Phase-sensitive Optical Time-domain Research Articles

An Ameliorated Denoising Scheme Based on Deep Learning for Φ-OTDR System With 41-km Detection Range

In recent years, denoising methods for improving the performance of the phase-sensitive optical time-domain reflectometry ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Phi $ </tex-math></inline-formula> -OTDR) system have been restricted by the deficiencies of time-consuming and limited denoising effect. In this work, a trained convolutional neural network (CNN)-based image denoising model is proposed to greatly eliminate the unwanted noises in the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Phi $ </tex-math></inline-formula> -OTDR-based sensing system. First, the given Rayleigh backscattering traces are acquired and preprocessed through adjacent differentiation and two-dimensionalization. Second, the 2-D preprocessed data are converted into a noisy gray-scale image and sent into the CNN model for training and testing. Third, the CNN model outputs a corresponding denoised gray-scale image, which can be further analyzed by reconverting it into a series of denoised Rayleigh backscattering traces. Finally, a series of experiments are carried out to demonstrate the effectiveness of the proposed denoising scheme. Experimental results show that, in allusion to the vibration signal with different intensities along the 41-km optical sensing fiber, the trained CNN model achieves a signal-to-noise ratio (SNR) enhancement of about 20 dB. Compared with the conventional methods based on wavelet and empirical mode decomposition (EMD), the proposed denoising scheme demonstrates characteristics of robustness, well spatial resolution reservation, and high efficiency. It is believed that the trained CNN model has great potential to be deployed on the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Phi $ </tex-math></inline-formula> -OTDR system for real-time denoising.

Denoising method based on VMD-PCC in φ-OTDR system

To improve the signal-to-noise ratio (SNR) of vibration signal in phase-sensitive optical time domain reflectometry (φ-OTDR) system, a vibration signal denoising method based on variational mode decomposition (VMD) and Pearson correlation coefficient (PCC) is proposed. The phase signal demodulated by I/Q demodulation is further processed by a denoising method consisting of K layers VMD, where K was determined by calculating the ratio of adjacent intrinsic mode functions (IMFs) center frequencies, the Pearson correlation coefficient (PCC) estimation between each component and phase signal, and distortion and noise casting by discarding the IMF components with the PCC less than a threshold of 0.4 and the residual component of decomposition. The denoising method was verified in a 2 km φ-OTDR system in the scenarios of single 200 Hz frequency vibration at one position, two single frequencies of 200 Hz and 500 Hz vibration at two positions with minimum separation distance defined by the pulse width, and a multi-frequency vibration of 200 Hz, 1200 Hz and 2500 Hz at one position. And the demodulated phase signal by I/Q demodulation was processed by the proposed method, and further compared with the wavelet decomposition, empirical mode decomposition (EMD), ensemble empirical mode decomposition (EEMD) and complementary ensemble empirical mode decomposition (CEEMD). Experimental results show that the VMD-PCC method is superior to the other methods compared in improving the SNR of vibration signals and has excellent adaptability and effectiveness in φ-OTDR system.

Distributed vibration sensor with a lasing phase-sensitive OTDR.

The authors experimentally demonstrate the operation of a lasing phase-sensitive optical time-domain reflectometer (Φ-OTDR) based on random feedback from a sensing fiber. Here, the full output of the laser provides the sensing signal, in contrast to the small backscattered signal measured in a conventional OTDR. In this proof-of-principle demonstration, the laser operates as a distributed vibration sensor with signal-to-noise ratio of 23-dB and 1.37-m spatial resolution.

Vectorial distributed acoustic sensing based on a multicore fiber and phase-sensitive optical time-domain reflectometry.

A vectorial distributed acoustic sensing (vDAS) system is proposed and demonstrated for distributed two-dimensional vector vibration measurements based on phase-sensitive optical time-domain reflectometry (ΦOTDR). An optical pulse compression (OPC) algorithm was used to achieve high spatial resolution and suppress fading noise, and a Rayleigh-enhanced seven-core fiber (eSCF) was used to magnify the differentials of Rayleigh backscattering (RBS) in various cores undergoing vibrations. A combination of OPC and eSCF allows the system to fully quantify perturbations with a spatial resolution of 1.1 m and a strain resolution of 1.1 pε/√Hz, achieving a maximum acceleration sensitivity of 1.04 mrad/g at 60 Hz and an orientation reconstruction error of 1.92°. The proposed vDAS system can achieve vectorial distributed vibration sensing without the need for specific fiber layouts or complex reconstruction algorithms, increasing its potential for applications in vertical seismic profiles or pipeline inspection.

