Tilted Fiber Bragg Grating Sensors Research Articles

Machine learning approach for automated data analysis in tilted FBGs

This paper presents the development of an approach for automatic estimation of the regression coefficients as a function of the refractive index (RI) in Tilted Fiber Bragg Gratings (TFBGs) based on their optical spectral characteristics. The methodology is based on machine learning approach, where different samples are analyzed in various test conditions to obtain a dataset for the training of a kNN regression algorithm. In this case, the spectral characteristics related to the wavelength spacing between adjacent cladding mode resonances, number of valleys and amplitude of the normalized spectra are correlated with the linear regression algorithm for RI estimation using the machine learning approach. The analysis are performed as a function of the wavelength shift and optical power, considering two regions on the transmitted spectrum and the envelope of the curve. Furthermore, the number of valleys and the area of the envelope curve are also analyzed as a function of the refractive index, which result in six analyzed features. The Principal Components Analysis (PCA) is used to reduce the number of features to two principal components. The results of the proposed algorithm are obtained with different trained and untrained samples, which indicates a root mean squared error (RMSE) of around 0.003RIU for the trained samples and 0.008 RIU for the untrained ones. Therefore, the results indicate the usefulness of the proposed approach, which can be used to enhance the reproducibility of large set of TFBG sensors in practical applications.

Polarization Influence on Algorithms of TFBG Sensors Data Analysis for Bending Application

In this article we deal with the influence of polarization on determining the bending radius in TFBG (tilted fiber Bragg grating) sensors. The spectrum of TFBG structures changes under the influence of many factors at the same time. In the case of bending radius measurements, additional factors may be the temperature and polarization state of the introduced light. So far, only the cladding mode envelope algorithm has been used to determine the bending radius. An interesting issue seems to be establishing the influence of cross-sensitivity of the spectrum on changes in polarization during bending measurements. In addition to the envelope algorithm, algorithms for spectral length and average deviation from the local mean were examined. As a result of the analysis of experimental data, it was found that the level of polarization’s influence on the result may be significant in determining the bending radius. Reducing the influence of polarization can be achieved by using algorithms providing wavelength parameters. Additionally, in order to reduce the influence of polarization, we proposed the use of the PLS (partial least squares regression) algorithm for the processed spectrum.

Tilted fiber Bragg grating surface plasmon resonance based optical fiber cadmium ion trace detection

A plasmonic tilted fiber Bragg grating (TFBG)-based sensor for the detection of cadmium ion (Cd2+) is proposed and demonstrated experimentally. The TFBG is coated with a thin gold film, where glutathione (GSH) can self-assemble on the gold coating to bond Cd2+ and form complexes. To enhance Cd2+ sensitivity, GSH-modified gold nanoparticles are used to amplify localized refractive index changes around TFBG surface, induced by Cd2+ concentration variations. The experimental results indicate a limit of detection of ∼0.06 nM, which is two orders better than reported fiber grating-based Cd2+ sensors. Additionally, the proposed TFBG sensor performs a wide dynamic range from 0.01 nM to 1000 nM for Cd2+ detections and temperature-compensated wavelength monitoring with Bragg resonance.

Decoupling bulk and surface characteristics with a bare tilted fiber Bragg grating.

The tilted fiber Bragg grating (TFBG) with dense comb-like resonances offers a promising fiber-optic sensing platform but could suffer from cross sensitivity dependent on bulk and surface environment. In this work, the decoupling of bulk and surface characteristics (indicated by bulk refractive index (RI) and surface-localized binding film) from each other is attained theoretically with a bare TFBG sensor. This is realized with the proposed decoupling approach based on differential spectral responses of cut-off mode resonance and mode dispersion represented as wavelength interval between P- and S-polarized resonances of the TFBG to the bulk RI and surface film thickness. The results demonstrate that with this method the sensing performance for decoupling bulk RI and surface film thickness is comparative to the cases in which either the bulk or surface environment of the TFBG sensor changes, with the bulk and surface sensitivities over 540 nm/RIU and 12 pm/nm, respectively.

