Design of pH Sensors in Long-Period Fiber Gratings Using Polymeric Nanocoatings

In this paper, two different pH sensors based on the deposition of nanometric scale polymeric films onto the surface of a long-period fiber grating (LPFG) have been studied and compared. An electrostatic self-assembled (ESA) method has been used to create sensitive films with an optimal overlay thickness. Two types of sensors have been designed: The first one is based on polyallylamine hydrochloride (PAH), polyacrylic acid (PAA), and the second one was done incorporating the pigment Prussian blue (PB) in the PAH/PAA matrix. A theoretical model of multilayer cylindrical waveguides based on coupled-mode theory has been used to predict the position of the attenuation bands as a function of the overlay thickness. Both sensors were tested and compared in terms of sensitivity and response time. A faster response was obtained with the introduction of PB particles in the polymeric matrix. Linear sensors in the pH range 4-7 were obtained, showing good repeatability and high sensitivity     

Evanescent Field Fiber-Optic Sensors for Humidity Monitoring Based on Nanocoatings

The transmitted optical power of two different fiber optic based structures when a nanofilm is being deposited onto them is experimentally studied. The technique used to build the nanofilms is Electrostatic Self Assembly (ESA), which has been widely reported in the literature. For the shake of comprehensibility, the comparative analysis of this phenomenon is accomplished for a particular sensing measure, humidity. The two structures selected towards development of practical humidity evanescent field sensors are hollow core fibers and tapered optical fibers. Some preliminary experimental studies of depositing humidity sensitive thin films and demonstrating their feasibility are presented. Depending on the working point selected, up to 10dB of variation in the optical output power is obtained when the environmental humidity changes. Both configurations exhibit similar dynamic behavior and response times shorter than 300msec, making these evanescent field sensors good candidates to monitor human breathing     

Multiplexing Scheme for Self-Interfering Long-Period Fiber Gratings Using a Low-Coherence Reflectometry

A novel multiplexing scheme based on a low-coherence reflectometry (LCR) is proposed for a sensing array (in parallel) of self-interfering long-period fiber gratings (SI-LPGs). Each SI-LPG sensor consists of an LPG and a section of fiber with a highly reflective end (mirror). The spectral information of each LPG is sensitive to some parameters of the surrounding measurand and can be reconstructed from the corresponding subreflectograms (obtained by the LCR) through a fast Fourier transformation. The sensing signals of multiple SI-LPG sensors can be multiplexed if the length of the fiber section in each sensor is set to a different value. Experiments of measuring the surrounding temperature at different sensors are demonstrated to show the good performance of our multiplexing system.     

Capacitive Humidity Sensors With High Sensitivity and Subsecond Response Times

Capacitive-based humidity sensors were fabricated using coplanar interdigitated electrodes coated with nanostructured TiO₂ thin films produced by glancing angle deposition. In this letter, we show that increased sensitivity (nF/%RH) is obtained by decreasing the electrode periodicity or by increasing the planar area of the electrodes, or both. The devices were sensitive over a wide range of relative humidity levels (<1% to >92%) and exhibited extremely fast, subsecond response times. Typical adsorption and desorption response times were measured to be <220 and >400 ms, respectively     

树脂传递模塑成型的光纤光栅监测

复合材料固化过程对于生产高质量复合材料部件十分必要。文中利用光栅传感器监测树脂传递模塑(RTM)复合材料层板制造过程中内应变及温度,根据复合材料内应变/温度关系曲线的突变点,获得复合材料的材料转变点(凝胶点、玻璃化转变温度)信息。在复合材料降温阶段,利用光栅监测的应变/温度值计算RTM成型复合材料的内层和外层热膨胀系数。监测结果与传统检测方法相比十分一致。     

Design of Long-Period Fiber Grating Refractometric Sensors With Linear Response by a Genetic Algorithm

A new method for the design of optical fiber refractometric sensors based on nonuniform long-period fiber gratings (LPFGs) is presented. A specialized genetic algorithm (GA) with properly designed operators is a fundamental element of this method and performs the theoretical synthesis of a necessary nonuniform LPFG period profile. Such a profile yields a linear response of the sensor to the refractive index of the external medium. A distinctive feature of the designed LPFG is a linear gradient of the core and cladding refractive index along the grating length. Also, a new mathematical model of nonuniform LPFGs is presented, the model permits to take into account the effect of some LPFG sections of a lower and others of a higher cladding refractive index than that of the external medium. The application of both the design method and the mathematical model is illustrated with a numerical example.     

