A Calibration Method Based on Linear InGaAs in Fiber Grating Sensors Interrogation System

In accordance with the characteristics of wavelength shift detection in fiber grating sensor interrogation system,the wavelength interrogation system which uses linear InGaAs as the spectrum receiver is proposed.Orientation of optic spectrum line affects the silt of volume phase grating and size of InGaAs photosensitive unit,thus the calibration method is needed.Based on an analysis of InGaAs imaging model,least square curve fitting method is proposed to detect spectrum wavelength and InGaAs photosensitive unit position.The experimental results show that the methods are effective and the demodulation system precision is improved.

The Design of Bending Long Period of Photonic Crystal Fiber Grating Sensors

The bending photonic crystal fiber grating sensor is an important role in underwater monitoring and fire alarm systems. It is studied that the resonant wavelength expression of bending long period photonic crystal fiber gratings is deduced, it is designed that a bending long period photonic crystal fiber grating sensor system, it is calculated in theory that between the bending long period photonic crystal fiber gratings sensor resonance wavelength and the grating period and the bending strain. The result is shown by calculating and analysing in theory, the grating curvature is increased by the increase of the bending strain of the grating, and the resonance wavelength of the grating sensor is drifted, the drift amount is increased, one in this grating, the drifted amount of the resonant wavelength is 0.014 nm.

Magneto-Optic Fiber Bragg Gratings with Application to High-Resolution Magnetic Field Sensors

Magneto-optic fiber Bragg gratings （MFBG） based on magneto-optic materials have a lot of potential applications for sensing and optical signal processing. The transmission and reflection spectra of guided optical waves in the MFBG are investigated. According to the sensitivity of MFBG spectral lines to the magneto-optic coupling intensity varying with applied magnetic field, a novel magnetic field sensor of high-resolution up to 0.01 nm/（kA/m） is predicted.

A Fiber-Optical Intrusion Alarm System Based on Quasi-Distributed Fiber Bragg Grating Sensors

A fiber-optical intrusion alarm system based on quasi-distributed fiber Bragg grating （FBG） sensors is demonstrated in this paper. The algorithms of empirical mode decomposition （EMD） and wavelet packet characteristic entropy are adopted to determine the intrusion location. The intrusion alarm software based on the Labview is developed, and it is also proved by the experiments. The results show that such a fiber-optical intrusion alarm system can offer the automatic intrusion alarm in real-time.

Packaging Effects on Fiber Bragg Grating Sensor Performance

In this paper, the effects of packaging material and structure of fiber Bragg grating sensor performance are investigated. The effects of thermal expansion coefficient of different embedding materials on the temperature sensitivities of the FBG sensors are studied both theoretically and experimentally with good agreement, which provides a means for selection of FBG packaging material to achieve desired temperature sensitivity. We also demonstrate a 4-point bending structured FBG lateral force sensor that measures up to 242N force with well-preserved reflection spectrum, whereas for 3-point bending structure, multiple-peaks start to occur when applied force reaches 72N.

Interrogating a Fiber Bragg Grating Vibration Sensor by Narrow Line Width Light

A method to interrogate fiber Bragg grating vibration sensor by narrow line width light is demonstrated. The interrogation scheme takes advantage of the intensity modulation of narrow spectral bandwidth light, such as distributed feedback laser, when a reflection or transmission spectrum curve of an fiber Bragg grating （FBG） moves due to the strain which is applied on the sensor. The sensor＇s response to accelerating frequency and amplitude is measured by experiment. The factors which have impacts on the sensitivity of the interrogation system are also discussed.

Fiber Bragg Grating Sensor Array System Based on Digital Phase Generated Carrier Demodulation and Reference Compensation Method

A novel fiber Bragg grating （FBG） sensor array system based on digital phase generated carrier （PGC） demodulation and reference compensation method is proposed and set up. Experimental results confirm that the digital PGC demodulation can be used for wavelength-division-multiplexed FBG sensor array and the reference compensation method can reduce the environmental interference by approximately 40 dB in the frequency range from 20 Hz to 2 kHz. The minimum detectable wavelength-shift of the sensor system is 1 × 10^-3 pm/Hz^1/2.

Characteristics of Smart Concrete with Fiber Optical Bragg Grating Sensor

Based on the advantages of the fiber Bragg grating sensing technology,this paper presents a principle of a novel smart concrete with fiber optical Bragg grating sensor,analyses the theory and characteristics,illustrates the key technology and method to make the fiber Bragg grating sensor for the smart concrete,and proves the feasibility with experiments.The results indicate that the smart concrete with fiber Bragg grating sensors is feasible in the structure monitoring and damage diagnosing in the long run.

