Brillouin Optical Time Domain Reflectometer Research Articles

Rayleigh and Brillouin self-heterodyne detection Brillouin optical time domain reflectometer system based on phase shift keying pulse encoding

Rayleigh and Brillouin self-heterodyne detection Brillouin optical time domain reflectometer system based on phase shift keying pulse encoding

Brillouin frequency shift extraction by bidirectional integration of Brillouin gain spectrum

A novel Brillouin frequency shift (BFS) extraction approach based on bidirectional integration is proposed and investigated for mitigating the fluctuations in Brillouin optical time domain reflectometer (BOTDR) system. The proposed method involves calculating the BFS through the forward and backward integrations of the original Brillouin gain spectrum (BGS) data. The findings from both simulations and experiments demonstrate that bidirectional integration achieves superior BFS accuracy compared to Lorentzian curve fitting. To further evaluate its performance, the root mean square error of BFS is measured under the different signal-to-noise ratios (SNRs) and frequency steps and the results show that bidirectional integration has higher tolerance for low SNR. Specifically, bidirectional integration achieves a BFS accuracy of 0.853 MHz under SNR of 5.22 dB and significantly reduces the measurement time by 85.86 %. Additionally, the introduced coefficient k characterizes the influence on the BGS compression, with bidirectional integration exhibiting a narrower full width at half maximum of BGS (52.74 MHz) compared to Lorentzian curve fitting (67.45 MHz).

Research on fast and accurate extraction method of Brillouin frequency shift based on window-weighted centroid algorithm

Curve fitting method (CFM) is the primary method for extracting Brillouin frequency shift (BFS) in Brillouin sensing system. However, the accuracy of extraction result is affected by the initial parameter settings, the spectral shape of the Brillouin power spectrum (BPS) and the fitted curve model. Additionally, the fitting process is time-consuming. In view of this problem, this paper proposes a fast and accurate extraction method of BFS based on the window-weighted centroid algorithm (WCA). This method achieves extraction of BFS by selecting data window and calculating weighted centroid for BPS. The proposed method was experimentally verified using a Brillouin Optical Time Domain Reflectometer (BOTDR) temperature sensing system for extracting the temperature information of the 50 m heating section near the end of a 9.4 km optical fiber. Compared with CFM, cross-correlation method (XCM), backward propagation neural network (BP-NN) and centroid algorithm (CA), the proposed method has a higher accuracy and faster processing speed and is suitable for dealing with distorted non-ideal spectral shape. The proposed WCA method is an alternative method for extracting BFS information, especially in situations where the real-time requirement is high and the spectral shape is complex.

Towards single-photon Brillouin optical time domain reflectometry.

We investigate a novel distributed Brillouin optical time domain reflectometer (BOTDR) using standard telecommunication fibers based on single-photon avalanche diodes (SPADs) in gated mode, ν -BOTDR, with a range of 120 km and 10 m spatial resolution. We experimentally demonstrate the ability to perform a distributed temperature measurement, by detecting a hot spot at 100 km. Instead of using a frequency scan like conventional BOTDR, we use a frequency discriminator based on the slope of a fiber Bragg grating (FBG) to convert the count rate of the SPAD into a frequency shift. A procedure to take into account the FBG drift during the acquisition and perform sensitive and reliable distributed measurements is described. We also present the possibility to differentiate strain and temperature.

SNR Enhancement for BOTDR With Spatial-Adaptive Image Denoising Method

A spatial-adaptive image denoising (SAID) method is proposed to enhance the signal-to-noise ratio (SNR) of Brillouin optical time domain reflectometer (BOTDR) without deterioration of spatial resolution. Through analyzing the principle of denoising, the spatial-adaptive denoising network structure is constructed, which can effectively reduce noise and avoid over-smoothing. Moreover, a high-similarity dataset generation method is designed to further enhance the denoising effect. The SAID method is experimentally implemented on two-dimension Brillouin gain spectrum matrix over a 25.1 km-long optical fiber with the spatial resolution of 2 m. The experimental results show that the SNR enhancement of SAID method is 21.92 dB, and the data processing time is only 2.48 s. Whereas the SNR enhancement of conventional 10000 times trace averaging method is merely 9.98 dB, but the data processing time is increased to 174.52 s. Therefore, it is demonstrated that the SAID method has obvious advantages in SNR enhancement and data processing speed without deteriorating the spatial resolution of BOTDR.

Distributed Acoustic Sensing Based on Microtremor Survey Method for Near-Surface Active Faults Exploration: A Case Study in Datong Basin, China.