Sensitivity Tuning in Terfenol-D Based Fiber Bragg Grating Magnetic Sensors

In this work, the dependence of the magnetostrictive response on the prestress has been used to improve and fit the performance of Terfenol-D based fiber Bragg grating magnetic sensor. The possibility to tune sensitivity allows to work at different operative conditions and to develop advanced sensors with reconfigurable sensitivity. Performance improvements in terms of magnetic resolution up to 0.0116 A/m have been demonstrated.      

Construction of Dual-Channel Water Transport in Mesoporous Silica Low Humidity Sensors to Achieve High Sensitivity

In this paper, strong hydrophilic poly(ionic liquid)s (PILs) are selectively grafted on different positions (mesoporous channels and outer surface) of mesoporous silica via thiol-ene click chemical reaction. The purposes of selective grafting are on the one hand, to explore the differences of adsorption and transportation of water molecules in mesoporous channels and on the outer surface, and on the other hand, to combine the two approaches (intra-pore grafting and external surface grafting) to reasonably design SiO₂@PILs low humidity sensing film with synergetic function to achieve high sensitivity. The results of low relativehumidity （RH） sensing test show that the sensing performance of humidity sensor based on mesoporous silica grafted with PILs in the channels is better than that of humidity sensor based on mesoporous silica grafted with PILs on the outer surface. Compared with water molecules transport single channel, the construction of dual-channel water transport significantly improves the sensitivity of the low humidity sensor, and the response of the sensor is up to 4112% in the range of 7–33% RH. Moreover, the existence of micropores and the formation of dual-channel water transport affect the adsorption/desorption behaviors of the sensor under different humidity ranges, especially below 11% RH.     

Research Advances in Microfiber Humidity Sensors

An overview of the numerous latest research in microfiber humidity sensors is carried out with a specific focus on measurement methods, humidity sensitive materials, probe structures, and sensing properties of different sensors. First, five mainstream measurement structures, including taper, fiber grating, coupler, resonator, and interferometer are reviewed. It is concluded that these measurement structures sense the physicochemical property variations of microfibers or sensitive films and exhibit the change of optical signal when exposed to environment. Second, the basic preparation methods, humidity‐sensing properties, and their advantages and disadvantages as humidity sensitive material are addressed. Then, the advantages and disadvantages of all the above sensing structures are also discussed and compared. Finally, the main existing problems and potential solutions of microfiber humidity sensors are pointed out.     

Nanostructured Metal Oxide Thin Films for Humidity Sensors

Capacitive humidity sensors were fabricated using countersunk interdigitated electrodes coated with amorphous nanostructured TiO₂, SiO₂, and Al₂O₃ thin films grown by glancing angle deposition. The capacitive response and response times for each sensor were measured. The sensor utilizing TiO₂ exhibited the largest change in capacitance, increasing exponentially from ~ 1 nF to ~ 1muF for an increase in relative humidity from 2% to 92%. Adsorption and desorption response times were measured using flow rates of 2.5 l/min and were between 90 ms and 300 ms for the sensors studied here. A simple model of the capacitive response of the devices has been developed and used to calculate the dielectric constant of the combined system of our films and adsorbed water. The obtained dielectric constants are found to be much higher than bulk or literature values for similar systems.     

Passive Temperature-Compensating Technique for Microstructured Fiber Bragg Gratings

The thermal drift of the characteristic wavelength of fiber Bragg gratings (FBGs) photowritten in the core of microstructured fibers (MOFs) is significantly reduced by inserting a liquid of suitable refractive index into their holes. For instance, the spectral range of variations is divided by a factor of 4 over a temperature range larger than 20degC in a six-hole MOF, and the maximum sensitivity is reduced. Such passive FBG temperature compensation technique is of great interest for applications involving accurate sensing free of thermal effects.     

Ion-Beam Assisted Glancing Angle Deposition for Relative Humidity Sensors

Ion-beam assisted glancing-angle deposition is used to fabricate relative humidity sensors. Ion currents of 3, 5, and 7 mA were tested. The morphology and capacitance of the sensors are found to be dependent on the ion current density. The capacitance increases with increasing ion current, especially for the 7 mA case which exhibits a capacitance approximately one order of magnitude greater than a standard glancing-angle deposited film over much of the sensor range.      

