Phase-sensitive Optical Time-domain Reflectometer Research Articles

Distributed Vibration Sensor Based on Space-Division Multiplexed Reflectometer and Interferometer in Multicore Fiber

The maximum detectable vibration frequency by phase-sensitive optical time-domain reflectometer (Φ-OTDR) is limited by the repetition rate of the pump pulse. Additionally, the intensity-demodulation based Φ-OTDR sensor suffers from the generally nonlinear dependency of the backscattered optical intensity on vibration. This makes quantitative measurement of vibration frequency difficult. In order to mitigate these limitations, we propose and demonstrate a multicore fiber (MCF) based space-division multiplexed (SDM) Φ-OTDR and Mach–Zehnder interferometer (MZI) hybrid sensor, enabling truly uninterrupted distributed vibration sensing with broad vibration frequency response range and high spatial resolution. By taking advantage of highly integrated multiple spatial cores offered by an MCF, Φ-OTDR implemented in one of the cores is used to locate the vibration. Meanwhile, an MZI is constructed using another two parallel spatial cores with sufficient continuous-wave optical power from a coherent narrow linewidth laser, which is used to retrieve the vibration frequency by processing the acquired interference signal. Compared with the single mode fiber based hybrid systems, the proposed SDM configuration allows implementation of Φ-OTDR and MZI independently without any crosstalk between them. Consequently it offers many unique advantages, such as simple data processing procedure, good signal-to-noise ratio of the demodulated vibration frequency spectrum, no frequency dead zone, and single-end access. This cost-effective SDM hybrid sensor provides significant potential for distributed and accurate monitoring of vibrations.

Pulse-Width Multiplexing ϕ-OTDR for Nuisance-Alarm Rate Reduction.

A pulse-width multiplexing method for reducing the nuisance-alarm rate of a phase-sensitive optical time-domain reflectometer (-OTDR) is described. In this method, light pulses of different pulse-widths are injected into the sensing fiber; the data acquired at different pulse-widths are regarded as the outputs of different sensors; and these data are then processed by a multisensor data fusion algorithm. In laboratory tests with a sensing fiber on a vibrating table, the effects of pulse-width on the signal-to-noise ratio (SNR) of the -OTDR data are observed. Furthermore, by utilizing the SNR as the feature in a feature-layer algorithm based on Dempster–Shafer evidential theory, a four-pulse-width multiplexing -OTDR system is constructed, and the nuisance-alarm rate is reduced by about 70%. These experimental results show that the proposed method has great potential for perimeter protection, since the nuisance-alarm rate is significantly reduced by using a simple configuration.

Event locating strategy in φ-optical time-domain reflectometer based on spatial frequency energy distribution and response section edge determination

Determination of intrusion location in a phase-sensitive optical time-domain reflectometer is important and fundamental. In general, the spatial resolution of event location is limited by the temporal width of the probe pulse. We found that the location of a single intrusion event could be precisely determined with high spatial resolution regardless of the pulse width. First, the concept of “response section” was proposed and then it was proved that its left edge indicated the exact location of the intrusion event. Then, the spatial frequency energy distribution was used to achieve explicit edge determination. Three experiments were conducted to demonstrate the superiority of the proposed method in terms of locating the events compared with the traditional trace differential method.

Sensitivity of a phase-sensitive optical time-domain reflectometer with a semiconductor laser source

In the present paper we perform, for the first time, an analysis of the average sensitivity of a coherent phase-sensitive optical time-domain reflectometer (phase-OTDR) with a semiconductor laser source to external actions. The sensitivity of this OTDR can be defined in a conventional manner via average SNR at its output, which in turn is defined by the average useful signal power and the average intensity noise power in the OTDR spatial channels in the bandwidth defined by the OTDR sampling frequency. The average intensity noise power is considered in detail in a previous paper. In the current paper we examine the average useful signal power at the output of a phase-OTDR.The analysis of the average useful signal power of a phase-OTDR is based on the study of a fiber scattered-light interferometer (FSLI) which is treated as a constituent part of a phase- OTDR. In the analysis, one of the conventional phase-OTDR schemes with a rectangular dual-pulse probe signal is considered. The FSLI which corresponds to this OTDR scheme has two scattering fiber segments with additional time delay, introduced between backscattered fields. The average useful signal power and the resulting average SNR at the output of this FSLI are determined by the degree of coherence of the semiconductor laser source, the length of the scattering fiber segments, and by the additional time delay between the scattering fiber segments. The average useful signal power characteristic of the corresponding phase-OTDR is determined by analogous parameters: the source coherence, the time durations of the parts constituting the dual-pulse, and the time interval which separates these parts.In the paper an expression for the average useful signal power of a phase-OTDR is theoretically derived and experimentally verified. Based on the found average useful signal power of a phase-OTDR and the average intensity noise power, derived in the previous paper, the average SNR of a phase-OTDR is defined. Setting the average signal SNR to 1, at a defined spectral band the minimum detectable external action amplitude for our particular phase-OTDR setup is determined. We also derive a simple relation for the average useful signal power and the average SNR which results when making the assumption that the laser source coherence is high. The results of the paper can serve as the basis for further development of the concept of phase-OTDR sensitivity.

