performed to investigate both static and dynamic performance to aid in structural design. Simulation-enabled structural optimization design has also been implemented to further improve the proposed design and the sensor's sensitivity has been increased. The optimized sensor design has been prototyped and calibrated to demonstrate an excellent linearity with a small linearity error of 0.97% and achieve a high resolution of 2.55 mN within a relatively large measurement range of 0-5N. Dynamic force stimulation experiments, in vitro palpation implementation on a silicone phantom embedded with simulated tumors and ex vivo indentation experiments on a porcine liver have validated the effectiveness of the presented sensor design.

Enhancing Sensitivity of Fiber Bragg Grating-Based Temperature Sensors through Teflon Coating

As global warming and climate change persistently threaten our planet, especially in developing countries, environmental monitoring becomes more and more imperative. One of the key parameters in estimating ecological disturbance due to climate change is temperature. In this work, we design a temperature sensor based on a fiber Bragg grating (FBG) and investigate the effect on its sensitivity upon applying a polymeric coating, and contrast it with different metal coatings. The reflected spectrum of an FBG has a narrow peak, corresponding to the Bragg wavelength, which is contingent on the grating period, grating length and effective refractive index. We simulate an outdoor ambient environment and examine the change in the peak reflected wavelength on variation in temperature. The shift in the peak for a unit change in temperature can be defined as the sensitivity of the FBG sensor. To enhance the sensitivity, we apply a uniform coating of Polytetrafluoroethylene (PTFE), more popularly known as Teflon. We also compare the sensitivity obtained on using PTFE coating with that through other materials, such as zinc, aluminum and stainless steel. It is visibly evident from this assessment that the coefficient of thermal expansion (CTE) has a significant effect on the sensitivity, when other physical and mechanical parameters are maintained constant. Since the CTE value is much higher for polymers than it is for metals, PTFE is able to provide a sensitivity of 0.3 nm/ °C, which is impressive when compared to zinc, the metal offering the highest sensitivity of around 0.079 nm/ °C. However, the CTE of PTFE itself varies with temperature, which is why we predict a sudden nonlinearity in the temperature dependence of the reflected wavelength, in an experimental scenario. Investigating this deviation leads us to the conclusion that, in the temperature range of 19–25 °C, PTFE undergoes two phase transitions from a triclinic to a hexagonal crystal phase, and subsequently to a pseudo-hexagonal phase. We calculate the sensitivities for the various phases of PTFE, and conclude that the high numerical and simulated values make this technology a promising application for futuristic purely optical sensors.

Reliability of stable fiber Bragg grating sensor system for monitoring temperature and strain individually

In the investigation, the fiber Bragg grating-based sensor system is proposed and demonstrated to monitor temperature and strain individually. In order to obtain a stable and accurate sensing signal, an erbium-doped fiber (EDF) ring laser scheme with a 2 m long unpumped EDF as the saturable absorber is also proposed. Hence, the observed stabilization of output power and sensing wavelength can be less than 0.1 dB and 0.02 nm, and 0.3 dB and 0.02 nm in the sensor system, respectively, in the back-to-back state and 25 km fiber link. Hence, this sensor system can improve the output stability of the sensing signal. In addition, the temperature- and strain-induced wavelength shifts can be operated in the range of 20 °C–70 °C and 0–37 500 µε, respectively.

Multiplexing fiber-optic ultrasound sensors using laser intensity modulation.

We propose and demonstrate the use of laser intensity modulation for the multiplexing and demultiplexing of fiber Bragg grating-based ultrasound sensors. The method utilizes an intensity modulator to modulate the output power of the laser with a frequency much higher than that of the ultrasounds. The high-frequency-modulated optical signal serves as a carrier signal. Ultrasonic signals impinged onto the sensor appear as the envelope of the carrier signal. In the frequency domain, the carrier signal and the sidebands encoded with the ultrasonic signal are separated from those of other channels and, thus, can be isolated using an electronic bandpass filter for crosstalk-free ultrasound detection. Each laser can be tuned to demodulate any sensor covered by the wavelength range of the laser, and a common photodetector is used for all channels. Both a theoretical analysis and experimental verification are provided to demonstrate the concept.