Active fault detection has an important significance for seismic disaster prevention and mitigation in urban areas. The high-density station arrays have the potential to provide a microtremor survey solution for shallow seismic investigations. However, the resolution limitation of the nodal seismometer and small-scale lateral velocity being inhomogeneous hinder their application in near-surface active fault exploration. Distributed acoustic sensing (DAS) has been developed rapidly in the past few years; it takes an optical fiber as the sensing medium and signal transmission medium, which can continuously detect vibration over long distances with high spatial resolution and low cost. This paper tried to address the issue of near-surface active fault exploration by using DAS. We selected a normal fault in the southern Datong basin, a graben basin in the Shanxi rift system in north China, to carry out the research. Microtremor surveys across the possible range of the active fault were conducted using DAS and nodal seismometers, so as to obtain a shallow shear wave velocity model. Meanwhile, we applied a Brillouin optical time domain reflectometer (BOTDR) and distributed temperature sensing (DTS) to monitor the real-time fluctuation of ground temperature and strain. Our results show that the resolution of the deep structures of the fault via the microtremor survey based on DAS is lower than that via the seismic reflection; whereas, their fault location is consistent, and the near-surface structure of the fault can be traced in the DAS results. In addition, both the BOTDR and DTS results indicate an apparent consistent change in ground temperature and strain across the fault determined by the DAS result, and the combination of surface monitoring and underground exploration will help to accurately avoid active faults and seismic potential assessment in urban areas.

Distributed Brillouin optical fiber sensors assisted by first‐order Raman amplification with window functions

Raman amplification can extend the sensing distance of distributed Brillouin optical fiber sensor, and there is a corresponding relationship between the optimal Raman pump, the sensing distance, and the lanched probe power. We propose a scheme of co-directional Raman pumping Brillouin optical time-domain reflectometer (BOTDR) with window functions to make a better tradeoff between sensing distance and signal-to-noise ratio (SNR). The mathematical model of window functions is designed based on the relationship between the sensing distance and Raman pump power. Both simulations and experiments analyze the distributions of Brillouin scattering power, which are detected under the fixed Raman pump and processed by the window functions. We then obtain the Brillouin frequency shift (BFS) distribution curve by Lorentzian curve fitting. The experimental results show that the system SNR is increased by 3.3 dB at the near-end of the 78.8 km long sensing fiber. Moreover, the accuracy of temperature and strain at the far-end is enhanced by 2.75 °C and 389 με by using window functions, respectively.

A wavelength scanning BOTDR sensing system based on Rayleigh and Brillouin self-heterodyne detection

We propose a wavelength scanning Brillouin Optical Time Domain Reflectometer (WS-BOTDR) sensing system based on Rayleigh and Brillouin (RB) self-heterodyne detection for high-accuracy temperature measurement. This method scans the power spectrum by adjusting the wavelength of laser rather than the frequency, which possesses the benefits of both electric frequency difference scanning and optical frequency difference scanning. Furthermore, the adverse effects of laser wavelength instability, polarization fading and phase fluctuation can be effectively eliminated by employing RB self-heterodyne detection. The experimental results demonstrate that the proposed method can achieve a high-accuracy measurement of 0.8 °C in the 50 m heating section of 9.5 km optical fiber end. It is also found that, from a practical point of view, the proposed method has prospective applications where both high-accuracy measurement and long-distance sensing are the primary concerns.

Single-photon detector-based long-distance Brillouin optical time domain reflectometry

We present a long-range Brillouin optical time domain reflectometer (BOTDR) based on photon counting technology. We demonstrate experimentally the ability to perform a distributed temperature measurement, by detecting a hot spot in a thermal bath at 100 km, and the possibility to achieve measurement until 120 km with a spatial resolution of 10 m. We use the slope of a fiber Bragg grating (FBG) as a frequency discriminator, to convert count rate variation into a frequency shift. A performance study of our distributed sensor as a function of spatial resolution is also presented.

High-Precision Measurement of Brillouin Frequency Shift in Brillouin Optical Time-Domain Reflectometer Based on Half-Peak Fitting Algorithm of Power Spectrum

The Brillouin optical time-domain reflectometer (BOTDR) system, which is based on the Brillouin frequency shift (BFS) of the scattering spectrum, has been widely applied to monitor the temperature and strain in many fields. The measurement accuracy of the BFS directly affects the monitoring and location of the temperature change or strain events. In this study, the characteristics of the least-squares fitting optimal solution of the Brillouin power spectrum are theoretically investigated, and a half-peak fitting algorithm is proposed to measure the BFS with high accuracy and stability. In particular, it can precisely determine the double-peak spectrum and detect the position in the transition section of a temperature change or strain event. Furthermore, it can reduce calculation complexity and enhance measurement speed by dropping most of the data. For the fiber under test (FUT) with a length of 12-km, 200 groups of time-domain data were processed using a probe pulse of width 50 ns and sampling rate 1 GHz. The half-peak fitting algorithm increased the temperature measurement accuracy by ~1 fold, with ~20% calculation of full fitting. Meanwhile, it effectively eliminated the influence of minor peak in the double-peak spectrum, and optimized the spatial resolution of the temperature change position to 0.1-m, which is the maximum limitation by sampling rate.

