Bragg Grating Research Articles

Advanced data-driven FBG sensor-based pavement monitoring system using multi-sensor data fusion and an unsupervised learning approach

Pavement health monitoring and assessment using sensor technology provides valuable insights into the condition of roads and helps in effective maintenance and management. This paper presents an advanced fiber Bragg grating (FBG) sensor-based pavement monitoring system that leverages multi-sensor data fusion techniques and an unsupervised learning approach for enhanced structural health assessment. The study commences with the fusion of data from various FBG sensors based on specific considerations and employing a novel function to improve the accuracy of pavement condition assessment. A dedicated data management system is introduced to address the challenges associated with continuous monitoring. This system automates data organization, pre-processing, and storage, facilitating historical trend analysis and post-processing. Then, the research introduces a novel unsupervised learning approach utilizing the density-based spatial clustering of applications with noise (DBSCAN) algorithm. This algorithm identifies abnormal patterns in the FBG sensor data, which may indicate structural anomalies or unexpected events, aiding the operator by automatically flagging abnormal data. By incorporating principle component analysis (PCA)-based feature fusion, the system reduces the dimensionality of the data, which enhances the efficiency of the clustering process. Various anomalies were detected in the data collected for four months from the test track. The cause and relevance of these anomalies were assessed to facilitate informed decision-making. This approach enables the operator to achieve early detection of potential structural issues and distinguish abnormal damage data from other anomalous data, thereby contributing to the overall success of the pavement monitoring system. In conclusion, the integration of advanced FBG sensor technology with multi-sensor data fusion and unsupervised learning techniques results in a sophisticated and robust pavement monitoring system. The presented approach enhances accuracy by detecting anomalies at any strain level, computational time using multi-sensor data fusion, and reliability of structural health assessment through anomaly evaluation. It also provides a framework for proactive decision-making, ultimately improving the longevity and structural integrity of critical infrastructure.

Simulation and Measurement of Strain Waveform under Vibration Using Fiber Bragg Gratings.

The work is devoted to the consideration of methods for determining the strain of objects using fiber Bragg gratings under a high-frequency vibration or pulsed mechanical action, which is difficult to perform using widespread methods and devices. The methods are based on numerical processing of the time dependence of the radiation power reflected from the fiber Bragg grating at various wavelengths, which makes it possible to measure strain parameters in a wide range of magnitude and frequencies. The efficiency of the proposed methods is demonstrated by numerical simulation. It is shown that it is possible to restore the strain dependence on time in the range ±1000 μϵ or more from simultaneously measured power dependencies reflected by the fiber Bragg grating using common fiber-optic components. The case of sequential registration of reflected radiation power at different wavelengths to determine the probability density of the distribution of the strain values is also considered. The results of signal processing obtained both by numerical simulation and experimentally for the case of a linear vibration are presented. The technical problems of using the proposed methods are discussed.

Experimental Study of Sinkhole Propagation Induced by a Leaking Pipe Using Fibre Bragg Grating Sensors.

Sinkhole formation caused by leaking pipes in karst soluble rocks is a significant concern, leading to infrastructure damage and safety risks. In this paper, an experiment was conducted to investigate sinkhole formation in dense sand induced by a leaking pipe. Fibre Bragg grating (FBG) sensors were used to record the strain. A balloon was gradually deflated within a bed of wet silica sand to create an underground cavity. Eighteen FBG sensors, with a wavelength range between 1550 nm and 1560 nm, were embedded horizontally and vertically in the physical model at different levels to monitor deformation at various locations. A leaking pipe was installed to induce the collapse of the formed arch above the cavity. The strain measurements suggested the following four phases in the sinkhole formation process: (1) cavity formation, (2) progressive weathering and erosion, (3) catastrophic collapse, and (4) subsequent equilibrium conditions. The results showed differences in the strain signatures and distributions between the horizontal and vertical measurements. During the critical phase of the sinkhole collapse, the horizontal measurements primarily showed tension, while the vertical measurements indicated compression. This investigation demonstrates the effectiveness of FBGs as advanced monitoring tools for sinkhole precursor identification. The study also suggests using FBGs in geotechnical monitoring applications to improve the understanding and mitigation of sinkholes and related geohazards.