A Review of Recent Results on Simultaneous Interrogation of Multiple Fiber Bragg Grating-Based Sensors Using Microwave Photonics

We review recent results on exploiting microwave photonics to enable simultaneous interrogation of multiple fiber Bragg grating (FBG)-based sensors. In particular, we describe the use of (1) microwave photonic filtering and (2) chirped microwave pulse generation and compression as a means to map the wavelength (spectral) changes in the response of FBG-based sensors (specifically, an in-fiber Fabry-Pérot cavity sensor based on FBGs, FBG sensors directly, and a linearly chirped FBG sensor) to applied temperature (or strain) to the power of a radio-frequency signal (i.e., a wavelength-to-power mapping) or to the correlation peak of the compressed microwave signal. The approaches support high-resolution and high-speed interrogation and can be suitable for large scale sensing networks.

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

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

Fiber Bragg Grating-Based Triaxial Force Sensor With Parallel Flexure Hinges

During cardiac surgery, the excessive or insufficient contact force between the tip of catheter and heart tissue will lead to the cardiac penetration or noneffective ablative lesion. Usually, the surgeon can only estimate the force at the distal end roughly from their experience by operating the catheter's proximal handle. In this paper, we design a fiber Bragg grating-based triaxial force sensor with parallel flexure hinges to sense the contact force. The proposed mechanism integrated with parallel flexure hinges is capable of achieving an excellent lateral and axial stiffness balance to guarantee the resolution at each direction with the same order of magnitude. The mathematical model is built to design a suitable stiffness configuration and validate the design reasonability. The model-based and model-free methods are adopted to decouple lateral and axial force components. Experimental results demonstrate that the developed 8 French size force sensor can successfully achieve the triaxial force prediction with <1 g resolution using both decoupling methods. And the model-free method can accomplish a higher accuracy with RMS error <1% of full scale of [–100 g, 100 g] along the lateral direction and [0 g, 100 g] along the axial compression direction.

Research on new FBG displacement sensor and its application in Beijing Daxing Airport project

In order to overcome the shortage of the existing fiber grating displacement sensor in the process of measuring bidirectional deformation of the foundation pit, which are the reduction of precision and the decrease of the range, a Fiber Bragg grating-based wire-pull bidirectional displacement sensor is designed. There are two wedge-shaped sliding surfaces and an integrated equal-strength beam structure, which make the sensor have the advantages of anti-sliding interference and high sensitivity of beam deflection displacement measurement. The sensor prototype is manufactured and the related performance tests show that the sensitivity of the sensor is 11.921731 pm/mm in the range of −100 mm to 100 mm, and the correlation coefficient of the fitting curve has reached 0.9996. Compared with the theoretical calculation value, the measured wavelength drift of the full scale deviates by about −0.92% and 0.42%, which can meet the accuracy requirements of structural health monitoring of major facilities such as civil engineering for displacement and deformation measurement. Four calibrated displacement sensors are installed on the top beam of the foundation pit of Beijing Daxing International Airport, the world's largest aviation hub, to monitor the deformation of foundation pit in real time. The reason of the displacement change of each monitoring section is analysed and the actual working condition on the site is checked. The effectiveness of the FBG bidirectional displacement sensing in the displacement monitoring of foundation pit engineering is verified.

Fiber Bragg grating-based sensor for torque and angle measurement in a series elastic actuator's spring.

Conventional technologies to monitor torque feedback and angle in exoskeleton actuators are bulky and sensitive to misalignments, and do not allow for multiplexed operation. Fiber Bragg grating (FBG)-based sensors are a robust sensing approach that are desirable for multi-parametric monitoring. Temperature, strain, torque, and angle are widely studied in human-robot interaction. In order to acquire the torque and angle of deflection in the torsional spring of a series elastic actuator, an experimental setup with the spring and an array of three FBGs is submitted to repeated torques and angles. This paper presents the characterization and validation of the FBG-based sensor for measuring by torque and angle variations. Temperature cross-sensitivity is derived by the use of a non-strain FBG. The developed sensor presented high linearity and small error for torque and angle measurements.

A Fiber Bragg Grating-Based Thin-Film Sensor for Measuring Dynamic Water Pressure

We present a fiber Bragg grating (FBG)-based thin-film sensor for monitoring dynamic response of water pressure. The proposed pressure sensor is simply composed of a round thin film substrate and an FBG glued on the film surface across the center. The sensing characteristic of the proposed pressure sensor is analyzed from the strain distribution on the thin-film substrate with the couple-mode theory and the transfer-matrix formulation. The thin-film substrate of the FBG allows the proposed sensor to perform dynamic pressure measurement with high sensitivity. Three demodulation techniques, based on the spectrometer, the matching FBG filter, and the chirped FBG filter, are employed with the proposed FBG pressure sensor. By comparison with a commercial pressure sensor, we first demonstrate that the proposed pressure sensor is capable of detecting static pressure within 5 Pa. Then, the proposed pressure sensor is employed to measure dynamic water pressure with a resolution of a 2 Pa inside a tank of water subjected to point-wise impacts at the water surface. The experimental results clearly indicate that the proposed FBG-based thin-film sensor is capable of monitoring dynamic water pressure with high sensitivity and resolution.

