用于环氧树脂固化监测的自参考型光纤固化传感器

研究了一种新型的自参考型光纤固化传感器,用于环氧树脂固化过程的实时监测。用光纤芯折射率为1.558的多模光纤制成端面反射型折射率传感器,利用环氧树脂的折射率与固化度之间的对应关系,通过实时测量环氧树脂的折射率进行固化监测。在固化过程中,当环氧树脂的折射率变化到与光纤芯的折射率相等时,传感界面的反射信号为零,传感器可对零点偏置和灵敏系数进行校准,在此后的固化过程中,传感器能够定量地测量环氧树脂的固化度。在用多个传感器对树脂内不同部位的固化过程同时进行监测的场合,各传感器经实时自参考校准后,可根据各自的灵敏系数对输出信号统一定标,从而可以相互比较,实时地反映不同部位的固化反应过程之间的差异。      

Extrinsic fiber-optic Fabry-Perot interferometer sensor for refractive index measurement of optical glass

A simple fiber-optic sensor based on Fabry-Perot interference for refractive index measurement of optical glass is investigated both theoretically and experimentally. A broadband light source is coupled into an extrinsic fiber Fabry-Perot cavity formed by the surfaces of a sensing fiber end and the measured sample. The interference signals from the cavity are reflected back into the same fiber. The refractive index of the sample can be obtained by measuring the contrast of the interference fringes. The experimental data meet with the theoretical values very well. The proposed technique is a new method for glass refractive index measurement with a simple, solid, and compact structure.     

Refractometric sensor based on a phase-shifted long-period fiber grating

A refractometric sensor based on a phase-shifted long-period fiber grating written by electric-arc discharges is presented. Transmission and reflective configurations for refractive index measurements are studied. It is observed that the reflective topology permits better performance compared with the transmission one, which is the approach normally utilized in the context of long-period fiber sensing. The resolution achieved in the measurement of refractive index enables the application of this sensing head structure in demanding situations, such as the measurement of the level of salinity of water.     

Optical fiber relative-humidity sensor with polyvinyl alcohol film

We describe a fiber-optic relative-humidity (RH) sensor comprising a moisture-sensitive overlay on a single-mode side-polished fiber. The hygroscopic polymeric material deposited was polyvinyl alcohol (PVA), which proved to have good adherence and stability. The film reached a fast equilibrium with atmospheric moisture (in less than 1 min), inducing changes in the output optical power of approximately 10 dB for the 70%-90% RH range. To yield a low-cost device, single-mode standard communication fibers were used; therefore all the components of the sensor can be commercial, mass-produced telecommunication devices. The experimental results obtained are consistent with the expected behavior of the system; the output power decreases because of losses in the polished region of the fiber as the refractive index of its external medium approaches the fiber core value. Because the external medium is PVA film, its refractive index changes in response to its water content.     

Long-period grating refractive index sensor with a modified cladding structure for large operational range and high sensitivity

What is believed to be a novel long-period grating (LPG) refractive index sensor with a modified cladding structure is proposed and studied. In the proposed structure, the cladding of the fiber has a two-layer structure, i.e., a cladding layer of low refractive index with a reduced radius and an overlay of high refractive index. The sensitivity of the structure-modified LPG sensor to the ambient refractive index change as a function of the cladding layer and overlay parameters is investigated by way of modeling. It is found that an increase of the ambient refractive index causes a field redistribution of the cladding mode into the overlay when the parameters of the overlay are properly selected. It is shown that by reducing the radius of the cladding layer, the operational range of the LPG refractive index sensor can be as large as 0.195 (from 1.244 to 1.440) with a minimum sensitivity of 660 nm/refractive index, which represents a 31% increase of operational range in comparison with the operational range obtained from the reported structure. The design guidelines for achieving this large operation range and high sensitivity are explained by investigating the dependence of the cladding modes on the radius of the cladding layer.     

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....     

