Ultra-weak Fiber Bragg Gratings Research Articles

The Impact of Rayleigh Scattering in UWFBG Array-Based Φ-OTDR and Its Suppression Method.

Ultra-weak fiber Bragg grating (UWFBG) array-based phase-sensitive optical time-domain reflectometry (Φ-OTDR) utilizes the interference interaction between the reference light and the reflected light from the broadband gratings for sensing. It significantly improves the performance of the distributed acoustic sensing system (DAS) because the intensity of the reflected signal is much higher than that of the Rayleigh backscattering. This paper shows that Rayleigh backscattering (RBS) has become one of the primary noise sources in the UWFBG array-based Φ-OTDR system. We reveal the impact of the Rayleigh backscattering signal on the intensity of the reflective signal and the precision of the demodulated signal, and we suggest reducing the pulse duration to improve the demodulation accuracy. Experimental results demonstrate that using light with a 100 ns pulse duration can improve the measurement precision by three times compared with the use of a 300 ns pulse duration.

Thermo-hydro-poro-mechanical responses of a reservoir-induced landslide tracked by high-resolution fiber optic sensing nerves

Thermo-poro-mechanical responses along sliding zone/surface have been extensively studied. However, it has not been recognized that the potential contribution of other crucial engineering geological interfaces beyond the slip surface to progressive failure. Here, we aim to investigate the subsurface multi-physics of reservoir landslides under two extreme hydrologic conditions (i.e. wet and dry), particularly within sliding masses. Based on ultra-weak fiber Bragg grating (UWFBG) technology, we employ special-purpose fiber optic sensing cables that can be implanted into boreholes as “nerves of the Earth” to collect data on soil temperature, water content, pore water pressure, and strain. The Xinpu landslide in the middle reach of the Three Gorges Reservoir Area in China was selected as a case study to establish a paradigm for in situ thermo-hydro-poro-mechanical monitoring. These UWFBG-based sensing cables were vertically buried in a 31 m-deep borehole at the foot of the landslide, with a resolution of 1 m except for the pressure sensor. We reported field measurements covering the period 2021 and 2022 and produced the spatiotemporal profiles throughout the borehole. Results show that wet years are more likely to motivate landslide motions than dry years. The annual thermally active layer of the landslide has a critical depth of roughly 9 m and might move downward in warmer years. The dynamic groundwater table is located at depths of 9–15 m, where the peaked strain undergoes a periodical response of leap and withdrawal to annual hydrometeorological cycles. These interface behaviors may support the interpretation of the contribution of reservoir regulation to slope stability, allowing us to correlate them to local damage events and potential global destabilization. This paper also offers a natural framework for interpreting thermo-hydro-poro-mechanical signatures from creeping reservoir bank slopes, which may form the basis for a landslide monitoring and early warning system.

Vehicle identification using deep learning for expressway monitoring based on ultra-weak FBG arrays.

A deep learning with knowledge distillation scheme for lateral lane-level vehicle identification based on ultra-weak fiber Bragg grating (UWFBG) arrays is proposed. Firstly, the UWFBG arrays are laid underground in each expressway lane to obtain the vibration signals of vehicles. Then, three types of vehicle vibration signals (the vibration signal of a single vehicle, the accompanying vibration signal, and the vibration signal of laterally adjacent vehicles) are separately extracted by density-based spatial clustering of applications with noise (DBSCAN) to produce a sample library. Finally, a teacher model is designed with a residual neural network (ResNet) connected to a long short-term memory (LSTM), and a student model consisting of only one LSTM layer is trained by knowledge distillation (KD) to satisfy the real-time monitoring with high accuracy. Experimental demonstration verifies that the average identification rate of the student model with KD is 95% with good real-time capability. By comparison tests with other models, the proposed scheme shows a solid performance in the integrated evaluation for vehicle identification.

Distributed dynamic strain measurement with a direct detection scheme by using a three-step-phase-shifted double pulse in a UWFBG array.