Distributed multiple vibrations detection with high frequency response assisted by dual NLL sources

Distributed multiple vibrations detection with high frequency response is still a challenging task in distributed optical fiber vibration sensors. In terms of optical interferometer based vibration sensors, the location information can not be directly demodulated. Moreover, the detectable frequency response in the optical reflectometer based vibration sensors is limited by the repetition frequency of the modulated optical pulse. In this paper, an integrated vibration sensing scheme based on two dual narrow line-width laser (NLL) sources with different wavelengths has been theoretically proposed and experimentally demonstrated. In the scheme, one NLL source is used for constructing the phase sensitive optical time domain reflectometry (Φ-OTDR), and the other one is used to build the Michelson interferometer. These two vibration sensors are integrated by using a dense wavelength division multiplexer. Based on the sensing mechanism of the integrated vibration sensor, the multiple simultaneous vibration points can be located by the Φ-OTDR sensing part, meanwhile its corresponding frequencies information can be fully acquired by the Michelson part. Experiments on detection of the piezolelectric transducer (PZT) show that a spatial resolution of ∼20 m is achieved in a 4.95 km sensing fiber link based on the width of the modulated optical pulse in Φ-OTDR part was set as 200 ns. And a maximum frequency response can reach 45 kHz restricted by the vibration source of the PZT. Compared to conventional vibration structure, the proposed vibration sensing scheme can offer a series of unique advantages, such as simple signal demodulation process, high signal to noise ratio of the demodulated frequency spectrum, no high order harmonics in the frequency spectrum, no dead zone in the detectable frequency range and it can be achieved with a common sensing fiber.

NLM Parameter Optimization for $\varphi$-OTDR Signal

The filtering parameters of non-local means (NLM) that seriously affect the denoising results are usually optimized based on the noise level estimation. But the noise level is unknown in many cases. In this paper, a simple method for optimizing NLM parameters based on autocorrelation function (ACF) is proposed from the perspective of signal estimation, which uses the periodic lag of the autocorrelation function to estimate the vibration signal. This method is verified with a hypothetical clean image corrupted by additive noise, and the NLM parameter optimization results are compared with those obtained by two traditional methods based on peak signal-to-noise ratio (PSNR) and structural similarity (SSIM) respectively. Then it is tested with the practical vibration signal of a phase-sensitive optical time-domain reflectometer ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\varphi$</tex-math></inline-formula> -OTDR). The fidelity and spatial resolution of the denoised practical signal are analyzed. This work helps to improve the performance of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\varphi$</tex-math></inline-formula> -OTDR and signal recognition.

Data Reduction in Phase-Sensitive OTDR with Ultra-Low Sampling Resolution and Undersampling Techniques.

Data storage is a problem that cannot be ignored in the long-term monitoring of a phase-sensitive optical time-domain reflectometry (Φ-OTDR) system. In this paper, we proposed a data-reduction approach for heterodyne Φ-OTDR using an ultra-low sampling resolution and undersampling techniques. The operation principles were demonstrated and experiments with different sensing configurations were carried out to verify the proposed method. The results showed that the vibration signal could be accurately reconstructed from the undersampled 1-bit data. A space saving ratio of 98.75% was achieved by converting 128 MB of data (corresponding to 268.44 ms of sensing time) to 1.6 MB. The proposed method led to a potentially new data-reduction approach for heterodyne Φ-OTDR, which also provided economical guidance for the selection of the data-acquisition device.

Lamb Wave Detection for Structural Health Monitoring Using a ϕ-OTDR System.

