Phase-sensitive Optical Time-domain Reflectometer Research Articles

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.

A harmful-intrusion detection method based on background reconstruction and two-dimensional K-S test in an optical fiber pre-warning system

The key technology and main difficulty for optical fiber intrusion pre-warning systems (OFIPS) is the extraction of harmful-intrusion signals. After being processed by a phase-sensitive optical time-domain reflectometer (Φ-OTDR), vibration signals can be preliminarily extracted. Generally, these include noises and intrusions. Here, intrusions can be divided into harmful and harmless intrusions. With respect to the close study of signal characteristics, an effective extraction method of harmful intrusion is proposed in the paper. Firstly, in the part of the background reconstruction, all intrusion signals are first detected by a constant false alarm rate (CFAR). We then reconstruct the backgrounds by extracting two-part information of alarm points, time and amplitude. This ensures that the detection background consists of intrusion signals. Secondly, in the part of the two-dimensional Kolmogorov-Smirnov (K-S) test, in order to extract harmful ones from all extracted intrusions, we design a separation method. It is based on the signal characteristics of harmful intrusion, which are shorter time interval and higher amplitude. In the actual OFIPS, the detection method is used in some typical scenes, which includes a lot of harmless intrusions, for example construction sites and busy roads. Results show that we can effectively extract harmful intrusions.

Experimental investigation on lower nuisance alarm rate phase-sensitive OTDR using the combination of a Mach–Zehnder interferometer

Phase-sensitive optical time domain reflectometer (φ-OTDR) currently suffers from high nuisance alarm rate (NAR) due to fluctuation of scattering light intensity, nonlinear coherent addition, and ambient noises in complicated noisy environments. In order to lower NAR, a φ-OTDR combined with a Mach–Zehnder interferometer (MZI) is proposed and experimentally demonstrated. In this system, φ-OTDR takes the role of detecting and locating the disturbance and MZI verifies the real disturbance by using an alarm threshold. Experimental results demonstrate that NAR of φ-OTDR with MZI can be efficiently reduced to ∼1% from ∼15% of a single φ-OTDR and a high probability of detection of ∼97% is achieved. Furthermore, the dependences of NAR and probability of detection on the pulse width and sampling rate is investigated. When the pulse width is changed from 0.1 to 1μs and sampling rate vary in the range from 10 to 50MS/s, NAR and probability of detection only exist a little fluctuation. The proposed scheme is helpful to reduce NAR of the current single φ-OTDR without introduction of complicated algorithm and also have simple configuration with low cost.

Phase-sensitive correlation optical time-domain reflectometer using quantum phase noise of laser light.

We propose and experimentally demonstrate a simple approach to realize a phase-sensitive correlation optical time-domain reflectometer (OTDR) suitable for detection and localization of dynamic perturbations along a single-mode optical fiber. It is based on the quantum phase fluctuations of a coherent light emitted by a telecom DFB diode laser. Truly random probe signals are generated by an interferometer with the optical path difference exceeding the coherence length of the laser light. Speckle-like OTDR traces were obtained by calculating cross-correlation functions between the probe light and the light intensity signals returned back from the sensing fiber. Perturbations are detected and localized by monitoring time variations of correlation amplitude along the fiber length. Results of proof-of-concept experimental testing are presented using an array of ultra-low-reflectivity fiber Bragg gratings as weak reflectors.

Influences of intrusion signal’s characteristics on detection performances of the distributed sensor based on phase-sensitive optical time domain reflectometer

Phase-sensitive optical time domain reflectometer (Φ-OTDR) has been a research hotspot in recent years. For Φ-OTDR, the Rayleigh backscattering light is monitored with a photodetector, and the plotted trace is a curve of the returned optical power versus time. The time is proportional to the distance from the location along the sensing fiber to the optical pulses’ incident end. The trace presents the form of speckle-like profile owing to the coherent addition of the amplitudes of the light backscattered from different locations of the fiber. Based on this, the influences of the intrusion signal’s characteristics (occurrence location and phase variation) on the detection performances of the distributed sensor based on Φ-OTDR are investigated theoretically and demonstrated experimentally, especially on signal-to-noise ratio (SNR), missing report, and location accuracy. Results show that if intrusions occur at locations corresponding to the curve’s peak or subpeak and when its phase variation is (2k+1)π(k=1,2,3⋯), SNR will be the maximum. If intrusions occur at locations corresponding to the curve’s troughs, a missing report will occur no matter what value is taken by the phase variation of the intrusion signal. On the other hand, experimental results show most missing reports can be uncovered by using a frequency tunable laser and/or optical pulses with an alternative repetition frequency.

