Φ-OTDR System Research Articles

Low-noise distributed acoustic sensing using enhanced backscattering fiber with ultra-low-loss point reflectors.

We present a low-noise distributed acoustic sensor using enhanced backscattering fiber with a series of localized reflectors. The point reflectors were inscribed in a standard telecom fiber in a fully automated system by focusing an ultra-fast laser through the fiber cladding. The inscribed reflectors provided a reflectance of -53 dB, significantly higher than the Rayleigh backscattering level of -70 dB/m, despite adding only 0.01 dB of loss per 100 reflection points. We constructed a coherent φ-OTDR system using a double-pulse architecture to probe the enhanced backscattering fiber. Using this system, we found that the point reflectors enabled an average phase noise of -91 dB (re rad2/Hz), 20 dB lower than sensors formed using Rayleigh backscattering in the same fiber. The sensors are immune to interference fading, exhibit a high degree of linearity, and demonstrate excellent non-local signal suppression (>50 dB). This work illustrates the potential for low-cost enhanced backscattering fiber to enable low-noise, long-range distributed acoustic sensing.

Performance Enhancement of the Location and Recognition of a Φ-OTDR System Using CEEMDAN-KL and AMNBP

It is commonly known that for characteristics, such as long-distance, high-sensitivity, and full-scale monitoring, phase-sensitive optical time-domain reflectometry (Φ-OTDR) has developed rapidly in many fields, especially with the arrival of 5G. Nevertheless, there are still some problems obstructing the application for practical environments. First, the fading effect leads to some results falling into the dead zone, which cannot be demodulated effectively. Second, because of the high sensitivity, the Φ-OTDR system is easy to be interfered with by strong noise in practical environments. Third, the large volume of data caused by the fast responses require a lot of calculations. All the above problems hinder the performance of Φ-OTDR in practical applications. This paper proposes an integration method based on a complete ensemble empirical mode decomposition with adaptive noise and Kullback–Leibler divergence (CEEMDAN-KL) and an adaptive moving neighbor binary pattern (AMNBP) to enhance the performance of Φ-OTDR. CEEMDAN-KL improved the signal characteristics in low signal-to-noise ratio (SNR) conditions. AMNBP optimized the location and recognition via a high calculation efficiency. Experimental results show that the average recognition rate of four kinds of events reached 94.03% and the calculation efficiency increased by 20.0%, which show the excellent performance of Φ-OTDR regarding location and recognition in practical environments.

Performance enhancement method for phase-sensitive optical time-domain reflectometer system based on suppression of fading induced false alarms

The phase-sensitive optical time-domain reflectometry (Φ-OTDR) has been developed rapidly as a fully fiber-optic distributed vibration sensing technology. However, the demodulation technique based on the phase term would induce a serious false alarm problem due to the signal fading effect. An effective method to suppress fading-induced false alarms in the Φ-OTDR system is proposed, which is based on the suppression mask and numerical relationship between phase and amplitudes of Rayleigh backscattering. The performance of the proposed method has been experimentally demonstrated in both laboratory environment and in-field situation test. Without any hardware addition in a traditional Φ-OTDR equipment, false alarms rate can be reduced from 4.81% to 0.15%, whereas low missing alarms rate can be achieved at the same time. In-field results show that this work provides a low-cost solution to enhance the performance for real-life engineering application of the phase-discrimination Φ-OTDR system.

Adaptive Pulse Period Method for Low-Frequency Vibration Sensing With Intensity-Based Phase-Sensitive OTDR Systems

The use of intensity-based phase-sensitive optical time-domain reflectometry (φ-OTDR) system to measure low-frequency vibration is the aim of this paper. It is well-known that the laser frequency drift (LFD) and other system noise act as internal vibration that challenge φ-OTDR system to capture external low-frequency vibration. We proposed theoretically and demonstrated experimentally that by adapting the probe pulse repetition rate/pulse period to the frequency of vibration to be measured, consistent intensity change should be always realized in the disturbance region compared to the induced phase-noise change from pulse to pulse. In particular, two different pulse periods were used to demonstrate the effectiveness of the adaptive pulse period method (APP) for detection of low-frequency vibration. Experimentally, the sensing of external vibrations of 5; 1; 0.5 and 0.1 Hz frequencies with intensity-based φ-OTDR system was achieved with good signal-to-noise ratio. Moreover, an approach suitable for measuring low-frequency vibration induced by a PZT through direct acquisition of the Rayleigh intensity in the disturbance region was established. Various driving voltages applied to the PZT confirmed the consistency of the method.

