Brillouin Optical Time-domain Analysis Research Articles

Dynamic Distributed Pressure Measurement Using Frequency-Agile Fast BOTDA

A distributed pressure sensor is proposed and designed for dynamic pressure measurement by using frequency-agile fast Brillouin optical time-domain analysis (BOTDA). A double coated single-mode fiber (SMF) is used as the sensing fiber because of its 1500- $\mu \text{m}$ thickness outer coating that can significantly improve the dependence of the Brillouin frequency shift on pressure. Then, the frequency-agile technique is used to generate frequency-agile pump pulse in BOTDA system so that a fast Brillouin gain spectrum distribution measurement can be achieved. An injection locked scheme is used as both optical filter and optical amplifier to improve the measurement accuracy. As a result, the static pressure sensitivity of the double coated fiber is −3.46 MHz/MPa with the pressure ranging from 0 to 30 MPa. Subsequently, a large dynamic range pressure measurement is performed with a sampling rate of 33.3 kHz and maximum sensing distance of 25 m. Furthermore, it is proved that the uncertainty of dynamic pressure measurement system is 0.03 MPa.

Single-shot chirped pulse BOTDA for static and dynamic strain sensing.

Driven by the strong need for distributed high-frequency dynamic strain sensing, ultra-fast Brillouin optical time-domain analysis is rapidly becoming a vital technique. Thus, in this Letter, we propose and demonstrate a novel method by using a chirped pulse as a pump signal to extract the relative Brillouin frequency shift (BFS) changes through the real-time delays between adjacent Brillouin traces; this enables static and dynamic strain measurement without time-consuming frequency sweeping process. Benefiting from single-shot measurement based on Brillouin traces, the system has a high acquisition rate that is only limited by the sensor length without averaging, and is immune to the polarization fading problem, thanks to electrical delay time measurement. The pump source is a 1 MHz linewidth laser without a phase-locking loop; the laser frequency drifting noise could be compensated by the signal from the non-disturbed fiber section. In the experiments, BFS measurement resolution of 0.42 MHz with 4.5 m spatial resolution is demonstrated over a 5 km non-uniform fiber.

Flexural Behavior of Natural Hybrid FRP-Strengthened RC Beams and Strain Measurements Using BOTDA.

Experimental and finite element analysis results of reinforced concrete beams under monotonic loading were presented in this study. In the experimental program, one beam was tested in an as-built condition. The other two beams were strengthened using natural hybrid FRP layers in different configurations. The natural hybrid FRP composite was developed by using natural jute FRP and basalt FRP. One of the most appealing advantages of natural fiber is its beneficial impact on the environment, which is necessary for the sustainability recognition as an alternative to synthetic FRP. The hybrid FRP was applied to the bottom concrete surface in one beam, while a U-shaped strengthening pattern was adopted for the other beam. The flexural behavior of each beam was assessed through strain measurements. Each beam was incorporated with conventional strain gages, as well as the Brillouin Optical Time Domain Analysis (BOTDA) technique. BOTDA has its exclusive advantages due to its simple system architecture, easy implementation, measurement speed, and cross-sensitivity. The experimental results revealed that the beam strengthened with the U-shaped hybrid FRP composite pattern had a better flexural response than the other counterpart beams did both in terms of peak loads and maximum bottom longitudinal steel bar strains. Beams B-01 and B-02 exhibited 20.5% and 28.4% higher energy dissipation capacities than the control beam did, respectively. The ultimate failure of the control beam was mainly due to the flexural cracks at very low loads, whereas the ultimate failure mode of FRP composite-strengthened beams was due to the rupture of the hybrid FRP composite. Further, strain measurements using BOTDA exhibited similar patterns as conventional strain gage measurements did. However, it was concluded that BOTDA measurements were substantially influenced by the bottom flexural cracks, ultimately resulting in shorter strain records than those of conventional strain gages. Nonlinear structural analysis of the beams was performed using the computer program ATENA. The analytical results for the control beam specimen showed a close match with the corresponding experimental results mainly in terms of maximum deflection. However, the analytical peak load was slightly higher than the corresponding experimental value.