In this paper, the use of a phase-sensitive optical time-domain reflectometry (ϕ-OTDR) sensor for the detection of the Lamb waves excited by a piezoelectric transducer in an aluminum plate, is investigated. The system is shown to detect and resolve the Lamb wave in distinct regions of the plate, opening the possibility of realizing structural health monitoring (SHM) and damage detection using a single optical fiber attached to the structure. The system also reveals the variations in the Lamb wave resulting from a change in the load conditions of the plate. The same optical fiber used to detect the Lamb waves has also been employed to realize distributed strain measurements using a Brillouin scattering system. The method can be potentially used to replace conventional SHM sensors such as strain gauges and PZT transducers, with the advantage of offering several sensing points using a single fiber.

Fading-free Φ-OTDR evaluation based on the statistical analysis of phase hopping.

In phase-sensitive optical time domain reflectometry (Φ-OTDR), false phase peaks caused by interference fading have been observed experimentally; however, the statistical law has not yet been disclosed. In this work, after clarifying that the false phase peaks originate from the phase hopping of demodulated phase noise during the unwinding process, we define the phase hopping rate (PHR) to evaluate the degree of fading and study the quantitative relationship between the PHR and signal-to-noise ratio (SNR) of the measured signal through theoretical derivation and experimental verification. In addition, a moving rotated-vector-average (MRVA) method is proposed to suppress the fading and eliminate the false phase peaks. In the experiment, after MRVA processing with a 25 ns sliding window, the lowest SNR is pulled from 0.003 to 14.9, and the corresponding PHR is reduced from 0.237 to less than 0.0001, which is consistent with the theoretical analysis.

Multiresolution Phase Compensation for Phase-Sensitive OTDR

In order to suppress the phase distortion caused by fading in phase-sensitive optical time-domain reflectometer ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\varphi $ </tex-math></inline-formula> -OTDR) system, a new multi-resolution phase compensation algorithm is proposed in this paper. When the intensity of the beat signal approaches the system floor noise, the phase variation will gradually follow the uniform distribution, and fading occurs. The fading region in the spatiotemporal phase plane would be determined subsequently by synthesizing the differential section intensity. The redundancy and local correlation of the phase plane is thoroughly utilized by the patch searching algorithm in multi-resolution. Hence, the marked fading region would be compensated by the fading free region in the phase plane. The proposed method has been proved and analyzed that the fading can be effectively suppressed in the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\varphi $ </tex-math></inline-formula> -OTDR system in the experiment. The fading region can be accurately compensated whether the disturbance exists. The disturbance can be restructured effectively with SNR of 31.2 dB. Furthermore, the strain resolution was achieved to 7.685 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{p}\varepsilon /\surd $ </tex-math></inline-formula> Hz. Therefore, the proposed method provides a promising approach to realizing fading elimination without additional structure in the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\varphi $ </tex-math></inline-formula> -OTDR system.

High-efficient disturbance event recognition method of ϕ-OTDR utilizing region-segmentation differential phase signals.

We demonstrate a disturbance event recognition method based on region segmentation, which utilizes differential phase signals of a phase-sensitive optical time-domain reflectometer (ϕ-OTDR) to recognize disturbance events efficiently. The long-haul sensing fiber is divided into subsensing regions; whereas the phase signals at the two end points of the subsensing regions are subtracted, unwrapped, and differenced to represent the disturbance information. Feature extraction and classification are performed separately on the subsensing regions datasets. The experimental results indicate that the average recognition accuracy of the region-segmentation-based event recognition method is up to 92.9%. Compared to the method without region segmentation, this proposed method improves the average recognition accuracy by 8%; whereas the recognition time of three disturbance events on a 14.8-km sensing system is only 0.39 s. The proposed method provides significant support for the development of disturbance event recognition of the ϕ-OTDR sensor system.

Hybrid B-OTDR/Φ-OTDR for multi-parameter measurement from a single end of fiber.

