Wavelength Shift Of Fiber Bragg Grating Research Articles

Low velocity impact localization system of CFRP using fiber Bragg grating sensors

Carbon fiber reinforced plastics (CFRP) structures are very susceptible to barely visible impact damage induced by low velocity impact. Thus knowledge of the position of an impact event on a CFRP structures can be useful information for detecting this type of damage. In this paper, low velocity impact localization system using fiber Bragg grating (FBG) sensors and localization algorithm for CFRP structures were investigated. The FBG wavelength shifts, caused by impact, were captured by a high speed FBG interrogation system. Wavelet transform was used to weaken signal noise. After that, the arrival time differences of each leading wave were calculated to produce the input data sets for least squares support vector machines (LS-SVM) model training. Finally, FBG impact localization system was constructed and validated on a CFRP plate with 300mm*300mm*2mm experiment area. The experimental results showed that the impact position abscissa and ordinate localization errors were all less than 5mm, and the maximum and average impact localization errors were 5.8mm and 3.7mm, respectively. This paper provided a reliable method for CFRP structures impact localization.

Experimental Study and Analysis of a Polymer Fiber Bragg Grating Embedded in a Composite Material

The characteristics of polymer fiber Bragg gratings (FBGs) embedded in composite materials are studied in this paper and are compared with characteristics of their silica counterparts. A polymer FBG of 10 mm length which exhibits a peak reflected wavelength circa 1530 nm is fabricated and characterized for this purpose. A silica FBG with a peak reflected wavelength circa 1553 nm is also embedded in the composite material for a comparison study. The fabricated composite material sample with embedded sensors is subjected to temperature and strain changes and the corresponding effects on the embedded polymer and silica FBGs are studied. The measured temperature sensitivity of the embedded polymer FBG was close to that of the same polymer FBG in free space, while the silica FBG shows elevated temperature sensitivity after embedding. With an increase in temperature, spectral broadening was observed for the embedded polymer FBG due to the stress induced by the thermal expansion of the composite material. From the observed wavelength shift and spectral bandwidth change of the polymer FBG, temperature and thermal expansion effects in the composite material can be measured simultaneously.

基于光纤Bragg光栅的量测锚头压力研究

The force of the anchor head has a significant impact on the safety of the anchor rod.In order to reflect the force status of the anchor head on-line,the two optical fiber Bragg grating along 45°are pasted on the surface at the midpoint of the two symmetry spokes,forming the spoke-and-wheel structure.The stress of anchor causes the shear deformation of spokes,and the deformation leads to the wavelength-shift of fiber Bragg grating.By measuring the center wavelength-shift of fiber Bragg grating,axial force at the anchor head on-line is monitored.The press experiment shows that the strain sensitivity of grating 1is 108.8pm/kN,grating 2is 115pm/kN;Linearity of grating 1is 0.042% FS,grating2 is 0.023% FS.

基于互谱相关的光纤光栅波长位移计测方法

In order to improve the accuracy of wavelength shift measurement which is limited by the scanning wavelength step in the traditional measurement,a method which combinates a coarse measurement and a fine one was present based on the cross-spectrum detection and cross-correlation technology.The coarse wavelength shift measurement which is based on the principle of cross-correlation detection has achieved a wide range of measurement,and the fine wavelength shift mesurement which is based on the principle of cross-spectrum detection has achieved a high-precision measurement.Both the range and the precision are improved with the combination of the coarse and the fine measrurement.The smoothing tecnlogy is applied for data processing,which has eliminated the influence of random noise in the detection system.The fiber grating wavelength shift is measured directly to reduce the system error which has avoided the measurement of the absolute values of FBG center wavelength.Simulation results show that the method breaks the limit of the scanning wavelength step,and the measurement accuracy is improved about three orders.FBG longitudinal load measuring experiment with the load range of 0~60g was carried out and the new method was applied to process the experiment data,the results show that the FBG longitudinal load sensitivy is 13.2pm/g,the new method has a better applicability.

