Brillouin Optical Time-domain Analysis Research Articles

Laboratory Measurement of Excavation Disturbance Effect Due to Trenchless Construction Using Distributed Fiber Optics Strain Sensing

Abstract Excavation disturbance induced by trenchless construction may lead to underground movement and deformation, which can be a potential hazard to the safety of overlying pavement structure. However, the development of a fiber optic–distributed sensing approach in recent years has provide an effective way for continuous measurement of soil strain. In this study, the mechanism of the excavation disturbance effect that is due to trenchless construction was investigated based on laboratory experiments. The sensing fibers were embedded within the overburden layer above the excavation surface to measure the distributed soil strain using Brillouin optical time-domain analysis. The relationships between ground settlements and soil strains were analyzed. The partition theory of excavation disturbance zone was verified based on test results. The influence of excavation on mechanical responses of overlying model pavement, including bending deformation and soil pressure, was measured and discussed. Laboratory test results indicated that soil strains were nearly symmetrically distributed around the excavation area, which was compressive close to the centerline and then became tensile on both sides of the excavation area. The width of the excavation disturbance zone derived from the soil strain measurements was found close to the calculation results based on the partition theory. Because of the uneven distribution of the support force that was caused by foundation settlement, the model pavement showed notable bending deformation after excavation. Therefore, the combined effects of traffic loading and excavation disturbance should be considered in the safety evaluation of trenchless construction.

PPP-BOTDA Distributed Optical Fiber Sensing Technology and Its Application to the Baishuihe Landslide

Serious landslide hazards are prevalent along the Yangtze River in China, particularly in the Three Gorges Reservoir area. Thus, landslide monitoring and forecasting technology research is critical if landslide geological hazards are to be prevented and controlled. Pulse-prepump brillouin optical time domain analysis (PPP-BOTDA) distributed optical fiber sensing technology is a recently developed monitoring method with evident advantages in precision and spatial resolution. Herein, fixed-point immobilization and direct burying methods were adopted to arrange parallel distribution of the strain and temperature-compensated optical fibers along the Baishuihe landslide’s front edge, in order to carry out ground surface deformation monitoring. The strain data acquired from both optical fibers were processed with temperature compensation to obtain the actual optical fiber strain produced by deformation. Butterworth low-pass filter denoising method was employed to determine the filter order (n) and cut-off frequency (Wn). The area differences between the two optical fiber monitoring curves and the fixed horizontal axis were selected as evaluation indexes to obtain the area difference along the optical fiber. This data were then leveraged to determine the positive correlation between the area difference and the optical fiber strain variation degree. Finally, these results were compared with the GPS and field measured data. This study shows that when PPP-BOTDA technology is used for landslide surface deformation monitoring in conjunction with Butterworth filter denoising and strain area difference, the optical fiber strain variation degree analysis results are consistent with the GPS monitoring data and the actual landslide deformation. As such, this methodology is highly relevant for reducing the workload and improving the monitoring precision in landslide monitoring, which in turn will protect lives and property.

Strain Test Performance of Brillouin Optical Time Domain Analysis and Fiber Bragg Grating Based on Calibration Test

Strain Test Performance of Brillouin Optical Time Domain Analysis and Fiber Bragg Grating Based on Calibration Test

System design and demonstration of performance monitoring of a butterfly‐shaped arch footbridge

This paper presents an overview of the structural performance monitoring system for a butterfly-shaped arch footbridge located at The Hong Kong Polytechnic University. The system is the product of a multidisciplinary academic collaboration of the pooling expertise of four academic departments, namely, structural engineering, traffic engineering, surveying, and optical communication engineering. This system consists of 88 sensors of 13 types, including an advanced Brillouin optical time-domain analysis technology, an optical fiber Bragg grating technology, a global navigation satellite system (GNSS), and high-performance video cameras. The paper summarizes the modular design, subsystem framework, and advanced features of the monitoring system. Preliminary monitoring results, such as high spatial resolution distributed temperature/strain measurement, global heat-transfer analysis, pedestrian traffic analysis, dynamic characteristics, and GNSS measurements, are introduced. A fire accident is employed to demonstrate the function and value of the monitoring system and provided a rare opportunity to detect damages in a real structure. The monitoring system not only monitors the performance of the bridge in real-time but also provides a multifunctional platform, including an Internet-based software for research and teaching and a touchscreen visualization platform for public education.

