Optical Time Domain Reflectometer Research Articles

Selective distributed optical fiber sensing system based on silicone cladding optical fiber and Rayleigh backscattering reflectometry for the detection of hydrocarbon leakages

A new system that exploits optical fiber distributed sensing for the detection of hydrocarbons is presented. The system relies on a custom-designed silica core/thin silicone cladding optical fiber to provide a quick and selective absorption of oil products through the cladding; moreover, it exhibits insensitivity to water. Optical fiber sensor interrogation is performed by detecting Rayleigh backscattering using an optical time domain reflectometer or, in a high-resolution version, an optical frequency domain reflectometer. The Rayleigh backscattering signal is influenced by a change in the refractive index of the cladding, which locally modifies the guiding properties of the fiber; and by the swelling of the cladding, resulting from hydrocarbon diffusion, that produces local stress and results in an associated increased reflection. The system has been tested with different solvents and has exhibited selectivity and a rapid response to exposure to high refractive index hydrocarbons, with the sensing fiber yielding a response time of the order of 1 s. The system, in its high-resolution version, is able to accurately locate leakages with an accuracy of 14 cm.

Automated Damage Detection Using Lamb Wave-Based Phase-Sensitive OTDR and Support Vector Machines.

In this paper, we propose and demonstrate a damage detection technique based on the automatic classification of the Lamb wave signals acquired on a metallic plate. In the reported experiments, Lamb waves are excited in an aluminum plate through a piezoelectric transducer glued onto the monitored structure. The response of the monitored structure is detected through a high-resolution phase-sensitive optical time-domain reflectometer (ϕ-OTDR). The presence and location of a small perturbation, induced by placing a lumped mass of 5 g on the plate, are determined by processing the optical fiber sensor data through support vector machine (SVM) classifiers trained with experimental data. The results show that the proposed method takes full advantage of the multipoint sensing nature of the ϕ-OTDR technology, resulting in accurate damage detection and localization.

Real-time classification for Φ-OTDR vibration events in the case of small sample size datasets

Efficient classification of vibration signals detected by phase-sensitive optical time domain reflectometer (Φ-OTDR) based on small samples is an effective method to reduce the false alarm rate without GPU or large data sets. This paper proposes a fiber optic system vibration event recognition method based on a combination of image segmentation pre-processing, texture, statistical, morphological feature extraction, and weighted support vector machine (WSVM), which can effectively classify-five types of vibration events in high-speed railway perimeter intrusion detection with small sample data and no parallel processing units. Erosion and dilation operations are applied to vibration signal image feature enhancement in image pre-processing. The vibration signal region and background are separated by the maximum inter-class variance method, then 35 features of the vibration signal region are calculated and finally employed to construct a WSVM. Experiments show that the method achieves 99 FPS and 98.8% accuracy on the test set with 330 vibration images as the training set to build the model without GPU and in the presence of interference signals. It provides a generalized Φ-OTDR vibration event recognition method for small samples.

Efficient Fault Detection Algorithm in Fiber Optic Communication Systems

Primary concern of optical communication system is the signal loss, which need to be traced precisely to enhance the efficiency of the system. Among numerous available techniques, optical time domain reflectometer (OTDR) is the most suitable one for characterizing and tracing the losses. It measures the fraction of a probe pulse that scatters back from the fiber optic cable. Due to the presence of small levels of backscattering in single-mode fiber at long wavelengths, very sensitive optical detector is necessary to achieve sufficient range performance. In the present research, a novel yet simple approach has been demonstrated to understand the range of optical fiber cable feasibility on fault detection and rectification technique. The actual place of fault is marked with the variation in the measured values of OTDR. Thus, it becomes a difficult task to trace and locate the actual place of fault detection, which leads to an inefficient rectification method. The observed values of OTDR shows a gradual decrease of accuracy in locating the actual place of fault. To resolve these issues, here we try to develop an algorithm which is able to easily and efficiently trace the exact location of fault on earth.

Concise method for high-precision vibration recovery of Φ-OTDR based on the GF-FastICA algorithm.

To efficiently restore the vibration signals of a phase-sensitive optical time domain reflectometer (Φ-OTDR), the GF-FastICA joint algorithm is proposed, which combines guided filtering with fast independent component analysis (FastICA). The marked region of vibration is precisely located by guided filtering. FastICA deals with the optimal phase mixing matrix of the marked region to separate the vibration signals from the noise-containing phase signals. The experimental results show that the GF-FastICA achieves a correlation coefficient of 0.998 for 5-Hz vibration signal recovery from a 14.3-km sensing fiber, verifying the potency of the algorithm. Compared with the traditional method and FastICA only, GF-FastICA improves the root mean square error (RMSE) metric by an order of magnitude, which is approaching an experience value of 10-3.

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

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

A real-time phase processing system for phase sensitive optical time domain reflectometer.

In order to improve the real-time performance of a phase-sensitive optical-time-domain reflectometer (Φ-OTDR), a fully digital phase processing system is demonstrated. Using digital down-conversion and field programmable gate array (FPGA) hardware calculation, the phase of the Rayleigh scattering light can be demodulated in real-time. Benefiting from the pipeline calculation in the FPGA, the real-time performance of this system is not affected by the interrogation rate and fiber sensing distance. Experiments show that this real-time detection system has the same vibration detection performance as an offline data process and has good stability. The dynamic strain sensing with a sensing distance of 50 km, an interrogation rate of 1kHz, and a spatial sampling interval of 1m is experimentally demonstrated. The obtained results indicate that the fully digital signal processing system can ensure the real-time detection of Φ-OTDR under the condition of long distance and high resolution.

