Single Fiber Bragg Grating Research Articles

Simultaneous Vibration and Temperature Real-Time Monitoring Using Single Fiber Bragg Grating and Free Space Optics

Simultaneous monitoring of temperature and vibration of electromechanical systems, public buildings, and volcanic terrain is an important indicator of preventing accidents. This work showcases the simultaneous temperature and vibration measurement using a single-fiber Bragg grating (FBG) sensor. The concurrent interrogation of vibration and temperature by the FBG sensing system can be integrated with free space optics (FSO), which saves on the costs associated with fiber optic cables and overcomes terrain barriers. Furthermore, the real-time measured vibration frequencies and different temperature levels can be recognized by a deep neural network (DNN) sequential model. In addition to the FSO support, the sensing system can be arranged in multiple sensing locations to form a wide range of measurements, offering a highly cost-effective solution.

A 3D-printed sensing element based on fiber Bragg grating technology for grasping force measurement in surgical forceps

This study introduced a 3D-printed sensing element based on fiber Bragg grating (FBG) technology to measure grasping forces when integrated into surgical forceps during minimally invasive surgery (MIS). Its design mainly consists of an octagonal-shaped structure with a single FBG suspended within, the first to use fused deposition modeling technique in this application scenario. This novel solution allows improving the design customization and the structural robustness of the proposed sensing element. A finite element model was developed to study the mechanical behavior of the structure under transversal force (F) values experienced during grasping. Thermoplastic polyurethane material as printing filament enhanced the deformation capabilities of the sensor, thereby optimizing its response to F as evidenced by the value of sensitivity to F (i.e., 0.11 nm·N−1). Additionally, the sensing element exhibited a low hysteresis error (always below 14 %). Tests in controlled conditions confirmed the sensor capability to discriminate materials with different stiffness. Furthermore, the sensor was used as the sensing core of a surgical forceps prototype and tested during ex vivo experiments on hepatic, myocardial and spleen tissues simulating a MIS environment. The system was able to measure F during tissue grasping and the real-time feedback improved the tissue holding stability, important for reducing tissue damage risk during MIS procedures.

Optimal Design of a Sensor Network for Guided Wave-Based Structural Health Monitoring Using Acoustically Coupled Optical Fibers.

Guided waves (GW) allow fast inspection of a large area and hence have received great interest from the structural health monitoring (SHM) community. Fiber Bragg grating (FBG) sensors offer several advantages but their use has been limited for the GW sensing due to its limited sensitivity. FBG sensors in the edge-filtering configuration have overcome this issue with sensitivity and there is a renewed interest in their use. Unfortunately, the FBG sensors and the equipment needed for interrogation is quite expensive, and hence their number is restricted. In the previous work by the authors, the number and location of the actuators was optimized for developing a SHM system with a single sensor and multiple actuators. But through the use of the phenomenon of acoustic coupling, multiple locations on the structure may be interrogated with a single FBG sensor. As a result, a sensor network with multiple sensing locations and a few actuators is feasible and cost effective. This paper develops a two-step methodology for the optimization of an actuator-sensor network harnessing the acoustic coupling ability of FBG sensors. In the first stage, the actuator-sensor network is optimized based on the application demands (coverage with at least three actuator-sensor pairs) and the cost of the instrumentation. In the second stage, an acoustic coupler network is designed to ensure high-fidelity measurements with minimal interference from other bond locations (overlap of measurements) as well as interference from features in the acoustically coupled circuit (fiber end, coupler, etc.). The non-sorting genetic algorithm (NSGA-II) is implemented for finding the optimal solution for both problems. The analytical implementation of the cost function is validated experimentally. The results show that the optimization does indeed have the potential to improve the quality of SHM while reducing the instrumentation costs significantly.

Multi-Directional Strain Measurement in Fiber-Reinforced Plastic Based on Birefringence of Embedded Fiber Bragg Grating.

Embedded fiber Bragg gratings are increasingly applied for in-situ strain measurement in fiber-reinforced plastics, integral to high-end aerospace equipment. Existing research primarily focuses on in-plane strain measurement, limited by the fact that fiber Bragg gratings are mainly sensitive to axial strain. However, out-of-plane strain measurement is equally important for comprehending structural deformation. The birefringence of fiber Bragg gratings shows promise for addressing this problem; yet, the strain transfer relationship between composites and optical fibers, along with the decoupling method for multi-directional strains, remains inadequately explored. This study introduces an innovative method for multi-directional strain measurement in fiber-reinforced plastics using the birefringence of a single-fiber Bragg grating. The strain transfer relationship between composites and embedded optical fibers was derived based on Kollar's analytical model, leading to the development of a multi-directional strain decoupling methodology. This method was experimentally validated on carbon fiber/polyetherimide laminates under thermo-mechanical loading. Its reliability was confirmed by comparing experimental results and finite element simulations. These findings significantly broaden the application scenarios of fiber Bragg gratings, advancing the in-situ measurement technology crucial for the next generation of high-end aerospace equipment.