Simulation Analysis on the Simultaneous Deployment of Brillouin Gain and Loss in Coded Brillouin Optical Time Domain Analysis (BOTDA) Fiber Sensor

We report in this paper the simulation analysis on the simultaneous deployment of Brillouin gain and Brillouin loss in the Golay coded phase-shift pulse Brillouin optical time domain analysis (PSP-BOTDA) fiber optic sensor for improving the sensing distance and at the same time obtaining high spatial resolution measurement. In this technique, the Golay coded pump pulse is alternately frequency modulated for the purpose of generating Stokes and Anti-Stokes scattering. The technique also modulates the coded pump with the return-to-zero (RZ) formats. Simulation analysis has revealed that the simultaneous use of Brillouin gain- and loss and the pulse coding technique in the PSP-BOTDA has improved the dynamic sensing range and the signal-to-noise improvement ratio (SNIR) of the sensor. By performing simulation over 500 m of fiber, we have also successfully demonstrated 10-cm of high spatial resolution measurement with the use of 1ns of coded pulse duration.

Single-Fiber-Based Brillouin Optical Time Domain Analysis With Far-End Modulation

The loop configuration consisting of a lead fiber and a sensing fiber is a common approach to realize Brillouin Optical Time-Domain Analysis (BOTDA), which is however impractical to implement for some field scenarios where only a single fiber is physically available. In this paper, a novel single-fiber-based BOTDA with far-end modulation is proposed and experimentally demonstrated, in which an auxiliary light and a pulse are injected from the near-end of the fiber, and the counter-propagating probe light is generated by modulating the auxiliary light from the far-end. By dedicatedly designing the frequency and temporal profile of each lightwave, Brillouin interaction takes place only between the pulse and the probe light, while stray effects such as four wave mixing and Rayleigh beating noise are avoided, which constitutes the key advantage of the propose technique with respect to traditional single-fiber-based BOTDA techniques. A 80-km-long sensing range, 1.73 MHz frequency accuracy and 2 m spatial resolution is experimentally demonstrated with 4000 trace averages.

DWI-Assisted BOTDA for Dynamic Sensing

A co-propagating dual-wave interferometer- assisted Brillouin optical time domain analysis (DWI-BOTDA) integrated sensing system is proposed and experimentally demonstrated for high sampling rate dynamic sensing. Fast dynamic strain measurement is achieved by the DWI based on analyzing the relative phase variation between two co-propagating light waves with significantly improved dynamic strain measurement range (DSMR). Meanwhile, the BOTDA with high spatial resolution is employed to locate the dynamic events accurately. In the experiment, the enhancements on the DSMR and sensing stability in the DWI are validated. Then, the DWI-BOTDA is implemented to measure periodical mechanical vibration over 2 km sensing range with 10 kHz sampling rate and 2 m spatial resolution.

Ultrafast Distributed Brillouin Optical Fiber Sensing Based on Optical Chirp Chain

In the past few years, Brillouin-based optical fiber sensors have gained significant ground for distributed temperature and strain measurements in the real world. Among these sensors, the optical chirp chain (OCC) based Brillouin optical fiber sensor is a good candidate to realize ultrafast distributed sensing, which is of great importance to distinguish quick-changing events in practical applications. In this paper, the concept, principle and sensing scheme of OCC are introduced, including the OCC Brillouin optical time-domain analysis and OCC Brillouin optical time-domain reflectometry. In OCC Brillouin optical time-domain analysis, the influence of transient interaction is reflected as three types of spectral distortions, i.e., the back end distortion, the frequency lag of main peak and the frequency saltation distortion, which are verified in the simulation and experiment and attributed to the rapid frequency sweeping. In addition, this paper reviews the OCC based cutting-edge schemes and technologies aspects to upgrade Brillouin sensors with ultra-fast and high-performance sensing capability. In the future, there are still many challenges in OCC based distributed sensing, such as high spatial resolution, real-time demodulation and low-cost scheme.