Phase-sensitive optical time domain reflectometer with ultrafast data processing based on GPU parallel computation.

The sensing performance of a phase-sensitive optical time domain reflectometer (ϕ-OTDR) has been sufficiently improved, thanks to plenty of valuable research in recent years. However, in the literature, there is hardly any attention aimed at enhancing the data processing capability of the system, the necessity and significance of which are undisputed. This paper, for the first time to the best of our knowledge, analyzed the intrinsic superiority of employing GPU parallel computation in ϕ-OTDR for improving the data processing capability and presented a comprehensive performance evaluation. Three typical, frequently implemented algorithms in ϕ-OTDR-moving average, batch fast Fourier transform, and batch correlation dimension computation-are carried out where CPU-based programs and counterpart GPU-based programs are, respectively, developed. Their time-consuming performances in different data scales are experimentally tested and compared. The experiment results show that in each case, employing GPU parallel computation can significantly enhance the system's data processing capacity, thus providing a feasible and efficient way of guaranteeing real-time operation with the growing data scale.

151-km single-end phase-sensitive optical time-domain reflectometer assisted by optical repeater

A phase-sensitive optical time-domain reflectometry (ϕOTDR) system that can detect intrusion over 150 km is presented. The ϕOTDR system uses nonbalanced optical repeaters to extend the sensing distance. The repeater consists of two erbium-doped optical fiber amplifiers (EDFAs) and one Raman amplifier (RA). One EDFA power amplifier amplifies the forward-transmitting pulse, and one EDFA preamplifier is used for the backscattering signal, respectively. The RA helps keeping the power along the fiber stable. The optical repeater is installed at the connection of two adjacent fibers to compensate the power decline due to fiber loss. It is easy to install the repeater midway among the fiber links in the system for longer-distance sensing since there is no need of modifying the original sensing system. The theoretical analysis of the repeater is given to describe its effect on the distributed sensing. In experiments, several ϕOTDR traces show a good agreement with theoretical results. Using the optical repeater, 35-Hz vibration at 151 km is successfully measured with signal-to-noise ratio of 8 dB without extra signal processing.

True Phase Measurement of Distributed Vibration Sensors Based on Heterodyne $\varphi $-OTDR

We demonstrate a method to truly measure the phase change originated from external vibration by heterodyne phase-sensitive optical time-domain reflectometer (φ-OTDR). The method of differential phase between sections of a sensing fiber is explored. In order to truly measure the phase change induced by external vibration, we point out that such sections of the fiber to calculate differential phase must cover at least the whole vibration region together with a section of fiber spaced by a pulse duration. The resultant differential phase is theoretically found to be twice the value of the real phase induced by external vibration. We further experimentally investigate a distributed sensor with a sensing fiber of 41.2 km based on φ-OTDR system. The experimental results are well consistent with the simulation results, which provides a general guidance for quantitative phase measurement of the external excitation such as slow variations based on heterodyne φ-OTDR technique.

Fast coarse-fine locating method for φ-OTDR.

We proposed and demonstrated a coarse-fine method to achieve fast locating of external vibration for the phase-sensitive optical time-domain reflectometer (φ-OTDR) sensing system. Firstly, the acquired backscattered traces from heterodyne coherent φ-OTDR systems are spatially divided into a few segments along a sensing fiber for coarse locating, and most of the acquired data can be excluded by comparing the phase difference between the endpoints in adjacent segments. Secondly, the amplitude-based locating is implemented within the target segments for fine locating. By using the proposed coarse-fine locating method, we have numerically and experimentally investigated a distributed vibration sensor based on the heterodyne coherent φ-OTDR system with a 50-km-long sensing fiber. We find that the computation cost of signal processing for locating is significantly reduced in the long-haul sensing fiber, showing a potential application in real-time locating of external vibration.