Fabrication of high temperature chirped FBG by thermal stretching of regenerated FBG

This paper presents an innovative method of high temperature sustainable chirped fiber Bragg grating (FBG) fabrication from the uniform regenerated FBG or their combination written by a single uniform phase mask. A thermal stretching technique is employed that modifies the grating structure of uniform FBGs, resulting in chirped FBGs capable of withstanding high temperatures without compromising their sensing capabilities. Thermal regeneration of uniform FBG was carried out at 850 °C by applying step annealing schedule. Thermal stretching of single regenerated FBG and dual regenerated FBG was carried out for the fabrication of high temperature chirped FBG. For both approaches, the FBG was stretched by applying strain ∼1200 µε along the length of the grating. In the single FBG approach, the chirped grating with reflection intensity 4500 (A.U.) and bandwidth ∼ 7.1 nm was fabricated by applying thermal gradient from 860 °C to 900 °C across the length of strained thermally regenerated FBG. In double grating approach, the chirped FBG with reflection intensity 6000 (A.U.) and bandwidth 12.7 nm was fabricated. The fabricated chirped FBG can sustain temperature upto 900 °C. This innovative method not only extends the operating range of chirped FBGs to high temperatures but also opens up new possibilities for fabricating chirped FBGs of longer length and bandwidth using low-cost uniform FBGs.

Machine learning-augmented multi-arrayed fiber bragg grating sensors for enhanced structural health monitoring by discriminating strain and temperature variations

AbstractFiber optic sensors (FOS) in long-term structural health monitoring (SHM) have drawn significant attention due to their pivotal role in detecting defects and measuring structural performance in diverse infrastructures. While using FOS, temperature variation due to environmental factors is still considered one of the major challenges to isolating sensing parameters. To address this issue, we reported a machine learning (ML)-augmented multi-parameter sensing system that enables simultaneous detection of strain and temperature effects based on one single fiber Bragg gratings (FBGs) sensor for SHM. The initial phase entailed designing, fabricating, and characterizing a novel FBG sensor in the laboratory, incorporating a set of four FBGs, each distinguished by distinct Bragg wavelengths. In the next phase, ML algorithms are employed to separate temperature effects from strain variations. As a proof of concept, mechanical loading tests are conducted on the sensor, exposing the FBG portion to various temperature conditions. In the final phase, data collected from a post-tensioned concrete bridge embedded with both strain and temperature FBG sensors are utilized, and the developed ML models are applied to observe real-environment outcomes. Despite the limited feature points of collected FBG spectrums, the developed ML models effectively address cross-sensitivity issues induced by temperature perturbations. The long-term benefit of using FOS is that it will enable a better understanding and utilization of aging infrastructure. This will potentially reduce embodied carbon of infrastructure in the future and assist in the global efforts to achieve Net-Zero.

Investigation of Transmission and Reflection of Single Mode Fiber Bragg Grating

The use of Single Mode Fiber Bragg Grating (SMFBG) has been increasing in recent years due to its compact size, low cost, fast response and immunity to electromagnetic interference. It is commonly integrated into medical devices for long-distance light transmission and collection due to its high flexibility, low propagation loss, compatibility and tolerance to electromagnetic interference. SMFBG is a device made of thin glass material that is used as a medium for transmitting information in the form of light signals sourced from lasers or LEDs from one location to another. It consists of 3 main components, namely core with a certain grating, blanket (cladding) and jacket (coating). The advantage of optical fiber is that the data when transmitted is converted into light so as to reduce the risk of data damage. Other advantages include very small size, minimal interference with electromagnetic waves, resistance to temperature changes, attenuation when the transmission process is small enough, and large enough bandwidth. The orientation of this literature review is to understand the concept of optical fiber, the concept of reflection and refraction, and how light propagates in optical fiber.

Modified CFBG-based approach for chromatic dispersion compensation in high speed DWDM links.