Damage detection of tunnel based on the high-density cross-sectional curvature obtained using strain data from BOTDA sensors

Fully distributed Brillouin-scattering-based optical sensors—e.g., Brillouin optical time-domain analysis (BOTDA) sensors—appear to be promising supplementary tools for the SHM of actual tunnels. As far as we know, there is little research on detecting the damage of tunnels by using the strain continuous monitoring data obtained from BOTDA sensors. To address this issue, a method based on high-density cross-sectional curvature is proposed for the damage detection of operating tunnel. First, considering the long-distance and large-scale characteristics of actual tunnels, the strain is not sufficiently sensitive to structural damage; thus, the structural strain is converted into the cross-sectional curvature of the tunnel in order to improve this sensitivity. The condition required for converting the strain at the measured points into corresponding measurements of the cross-sectional curvature of the tunnel is discussed, and on this basis, a discriminative approach to the abovementioned conversion is presented. Second, a damage detection index based on high-density cross-sectional curvature is established, and combined with the novel detection approach, the proposed index is implemented to detect the damage of the tunnel. Finally, numerical examples are presented to analyze and discuss the noise resistance performance and structural damage sensitivity of the proposed method. Moreover, the effectiveness of the proposed method is demonstrated using strain monitoring data obtained from an actual tunnel with a box-type structure.

Distributed optomechanical fiber sensing based on serrodyne analysis

Distributed measurement of forward stimulated Brillouin scattering (FSBS) attracted substantial attention for its ability to probe media surrounding optical fibers. Currently, all techniques extract the information from the FSBS-induced local energy transfer among distinct optical tones, this transfer being fundamentally sensitive to intensity perturbations imposed by nonlinear effects. Instead, here we propose to extract the local FSBS information by measuring the frequency shift of a short optical pulse subject to the phase chirp modulation caused by harmonic FSBS oscillation. In full contrast with existing techniques, the optical pulse is much shorter than the period of the acoustic oscillation, enabling ultrashort spatial resolutions, and its frequency shift is precisely probed by a standard Brillouin optical time-domain analyzer. The proposed technique is validated in both remote and integrally distributed sensing configurations, demonstrating spatial resolutions of 0.8 m and 2 m, respectively, substantially outperforming state-of-the-art techniques.

Experimental study on mechanical properties of prestressed concrete cylinder pipes (PCCPs) under external load

Prestressed concrete cylinder pipes (PCCPs) have been applied for various water conveyance projects recently, especially in long distance. PCCPs not only bear internal water pressures, but also external loads. The excess of external loads may cause major accidents. However, the mechanical behavior of PCCPs under external load is different from that of PCCPs under internal pressure. In this study, a three-edge bearing test (TEBT) of a PCCP was performed to study its mechanical properties under external load. Meanwhile, the fiber Bragg grating (FBG) sensing technology and pulse pre-pump Brillouin optical time-domain analysis (PPP-BOTDA) optical fiber sensing technology were successfully employed to monitor the deformation of PCCPs. The test results show that the deformation of five components of the pipe is proportional to the external load. At the springline of the pipe, the outer concrete core and the prestressing wire are in tension, the inner concrete core and the steel cylinder are in compression. The loading status at the crown and invert of the pipe are opposite with those at the springline of the pipe, but the deformation coordination is consistent. Moreover, the test results demonstrate that PPP-BOTDA-based distributed optical fiber sensors can not only accurately monitor the real-time loading response of the PCCP under the external load like FBG point optical fiber sensors, but also obtain the dynamic and continuous strain distribution of the circumferential section of the pipe. This study has identified the relationship of the external load and deformation, the interaction and strain distribution of each component of PCCPs. This study also confirms that the distributed optical fiber sensing technology can provide an effective means for the structural health monitoring in long-term operation of PCCPs.

Benefits of Spectral Property Engineering in Distributed Brillouin Fiber Sensing.

As one of the most consolidated distributed fiber sensors based on stimulated Brillouin scattering, the Brillouin optical time-domain analyzer (BOTDA) has been developed for decades. Despite the commercial availability and outstanding progresses which has been achieved, the intrinsic Lorentzian gain spectrum restricts the sensing performance from possible further enhancements and hence limits the field of validity of the technique. In this paper, the novel method of engineering the gain spectral properties of the Brillouin scattering and its application on static and dynamic BOTDA sensors will be reviewed. Such a spectral property engineering has not only provided improvements to BOTDA, but also might open a new way to enhance the performance of all kinds of distributed Brillouin fiber sensors.