Optical Fiber Frequency Shift Characterization of Overburden Deformation in Short-Distance Coal Seam Mining

Overburden deformation is an important concern for the safe and green mining of coal resources. Similarity simulation testing is the main approach used to study the deformation characteristics of the overburden in coal mining. In the application of Brillouin optical time-domain analysis (BOTDA) in similarity simulation tests, the capability of distributed optical fiber sensing (DOFS) to detect the characteristics of the overburden deformation and the evolution is a key factor affecting the testing accuracy. In this study, the relationships between DOFS and overburden deformation and the face impact pressure under geological conditions in short-distance coal seam mining were explored. The results show that DOFS can be used to monitor the strain conditions of the overburden during the entire mining process and can provide the peak positions of the advance support pressure on the face. A DOFS characterization model for investigating the spatial and temporal evolutions of overburden deformation was established. A new method of characterizing the face impact pressure based on the fiber frequency shift variation was developed. The method was demonstrated to be effective through comparison of monitored results of impact pressure counts detected using pressure sensors. The characteristics of the face impact pressure in short-distance coal seam mining were obtained. The results of this study provide valuable guidance for the development of similarity simulation testing and intelligent mining engineering techniques.

Enhanced BOTDA Sensors Based on Brillouin Phase Recovery Using Kramers-Kronig Relation

We have proposed and demonstrated an enhanced Brillouin optical time-domain analysis (BOTDA) sensing system with fast measurement speed based on the Brillouin phase recovery using Kramers-Kronig (KK) relation. The non-Lorentz Brillouin gain spectrum (BGS) and Brillouin phase spectrum (BPS) measured by BOTDA are theoretically validated to satisfy KK relation and hence the BPS is digitally recovered by performing Hilbert Transform (HT) to the measured BGS. The recovery of the whole BPS along the 45km fiber only consumes 0.3s. Both simulation and experiment have been carried out to analyze the quality of the recovered BPS under various conditions. By making use of the excellent linearity of the recovered BPS, we accelerate the data processing speed by three times through Linear Fitting (LF) of BPS, without compromise of the sensing accuracy. With implementation of simple HT and LF, this scheme does not need modification of the BOTDA setup and complicated data processing algorithm, which would be a practical way for applications where fast measurement speed is needed, especially in long-distance and high-spatial resolution sensing.

A Dual-Adaptive Denoising Algorithm for Brillouin Optical Time Domain Analysis Sensor

A dual-adaptive denoising algorithm based on complementary ensemble empirical mode decomposition (CEEMD) and particle swarm optimization (PSO) is proposed to improve the signal-to-noise ratio (SNR) of Brillouin optical time domain analysis (BOTDA) sensor. Here, CEEMD is employed to decompose the measurement signal into a series of intrinsic mode functions (IMFs) adaptively and accurately. The spectrum centroid method is applied to quantify objectively the range of denoising. Then PSO threshold algorithm is dedicatedly designed to search for the optimal/sub-optimal denoising threshold for noised IMFs, which can adjust the threshold size adaptively according to the noise level of input signal. The results show that the SNR can be improved by more than 14 dB and the Brillouin frequency shift (BFS) accuracy is optimized by 1.12 MHz in 22.5 km sensing fiber. This algorithm does not need any condition prediction in the process of denoising, which offers an adaptation and high SNR demodulation scheme for BOTDA sensor with only modifying the software.

Application of Dual-Frequency Self-Injection Locked DFB Laser for Brillouin Optical Time Domain Analysis.

Self-injection locking to an external fiber cavity is an efficient technique enabling drastic linewidth narrowing of semiconductor lasers. Recently, we constructed a simple dual-frequency laser source that employs self-injection locking of a DFB laser in the external ring fiber cavity and Brillouin lasing in the same cavity. The laser performance characteristics are on the level of the laser modules commonly used with BOTDA. The use of a laser source operating two frequencies strongly locked through the Brillouin resonance simplifies the BOTDA system, avoiding the use of a broadband electrooptical modulator (EOM) and high-frequency electronics. Here, in a direct comparison with the commercial BOTDA, we explore the capacity of our low-cost solution for BOTDA sensing, demonstrating distributed measurements of the Brillouin frequency shift in a 10 km sensing fiber with a 1.5 m spatial resolution.