FBG Earth Pressure Sensors Applied in Surrounding Rock Pressure of Tunnel

During the period of tunnel excavation, shoring, forming and long-term operation, stress changes of tunnel surrounding rock are complex, the real-time monitoring of surrounding rock pressure is the key factor in ensuring long-term stability in tunnel. Fiber Bragg grating earth pressure sensors apply in surrounding rock pressure of tunnel which can change the pressure of the surrounding rock into fiber Bragg grating wavelength shift. According to the feature of pressure and temperature in Tian Xin Tunnel, 40 earth pressure sensors are embedded in 20 representative sections and one earth pressure sensor is embedded in each arch shoulder. In addition, one temperature compensation sensor is embedded in each arch crown. During the 235 monitoring days, the biggest daily change of surrounding rock pressure reaches 800 KPa. In 3 months of the sensor installation, the average monthly variation is within 50 KPa. The long-term measurement results indicate that the changes of surrounding rock pressure are different in different locations. When the surrounding rock is close to the excavated and blasted surface the surrounding rock pressure changes largely.

Determination of Wavelength Shift of Fiber Bragg Grating Sensors by Tunable Single-mode Diode Laser

We propose a novel principle of determination of fiber Bragg grating (FBG) wavelength shift which is impacted by a variation of physical parameters such as temperature, pressure and/or strain. In common case the wavelength shift of FBG was monitored by wavelength measurement using a high-cost spectrometer and a broad band light source. In our proposed technique the wavelength shift of FBG can be determined by change of lasing wavelength of distributed feedback laser (DFB-laser) due to the change of laser substrate temperature. The maximal opto-electrical intensity of photodetector would be obtained when the laser wavelength and FBG reflection wavelength are coincided. The FBG sensor prototype has shown excellent response for laser temperature change in the range of \(10^{\circ}C-50^{\circ}C\) with the ratio \(\Delta \lambda /\Delta T\) of the DFB laser is of 77.5 pm.K\(^{ - 1}\). Key features of the proposed technique are fabrication of low-cost FBG sensors for civil engineering.

Theory, experiment, and application of optical fiber etching

Based on fiber Bragg grating (FBG), an online monitoring system for the etching process of optical fiber in a hydrofluoric (HF) acid solution has been designed. The variation curves of the wavelength shifts of FBGs with etching time at three different temperatures have been obtained and analyzed theoretically. The results show that the etching process of optical fiber in HF acid solution can be understood by the variation of the wavelength shift of FBG with etching time. Finally, required tapered fiber tips can be made by controlling the etching velocity and the pulling velocity of optical fiber from the etching solution.

Real-time impact identification algorithm for composite structures using fiber Bragg grating sensors

SUMMARY In composite structures, low-velocity impact-induced damage such as delamination are mostly hidden inside laminates or leave a small dent on the impact side. Thus, detecting this type of damage using conventional inspection methods is not easy and requires much time and cost. To enhance efficiency of these methods, information on the estimated impact locations must be provided with a high accuracy. In this way, unnecessary inspections for large intact regions can be reduced. In this study, impact localization algorithms for various composite structures were developed using the impact induced acoustic signals acquired by multiplexed fiber Bragg grating (FBG) sensors. The acoustic waves from a given impact were transmitted to each FBG sensor, and incurred FBG wavelength shifts were captured by a high speed multiplexible FBG interrogation system with a sampling frequency of 100 kHz. After acquisition of the FBG sensor signals at all the training points in the target section, the impact wave arrival time differences between each FBG signal were calculated to produce the input data sets for neural network training. To reliably use the neural network algorithm for impact identification, high reproducible arrival time determination algorithms are essentially required. In this study, such arrival time determination algorithms were developed through various types of structures. Finally, we evaluated the performances of the suggested impact identification algorithms for a composite wing box structure. Copyright © 2012 John Wiley & Sons, Ltd.

