Fiber Bragg Gratings For Applications Research Articles

Multilayer metal-coated fiber Bragg grating for high-temperature sensing

In this study, a copper coating was deposited on the surface of a fiber Bragg grating (FBG) via electroplating. We compared the temperature responses of the uncoated and copper-coated FBGs from 20 °C to 1000 °C. Under high-temperature conditions, copper diffused into the FBG to form copper crystals, and the copper coating gradually underwent high-temperature oxidation. This reduced the sensitivity and spectral consistency of the copper-coated FBG. A multilayer metal-coated FBG was prepared with an inner layer of nickel, outer layer of gold, and copper layer in the middle. The multilayer FBG exhibited a temperature sensitivity of 23.1–23.53 pm/°C. Furthermore, it showed the coating-enhancement effect and good linearity after multiple thermal cycles. The multilayer metal-coated FBG not only shows high-temperature sensitization but also solves the problem of bare FBGs being extremely fragile after experiencing high temperatures. This study provides a feasible solution for the application of FBGs in extremely high-temperature environments.

Elastomeric-coated FBGs for point-of-care diagnostics

This study deploys the application of Fiber Bragg Gratings (FBGs) in physiological pressure monitoring by integrating an elastomeric, biocompatible coating ranging from 300-500μm, designed to improve sensor functionality for in-vivo pressure monitoring applications. FBGs are favored for their sensitivity, immunity to electromagnetic interference, and compact size, making them ideal for embedding within medical devices such as catheters and guidewires. However, their use has been limited by low inherent pressure sensitivity (3.14 pm/MPa) and the impracticality of thicker coatings described in previous studies. Our approach demonstrates that this unique coating not only boosts the pressure sensitivity significantly—surpassing 1.63 orders of magnitude (43.10 times)—but also enhances the signal-to-noise ratio of the optical signal. These advancements enable potential applications in high-resolution manometry, gastrointestinal pressure monitoring, intracranial and intracoronary blood pressure measurements, marking a significant step forward in medical diagnostics and monitoring.

Influence of Annealing on Polymer Optical Fiber Bragg Grating Inscription, Stability and Sensing: A Review.

This article reviews recent research progress on the annealing effects on polymer optical fibers (POFs), which are of great importance for inscription, stability and sensing applications of fiber Bragg gratings (FBGs) in POFs due to their unique properties related to polymer molecular chains. In this review, the principle of annealing to reduce frozen-in stress in POFs drawing and different annealing timings are firstly summarized. Then, the annealing methods for POFs are introduced under several different conditions (temperature, humidity, strain, stress and solution). Afterwards, the principle of FBGs and several inscription techniques are reported. Subsequently, the annealing effects on the properties of POFs and polymer optical fiber Bragg gratings (POFBGs) quality are discussed. Finally, the influence of annealing on POFBG sensitivity is summarized. Overall, this paper provides a comprehensive overview of annealing techniques and their impact on both POFs and POFBGs. We hope that it will highlight the important progress made in this field.

Integrating hybrid piezoelectric actuators with a single fiber Bragg grating sensor for online monitoring of structures

The application of fiber Bragg grating (FBG) sensors for detecting ultrasonic guided waves (GW) in a hybrid system with piezoelectric transducers (PZT) is a promising area for structural integrity analysis due to FBG’s potential for embedding in structures. However, the directional sensitivity of FBG sensors presents a challenge for signal processing and the direct application of damage mapping methods designed for omnidirectional PZT sensors is not feasible. The novelty of this study lies in the development of a proof-of-study damage mapping method that uses an improved cosine distance (CD) formulation to detect damage paths while accounting for the directional sensitivity of FBG sensors. Additionally, a sectorial elliptical code (SEC) based on CD was used to localize damages in a hybrid FBG-PZT network. This approach was validated with numerical simulations, demonstrating its effectiveness in accurately detecting and localizing damages in structures. The damage detection with the CD method located all the damage paths which help to identify the probable damage region. The SEC method localized damages not only in the inner sensor zone but also in the outer zone. The CD method successfully predicted the probable damage zones overcoming the directional sensitivity issue. The error predicted with the SEC method was as low as 0.7 cm in the experimental and 0.1 cm in numerical studies.