The measurement of strain of a prototype pulley system using a Brillouin optical time domain analysis

AbstractIn the chain and rope hoist systems of large cranes, instant and monitoring of the strain caused by different loads is of great importance. Strain is a size that cannot be measured directly and whose effect can be measured. Strain can be measured point by point with the help of electronic sensors. However, increasing the number of measuring points causes complexity in electronic systems. Thanks to distributed fiber sensors, this problem has been overcome and standard SMF has been used as tens of thousands of measuring points. In this study, a model of the pulley system used in large cranes is created and the strain occurring at a certain point of a SMF was experimentally measured using the BOTDA technique. SNR was improved by using the wavelet noise reduction technique in the processing of the received signals. In order to monitor the measured strain, a user‐friendly interface was designed.

Rapid noise removal based dual adversarial network for the Brillouin optical time domain analyzer

We propose a dual adversarial network (DANet) to improve the signal-to-noise ratio (SNR) of the Brillouin optical time domain analyzer. Rather than inferring the conditional posteriori distribution in the conventional maximum a posteriori (MAP) framework, DANet constructs a joint distribution from two different factorizations corresponding to the noise removal and generation tasks. This method utilizes all the information between the clean-noisy image pairs to preserve data completely without requiring traditional image priors and noise distribution assumptions. Additionally, the clean-noisy image pairs produced by the generator can expand the original dataset to retrain and enhance the denoising effect. The performance of DANet is verified using the simulated and experimental data. Without spatial resolution deterioration, an SNR improvement of 35.51 dB is observed in the simulation, and the Brillouin frequency shift (BFS) uncertainty along the fiber is reduced by 3.56 MHz. Experiments yield a maximum SNR improvement of 19.08 dB, with the BFS uncertainty along the fiber reduced by 0.93MHz. Significantly, DANet has a processing time of 1.26 s, which is considerably faster than conventional methods, demonstrating its potential for rapid noise removal tasks.

Experimental study on a new method to forecasting goaf pressure based key strata deformation detected using optic fiber sensors

The spatial distribution of rock mass stress in goaf seriously affects the stable operation of underground reservoir. Measurement of stress distribution in the goaf of coal mines is difficult due to the complex internal environment and other restrictive conditions of goaf. This paper presents a new method of monitoring the deformation of key stratum in overburden by optical fiber sensing, and then characterizing the pressure of goaf. The internal mechanism of key stratum breakage and the variation of goaf stress is investigated by physical model with the geometric dimension of 3 (length) × 1.3 (height) × 0.4 (width) m), in which the Brillouin Optical Time Domain Analysis (BOTDA) fiber-optic sensing technology and Fiber Bragg Grating (FBG) technology are used to monitor the stability of key stratum and the non-contact Digital Image Correlation (DIC) technique is employed to provide real-time deformation measurements. Combining the fracture mechanics model of key strata with the parabolic function model of subsidence trajectory, the stress evolution model of coal seam mining space is established for the first time, the goaf pressure change and the corresponding relation between the key layer breakage was quantitatively characterized. The research results show that: The fracture location of the key strata is consistent with the peak stress location of the goaf. The breaking limit strain threshold value of the key strata based on optical fiber sensing detection is 4000 με. Through the inversion of optical fiber monitoring results, the fracture degree and range of overburden rock in goaf are obtained, thus reflecting the position and change of the peak pressure in goaf. The consistency between key layer deformation and goaf pressure variation is verified.

Hybrid aperiodic coding for SNR improvement in a BOTDA fiber sensor.

The measurement accuracy of a Brillouin optical time domain analysis (BOTDA) fiber sensor is determined by the signal-to-noise ratio (SNR) of the received sensing signal. Here, a new hybrid aperiodic coding method is proposed to improve the SNR. In the proposed method, two pre-discovered short seed aperiodic codes (SA-codes) are used to construct a new hybrid aperiodic code (HA-code) in a nested way. The HA-code inherits the good denoising capabilities of the two SA-codes and features a high coding gain. In the proof-of-concept experiment, a SNR improvement up to 8 dB is obtained, which improves the measurement certainty to 1.67 MHz over a 117.46 km sensing range under a spatial resolution of 2.6 m.

Review of Specialty Fiber Based Brillouin Optical Time Domain Analysis Technology

Specialty fibers have introduced new functionalities and opportunities in distributed fiber sensing applications. Particularly, Brillouin optical time domain analysis (BOTDA) systems have leveraged the unique features of specialty fibers to achieve performance enhancement in various sensing applications. This paper provides an overview of recent developments of the specialty fibers based BOTDA technologies and their sensing applications. The specialty fibers based BOTDA systems are categorized and reviewed based on the new features or performance enhancements. The prospects of using specialty fibers for BOTDA systems are discussed.