Application of Fiber Bragg Grating Level Sensor and Fabry-Pérot Pressure Sensor to Simultaneous Measurement of Liquid Level and Specific Gravity

This study is proposed for obtaining simultaneous measurements of the level and specific gravity of a liquid using a dual-optical-fiber-sensor system comprising a fiber Bragg grating (FBG) level sensor and a Fabry-Perot (FP) pressure sensor. In the FBG level sensor, the liquid level is derived from the FBG wavelength shift induced when the buoyancy is applied on a cantilever beam. Meanwhile, in the FP sensor, the pressure is calculated from the change in cavity length which takes place when the sensor is immersed. The experimental results show that the FBG level and FP pressure sensors have sensitivities of 0.01491 nm/cm and 0.1569 μm/kPa, respectively. Analytical formulae are derived for the level and specific gravity of the liquid in terms of the Bragg wavelength shift, the change in cavity length, and the vertical separation distance between the two sensors. The average measurement errors of the liquid level and specific gravity are found to be 0.0323 and 0.0528 m, respectively.

Application of Fabry–Pérot and fiber Bragg grating pressure sensors to simultaneous measurement of liquid level and specific gravity

An approach is proposed for obtaining simultaneous measurements of the level and specific gravity of a liquid using a dual-pressure-sensor system comprising a fiber Bragg grating (FBG) pressure sensor and a Fabry–Pérot (FP) pressure sensor. In the FBG sensor, the pressure is derived from the FBG wavelength shift induced when the sensor is immersed in the liquid. Meanwhile, in the FP sensor, the pressure is calculated from the change in cavity length which takes place when the sensor is immersed. The advantageous concept of the dual-pressure-sensor system is atmospheric pressure compensation. The experimental results show that the FBG and FP pressure sensors have sensitivities of 0.1495nm/kPa and 0.1569μm/kPa, respectively. Analytical formulae are derived for the level and specific gravity of the liquid in terms of the FBG wavelength shift, the change in cavity length, and the vertical separation distance between the two sensors.

带双FBG的高功率非致冷980 nm半导体激光器的波长锁定研究

An optimized dual fiber Bragg grating (FBG) is proposed for 980-nm semiconductor lasers without thermoelectric coolers to restrict temperature-induced wavelength shift. The mathematical model of the temperature-induced wavelength shift of the laser with the dual FBG is built using the external cavity feedback rate equations. The external cavity parameters are optimized for achieving the stability mode-locking laser output. The spectral characteristics of the dual FBG stabilized laser are measured to range from 0 to 70 oC. The side mode suppression ratio (SMSR) is more than 45 dB, while the full-width at half-maximum (FWHM) is less than 1 nm. The peak wavelength shift is less than 0.1 nm. The dual FBG wavelength shift proportional coefficient is between 0.1086 and 0.4342.

Vibration Monitoring of Wind Turbine Blade using Fiber Bragg Grating

Rapid growth in generating power from wind turbines led to application of long laminated composite blades. However, as the length of blades increases, the risk of failure due to excessive vibration increases. Therefore, a reliable wind power generator requires an efficient and accurate, yet economical structural health monitoring system to detect vibration and apply a reliable control to prevent from unpredicted failure. This paper presents an optimal, low cost and continuous vibration monitoring system for laminated composite blades using Fiber Bragg Grating (FBG). Utilizing a layerwise displacement theory, the Finite Element (FEM) model has been developed for the wind turbine blade. The optomechanical relations between strains and FBG wavelength shift have been presented to study the dynamic of a wind turbine blade. The reflected spectrum caused by modal displacement, interrogated by WDM method has been studied for vibration control. Numerical examples have been presented to compare the FEM and FBG results which show a very good agreement for the first 3 vibration modes.

Serially multiplexed FBG accelerometer for structural health monitoring of bridges

This article describes the development of a fiber optic accelerometer based on Fiber Bragg Gratings (FBG). The accelerometer utilizes the stiffness of the optical fiber and a lumped mass in the design. Acceleration is measured by the FBG in response to the vibration of the fiber optic mass system. The wavelength shift of FBG is proportional to the change in acceleration, and the gauge factor pertains to the shift in wavelength as a function of acceleration. Low frequency version of the accelerometer was developed for applications in monitoring bridges. The accelerometer was first evaluated in laboratory settings and then employed in a demonstration project for condition assessment of a bridge. Laboratory experiments involved evaluation of the sensitivity and resolution of measurements under a series of low frequency low amplitude conditions. The main feature of this accelerometer is single channel multiplexing capability rendering the system highly practical for application in condition assessment of bridges. This feature of the accelerometer was evaluated by using the system during ambient vibration tests of a bridge. The Frequency Domain Decomposition method was employed to identify the mode shapes and natural frequencies of the bridge. Results were compared with the data acquired from the conventional accelerometers.