Simultaneous Measurement of Strain and Displacement for Railway Tunnel Lining Safety Monitoring

This paper proposes a dual-parameter strain/displacement safety monitoring technology for railway tunnel lining structures. An integrated monitoring system with FBG (Fiber Bragg grating) and VDM (video displacement meter) components was used to monitor both the strain and deformation of the tunnel cross-section. Initially, a comprehensive experimental study was carried out using FBG strain sensors with temperature-compensated grating. The temperature-compensated grating was used to further improve the monitoring accuracy. The data show that the stability and accuracy were better than the traditional electronic strain sensor. Secondly, high-precision and multipoint monitoring of railway tunnel lining deformation was achieved by using VDM technology. Three months of case study results taken from the Gansu Railway Tunnel in China demonstrated a tunnel cross-section strain accuracy for microstrain and crown deformation at the submillimeter level, respectively. The technology provides a new high-precision way to monitor the condition of tunnel lining structures.

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.

Detection of riboflavin concentration by sol-gel silica coated dual-peak long period fiber grating sensor

A dual-peak long period fiber grating (DP-LPFG)-based sensor was designed and fabricated for the sensitive detection of riboflavin concentration. LPFGs with a period of 163 μm and a cladding mode of LP0, 12 were inscribed on a standard single-mode silica fiber using a femtosecond laser direct writing technique. Coatings of silica-based materials doped with β-cyclodextrin (β-CD), prepared via the sol-gel method, were applied to the surface of the LPFGs for the detection of riboflavin. To investigate the impact of coating thickness on the sensing sensitivity, sol-gel coatings of different thicknesses were applied to the surface of the LPFG sensor. Experimental findings revealed that the sensor with a thickness of approximately 540 nm sol-gel coatings exhibited superior sensing performance. The detection limit of the prepared DP-LPFG sensor was tested to be 0.08 nM, and it exhibits a response time of less than 3 minutes, along with high sensitivity, excellent repeatability, and selectivity. Thus, this sensor holds significant promise for applications in disease treatment and diagnosis, as well as in ensuring food quality.

Characterization of Thermal Expansion Coefficient of 3D Printing Polymeric Materials Using Fiber Bragg Grating Sensors.

This work aims at the determination of the coefficient of thermal expansion (CTE) of parts manufactured through the Fused Deposition Modeling process, employing fiber Bragg grating (FBG) sensors. Pure thermoplastic and composite specimens were built using different commercially available filament materials, including acrylonitrile butadiene styrene, polylactic acid, polyamide, polyether-block-amide (PEBA) and chopped carbon fiber-reinforced polyamide (CF-PA) composite. During the building process, the FBGs were embedded into the middle-plane of the test specimens, featuring 0° and 90° raster printing orientations. The samples were then subjected to thermal loading for measuring the thermally induced strains as a function of applied temperature and, consequently, the test samples' CTE and glass transition temperature (Tg) based on the recorded FBG wavelengths. Additionally, the integrated FBGs were used for the characterization of the residual strain magnitudes both at the end of the 3D printing process and at the end of each of the two consecutively applied thermal cycles. The results indicate that, among all tested materials, the CF-PA/0° specimens exhibited the lowest CTE value of 14 × 10-6/°C. The PEBA material was proven to have the most isotropic thermal response for both examined raster orientations, 0° and 90°, with CTE values of 117 × 10-6/°C and 108 × 10-6/°C, respectively, while similar residual strains were also calculated in both printing orientations. It is presented that the followed FBG-based methodology is proven to be an excellent alternative experimental technique for the CTE characterization of materials used in 3D printing.

A non-invasive pipeline heat flux measurement method and application based on CFBG

A non-invasive, real-time monitoring, spectral analysis heat flux sensor with single measuring point based on chirp fiber Bragg grating (CFBG) has been proposed in this paper. When CFBG senses heat flux, the linear increase of axial thermal expansion leads to spectral broadening. The measurement of heat flux can be realized by the measurement of the full width at half maximum (FWHM). The theoretical model and working principle of CFBG heat flux sensor are established to determine the heat flux sensing characteristics of CFBG. The numerical analysis and theoretical calculation show that CFBG heat flux sensor can realize stable transformation of heat flux and FWHM. A heat flux calibration platform was built and its feasibility was verified by simulation. The sensitivity of CFBG sensor to monitor heat flux in a wide temperature range is 1.078 pm/(W/m2). The heat flux of the pipe wall at the outlet of the axial variable piston pump in the hydraulic system was quantitatively measured by the CFBG heat flux sensor. The sensor has a spatial resolution of 1 cm, a length of 10 mm, a diameter of 0.125 mm, and a sensitivity of 1.078 pm/(W/m2). It has the advantages of high spatial resolution, single measurement point, real-time measurement, small installation space, and non-invasiveness. The theoretical model can provide theoretical guidance for the design of fiber grating heat flux sensor, and it is convenient to design a series of sensors for specific measurement requirements.