用于环氧树脂固化过程实时监测的光纤折射率传感器

研究了应用光纤传感技术对环氧树脂固化过程进行实时监测的方法。通过将一段光纤的包层去掉构成光纤折射率传感器, 利用环氧树脂在固化过程中折射率随固化度增大而增大的特性进行固化监测。应用光线追迹法和局部平面波理论导出光纤固化传感器的理论模型。通过在光纤传感区一端的光纤端面镀全反射膜形成探针型传感结构。对E51 环氧树脂的等温固化过程进行了实时监测实验。      

多模光纤传感器复合材料固化实时监测研究

设计了一种新型光纤传感器，提出通过测量光纤末端近场模斑谱监测光纤埋置周转树脂折射率的变化。     

Two-Layer Nanocoatings in Long-Period Fiber Gratings for Improved Sensitivity of Humidity Sensors

A relative humidity sensor based on the deposition of electrostatic self-assembled alumina ( $hbox{Al}_{2}hbox{O}_{3}$) and poly(sodium 4-styrenesulfonate) on the cladding of a long-period fiber grating (LPFG) has been designed. The sensitive material has a lower refractive index than that of the fiber cladding, which limits the sensitivity of the LPFG response. In order to enhance its sensitivity, a previous high refractive index coating has been deposited. The overlay thickness is of the order of magnitude of the light wavelength used to interrogate the sensor. A theoretical model of multilayer cylindrical waveguides based on coupled-mode theory has been used to predict the phenomenon. Experimentally, an increased wavelength shift of the attenuation bands (75%) was obtained during the fabrication of the sensor, and, what is more important, the sensitivity was improved by a ratio of almost four. The proposed method improves the performance of LPFG-based sensors characterized by overlays of low refractive index.      

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     

High sensitivity of taper-based Mach-Zehnder interferometer embedded in a thinned optical fiber for refractive index sensing

A taper-based Mach-Zehnder interferometer (MZI) embedded in a thinned optical fiber is demonstrated as a highly sensitive refractive index (RI) sensor. A RI sensitivity of 2210.84 nm/RIU (refractive index unit) is obtained at the external RI of 1.40, which is ten times higher than that of normal taper- and long-period fiber grating (LPFG)-based sensors. The sensitivity can be further improved by decreasing the diameter of the thinned fiber and increasing the interferometer length of the MZI. The proposed MZIs have lower temperature sensitivities compared with normal fiber sensors, which is a desirable merit for RI sensors to reduce the cross sensitivity caused by thermal drift.     

保偏微结构光纤光栅 F-P腔折射率传感特性分析

针对提高光纤光栅折射率传感器抗干扰能力以及增加反射率的需求,本文提出了一种基于Fabry-Perot腔的保偏微结构光纤(PM-MOF)布拉格光栅折射率传感器.根据传榆矩阵法和有限元方法,分析了微结构光纤光栅F-P腔中被测物折射率与F-P腔反射谱中两个偏振模谐振波长差的关系,在此基础上讨论了中心孔直径、F-P腔长度等参数对传输特性的影响.研究结果表明,随着空气孔中填充物折射率的增加,保偏微结构光纤光栅F-P腔的两个偏振态的谐振波长差将逐渐减小;F-P腔的干涉作用使反射率较单个光栅有很大提高,便于长距离传输和实时解调;两个偏振模对外界干扰具有相似的响应,因此该传感器具有更强的抗干扰能力.本文研究结果为保偏微结构光纤光栅在折射率传感器及其生物传感器方面的应用提供了理论依据.     

Colloidal gold-modified optical fiber for chemical and biochemical sensing

A novel class of fiber-optic evanescent-wave sensor was constructed on the basis of modification of the unclad portion of an optical fiber with self-assembled gold colloids. The optical properties and, hence, the attenuated total reflection spectrum of self-assembled gold colloids on the optical fiber changes with different refractive index of the environment near the colloidal gold surface. With sucrose solutions of increasing refractive index, the sensor response decreases linearly. The colloidal gold surface was also functionalized with glycine, succinic acid, or biotin to enhance the selectivity of the sensor. Results show that the sensor response decreases linearly with increasing concentration of each analyte. When the colloidal gold surface was functionalized with biotin, the detection limit of the sensor for streptavidin was 9.8 x 10(-11) M. Using this approach, we demonstrate proof-of-concept of a class of refractive index sensor that is sensitive to the refractive index of the environment near the colloidal gold surface and, hence, is suitable for label-free detection of molecular or biomolecular binding at the surface of gold colloids.     