Experimental Study of Deformation Measurement of Bored Pile Using OFDR and BOTDR Joint Optical Fiber Sensing Technology

Pile foundation is the most common foundation form in geotechnical engineering; it is very important for engineering safety. In order to accurately grasp the deformation of pile foundation, OFDR (optical frequency domain reflectometer) and BOTDR (Brillouin optical time domain reflectometer) optical fiber sensing technologies are used to measure the strain variation of pile foundation. The measurement results of the two technologies are analyzed, and different data processing methods are used. The ability of the two methods to measure the strain of pile foundation is evaluated. The results show that OFDR technology can achieve high-precision and distributed measurement of strain of pile; BOTDR technology can achieve the monitoring effect of OFDR to a certain extent using appropriate data processing methods; the combination of the two methods can make up for the shortcomings of the short monitoring distance of the OFDR technique and the low accuracy of the BOTDR technique; by comparing the application effect with the two technologies in geotechnical engineering, the application prospect of OFDR–BOTDR joint optical fiber sensing technology in geotechnical engineering is discussed.

In situ behaviour of an instrumented ring subjected to active eccentric tail seal passage

With common TBM configurations, the last assembled ring is contemporarily loaded by the thrust jacks and a generally off-centered tail seal system. In active eccentric scenarios of tail seal passage, significant tail-ring eccentricities can be locked by the opposing action of sealing pressures and thrust jacks (Gil Lorenzo, 2022). In this paper, the in situ response of the CAM3 instrumented ring in the Crossrail’s Thames tunnel (CTT) during active eccentric tail seal passage is examined in detail. The field data comprises selected TBM data, the distributed strain measurements of a Brillouin Optical Time Domain Reflectometer (BOTDR), plus the output quantities of vibrating wire strain gauges (VWSGs) and biaxial micro-electro-mechanical system (MEMS) tiltmeters. The field data was completed with relevant analytical models (Gil Lorenzo, 2019a; 2022) and compared with the outputs of the finite element (FE) model conducted alongside (Gil Lorenzo, 2021a; 2021b), showing good agreement with the FE-computed deformational quantities. It was found that the eccentric tail seal passage was the main cause of the high and uneven lining pressures detected at the early stages of tunnelling. The active eccentric tail seal passage, with upwards seal pressure gradients and downwards transverse jack forces, triggered the ring deformations in the unsupported tunnel, which became essentially irrecoverable. The radial rotations of the outer springline segment were largely affected by the interfacial quality between ram shoe and segment first, and between segments once the next ring was erected. The high sealing pressure gradients yielded a 0.12% ring distortion ratio increment already close to recommended construction limits. The top half of the non-bedded ring behaved as an arch-like structure with the crown segment supported by the neighbouring rings. At the lower segments, the hoop curvatures were the result of the ring ovalisation whereas, at the upper segments, the hoop curvatures were primarily induced by the inward radial jack forces. The outer springline segment worked under considerable torsion in the first TBM cycle. Initial in-plane angularities in longitudinal joints with contact at the rear can be accentuated by the advance of the sealing pressures from rear to front and become irreversible. Longitudinal cracking is more likely in the first TBM cycle while the ring is radially loaded at the rear only. In addition, a field example of concrete cracking in a segment with a defective packer support after ring erection is provided.

An improved lorentz fitting algorithm for BOTDR using SVM model to capture the main peak of power cumulative average data

The Brillouin scattering signal is weak and the signal-to-noise ratio (SNR) is low in Brillouin optical time domain reflectometer (BOTDR), these result in a difficulty to improve the spatial resolution (SR) and measurement accuracy at the same time. In this paper, the denoising principle of power cumulative average (PCA) is analyzed theoretically, then an improved Lorentzian-curve fitting (ILCF) algorithm based on PCA data is proposed, in which the support vector machine (SVM) model is adopted to capture the main peak, and the Brillouin frequency shift (BFS) of double-peak spectrum is determined precisely. Furtherly, by adjusting the step of the sliding window, the precision of edge detection in the transition section is improved to 0.1-m. For the fiber under test (FUT) with a length of 12-km, 500 groups of time-domain data are processed under the probe pulse with 50 ns width and 1 GHz sampling rate. The average error of temperature accesses to 0.8 °C and the SR achieves 6.4-m simultaneously. The algorithm in this paper can reduce BFS standard deviation from 28.21 MHz to 1.27 MHz, improve the SR to the maximum limitation by the probe pulse, determine the BFS of double-peak spectrum and reduce the measurement time. Therefore, the proposed method can improve SR and measurement accuracy at the same time.

