Structural Health Monitoring by Using Fiber-Optic Distributed Strain Sensors With High Spatial Resolution

In this paper, we review our researches on the topics of the structural health monitoring （SHM） with the fiber-optic distributed strain sensor. Highly-dense information on strains in a structure can be useful to identify some kind of existing damages or applied loads in implementation of SHM. The fiber-optic distributed sensors developed by the authors have been applied to the damage detection of a single-lap joint and load identification of a beam simply supported. We confirmed that the applicability of the distributed sensor to SHM could be improved as making the spatial resolution higher. In addition, we showed that the simulation technique considering both structural and optical effects seamlessly in strain measurement could be powerful tools to evaluate the performance of a sensing system and design it for SHM. Finally, the technique for simultaneous distributed strain and temperature measurement using the PANDA-fiber Bragg grating （FBG） is shown in this paper, because problems caused by the cross-sensitivity toward strain and temperature would be always inevitable in strain measurement for SHM.

Fiber-optic Fabry-Prot strain sensor based on graded-index multimode fiber

By using a graded-index multimode fiber （GI-MMF） with a relatively flat index profile and high refractive index of the fiber core, a microextrinsic fiber-optic Fabry Prot interferometric （MEFPI） strain sensor is fabricated through chemical etching and fusion splicing. Higher reflectance of the microcavity is obtained due to the less-curved inner wall in the center of the fiber core after etching and higher index contrast between the GI-MMF core and air. The maximum reflection of the sensor is enhanced 12 dB than that obtained by etching of the Er- or B-doped fibers. High fringe contrast of 22 dB is obtained. The strain and temperature responses of the MEFPI sensors are investigated in this experiment. Good linearity and high sensitivity are achieved, with wavelength-strain and wavelength-temperature sensitivities of 7.82 pm/με and 5.01 pm/°C, respectively.

Strain Sensor Based on Embedded Fiber Bragg Grating in Thermoplastic Polyurethane Using the 3D Printing Technology for Improved Sensitivity

A new and easy-to-fabricate strain sensor has been developed,based on fiber Bragg grating(FBG)technology embedded into a thermoplastic polyurethane filament using a 3-dimensional(3D)printer.Taking advantage of the flexibility and elastic properties of the thermoplastic polyurethane material,the embedding of the FBG provides durable protection with enhanced flexibility and sensitivity,as compared to the use of a bare FBG.Results of an evaluation of its performance have shown that the FBG sensors embedded in this way can be applied effectively in the measurement of strain,with an average wavelength responsivity of 0.0135 nm/cm of displacement for tensile strain and -0.0142 nm/cm for compressive strain,both showing a linearity value of up to 99%.Furthermore,such an embedded FBG-based strain sensor has a sensitivity of~1.74 times greater than that of a bare FBG used for strain measurement and is well protected and suitable for in-the-field use.It is also observed that the thermoplastic polyurethane based(TPU-based)FBG strain sensor carries a sensitivity value of~2.05 times higher than that of the polylactic acid based(PLA-based)FBG strain sensor proving that TPU material can be made as the material of choice as a“sensing”pad for the FBG.

Resolving the problem of cross sensitivity in fiber Bragg grating sensor based on the principle of polarized-light interference

The article introduces an advanced approach of fiber grating strain demodulation based on the principle of polarized-light interference.This method can solve the problem of cross sensitivity in fiber Bragg grating sensor.As for the yttrium vanadate(YVO4)crystal polarized-light interferometer,if there are two different fiber Bragg gratings that have a p rad phase difference,one is used for strain sensor and the other is used for temperature compensating.Then the problem of cross sensitivity can be overcome.The analyses of the strain demodulation resolution show that with a decrease in crystal thickness the resolution increases.The experiment shows that when the thickness of YVO_(4) crystal is 0.5 mm,the effect of cross-sensitivity will be reduced to 0.13μe/°C,which is 1.6%of the effect used by only one fiber Bragg grating for strain measure.And when the thickness is 0.1 mm the cross-sensitivity will be reduced to 0.0067μe/°C,which is less than 0.08%.Less thickness of crystal is beneficial for the resolution of strain measure,but the machining of small thickness crystal is difficult.To solve this problem,a new scheme of polarized-light interferometer with the structure of double layer crystal was analyzed.

钢筋混凝土锈蚀诱导的膨胀监测及评估——低相干光纤应变传感器的应用

针对钢筋混凝土中锈蚀问题,提出了一种基于低相干光纤应变传感器的钢筋混凝土锈蚀进程监测方法,设计中将传感光纤环绕在待测结构易锈蚀部分,通过测量钢筋锈蚀诱导膨胀所造成的结构微形变来反映其锈蚀进程.基于Michelson干涉仪的信号解调方法可以测得任意长传感光纤上5μm的绝对形变.将布里渊分布式传感器和低相干光纤应变传感器布置在同一钢筋混凝土试件结构中,进行模拟海洋环境的电化学加速锈蚀实验.结果表明,分布式传感器在多点测量方面有优势,但其信号强度在锈蚀的后期衰减较快,在18mm钢筋直径的锈蚀实验中测得的上限锈胀应变为1000με,这难以满足结构耐久性监测的需要.而低相干光纤应变传感器可以长寿命测量出锈蚀的产生及发展,根据这些实验数据对钢筋混凝土的锈蚀膨胀应变与结构性能作了相关分析,为钢筋混凝土结构的耐久性监测提供了一种可行的实验室研究手段.

Dynamic Gratings in Optical Fibers： Synthesis and Sensing Applications

We proposed two schemes of generating and localizing dynamic gratings in optical fibers： one is based on the gain saturation in erbium-doped fiber; the other is based on Brillouin scattering in the fiber. By using these dynamic gratings, fully distributed strain/temperature sensors have been demonstrated. In this presentation, we review the principles, basic schemes, and experimental demonstrations of the novel dynamic grating techniques.

Photonic Crystal Fiber Fabry-Perot Interferometers With High-Reflectance Internal Mirrors

We demonstrated an in-line micro fiber-optic Fabry-Perot interferometer with an air cavity which was created by multi-step fusion splicing a muti-mode photonic crystal fiber （MPCF） to a standard single mode fiber （SMF）. The fringe visibility of the interference pattern was up to 20 dB by reshaping the air cavity. Experimental results showed that such a device could be used as a highly sensitive strain sensor with the sensitivity of 4.5 pm/με. Moreover, it offered some other outstanding advantages, such as the extremely compact structure, easy fabrication, low cost, and high accuracy.