Single-pulse fiber Bragg gratings and specific coatings for use at elevated temperatures

The technique of recording fiber Bragg gratings (FBGs) with single exposure pulses during the fiber drawing process allows production of such gratings in complex array structures, with high mechanical strength of the fiber and in a simple and cost-efficient way. This is of special interest for the growing field of fiber sensor applications with FBGs. A general advantage of fiber sensor systems is their ability to be used also at elevated temperatures compared with conventional electric or electronic sensors. For this purpose, the fiber itself as well as the grating structure and the fiber coating should be stable under such elevated temperature conditions. We have investigated different coating materials and possibilities of making temperature-stable FBGs of types I and II in the range of 100 degrees C-1000 degrees C with good reflection efficiency by single-pulse exposure during the fiber drawing process.     

Surface-relief fiber Bragg gratings for sensing applications

We present a new type of fiber Bragg grating (FBG) in which we etch the grating into the flat surface of a D-shaped optical fiber. Instead of being written in the core of the fiber, as are standard FBGs, these surface-relief FBGs are placed in the cladding above the core. These gratings are a viable alternative to standard FBGs for sensing applications. We describe the fabrication process for etching Bragg gratings into the surface of D-fibers and demonstrate their performance as temperature sensors.     

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.     

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....     

Transparent network for hybrid multiplexing of fiber Bragg gratings and intensity-modulated fiber-optic sensors

A network for multiplexing fiber Bragg gratings (FBGs) and intensity-modulated fiber-optic sensors with no need to distinguish between the two kinds of sensor is proposed and experimentally demonstrated. Two FBG sensors and two intensity-modulated sensors are wavelength-division multiplexed; the electrical phase of the output signal is measured as a common parameter for both types of sensor. Furthermore, the intensity sensors become power referenced, and the FBG sensors are interrogated by a low-cost technique. Low cross talk is achieved by use of a tunable optical filter at the detector.     

新型高灵敏度微纳光纤应变传感器

本文使用火焰熔融拉锥的方法,通过控制火焰的高度及拉锥速度,成功制备了具有微拱型渐变区的新型微纳光纤器件。理论计算表明,微拱型渐变区有利于激发出强度相当的高阶微纳光纤传输模式,从而增加了传输光谱中由模间干涉导致的透射谷的深度。实验表明,该新型微纳光纤器件透射谷深度达到18 dB,当轴向应变量增加时,透射谷向短波长方向移动,轴向应变灵敏度为-13.1 pm/με,比光纤光栅应变传感器提高一个数量级,是传统直线型微纳光纤灵敏度的3倍,线性度为99.15%。这种具有微拱型渐变区的微纳光纤器件具有灵敏度高、机械性能好以及便于与现有光纤系统集成等优点。并且结构简单,易于制备,可广泛应用于各种物理、化学和生物传感和探测领域。     

Analysis of strain transfer of six-layer surface-bonded fiber Bragg gratings

A theoretical analysis of strain transfer of six-layer surface-bonded fiber Bragg gratings (FBGs) subjected to uniform axial stress is presented. The proposed six-layer structure consists of optical fiber, protective coating, adhesive layer, substrate layer, outer adhesive layer, and host material, which is different from the four-layer case of common acknowledgement. A theoretical formula of strain transfer rate from host material to optical fiber is established to provide an accurate theoretical prediction. On the basis of the theoretical analysis, influence parameters of the middle layers that affect the average strain transfer rate of the six-layer surface-bonded FBG are discussed. After the parametric study, a selection scheme of sensor parameters for numerical validation, which makes the average strain transfer rate approach unity, is determined. Good agreement is observed between numerical results and theoretical predictions. In the end, the six-layer model is extended to the general situation of multiple substrate layers, which lays a theoretical groundwork for the research and design of surface-bonded FBGs with substrate layers in the future.     

Miniaturized Optical Fiber Sensor Interrogation System Employing Echelle Diffractive Gratings Demultiplexer for Potential Aerospace Applications

To enable the application of optical fiber sensors to aerospace vehicles, the sensor interrogation or data acquisition system has to meet small size and low weight requirements. This paper presents the developmental work of an echelle diffractive gratings (EDGs) demultiplexer-based optical fiber sensor interrogation system. The operation principle of this system and its application to fiber Bragg grating (FBG) sensor interrogation are presented. The experimental results have shown that the developed interrogator (not including the electronic controller) weighs less than 60 g and provides better than 1 pm measurement resolution and better than plusmn10 pm measurement accuracy.     