Long-Range and High Spatial Resolution Brillouin Time Domain Sensor Using Oversampling Coding and Deconvolution Algorithm

Brillouin optical time domain reflectometer is widely used due to the advantage of single-ended input, but facing the trade-off between sensing distance, spatial resolution, and measurement time in long range application. Harnessing the co-propagating Raman amplification, we introduce Golay code under oversampling conditions to enhance the performance of the sensor. The relationship between the coding gain and the oversampling rate is investigated to determine the optimal sampling rate. At the sampling rate of 250 MSa/s, long distance experiments are conducted over a 100 km sensing fiber with 64-bit return to zero Golay coding using 100 ns optical pulses, yielding a BFS uncertainty of about 1 MHz along the fiber and complete rising edge of 20 m. At the same time, the total variation regularization-based deconvolution algorithm is proposed to resolve the deterioration of spatial resolution due to direct decoding. The robustness of the algorithm over different signal-to-noise ratio, temperature gradients, and sampling rates is investigated through simulations and experiments. Finally, the spatial resolution of the experimental results is improved to be higher than 6 m, which is one third of the original result. These methods greatly facilitate optical pulse coding in long range and high-resolution sensing.

Discussion on pile axial load test methods and their applicability in cold regions

The measurement of pile axial load is of great significance to determining pile foundation design parameters such as skin friction and end bearing capacity and analyzing load transfer mechanisms. Affected by the temperature and ice content of frozen ground, the interface contact relationship between pile foundation and frozen soil is complicated, making pile axial load measurements more uncertain than that in non-frozen ground. Therefore, it is necessary to gain an in-depth understanding of the current pile axial load test methods. Four methods are systematically reviewed: vibrating wire sensors, strain gauges, sliding micrometers, and optical fiber strain sensors. At the same time, the applicability of the four test methods in frozen soil regions is discussed in detail. The first two methods are mature and commonly used. The sliding micrometer is only suitable for short-term measurement. While the Fiber Bragg grating (FBG) strain gauge meets the monitoring requirements, the Brillouin optical time-domain reflectometer (BOTDR) needs further verification. This paper aims to provide a technical reference for selecting and applying different methods in the pile axial load test for the stability study and bearing capacity assessment of pile foundations in cold regions.

Brillouin frequency shift error estimation formula for distributed optical fiber sensing technology based on Brillouin scattering.

In this work, the problems in the existing Brillouin frequency shift (BFS) error estimation formulas for distributed optical fiber sensing technology based on Brillouin scattering are discussed. Based on the analysis, a new, to the best of our knowledge, BFS error estimation formula is proposed. To validate the proposed formula, a large number of Brillouin spectra with different frequency sweep spans, signal-to-noise ratios, linewidths, and frequency steps are numerically generated, and at the same time, Brillouin spectra with different values of incident light pulse widths and frequency sweep spans are measured with a Brillouin optical time domain reflectometer. Based on those Brillouin spectra, the errors of the proposed formula and existing formulas are systematically compared. The results reveal that the proposed formula generally has a higher accuracy than the existing typical formulas, especially when the frequency sweep span or incident light pulse width is large. Therefore, it has a much wider application range.

Random coding method for SNR enhancement of BOTDR.

A random coding method for a Brillouin optical time domain reflectometer (BOTDR) fiber sensor is proposed. In this method, a series of pulses modulated by random code are injected into the optical fiber to enhance the signal-to-noise ratio (SNR) and further improve the measurement accuracy. Random coding method allows the sensing range to be extended to several tens of kilometers while maintaining meter-scale spatial resolution and lower detection peak power, without modifying the conventional configuration of BOTDR. The decoding principle and the coding gain of random coding method are analyzed and simulated. We experimentally implement the method and evaluate its influence on the performance optimization of BOTDR. Compared with the single pulse with peak power of 10 mW, the measured BFS uncertainty over 4.93 km sensing fiber is reduced from 5.34 MHz to 0.38 MHz when 512-bit random coding pulses with the same peak power are utilized. The experimental results show that the coding gain of 11.93 dB is obtained by 512-bit random coding. Benefitting from the SNR enhancement, the sensing range is extended from 4.93 km to 64.76 km within a root-mean-square error (RMSE) of 3 MHz, when the pulse peak power is only 10 mW and the spatial resolution is 2 m.