We demonstrate a stable homodyne phase demodulation method with a double pulse based on an ultra-weak fiber Bragg grating (UWFBG) array. The technique divides one of the probe pulses into three sections and introduces successive 2π/3 phase differences into each section. By using a simple direct detection scheme, it can achieve distributed and quantitative vibration measurement along the UWFBG array. Compared to the traditional homodyne demodulation technique, the proposed technique is more stable and easier to accomplish. Moreover, the reflected light from the UWFBGs can provide a signal that is modulated uniformly by the dynamic strain and multiple results for averaging, resulting in a higher signal-to-noise ratio (SNR). We experimentally demonstrate the technique's effectiveness by monitoring different vibrations. The SNR for measuring a generic 100 Hz, 0.08 rad vibration in a 3 km UWFBG array with a reflectivity of -40 to -45 dB is estimated to be ∼44.92 dB.

Anti-noise UWFBG-array enhanced DAS system using double-pulse-based time-domain adaptive delay interference.

An anti-noise interrogation technique for ultra-weak fiber Bragg grating (UWFBG)-based distributed acoustic sensing (DAS) systems is proposed and demonstrated using double-pulse-based time-domain adaptive delay interference. This technique breaks the limitation that the optical path difference (OPD) between the two arms of the interferometer should be completely matched with the entire OPD between the adjacent gratings in the traditional single-pulse system. The length of the delay fiber in the interferometer can be reduced, and the double-pulse interval can adapt flexibly to the UWFBG array with different grating spacing. The acoustic signal is restored accurately when the grating spacing is 15 m or 20 m by the time-domain adjustable delay interference. Moreover, the noise induced by the interferometer can be suppressed significantly as compared to using a single pulse, and above 8-dB signal-to-noise ratio (SNR) enhancement can be obtained without any extra optical devices when the noise frequency and the vibration acceleration are below 100 Hz and 0.1 m/s2, respectively.

Evaluating the effect of post-grouting on long bored pile based on ultra-weak fiber Bragg grating array

Fiber optic sensing has been widely acknowledged as a dependable tool to monitor pile performance. However, its applicability is restricted by the limited multiplexing capacity of fiber Bragg gratings (FBG) or the high equipment cost and poor real-time performance of distributed fiber optic sensing systems. In this paper, a novel sensing cable and system were developed based on ultra-weak FBG array for strain measurement of large-scale engineering structures. Through the laboratory calibration and field test of a post-grouting pile, the good strain measurement performance of the sensing cable and system were confirmed. Based on the distributed strain data obtained by the UWFBG array, the strengthening mechanism of the pile was well explained. The UWFBG array and sensing system could realize distributed, high-precision, and real-time strain measurement with a spatial resolution of 1 m within 1.5 km, which has great potentials in the monitoring of geo-environment and engineering structures.

Ultra-weak FBG sensing for identification and analysis of plastic zone of soil caused by supported excavation

Deformation caused by supported excavation makes significant impact on foundation pit stability. Evaluation of potential plastic zone is crucial for early warning of foundation pit instability. To address the shortcomings of existing evaluation methods, a new identification method based on the ultra-weak fiber Bragg grating (UWFBG) technology was proposed. To validate this approach, a UWFBG monitoring program was implemented in the boreholes as part of the Shanghai Metro Line 18 project. The monitoring results were utilized to determine the range of plastic zone for each borehole. It was noted that there are two plastic deformation zones formed with different mechanisms, one located near the base of the diaphragm wall (DW) and the other is distributed within the range of 6–19 m below the excavation face. Lastly, a criterion of early warning of foundation pit instability was presented. By analyzing the monitoring data, the highest risk level of this project was determined as “Attention”, implying the necessity of taking measures to prevent the local plastic zone from evolving into a full-scale sliding surface.

A Continuous Tilt Sensor Based on UWFBG Technology for Landslide Real-Time Monitoring

Displacement monitoring is the key work of landslide disaster early warning. Distributed and real-time displacement monitoring can provide a more accurate and timely reference for landslide instability evaluation. However, existing technologies have deficiencies in combining distributed and real-time capabilities. Given this, a novel continuous ultra-weak fiber Bragg grating (UWFBG)-based tilt sensor is proposed. First, the tensile test is designed and carried out to investigate the strain transfer mechanism of the UWFBG cable. Then, calibration experiments are used to verify the monitoring performance of the sensors. Finally, the UWFBG tilt sensors are applied in the Majiagou landslide, a typical reservoir landslide located in Three Gorges Reservoir Area (TGRA). The results show that the sliding surface of the landslide is located at 33 m, and the maximum displacement value is 8.1 mm during the monitoring period. When the landslide is in a stable deformation state, the dynamic range of displacement is 1.4 mm. The UWFBG tilt sensors with distributed and real-time monitoring characteristics can achieve accurate landslide displacement measurement and provide outlets for early warning.