Design and Investigation of a Fiber Bragg Grating Tilt Sensor With Vibration Damping

Fiber Bragg grating (FBG) sensors play an increasingly important role in structural health monitoring, and tilt sensors are one of the most commonly used among them. Traditional FBG tilt sensors are easily affected by external vibration, and to overcome such a problem, this work proposes a novel FBG tilt sensor with a vibration damping feature. Based on the classical equal strength beam structure, a vibration-isolating spring is used as the vibration-damping connection between the beam and the mass block, while a damping fluid is filled inside the sensor housing as the vibration-damping material. After elaborating on the details of sensor structural design and measurement theory, five sensors were set up for relevant performance tests, and the controlled experiments were carried out for the FBG sensors with and without the vibration isolation springs. Meanwhile, for sensors with vibration isolation springs, the viscosity of filled damping fluid was also varied in the experiments, to better reveal its influence on the sensor performance. The experimental results show that the proposed sensors have a high sensitivity of 112.743 pm/° within a range of ±15° and also have superior creep resistance as well as good temperature compensation capability. From controlled experiments, it is evident that the proposed sensor with a vibration damping structure offers better vibration stability and monitoring accuracy and can be effectively adapted to safety monitoring tasks in different engineering applications by simply adjusting the viscosity of filled damping fluid.

Research on Hydraulic Support Attitude Monitoring Method Merging FBG Sensing Technology and AdaBoost Algorithm

The hydraulic support is the key equipment of surrounding rock support in a stope, and thus monitoring the attitude of the hydraulic support has an important guiding role in the support selection, operation control and rock pressure analysis of the working face. At present, attitude monitoring technology for hydraulic support mainly includes inertial measurement, contact measurement and visual measurement. Aiming at the technical defects of imperfect attitude perception models, incomplete perception parameters and the low decision-making ability of such systems, the fiber Bragg grating (FBG) pressure sensor and the FBG tilt sensor are developed independently by combining with FBG sensing theory. The pressure sensitivity of the FBG pressure sensor is 35.6 pm/MPa, and the angular sensitivity of the FBG tilt sensor is 31.3 pm/(°). Additionally, an information platform for FBG sensing monitoring for hydraulic support attitude is constructed based on. NET technology and C/S architecture. The information platform realizes real-time monitoring, data management, report management, production information management and data querying of hydraulic support attitude monitoring data. An AdaBoost neural network hydraulic support working resistance prediction model is established using MATLAB. The AdaBoost neural network algorithm successfully predicts the periodic pressure of the coal mining face by training with the sample data of the working resistance of the hydraulic support. The predicting accuracy is more than 95%.

All-Silicon Packaging Technology for Fiber Bragg Gratings and Its Application in Tilt Sensor

A fiber Bragg grating (FBG) sensor includes three main parts, an FBG, a sensor substrate, and a packaging material. The most commonly used packaging material is epoxy resin adhesive, which is prone to aging and failure, which further affects the performance and life of a sensor. This research provides an innovative approach that enables all-silicon sensor fabrication, where the substrate and packaging material are prepared by silicon glass, which is consistent with silicon optical fiber. In the packaging process, there are three main bottleneck problems, which are considered in this study: 1) highly elastic processing of glass used in the sensor elastic element; 2) thermal fixation between the silicon packaging material and substrate; and 3) selecting appropriate FBG parameters and fixing methods. In the proposed approach, an elastic glass cantilever beam and an FBG are packaged with molten glass powder. The proposed approach is verified by tests, and the test results show that the packaged FBG has good package quality and sensing performance. The theoretical analysis of the wavelength drift phenomenon during the packaging is presented. Finally, the introduced packaging method is applied to the fabrication of an FBG tilt sensor. The sensor shell, cantilever beam, temperature compensation beam, mass block, and other components are designed and made of silicon glass. The sensing principle and performance tests of the tilt sensor have been described in detail. The tilt sensor shows good linearity and repeatability. The presented packaging technology shows good applicability and has promising applications in the fabrication of high-performance FBG sensors.

Design and fabrication of an ultra-sensitive Ta2C MXene/Au-coated tilted grating sensor

Accurate evaluation of trace biochemical molecular signals by fiber-optic sensors is highly challenging. Recently, MXene, as one of the most promising 2D materials with unique optoelectronic properties and structural advantages, has attracted numerous concerns about the sensitization of optical fiber sensors. This paper proposed an innovative Ta2C MXene (Ta2C-M) sensitized Au-coated tilted fiber Bragg grating (TFBG) SPR sensor. We achieved the optimal deposition scheme for Ta2C-M by varying the laser induction time. The introduction of Ta2C-M brought a 23.3% increase in sensitivity and exhibited an excellent figure of merit (FOM=2586.11 RIU-1) and linearity (R2 =0.99903). Based on COMSOL, a 3D diversity analysis model was proposed, which validated the polarization response of the TFBG sensor, the electromagnetic gain of the Ta2C-M coating and enabled the fitting evaluation of the SPR behavior. The sensitization mechanism of the Ta2C-M coating was further analyzed utilizing energy band theory based on the Vienna ab initio Simulation Package (VASP). The assessment of two carcinogenic food additives, Saccharin and BHT, demonstrated the extreme SPR signal resolution and accurate trace detection performance (10-11 M) of the Ta2C-M/Au/TFBG sensor. These findings indicate that the proposed sensor will hopefully open new biotechnology, medicine, and food safety detection avenues.