Transient nanostrain detection in phi-OTDR using statistics-based signal processing

Phase-sensitive optical time-domain reflectometry (Φ-OTDR) is capable of detecting acoustic emission induced small strain with high sensitivity. However, there is a tradeoff between sensitivity, bandwidth and detection range, which makes the detection of a transient weak signal challenging. In this work, we focus on transient weak signal detection using Φ-OTDR. To achieve this aim, we propose a cascaded statistics-based signal-processing framework in a Φ-OTDR system to fetch the transient weak signal from a noisy background. Our framework is based on two key elements, including an estimator that is derived based on the probability characteristic of the Rayleigh backscattered light, and a time-frequency feature extraction process that maps the signal to the time-frequency domain. Using our statistics-based signal processing Φ-OTDR, we demonstrate experimentally the detections of, firstly a weak persistent signal with a magnitude down to 4 nε and a frequency up to 40 kHz, and then a weak transient acoustic tone-burst signal. Our proposed scheme is promising for Φ-OTDR performance improvement particularly in weak signal detections, and it will find new applications in the future systems.

Quantitative amplitude measuring φ-OTDR using multiple uncorrelated Rayleigh backscattering realizations.

We propose and demonstrate a technique to perform quantitative strain sensing using the amplitude of the Rayleigh backscattered light in a modified φ-OTDR system. While standard amplitude measuring φ-OTDR sensors can identify the presence of strain, they cannot perform quantitative measurements because the amplitude of the Rayleigh backscattered light exhibits a non-linear and unpredictable strain response. Here, we demonstrate a technique to computationally recover a linear strain response from a set of uncorrelated Rayleigh backscattering measurements. Using a combination of frequency and polarization multiplexing, we constructed a φ-OTDR system capable of recording 18 uncorrelated Rayleigh backscattering measurements in parallel. By combining information from these 18 measurements, the sensor achieves a linear strain response with total harmonic distortion below -35 dB. The sensor is immune to signal fading, has a minimum detectable strain of 5 pɛ/√Hz and a bandwidth of 500 kHz.

Compensation of optical path difference in heterodyne Φ-OTDR systems and SNR enhancement by generating multiple beat signals.

In conventional heterodyne phase-sensitive optical time domain reflectometry (Φ-OTDR), the optical path difference (OPD) between sensing and reference arms deteriorates the signal-to-noise ratio (SNR). We counteract this effect by introducing a loop path in reference arm. This facilitates the use of laser source with low coherence length. Compared with an ordinary Φ-OTDR system, SNR increased from ~1 dB to 5 dB. Regardless of OPD compensation features, the probability to getting a better output increases by generation of multiple reference beams (and consequently multiple beat signals), as compared to conventional systems. The method is cost efficient and suitable for practical purposes.

Nuisance alarm rate reduction using pulse-width multiplexing [formula omitted]-OTDR with optimized positioning accuracy

High nuisance alarm rates and missing alarm rates in phase-sensitive optical time-domain reflectometer (ϕ-OTDR) greatly restrict their practical application. In this paper, an asynchronous sampling pulse width multiplexing ϕ-OTDR is proposed to reduce both of these rates. Intersection and inclusion relationships among multiple positioning intervals are used to optimize the positioning accuracy. We construct an eight-pulse-width multiplexed ϕ-OTDR system and adopt a multisensor information fusion algorithm to verify the feasibility of the method. The experimental results show that this method can reduce the missing alarm rate by 93% and the number of nuisance alarms by 97%. The average positioning accuracy of the system is 21.4 m, which is equivalent to the spatial resolution level corresponding to the minimum pulse width in the optical pulse sequence. The positioning accuracy of some groups was as low as 6.4 m, far less than 20m. The proposed method provides a simple and feasible new approach for reducing nuisance alarm and missing alarm rates and optimizing the positioning accuracy of ϕ-OTDR.