Axial strain monitoring method of cast-in-place piles based on ultra-weak fiber Bragg grating

The axial strain distribution of cast-in-place piles under the static load test is a reliable basis for analyzing the compressive bearing capacity of the pile foundation. However, it is still difficult to achieve high-precision, high-sensitivity, real-time, and distributed monitoring of the pile foundation at the same time. To improve the monitoring of the stress distributions of the pile foundation, a fixed-point ultra-weak fiber Bragg grating (UWFBG) strain-sensing optical cable is designed on the basis of the large capacity characteristic of UWFBG. The strain sensitivity of this optical cable is 1.15 pm μϵ −1 within the range of 10 000 μϵ, which meets the accuracy requirements of pile health monitoring. The effectiveness of the designed UWFBG in pile foundation monitoring is verified through a static load test of the cast-in-place pile. The results show that the measured results of UWFBG and BOTDA (Brillouin optical time-domain analysis) have good consistency, and their average error is less than 7.5%. Compared with BOTDA, the UWFBG sensing system exhibits stronger anti-interference capability and faster response. The monitoring method proposed in this paper overcomes the shortcomings of previous monitoring methods in the static load test of the pile. The measured data can be used to calculate the detailed axial strain distribution of piles and analyze the distribution of axial force and side friction resistance of the pile. It not only provides a new monitoring method for static load test of cast-in-place piles, but also has great potential in monitoring large diameter pile.

High-precision continuous deformation monitoring method based on ultra-weak FBG array

Deformation monitoring is an important part of structural health monitoring. A high-precision continuous deformation monitoring method based on ultra-weak fiber Bragg grating (FBG) array sensing technology is proposed in this paper. The ultra-weak FBG array sensing fiber is continuously arranged by cross-type. The wavelength shift difference of the corresponding two ultra-weak FBGs is monitored in real-time to obtain the relative deformation between the monitoring nodes, so as to obtain the absolute deformation of all monitoring nodes. The mathematical model of continuous deformation based on cross-type is established and verified by experiments. The results show that this method has the advantages of good linearity, high-sensitivity, and temperature-insensitive. In the continuous deformation simulation experiment, the tiny deformation of the monitoring node is accurately obtained, and the error between the monitoring results and the actual deformation results is less than 5%. The proposed monitoring method is particularly suitable for continuous small deformation monitoring of large-scale metal structures.

Pretreatment of Ultra-Weak Fiber Bragg Grating Hydrophone Array Based on Cubic Spline Interpolation Using Intensity Compensation.

The demodulation algorithm based on 3 × 3 coupler in a fiber-optic hydrophone array has gained extensive attention in recent years. The traditional method uses a circulator to construct the normal path-match interferometry; however, the problem of increasing the asymmetry of the three-way signal to be demodulated is easily overlooked. To provide a solution, we report a pretreatment method for hydrophone array based on 3 × 3 coupler demodulation. We use cubic spline interpolation to perform nonlinear fitting to the reflected pulse train and calculate the peak-to-peak values of the single pulse to determine the light intensity compensation coefficient of the interference signal, so as to demodulate the corrected three-way interference signal. For experimental verification, ultra-weak fiber Bragg gratings (uwFBGs) with reflectivity of −50 dB are applied to construct a hydrophone array with 800 sensors, and a vibratory liquid column method is set up to generate a low-frequency hydroacoustic signal. Compared to the traditional demodulation algorithm based on a 3 × 3 coupler, the pretreatment method can improve the consistency of interference signals. The Lissajous figures show that cubic spline interpolation can improve the accuracy of monopulse peak seeking results by about 1 dB, and intensity compensation can further lead to a much lower noise density level for the interference pulse amplitude—specifically, more than 7 dB at 5~50 Hz—and the signal-to-noise ratio is improved by approximately 10 dB at 10 Hz. The distinct advantages of the proposed pretreatment method make it an excellent candidate for a hydrophone array system based on path-match interferometry.