Type-II Bragg gratings with a 300 nm period fabricated using tightly focused femtosecond pulses and the phase mask technique

1000°C-resistant Bragg gratings (i.e., Type-II Bragg gratings) with a 300 nm period are fabricated in non-photosensitized silica-based optical fibers using tightly focused ultraviolet/visible (400 nm) femtosecond pulses and a phase mask. Five millimeter-long Bragg gratings with a -10 dB transmission dip and -0.1 dB off-resonance insertion loss are demonstrated. Inscription of such gratings is also possible through protective acrylate coatings on the fiber. The Bragg wavelength drift and changes in the transmission of the fiber Bragg gratings are monitored in the course of isothermal annealing performed at 1000°C for 120 hours.

A comparative study of Fibre Bragg Grating for spatially and temporally resolved gas temperature measurements in cold atmospheric pressure plasma jets

Abstract Cold atmospheric pressure plasma jets (CAP-Jet) are successfully used in medical ther-apy for healing of chronic wounds and are widely researched in inactivation of patho-gens and in assisting in cancer therapy. A crucial parameter for these plasma applica-tions is that CAP-Jets operate at temperatures that are tolerable for biological tissues. While tools characterizing the plasma's gas temperature are well developed, there are only a few methods that work with an agreeable limit of uncertainty, complexity and limited perturbation properties to accurately determine that the studied plasma jet operates at tissue tolerable temperatures at all times. In the current work, time resolved measurements of the gas temperature in the effluent of a CAP-Jet are performed using the innovative technique of a Fibre Bragg Grating (FBG), in which the temperature dynamics is measured by a shift of the FBGs resonant wavelength through its thermo-optic coefficient. Comparing with other temporal and spatial diagnostic tools such as thermocouple measurement, Schlieren imaging, and optical emission spectroscopy, we demonstrate reliable calorimetric measurements at different plasma duty cycles. The plasma source maintains tissue tolerable temperatures inside the plasma active zone with values below 35°C at 1 cm distance from the jet nozzle. The calorimetric measure-ments have revealed that the heat power dissipation in comparison to electric energy of our plasma source is at least 50%.

Fiber Bragg grating sensing multiplexing method using photonic lantern spatial mode diversity

This paper introduces a method for fiber Bragg grating (FBG) multiplexing based on spatial mode diversity. By introducing the dimension of spatial modes in few-mode fibers (FMF), the FBGs are allocated into multiple optical paths carried by different spatial modes. The method involves multiplexing and detecting the reflected spectra of FBGs in various spatial modes, effectively enhancing the system's sensing capacity. The paper establishes a mode-diversity multiplexing sensing system using mode-selective photonic lanterns and FMF circulator. As a proof of concept, the system is experimentally validated for multipoint temperature sensing using four spatial modes LP01, LP11, LP21 and LP02, and its performance is compared with that of traditional multiplexing methods. The experimental results indicate that the proposed mode division multiplexing (MDM) and hybrid MDM/wavelength division multiplexing (WDM) schemes effectively enhance the sensing capacity. Furthermore, the sensitivity to temperature response for the LP01, LP11, LP21 and LP02 modes in the spatial mode diversity multiplexed sensing surpasses 0.0096 nm/°C, maintaining linearity above 97%, and the maximum error contained within 5%. The proposed method offers a novel approach to expand the capacity of sensor networks.