A visibility enhanced broadband phase-sensitive OTDR based on the UWFBG array and frequency-division-multiplexing

Abstract The phase-sensitive optical time domain reflectometers (Φ-OTDR), which can realize distributed acoustic sensing, has been widely applied in health monitoring of civil structures. To break through the limit of the frequency response bandwidth (FRB) and to reduce the fading positions of traditional Φ-OTDR systems, we propose an Φ-OTDR system based on the ultra-weak fiber Bragg grating (UWFBG) array and the frequency-division-multiplexing (FDM) scheme. Taking advantage of UWFBGs, the probability of fading positions occurred on the Rayleigh backscattering (RBS) waveforms can be dramatically reduced, which means the visibility of the system can be enhanced. The FDM structure can enhance the sampling rate by interleaving algorithm, which helps the Φ-OTDR system to expand its FRB. The experimental results show that the frequency of vibration up to 440 kHz can be detected along 330 m UWFBGs, which is approximately three times higher than the upper limit of the FRB of traditional systems. When compared with the traditional scheme that adopts ordinary single-mode fiber as the sensing part, the probability of fading occurrence in the proposed system has been reduced from 10−2 to 10−10. The proposed sensing system in this paper reveals a promising option for the performance enhancement of the Φ-OTDR system, which can offer wider FRB as well as the enhanced visibility characteristic.

Phase Sensitive Optical Time Domain Reflectometry Based on Compressive Sensing

A sub-Nyquist random single-pulse sampling method based compressive sensing technique is proposed firstly to reconstruct signals under test in phase-sensitive optical time domain reflectometry (ϕ-OTDR). The key information of Rayleigh backscattering curves can be recognized after sparse transformation, which can be successfully reconstructed with far smaller number of measurements by orthogonal matching pursuit algorithm. Unlike the uniform sampling in conventional ϕ-OTDR system, pulse intervals are randomly modulated to generate a random single-pulse sequence in the proposed method in order to achieve random single-pulse sampling along every sensing point of the fiber. Experimental results verify that pulse sequences with average pulse repetition rate less than 2-kHz can be efficiently utilized to reconstruct an 8-kHz signal under test and obtain its frequency information, as well as to realize the detection of high frequency signals at different locations along the sensing fiber. It is demonstrated that the sampling rate is reduced greatly without any hardware modification compared to that required in the conventional method. This method can break the limits of the traditional Shannon-Nyquist sampling theorem to reconstruct signal, which is of great significance for the sensor performance enhanced in traditional ϕ-OTDR system.

Effects of weak fiber Bragg gratings on a distributed vibration sensing system based on phase-sensitive optical time-domain reflectometry

A distributed fiber sensing system based on weak fiber Bragg gratings (wFBGs) enhanced phase-sensitive optical time-domain reflectometry (Φ-OTDR) has improved signal-to-noise ratio (SNR) and sensitivity. The effects of wFBGs on a Φ-OTDR system are investigated theoretically and experimentally. The SNR improvement induced by wFBGs, which have higher reflectivity compared with the Rayleigh scattering, has been analyzed in systems with different laser source linewidths. The experimental results show that Φ-OTDR assisted by wFBGs with −35-dB reflectivity has a lower noise level and its SNR is improved by 27 dB. By comparing laser sources with different linewidths, it is proved that the enhanced system no longer requires an ultranarrow linewidth laser. The SNR in different locations of a 2.4-km-wFBG-array remains at a high level.

Characterization of vibration relative amplitude in direct detection Φ-OTDR using variable gain method

Vibration relative amplitude characterization has always been a tough problem in direct detection phase-sensitive optical time-domain reflectometry (Φ-OTDR) system due to the nonlinear relationship between the optical intensity and vibration amplitude. In this paper, a novel variable gain method is proposed to characterize the relative amplitude of external vibration. The mathematical model of Φ-OTDR is established, and the influence of external vibration on detected coherent Rayleigh back-scattering light in a single-mode fiber is simulated. Then, the vibration signals of different amplitudes imposed on the sensing fiber are further investigated, and the relationship between the interference amplitude peak power and vibration amplitude is derived. On the basis of the theoretical analysis, the variable gain method is carried out and implemented in Φ-OTDR system. The experimental results demonstrate that the calibrated interference peak power increases linearly with the applied vibration amplitude, and a high correlation coefficient (R2 = 0.93071) for linear fitting is obtained.