Chromatic dispersion (CD) is a major concern in high speed fiber optics communication systems. This work presents an efficient optical compensation technique with reduced complexity to mitigate CD effects in single mode fiber (SMF) at high data rates. The proposed method includes a post-dispersion mitigation system using a single linear chirped fiber Bragg grating (CFBG) with tanh apodization. Here, various parameters of CFBG, such as grating length, effective refractive index, and apodization function, are optimized to achieve effective CD compensation. Then, the system is extended to a 16-channel WDM link with each channel carrying data at 25Gbps over 100km. The simulation results show that the proposed model enhances the system performance while minimizing overall system complexity. The proposed system is also compatible with the prevailing fiber optics networks. The results are consistent with the findings published in recent studies.

Au coated etched FBG SPR sensor for the detection of ethanol in aqueous solution

Over the past few years, there has been a growing need for reliable ethanol sensors in the pharmaceutical and food industries. These industries require constant monitoring to ensure that the acceptable ethanol levels are maintained. This study reports a new fiber optic sensor that uses etched single mode fiber Bragg grating (FBG) coated with the thin layer of 45 nm gold to produce Surface Plasmon Resonance (SPR). A thin layer of 45 nm gold improves the sensitivity of sensor by improving the evanescent wave around the fiber, resulting in better interaction with the analyte. The sensing performance of developed structure was examined by analyzing the change in wavelength shift observed by varying ethanol concentration in aqueous solution from 0 to 100 %. The performance of developed sensor in terms of sensitivity is 20 pm/RIU. Which is expected to play an important role in food and pharmaceutical industries to protect human health from detrimental affects due to high concentration of ethanol.

High-efficiency continuous-wave Tm-doped fiber laser with a single fiber Bragg grating at 1942 nm

We report a continuous-wave Tm-doped fiber (TDF) laser with a single fiber Bragg grating (FBG). The Fresnel effect presenting in the passive fiber end-face and high-reflectivity FBG established the resonator. The peak wavelength of the laser was 1942.25 nm with a spectral linewidth of 194 pm. The maximum output power was 37.5 W, corresponding to a slope efficiency of 63.7% with a beam quality factor of M 2 ∼ 1.51. To the best of our knowledge, this is the highest slope efficiency of a TDF laser pumped by 793 nm laser diode.

In-Bulk Temperature Profile Mapping Using Fiber Bragg Grating in Fluids.

The capabilities of Fiber Bragg Grating (FBG) sensors to measure temperature variations in the bulk of liquid flows were considered. In the first step of our research project, reported in this paper, we investigated to what extent the use of thin glass fibers without encapsulation, which only minimally disturb a flow, can fulfill the requirements for robustness and measurement accuracy. Experimental tests were performed in a benchmark setup containing 24 FBG measuring positions, which were instrumented in parallel with thermocouples for validation. We suggest a special assembly procedure in which the fiber is placed under a defined tension to improve its stiffness and immobility for certain flow conditions. This approach uses a single FBG sensor as a reference that measures the strain effect in real time, allowing accurate relative temperature measurements to be made at the other FBG sensor points, taking into account an appropriate correction term. Absolute temperature readings can be obtained by installing another well-calibrated, strain-independent thermometer on the reference FBG. We demonstrated this method in two test cases: (i) a temperature gradient with stable density stratification in the liquid metal GaInSn and (ii) the heating of a water column using a local heat source. In these measurements, we succeeded in recording both spatial and temporal changes in the linear temperature distribution along the fiber. We present the corresponding results from the tests and, against this background, we discuss the capabilities and limitations of this measurement technique with respect to the detection of temperature fields in liquid flows.

Continuous Fiber Bragg Grating Optical Sensors for Superconducting Magnet Quench Detection: The Effects of Attenuation and Position in the Array

Quasi-continuous arrays of fiber Bragg gratings (FBGs) show great promise for quench protection in superconductors. The number of identical gratings and the reflectivity of the individual gratings are important parameters determining the sensitivity of the system to a local temperature or strain perturbation. We apply strain to shift the Bragg wavelength of a single FBG through the spectrum of an array of up to 1000 identical gratings, and map out the detection sensitivity for a given <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\Delta \lambda$</tex-math></inline-formula> relative to the peak reflectance of the FBG array, and show that the position of the perturbed FBG matters only due to the broadband attenuation of the probe light down the array. Varying the number of gratings in the array shows the interplay between the number of gratings, the reflectivity of the gratings, and the sensitivity of the system. We demonstrate that the ability to model the spectrum of the FBG array is crucial to predicting system performance.