Design and fabrication of Gr/Ag-coated tilted grating sensor for ultra-sensitive detection of DNA hybridization

A graphene sensitized biosensor based on 6° tilted fiber Bragg grating (TFBG) was proposed. The introduction of the graphene coating boosted the sensitivity and stability of the Ag/TFBG sensor, and improved the probe loading and molecular capture of the sensitized layer. A quantitative analytical model was proposed to calculate the hybridization efficiency and maximum sensor response of DNA single base mismatch. The results showed that the kinetic affinity response of the Gr/Ag/TFBG sensor was stably demonstrated with a dissociation constant (kd=9.392 ± 0.178·10−3 min−1) and the detection limit of DNA was as low as 3 pM. This indicates that the sensor has the ability to distinguish single base mutations quantitatively in real time. The surface modification, probe binding and DNA hybridization were characterized based on atomic force microscopy (AFM) and Raman detection spectrum detection system. A 3D diversity analysis model for TFBG sensors was innovatively proposed based on COMSOL. The multi-mode coupling and polarization response analysis of the inner core, the electric field distribution and polarization transmission mode of the sensitized layer, and the polarization gain brought by graphene are systematically described. Excellent bioanalytical capabilities combined with well-established analytical methods make prepared sensor promising for high-throughput screening of genetic variants and disease markers.

Novel Fiber Bragg Grating Tilt Sensor with High Sensitivity

Novel Fiber Bragg Grating Tilt Sensor with High Sensitivity

Graphene oxide modified tilted fiber Bragg grating for 10<sup>–12</sup> level heavy metal ion sensing

Graphene oxide (GO) is an ideal label-free sensing material with its super large specific surface area and abundant surface functional groups. Considering its insulating characteristic, the GO is suitable for optics-based heavy metal ion sensing. However, given the large surface tension of water and the hydrophilicity of GO, the agglomeration or wrinkles of GO nanosheets is usually inevitable during coating with aqueous dispersion. This reduces the accessible surface area and surface functional groups of GO, thereby degrading the sensing performance. Here, an ultra-sensitive GO functionalized tilted fiber Bragg grating (TFBG) sensor is designed to detect heavy metal ions in aqueous solutions. Firstly, a strategy of free energy manipulation is employed to avoid the wrinkles and agglomeration of GO nanosheets. In the scenario of aqueous dispersion, the GO nanosheets will wrinkle as the water droplets evaporate and shrink. In contrast, using the lower-surface-tension ethanol as the dispersant and a high-surface-energy substrate processed by oxygen plasma, the dispersion will evenly spread on the substrate instead of forming droplets. When ethanol evaporates, GO nanosheets are attached to the substrate in largest possible area to reduce the free energy of the system, by which a GO film without agglomeration or wrinkles can be obtained. Secondly, the intrinsic sensitivity of TFBG is conducive to the detection of heavy metal ions in water. Mode interference occurs between the cladding mode and the core mode in the TFBG, and the wavelength and intensity of the interference are highly sensitive to the surrounding temperature, stress, and refractive index. Combining the above characteristics, the GO functionalized TFBG is highly sensitive to Pb<sup>2+</sup>, Cd<sup>2+</sup>, and Cu<sup>2+</sup> ions in water. These heavy metal ions are adsorbed by the GO, and thus causing the effective refractive index to increase. The results show that the adsorption of heavy metal ions makes the interference peaks red-shifted in the transmission spectrum. The lowest detection limit for Pb<sup>2+</sup> and Cd<sup>2+</sup> can reach 10<sup>–10</sup> mol/L (ng/L level), and the corresponding sensitivities are 0.426 and 0.385 dB/(nmol·L<sup>–1</sup>) (2.06 and 3.43 dB/(μg·L<sup>–1</sup>)), respectively. These superior sensing performances benefit from the high specific surface area and accessible carbonyl groups of the unfolded GO, and also rely on the excellent intrinsic sensitivity of TFBG. The GO functionalized TFBG sensor has a promising potential application in environment monitoring.

