Fiber Bragg Grating Sensor System With Two-Level Ring Architecture

This investigation proposes a fiber Bragg grating (FBG) sensor system with a two-level ring architecture. The survivability and capacity of a FBG for a multipoint sensor system are enhanced by adding remote nodes and optical switches in the two-level ring architecture. Additionally, to enhance the signal-to-noise ratio (SNR) of the sensor system, a fiber ring laser approach is utilized to construct the proposed two-level ring architecture. The fiber ring laser adopted herein yields the high SNR of the sensor system. The proposed system can increase the reliability of FBG sensor systems for multipoint smart structures.      

Absolute strain measurements made with fiber bragg grating sensors

A strain sensor system based on optical fiber Bragg gratings (FBGs) is proposed with a new matched-filter design. The strain variation on the sensor FBG is continuously followed and matched by a filter FBG by use of a feedback control loop that produces an identical strain condition on the filter FBG. The matched strain on the filter FBG is then determined from the resonance vibration of the fiber piece embedding the filter FBG. The implementation and the performance of the proposed system are described. It is demonstrated that the proposed system can distinguish strain variation on the sensor FBG with resolution of one microstrain.     

纳米坐标测量机的三维接触式测头机构

介绍了一种新型的用于纳米坐标测量机的三维微纳米接触触发式测头机构.本测头以灵敏度高、抗干扰性强的布拉格光纤光栅(FBG)为测量的敏感元件,根据FBG对轴向应变变化敏感的特点,开发了一套有效触发测量力小的柔性悬架机构,该机构为三悬丝-六边中心连接体的悬架结构,相间隔的3边延伸悬臂与3根布拉格光纤光栅相连,当测球发生预行程变化时,由测杆带动柔性悬架机构产生偏摆,从而带动3根FBG发生轴向的拉伸或压缩,进而产生传感信号的输出.由于测头结构复位性是衡量测球和工件分离后能否回到初始位置的标准,是测头其他各项指标的基础,因此结合激光干涉仪和精密微动平台,采用光学非接触干涉测量方法对该测头机构的实际复位性能进行了测量.结果表明,测头系统采用15 N的预紧力安装悬丝,可得到较好的复位性和灵敏度,该测头机构复位性精度在20 nm以内,满足微纳米量级高精度测量的需要.     

布拉格光栅监测不同厚度方向的固化残余应力

复合材料在固化过程中产生的残余应力会严重影响材料的使用和安全。首先在研究和测试了布拉格光栅温度和应力灵敏度的基础上,将3根光栅分别以平行于碳纤维的方向置入预浸料的上、中、下3个位置,以此获得复合材料不同厚度位置的残余应力变化情况。监测结果表明,上、下层光栅监测到的预浸料固化过程几乎完全一致,而在降温时下层的布拉格光栅残余应力则比上层的要大得多,中间层布拉格光栅在固化过程中测得的残余应力最小,但在降温过程中的热残余应力不易释放而导致最后的残余应力也较大。     

Comparison of nondestructive microfailure evaluation of fiber-optic Bragg grating and acoustic emission piezoelectric sensors using fragmentation test

Nondestructive evaluation of microfailure mechanisms in two-diameter SiC fibers/epoxy composites is investigated using a directly embedded fiber-optic sensor attached with an acoustic emission piezoelectric (AE-PZT) sensor. Interfacial shear strength by fragmentation test, and optical failure observation inside microcomposite can contribute to analyze two sensors quantitatively. Although fiber Bragg grating (FBG) sensor exhibits sudden wavelength shift due to plastic deformation by larger diameter SiC fiber breakage, AE-PZT monitors much more precise microfailure process, such as the fiber break or matrix cracking. Since the FBG sensor can measure the strain at only a single point, whether it can detect a fiber break in single-fiber composite specimen depends on its proximity to the failure location. In addition, micro-strain measurement at one single point may not provide enough information on the whole microfailure process including multiple fiber breakage and matrix crack. It can be considered that FBG sensor can be somewhat effective in measuring the continuous micro-strain change due to the internal disturbance such as resin curing, whereas AE-PZT sensor can be effective in detecting the microfailure by elastic wave propagation through the composite materials.     

High-Temperature Resistance Fiber Bragg Grating Temperature Sensor Fabrication

Fiber Bragg grating (FBG) temperature sensor and sensor arrays were applied widespread particularly in harsh environments. Although FBGs are often referring to permanent refractive index modulation in the fiber core, exposure to high-temperature environments usually results in the bleach of the refractive index modulation. The maximum temperature reported for the conventional FBG temperature sensor is around 600 degC due to its weak bonds of germanium and oxygen. In this paper, we report design and development of a novel high-temperature resistance FBG temperature sensor, based on the hydrogen-loaded germanium-doped FBG. The refractive index modulation in the FBG is induced by the molecular water. The results of our experiments have shown that the stability of the device is substantially increased at high temperature range. Due to the high bonds energy of hydroxyl and the low diffusivity of the molecular water, the thermal testing results of this temperature sensor show the thermal stability of hydrogen-loaded FBG can be increased by using annealing treatment; moreover, the highest erasing temperature for the device could reach to 1100 degC or more. The reflectivity of this new FBG depends on the concentration of Si-OH and indirectly related to the reflectivity of hydrogen-loaded FBG. Furthermore, the experimental results have provided a better understanding of the formation of the hydrogen-loaded FBGs and the chemical transfers at elevated temperatures in the fiber core     