Discrimination method using higher-order harmonic frequencies for two close perturbations in phase-sensitive optical time domain reflectometry

This paper proposed a method for discriminating two close perturbations, whose interval is less than spatial resolution, in Φ-OTDR system. The mathematical model under two close perturbations is studied and the frequency features of temporal signal from every sensing positions are obtained. The model shows that if the two close perturbations have different frequencies, they can be distinguished through spatial-frequency analysis. When the two perturbations have even the same frequency, they can still be distinguished by inspecting higher-order harmonic frequencies. Both computer simulation and experiments have been carried out to support the effectiveness of this method.

Performance Enhancement Methods for the Distributed Acoustic Sensors Based on Frequency Division Multiplexing

The last years have witnessed the wide application of Distributed Acoustic Sensor (DAS) systems in several fields, such as submarine cable monitoring, seismic wave detection, structural health monitoring, etc. Due to their distributed measurement ability and high sensitivity, DAS systems can be employed as a promising tool for the phase sensitive optical time domain reflectometry (Φ-OTDR). However, it is also well-known that the traditional Φ-OTDR system suffers from Rayleigh backscattering (RBS) fading effects, which induce dead zones in the measurement results. Worse still, in practice it is difficult to achieve the optimum matching between spatial resolution (SR) and signal to noise ratio (SNR). Further, the overall frequency response range (FRR) of the traditional Φ-OTDR is commonly limited by the length of the fiber in order to prevent RBS signals from overlapping with each other. Additionally, it is usually difficult to reconstruct high frequency vibration signals accurately for long range monitoring. Aiming at solving these problems, we introduce frequency division multiplexing (FDM) that makes it easier to improve the system performance with less system structure changes. We propose several novel Φ-OTDR schemes based on Frequency Division Multiplexing (FDM) technology to solve the above problems. Experimental results showed that the distortion induced by fading effects could be suppressed to 1.26%; when the SR of Φ-OTDR is consistent with the length of the vibration region, the SNR of the sensing system is improved by 3 dB compared to the average SNR with different SRs; vibration frequencies up to 440 kHz have been detected along 330 m artificial microstructures. Thus, the proposed sensing system offers a promising solution for the performance enhancement of DAS systems that could achieve high SNR, broadband FRR and dead zone-free measurements at the same time.

80 km Fading Free Phase-Sensitive Reflectometry Based on Multi-Carrier NLFM Pulse Without Distributed Amplification

Long range phase-sensitive optical time domain reflectometer (ϕ-OTDR) sensing mainly employs distributed amplification in the sensing fiber. It requires the light to be injected into both ends of the sensing fiber, which reduces the degree of freedom in embedding the fiber into structures. To overcome this problem, the key factors that affects the signal-to-noise ratio (SNR) in ϕ-OTDR system based on matched filter is analyzed thoroughly, and a single-ended long-range ϕ-OTDR that does not require distributed amplification is proposed. In this system, two key techniques are adopted for SNR improvement. To boost the pulse energy and suppress the self-phase modulation, the distortion of the amplified pulse is rectified by using iterative predistortion method; to mitigate the influence of the interference fading and the stimulated Brillouin backscattering, a three-carrier pulse is employed. In combination with the non-linear frequency modulation technique which yields a 42.7 dB side lobe suppression ratio, these approaches guarantee an achievable ϕ-OTDR of 80 km sensing range, 2.7 m spatial resolution, 49.6 dB dynamic range in the experiment. To the best of authors’ knowledge, this is the first time that a ϕ-OTDR without optical amplification in the sensing fiber is realized over such a long sensing range.