Spectral analysis for tilted fiber Bragg gratings in the corrosion detection for concrete structure

An effective corrosion monitoring technique is sought after by the engineers for assessing the steel bar corrosion at the early stage for the maintenance and repair works, especially in the corrosive environments, such as coastal and marine. In this work, tilted fiber Bragg grating (TFBG) with the optical sensor is employed in corrosion monitoring of a reinforced concrete structure. Taking advantage of the high sensitivity of TFBG cladding resonance wavelengths to the change in the surrounding medium, the sensor is mounted on the steel bar that is embedded in a concreted block during an accelerated corrosion process. The acquired transmission spectrum of the TFBG during the procedure is digitally processed using Fourier Transform to produce an index that is sensitive to the generated corrosion product surrounding the TFBG sensor. This eases the analysis of the sophisticated TFBG transmission spectra. The generated index can be used as an indicator (indicator J) for the corrosion process of the embedded steel bar in the concrete structure. This indicator J can act as an indicator to describe the corrosion activity and corrosion level at a specific point of the steel bar in concrete structures.

Demodulation of a tilted fibre Bragg grating transmission signal using α-shape modified Delaunay triangulation

Reflective Tilted Fiber Bragg Grating (TFBG) sensors have intriguing sensing capabilities due to the resonance-peaks present in their transmitted spectrum. Previous works measured the external refractive index (ERI) in which the TFBG sensor is placed, by considering the wavelengths or the envelope of the cladding-modes resonances. In this paper, primarily, we demonstrate the effectiveness of an alternative global technique, based on Delaunay triangulation, to analyze the TFBG spectrum for refractometer purposes. Hence, we performed the correlation between the area subtended by the upper and lower cladding-modes peaks and the ERI. An investigation on the goodness-of-fit correlation functions is also presented for TFBG sensors written in standard- and thin-optical fibres and considering different values of the fundamental triangulation parameter α.

Internal Residual Strain Measurements in Carbon Fiber-Reinforced Polymer Laminates Curing Process Using Embedded Tilted Fiber Bragg Grating Sensor.

Carbon fiber reinforced plastics (CFRP) have many mechanical properties that are superior to those of conventional structural materials and are becoming more and more widely used. Monitoring the curing process used to produce such composite material is important to ensure the quality of the process, especially for the characterization of residual strains after the material has been manufactured. In this study, we present a tilted fiber Bragg grating (TFBG) sensor used to monitor the curing of CFRP composite materials. The TFBG sensor was embedded into the layers of CFRP laminates to study the curing residual strain of the laminates. The experimental results showed that the curing residual stress was about −22.25 MPa, the axial residual strain was −281.351 με, and lateral residual strain of 89.91 με. The TFBG sensor was found to be sensitive to the curing residual strain of the CFRP, meaning that it has potential for use in applications involving composite curing processes. Moreover, it is indeed possible to improve the properties of composite materials via the optimization and monitoring of their curing parameters.

Comparison of the sensing mechanisms and capabilities of three functional materials surface-modified TFBG sensors

Three functional materials were coated on tilted fiber Bragg grating (TFBG) sensors, and through the analysis of the characteristics of TBFG sensors’ amplitudes in the core and cladding modes, their abilities to sense humidity were compared. The mechanisms by which physical moisture absorption sensing was achieved through the three materials were investigated. The degree of sensitivity in terms of resonance wavelength in the cladding mode for the three coating materials was PAHP4 > PEDOT:PSS > GO. The sensitivity in the cladding modes of the tested sensor showed that the best resonance wavelength was as high as 0.0112 nm/% RH and was achieved for the TFBG coated with PAHP4 due to the hydrophilic groups around the material, which are good at forming hydrogen bond interactions with H2O.

Wide-Range Fiber Bragg Grating Tilt Sensor Based on a Cam Structure

This work presents a fiber Bragg grating (FBG) wide-range tilt sensor based on a cam structure. Two FBGs are fixed on a specially-designed sensor shell. When one FBG is stretched by the rotating cam driven by tilt, the other FBG moves along a standard semicircle with a constant radius, and compensates for temperature variations of the stretched FBG. By detecting the difference of the central wavelength shifts of two FBGs, variations of the tilt angle can be measured. The detailed mechanical structure design and measurement theory of the sensor is presented, and the sensor prototype is discussed. Testing results of comprehensive performance show that there is a good linear relationship between the tilt angle and the central wavelength of the FBG over three intervals from -90° to 90°. The repeatability and creep resistance of the FBG tilt sensor are good, the measuring accuracy is 1.516°, and the measuring resolution is 0.003°. The measurement range is much larger than comparable existing sensors, and it avoids cross-sensitivity between temperature and strain for FBG. The sensitivity of the sensor can be easily adjusted by changing the outline of the cam, or the length of the optical fiber between two fixed points.

