Fiber Bragg grating cryogenic temperature sensors

Temperature sensing to as low as 80 K was demonstrated with 1.55-μm fiber Bragg gratings. The gratings were bonded on substrates to increase sensitivity, and a shift of the reflection wavelength was measured. The temperature sensitivity was 0.02 nm/K at 100 K when an aluminum substrate was used and 0.04 nm/K at 100 K when a poly(methyl methacrylate) substrate was used. These values are smaller than those at room temperature because of the nonlinearity of both the thermal expansion and the thermo-optic effect. Extension to the liquid helium temperature is also discussed.     

人体测温光纤光栅温度传感器的研制

针对医疗行业对人体测温的需要,研制了一种以裸光纤Bragg光栅为传感元件的光纤光栅温度传感器,并对其温度特性进行了研究.采用恒温水浴对工作波长为1 557 nm和1 547 nm附近的2只光纤光栅传感器进行温度定标.结果表明,在34～48℃范围内,光纤光栅的中心波长随温度的变化呈现良好的线性关系,相关系数分别为0.998 3和0.997 4,光纤光栅温度传感器的测温标准偏差分别为0.239℃和0.245℃,可应用于医疗中人体温度的实时监测.     

Design,fabrication, and testing of fiber Bragg grating sensors for cryogenic long-range displacement measurement

It is essential to measure the shrinkage/expansion and positioning/aligning of magnets and to control valve displacement which plays a vital role in experiments like the Karlsruhe tritium neutrino experiment beam tube and Cryo pumps. Hence, a displacement sensor which, over a long working range, can be operated under extreme environmental conditions needs to be developed. Fiber Bragg gratings (FBG) have been considered to be excellent sensor elements useful for a variety of applications. This paper will discuss a long range displacement sensors based on fiber Bragg gratings for cryogenic temperature applications. The cryo pump inlet valve control requirements have been taken as example specifications for sensor design. To achieve the development goal, a proper signal transducer and sensor package were designed. A study of the strain transmission of surface-bonded FBG was conducted. The influence of bonding thickness and bonding length was reported. The design, fabrication, and performance were tested at low temperature of around 77 K. The sensor performance was found to be satisfactory at both room temperature and 77 K and linearly for long-range displacement of 550 mm with 14 pm/mm sensitivity and 0.142 mm accuracy.     

Vernier-effect optical interrogation technique for fiber Bragg grating sensors

This work demonstrates a new technique for real-time optical sensor interrogation by exploiting a novel Vernier effect between the multiple (comb) wavelength responses of a multiplexed fiber Bragg grating array and the fixed discrete wavelengths of an all-solid-state tunable laser. Sets of output photodetector voltages serve as high-resolution optical "signatures" to determine uniquely the strain in the single fiber section. The sensor demonstrated here is compact, lightweight, and is specifically intended for remote operability in harsh (vibrational) environments. In this proof of concept, strain values over a range of nearly 500 microepsilon can be easily resolved to better than 5.9 microepsilon, which is the incremental limit of the mechanical test fixture used to induce strain in the experiment.     

Effects of metal coating on the fiber Bragg grating temperature sensing characteristics

Effects of the metal coatings on the fiber Bragg grating (FBG) temperature sensing characteristics were simulated and experimentally investigated. Temperature sensitivity of the coated FBG as a function of the coating material properties was simulated using MATLAB software based on the temperature sensing model. The simulation results show that the elastic modulus, Poisson’s ratio, thermal expansion coefficient and coating thickness of the coatings have noticeable impacts on the sensing characteristics of the coated FBG. It is noted that (i) there is an extreme value for the influence of the elastic modulus on temperature sensitivity; (ii) temperature sensitivity increases with the increase of Poisson’s ratio and thermal expansion coefficient; (iii) with the increase of the coating thickness, the temperature sensitivity increases first then reaches a plateau asymptotically. To validate the simulation results, several FBGs were coated with copper, nickel, cobalt, copper–zinc, and Ni–ZrO₂, and subsequently the associated temperature sensitivities were measured using a network analyzer as well as a water bath. The results show that the simulation results agree well with that of the experiments, with the errors up to 5.49%.     