Wavelength Detection of Coherence-Multiplexed Fiber-Optic Sensors Based on Long-Period Grating Pairs

Multiplexed fiber-optic sensors based on long-period grating pairs (LPGPs) are presented. With a low-coherence reflectometry, the spectral responses of the specially configured LPGP sensors are precisely quantified from the corresponding sub-reflectograms through a fast Fourier transformation (FFT). By monitoring the wavelength shift of the LPGP sensors, temperature is measured experimentally, and good performance is obtained     

石墨烯湿敏性能研究进展

湿度传感器与大气监测、工业生产和生物医疗等领域息息相关。随着科技的不断发展,人们对高性能湿度传感器的需求不断增加,这为湿度传感器行业的发展带来了前所未有的机遇和挑战,其中高性能湿敏材料的开发尤为关键。在诸多湿度传感器中,金属氧化物或金属氧化物/聚合物复合材料湿度传感器因其敏感元件选择的多样性、易于后加工处理和响应特性高等特点而受到广泛关注。与聚合物湿度传感器相比,陶瓷材料的合成过程更简便,响应也通常更为迅速,且聚合物的成本更低。近些年,新型纳米材料被广泛应用于湿度传感器领域,逐渐成为湿敏材料的主要发展方向及研究热点。零维和一维纳米碳质材料,如富勒烯、碳纳米管作为湿敏活性层制备的传感器通常具有大比表面积、可室温下工作、易于实现微型化、稳定性好等诸多优点,但它们的零维或一维结构与现有的平面电子器件加工工艺不相匹配。石墨烯是由sp2杂化的碳原子紧密排列构成的二维蜂巢晶格结构的单层石墨,其独特的二维结构适用于现有的平面电子器件加工工艺。石墨烯材料作为湿敏活性层受到研究者们的广泛关注是因为它具备诸多优异特性:(1)石墨烯的所有原子都在表面,具有超大的比表面积,原则上,石墨烯传感器的动态检测范围可以从单个分子到很高的浓度水平;(2)利用石墨烯的电学特性和力学特性可以很好地进行传感信号的转换;(3)金属、聚合物或其他修饰剂功能化的石墨烯能与特定分子发生相互作用,大大增强传感器的选择性;(4)石墨烯单晶可以制作四探针装置,从而能够避免接触电阻的影响,并大大提高灵敏度;(5)与其他纳米碳材料如碳纳米管相比,石墨烯和氧化石墨烯制备成本更低。本文综述了石墨烯湿敏性能及其应用的研究进展,着重讨论了本征石墨烯、氧化石墨烯和改性石墨烯的湿敏特性。文章最后分析了石墨烯基湿度传感器未来的发展方向和面临的挑战。     

Highly Sensitive MoS2 Humidity Sensors Array for Noncontact Sensation

Recently, 2D materials exhibit great potential for humidity sensing applications due to the fact that almost all atoms are at the surface. Therefore, the quality of the material surface becomes the key point for sensitive perception. This study reports an integrated, highly sensitive humidity sensors array based on large‐area, uniform single‐layer molybdenum disulfide with an ultraclean surface. Device mobilities and on/off ratios decrease linearly with the relative humidity varying from 0% to 35%, leading to a high sensitivity of more than 10⁴. The reversible water physisorption process leads to short response and decay times. In addition, the device array on a flexible substrate shows stable performance, suggesting great potential in future noncontact interface localization applications.     

Spectroscopy for the Analysis of Nanoporous Silicon Gas and Humidity Sensors

In this work, a Raman spectroscopic study of nanoporous silicon sensor samples demonstrated its use as a method of gauging the sensor potential via quantitative data it provides on the sensor nanostructure dimensions. This special property of the Raman spectroscopy technique also showed its potential to determine mechanical stability of the samples over 3 months. This work also shows that the Raman spectroscopy technique is sensitive to step changes in relative humidity in all the sensor samples via its measurement of the strain-free crystalline silicon (c-Si) Raman peak. Since the Raman technique is non-destructive and senses remotely on the fragile nanoporous sensor samples it will be the ideal replacement of the presently used electrical capacitance techniques as the primary determination of relative humidity.     

光纤布拉格光栅的热致衰减研究

光纤布拉格光栅的工作寿命是影响光纤光栅传感器性能的关键因素。从光纤布拉格光栅的热致衰减特性出发,以界面阻隔能模型为理论基础,以光纤光栅的反射峰值为研究对象,探讨了光纤布拉格光栅的热致衰减行为。并通过对比试验对3支不同的光纤布拉格光栅进行了600℃热加速衰减实验,初步证明即使进行了退火处理,光纤布拉格光栅的性能仍然存在热致衰减蜕化。从实验数据中得到了光纤布拉格光栅的热振动频率,建立了热致衰减曲线。预测了30℃、50℃、100℃条件下光纤布拉格光栅的反射率衰减到90%的时间分别为23年、11年和3年。     

光纤Bragg光栅应变传感中的温度补偿技术

本文对光纤光栅传感中的应变和温度交叉敏感问题进行了讨论,在此基础上提出了采用参考光栅实现光纤光栅应变传感时的温度补偿的基本原理和方法,并通过实验进行了验证.理论和实验均证明,本文提出的参考光栅温度补偿法原理简单,而且补偿效果好.