Neural-Network-Optimized Vehicle Classification Using Clustered Image and Fiber-Sensor Datasets

Internet of Things (IoT) becomes indispensable for transport and automotive industry to advance functions in on-road traffic monitoring. Indeed, smart management tools and machine learning concepts are inevitable in vehicle categorization systems. However, existing systems are only built on individual platforms, and for a most part, the classification accuracy remains limited. In this work, these challenges are tackled by designing a novel convolutional neural network (CNN) that substantially improves on-road vehicle classification. In particular, we experimentally harness, to the best of our knowledge for the first time, two different datasets from separated technological platforms based on close-circuit television (CCTV) and fiber Bragg grating (FBG) sensors, respectively. Standard neural networks in single FBG platform yield limited classification accuracy of only 34% - 62% for <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">AlexNet</i> , 51% - 77% for <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">GoogleNet</i> , 57% - 78% for <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ResNet-50</i> , and 59% - 86% for <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ResNet-101</i> . In contrast, hybrid CNN classification with individual CCTV and FBG datasets substantially improves detection levels, reaching in-class accuracy up to 90% - 97%. Moreover, this classification concept includes an intrinsic back-up verification with respect to each platform compensating the shortcomings of individual technologies. Our demonstration can make key advances towards near-unity accuracy in vehicle classifications for IoT systems, capitalizing on cost-effective and well-established platforms.

Enhancing the Performance of FBG Accelerometers by Using In-Fiber Fabry Perot Interferometers

An in-fiber Fabry–Perot interferometer (FPI) is proposed and designed for performance improvement of a fiber Bragg grating (FBG) accelerometer. It consists of two FBGs which are inscribed using the femtosecond laser point-by-point technology. The spectral characteristics of the FBG Fabry–Perot (FBG-FP) were studied. Edge filter interrogation configuration was employed in the vibration measurement system. In contrast to the conventional FBG accelerometer used in wavelength interrogation or edge filter interrogation configurations, the scheme has a higher sensitivity and lower detection threshold which are achieved improving the optical properties of the accelerometer. We fabricated a high-frequency accelerometer and experiments were carried out to verify its performance. The sensitivity and detection threshold were 27.3 mV/g and 1.5 mg, respectively. This study presents a feasibility analysis of the short cavity FBG-FP, as opposed to a single FBG, for the vibration accelerometer.

Performance characteristics of single Bragg grating fiber under various filter impulse functions and grating filter profiles

Abstract This work has demonstrated the performance characteristics of single Bragg grating fiber under various filter impulse functions and grating filter profiles. Single fiber Bragg grating (FBG) power propagation, grating index change, coupling coefficient and apodization variations versus grating length variations is demonstrated at the central wavelength of 1550 nm by using both Hamming, Blackman impulse filter function and Hamming, Blackman grating profile. As well as the single FBG power spectrum, delay, dispersion, transmittivity/reflectivity are simulated and clarified against wavelength variations using both Hamming, Blackman impulse filter function and Hamming, Blackman grating profile. The Hamming impulse function filter and Hamming grating profile have clarified better performance than Blackman impulse function filter and Blackman grating profile.

Monitoring Epoxy Coated Steel under Combined Mechanical Loads and Corrosion Using Fiber Bragg Grating Sensors.

Fiber Bragg grating (FBG) sensors have been applied to assess strains, stresses, loads, corrosion, and temperature for structural health monitoring (SHM) of steel infrastructure, such as buildings, bridges, and pipelines. Since a single FBG sensor measures a particular parameter at a local spot, it is challenging to detect different types of anomalies and interactions of anomalies. This paper presents an approach to assess interactive anomalies caused by mechanical loading and corrosion on epoxy coated steel substrates using FBG sensors in real time. Experiments were performed by comparing the monitored center wavelength changes in the conditions with loading only, corrosion only, and simultaneous loading and corrosion. The theoretical and experimental results indicated that there were significant interactive influences between loading and corrosion for steel substrates. Loading accelerated the progress of corrosion for the epoxy coated steel substrate, especially when delamination in the epoxy coating was noticed. Through the real-time monitoring from the FBG sensors, the interactions between the anomalies induced by the loading and corrosion can be quantitatively evaluated through the corrosion depth and the loading contact length. These fundamental understandings of the interactions of different anomalies on steel structures can provide valuable information to engineers for better management of steel structures.