Optical Fiber -Based Needle Shape Sensing: Three-channel Single Core vs. Multicore Approaches.

Bevel-tip needles are commonly utilized in percutaneous medical interventions where a curved insertion trajectory is required. To avoid deviation from the intended trajectory, needle shape sensing and tip localization is crucial in providing the operator with feedback. There is an abundance of previous work that investigate the medical application of fiber Bragg grating (FBG) sensors, but most works select only one specific type of fiber among the many available sensor options to integrate into their hardware designs. In this work, we compare two different types of FBG sensors under identical conditions and application, namely, acting as the sensor for needle insertion shape reconstruction. We built a three-channel single core needle and a seven-channel multicore fiber (MCF) needle and discuss the pros and cons of both constructions for shape sensing experiments into constant curvature jigs. The overall needle tip error is 1.23 mm for the single core needle and 2.08 mm for the multicore needle.

Application of Fiber Bragg Gratings as a Sensor of Pulsed Mechanical Action.

The pulsed elongation of fiber Bragg gratings is considered in order to be used to measure the displacement or deformation rate of objects. Optimal measurement modes were determined, numerical simulation of the output signal was performed during pulsed elongation or compression of the fiber grating, and the main patterns were analyzed. The results of the application of the Bragg gratings for the experimental determination of the deformation rate of materials under pulsed magnetic action are presented. Experimentally obtained and theoretical dependencies are compared. The dependencies of the change in the grating parameters—the coefficient and the half-width of the reflection spectrum with successive shortening of the grating—are given.

Development of a monitoring and warning system based on optical fiber sensing technology for masonry retaining walls and trees

Hong Kong has a long history of applying masonry retaining walls to provide horizontal platforms and stabilize man-made slopes. Due to the sub-tropical climate, some masonry retaining walls are colonized by trees. Extreme weather, such as typhoons and heavy rains, may cause rupture or root failure of those trees, thus resulting in instability of the retaining walls. A monitoring and warning system for the movement of masonry retaining walls and sway of trees has been designed with the application of fiber Bragg grating (FBG) sensing technology. The monitoring system is also equipped with a solar power system and 4G data transmission devices. The key functions of the proposed monitoring system include remote sensing and data access, early warning, and real-time data visualization. The setups and working principles of the monitoring systems and related transducers are introduced. The feasibility, accuracy, serviceability and reliability of this monitoring system have been checked by in-site calibration tests and four-month monitoring. Besides, a two-level interface has been developed for data visualization. The monitoring results show that the monitored masonry retaining wall had a reversible movement up to 2.5 mm during the monitoring period. Besides, it is found that the locations of the maximum strain on trees depend on the crown spread of trees.

Determination Of Flow Velocities Using Fiber-Optic Temperature Measurements

A novel flow measuring technique is introduced to measure under harsh circumstances in environments with dirt, high pressures and elevated temperatures as in boreholes within the earth's crust. A glass fiber embedded in a cable with heating wires measures the temperature within the heated cable with distributed temperature sensing (DTS). Similar to Hot Wire Anemometry, the velocity dependence of convective heat transfer is exploited to measure the velocity of the cable as a cylinder in crossflow. A borehole-mimicking test rig and a realistic prototype of a borehole probe were built and the flow along the borehole axis was investigated. The expected Nusselt Reynolds characteristic of a cylinder in crossflow has been measured which proves the concept of this novel measurement technique. Challenges arise with the insufficient spatial resolution of DTS measurements and in the heat transfer modeling because the temperature profile of the cables cross-section must be taken into account. More detailed investigations and developments are planned to elevate this measurement technique from the current proof-of-concept stage to a reliable flow measurement technique. As a first step, the DTS technique will be extended the application of fiber Bragg grating (FBG) temperature measurements at specific points along the glass fiber. Subsequently a straight segment of a new hybrid cable will be placed in a water channel perpendicular to the flow direction. The Flow will be precisely specified using Particle Image Velocimetry and multiple temperature sensors in the channel and on the cable's sheath will deliver the information for enhanced heat transfer modelling.

