Ultra-weak fiber Bragg grating (UWFBG) sensing technology has considerable multiplexing capacity on one fiber. Therefore, UWFBG have the potential of distributed monitoring, and its monitoring accuracy is higher than that of distributed optical fiber sensing technology. In this work, a novel deformation monitoring method based on UWFBG for high-speed railway girders was proposed, and the deformation of a real high-speed railway girder was monitored by UWFBG. In the test, an UWFBG array was pasted on the bottom of a real high-speed railway beam by epoxy resin, and a vibrating wire strain gauge was pasted in the middle of the span for comparison. The beam was loaded step by step through a reaction frame. During the loading process, the deformation of the beam at different positions was recorded by the UWFBG array. The results show that this method can effectively monitor the deformation of high-speed railway girder, and provide safety guarantee for the normal operation of high-speed railway.
Distributed fiber optic sensing techniques are suitable for pipeline leakage monitoring owing to their long-distance sensing abilities. In this work, a novel pipeline leakage detection and localization method using a distributed fiber optic sensor is proposed. In contrast to conventional methods, the optical fiber is placed in the fluid inside the pipeline without any constraints. When leakage occurs, the local flow field close to the leakage position is disturbed and the fluid pressure caused by the fluid disturbance leads to optical fiber deformation or vibration. With the help of the phase-sensitive optical time domain reflection technique, the fluid disturbance can be detected over a long distance. This work first analyzed the fluid action on the optical fiber. Then, the effects on the fluid field of the local leakage are obtained through finite element simulations. Finally, the proposed method is demonstrated via a series of laboratory experiments.
Landslide disasters often cause huge property losses and casualties. Therefore, distributed real-time landslide deformation monitoring is of great significance to ensure the safety of people’s lives and property, and the stability evaluation of landslide. Ultra-weak fiber Bragg grating (UWFBG) sensing technology can realize the measurement distance of 20 km for single optical fiber, and the single measurement time is less than 1 s. Therefore, it has the potential of distributed realtime monitoring. In this work, a novel distributed real-time landslide deformation monitoring method based on UWFBG was proposed. And this method was applied to a real landslide in Yibin City, Sichuan Province, China, to monitor its deformation in situ. The feasibility of the monitoring method was verified and the landslide deformation was analyzed. The in-situ monitoring results show that on the time scale, the local deformation of the landslide is gradually increasing; on the spatial scale, there are some points with large local deformation, but the overall deformation in the monitoring area is small. The monitored results also suggest that the landslide is in a stable state during the monitoring period.
Phase-sensitive optical time domain reflectometry (Φ-OTDR) technology is a disturbance monitoring technology based on Rayleigh scattering. The principle is that when sensing fiber is disturbed, the refractive index at the disturbance point will change, which will affect the intensity of Rayleigh backscattered light, so as to locate disturbance point. This technology can be used for perimeter security, intrusion monitoring etc. For a landslide about to slide, timely and accurate detection of intrusion events on it is of great significance for the evaluation of landslide state and timely response. In this paper, we proposed a method for landslide intrusion monitoring based on Φ-OTDR. We first studied the performance of the instrument used in this work. Then, different intrusion behaviors were simulated and monitored. And the signal characteristics of different behaviors were analyzed. Test results show that different intrusion events can be distinguished by event frequency.
Continuous and distributed monitoring of ground settlement is of great significance for the evaluation of ground stability. To realize the distributed on-line and real-time monitoring of ground settlement, a ground settlement strain-displacement transformation algorithm based on distributed strain data was proposed in this work. The distributed strain data was obtained by distributed optical fiber sensing (DOFS) technology. The effectiveness of the algorithm was first verified by numerical simulation, and the factors affecting the accuracy of the algorithm were analyzed. Then, experiments were carried out to verify the effectiveness of the algorithm for actual settlement monitoring. Test results show that the ground settlement displacement obtained by the proposed algorithm is basically consistent with the actual ground settlement displacement, which can meet the needs of ground settlement monitoring. However, there are still errors in the results obtained by the algorithm, which mainly come from, strain monitoring accuracy, and spatial resolution of the demodulator (k value). The smaller the k value, the higher the strain accuracy, the higher the accuracy of the algorithm.
Soil moisture field has an obvious influence on the stability of reservoir bank slope. Thus, effective, real-time, reliable, and high precision monitoring methods are imperative. However, the existing soil moisture monitoring methods have more or less deficiencies. In this paper, a novel soil moisture monitoring and estimation method based on an ultra-weak fiber Bragg grating (UWFBG) array is proposed which is suitable for large-scale in-situ monitoring. A soil moisture monitoring device is designed and fabricated. This monitoring device consists of several monitoring units, and each monitoring unit consists of a soil container, protector and two UWFBG. Laboratory calibration tests were conducted to establish a relationship between wavelength change and soil moisture content. The test results show that the monitoring accuracy of the device is related to the quality of the loaded dry soil. When the mass of dry soil is 100 g, the monitoring accuracy of soil moisture content is 1.68 % (mass ratio).
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