Overcoming EDFA slow transient effect in a Golay-coded BOTDA sensor by a distributed depletion mapping method.

The erbium-doped-fiber-amplifier (EDFA), generally served as a pre-amplifier, could effectively raise the signal-to-noise ratio (SNR) of a Brillouin optical time-domain analysis (BOTDA) sensor. However, it also induces a distortion in the Brillouin gain spectrum and Brillouin frequency shift measurement errors due to the slow transient effect (STE) in the coded-BOTDA. We propose a distributed depletion mapping (DDM) method to overcome such an effect. A continuous light wave with a particular wavelength is injected to map the STE-induced depletion to compensate for the distortion. The proposed scheme is experimentally demonstrated along a 120-km sensing fiber with 2-m spatial resolution. Experimental results show that the conventional tail-alignment (TA) method cannot compensate for the STE over the whole fiber link, while the proposed DDM method compensates for over 7.69-MHz measurement errors.

Error Estimation of BFS Extraction With Optimized Neural Network & Frequency Scanning Range

We present and investigate a scheme for the error estimation formula derivation of Brillouin frequency shift (BFS) extraction based on an optimized neural network and frequency scanning range for a Brillouin optical time-domain analyzer (BOTDA) system. The system uses a general single mode optical fiber as the sensing medium, and the pump pulse duration is much longer than the phonon lifetime to ensure that the measured gain spectra are close to a Lorentzian shape. The performance of the network for extracting the BFS from f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">s</sub> to f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">e</sub> with different frequency scanning ranges is studied in detail, where f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">s</sub> ~f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">e</sub> is the measurement range needing high precision. The results show that the optimal frequency scanning range is f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">s</sub> -20~f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">e</sub> + 20 (MHz). Based on the optimized network, the influences of the signal-to-noise ratio (SNR), linewidth of the gain spectrum (Δv <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B</sub> ) and frequency step (δ) on the error of BFS extraction are each discussed in detail. The error declines exponentially as the SNR improves and rises linearly as Δv <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B</sub> increases. The linear interpolation method is used to obtain the desired input data for the network under different frequency steps, and the error shows a linear relationship with δ. Finally, a comprehensive error estimation formula for the BFS extraction of a BOTDA is constructed according to the above three relationships and completed by fitting the errors under various SNRs, Δv <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B</sub> and δ.

Calculation model of overburden rock failure volume in mined-out area based on Brillouin optical time-domain analysis technology

The determination of overburden failure volume is a great challenge because of the complexity and concealment of mining fractures. Taking the 61,601 working face of Longwanggou Coal Mine as the engineering background, this paper established a two-dimensional model of 3000 mm × 200 mm × 1350 mm (length × width × height) in the laboratory. By adopting the Brillouin optical time-domain analysis (BOTDA) technology, considering the fiber deformation and the coordination of rock strata, Of deformation and damage state of overburden strata above the mined-out area, discusses the strata deformation partitioning features and characteristics of fiber optic monitoring strain curve partition interaction relations, is derived based on the above theoretical calculation formula of the rock damage volume, comparing the calculated results with measured results, verify the accuracy of volume calculation formula of mined-out area. The results show a good correspondence between the strain zoning characteristics monitored by optical fiber and the deformation zoning characteristics of overburden. Based on considering the discordant deformation between the overlying rock mass and the optical fiber sensor, the range and degree of overlying mining rock can be obtained through the inversion of the horizontal optical fiber monitoring results to reflect the position and degree of rock strata movement and deformation with the optical fiber. By analyzing the development process of the vertical fiber optic strain curve, it can be found that the rock strata with different propelling distances and different heights bear different tensile stresses, which have remarkable range characteristics. By comparing the results of the calculation model proposed in this paper with the monitored values of the model, it is found that the void volume size of goaf is the same, and the relative error is less than 9%. The results show that the model is reliable in calculating the failure volume of overburden rock in goaf.

Millimeter-level recognition capability of BOTDA based on a transient pump pulse and algorithm enhancement.