Distributed Acoustic Sensing System Based on Inserting-Zero Golay Coding With Ultra-Weak Fiber Bragg Gratings

A distributed acoustic sensing (DAS) system based on ultra-weak fiber Bragg grating (UWFBG) array is proposed and experimentally demonstrated by Inserting-Zero (IZ) Golay coding. The system uses 1.5 km UWFBG as the sensing fiber, and extracts phase information through heterodyne detection and IQ demodulation. The Golay coding pulse sequence are used as the modulation signal to improve the system signal-to-noise ratio (SNR), which is benefited from the good autocorrelation characteristics of Golay code. The experimental results display that the DAS system based on Golay coding can effectively reduce the noise level, and the SNR of the system increases by 4.6 dB compared with the individual pulse system. Every bit of Golay coding sequence is inserting one bit “0” can decrease the inter-symbol crosstalk (ISC), and the SNR is further improved by 2.7 dB, and reaches 62.2 dB at 100 Hz. The proposed system has a good response ability of distributed sensing and a good linear characteristic with the coefficient of 0.9966.

Intrusion identification using GMM-HMM for perimeter monitoring based on ultra-weak FBG arrays.

Intrusion identification has been an intractable task for perimeter security. One of the primary challenges is to possess high identification rate over a long-distance range monitoring. This paper proposes an intrusion identification scheme based on ultra-weak fiber Bragg grating (UWFBG) arrays. The scheme is acquired by the combination of a Gaussian mixture model (GMM) and a hidden Markov model (HMM). The time dependencies are obtained by the analysis of relevant sensors in UWFBG arrays from the procedure of intrusions. The features extracted from vibration signals with time dependencies are used as the input of GMM-HMM. The GMM-HMM simultaneously analyzes features and time dependencies to identify intrusion. Experimental demonstration verifies that the proposed scheme can identify three intrusions (walking, knocking and climbing) and two non-intrusions (heavy truck passing and wind blowing) with the average identification rate of 98.2%. By the comparison tests with other six classifiers, the proposed GMM-HMM scheme shows a solid performance in the integrated evaluation for intrusion identification.

Large-capacity and long-distance distributed acoustic sensing based on an ultra-weak fiber Bragg grating array with an optimized pulsed optical power arrangement.

A large-capacity, long-distance distributed acoustic sensing (DAS) system without inline optical amplification was proposed and experimentally demonstrated using an ultra-weak fiber Bragg grating (UWFBG) array and coherent detection. The effect of the finite extinction ratio of an acousto-optic modulator and the Stokes signal of stimulated Brillouin scattering (SBS) in UWFBGs on the performance of DAS was simulated and revealed. A high extinction ratio and a balanced input pulsed optical power can improve the capacity and distance of the DAS. The dynamic acoustic signal can be well reconstructed for a serial array of 10828 near-identical UWFBG with a length of 54.14 km. An acoustic signal sensitivity of 189.54 pɛ/√Hz and a signal SNR of 40.01 dB with a spatial resolution of 5 m can be achieved at the far end.

Large deflection deformation detection method of steel strip based on ultra-weak fiber bragg grating

Large deflection deformation of a steel strip is an important part of structural deformation detection. Deformation detection has been achieved using ultra-weak fiber Bragg gratings (FBGs), and the deformation reconstruction inversion algorithm of the tangent recursive algorithm has been analyzed theoretically. The strain distribution of the steel strip (2.5 m × 0.050 m × 0.002 m) with large deflection under different loads was obtained by pasting eight ultra-weak FBG sensors with spacing of 0.3 m on the surface of the strip. The steel strip deformation was inversed by tangent recursive algorithm, and deformation error under loading condition was analyzed by finite element simulation. The inversion value is compared with the theoretical value and the measured value, which proves the effectiveness and accuracy of the ultra weak FBG method proposed in this paper. Results show that the large deflection deformation of a steel strip under different loads can be reconstructed using the tangent recursive algorithm, and the relative errors of the coordinates of the sampling points are less than 6.21%. This paper provides a reference for the detection of structural deformation.

Correlation-based multiplexing of spectral channels and fiber-optic sensors using unmodulated continuous-wave distributed feedback diode lasers.

In this Letter, we demonstrate a method for the multiplexing of spectral channels and fiber-optic sensors. The method makes use of the correlation-based demultiplexing technique. In our approach, each light source has an inherent phase noise which we propose to use as a "fingerprint" for correlation-based recognition and demultiplexing of spectral channels as well as signals from different sensors in each spectral channel. The proposed method requires for its implementation an extremely simple and robust scheme and can be attractive for budget-sensitive applications. As active elements, it uses only two standard free-running CW distributed feedback (DFB) diode lasers. No modulators or optical filters are required for operation. Results of proof-of-concept experiments are presented for the interrogation of several ultra-weak fiber Bragg gratings (FBGs) with reflectivity of 0.05% in a 4-km-long fiber for temperature measurements with the resolution of 0.1°C.