Controllable lasing characteristics in Brillouin random fiber lasers by tailoring gain and random feedback fibers

Brillouin random fiber lasers (BRFLs), the combination of stimulated Brillouin scattering (SBS) gain and random distributed feedback, offer narrow linewidth lasing with simplicity and flexibility. However, BRFLs may not gain broad acceptance unless the fundamental lasing mechanisms governing their operation are fully understood and the lasing properties are effectively manipulated. Here, we demonstrate the control of lasing characteristics in BRFLs by tailoring the SBS gain fiber and the scattering pattern of the random feedback fiber. Experimental results show that BRFLs with 5 cm random fiber gratings (RFGs) feedback exhibit lower intensity fluctuation, longer coherence time, and more stable frequency compared to those with 6 km Rayleigh scattering fiber (RSF) feedback thanks to more correlated phase, lower mode density, and weaker dependence on external variations of RFGs. The low-randomness RFG feedback further improves the coherence time and intensity fluctuation attributed to the small period variation of sub-gratings. Moreover, the BRFL based on the high gain fiber and the strong scattering RFG feedback with low loss achieves high lasing efficiency and low threshold. The frequency jitter, intensity noise, and coherence time are also improved by reducing the gain fiber from 20 km to 1 km due to decreased mode hopping from mode competition. These results clarify the impact of gain and random feedback fibers on BRFL performance, offering insights for optimizing complex laser design for applications requiring high frequency stability and long coherence time.

Observation of the enhanced dual-split photonic spin Hall effect in wavelength domain via a helical fiber grating

To date, the photonic spin Hall effect (PSHE) has been studied and observed only in the space and momentum domains. Direct observation of the PSHE in the wavelength domain remains unexplored. In this work, we demonstrate both theoretically and experimentally the enhanced PSHE in the wavelength domain via a helical long-period fiber grating (HLPG), where the spin-dependent dual-split in the transmission spectrum of the utilized HLPG has been observed. This is unlike the PSHEs reported thus far, where a mono-splitting of the light is observed in either the space or the momentum domains. The proposed method provides an additional degree of freedom to observe the PSHE, which paves the way for exploiting all fiber-based HLPGs in chiral photonics, chiral sensors, and fine precise measurements.

Sensitivity-Adjustable Strain Sensor Based on Shrinking Long Period Fiber Grating

Sensitivity-Adjustable Strain Sensor Based on Shrinking Long Period Fiber Grating

Dopant, coating, and grating effects in silica optical fibers under extreme neutron irradiation

Radiation effects on fiber Bragg gratings (FBGs) have been studied but data gaps related to the effects of displacement damage resulting from high fast neutron fluence remain. In this work, Type-I and Type-II FBGs inscribed in optical fibers with various core dopants (Ge and F) and fiber coatings (acrylate, polyimide) were monitored in situ during 75 days of neutron irradiation to a peak fast (>0.1 MeV) neutron fluence of 3×1021 nfast/cm2. The reflected intensity of the Type-I FBGs inscribed in a Ge-doped core fiber decreased by >30 dB within 7 h of irradiation (1019 nfast/cm2), whereas Type-II FBGs inscribed in a pure silica core fiber eventually approached >40 dB attenuation after accumulating a fast neutron fluence on the order of 1020 nfast/cm2. Type-II FBGs inscribed in F-doped core fiber improved stability: the attenuation approached an equilibrium value in the range of 10 to 20 dB.

Temperature self-compensated dual core fiber-optic sensor integrated with moisture sensitive MIL-96(Al) film for breath detection

In this paper, a dual-core fiber optic sensor has been proposed for dynamic monitoring of temperature and humidity. The side core is polished into a D-type optical fiber, and a layer of nano-silver film and a layer of polydimethylsiloxane (PDMS) are deposited to construct a surface plasmon resonance (SPR) temperature compensator. The middle core is constructed as the long-period fiber grating (LPFG) sensing channel. The mode conversion of LPFG is achieved by TiO2 film, and its refractive index (RI) sensitivity is increased by about 4 times. Meanwhile, a layer of moisture sensitive MIL-96(Al) film is grown outside the TiO2 film. Due to the particular pore structure for water molecules, it is possible to change the effective RI of MIL-96(Al) film by absorbing water molecules. Research has shown that the LPFG sensor can achieve a relative humidity sensitivity of −0.411 nm/%RH. Finally, the sensor successfully achieved real-time dynamic monitoring of respiratory rate.

