A novel time-division multiplexing fiber Bragg grating sensor interrogator for structural health monitoring

A novel fiber Bragg grating (FBG) sensor system for measurement of strain and temperature is proposed in this paper. The proposed sensor technique is based on time-division multiplexing (TDM). A semiconductor optical amplifier (SOA), connected in a ring cavity, is used to serve as a gain medium and switch. The SOA is driven by a pulse generator, which operates the SOA at different periods of time to select reflected pulses from a particular sensor. The FBG sensors have identical center wavelengths and can be deployed along the same fiber. This technique relieves the spectral bandwidth issue and permits the interrogation of up to 100 FBGs along a fiber, if the reflectivity of the individual sensors is sufficiently low to avoid shadowing effects. This system is particularly suitable for the application in structural health monitoring (SHM) where large numbers of sensors are required in wide measurement ranges.     

基于粗时分复用技术的光纤光栅传感系统研究

充分利用光纤光栅传感网络的频域和时域资源,综合考虑了光栅传感环境特点、传感点铺设和维护等因素,提出了一种新型的基于粗时分复用的TDM/WDM光纤传感网络设计方案。先按时分复用方式将光纤传感环境划分成不同时延的传感域,再将域内传感点按波分复用方式进行波分复用,并结合网络拓扑结构分析了网络中的影响测量结果的主要因素。方案引入了传感域和时延容差概念,可以放宽对传感点精确位置的要求,可对传感网进行分域管理,便于替换死亡的传感器,更加方便、简洁地实现大容量光纤光栅传感系统,可依据器件具体情况灵活分配传感域内域外的传感器复用数,使每个传感单元的传感成本显著降低。     

Metal-mirror-based grating for high-efficiency element in dense wavelength division multiplexing

It is desirable to obtain high efficiency with polarization-independence and wideband properties for incident wavelength. A metal-mirror-based grating is presented to diffract the incident wave into reflection orders with high efficiency for TE and TM polarization. The modal method and rigorous coupled-wave analysis (RCWA) are used together to optimize a metal-mirror-based grating effectively. From the analysis of the modal method, it is feasible to realize such a grating with the prescribed grating duty cycle and period. Accurate parameters of the grating depth and thickness of the connecting layer are optimized using RCWA. Compared with the reported binary simple grating, high efficiency can be improved greatly for the incident wavelength of 1550 nm in dense wavelength division multiplexing. The diffraction investigation indicates that a wideband property for incident wavelength can be obtained for such a novel metal-mirror-based grating.     

Partial scanning frequency division multiplexing for interrogation of low-reflective fiber Bragg grating-based sensor array

Interrogation using partial spectrum scanning for a low-reflective fiber Bragg grating (FBG)-based sensor array is proposed. The sensor head is a Fabry–Perot interferometer (FPI) consisting of low-reflective fiber Bragg gratings. An FPI consisting of low-reflective FBGs (FBG-FPI) has a spectrum with a sinusoidal structure with a period determined by the length of the FBG-FPI. Multiple point sensing is possible by installing multiple low-reflective FBG-FPIs on one fiber and analyzing the reflection spectrum of that fiber by the frequency division multiplexing method. A coherent wavelength-swept light source is necessary to acquire the reflection spectrum; however, but the sweep speed of a commercially available wavelength-swept light source that satisfies the condition is only approximately several tens of hertz. Therefore, we propose to conduct a wavelength sweep by injection current modulation of a laser diode. Wavelength sweeping with a modulated laser diode allows fast sweeping at 100 Hz or more, and the sweep range is as narrow as several hundred picometers. Reading only a part of the reflection spectrum of a low-reflective FBG-FPI sensor array using a modulated laser diode enables high-speed multipoint measurement through frequency analysis. Theoretical requirements for successful interrogation using the partial reflection spectrum are shown. A demonstration experiment to simultaneously measure strain applied to two low-reflective FBG-FPI sensors with a measurement time of 10 ms is reported.     

A fiber Bragg grating with triangular spectrum as wavelength readout in sensor systems

A wavelength readout for sensors is presented by a fiber Bragg grating (FBG) with triangular spectrum. The synthesis and fabrication of the triangular FBG is described. Experimental results showed that the wavelength readout device has advantages of high sensitivity, immunity from the light source power fluctuations, the uneven power distributions of source spectrum and micro-bend attenuation of the sensing FBG. Measurement resolution of 1.5 με under the condition of 10% fluctuation of the light source power was obtained for a FBG strain sensor.     