Accuracy Improvement of Residual Stress Measurements in the Tube by FBG Using the Genetic Algorithm

The genetic algorithm (GA) method is used to demodulate the strain distribution of fiber Bragg grating (FBG), which improves the residual stress measurement accuracy of the tube in the hole-drilling method. The accuracy of the method is verified by simulation and experiment. Compared with the traditional peak demodulation, the residual stress error is reduced from 10%–20% to 0%–5% by GA. The results show that the use of the GA method can effectively improve the accuracy of residual stress measurement of the tube based on FBG, which promotes the application of FBG in the measurement of residual stress in special-shaped components and expands the range of FBG residual stress measurement.

Sensing applications of fiber Bragg gratings in single mode fibers with as-drawn 25 μm diameter cladding

The sensing properties of fiber Bragg gratings (FBG) inscribed in single mode fiber with a 5 μm diameter core and 25 μm diameter cladding are studied experimentally for temperature, strain, bending, and surrounding refractive index. Compared to normal single mode fiber, the diameter of this fiber is 5 times smaller and it stretches 14.5 times more at the same applied load. Therefore, it is much more flexible and stretchable, while maintaining excellent optical quality at wavelengths near 1550 nm. In addition to a core mode back reflection resonance, strong FBGs inscribed in this fiber show a relatively small number of narrow bandwidth (0.7 nm) cladding mode resonances separated in wavelength by 2.5–6 nm. This relatively coarse spectral comb can then be used to sense many different kinds of perturbations involving core and cladding modes. In particular, unlike cladding-mode based sensors made from tilted FBGs, all resonances are of the same azimuthal order as the core mode (i.e. HE1m). This feature makes these gratings particularly sensitive to bending which causes the appearance of new resonances and reduced amplitudes of the original ones, each by up to 10 dB/mm−1 of curvature. On the other hand, the temperature sensitivities of all modes are similar to that of standard fiber (around 11 pm/oC) while strain sensitivities are somewhat higher (1.6–1.7 pm/μstrain). The surrounding refractive index sensitivity is also increased (by a factor of 3) over normal fiber, mostly due to the increased modal dispersion of the modes of the thinner cladding. Furthermore, it is possible to serially multiplex different gratings at different wavelengths by interleaving their resonance combs and preserving each grating identity in the combined spectrum.

Centrifuge application of fibre Bragg gratings: pile axial loads and wall bending moments

One challenge for geotechnical centrifuge testing of soil–structure interaction problems is the reliable measurement of induced structural strains/forces. This paper presents a novel application of fibre Bragg grating (FBG) sensors for strain measurement within geotechnical centrifuge tests. FBG sensors have several advantages for centrifuge testing, in particular their small size and minimal self-weight. This paper gives an overview of recently developed installation and calibration procedures for FBG sensors within buried centrifuge model structures. The effect of thermal expansion/contraction of the materials (including both the fibre and structures) is considered and assessed. The precision and reliability of the FBG sensors are demonstrated using verification tests. The application of the FBG sensors is considered for two geotechnical problems, namely, pile jacking and a ‘retaining wall’ adjacent to a tunnel (acting as a protective wall to prevent an adjacent structure from tunnelling-induced ground movement). The results demonstrate that the FBG sensors can provide reliable measurements of pile axial strains/forces and protective wall bending moments. The paper provides evidence to support the routine adoption of FBG sensors for strain/force measurement of structures in geotechnical centrifuge modelling.