Continuous Fading Suppression Method for Φ-OTDR Systems Using Optimum Tracking Over Multiple Probe Frequencies

The demand for distributed acoustic sensors, which are capable of reconstructing the amplitude, frequency, and phase of an acoustic field, is increasing. These sensors are typically realized by phase-sensitive optical time-domain reflectometry (Φ-OTDR). However, common Φ-OTDR systems suffer from a fading phenomenon, which causes amplitude fluctuation on Rayleigh backscattering traces, and the scattered light may not have enough intensity in some regions. These areas have low amplitude backscattering that may be even lower than the system noise floor during some periods of time. Therefore, we cannot reconstruct the phase signal properly in such low intensity areas. Systems with multiple frequencies have been proposed to achieve fading suppression. However, online data processing of such schemes remains a challenge, especially for continuous in-field applications. Prediction of fading must be performed in real time. Any delay caused by massive calculations is not acceptable. In this paper, a continuous fading suppression method based on a Φ-OTDR system with three different probe frequencies is presented as well as a tracking algorithm for selecting the optimum probe signal for any time, which predicts the occurrence of fading before it actually occurs. The performance of the proposed method has been experimentally evaluated and statistically analyzed. The distortion induced by the fading effect could be suppressed to 1.15% under continuous real-time running. The method is highly effective and repeatable, which makes it suitable for practical online purposes.

A $k$ -Nearest Neighbor Algorithm-Based Near Category Support Vector Machine Method for Event Identification of $\varphi$ -OTDR

In order to reduce the nuisance alarm rate (NAR) for the phase-sensitive optical time-domain reflectometer (φ-OTDR), we propose an event identification method based on the near category support vector machines (NC-SVM), which extends the current binary SVM classifier to multiclass problems by using k-nearest neighbor (kNN) algorithm. Five kinds of disturbance events, including watering, climbing, knocking, pressing, and false disturbance event, can be effectively identified for 25.05 km long φ-OTDR system. The experimental results demonstrate that the average identification rate of five disturbance events exceeds 94%, the identification time is 0.55s, and the NAR is 5.62%. Compared with the one against one multiclass SVM classifier, our proposed method has the distinguished advantage of higher identification rate, shorter identification time, and lower NAR.

Partial Discharge Recognition Based on Optical Fiber Distributed Acoustic Sensing and a Convolutional Neural Network

Fiber-optic distributed acoustic sensing (FDAS) with phase-sensitive optical time-domain reflectometry (Φ-OTDR) is a promising technique for high-sensitivity measurement. In this paper, an improved Φ-OTDR system with a weak fiber Bragg grating (wFBG) array for partial discharge (PD) detection in cross-linked polyethylene (XLPE) power cables is demonstrated; and an event recognition method based on a convolutional neural network (CNN) model is proposed to identify and classify different types of events, including internal PD, corona PD, surface PD, and noise. A multiscale wavelet decomposition and reconstruction method is used to extract PD signals and a two-dimensional spectral frame representation of the PD signals is obtained by the mel-frequency cepstrum coefficients (MFCC). The experimental results based on 1280 training samples and 832 test samples have demonstrated high values of precision, sensitivity, and specificity for each event (up to 96.3%, 96.4%, and 98.7%, respectively), which means that the combination of multiscale wavelet decomposition and reconstruction, the MFCC and CNN may be a promising event recognition method for the FDAS systems.

基于内调制啁啾脉冲的高信噪比低成本直接探测型φ-OTDR系统

基于内调制啁啾脉冲的高信噪比低成本直接探测型<i>φ</i>-OTDR系统

Performance comparison of combining algorithms for polarization-diversity receiving in phase-sensitive OTDR

We investigate three different combining algorithms for polarization-diversity receiving in phase-sensitive optical time-domain reflectometry (φ-OTDR) system. The algorithms are equal gain combining(EGC), selective combining(SC) and max-ratio combining(MRC). We compare the performance of the three combining algorithms in three aspects, which are the SNR of the locating signal, the SNR of the recovered vibration signal and the computation cost of the algorithms. The results show that the SC algorithm has a poor locating performance and is unsuitable for locating of the system. The EGC algorithm has the lowest complexity and general locating performance, which can be applied in real-time locating of external vibration. The performance of MRC algorithm is optimal, which is a promising tool in the high SNR intrusion location system. The proper combining algorithms we chosen will contribute to achieving optimal performance and optimal resource allocation of the φ-OTDR system.