Distributed fiber sensing system with wide frequency response and accurate location

A distributed fiber sensing system merging Mach–Zehnder interferometer and phase-sensitive optical time domain reflectometer (Φ-OTDR) is demonstrated for vibration measurement, which requires wide frequency response and accurate location. Two narrow line-width lasers with delicately different wavelengths are used to constitute the interferometer and reflectometer respectively. A narrow band Fiber Bragg Grating is responsible for separating the two wavelengths. In addition, heterodyne detection is applied to maintain the signal to noise rate of the locating signal. Experiment results show that the novel system has a wide frequency from 1Hz to 50MHz, limited by the sample frequency of data acquisition card, and a spatial resolution of 20m, according to 200ns pulse width, along 2.5km fiber link.

Optimization of a distributed optical fiber sensor system based on phase sensitive OTDR for disturbance detection

Purpose – The purpose of this paper is to theoretically analyze and experimentally demonstrate the investigation on and optimization of a distributed optical fiber sensor based on phase-sensitive optical time domain reflectometer (F-OTDR) for disturbance detection. Design/methodology/approach – The F-OTDR system is investigated and optimized in two aspects: the hardware parameter and the interrogation scheme. Findings – Based on the optimized hardware and the new interrogation scheme, the performances of the F-OTDR system have been improved greatly, compared with conventional F-OTDR system. A location accuracy of 2 m and a signal-to-noise ratio (SNR) of 16 dB have been achieved under a spatial resolution of 8 m. On the other hand, four disturbances at four different locations have been detected and located simultaneously, which is the most effective detection system with the maximum detection capability reported to date, to the best of the authors’ knowledge. Originality/value – Four disturbances at four different locations have been detected and located simultaneously, which is the most effective detection system with the maximum detection capability reported to date, to the best of the authors’ knowledge. With same hardware conditions, more existing disturbances can be detected by using the new interrogation scheme, which is helpful to reduce the miss report of disturbance.

Distributed phase birefringence measurements based on polarization correlation in phase-sensitive optical time-domain reflectometers.

In this paper a technique to measure the distributed birefringence profile along optical fibers is proposed and experimentally validated. The method is based on the spectral correlation between two sets of orthogonally-polarized measurements acquired using a phase-sensitive optical time-domain reflectometer (ϕOTDR). The correlation between the two measured spectra gives a resonance (correlation) peak at a frequency detuning that is proportional to the local refractive index difference between the two orthogonal polarization axes of the fiber. In this way the method enables local phase birefringence measurements at any position along optical fibers, so that any longitudinal fluctuation can be precisely evaluated with metric spatial resolution. The method has been experimentally validated by measuring fibers with low and high birefringence, such as standard single-mode fibers as well as conventional polarization-maintaining fibers. The technique has potential applications in the characterization of optical fibers for telecommunications as well as in distributed optical fiber sensing.

A Long Distance Phase-Sensitive Optical Time Domain Reflectometer with Simple Structure and High Locating Accuracy.

A phase-sensitive optical time domain reflectometer (Φ-OTDR) can be used for pipeline security. However, the sensing distance (less than 20 km) of traditional Φ-OTDR is too short for the needs of typical oil and gas pipeline monitoring applications (30–50 km). A simple structure Φ-OTDR system utilizing long pulse, balanced amplified detector and heterodyne detection is proposed in this paper and the sensing range is thereby increased to 60 km. Through analyzing the sensing principle of Φ-OTDR, a novel locating strategy is proposed to maintain the locating accuracy at a few meters when a long pulse (5 µs) is used. The increased pulse width deteriorates the time series of each sensing point seriously. In order to eliminate the deterioration, a data processing technique combining wavelet and empirical mode decomposition is applied in this system. The experiment results show that the sensing distance can be increased to 60 km and the locating accuracy is maintained at 6.8 m.

Recognition of a Phase-Sensitivity OTDR Sensing System Based on Morphologic Feature Extraction

This paper proposes a novel feature extraction method for intrusion event recognition within a phase-sensitive optical time-domain reflectometer (Φ-OTDR) sensing system. Feature extraction of time domain signals in these systems is time-consuming and may lead to inaccuracies due to noise disturbances. The recognition accuracy and speed of current systems cannot meet the requirements of Φ-OTDR online vibration monitoring systems. In the method proposed in this paper, the time-space domain signal is used for feature extraction instead of the time domain signal. Feature vectors are obtained from morphologic features of time-space domain signals. A scatter matrix is calculated for the feature selection. Experiments show that the feature extraction method proposed in this paper can greatly improve recognition accuracies, with a lower computation time than traditional methods, i.e., a recognition accuracy of 97.8% can be achieved with a recognition time of below 1 s, making it is very suitable for Φ-OTDR system online vibration monitoring.