A Fibre-optic Strain Measurement System to Monitor the Impact of Tunnelling on Nearby Heritage Masonry Buildings

ABSTRACT Fibre-optic sensing technologies provide the opportunity to measure detailed structural response in real time over long durations. This paper describes a first-of-its-kind application of Fibre Bragg Grating (FBG) measurement to monitor the impact of tunnelling works on two Grade-I listed buildings in London, UK. After describing the principles behind the strain sensing technology, laboratory tests are discussed. These tests are used to calibrate strain sensors and validate the newly developed temperature compensation technique. The arrangement of sensors and their installation on the buildings is then presented. Fibre-optic temperature and strain measurements from the ensuing three-year monitoring period are compared with other independent measurements. It is demonstrated that the system enabled direct real-time monitoring of both the short- and long-term impact of tunnelling on the historic assets in a reliable manner.

Design of storage and management device of packaged FBG sensors based on RFID

In the application of fiber Bragg grating(FBG) sensor in structural condition monitoring, the reasonable layout scheme is designed according to its central wavelength. However, the paper label with FBG central wavelength information may be lost when FBG is used, therefore it is difficult to store and manage FBG sensors. This paper designs a storage device and its management system of packaged independent FBG sensor based on RFID technology. The oblique grid structure can store the FBG sensors with the same central wavelength. The RFID reader module is arranged on the top of the box-type device with multi-layer oblique grid. The packaged FBG sensors are stored, read and classified with the help of special software.

Design of a management system of packaged FBG sensor based on RFID for wind turbine blade

With the application of fiber bragg grating (FBG) sensors in the condition monitoring of wind turbine blades, the layout management of FBG sensors with different central wavelengths and manufacturing processes has attracted much attention. A management system of FBG sensor based on RFID technology is proposed. The management system includes three parts: packaging process of FBG sensor with RFID chip, RFID chip stored information and the FBG sensors status monitoring system, the FBG sensor installation on full scale field wind turbine blade and its maintenance. The proposed management system introduces RFID technology into the blade monitoring using FBG sensor. It provides effective digital management method for the manufacturing, the layout, the installation and the maintenance process of the FBG sensor on the wind turbine blade.

Study of Fiber Bragg Gratings Embedded in 3D-Printed Patches for Deformation Monitoring

Aim of this work is the analysis of the 3D printed patches embedding fiber Bragg gratings (FBGs) for deformation monitoring. In particular, the paper first describes the manufacturing process and the performances of FBGs embedded in PLA and ABS rectangular patches, showing that the sensitivity to temperature is strongly dependent on the material and on the dimension of the patches. Then, the deformation characteristics are, for the first time, explored showing that the thickness of the patch, influencing the neutral axis position, acts as gaining factor on the deformation sensitivity of the FBG sensor. Such results enable the application of FBG embedded in 3D-printed patches for deformation monitoring taking in proper consideration the dimension of the patches, in particular its thickness, that allows to enhance the wavelength shift of the applied FBG sensors.

Critical Issues of Double-Metal Layer Coating on FBG for Applications at High Temperatures

Use of fiber Bragg gratings (FBGs) to monitor high temperature (HT) applications is of great interest to the research community. Standard commercial FBGs can operate up to 600 C. For applications beyond that value, specific processing of the FBGs must be adopted to allow the grating not to deteriorate. The most common technique used to process FBGs for HT applications is the regeneration procedure (RP), which typically extends their use up to 1000 C. RP involves a long-term annealing of the FBGs, to be done at a temperature ranging from 550 to 950 C. As at that temperature, the original coating of the FBGs would burn out, they shall stay uncoated, and their brittleness is a serious concern to deal with. Depositing a metal coating on the FBGs prior to process them for RP offers an effective solution to provide them with the necessary mechanical strengthening. In this paper, a procedure to provide the FBG with a bimetallic coating made by copper and nickel electrodeposition (ED) is proposed, discussing issues related to the coating morphology, adherence to the fiber, and effects on the grating spectral response. To define the processing parameters of the proposed procedure, production tests were performed on dummy samples which were used for destructive SEM–EDS analysis. As a critical step, the proposed procedure was shown to necessitate a heat treatment after the nickel ED, to remove the absorbed hydrogen. The spectral response of the FBG samples was monitored along the various steps of the proposed procedure and, as a final proof test for adherence stability of the bimetallic coating, along a heating/cooling cycle from room temperature to 1010 C. The results suggest that, given the emergence of Kirkendall voids at the copper–nickel interface, occurring at the highest temperatures (700–1010 C), the bimetallic layer could be employed as FBG coating up to 700 C.