Theoretical and experimental study on fiber Bragg grating sensing of rock strata settlement deformation

Based on the theory of fiber Bragg grating sensing, and using a 2 m plane stress material model of the simulation experiment, the wavelength shift regulations of fiber Bragg gratings under rock settlement deformation conditions were got, which provided reliable basis for the design of the wavelength bandwidth of sensing gratings. The wavelength increase when the rock layers are Compressive strain sensitivity is bigger under tension, and decrease under compression. than tensile strain sensitivity when the load is lower. However compressive strain sensitivity is smaller than tensile strain sensitivity when the load is higher. Experimental result coincides well with theory. The wavelength bandwidth of two neighboring fiber Bragg gratings of the gratings used for monitoring the deformation of rock layer should be no less than 6 nm. An effective new approach for detecting the deformation of rock settlement with fiber Bragg gratings is provided.

Dissolution and Deformation Characteristics of Limestones Containing Different Calcite and Dolomite Content Induced by CO_(2)-Water-Rock Interaction

To investigate the impacts of mineral composition on physical and mechanical properties of carbonate rocks,limestone specimens containing different contents in calcite and dolomite are selected to perform CO_(2)-water-rock reaction experiments.The X-ray Diffraction(XRD) and Nuclear Magnetic Resonance(NMR) are carried out to examine the change characteristics of mineral dissolution and pore structure after reaction.The core flooding experiments with Fiber Bragg gratings are implemented to examine the stress sensitivity of carbonate rocks.The results show that the limestones containing pure calcite are more susceptible to acid dissolution compared to limestone containing impure dolomite.The calcite content in pure limestone decreases as the reaction undergoes.The dissolution of dolomite leads to the formation of calcite in impure limestone.Calcite dissolution leads to the formation of macropore and flow channels in pure limestone,while the effects of impure dolomite in impure limestone results in mesopore formation.When confining pressure is lower than 12 MPa,pure limestones demonstrate higher strain sensitivity coefficients compared to impure limestone containing dolomite after reaction.When confining pressure exceeds 12 MPa,the strain sensitivity coefficients of both pure and impure limestones become almost equal.

Long-term performance of packaged fiber Bragg grating sensors for strain monitoring inside creep medium

To investigate the long-term performance of the packaged fiber Bragg grating(FBG)sensors embedded in civil infrastructure for strain monitoring,in this paper,the influence of host matrix’s creep effect on the behavior of the FBG sensors was systematically studied through theoretical,numerical,and experimental analysis.A theoretical strain transfer analysis between the optic fiber,packaging layer,and host matrix to consider the creep effect of the host matrix was performed accordingly for long-term strain monitoring.Parametric studies were carried out using numerical analysis for FBG sensors packaged with glass fiber reinforced plastic(GFRP),also known as FBG-GFRP sensors in concrete,as an example.The results show that embedded in a creep medium,an acceptable long-term performance of packaged FBG sensors requires the packaging layer to have a minimum length and maximum thickness.Laboratory long-term creep tests using epoxy resin and concrete as host matrix for FBG-GFRP sensors also clearly demonstrated that the influence of creep effect cannot be ignored for strain measurements if the host matrix has a creep potential and the developed correction model performed well to reduce measurement errors of such sensors in creep medium.

Use of fiber Bragg grating sensors for monitoring delamination damage propagation in glass-fiber reinforced composite structures

Embedded fiber Bragg grating（FBG）sensors have been widely used for damage monkoring of fiber composite structures for a few decades.However,many remaining engineering challenges have delayed FBG based in situ structural health monitoring（SHM）systems.One of the major problem associated with FBG based SHM system is the unavailability of reliable data processing algorithms.The present work details a study which has been undertaken for identification of delamination crack propagation in composite plate under fiber reinforced polymer（FRP）uniaxial loading.The strain measured by embedded FBG sensors closer to the crack tip was used to qualitatively and quantitatively analyze delamination damage propagation using recently proposed elasto-plastic model.Strain energy release rate was calculated and compared with the model prediction.The study has concluded that the delamination crack propagation in a FRP composite can be monitored successfully using an integral approach of FBG sensors measurements and the predictions of proposed elasto-plastic model.

Experimental study on cross-sensitivity of temperature and vibration of embedded fiber Bragg grating sensors

In view of the principle for occurrence of cross-sensitivity, a series of calibration experiments are carried out to solve the cross-sensitivity problem of embedded fiber Bragg gratings(FBGs) using the reference grating method. Moreover, an ultrasonic-vibration-assisted grinding(UVAG) model is established, and finite element analysis(FEA) is carried out under the monitoring environment of embedded temperature measurement system. In addition, the related temperature acquisition tests are set in accordance with requirements of the reference grating method. Finally, comparative analyses of the simulation and experimental results are performed, and it may be concluded that the reference grating method may be utilized to effectively solve the cross-sensitivity of embedded FBGs.