Brillouin optical time-domain analysis requires a pulsed pump to obtain a distributed Brillouin gain spectrum (BGS) containing environmental information, whose width corresponds to spatial resolution (SR). We propose a rising edge demodulation (RED) algorithm acting on Brillouin information generated by a transient pump pulse (<phonon lifetime) via a nonlinear weight matrix to enhance SR. The distributed BGS generated by using an 8-ns transient pump pulse is processed by the RED algorithm, and its SR is enhanced from 0.8 m to 1 mm (i.e., 800-fold improvement) in the simulation and 8 mm (i.e., 100-fold improvement) in the experiment.

Bending-loss-resistant distributed Brillouin curvature sensor based on an erbium-doped few-mode fiber.

We developed a bending-loss-resistant distributed Brillouin curvature sensor based on an erbium-doped few-mode fiber (Er-FMF) and differential pulse-width pair Brillouin optical time-domain analysis. With Er ion amplification compensating for bending-induced optical loss, radii in the ∼0.3 to 2.02 cm range could be monitored correctly. The corresponding Brillouin frequency shift variations were in the range of 91.7 to 9 MHz, which agrees well with theoretical calculations. The sensitivity of the Er-FMF device increased inversely with the bending radius, with the optimal sensitivity of 292.7 MHz/cm obtained at a radius of 0.3 cm. To the best of our knowledge, this is the smallest radius of curvature detected and highest sensitivity reported to date, indicating potential applications in distributed sharp-bend sensing, such as intelligent robotics and structural health monitoring.

Hybrid Fiber-Optic Sensing Integrating Brillouin Optical Time-Domain Analysis and Fiber Bragg Grating for Long-Range Two-Parameter Measurement.

Distributed fiber sensing (DFS) can provide real-time signals and warnings. The entire length of fiber optic cable can act as a sensing element, but the accuracy is sometimes limited. On the other hand, point-to-point fiber sensing (PPFS) is usually implemented using one or more fiber Bragg gratings (FBGs) at specific positions along with the fiber for the monitoring of specific parameters (temperature, strain, pressure, and so on). However, the cost becomes expensive when the number of FBGs increases. A hybrid fiber sensing scheme is thus proposed, combining the advantages of DFS and PPFS. It is based on a Brillouin optical time-domain analysis (BOTDA) fiber system with additional FBGs embedded at certain positions where it is necessary to detect specific parameters. The hybrid fiber sensing system has the advantages of full sensing coverage at essential locations that need to be carefully monitored. In our work, the test results showed that the proposed system could achieve a sensing distance of 16 km with the single-mode fiber with a 2 m spatial resolution. For FBG parameter measurements, the temperature variation was 52 °C, from 25 °C to 77 °C, with a temperature sensitivity of 23 pm/°C, and the strain was from 0 to 400 µε, with a strain sensitivity of 0.975 pm/µε, respectively, using two FBGs.

Distributed Optical Fiber Sensing Assisted by Optical Communication Techniques

The development of optical fiber technology has facilitated the technological innovation in optical fiber communication and sensing systems over the past decades. Among all the fiber sensing technologies, distributed optical fiber sensing (DOFS) has attracted great research interests and has been extensively investigated. In optical fiber communication systems, optical parameters such as intensity, phase, polarization, or frequency are modulated to realize data transmission, while for DOFS they are employed to measure the distributed physical parameter variation along the sensing fiber. Due to the similarities shared by optical fiber communication systems and DOFS systems, optical communication techniques, including coherent detection, polarization diversity and multicarrier signaling have been widely used in the DOFS to improve the sensing performance. In this article, principles of coherent detection, polarization diversity and multicarrier signaling are introduced and their applications in phase sensitive optical time domain reflectometry and Brillouin optical time domain analyzer are reviewed. A newly proposed unidirectional forward transmission based DOFS is also presented.

Distributed Optical Fiber Sensor for Dynamic Measurement

Distributed optical fiber sensors (DOFS), which can distributed measure static parameters such as temperature and strain, have been extensively researched and applied in industrial fields over the past several decades. Recently, in consideration of the explosive demand for dynamic sensing, the DOFS with the capability of dynamic phenomena measurement has attracted amounts of attentions. Various techniques that could realize and enhance the dynamic performance of DOFS have been proposed. In this article, DOFS for dynamic measurement is discussed. Firstly, the principles of DOFS based on Rayleigh and Brillouin scattering are introduced. Then, the most recent advances of fast DOFS especially those based on phase sensitive optical time domain reflectometry (φ-OTDR) and Brillouin optical time domain analysis (BOTDA), are analyzed. Furthermore, DOFS for multi-parameter sensing is also reviewed.