The multi-parameter sensing is in great demand for comprehensive recognition in various application scenarios. We demonstrate a distributed optical fiber sensor (DOFS) for simultaneous vibration and temperature/strain sensing based on the integration of Brillouin optical time-domain reflectometry (BOTDR) and phase-sensitive optical time-domain reflectometry (Φ-OTDR). A double heterodyne detection configuration is set up to receive the Rayleigh scattering and Brillouin scattering simultaneously, which can enhance the signal-to-noise ratio (SNR) and reduce the polarization fading noise for the Brillouin signal at the same time. Most components in the setup are multiplexed for the Brillouin and Rayleigh signals. The overall cost is much lower than the summation of the BOTDR and the Φ-OTDR setup. In experiments, the setup is capable of measuring temperature change near the end of the 49.9 km long fiber with 0.381 MHz measurement uncertainty and restore different vibration patterns correctly by demodulating the phase variation of Rayleigh scattering signal, owning a 20 m spatial resolution. The sampling rate of the vibration measurement is 1.8 kHz, and the achieved dynamic strain resolution is 1.235 nɛ/√Hz with 100 Hz vibration.

Temporal convolution network with a dual attention mechanism for φ-OTDR event classification.

We propose a hybrid model named channel attention based temporal convolutional network combined with spatial attention and bidirectional long short-term memory network (ATCN-SA-BiLSTM) for phase sensitive optical time domain reflectometry signal recognition. This hybrid model consists of three parts: ATCN, which extracts temporal features and preserves causality of time domain signals, the SA mechanism, which re-weights spatial sequences for better feature extraction, and BiLSTM, which extracts spatial relationships considering the bidirectional propagation characteristics of disturbances in space domain signals. Experimental results show that our method achieves better classification performance with an accuracy of 93.4% and zero nuisance alarm rate.

Ultra-low sampling resolution technique for heterodyne phase-OTDR based distributed acoustic sensing.

Phase-sensitive optical time-domain reflectometry (Φ-OTDR) based on heterodyne detection is widely used for its simple structure and high signal-to-noise ratio (SNR). However, the large amount of raw data of Φ-OTDR places a heavy burden on the storage device and also limits the transferability of the data. In this Letter, we propose an ultra-low sampling resolution technique to solve the data storage problem in heterodyne Φ-OTDR. Experimental results show that the optical phase variations induced by external vibrations can be successfully demodulated from the 1-bit-resolution raw data, and a vibration SNR of 58.03 dB is achieved. In addition, this work also reveals that a data acquisition device with extremely low sampling resolution is sufficient for heterodyne Φ-OTDR, signifying that the cost of the system can be further decreased.

Golay Coding Φ-OTDR With Distributed Frequency-Drift Compensation

The performance of optical-pulse-coding (OPC) phase-sensitive optical time-domain reflectometry ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\boldsymbol {\Phi }$ </tex-math></inline-formula> -OTDR) has been improved significantly in recent years. However, laser frequency drift is still a crucial issue, as it reduces the peak to side-lobe ratio (PSR) in Golay coding <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\boldsymbol {\Phi }$ </tex-math></inline-formula> -OTDR. In this paper, a novel and simple distributed frequency-drift compensation method is proposed. As the phase accumulations of the reference fiber and the sensing fiber are consistent through theoretical analysis and experimental verification, the frequency drift can be compensated by injecting the modulated Golay coding pulses into two fibers at the same time, and subtracting the phase drift of the sensing fiber and the reference fiber. In order to verify the distributed compensation effect, a two-point perturbation measurement experiment is carried out in 9.7 km sensing range. As a result, the far-end signal-to-noise ratio (SNR) is enhanced and the demodulation perturbation SNRs are increased by 2.8 dB and 18.9 dB separately compared with the case without frequency drift compensation. Furthermore, this scheme can also be used in other complex coding systems, providing more possibilities for system designs.

Time-Expanded Φ-OTDR Based on Binary Sequences

In this letter, the capabilities of time-expanded phase-sensitive optical time-domain reflectometry (TE <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Phi $ </tex-math></inline-formula> -OTDR) using binary sequences are demonstrated. We present a highly flexible and integrable TE <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Phi $ </tex-math></inline-formula> -OTDR approach that allows a customized distributed optical fiber sensor (range, spatial resolution, and acoustic sampling) by simply changing the length of the binary sequence and the reference clock frequencies of the binary sequence generators. The here presented architecture eliminates the need for the cumbersome arbitrary signal generators used to date to create the dual-comb spectra for interrogating the fiber. In this approach, the use of large binary sequences allows us to obtain dual combs in a simple and cost-effective way. Spatial resolution of ~1 cm is achieved, attaining ~15,000 independent measurements points along the interrogated fiber, with a capability of sensing ~30,000 measurements points.