Optische Schnittstellen

Optical interfaces: Viewing into the vacuum The requirements concerning in‐vacuum process diagnostics are more and more challenging. On the one hand, the setup itself has to be decoupled from the process to avoid any external impact on the process. On the other hand, the process parameters have to be in‐situ characterized and controlled by an in‐vacuum monitoring system. In the vicinity of these two contradictive requirements optical fiber applications gain more and more impact, since optical fibers cover several advantages: high transmission signal, inherent immunity against external electromagnetic forces, high durability and high mechanical as well as optical flexibility. The focus of the contribution is a new in‐vacuum optical temperature sensor combined with high‐temperature fiber optical feedthroughs. The sensor based on fiber Bragg gratings (FBG) in the optical fiber, which are extremely sensitive to temperature variations. In a first case study, the application of FBG with the feedthrough as an in‐vacuum optical temperature sensor is demonstrated. Thus an optical fiber containing a bunch of several FBG becomes a compact, robust, and flexible in‐vacuum network of local temperature sensors. Each individual sensor captures temperature variations over a signal path that is several km long with a remarkable accuracy up to 0.1 K and with speed of light.     

Pointwise fiber Bragg grating displacement sensor system for dynamic measurements

A method for setting up a fiber Bragg grating (FBG) sensor which can measure the pointwise, out-of-plane or in-plane dynamic displacement is proposed. The proposed FBG sensor is reusable. A multiplexing demodulation system based on a single long-period fiber grating is used in this study. The experimental results of the steady-state motion for a multilayer piezoelectric actuator and the dynamic response of a cantilever beam subjected to impact loadings are presented. These results indicate that the proposed displacement sensor has the ability to measure the out-of-plane dynamic displacement with high sensitivity. Measurements for a piezoceramic plate excited by high frequency show that the proposed displacement sensor also has the ability to provide the in-plane dynamic displacement up to 20 kHz.     

基于FPGA的多通道高速光纤光栅解调系统

研发了一种基于波长扫描光源和嵌入式解调的多通道高速光纤布拉格光栅解调系统。该系统采用半导体光放大器(SOA)结合可调谐F-P滤波器产生频率为1 kHz的稳定波长扫描光。通过使用对称三角波技术和快速FPGA解调算法,系统最终能够同时实现16个通道以及2 kHz的解调速率,测量范围80 nm。     

Damage detection in holed composite laminates using an embedded FBG sensor

This paper discusses damage detection in a holed CFRP laminate under static and cyclic loading using an embedded fiber Bragg grating (FBG) sensor. In order to detect the damage extension in the laminate, the change in the spectrum shape was measured using an embedded FBG sensor and was compared with that obtained by numerical simulation. The shape of the reflection spectrum did not change during the cyclic load test; however, it did change with increased strain in the static load test, due to damage around the hole. To clarify this difference, the polished surface of the cross section of the specimen was analyzed. Debonding was observed between the optical fiber and matrix during the cyclic load test. These results lead us to conclude that fatigue damage around a hole in a composite laminate may not be detected with an FBG sensor due to the debondings.     

复合材料胶层光纤布格光栅应变监测优化配置研究

航空航天用复合材料粘接胶层在制造和服役过程中不可避免会产生各种缺陷和损伤。针对其粘接胶层缺陷和损伤检测,基于光纤光栅应变传感原理,提出一种针对复合材料胶接层结构应变监测的光纤光栅传感网络优化配置的方法。通过仿真和实验研究了埋入复合材料构件胶接层的光纤布拉格光栅传感器在静载作用下的敏感区域分布特性,分析了载荷位移与布拉格波长漂移的关系,建立符合光纤布拉格光栅传感器实际特点的探测模型,采用粒子群优化算法进行光纤光栅传感网络的优化布置,研究结果表明光纤布拉格光栅传感器位置经优化后,覆盖率得到明显提高。     

光纤光栅传感器在CFRP冲击载荷监测中的应用研究

提出了一种用光纤光栅传感器监测CFRP结构冲击载荷的方法,构建了冲击监测系统。将FBG传感器粘贴于CFRP试件表面构成传感网络,针对不同冲击点,同一传感器监测的冲击响应信号进行分析,CFRP试件的一阶、二阶谐振频率为53 Hz、100 Hz,不同谐振频率对应的幅值随冲击点的改变而不同。冲击CFRP试件,对FBG传感网络感知的冲击响应信号进行小波包分解,获得各传感器的小波包能量谱图。结果表明传感器信号的第6阶小波包能量变化明显,能量大小与冲击点和传感器之间的距离及其夹角有关。研究结果将为CFRP结构的动态监测领域提供了实验参考。     

Fiber-Bragg-grating force sensor based on a wavelength-switching actively mode-locked erbium-doped fiber laser