Improvement of Performance for Raman Assisted BOTDR by Analyzing Brillouin Gain Spectrum.

We propose a simplified partitioned Brillouin gain spectrum (BGS) analysis method to enhance the spatial resolution and measurement accuracy of a Brillouin optical time-domain reflectometer (BOTDR) assisted by a first-order Raman pump. We theoretically derive the mathematical model of the partitioned BGS and analyze the superposition process of sub-Brillouin signals within a theoretical spatial resolution range. We unified all the unknown constant parameters of the calculation process to simplify the partitioned BGS analysis method and the value of the uniform parameter is attained through the system test data and numerical analysis. Moreover, to automate data processing, the starting point of the temperature/strain change is determined by the first occurrence of the maximum Brillouin frequency shift (BFS), then the position where the partitioned BGS analysis method calculation begins is obtained. Using a 100 ns probe pulse and partitioned BGS analysis method, we obtain a spatial resolution of 0.4 m in the 78.45-km-long Raman-assisted BOTDR system, and the measurement accuracy is significantly improved. In addition, we achieve a strain accuracy of 5.6 με and a spatial resolution of 0.4 m in the 28.5-km-long BOTDR without Raman amplification.

In situ behaviour of an instrumented ring subjected to incipient TBM steering around a curve

During the TBM tunnelling of curved tunnel sections with passively articulated shield tails, the concrete segmental rings are susceptible to ring deformation and structural damage. In this paper, the in situ loading condition and response of the CAM4 instrumented ring in the Crossrail’s Thames tunnel (CTT) during tail seal passage at incipient TBM steering around a left curve are examined in detail. The field data comprises selected TBM data, the distributed strain measurements of a Brillouin Optical Time Domain Reflectometer (BOTDR) and the output quantities of vibrating wire strain gauges (VWSGs) and biaxial micro-electro-mechanical system (MEMS) tiltmeters. The field data was completed with relevant analytical models (Gil Lorenzo, 2019a, 2021c) and compared with the outputs of the finite element modelling conducted alongside (Gil Lorenzo, 2021a, 2021b). It was found that the eccentric tail seal passage was the main cause of the highest lining pressures and pressure gradients detected at the early stages of tunnelling. The machine turned around the thrust symmetry axis so that the shield posture changed from vertical and upwards to left and inwards. The transverse jack forces near the CAM4 ring switched orientation from downwards to rightwards. The locking mechanism of opposing TBM transverse actions during the eccentric tail seal passage triggered the ring deformations in the unsupported tunnel, which became essentially irrecoverable. The radial rotations of the outer springline segment were largely affected by the interfacial quality between ram shoe and segment first and between segments once the next ring was erected. The moderate sealing pressure gradients and the effective ring coupling ensured by the high thrust loads translated into a very limited ring distortion. The top half of the unbedded ring behaved as an arch-like structure with the crown segment being supported by the neighbouring segments. The in situ hoop curvatures were the result of ring ovalisation developed under the sealing pressure gradients and, in the first TBM cycle, the segment interaction with the hydraulic jacks. A temporary intrados front longitudinal crack near the mid length of the outer springline segment was detected at the time of incipient machine turning. The crack was caused by the combination of the spalling stresses between ram shoes with the front hoop tensile stresses typical of segmental rings loaded at the rear. Initial in-plane angularities of longitudinal joints with contact at the segment front can be corrected by the sequential loading of the ring from rear to front. There was good agreement between the in situ and FE-computed deformational quantities.

Distributed Transmission Line Ice-Coating Recognition System Based on BOTDR Temperature Monitoring

An optical fiber composite overhead ground wire (OPGW) distributed ice-coating monitoring scheme based on Brillouin optical time domain reflectometer (BOTDR) temperature measurement is proposed and demonstrated. The thermodynamic factors affecting the temperature of the OPGW cable are analyzed, and the temperature variation characteristics of the icing and non-icing sections of the OPGW cable with different icing thicknesses are analyzed via finite-element analysis. The results of an OPGW icing experiment indicate that when the ambient temperature changes, there is an obvious hysteresis characteristic between the temperature changes of the icing and non-icing sections of the OPGW cable, which is related to the thickness of the ice. Thus, we propose a distributed OPGW cable icing monitoring system based on BOTDR temperature monitoring.