Design of a Fiber Temperature and Strain Sensor Model Using a Fiber Bragg Grating to Monitor Road Surface Conditions

In this paper, the types and principles of operation of fiber sensors based on fiber Bragg gratings (FBGs) are investigated. The influence of strain and temperature on the characteristics of FBGs is considered, and a method for the simultaneous measurement of these parameters is presented. Laboratory studies were carried out in the temperature range from +18 °C to +135 °C with an incremental step of 5 °C, with the actual temperature not deviating by more than ±0.5 °C. From the data obtained, the Bragg wavelength–temperature relationships were plotted, which showed a linear increase in wavelength with increasing temperature. This study shows that the use of two FBGs with a different sensitivity to temperature and strain allowed for the simultaneous measurement of both parameters. Numerical models created in the MATLAB R2022b environment confirmed the high accuracy and precision of the measurements. The FBG-based sensors demonstrated a robust performance in harsh environments, withstanding temperatures of up to 160 °C and high humidity, making them applicable in various industries and sciences. This work confirms that FBGs are a promising tool for accurate temperature and strain measurements, providing reliable results in harsh environments.

Research on vector bending sensors based on taper-drawn seven-core fiber Bragg grating

We present a novel vector bending sensor that enables simultaneous measurement of the bending response in six outer cores of a seven-core fiber (SCF) without the need for fan-in/fan-out configurations. Utilizing femtosecond laser technology, we inscribe seven Fiber Bragg Gratings (FBGs) with distinct wavelengths across the SCF cores. The sensor’s tapering process, tailored for the bending sensing scenario, effectively combines the reflection peaks of the FBGs into one detection channel, thereby substantially improving the measurement efficiency. Our experimental results demonstrate a strong directional dependence of the bending response, with a maximum sensitivity of 127 pm/m−1. The bending angle and curvature magnitude are reconstructed by analyzing the wavelength shifts of any two non-diagonal outer cores, offering a versatile solution for real-time monitoring applications. This sensor design, by eliminating the need for additional fan-in/fan-out devices, simplifies the system architecture and reduces both measurement time and sensor size, making it highly suitable for applications in precision manufacturing, environmental monitoring, and robotics.

Research on FBG curvature sensor based on PET substrate and silicone package

To address the low repeatability and accuracy in single package form and lack of strain transfer theory analysis in composite material form of fiber Bragg grating (FBG) curvature sensors, a flexible curvature sensing test scheme for FBG based on polyethylene terephthalate (PET) substrate and silicone package is proposed. Firstly, the factors affecting the strain response of the FBG are obtained through the analysis of the multilayer strain transfer theory, and the relationship between different material encapsulation forms and the strain response of the FBG is explored through finite element simulation. Secondly, the FBG curvature sensor with optimized simulation parameters is fabricated in package, and the curvature calibration verification experiments are performed on the sensor. Finally, the experimental results show that the deviation index of the FBG curvature sensor is between −1.89 % and 1.62 %, which indicates a better repeatability consistency. Based on the computation, the maximum sensitivity of the FBG curvature sensor is 13.32 µε/m−1 when the spacing between the fiber and the substrate layer is 2.50 mm, and its error is 0.30 % comparing with the simulation of the same condition, which supports the accuracy of the theoretical analysis and simulation of the multilayer strain transfer of the encapsulated FBG sensor conducted in this paper, meanwhile, the optimal encapsulation process of the FBG curvature sensor is obtained.

Accurate high-temperature profile sensing with dense multipoint arrays of regenerated fiber Bragg gratings

A spectrally and spatially dense sensor array consisting of 15 regenerated fiber Bragg gratings (RFBGs) over a length of 30 mm is presented for precise and fast multipoint temperature sensing up to 700°C. For the first time, it could be shown that with a dense fiber Bragg grating (FBG)-based sensor array the accuracy requirements of Class 1 thermocouples could be achieved and even exceeded. This also represents the highest spatial density of FBG-based high-temperature multipoint sensing reported so far. The mitigation of broadband losses during the regeneration process was studied, revealing the advantages of the low broadband loss characteristics of the RFBGs, especially when larger numbers of measuring points are required. Low measurement uncertainties were achieved by a new, improved calibration methodology and by an analysis of interferences of an FBG with the side lobes of spectrally neighboring FBGs as well as their suppression by a suitable arrangement of the Bragg wavelengths within the array and corresponding data processing. The capabilities of this multipoint sensor technique were demonstrated by resolving the temperature profile within the calibration volume of a calibration furnace and by resolving the temporal and spatial temperature gradients within the flame of a Bunsen burner. The results are of great importance for fiber-optic sensing of high-temperature profiles in real-world applications.