Bragg Grating-based Sensor Research Articles

Heterodyne detection applied to a fiber Bragg grating-based sensor using a directly modulated distributed feedback laser.

The heterodyne detection technique is applied to a fiber Bragg grating (FBG)-based sensor. Specifically, a directly modulated distributed feedback laser array is used as a frequency scanning light source. Weak reflections from the FBG are recorded as a beat signal with a high signal-to-noise ratio using heterodyne detection. A reflection signal is clearly observed from the FBG with a reflectivity of 1% located at 60 km from the detection port.

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.

Extended Study of Fiber Optic-Based Humidity Sensing System Performance for Sewer Network Condition Monitoring

This article reports on an extended (20-month) period of monitoring of humidity in situ at two locations in the sewer network operated by Sydney Water using a fibre optic network into which a series of Bragg Grating-based sensors had been installed. The locations (Eustace Street in Manly, Sydney and Old Toongabbie at Oakes Reserve, western Sydney, Australia) both had different operating environments and thus conditions for evaluating the sensor system. It was designed to provide a solution to enable long term, low cost and more reliable monitoring in the harsh conditions of the sewer environments in terms of high relative humidity >95% and a broad range of hydrogen sulfide levels. The results of the study show that even after ~20 months of use, the same sensor is reliably recording humidity and temperature in the sewer environment-overcoming the problems seen with conventional electrical sensors, which typically fail within a couple of weeks of use in this continuous high acid/high humidity environment. The data, recorded constantly from the sensor system, were stable throughout the full monitoring period and further, a comparison with the changing weather conditions was made over the different seasons during the study. The sensor system developed was battery operated and had 4G connectivity for data transfer and debugging. These features have enabled the system to be installed in situations where power is not available and operate successfully with minimal human operation, thus allowing for additional systems to be integrated to the measurement system in the future.

Practicalities of BVID detection on aerospace-grade CFRP materials with optical fibre sensors

Several reports have recently shown the capability of optical fibre sensors for the detection of barely visible impact damage (BVID) on CFRP laminates, based on changes in the optical signal before and after impact. Although this approach was shown to work well in lab conditions, little attention has been paid so far to its practicality when considering realistic ambient conditions or to the detection range. In this work we attempt to derive BVID detection thresholds for fibre Bragg grating-based sensors, by considering standardized temperature and vibration levels for aerospace applications. To the best of our knowledge this is the first practical threshold assessment for BVID detection on aerospace-grade CFRP for such sensors. We have put our detection thresholds to the test by analysing a total of 24 BVIDs on 12 panels made from 4 different CFRP material systems and derived commendable sizes for the detection range on the different material systems.

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.

Evaluation of optical fibre sensors in the electrical domain

Abstract. Optical fibre sensors cover a wide range of applications. They offer versatile advantages including resilience to electromagnetic interference, biocompatibility and chemical resistivity. Even in environments with restricted accessibility, integration difficulties can be overcome by using radio-over-fibre (RoF) technology that allows a wireless read-out. Conventionally, optical fibre sensors are evaluated in the optical domain by analysing the amplitude or spectrum of either the transmitted or the reflected light. A novel approach is to feed a radio frequency-modulated laser into the optical sensor and carry out a full electrical analysis of the resulting radio frequency (RF) signal, which is changed by the sensor's characteristics. This method will be investigated in this paper for fibre Bragg grating-based and chirped fibre Bragg grating-based sensors in reflection and transmission configuration. Their applicability for this new evaluation scheme will be discussed. Subsequent studies may cover additional types of sensors and the testing of the novel evaluation method within an application-related scenario, including packaging.

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.

Sensitivity enhancement analysis due to different coating materials of Fibre Bragg Grating-based depth sensor for underwater applications

Ocean depth is a critical parameter for several maritime applications, ranging from surveillance to navigation. One method commonly employed is to measure the pressure at an unknown depth, and compare it with that at the surface. Fibre-optic sensors have made significant advances in this direction, conferring exceptional sensitivity, resolution and compactness. In this work, we present a simulation of an ocean depth sensor based on a uniform Fibre Bragg Grating. We study the effect of introducing a uniform layer of coating on the sensitivity. A range of different materials, including metals and polymers, are studied and compared for their sensitivity performance and general feasibility. Our results show that a coating of Polytetrafluoroethylene (PTFE) provides the highest sensitivity, in terms of wavelength shift per unit change in depth. This sensitivity can be further augmented by introducing modulations in the grating, such as chirp or apodization. We find that apodized PTFE-coated gratings are most suitable for our application, and report a peak sensitivity of 24.54 pm/m. The choice of an optimum coating is critical not only for the sensitivity, but also for parameters such as operating lifetime, robustness and linearity. Our results are anticipated to pave the way to more sensitive, flexible and precise optical sensors for underwater depth gauges.

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.

Regenerated Fiber Bragg Grating Sensing System for Ultrasonic Detection in a 900 °C Environment

Heat-resistant composites, such as ceramic matrix composites and heat-resistant carbon fiber reinforced plastics (CFRPs), are expected to be used for aircraft engine parts. The development of reliable heat-resistant composite materials requires the use of nondestructive test techniques for evaluating the progression of damage during material testing at elevated temperatures. Furthermore, structural health monitoring (SHM) technologies that operate under harsh environments are expected to be realized for monitoring heat-resistant composite structures. To provide potential solutions for the establishment of such technologies, this research developed a heat-resistant ultrasonic sensor based on a regenerated fiber-optic Bragg grating (RFBG). First, we fabricated an RFBG by annealing a normal fiber-optic Bragg gratings (FBG) sensor. Because the RFBG exhibits high heat resistance at temperatures of 1000 °C, the sensor achieved stable ultrasonic detection at an elevated temperature. In addition, we attempted to use a π-phase-shifted FBG (PSFBG) as the seed grating to construct an ultrasonic sensor with enhanced performance. As a result, the regenerated phase-shifted fiber-optic Bragg grating (R(PS)FBG) sensor possessed a very short effective gauge length and achieved a broad frequency response to ultrasonic waves with frequencies greater than 1.5 MHz. The broadband detectability enables the R(PS)FBG sensor to acquire an accurate response to ultrasonic waves. Hence, we believe the regenerated Bragg grating-based ultrasonic sensors can contribute to establishing an effective nondestructive evaluation method for composite materials, thereby enabling a structural health monitoring system for a composite-made structure operating under extreme high-temperature environments.

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.