Pattern recognition based on time-frequency analysis and convolutional neural networks for vibrational events in φ-OTDR

Based on vibration signals detected by a phase-sensitive optical time-domain reflectometer distributed optical fiber sensing system, this paper presents an implement of time-frequency analysis and convolutional neural network (CNN), used to classify different types of vibrational events. First, spectral subtraction and the short-time Fourier transform are used to enhance time-frequency features of vibration signals and transform different types of vibration signals into spectrograms, which are input to the CNN for automatic feature extraction and classification. Finally, by replacing the soft-max layer in the CNN with a multiclass support vector machine, the performance of the classifier is enhanced. Experiments show that after using this method to process 4000 vibration signal samples generated by four different vibration events, namely, digging, walking, vehicles passing, and damaging, the recognition rates of vibration events are over 90%. The experimental results prove that this method can automatically make an effective feature selection and greatly improve the classification accuracy of vibrational events in distributed optical fiber sensing systems.

Intensity noise limit in a phase-sensitive optical time-domain reflectometer with a semiconductor laser source

In the present paper we perform, for the first time, the analysis of the average intensity noise power level at the output of a coherent phase-sensitive optical time-domain reflectometer (phase-OTDR) with a semiconductor laser source. The origin of the considered intensity noise lies in random phase fluctuations of a semiconductor laser source field. These phase fluctuations are converted to intensity noise in the process of interference of backscattered light. This intensity noise inevitably emerges in every phase-OTDR spatial channel and limits its sensitivity to external phase actions. The analysis of intensity noise in a phase-OTDR was based on the study of a fiber scattered-light interferometer (FSLI) which is treated as the constituent part of OTDR. When considered independently, FSLI has a broad intensity noise spectrum at its output; when FSLI is treated as a part of a phase-OTDR, due to aliasing effect, the wide FSLI noise spectrum is folded within the spectral band, determined by the probe pulse repetition frequency. In the analysis one of the conventional phase-OTDR schemes with rectangular dual-pulse probe signal was considered, the FSLI, which corresponds to this OTDR scheme, has two scattering fiber segments with additional time delay introduced between backscattered fields. The average intensity noise power and resulting noise spectrum at the output of this FSLI are determined by the degree of coherence of the semiconductor laser source, the length of the scattering fiber segments and by the additional time delay between the scattering segments. The average intensity noise characteristics at the output of the corresponding phase-OTDR are determined by the analogous parameters: the source coherence, the lengths of the parts constituting the dual-pulse and the time interval which separates the parts of the dual-pulse. In the paper the expression for the average noise power spectral density (NPSD) at the output of FSLI was theoretically derived and experimentally verified. Based on the found average NPSD of FSLI, a simple relation connecting the phase-OTDR parameters and the limiting level of full average intensity noise power at its output was derived. This relation was verified by experimental measurement of the average noise power at the output of phase-OTDR. The limiting noise level, considered in the paper, determines the fundamental noise floor for the phase-OTDR with given parameters of the source coherence, probe pulse length and time delay between two pulses constituting the dual-pulse.

High spatial resolution phase-sensitive optical time domain reflectometer with a frequency-swept pulse.

A high spatial resolution phase-sensitive optical time domain reflectometer (ϕ-OTDR) with an optical frequency-swept pulse (FSP) is proposed, and the experimental results are presented in the Letter. The FSP ϕ-OTDR uses optical pulses with linear frequency modulation with higher pulse energy for longer sensing fiber and uses matched filter in the receiver to compress the processed pulse width. Thus, the contradiction between spatial resolution and the working distance in ordinary ϕ-OTDR is relaxed. A spatial resolution of 30cm, a sensing distance of 19.8km, and a signal-to-noise ratio of 10dB for vibration sensing were obtained experimentally. To our best of our knowledge, this is the first time that a sub-meter spatial resolution over such a long sensing range has been reported in ϕ-OTDR sensors.

Recent progress in ultra-long distributed fiber-optic sensing

Distributed fiber-optic sensing (DFOS) is one of the most important parts in the fiber-optic sensing field, due to the following advantages:1) there is no need to manufacture sensors on the fiber; 2) fibers are able to realize transmission and detection simultaneously; 3) long-distance/large-scale sensing and networking can be accomplished prospectively; 4) the spatial distribution and measurement information of physical parameters such as temperature, strain and vibration, can be obtained continuously along the fiber link, and the number of sensing points on a single fiber can be up to several tens of thousands. Due to the above tremendous superiority, DFOS has found wide application prospects, including perimeter security, oil/gas exploration, electrical facilities and structure monitoring, etc. This paper overviews recent progress in ultra-long distributed fiber-optic static (Brillouin optical time-domain analyzer) and dynamic (phase-sensitive optical time-domain reflectometer) sensing at Key Laboratory of Optical Fiber Sensing and Communications, UESTC. This paper summarizes our work on both basic and application studies.