Phase-Sensitive Optical Time Domain Reflectometer with Dual-Wavelength Probe Pulse

A dual-wavelength pulse strategy is proposed to reduce the fading phenomenon in a phase-sensitive optical time domain reflectometer ( ϕ-OTDR). The theoretical basis behind this dual-wavelength pulse strategy is presented and an experimental setup is described to help validate the proposed strategy. Through the experimental tests, a 37.79% improvement in detection for available points along the fiber is achieved when 1550.000 nm and 1550.138 nm wavelength signals are applied. The different wavelength tests show that at least a 0.012 nm or 1.498 GHz difference should be applied in a dual-wavelength pulse and an average 30% detecting improvement can be achieved when one wavelength changes from 1549.950 nm to 1550.100 nm. In addition, the dual-wavelength strategy makes the system response more stable and consistent along the sensing optical fiber.

A phase-sensitive optical time-domain reflectometer with dual-pulse diverse frequency probe signal

In the present communication we propose a novel approach to the realization of a phase sensitive optical time-domain reflectometer (OTDR) which is capable of a precise reconstruction of the phase signal which impacts the arbitrary point of a fiber-optic line. The method uses a dual-pulse probe signal with diverse carrier optical frequency within each half of the double pulse. The quasi-periodic intensity pattern which emerges as a result of double frequency backscattered signal interference contains the information of the external action over the fiber. The phase signal is extracted with the aid of an I/Q quadrature demodulation scheme, realized at the receiving side of the OTDR. The feasibility and limitations of the proposed scheme are theoretically proved and experimentally demonstrated.

Phase-Sensitive Optical Time Domain Reflectometer Based on Phase-Generated Carrier Algorithm

We propose a novel approach to generate distributed fiber sensing system based on phase-sensitive optical time domain reflectometer (Φ-OTDR) and phase-generated carrier demodulation algorithm. An unbalanced Michelson interferometer is introduced at the receiving end of the system. The back Rayleigh scattering light from a certain position along the sensing fiber would interfere to generate interference light signal versus time, whose phase carries the sensing information. Phase-generated carrier demodulation algorithm is proposed and carried out to recover the phase information. A single frequency vibration event is applied to a certain position along the sensing fiber and we realize to demodulate it correctly. The noise level of the phase sensitive OTDR system is about 3 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> rad/VHz and a signal to noise ratio about 30.45 dB is achieved. The maximum sensing length and the spatial resolution of the Φ-OTDR system are 10 km and 6 m with pulse repetition rate at 10 kHz and 6 m fiber delay in MI with interrogating pulse width of 30 ns.

Experimental study of influence of nonlinear effects on phase- sensitive optical time-domain reflectometer operating range

This paper presents experimental results of a study the influence of nonlinear effects on the range of the phase-sensitive optical time-domain reflectometer. Results from measurements of the impact of the effects of stimulated Raman scattering, stimulated Brillouin scattering, self-phase modulation, and modulation instability are presented. It is shown that the power of the probe pulse for 200 ns does not exceed ≈300 mW.

Distributed Vibration Sensing Over 125 km With Enhanced SNR Using Phi-OTDR Over a URFL Cavity

We describe the use of a phase-sensitive optical time domain reflectometer ( $\phi {\bf OTDR}$ ) over an ultra-long Raman fiber laser cavity allowing fully distributed detection of vibrations over 125 km. Compared to a first-order Raman-assisted $\phi {\bf OTDR}$ , this scheme shows an enhanced signal-to-noise ratio (SNR). This is due to the fact that the relative intensity noise introduced by the Raman amplification is mostly transferred to a lower frequency range, where the balanced detection implemented in the setup provides better suppression of the common-mode noise. The sensor was able to measure vibrations of up to 380 Hz (limit set by the time of flight of light pulses) in a distance of 125 km with a resolution of 10 m and an average SNR of 8 dB with no postprocessing. This implies a >3 dB improvement in SNR over a first-order Raman-assisted setup with similar characteristics.

Novel distributed passive vehicle tracking technology using phase sensitive optical time domain reflectometer

A novel distributed passive vehicle tracking technology is proposed and demonstrated. This technology is based on a phase-sensitive optical time domain reflectometer (Φ-OTDR) that can sense and locate vibrations. Two algorithms, dynamic frequency-space image and 2D digital sliding filtering, are proposed to distinguish a car’s moving signals from severe environmental noises and disturbances. This technology is proved effective by field experiments for tracking a single car and multiple cars. This work provides a new distributed passive way for real-time vehicle tracking and this technology will be extremely important for traffic controlling and public safety in modern society.