Demodulation of the multi-peak fiber Bragg grating sensor based on partial wavelength scan

Until now, a high-efficiency demodulation method for fiber Bragg grating(FBG) sensors has been a challenge.In this Letter, by employing multi-peak FBGs, an FBG sensor with a partial wavelength scan is proposed and initially demonstrated. By demodulating a near-symmetrical multi-peak FBG and an asymmetrical multi-peak FBG in the strain experiment, sensor sensitivities of 1.02 pm/με and 1.01 pm/με are measured for the interrogation system, respectively. The average demodulation deviations for the two sensors are 1.81% and 0.4%,respectively. The proposed method is expected to realize high-efficiency and low-cost FBG interrogators.     

Oscillation wavelength of fiber Bragg grating semiconductor lasers

Based on the physical fact that a laser containing a gain medium with homogenous line broadening oscillates at the wavelength which requires the smallest threshold gain due to the mode competition effect, after taking into account the wavelength-depended reflectivity distribution profile of a fiber Bragg grating, the oscillation wavelength λ_l of fiber Bragg grating semiconductor lasers has been investigated theoretically. The results show that the laser oscillation wavelength λ_B is not fixed at the Bragg reflection wavelength λ_l of fiber Bragg grating, and the offset between λ_B and λ_l depends on the reflectivity distribution profile of fiber Bragg grating and the gain profile of semiconductor gain medium.     

Four-element fiber Bragg grating acceleration sensor array

A four-element fiber Bragg grating acceleration sensor array is presented in this paper. The system combines an unbalanced Mach–Zehnder interferometer and three coarse wavelength division multiplexers (WDMs). The totally passive scheme has the advantages of high operation frequency and loose requirements for the working environment. An acceleration resolution of better than for the four channels is achieved.     

自锁紧式光纤Bragg光栅管式应变传感器

为了便于实现应变传感器两端铠装光缆与传感器自身牢固的连接,提高其在工程中抗拉拽等能力,研制了一种由不锈钢管、法兰盘和自锁紧螺帽等元件制作的自锁紧式光纤Bragg光栅管式应变传感器。光纤光栅通过限位堵头内的环氧树脂固定于不锈钢管内,应变作用于拧在不锈钢管上的法兰盘,带动不锈钢管和粘贴在上面的光纤光栅发生形变,检测出光纤光栅的波长变化量就可以反算出应变的大小。使用万能拉力试验机对其进行拉力荷载试验,结果表明,该传感器的灵敏度为0.45pm/με,线性度为3.71%FS,重复性为7.43%FS,迟滞性为3.85%FS。     

一种新型的Bragg光纤光栅电流传感器的设计

提出了一种新型的光纤光栅高压电流传感器。该系统由Bragg光纤光栅电流测量部分和温度补偿部分组成。在测量过程中,采用了匹配光栅解调方法,其动态测量范围比通常的干涉法光纤电流传感器大,且系统不受外界电磁干扰的影响,可实现高压电流的在线测量。     

A novel fiber Bragg grating high-temperature sensor

A novel fiber Bragg grating (FBG) sensor for the measurement of high temperature is proposed and experimentally demonstrated. The interrogation system of the sensor system is simple, low cost but effective. The sensor head is comprised of one FBG and two metal rods. The lengths of the rods are different from each other. The coefficients of thermal expansion of the rods are also different from each other. The FBG will be strained by the sensor head when the temperature to be measured changes. The temperature is measured basis of the wavelength shifts of the FBG induced by strain. A dynamic range of 0–800 °C and a resolution of 1 °C have been obtained by the sensor system. The experiment results agree with theoretical analyses.     

Bragg光纤光栅法布里-珀罗传感器的应变测量

提出了一种对静态应变进行高分辨率测量的方法。该方法是把Bragg光纤光栅(FBG)与全光纤法布里_珀罗传感器(FFPI)组合起来,利用FBG反射波长的漂移特性来测量干涉条纹的数目。通过利用FFPI和伪外差解调法对小数条纹进行解调,可实现高分辨率的测量。结果表明,静态应变的测量分辨率达到了33×10-9ε(ε为应变)。     

Elimination of environmental noise in interferometric wavelength shift demodulation for dynamic fiber Bragg grating sensor array

We present a novel reference compensation method for eliminating environmental noise in interferometric wavelength shift demodulation for dynamic fiber Bragg grating (FBG) sensors. By employing a shielded wavelength-division-multiplexed reference FBG in the system, the environmental noise is measured from the reference channel, and then subtracted from the demodulation result of each sensor channel. An approximate 40 dB reduction of the environmental noise has been experimentally achieved over a frequency range from 20 Hz to 2 kHz. This method is also suitable for the elimination of broadband environmental noise. The corresponding FBG sensor array system proposed in this paper has shown a wavelength resolution of .     