Application of Fractal Theory in Brick-Concrete Structural Health Monitoring

In order to monitor and forecast the deformation of the brick-concrete building, by taking a brick-concrete building as research object, fiber grating sensors were used to collect the monitoring data and double logarithmic curve of limit value characteristic and monitoring data were obtained based on the fractal theory. Constant dimension fractal method cannot be used to analyze the data directly. With the method of variable dimension fractal, we accumulate data, and the double logarithmic curve is smooth. Piecewise fractal dimensions are close. The outer interpolation method is used to calculate the fractal dimension of the next point and then back calculate the vertical displacement. The relative errors are calculated by comparing the forecast values and monitoring values, and the maximum relative error is 5.76%. The result shows that the fractal theory is suitable to use in the forecast of the deformation and the accuracy is good.

Application of Tube-Packaged FBG Strain Sensor in Vibration Experi ment of Submarine Pipeline Model

Optical fiber sensors have received increasing attention in the fields of aeronautic and civil engineering for their superior ability to stand explosion, immunity to electromagnetic interference and high accuracy, especially fit for measureroent applications in harsh environment. In this study, a novel FBG （fiber Bragg grating） strain sensor, which is packaged in a 1.2 mm stainless steel tube with epoxy resin, is developed. Experiments are conducted on the universal material testing machine to calibrate its strain transferring characteristics. The sensor has the advantages of small size, high precision and flexible use, and exhibits promising potentials. Five tube-packaged strain FBG sensors have been applied to the vibration experiment of a submarine pipeline model. The strain measttred with the FBG sensor agrees well with that measttred with the electric resistance strain sensor.

Field measurement of strain response for typical asphalt pavement

In order to reveal the changing law of the mechanical response of asphalt pavements under the action of vehicle load and provide references for the design of durable pavements,three typical asphalt pavement structures with flexible base(S1),combined base(S2),and semi-rigid base(S3)were selected to perform field strain tests under static and dynamic load using the fiber Bragg grating optical sensing technology.The changing characteristics of the strain field along the horizontal and depth directions of pavements were analyzed.The results indicate that the most unfavorable asphalt pavement layers were the upper-middle surface layer and the lower base layer.In addition,the most unfavorable loading positions on the surface layer and the base layer were the center of wheel load and the gap center between two wheels,respectively.The most unfavorable layer of the surface layers gradually moved from the lower layer to the upper layer with the increase of base layer modulus.The power function relationships between structural layer strain and vehicle speed were revealed.The semi-rigid base asphalt pavement was the most durable pavement type,since its strain value was lower compared to those of the other structures.

Influence of differential settlement on pavement structure of widened roads based on large-scale model test

This study introduced at first the background of numerous highway widening projects that have been developed in recent years in China.Using a large ground settlement simulator and a fiber Bragg grating (FBG) strain sensor network system,a large-scale model test,with a similarity ratio of 1:2,was performed to analyze the influence of differential settlement between new and old subgrades on pavement structure under loading condition.The result shows that excessive differential settlement can cause considerable tensile strain in the pavement structure of a widened road,for which a maximum value (S) of 6 cm is recommended.Under the repetitive load,the top layers of pavement structure are subjected to the alternate action of tensile and compressive strains,which would eventually lead to a fatigue failure of the pavement.However,application of geogrid to the splice between the new and the old roads can reduce differential settlement to a limited extent.The new subgrade of a widened road is vulnerable to the influence of dynamic load transferred from the above pavement structures.While for the old subgrade,due to its comparatively high stiffness,it can well spread the load on the pavement statically or dynamically.The test also shows that application of geogrid can effectively prevent or defer the failure of pavement structure.With geogrid,the modulus of resilience of the subgrade is increased and inhomogeneous deformation can be reduced;therefore,the stress/strain distribution in pavement structure under loading condition becomes uniform.The results obtained in this context are expected to provide a helpful reference for structural design and maintenance strategy for future highway widening projects.

Fiber Bragg grating pressure sensor with enhanced sensitivity

A novel fiber Bragg grating （FBG） pressure sensor with the enhanced sensitivity has been demonstrated. A piston-like diaphragm with a hard core in the center is used to enhance the sensitivity. Both the theoretical analysis and the experimental result show that the radius of the hard core has significant effect on the pressure sensitivity. When the radius of the hard core is 1.5 mm, a pressure sensitivity of 7.23 nm/MPa has been achieved.

Hybrid fiber Bragg grating sensor for vibration and temperature monitoring of a train bearing

We report a fiber Bragg grating（FBG）-based sensor for the simultaneous measurement of a train bearing’s vibration and temperature. A pre-stretched optical fiber with an FBG and a mass is designed for axial vibration sensing. Another multiplexed FBG is embedded in a selected copper-based alloy with a high thermal expansion to detect temperature. Experiments show that the sensor possesses a high resonant frequency of 970 Hz, an acceleration sensitivity of 27.28 pm/g, and a high temperature sensitivity of 35.165 pm/℃. A resonant excitation test is also carried out that demonstrates the robustness and reliability of the sensor.

A Novel Method of Edge Filter Linear Demodulation Using Long Period Grating in Fiber Sensor System

A novel method of linear demodulation based on edge filter is presented. An experimental system is built up in which LPG is used as the edge filter. We achieve linear demodulation with a bandwidth of 5nm.