State-of-strain evaluation with fiber Bragg grating rosettes: application to discrimination between strain and temperature effects in fiber sensors

An optical rosette that incorporates fiber Bragg gratings as strain gauges has been designed, fabricated, and tested. We investigated it by measuring the state of strain of a thin plate as the test structure submitted to an increasing load in a four-point bending configuration and for various angular orientations. This device has also been successfully investigated as a self-temperature-compensated in situ uniaxial strain sensor without any angular dependence and with high accuracy in recovery analysis, leading us to expect many industrial applications. Printed circuit processes or integrated optics on polymers would provide a means for accuracy, reproducibility, and integration in a mass-produced process.     

Instrumentation design for bridge scour monitoring using fiber Bragg grating sensors

Scour is one of the main causes of bridge failures. In order to measure and monitor scour depth variations including deposition (refilling) process, three designs for a scour monitoring system using fiber Bragg grating (FBG) sensors are discussed in the present study. By a comparative study, one of them is recommended in the present study and its instrumentation manufacture process is also introduced in detail. Using this recommended design, the advantages of FBG sensors for monitoring, such as immunity from electromagnetic interference and multiplexing capability, can be fully utilized. Both scour depth variations and entire scour development process including deposition process can be correctly monitored in real-time by continuously identifying the locations of emerging FBG sensors from the riverbed. A reliable sensor protection measure is also designed for FBG sensors in harsh environments, especially in floods. Finally, a verification test using a flume is carried out in the laboratory and three experimental cases are conducted to demonstrate the capability of FBG sensors and applicability of the recommended scour monitoring system. It can be concluded that the recommended scour monitoring system using FBG sensors is capable of measuring the water level, (maximum) scour depth, entire process of scour development, and deposition height due to refilling process. The advantages over other conventional scour monitoring systems are clearly demonstrated.     

毛细铜管封装的内嵌式镀金光纤布拉格光栅温度和应力传感器

为了实现复杂、恶劣环境下工程机械表面无损的应力监测方式,实现对大型工程机械的实时动态监测,提出了基于磁控溅射技术的光纤布拉格光栅(FBG)应力传感器封装方法。并对完全嵌套(整个栅区嵌套毛细铜管)和两端嵌套(栅区两端嵌套毛细铜管)两种封装方法开展了研究。从理论分析和有限元仿真的角度比较了传感器的增敏效果,前后结果一致。制备了传感器实物并进行了温度、应力和对比实验。仿真实验结果表明,该模型下FBG传感器能提高约7.5%的灵敏度。温度实验表明第二种封装结构的温度反馈相关系数R2达到了0.99948,在30℃～80℃范围内呈现良好的线性度;应力实验的相关系数R2也达到0.99924,灵敏度为6.14 pm/MPa,在该实验搭建的解调系统下精度达到0.05 MPa,可以快速、精确地解调应力。对比实验表明,光栅解调仪组成的监测系统比应变片组成的监测系统具有更高的精度,最大偏差值减小了59.8%。嵌套毛细铜管的金属化方式结合有机胶固定的封装结构简单、灵敏度和精度高,可以满足大型工程机械表面无损实时健康监测的需求。     

Mechanical characterization of bone cement using fiber Bragg grating sensors

The aim of this work was the study and understanding of the behavior and linearity of an optical fiber Bragg grating (FBG) sensor embedded in bone cement. Test its ability to monitor strains inside bone cement during different mechanical tests, at real-time. Bone cement is a biomaterials based on polymethacrylate used as fixation method in artificial joints. Work as a bonding, load transfer and optimal stress/strain distribution inside the complex human body environment. Bone cement is the weakest element in a joint implant, being considered the main reason of prosthesis loosening.Inside the bone cement, its temperature, longitudinal strain and load were measured using fiber Bragg gratings. All the measurements report a linear response showing a good adaptation and optimization of the load transfer between the biomaterial and the embedded optical sensor.     