靶式光纤布拉格光栅流量传感器的设计及实验

The target type flowmeter based on fiber Bragg gratings (FBGs) is experimentally studied. The relationship between the central wavelength shift of FBG and the flux is derived and the analytic expression is also given. Simulation and preliminary experiments have been carried out, and experimental validation of the water further proves the feasibility of the sensor. The experimental results verify the proposed sensor which can measure flux range from 200 to 1200 cm3/s. And on this basis, the improvement program is raised.

Strain dependence of fiber Bragg grating sensors at low temperature

The strain and temperature response of fiber Bragg grating (FBG) sensors has been measured from 123 to 317 K. The wavelength shift of FBGs is found to be linear with applied tensile strain at a constant temperature. On the other hand, their strain sensitivity is found to increase linearly with decreasing temperature. The nonlinear temperature dependence of FBG sensors is confirmed.

Novel current measurement method based on fiber Bragg grating sensor technology

Experimental setup for current measurement and a novel signal detection method for demodulation of fiber Bragg grating wavelength shift are developed. Being attached respectively on the upper and lower surface of a pendulum-type cantilever element, only a couple of matched FBGs are used for both current sensing and wavelength shift signal demodulating. So, the received light power will change due to the split of the two reflected spectrum of FBGs’, which is corresponding to the electromagnetic force caused by the measured current. In addition, cross-sensitivity effect of FBG-based sensor automatically solved due to a differential signal process method. Static and quasistatic measurement results indicate the feasibility of the developed sensor system. Measurement error of ±1 mA is obtained for most measurement data in the whole range from 0 to 400 mA based on the present experiment setup.

A novel demodulation scheme for fiber Bragg grating sensor system

A novel fiber Bragg grating (FBG) wavelength demodulation scheme based on a compound interference configuration is introduced. This scheme measures the strain-induced wavelength-shift of the FBG in terms of phase-shift signal. The sensing resolution and sensitivity were demonstrated to be 6.2 n/spl epsiv/ and 1.59/spl deg///spl mu//spl epsiv/, respectively, which match the theoretical value of 1.61/spl deg///spl mu//spl epsiv/ very well.

Temperature-compensated high pressure FBG sensor with a bulk-modulus and self-demodulation method

A free active element bulk-modulus based fiber Bragg grating (FBG) high pressure sensor and self-demodulation method for FBG wavelength shift signals are presented. With a simple isosceles triangle cantilever structure, problems of cross-sensitivity and chirped-signal in FBG sensor and are solved. In addition, unlike other FBG-based sensors which have separated sensing and demodulating element, this FBG-based high pressure sensor has only a single structure unit for both sensing and demodulating. Measurement principle and theoretical analysis are introduced. Simulation results and preliminary experimental results indicate that the measurement resolution of pressure can reach 0.1 MPa in a range from standard atmosphere pressure to 100 MPa.

Fiber Bragg grating strain sensor demodulator using a chirped fiber grating

A fiber Bragg grating (FBG) sensor demodulator using a chirped fiber grating is proposed. The demodulator uses UV-induced birefringence of chirped fiber grating to interrogate the wavelength shift of a sensor FBG. The demodulator is composed of a polarizing beam splitter, a polarization controller, a single-mode fiber, and a chirped fiber grating. The proposed demodulator is immune to light power fluctuation and is cost-effective.

Simultaneous displacement and temperature measurement with cantilever-based fiber Bragg grating sensor

Fiber Bragg grating (FBG) is demonstrated to measure displacement, which relies on attaching the FBG on a cantilever structure, so that the vertical displacement of the cantilever's free end can be obtained by measuring the wavelength shift of FBG. A novel technique is then presented to measure displacement and temperature simultaneously with a single FBG. The measured displacement and temperature sensitivities of this cantilever-based FBG sensor are 8.22×10 −4 mm −1 and 8.86×10 −5 (°C −1), respectively. And the measurement range can be up to 10.5 mm.