A PON Monitoring System Integrating Fault Detection and Localization

We propose and experimentally demonstrate an integrated passive optical network monitoring system for fault detection and localization. Fault detection technology is based on the recognition of reflection spectrum generated by each single fiber Bragg grating, which can support large-capacity user fiber fault monitoring. Fault localization method uses two different total received powers measured by an optical power meter to solve the established equation, and it shows a good accuracy compared with the OTDR measurement results. Fault detection and localization are carried out separately but share the same transmitter module. The introduction of the additional module ensures that the troubleshooting does not affect other normal user data communication. We also investigate the dynamic range requirements of the fault detection technology and the measurement error of the fault localization method. The calculation results show the required dynamic range for the recognition device can be maintained within a small range and the use of high-accuracy optical power meter can reduce the measurement error. The proposed monitoring system can simultaneously provide a large-capacity fault detection technology and high-accuracy fault localization method in a cost-sensitive passive optical network market.

Guided waves based damage localization using acoustically coupled optical fibers and a single fiber Bragg grating sensor

Fiber Bragg grating (FBG) sensors have long been thought of as the ideal sensors for structural health monitoring (SHM) due to their small size, light weight, ability to be embedded and ability to be multiplexed. So, FBG sensors have been commonly used for strain based SHM. In recent times, a renewed interest is seen in the use of FBG sensors for GW measurements using the edge filtering approach which increases the sensitivity several fold. The remote bonding configuration has also been proposed to enhance this sensitivity further. But in order to achieve damage localization, a multi-sensor network is needed. The FBG sensors and the equipment for employing them in the edge filtering configuration is expensive. As a result, their use for SHM in large structures is still limited. Recent studies have shown that the acoustic wave in an optical fiber may be transferred from one optical fiber to another through an adhesive based acoustic coupler. In this study, this phenomenon is used to develop an SHM system which is capable of damage localization using a single FBG sensor. The paper presents a proof-of-concept of the use of the acoustically coupled optical fibers and single FBG for damage localization. The paper also highlights the design considerations of an acoustic coupler and discusses them in detail. The proposed approach has a potential to radically reduce the equipment costs (factor of 3) which is one of the limiting factors in the widespread acceptance of SHM systems in structures.

Gel Point Determination in Resin Transfer Molding Process with Fiber Bragg Grating Inscribed in Side-Hole Elliptical Core Optical Fiber.

Material as well as process variations in the composites industry are reasons to develop methods for in-line monitoring, which would increase reproducibility of the manufacturing process and the final composite products. Fiber Bragg Gratings (FBGs) have shown to be useful for monitoring liquid-composite molding processes, e.g., in terms of online gel point detection. Existing works however, focus on in-plane strain measurements while out-of-plane residual strain prevails. In order to measure out-of-plane strain, FBG inscribed in highly birefringent fiber (HB FBG) can be used. The purpose of this research is the cure stage detection with (a) FBG inscribed in single mode and (b) FBG inscribed in highly-birefringent side-hole fiber in comparison to the reference gel point detected with an in-mold DC sensor. Results reveal that the curing process is better traceable with HB FBG than with regular FBG. Thus, the use of HB FBG can be a good method for the gel point estimation in the RTM process.

Distributed strain sensing, employing apodized π-phase shifted FBG: Application in power transformer oil breakdown detection

Early detection of power transformer oil breakdown can save the transformer and power grid from any potential damages. In this article, a novel distributed strain sensing system based on apodized π-phase-shifted Fiber Bragg Grating (FBG) is presented in order to detect power transformer oil breakdown. The main objective is to detect high-frequency acoustic waves and ultra-small strains resulting from partial discharge (PD) occurrence. To this end, optical fiber under various time and position dependent strain conditions is simulated by employing transfer matrix method. Strain is incorporated in the equations by employing photo-elastic constants. For a single FBG sensor case employing a laser at a fixed wavelength of 1550 nm, it is shown that incorporation of a π phase shift drastically increases the sensitivity, reaching values as high as 60 %/0.1μstrain in reflected light intensity. However, maximum measurable strain is limited to only 0.25 μstrain. Afterwards, a suitable Gaussian apodization function is applied to the FBG which drastically increased the strain measurement range to almost 0.45 μstrain by a complete suppression of sidelobes in the reflected light spectrum. It is shown that the proposed system is capable of detecting high-frequency PD-induced acoustic waves of up to 500 kHz.