Fibre bragg gratings optical sensing technology

Fibre optic technology is a well established field in the world of smart structures. While optical communications systems date back as far as the late 1700s when the French engineer Claude Chappe invented the ‘optical telegraph’, advances in optical fibres did not really take off until Corning Glass Works (now Corning, Inc.) developed single-mode fibres with attenuation below 20 dB/km in 1970¹. Now fibre optics are widely used in the telecommunications industry. This breakthrough also led the way for the development of fibre optic sensors. While there are many types of optical sensors, the focus of this article is on the fibre Bragg grating sensor. In-line fibre Bragg gratings (FBGs) were developed as narrow band optical filters for the telecommunications industry². Television commercials by the major telephone companies describe the capability of transmitting as many as 16 telephone calls on the same optical fibre link, a capability made possible with wavelength division multiplexing using Bragg grating or equivalent spectral filtering technology. This intrinsic multiplexing capability and their sensitivity to thermo-mechanical loads also render in-fibre Bragg gratings one of the most promising distributed sensor technologies for structural systems. There are many other applications of FBGs including shape, moisture and corrosion sensing.     

Acousto-Ultrasonic Optical Fiber Sensors: Overview and State-of-the-Art

This paper gives a review of acoustic and ultrasonic optical fiber sensors (OFSs). The review covers optical fiber sensing methods for detecting dynamic strain signals, including general sound and acoustic signals, high-frequency signals, i.e., ultrasonic/ultrasound, and other signals such as acoustic emissions, and impact induced dynamic strain. Several optical fiber sensing methods are included, in an attempted to summarize the majority of optical fiber sensing methods used to date. The OFS include single fiber sensors and optical fiber devices, fiber-optic interferometers, and fiber Bragg gratings (FBGs). The single fiber and fiber device sensors include optical fiber couplers, microbend sensors, refraction-based sensors, and other extrinsic intensity sensors. The optical fiber interferometers include Michelson, Mach-Zehnder, Fabry-Perot, Sagnac interferometers, as well as polarization and model interference. The specific applications addressed in this review include optical fiber hydrophones, biomedical sensors, and sensors for nondestructive evaluation and structural health monitoring. Future directions are outlined and proposed for acousto-ultrasonic OFS.     

Studies on bridging tractions – simultaneous bridging tractions and COD measurements

The main objective of this work is to investigate the bridging tractions in a model composite using optical fiber Bragg grating (FBG) sensors written into selected reinforcing fibers. Simultaneously, the crack opening displacement (COD) is measured using a speckle interferometry technique. The measurements are useful in the verification of the relation between the COD and bridging tractions established with the use of the weight function method. Center crack specimens made of epoxy and reinforced with one layer of optical fibers are prepared and tested under remote tension parallel to the fibers. Bragg gratings of 0.17 to 0.38 mm in length are introduced in selected fibers for direct, non invasive, local measurements of axial strains in these fibers. A controlled central crack, bridged by intact fibers, is introduced by a laser technique such that the FBGs are located between the crack faces. The results on the forces obtained from the FBGs and the COD-weight function method show good agreement. The experimental results also compare very well with 3-dimensional numerical simulations of the actual specimen geometry and loading configuration.     

Fiber Bragg grating strain measurements in comparison with additional techniques for rock mechanical testing

Fiber Bragg gratings (FBGs) have been used as strain transducers for deformation measurements on two types of granite in uniaxial compression (UC) tests. A comparison of the strain results is given for mechanical extensometers based on cross-flexure strain gages, surface-mounted FBG sensors, and a noncontacting laser extensometer measuring system as additional optical reference.     

Multiplexed Fiber Bragg Grating Interrogation System Using a Microelectromechanical Fabry–Perot Tunable Filter

This paper describes a fiber Bragg grating strain sensor interrogation system based on a microelectromechanical systems tunable Fabry–Perot filter. The shift in the Bragg wavelength due to strain applied to a sensor fiber is detected by means of a correlation algorithm which was implemented on an embedded digital signal processor. The instrument has a 70 nm tuning range, allowing multiple strain sensors to be multiplexed on the same fiber. The performance of the interrogator was characterized using an optical fiber containing six grating strain sensors embedded in a fiberglass test specimen. The measured root mean square (RMS) strain error was 1.5 microstrain, corresponding to a 1.2 pm RMS error in the estimated wavelength shift. Strain measurements are produced with an update rate of 39 samples/s.      

Multipoint temperature-independent fiber-Bragg-grating strain-sensing system employing an optical-power-detection scheme

A temperature-independent fiber-Bragg-grating strains-sensing system, based on a novel optical-power-detection scheme, is developed and analyzed. In this system a pair of fiber Bragg gratings with reflection spectra either partially or substantially overlapping is placed side by side to form a temperature-independent strain-sensor unit. Conventional wavelength-interrogation techniques are not used here, and instead an optical-power-detection scheme is proposed to directly calibrate the measurand, i.e., the strain. Unlike the conventional approach in a multiplexed sensing system, the presented power-detection-based interrogation method does not need the fiber-Bragg-grating sensors to be spectrally separate. The only requirement is that the spectra of the two fiber Bragg gratings of each sensor unit in a multiplexed system be identical or slightly separate (slightly overlapping spectra would also work in the sensing scheme) and the source's optical power be sufficient for sensitive measurement. Based on a three-sensor-unit system, we demonstrate simple strain measurements of high linearity (+/- 0.4%), good sensitivity [2 microstrains (microS)], high thermal stability (+/- 0.8%), and zero cross talk. The effects of light source spectral flatness and fiber bending loss on measurement accuracy are also discussed.     