A Fading-Discrimination Method for Distributed Vibration Sensor Using Coherent Detection of $\varphi $ -OTDR

We have proposed an effective method to discriminate the false alarm peaks induced by fading effect in distributed fiber-optic vibration sensing systems based on coherent heterodyne detection of phase-sensitive optical time-domain reflectometer ( $\varphi $ -OTDR). By comparison of the demodulated phase of backscattered light returned from the location of the sensing fiber before and behind the alarm peaks, the false alarm peaks can be distinguished from real vibration alarm peaks. We have demonstrated a fiber-optic distributed vibration sensor with efficient fading discrimination based on $\varphi $ -OTDR system that has a measurement range of 2.5 km, a spatial resolution of 2.5 m, and a signal-to-noise ratio of 16 dB. The results provide a simple and low-cost approach for the false alarm discrimination in all-fiber $\varphi $ -OTDR systems.

Quantitative measurement of dynamic nanostrain based on a phase-sensitive optical time domain reflectometer.

A sensing system is proposed for quantitative measurement of large-range dynamic nanostrain based on a phase-sensitive optical time domain reflectometer, where the coherent detection and I/Q demodulation methods are employed to demodulate both the phase and the amplitude of the Rayleigh scattering light in real time. A nanopositioning translation stage is utilized to apply precise nanostrain to fiber. By measuring phase differences between two adjacent sections, the quantitative nanostrain with a large measurement range is demonstrated; this is also a method to measure the strain parameter of refractive index. For the Panda polarization-maintaining fiber under test in the experiment, the strain parameter of phase difference is measured to be 8.714 mrad/(nε·m), while the strain parameter of refractive index is measured to be -0.3751ε-1. As a proof of the concept, the dynamic strain sensing with a range of 10-1000 nε is experimentally demonstrated, and the strain resolution is 1 or 2 nε, corresponding to 5 or 2.5 m spatial resolution, respectively. The experimental measurement also shows a triangular wave with a 12-Hz vibrating frequency and a 100-nε strain amplitude as well as a 188-Hz resonant signal of the tensile section.

Vibration detection method for optical fibre pre‐warning system

The measurement of optical fibre vibration is a key part of optic fibre pre-warning system, which has gradually focused on phase-sensitive optical time-domain reflectometer. However, for this instrument, false alarm rate is very high and some unstable intrusion signals cannot be detected by using its fixed threshold method in the actual application. It needs to develop new vibration detection method to overcome the above defect. The vibration signals normally consist of three parts, that is, noise, interference and intrusion signals. After a large number of data analysis, the authors find that the system noise is time varying and follows the Rayleigh distribution. Hence, the authors innovatively use the constant false alarm rate (CFAR) method to detect this type of intrusion. Considering interference is also time varying and diverse, a good detection performance cannot be obtained only by using the conventional CFAR. For this reason, a background homogeneity adaptive CFAR (BHA-CFAR) method is further proposed to detect the vibration signals in this study. The BHA-CFAR consists of two detectors, cell averaging CFAR (CA-CFAR) detector and greatest-of/smallest-of CFAR (GO/SO-CFAR) detector. A parameter, homogeneity of background, is estimated first to classify the surrounding. Then CA-CFAR and GO/SO-CFAR are optionally used according to the surrounding is homogeneous or heterogeneous, respectively. This new detection method can adapt to any background surrounding and has a good detection performance. In order to check the feasibility and validity of the BHA-CFAR method, several experiments were carried out in Da Gang oilfield. The detection results show that the proposed method can provide a good tradeoff between the detection performance and computation time.

Distributed Fiber-Optic Sensors for Vibration Detection.

Distributed fiber-optic vibration sensors receive extensive investigation and play a significant role in the sensor panorama. Optical parameters such as light intensity, phase, polarization state, or light frequency will change when external vibration is applied on the sensing fiber. In this paper, various technologies of distributed fiber-optic vibration sensing are reviewed, from interferometric sensing technology, such as Sagnac, Mach–Zehnder, and Michelson, to backscattering-based sensing technology, such as phase-sensitive optical time domain reflectometer, polarization-optical time domain reflectometer, optical frequency domain reflectometer, as well as some combinations of interferometric and backscattering-based techniques. Their operation principles are presented and recent research efforts are also included. Finally, the applications of distributed fiber-optic vibration sensors are summarized, which mainly include structural health monitoring and perimeter security, etc. Overall, distributed fiber-optic vibration sensors possess the advantages of large-scale monitoring, good concealment, excellent flexibility, and immunity to electromagnetic interference, and thus show considerable potential for a variety of practical applications.