Estimation of the buried pipeline displacement using distributed fibre optic sensing: An experimental study

Pipe displacement (bending) is one of the most prominent risks caused by the soil movements in the landslide areas. The lateral soil movement causes damages to the buried pipelines, which might explode due to leakage of flammable materials. Employing fibre optic sensors to monitor the buried pipes can maintain the integrity of piping systems and the surrounding environment. The fibre optic sensor has already demonstrated its capabilities to provide a continuous monitoring system for the structures. This paper aims to evaluate the buried pipeline displacement caused by the lateral soil movement. Brillouin Optical Time Domain Analysis (BOTDA) is used to measure the strain values from the fibre optic cable. A small physical model is used to demonstrate the bending of High-density polyethylene (HDPE) pipe under the ground movement. This paper concludes that the fibre optic sensor could be employed to evaluate the horizontal and vertical displacements of the buried pipeline.

Phase fluctuation cancellation for coherent-detection BOTDA fiber sensors based on optical subcarrier multiplexing.

The employment of coherent detection in a Brillouin optical time domain analysis (BOTDA) fiber sensor brings benefits, including signal-to-noise ratio enhancement, non-local effect reduction, and sensing speed improvement. Recently, it was found that the performance of a coherent-detection BOTDA fiber sensor suffers from phase fluctuations introduced by the fiber group delay jitter. Here, we propose a phase fluctuation cancellation approach based on optical subcarrier multiplexing. In a proof-of-concept experiment, the phase stability for in-phase/quadrature demodulation reaches a standard deviation value of as small as 0.4 mrad. The variations in the Brillouin gain and phase spectra caused by the phase fluctuation are then effectively alleviated, resulting in an enhancement of the Brillouin frequency shift measurement certainty along the whole sensing fiber.

Fast extraction for Brillouin frequency shift in BOTDA system

In this paper, we introduce the basic principle of the Brillouin optical time domain analysis (BOTDA) system and simulate the gain-type BOTDA system. In order to extract the Brillouin frequency shift more accurately, the filtering method is vitally important. In this letter, Gaussian filtering, median filtering and hybrid filtering are used to remove the noise respectively. It is observed that hybrid filtering has the best performance among the three methods, which can improve the signal-to-noise ratio by 9.18 dB. Besides, a peak location method is proposed to quickly extract the center frequency of the filtered Brillouin gain spectrum. This method reduces processing complexity by omitting Lorentz fitting and it possesses the advantages of fast processing rate and high accuracy. Therefore, the peak location method we proposed can be considered as an attractive method for extracting temperature distributions.

Distributed fiber optic measurements of strain and temperature in long-span composite floor beams with simple shear connections subject to compartment fires

This study explores an instrumentation strategy using distributed fiber optic sensors to measure strain and temperature through the concrete volume in large-scale structures. Single-mode optical fibers were deployed in three 12.8 m long steel and concrete composite floor specimens tested under mechanical or combined mechanical and fire loading. The concrete slab in each specimen was instrumented with five strain and temperature fiber optic sensors along the centerline of the slab to determine the variation of the measurands through the depth of the concrete. Two additional fiber optic temperature sensors were arranged in a zigzag pattern at mid-depth in the concrete to map the horizontal spatial temperature distribution across each slab. Pulse pre-pump Brillouin optical time domain analysis (PPP-BOTDA) was used to determine strains and temperatures at thousands of locations at time intervals of a few minutes. Comparisons with co-located strain gauges and theoretical calculations indicate good agreement in overall spatial distribution along the length of the beam tested at ambient temperature, while the fiber optic sensors additionally capture strain fluctuations associated with local geometric variations in the specimen. Strain measurements with the distributed fiber optic sensors at elevated temperatures were unsuccessful. Comparisons with co-located thermocouples show that while the increased spatial resolution provides new insights about temperature phenomena, challenges for local temperature measurements were encountered during this first attempt at application to large-scale specimens.