A FBG Tilt Sensor Fabricated Using 3D Printing Technique for Monitoring Ground Movement

In this paper, 3D printing method and fiber Bragg grating (FBG) sensing technology were combined to fabricate a simple and small size tilt sensor. The raw material for creating different sensor components was Polylactic Acid (PLA) material. Bare FBG sensors were directly inscribed into PLA material using fused deposition modeling (FDM) process for complete protection. Printing test using FDM process indicate that bare optical fiber cables successfully survived the FDM process even though the melted PLA materials approached a maximum temperature of around 220°. Two FBG sensors characterized by different wavelength values were symmetrically arranged inside a sensing unit for the measurement of the tilt angle in terms of their wavelength shifts. The minimum resolution and the measurement range of this new FBG tilt sensor were 0.084° and ±75°, respectively. The measurement performance of this new tilt sensor was also verified with the conventional tilt sensor. A small slope model was built up in the laboratory to examine the measurement performance of three FBG tilt sensors which were mounted at different depths inside the slope model. The measured horizontal displacement data from FBG tilt sensors were verified with the output data of the Linear Variable Displacement Transducer (LVDT) and the obtained maximum error was less than 3% when the maximum displacements are approached.

Integrated Differential Area Method for Variable Sensitivity Interrogation of Tilted Fiber Bragg Grating Sensors

A novel demodulation method for tilted fiber Bragg grating (TFBG) refractometry is proposed and experimentally demonstrated. This “integrated differential area” method consists of calculating the difference between measured spectral transmission over a selected bandwidth and a reference spectrum and then integrating the absolute value of the result. This has the effect of multiplying the effect of the individual cladding mode resonance shifts relative to the background noise. The method further allows the use of spectrum analyzers with low resolution, including portable ones with spectral resolution of the order of 150-200 pm without loss of sensitivity. In practical measurements, a reflection configuration is demonstrated using a TFBG cascaded by a chirped-fiber Bragg grating with bandwidth of 19 nm. A refractive index sensitivity of 7.19% transmission change per refractive index unit is achieved for refractive indices ranging from 1.32 to 1.38, along with a limit of detection of 0.00026 refractive index units.

A fiber Bragg grating tilt sensor for posture monitoring of hydraulic supports in coal mine working face

Hydraulic supports are indispensable equipment in coal mining face. In order to establish an intelligent monitoring system for posture monitoring of hydraulic supports and realize posture monitoring of hydraulic supports online, the posture monitoring system structure of two columns shield hydraulic supports is established in theory. Then, the kinematics model of the hydraulic supports is analyzed and the fiber Bragg grating tilt sensor is designed. Besides, a method of error compensation for tilt monitoring is proposed. What’s more, the performance test is carried out. The test results indicated that the linear relationship between angles and the FBG center wavelength was good with an angular sensitivity of 32.4 pm/(°) and a linear fitting coefficient of 0.9992 in the range from −30° to 30°, and the repeatability of the FBG tilt sensor was superior. Finally, posture monitoring programs of the hydraulic supports is designed to test the performance of the FBG tilt sensor. The engineering monitoring instance is displayed, and the reliability of the hydraulic support monitoring system based on optical fiber sensing technology is analyzed, and the dynamic state of the hydraulic support posture during production process of the working face is obtained. Furthermore, such a posture monitoring system can be extensively used for quasi-distributed measurement and long-term online monitoring of other types of hydraulic supports such as four-column chock-shield hydraulic supports and standing supports.

Tilted Fiber Bragg Grating Sensor Using Chemical Plating of a Palladium Membrane for the Detection of Hydrogen Leakage.

A tilted fiber Bragg grating (TFBG) hydrogen sensor coated with a palladium (Pd) membrane by the electroless plating method is proposed in this paper. A uniform layer of Pd metal is fabricated in aqueous solutions by the chemical coating method, which is used as the sensitive element to detect the change of the surrounding refractive index (SRI) caused by hydrogen absorption. The change in SRI causes an unsynchronized change of the cladding modes and the Bragg peak in the TFBG transmission spectrum, thereby eliminating the cross-sensitivity due to membrane expansion and is able to simultaneously monitor the presence of cracks in the pipe, as well as the hydrogen leakage. By subtracting the wavelength shift caused by fiber expansion, the change of SRI, i.e., the information from the H2 level, can be separately obtained. The drifted wavelength is measured for the H2 concentration below the hydrogen explosion limit between 1% and 4%. The chemical-based coating has the advantages of a low cost, a simple operation, and being suitable for coating on long fiber structures. The proposed sensor is able to detect the H2 signal in 5 min at a 1% H2 concentration. The proposed sensor is proved to be able to monitor the hydrogen level without the cross-sensitivity of temperature variation and expansion strains, so could be a good candidate for security applications in industry.