Optimization of the moving averaging-moving differential algorithm for Φ-OTDR.

The moving averaging-moving differential (MA-MD) algorithm is widely adopted for event recognition and positioning in phase-sensitive optical time-domain reflectometry (Φ-OTDR) vibration sensing. In this paper, two theorems about the two critical parameters of this algorithm, the MA window width and the MD interval, are found, proved, and verified. According to them we can not only obtain an optimal signal-to-noise ratio and avoid missing the vibration signal but also greatly improve the signal-searching efficiency. Work in this paper is of essential importance for understanding the characteristics of the MA-MD algorithm and will remarkably benefit its practical applications.

Image Edge Detection Methods in Perimeter Security Systems Using Distributed Fiber Optical Sensing.

This paper aims to evaluate detection algorithms for perimeter security systems based on phase-sensitive optical time-domain reflectometry (-OTDR). Our own designed and developed sensor system was used for the measurement. The main application of the system is in the area the perimeter fencing intrusion detection. The system is unique thanks to the developed motherboard, which contains a field-programmable gate array (FPGA) that takes care of signal processing. This allows the entire system to be integrated into a 1U rack chassis. A polygon containing two different fence types and also cable laid underground in a plastic tube was used for testing. Edge detection algorithms using the Sobel and Prewitt operators are considered for post-processing. The comparison is made based on the signal-to-noise ratio (SNR) values calculated for each event. Results of algorithms based on edge detection methods are compared with the conventional differential method commonly used in -OTDR systems.

High-sensitivity distributed relative salinity sensor based on frequency-scanning φ-OTDR.

In this paper, a high-sensitivity distributed optical fiber salinity sensor based on frequency-scanning phase-sensitive optical time-domain reflectometry (φ-OTDR) and polyimide-coated single-mode fiber is proposed. Distributed salinity sensing over an 1100 m polyimide-coated fiber with a 1 m spatial resolution was demonstrated, and a sensitivity of 782.4 MHz/(mol/L) was achieved with the salinity changing from 0 mol/L to 1.61 mol/L. Then the measurement accuracies of frequency shift and salinity were evaluated theoretically and experimentally. Both theoretical and experimental results show that the measurement accuracy deteriorates as the pulse width decreases, resulting in a trade-off between the spatial resolution and measurement accuracy. The measurement uncertainty of salinity is 0.022 mol/L in the case of 30 cm spatial resolution. And when the spatial resolution is set to be 2 m, the measurement uncertainty of salinity decreases to 0.005 mol/L. The response time of the fiber to external salinity change has also been investigated, and it takes about 8 minutes for the fiber to reach a stable state. The proposed salinity sensor exhibits high sensitivity and long measurement range, which may be used for distributed marine environmental monitoring.

An improved high stability and low distortion phase generated carrier demodulation algorithm for phase-sensitive optical time-domain reflectometers

The phase generated carrier (PGC) demodulation technique for phase-sensitive optical time-domain reflectometry (Ф-OTDR) has significant potential in applications involving distributed optical fiber sensing and imaging. An improved PGC demodulation algorithm for a Ф-OTDR combined with an unbalanced Michelson interferometer system is presented in this letter. This modified PGC technique uses the one output signal as a fundamental carrier frequency signal with mathematical transformations, and another output is the modulated signal, which enable the two signals to be output synchronously from the interferometer. Theoretical simulations and experiments demonstrate that the linear and nonlinear distortion caused by phase modulation depth and carrier phase delay are well suppressed by the proposed algorithm with high stability and low distortion. The signal-to-noise-and-distortion ratio of sensors using the proposed method in the same conditions achieve gains of 12.98 dB, 30.82 dB and 6.93 dB over PGC-Arctan, PGC-differential-and-cross-multiplying and PGC-single-path differential division method, respectively. Experimental verification shows that the proposed demodulation system can exactly recover the original signals of multiple vibration positions simultaneously, and the system employing the ameliorated PGC demodulation algorithm has the noise level of about 1.83 × 10−4 rad (√Hz)−1 and a large dynamic range of 65.72 dB @ 800 Hz, which provides a new choice for distributed optical fiber sensing technology.