Elasto–optical behavior model of a step-index fiber under localized pressure

The intensity attenuation of a step-index optical fiber under localized pressure is investigated using the elasto–optical theory and the principles of geometrical optics. A model with a Gaussian variation of the refractive index in the axial direction is used to evaluate the effect of applied pressure on the external coating of the fiber. The results are compared with experimental measurements obtained using a prototype of an optical fiber pressure sensor.     

用光纤进行树脂基复合材料的成型过程监测

研究了用光纤对碳纤维环氧树脂复合材料成型监测的方法和系统。通过测量复合材料成型过程中树脂折射率的变化来反映树脂的粘度变化和固化过程。     

Simple fiber-optic-based sensors for process monitoring: an application in wine quality control monitoring

The goal of this research was to develop a simple and economical fiber-optic sensor technology for agrifood process monitoring. Toward this end, two fiber-optic sensors were developed to be used in combination: a single reflection V-bend sensor and a single fiber air-gap probe. The former is designed to be sensitive toward refractive index and the latter towards absorption. Experiments indicate that the micromachined V-bend fiber refractometer is most sensitive when the bend angle is centered around 140 degrees, at which angle the sensor may resolve changes in refractive index as small as 0.00015. Additionally, the V-bend sensor was found to be non-responsive toward sample absorption even in extremely absorbing solutions. The air-gap design absorption sensor, most commonly used for measurements in highly colored media, was found to be slightly sensitive towards refractive index. When the two sensors are used together, the response of the absorption sensor may be corrected for. This sensor combination is able to provide accurate measurements in situations where Beer's law is not obeyed. Results are presented that show that the sensor pair was successfully used to monitor wine sugar content (Brix), and color density and hue, parameters related to the age of the wine.     

Pulsed laser deposited ZnO film on side-polished fiber as a gas sensing element

A simple sensor element consisting of a side-polished single-mode fiber and a planar metal oxide waveguide is described. The thin ZnO planar waveguide was produced on the polished fiber surface by pulsed laser deposition at optimized processing parameters. A measurement scheme for in situ control of the film thickness during the deposition process was developed and used. X-ray diffraction measurements and scanning electron microscopy were used to characterize the structure and the surface morphology of the planar waveguide, respectively. The numerical evaluation of the sensor sensitivity predicts the possibility to detect refractive index changes of less than 10(-4). Furthermore, preliminary gas sensor tests were performed by using a mixture of 1.5% butane diluted in N(2) and pure butane. A shift of the spectral position of the resonance points was observed from 3 to 5 s after gas exposure, which corresponds to refractive index changes of 3 x 10(-5) and 1.2 x 10(-3) for 1.5% butane and for pure butane, respectively.     

微纳Taper光纤干涉传感器的折射率测量理论研究

利用微纳taper光纤干涉方程和微纳光纤传输模式函数关系,建立了微纳taper光纤干涉波长随环境折射率变化的数学模型,指出微纳taper光纤干涉波长的响应规律由基模高阶模有效折射率差、色散因子和环境折射率3个因素所决定.详细研究了微纳taper光纤传感器的灵敏度变化规律,结果表明,传感器的灵敏度随光纤半径变小而急速增大,并随环境折射率的增大而非线性增加,而且探测波长越长,其灵敏度越大.     

Tailoring fiber grating sensors for assessment of highly refractive fuels

Three approaches that allow the tailoring of long period gratings based refractometric sensors for concentration measurement in fuel blends are employed to assess the fuel quality in biodiesel and biodiesel-petrodiesel blend. To allow the analysis of fuel samples with refractive index higher than fiber cladding one, the samples refractive indices were changed by thermo-optic effect and by dilution in a standard substance with low refractive index. The obtained results show the sensor can detect oil concentration in biodiesel samples with resolution as better as 0.07% and biodiesel concentration in biodiesel-petrodiesel samples with average resolution of 0.09%.     

Revisiting twin-core fiber sensors for high-temperature measurements

A twin-core fiber Michelson interferometer is evaluated as a high-temperature sensor. Although linear and reproducible operation up to 300°C is obtained, at higher temperatures (700°C) the refractive index shifts plastically and hysteresis is observed, rendering an untreated sensor head unusable. The shift is shown to be greatly reduced by an annealing process of the fiber for 10 h at 900°C, with which the linear response is preserved.