Quadratic behavior of fiber Bragg grating temperature coefficients

We describe the characterization of the temperature and strain responses of fiber Bragg grating sensors by use of an interferometric interrogation technique to provide an absolute measurement of the grating wavelength. The fiber Bragg grating temperature response was found to be nonlinear over the temperature range -70 degrees C to 80 degrees C. The nonlinearity was observed to be a quadratic function of temperature, arising from the linear dependence on temperature of the thermo-optic coefficient of silica glass over this range, and is in good agreement with a theoretical model.     

利用WDM光纤耦合器的光纤光栅传感解调技术

根据 WDM 光纤耦合器波长解调方案的工作原理、偏振特性以及影响系统波长分辨力的因素,提出一种改进的利用 WDM 光纤耦合器的光纤光栅传感解调技术。该技术在原技术的基础上,采用偏振控制器控制入射光偏振状态,提高了解调的精度和稳定性。对 WDM 光纤耦合器的多次波长扫描结果表明,采用偏振控制器后,其波长误差可减小到 5pm 左右。实验采用 1540/1560nm的 WDM 光纤耦合器对单点光纤光栅应变传感器进行静态解调,结果表明:按此技术开发的解调系统具有 0.01nm 波长分辨力和 10nm 的波长线性解调范围。     

Analysis of the reflective-matched fiber Bragg grating sensing interrogation scheme

A technique for the demodulation of fiber Bragg grating (FBG) sensors based on the use of a second wavelength-matched FBG receiver to track wavelength shifts from the FBG sensor is analyzed, particularly regarding its sensitivity as determined by primary noise sources. Numerical and experimental results show that there is an optimum Bragg wavelength difference between the two FBG's that maximizes the sensitivity for this demodulation technique.     

光纤光栅传感技术在桥梁试桩中的应用

基于对光纤布拉格光栅(FBG)传感特性的理论分析,研制了可用于桥梁试桩应变监测的光纤光栅钢筋应变计,并进行了标定试验;研究了光纤光栅传感器的布设工艺,在某特大跨海大桥试桩项目中的钻孔灌注桩里成功地埋入了52支光纤光栅钢筋应变计,进行了荷载试验.研究结果表明,在桥梁试桩项目中利用光纤光栅传感技术可以方便有效地对试桩的应变进行多点实时监测,为桥梁桩基础的优化设计提供了可靠的依据.     

Application of chirped fiber Bragg grating sensors for identification of crack locations in composites

Chirped fiber Bragg grating (FBG) sensors were applied for the identification of crack locations in carbon fiber reinforced plastic (CFRP) laminates since the reflection spectrum from a chirped FBG was expressed as a function of the position along the grating. For the crack identification in the 90° ply of a cross-ply laminate, a chirped FBG sensor, whose grating length and spectrum width were 50 mm and about 5 nm respectively, was embedded into the 0° ply of the CFRP cross-ply laminate, and the reflection spectra were measured after tensile loadings were applied to the laminate. As a result, the spectra had dips corresponding to locations of transverse cracks. This change in the form of the spectrum was also confirmed by a theoretical calculation. Hence, the crack locations could be identified from the form of the spectrum. Furthermore, it was demonstrated that a small-diameter chirped FBG sensor could also be applied for the identification of the crack locations.     