Dentistry Applications of Fiber Bragg Gratings: Irradiation Protocols for Bulk Fill Flow Dental Composites

This study evaluates the polymerization shrinkage of a bulk fill flow composite photoactivated by continuous or ramped irradiation. Sample teeth were prepared with Class I occlusal cavity. The fiber Bragg grating sensors are placed along bucco-lingual and mesio-distal directions for measuring the shrinkage and temperature. The results reveal that ramp irradiation protocol showed a slower evolution of shrinkage polymerization curve, but the final values of shrinkage for continuous and ramp protocols are not significantly different. The ramp irradiation protocol showed higher temperature changes than that of the continuous irradiation procedure.

Quasi-Distributed Torque and Displacement Sensing on a Series Elastic Actuator’s Spring Using FBG Arrays Inscribed in CYTOP Fibers

We report the application of fiber Bragg grating (FBG) arrays in polymer optical fibers for the quasi-distributed displacement and torque sensing in a series elastic actuator’s spring. The arrays were inscribed in cyclic transparent optical polymer fiber through the direct-write, plane-by-plane inscription method using a femtosecond laser. Two arrays with four FBGs each were positioned on a rotary spring, and a numerical analysis using the finite-element method was performed to evaluate the sensors’ behavior under different loading conditions. Experimental tests were performed on the spring, and the results show that the FBGs responses follow the predicted numerical simulations. In addition, a Kalman filter was applied on the response of the FBGs for sensor fusion, leading to a more accurate torque estimation, where the relative error for the torque estimation was improved by a factor of 7. Furthermore, transverse forces were applied on the predefined points of the spring in order to show the ability of the quasi-distributed sensor approach to identify external mechanical disturbances on the spring.

Optische Schnittstellen

SummaryOptical interfaces: Viewing into the vacuumThe requirements concerning in‐vacuum process diagnostics are more and more challenging. On the one hand, the setup itself has to be decoupled from the process to avoid any external impact on the process. On the other hand, the process parameters have to be in‐situ characterized and controlled by an in‐vacuum monitoring system. In the vicinity of these two contradictive requirements optical fiber applications gain more and more impact, since optical fibers cover several advantages: high transmission signal, inherent immunity against external electromagnetic forces, high durability and high mechanical as well as optical flexibility. The focus of the contribution is a new in‐vacuum optical temperature sensor combined with high‐temperature fiber optical feedthroughs. The sensor based on fiber Bragg gratings (FBG) in the optical fiber, which are extremely sensitive to temperature variations. In a first case study, the application of FBG with the feedthrough as an in‐vacuum optical temperature sensor is demonstrated. Thus an optical fiber containing a bunch of several FBG becomes a compact, robust, and flexible in‐vacuum network of local temperature sensors. Each individual sensor captures temperature variations over a signal path that is several km long with a remarkable accuracy up to 0.1 K and with speed of light.

High Temperature Effects during High Energy Laser Strikes on Embedded Fiber Bragg Grating Sensors

As the applications of fiber Bragg gratings (FBGs) continue to grow and become more advanced, it becomes necessary to understand their behavior when exposed to high temperatures in unique situations. In these experiments, uniform 1530-nm fiber Bragg gratings and Type K Cr-Al thermocouples were embedded in three-ply carbon fiber composites. A 100 W high energy laser (HEL) heated the composites to high temperatures over timespans less than one second, and FBG spectral data and thermocouple temperature data were collected during each HEL heating test. The data from three high energy laser tests that represent different levels of damage to the FBG are analyzed to explore the spectral response and thermal decay of embedded FBG sensors when exposed to high temperatures over short timespans. Results are compared to a previously proposed power-law model describing the decay of FBGs in bare fiber when held at constant temperatures over much longer timespans.