Partial scanning frequency division multiplexing for interrogation of low-reflective fiber Bragg grating-based sensor array

Interrogation using partial spectrum scanning for a low-reflective fiber Bragg grating (FBG)-based sensor array is proposed. The sensor head is a Fabry–Perot interferometer (FPI) consisting of low-reflective fiber Bragg gratings. An FPI consisting of low-reflective FBGs (FBG-FPI) has a spectrum with a sinusoidal structure with a period determined by the length of the FBG-FPI. Multiple point sensing is possible by installing multiple low-reflective FBG-FPIs on one fiber and analyzing the reflection spectrum of that fiber by the frequency division multiplexing method. A coherent wavelength-swept light source is necessary to acquire the reflection spectrum; however, but the sweep speed of a commercially available wavelength-swept light source that satisfies the condition is only approximately several tens of hertz. Therefore, we propose to conduct a wavelength sweep by injection current modulation of a laser diode. Wavelength sweeping with a modulated laser diode allows fast sweeping at 100 Hz or more, and the sweep range is as narrow as several hundred picometers. Reading only a part of the reflection spectrum of a low-reflective FBG-FPI sensor array using a modulated laser diode enables high-speed multipoint measurement through frequency analysis. Theoretical requirements for successful interrogation using the partial reflection spectrum are shown. A demonstration experiment to simultaneously measure strain applied to two low-reflective FBG-FPI sensors with a measurement time of 10 ms is reported.

Highly dispersion tailored properties of few mode fiber Bragg grating-based vibration sensor due to a perturbed apodization profile

A dispersion tailored properties of fiber Bragg grating (FBG)-based vibration sensor has been explored, assuming a highly perturbed apodization profile. Most of the FBG designed so far for vibration sensing applications are nonuniform. This paper deals with the modeling of perturbation as a more complicated chirp-type nonlinear apodization profile. It can significantly reduce the dispersion-induced signal distortion and side lobe intensity followed by the improvement of sensor performance. The study reveals that an accurate modeling of vibration sensor is quite feasible while opting the proposed apodization profile. FBG sensor parameters are optimized in such a way that the sensor may operate only under lowest order modes. To the author’s knowledge, sensing capability of FBG-based microaccelerometer having arbitrary chirp type of apodization profile has not been discussed before. It is assumed that small perturbation theory is valid while keeping the influence of microbend geometry in analysis. The standard matrix method is used to study dispersion characteristics due to the complicated apodized profile. The simulation results are validated with the experimental result for the known cases.

A Millinewton Resolution Fiber Bragg Grating-Based Catheter Two-Dimensional Distal Force Sensor for Cardiac Catheterization

This paper presents the development of a novel 2-D fiber Bragg grating (FBG)-based micro-force sensing design for detection of catheter tip-tissue interaction forces. A miniature and symmetrical force-sensitive flexure-based catheter distal sensor has been prototyped, and four optical fibers inscribed with one FBG element each have been mounted on it for force and temperature decoupling and detection. The axial property of the tightly suspended fiber configuration has been utilized with a pre-tensioned force, and the embedded FBG element can be stretched and compressed to sense the force-induced and temperature-caused strain variations. The proposed configuration can achieve an improved resolution and sensitivity than the light intensity modulation-based approaches, and avoid the limitations closely associated with the commonly direct FBG-pasting methods, such as chirping failure and low repeatability. Finite-element modeling (FEM)-based simulation has been implemented to investigate the flexure performance and improve the design. The decoupling approach has been proposed based on the simulation results and implemented to separate and determine the force and temperature. The force-sensing flexure prototype has been calibrated to achieve a resolution of around 4.6 mN within the measurement range of 0 ~ 3.5 N. Both static calibration experiments and in-vitro dynamic experiments have been performed to prove the feasibility of the proposed design. The decoupling capacity of force and temperature will benefit its broad implementations in generalized intravascular catherization procedures.