体相位全息光栅式波分复用器的研究

利用体相位全息光栅的优良特性,研制光栅式密集型波分复用器(DWDM)。准直后的信道波长通过体相位全息光栅两次衍射,在焦面上实现信道波长的分复用。阐述了体相位全息光栅的独特结构特点,给出了体相位全息光栅式密集型光波分复用器件的原理图,计算出各个信道的位置及间隔,用Zemax仿真出设计模型。相比于薄膜滤光片式器件而言,具有许多独特的优良特性,能够实现更密集更多信道数量的分复用。     

Dual wavelength erbium-doped fiber laser with a lateral pressure-tuned Hi-Bi fiber Bragg grating

Tunable dual wavelength erbium-doped fiber laser (EDFL) with stable oscillation at room temperature is proposed and demonstrated. This laser utilizes a Bragg grating fabricated in a high birefringence fiber as the wavelength-selective component, and then achieves the stable dual wavelength oscillation by introducing the polarization hole burning effect. Furthermore, by applying lateral strain upon the fiber Bragg grating (FBG), the space of the laser dual wavelengths can be tuned continuously.     

Dual wavelength erbium-doped fiber laser with a lateral pressure-tuned Hi-Bi fiber Bragg grating

Tunable dual wavelength erbium-doped fiber laser (EDFL) with stable oscillation at room temperature is proposed and demonstrated. This laser utilizes a Bragg grating fabricated in a high birefringence fiber as the wavelength-selective component, and then achieves the stable dual wavelength oscillation by introducing the polarization hole burning effect. Furthermore, by applying lateral strain upon the fiber Bragg grating (FBG), the space of the laser dual wavelengths can be tuned continuously.     

Acoustic-induced switching of the reflection wavelength in a fiber Bragg grating

Coupling between copropagating core and cladding modes was implemented by acoustic generation of lateral vibration of an etched fiber. When these coupling processes were combined with counterpropagating coupling of a core mode and a cladding mode and the Bragg reflection of a fiber grating, switching of reflection wavelength between the Bragg wavelength and cladding-mode coupling wavelengths was achieved. We report the implementation of such acoustically induced switching behaviors and explain their operation principles. The implemented results can be used for wavelength-division multiplexed add-drop filtering.     

High sensitivity fiber Bragg grating pressure difference sensor

Based on the effect of fiber Bragg grating (FBG) pressure difference sensitivity enhancement by encapsulating the FBG with uniform strength beam and metal bellows, a FBG pressure difference sensor is proposed, and its mechanism is also discussed. The relationship between Bragg wavelength and the pressure difference is derived, and the expression of the pressure difference sensitivity coefficient is also given. It is indicated that there is good linear relation between the Bragg wavelength shift and the pressure difference of the sensor. The theoretical and experimental pressure difference sensitivity coefficients are 38.67 and 37.6 nm/MPa, which are 12890 and 12533 times of that of the bare FBG, respectively. The pressure difference sensitivity and dynamic range can be easily changed by changing the size, Young's modulus, and Poisson's ratio of the beam and the bellows.     

双套管式光纤Bragg光栅温度传感器

研制了一种双套管式光纤Bragg光栅温度传感器,以实现无外力作用的温度测量。其中外套管隔离外加应力应变,在内套管内松弛地放置光栅以隔离封装结构的表观热应变。引入引出尾纤穿过带孔螺栓,以探测多个串联光栅。为了避免光栅处于张拉状态,封装在外套管1、外套管2和内套管中的光纤余长应分别大于1.4mm,1.8mm和0.4mm。试验结果表明,该光栅传感器的温度响应灵敏度为9.671×10-3nm/℃,温度测量分辨率为0.1℃。当水温从20℃跃变到70℃时,该传感器的温度响应时间分别为t0.5=15±2s和t0.9=52±4s。当水温从70℃跃变到20℃时,该传感器的温度响应时间分别为t0.5=26±9s和t0.9=63±8s。     

Acceleration test of substrate-packaged fiber Bragg grating sensor北大核心CSCD

To test whether the fiber Bragg grating (FBG) sensor can endure the steady-state inertial loads caused by the acceleration and the sensing properties during the loads, a FBG strain and temperature sensor with aluminium alloy substrate package was designed, and the acceleration performance on the sensor was tested. The sizes of FBG strain and temperature sensor were designed and its package process was described. The strain and temperature sensing mechanisms of FBG sensor were analyzed, and the spectrum detection and demodulation system based on volume phase grating and linear array photodetector was developed. Finally, the acceleration test equipment was established, and the acceleration performance test of the selected FBG strain and temperature sensor was carried out in accordance with the requirements and methods of GJB150.15A acceleration test. The experimental results show that in the 2 min performance test before and after the acceleration test, the wavelength offset is below to ±50 pm, and the change of light intensity is below to 0.3 V. In acceleration test, the maximum fluctuation of wavelength offset is ±7 pm, and the light intensity is in the range of 1.3 V~4.003 V. It is proved that the designed FBG sensor has the ability to endure the acceleration loads and has the good sensing performance during the acceleration loads. Copyright ©2022 Journal of Applied Optics. All rights reserved.