A fiber-Bragg-grating (FBG) transverse-force sensor based on a wavelength-switching actively mode-locked erbium-doped fiber laser is proposed, in which a FBG is used as both the sensing element and the wavelength-selection element of the laser. When a force is applied to the FBG, the induced birefringence in the FBG causes the laser to emit pulses at two close wavelengths, whose separation is proportional to the applied force. To suppress the interference between the two wavelengths, the laser is made to emit at the two wavelengths alternately by use of a polarization-switching technique. The wavelength separation is converted into a time difference by transmission of the laser pulses through a dispersive single-mode fiber, so the wavelength measurement is replaced by the less-expensive time measurement. The output of the sensor is insensitive to temperature and axial strain changes along the FBG. To interrogate similar FBG sensing elements connected in series it is necessary only to change the modulating frequency of an electro-optic modulator to select the corresponding laser cavity. The practicability of this approach was demonstrated experimentally with two multiplexed sensing elements.     

基于长信号谱自相关的光纤光栅传感解调方法

针对光纤光栅传感领域波长高精度探测和传感复用光栅数量增多的需要,提出一种基于长信号相关谱的新型光纤光栅数字解调技术.该技术在可调谐滤波法的基础上,通过传感光栅与调制光栅反射谱卷积后的信号,即光电探测器的光强时间信号,进行自相关分析以实现对波长漂移的测量以及对传感光栅反射谱形状的识别,增强系统的复用能力,提高了性价比.模拟仿真表明,光纤光栅长信号相关数字解调方法可以准确测量光纤光栅波长的漂移,能更好实现传感复用.等强度悬臂梁实验验证该解调方法能实现光栅应变的高精度测量,优于传统的应变片测试.     

基于双模LPFG折射率不敏感双参量传感器

环境折射率和环境温度变化是影响光纤应变测量误差的主要因素.本文利用双模光纤纤芯双模式(LP01和LP11)支持特性设计了一款环境折射率不敏感的双模光纤(DMF)长周期光纤光栅LPFG)应变传感器.设计了传感器模型结构,制作了最优化参数的传感器样品.实验测试了DMF-LPFG传感结构对外部环境中应变、温度和折射率的响应....     

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.     

热残余应力对内埋光纤光栅传感器性能的影响

将布拉格光纤光栅(FBG)埋植于复合材料T型加筋板结构非干涉区—三角填充区作为应变传感器对复合材料加筋板在固化过程及冲击后压缩过程中的应变变化进行监测。对比了光纤刻栅区采用UV光固化树脂涂层保护和未保护的两种FBG传感器的波谱信号变化; 分析了复合材料在固化成型过程中产生的非轴对称热残余应力对FBG传感性能的影响。结果表明, 刻栅区采用聚合物涂层保护的FBG传感器的半峰宽(FWHM)在固化过程中未发生变化, 并且聚合物涂层可以有效地消除非轴对称热残余应力对光纤光栅反射波谱的影响。在冲击后压缩过程中, 采用聚合物涂层保护的FBG传感器测得的应变与贴于试样表面的应变片测得的应变数据一致性较好。本文对埋植于复合材料加筋板三角填充区的FBG传感器在复合材料固化过程及冲击后压缩过程中应变监测的有效性及可靠性进行了有益的探索。     

Fiber Bragg grating temperature sensor with controllable sensitivity

We demonstrate a fiber Bragg grating (FBG) sensor with controllable sensitivity by connecting two metal strips that have different temperature-expansion coefficients. By changing the lengths of the metal strips we successfully controlled and improved the temperature sensitivity to 3.3 times of that of bare FBG.     

Combined time- and wavelength-division-multiplexing demodulation technique of fiber grating sensor arrays using a tunable pulsed laser

Combined time- and wavelength-division-multiplexing demodulation technique of fiber Bragg grating (FBG) sensor arrays using a tunable pulsed laser is proposed and demonstrated. A tunable fiber laser based on a matched FBG is applied. The wavelengths of the sensing FBGs are obtained by detecting the maximum voltages with a photodiode that avoids the complex demodulation process. The advantages of this scheme are simple structure, high signal-to-noise ratio, and the sensing signals obtained by detecting the maximum voltages.     

Detection of edge delamination in CFRP laminates under cyclic loading using small-diameter FBG sensors

Small-diameter FBG sensors were applied for the detection of edge delamination in carbon fiber reinforced plastic (CFRP) quasi-isotropic laminates. Reflection spectra from the embedded fiber Bragg grating (FBG) sensor were measured at various lengths of delamination initiated from the edge of the specimen under cyclic loading. The form of the spectrum changed sensitively as the edge delamination grew. For confirmation of the measured results, the strain distribution in the FBG sensor was calculated by FEM analysis and the spectrum was simulated from the strain distribution theoretically. The change in the form of the measured spectrum was consistent with that of the calculated spectrum. From these results, the spectrum was found to depend on the size and location of the edge delamination. Moreover, the intensity ratio of the two peaks in the spectrum was proposed as an effective indicator for the quantitative evaluation of the edge delamination size.