Performance analysis of a fiber Bragg grating filter-based strain/temperature sensing system based on a modified Gaussian function approximation method

This paper analyzes the performance of a fiber Bragg grating (FBG) filter-based strain and/or temperature sensing system based on a modified Gaussian function (MGF) approximation method. Instead of using a conventional Gaussian function, we propose the MGF, which can capture the characteristics of the sidelobes of the reflected spectrum, to model the FBG sensor and filter. We experimentally demonstrate that, by considering the contributions of the sidelobes with the MGF approximation method, behaviors of the FBG filter-based FBG displacement and/or temperature sensing system can be predicted more accurately. The predicted behaviors include the saturation, the sensitivity, the sensing range, and the optimal initial Bragg wavelengths of the FBG sensing system.     

Method for determination of crack bridging parameters using long optical fiber Bragg grating sensors

The state of the local fiber–matrix interface highly influences the propagation of cracks in fiber-reinforced composites and thus the stress distribution in any bridging fiber. This paper demonstrates that by embedding a long optical fiber Bragg grating into a reinforcing fiber and using an established model of the grating response to non-uniform stress distributions, one can determine key parameters of a crack bridging model. The grating extending into the epoxy on each side of the crack is subject to a strain function as a result of all micro-mechanical phenomena acting along the fiber. Furthermore, this technique does not require that one knows a priori the exact location of the crack. Two types of central crack specimens with an artificial crack were fabricated and tested, one with a strong interface and one with a weaker interface resulting in frictional sliding. The results demonstrate that this technique is efficient for the measurement of the bridging forces through validation by previous measurements using short Bragg gratings and the deduction of interface parameters. Analysis also shows that the sensitivity of the Bragg grating sensor to the bridging force is sufficient, even for the more realistic case of an initially zero-width crack e.g. grown by fatigue.     

基于内埋光纤Bragg光栅传感器的复合材料固化过程监测

为了全面了解复合材料的固化特性,在对碳纤维增强树脂基复合材料固化变形进行数值仿真分析的基础上,将自行设计的光纤Bragg光栅（FBG）传感器埋入复合材料中,实时在线监测复合材料固化过程中温度和应变的演变。预浸料铺层方式为[0₁₁/90₁₁],分别在层合板0°和45°方向的典型位置埋入FBG温度和应变传感器,采用热模压方式固化成型复合材料层合板,并对成型后的层合板进行连续2次降温处理,实时记录固化过程中FBG传感器中心波长的变化。结果表明:在相同的温度条件下,复合材料在第1次降温初始阶段的压应变绝对值明显小于在第2次降温初始阶段的压应变绝对值,表明复合材料在第1次降温过程中仍在进行FBG传感器可检的“后固化”反应;此外,层合板变形的FBG传感器监测数据与有限元模拟结果吻合良好。因此,采用内埋FBG传感器的方法能够实时监测复合材料固化过程,为更全面地分析复合材料固化特性提供了一种可靠有效的方法。      

Delamination detection in CFRP laminates with embedded small-diameter fiber Bragg grating sensors

Newly developed small-diameter fiber Bragg grating (FBG) sensors were applied for the detection of the delamination in carbon fiber reinforced plastic (CFRP) cross-ply laminates. Since the diameter of the FBG sensor was very small, the sensor was easily embedded into the 0° ply along the reinforcing carbon fibers. The reflection spectra from the FBG sensor were measured at various delamination lengths through four-point bending test. The form of the spectrum changed sensitively, as the delamination length increased. Moreover, the spectrum was calculated theoretically for confirmation of the measured spectrum. The calculated result reproduced the change in the measured spectrum very well. Then, we proposed the intensity ratio in the spectrum as an effective indicator for the prediction of the delamination length. This indicator has a potential to be applied to other laminate configurations.     

基于光纤光栅传感器的结构故障定位方法

提出了一种实现结构故障定位和故障实时监测的新方法.该方法利用光纤光栅传感网络测量结构应变,得到结构的挠度矩阵,分析故障前后挠度矩阵变化,计算得出故障定位向量,进而确定损伤区域,实现了结构故障准确定位和健康状况实时监测.实验结果表明,上述方法大大提高了故障定位的准确度和实时性.