Comparative analysis of wavelength-multiplexed photonic-sensor networks using fused biconical WDMS

Three networks based on fused biconical wavelength division multiplexers (WDMs) and fiber Bragg gratings (FBGs) are theoretically and experimentally demonstrated for photonic-intensity-sensor multiplexing. The aim of replacing standard couplers for WDMs in the power division process is to reduce power losses and improve the robustness of the systems to FBG wavelength shifts. The different network topologies are analyzed both in terms of power budget and crosstalk noise, considering the multiplexing of two fiber-taper displacement sensors. The configuration with one detector for each sensor and the corresponding FBG at the detection end is proven to be the best topology in terms of crosstalk, doubling the peak-isolation value of the WDMs employed and yielding a 37.4 dB optical signal-to-noise ratio for a two-sensor network. Regarding power losses, the optimum configuration locates the FBGs at the sensor heads, thus improving power budget and avoiding additional couplers at the detectors. Both topologies are expanded to multiplex four sensors, with crosstalk identified as the critical factor in these networks. With this limiting parameter, the first configuration has been determined as the most suitable for multiplexing a high number of sensors.     

Strain and damage monitoring of CFRP in impact loading using a fiber Bragg grating sensor system

A fiber-optic system featuring strain measurement and ultrasonic detection was constructed with fiber Bragg gratings based on wavelength–light intensity conversion technique. This fiber Bragg grating sensing system consists of a broadband light source, a broadband optical filter for strain measurement and a narrowband tunable filter for ultrasonic detection. The system was applied to strain measurement in impact loading to carbon fiber-reinforced plastics and the subsequent impact damage detection. Experimental results demonstrated that fiber Bragg grating sensors could measure strain with higher resolution compared with conventional strain gauges. Furthermore, ultrasonic inspection in which ultrasonic sensitivity was maximized by controlling the transmissive wavelength of the tunable filter could detect a 6.3 × 9 mm² impact damage.     

Temperature-Independent Strain Sensor Based on a Hi-Bi Photonic Crystal Fiber Loop Mirror

This work presents an optical sensor based on a highly birefringent photonic crystal fiber (Hi-Bi PCF) loop mirror. The length of the sensing head is 380 mm and its corresponding wavelength spacing between two interferometer minima is 8 nm. The optical sensor was characterized in strain and in temperature with an uncoated Hi-Bi PCF and with an acrylate coated Hi-Bi PCF. Different results for strain and temperature sensitivity were obtained. Relatively to the strain measurement, the sensor with the uncoated Hi-Bi PCF presents slightly less sensitivity (1.11 ) when compared with coated Hi-Bi PCF (1.21 ). For the temperature measurement and with the uncoated Hi-Bi PCF, the optical sensor is insensitive to temperature (0.29 pm/K).     

一种用于低频测量的光纤光栅振动传感器

利用光纤光栅敏感技术,设计了一种用于低频测量的光纤光栅振动传感器.该传感器采用了等强度悬臂梁作为振动传感元件,并在等强度悬臂梁的上下双面优化布置性能匹配的两个光纤光栅,一个为振动敏感测量光栅,另一个为信号匹配滤波解调光栅.该结构设计可以补偿温度的不利影响,同时也提高了传感器信号检测的灵敏度.实测结果表明,该传感器可以用于25 Hz以下的低频目标的振动测量.     

A field deployable, multiplexed Bragg grating sensor system used in an extensive highway bridge monitoring evaluation tests

A multiplexed optical fiber Bragg grating sensor system with a measurement bandwidth of up to 200 Hz enabling dynamic loading events, e.g., road traffic, to be observed has been designed, installed, and tested over an 18-month period on a 346-m road bridge in Norway, for design verification and structural integrity monitoring purposes. A network of 32 fiber Bragg sensors was surface bonded along with a corresponding set of resistive strain gauges for comparative tests to be made. The wavelength data were calibrated against two thermally stabilized (/spl sim/0.15 pm) reference gratings, which rejected common mode noise and provided absolute wavelength scaling. These data provides independent strain and temperature information. Long-term test results showed good linearity and repeatability of <10 /spl mu//spl epsiv/ over the test period with a precision of /spl plusmn/5 /spl mu//spl epsiv/ and a resolution of /spl plusmn/1 /spl mu//spl epsiv/. The readings from the FBG sensors were comparable to those from the foil gauge sensors to within /spl plusmn/4 /spl mu//spl epsiv/.