Research on the performance of multifunctional distributed optical fiber sensing system based on the combination of Brillouin optical time-domain analysis and phase-sensitive optical time-domain reflectometer

There are many similarities between Brillouin optical time-domain analysis and phase-sensitive optical time-domain reflectometer in system topological structure. A multifunctional distributed optical fiber sensing system based on their similarities and the combination of their topological structure is presented. The system can monitor strain and temperature, as well as detect and locate the intrusions or disturbances along the sensing fiber. Suppose that X is the width of the optical pulse incident into the sensing fiber with the unit of nanosecond and Y is the minimum detectable distance with the unit of meter between two intrusions that are being detected simultaneously. The experiment results show that y changes linearly with x according to the equation: y=0.103x−0.452. By optimizing device parameters of the system, strain measurement accuracy of 3.17 μϵ and the temperature measurement accuracy of 0.45°C have been realized when the optical pulse width is 30 ns. On the other hand, two intrusions have been detected and located simultaneously with this multifunctional system. The results show that the pulse width and the minimum distance between two intrusions (Lmin) have a linear relationship. The theoretical analyses and the experiment results show it is applicable to use this multifunctional system for strain and temperature monitoring, and intrusion detecting.

Fading reduction in a phase optical time-domain reflectometer with multimode sensitive fiber

In the present paper we propose a novel type of a coherent phase-sensitive optical time-domain reflectometer (OTDR) that utilizes a multimode optical fiber as a sensitive element and is capable of considerable reduction of signal fading. Elimination of OTDR signal fading consequently removes randomly occurring insensitivity of the fiber regions to an external phase action. The backscattered light field at the input of OTDR sensitive multimode optical fiber is represented by a speckle-like pattern, due to a so called modal noise phenomenon. This speckle pattern randomly changes when an optical probe pulse propagates in the fiber line. The backscattered field intensity in every single speckle changes in time statistically independently from the intensity change in every other speckle remote enough from the first one. Thus, on the output of a multimode sensitive fiber, there exist several statistically independent reflectograms, and every single reflectogram contains the same information about external action. The joint independent analysis of these reflectograms can result in reduced or complete fading elimination.

Phase-Sensitive Optical Time-Domain Reflectometer Based on a 120°-Phase-Difference Michelson Interferometer**Supported by the National Natural Science Foundation of China under Grant Nos U0934001 and 11076028, the Science and Technology Commission of Shanghai Municipality under Grant Nos 11DZ1140202 and 13XD1425400, and the Pudong New Area Science and Technology Development Fund of China under Grant No PKJ2012-D04.

A phase-sensitive optical time domain reflectometer (φ-OTDR) based on a 120°-phase-difference Michelson interferometer is proposed. The Michelson interferometer with arm difference of 4m is used to test the phase difference between the Rayleigh scattering from two sections of the fiber. A new demodulation method called the inverse transmission matrix demodulation scheme is utilized to demodulate the distributed phase from the backward scattering along the long fiber. The experimental results show that the 120°-phase-difference interferometer φ-OTDR can detect the phase along the 3km fiber, and the acoustic signal within the whole human hearing range of 20 Hz–20 kHz is reproduced accurately and quickly.

Combination of Phase-Sensitive OTDR and Michelson Interferometer for Nuisance Alarm Rate Reducing and Event Identification

In order to obtain low nuisance alarm rate (NAR) and achieve the disturbance event identification, the wavelength-division multiplexing combination of a phasesensitive optical time-domain reflectometer (φ-OTDR) and a Michelson interferometer (MI) is proposed and demonstrated. The disturbance is synchronously detected by a φOTDR and an MI. A true alarm is presented only when a real disturbance is detected by both φ-OTDR and MI. Furthermore, the event identification of the confirmed real disturbance is recognized by an equivalent frequency distribution analysis on the MI signal. It is found that, by the experiment, NAR can be reduced to 1.2% from 13.5% via a single φ-OTDR, with a high detection probability of 92%. Furthermore, four disturbance events can be distinguished with a 90% identification rate. Our proposed wavelength-division multiplexing of a φ-OTDR and an MI is simple and low cost, no complicated algorithm is required, and only one sensing fiber is needed; therefore, it is an interesting exploration of multiplexing of φ-OTDR and interferometers.