Perimeter Security System Based on Ultra-Weak Fiber Bragg Grating Array

Perimeter Security System Based on Ultra-Weak Fiber Bragg Grating Array

Design of a Weak Fiber Bragg Grating Acoustic Sensing System for Pipeline Leakage Monitoring in a Nuclear Environment

We propose a design method of a weak fiber Bragg grating (FBG) array, in which the reflectivity of the FBG increases along the fiber, and this increase is limited by the dynamic range of the distributed optical fiber sensing (DOFS) system. For a DOFS system, this design method can not only well compensate for the impact of fiber loss, within a certain range and caused by radiation, on the deterioration of the system performance, but also can reduce the requirements for the dynamic range of the system in nuclear environments. Based on this design method, we propose a short-distance acoustic detection system based on broadband ultraweak FBGs assisted phase-sensitive optical time domain reflectometry ( $\Phi $ -OTDR), and assess the system’s ability to detect pipeline leakages. This work not only provides guidance for the design of FBG sensing systems but also provides a new promising scheme for the application of DOFS system in nuclear environments.

Femtosecond laser point-by-point inscription of an ultra-weak fiber Bragg grating array for distributed high-temperature sensing.

Ultra-weak fiber Bragg grating (UWFBG) arrays are key elements for constructing large-scale quasi-distributed sensing networks for structural health monitoring. Conventional methods for creating UWFBG arrays are based on in-line UV exposure during fiber drawing. However, the UV-induced UWFBG arrays cannot withstand a high temperature above 450 °C. Here, we report for the first time, to the best of our knowledge, a new method for fabricating high-temperature-resistant UWFBG arrays by using a femtosecond laser point-by-point (PbP) technology. UWFBGs with a low peak reflectivity of ∼ - 45 dB (corresponding to ∼ 0.0032%) were successfully fabricated in a conventional single-mode fiber (SMF) by femtosecond laser PbP inscription through fiber coating. Moreover, the influences of grating length, laser pulse energy, and grating order on the UWFBGs were studied, and a grating length of 1 mm, a pulse energy of 29.2 nJ, and a grating order of 120 were used for fabricating the UWFBGs. And then, a long-term high-temperature annealing was carried out, and the results show that the UWFBGs can withstand a high temperature of 1000 °C and have an excellent thermal repeatability with a sensitivity of 18.2 pm/°C at 1000 °C. A UWFBG array consisting of 200 identical UWFBGs was successfully fabricated along a 2 m-long conventional SMF with an interval of 10 mm, and interrogated with an optical frequency domain reflectometer (OFDR). Distributed high-temperature sensing up to 1000 °C was demonstrated by using the fabricated UWFBG array and OFDR demodulation. As such, the proposed femtosecond laser-inscribed UWFBG array is promising for distributed high-temperature sensing in hash environments, such as aerospace vehicles, nuclear plants, and smelting furnaces.

Deep displacement monitoring and foundation base boundary reconstruction analysis of diaphragm wall based on ultra-weak FBG

The instability or destruction of diaphragm walls in underground engineering will cause significant economic losses and casualties. The distributed and high-precision monitoring of diaphragm wall deformation can provide reliable and detail data for disaster early-warning as well as economical design. However, recording continuous deformation of the diaphragm wall using traditional monitoring technologies remain challenging. Moreover, the basement movement of diaphragm wall is rarely considered in previous studies. In order to overcome the shortcomings of the current displacement monitoring methods and deal with the basement boundary issues, a new base boundary reconstruction (BR) was proposed and the ultra-weak Fiber Bragg grating (UWFBG) technology was adopted to monitor the displacement of the diaphragm wall. For validation, a laboratory test based on ultra-weak FBG was carried out and the feasibility of the BR method was demonstrated. Subsequently, the ultra-weak FBG and the BR method were applied to Shanghai metro line 18. The results showed that the monitoring technology and analysis method proposed in this paper can obtain detailed deformation information of the diaphragm wall in real-time, which not only provides an advanced monitoring method for the deep deformation of diaphragm wall, but also solves the deformation calculation issue with the base boundary condition changes involved.