Optical characterization of strain sensing cables for Brillouin optical time domain analysis

Two innovative optical fibre cable layouts designed to improve BOTDA strain measurement accuracy through improved strain transfer efficiency are presented, discussed and tested through experiments, analytical and numerical modelling. The second improved design presents good features to minimize the mismatch between measured and actual strain.

Monitoring of Deformation Behaviour of Unsaturated Soil Slope using Distributed Optical Fibre Sensor

A distributed optical fibre sensing system named Brillouin Optical Time Domain Analysis (BOTDA) is used to monitor the strain development of a laboratory soil slope model. The technology is yet to be fully implemented due to uncertainties of attachment method or the best way to set up optical fibre sensors for geo-structure health monitoring. The aim of study is to evaluate the deformation behaviour subjected to the development of horizontal strains from Brillouin-based optical fibre sensor of a residual soil slope under loading impact using BOTDA technology. In this study, a soil-embedded strain sensor placement approach was proposed to be installed in the 1g model of soil slope which was achieved via the horizontal planting of a three-layered optical fibre cable in S-curve forming slope. In this paper, however, only pilot tests result is demonstrated for preliminary data interpretation purposes. From the preliminary laboratory tests, the results show the soil-embedded sensing fibre arrangement has efficiently detected and measured the horizontal strain deformation due to loading. Therefore, it can be concluded that the sensing fibre was well-responded with the soil movement under loading impact.

Improving the Spatial Resolution of a BOTDA Sensor Using Deconvolution Algorithm

Spatial resolution improvement from an acquired measurement using long pulse is developed for Brillouin optical time domain analysis (BOTDA) systems based on the total variation deconvolution algorithm. The frequency dependency of Brillouin gain temporal envelope is investigated by simulation, and its impact on the recovered results of deconvolution algorithm is thoroughly analyzed. To implement a reliable deconvolution process, differential pulse-width pair (DPP) technique is utilized to effectively eliminate the systematic BFS distortion stemming from the frequency dependency of temporal envelope. The width of the pulse pairs should be larger than 40 ns as is analyzed theoretically and verified experimentally. It has been demonstrated that the proposed method can realize flexible adjustment of spatial resolution with enhanced signal-to-noise ratio (SNR) from an established measurement with long pump pulse. In the experiment, the spatial resolution is increased to 0.5 m and 1 m with high measurement accuracy by using the deconvolution algorithm from the measurement of 60/40 ns DPP signals. Compared with the raw DPP results with the same spatial resolution, 9.2 dB and 8.4 dB SNR improvements are obtained for 0.5 m and 1 m spatial resolution respectively, thanks to the denoising capability of the total variation deconvolution algorithm. The impact of sampling rate on the recovery results is also studied. The proposed sensing system allows for distortion-free Brillouin distributed sensing with higher spatial resolution and enhanced SNR from the conventional DPP setup with long pulse pairs.

Structural Crack Detection Using DPP-BOTDA and Crack-Induced Features of the Brillouin Gain Spectrum

Structural damage generally initiates in the form of structural cracks. Thus, developing efficient crack detection techniques is of great importance for the structural health monitoring. In this paper, a new crack identification method is proposed, which is based on the differential pulse-width pair Brillouin optical time domain analysis (DPP-BOTDA) technology and the irregular features of Brillouin gain spectrum (BGS) in the fiber due to structural cracks. The proposed method provides a new way to detect and quantify structural cracks without knowing the strain in the structure. First, the working mechanism of DPP-BOTDA is introduced to illustrate the reason that the DPP-BOTDA, compared to traditional BOTDA technique, can significantly improve the spatial resolution of distributed strain sensing, which is critical for structural crack detection. Then, the BGSs in the fiber with the presence of structural cracks, measured by the DPP-BOTDA, are numerically simulated, from which the crack-induced irregular features of the BGS are summarized. Based these irregular features, new structural crack detection and quantification methods are proposed, which are found to be independent of structural stain. Finally, an experiment is conducted on a simple supported reinforced concrete (RC) beam. The results demonstrate that by using the BGS measured by the DPP-BOTDA, the proposed structural crack identification method successfully detects the occurrence of structural cracks and relatively accurately predicts the crack widths.