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

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

Single and composite disturbance event recognition based on the DBN-GRU network in φ-OTDR.

An end-to-end deep learning model based on the deep belief network (DBN) and gated recurrent unit (GRU) is proposed to recognize the single disturbance events and composite disturbance events in the phase-sensitive optical time-domain reflectometer (φ-OTDR). Making use of the DBN to fit the original data, five kinds of single disturbance events can be effectively recognized with the GRU network as the classifier. An average recognition accuracy of 96.72% with a short recognition time of 0.079s can be achieved for single disturbance events. Moreover, the proposed method is also applied for recognizing composite disturbance events. Four kinds of composite disturbance events can be recognized with an average recognition accuracy as high as 90.94%, and the corresponding recognition time is only 0.084s. Up until now, there have been fewer reports about the recognition of composite disturbance events in φ-OTDR systems. High recognition accuracy and short recognition time make the model based on DBN-GRU more capable in a high sensitivity, real-time φ-OTDR system.

Cascade Edge Filter for Demodulation of Quasi-Distributed FBG Sensor

The realization of multiple demodulation sche- mes of fiber Bragg grating (FBG) sensor is important for its engineering application. Here, we proposed a new edge filter that is formed by cascading dispersion compensation fiber (DCF) and no-core fiber (NCF) structure between two single-mode fibers (SMFs) for demodulation of quasi-distributed FBG sensor. The edge filter can be suitable for FBG with different center wavelengths by changing its structure parameters to achieve a wide range of FBG demodulation. In order to verify the performance of the edge filter, a demodulation system for quasi-distributed FBG sensor is successfully designed by introducing an optical time-domain reflectometer (OTDR), the wavelength shift of FBG caused by the change of temperature and strain is converted into power change through the edge filter, so as to realize temperature and strain demodulation. In the experiment, two FBG sensors separated by 500 m were set at the end of a 3000-m optical fiber for temperature and strain monitoring. The results show that the temperature sensitivity is 0.23 dB/°C from 50 °C to 100 °C, and the strain sensitivity is 0.01658 dB/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \epsilon $ </tex-math></inline-formula> from 0 to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$669.6\,\mu \epsilon $ </tex-math></inline-formula> . The edge filter for FBG demodulation has the advantages of high sensitivity, adjustable structural parameters, low cost, and independent multipoint/multiparameter demodulation.

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

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

Cascaded high-density multipoint gas detection with branched gas chambers

Distributed or multipoint gas detection is crucial for several industrial and civil applications. Due to various losses, the traditional series–parallel method has a restricted number of measurement points. In this paper, an innovative cascaded connection method has been introduced for measuring a large number of locations. It is basically a branching route that contains a gas cell and a reflecting mirror. A small amount of light is coupled to the branch channel, leading to limited effects on transmission power. Using a light source that corresponds to the gas absorption line, an optical time domain reflectometer (OTDR) device interrogates each cell by branching. After establishing a length of delay fiber at one of the branch locations, the signals coming from each air cell are recognized individually at the receiving end. In this experiment, 99:1 couplers are used, and only 1% of the total light is transferred to the branch route gas cell. The signal-to-noise ratio increases dramatically compared to previous studies based on Rayleigh scattering. This is because the amplitude of the reflected light is one-half order more than that of the scattered light. A 1530 nm laser for acetylene and a 1653 nm laser for methane with a short pulse width are used for evaluating the novel multipoint gas detection technique. There is an analysis and discussion of the number of measurement points. The proposed method is capable of optimally laying 220 cells at a maximum monitoring distance of 8.8 km within a dynamic range of ∼13.56 dB. Using the present wavelength power calibration method, the minimum detection limits for methane and acetylene along the 1% branching path are 0.093 low explosion level (LEL) and 0.321 LEL, respectively.

A Design Fiber Performance Monitoring Tool (FPMT) for Online Remote Fiber Line Performance Detection

A new technique for fiber faults events detection and monitoring in optical communication network systems is proposed. The fiber performance monitoring tool is a new proposed technique designed to detect, locate, and estimate the fiber faults without interrupting the data flow with efficient costs and to improve the availability and reliability of optical networks as it detects fiber faults remotely in real time. Instead of the traditional old method, the new proposed FPMT uses an optical time domain reflectometer to detect multiple types of fiber failures, e.g., fiber breaks, fiber end face contamination, fiber end face burning, large insertion losses on the connector and interconnection, or mismatches between two different types of fiber cables. The proposed technique methodology to detect the fiber failures depends on analyzing the feedback of the reflected signal and the pattern shape of the reflected signal over network fiber lines, supports a higher range of distance testing and performance monitoring, and can be performed inside an optical network in real time and remotely by integrating with an OSC board. The proposed technique detects fiber faults with an average accuracy of measurement up to 99.8%, the maximum distance to detect fiber line faults is up to 150 km, and it can improve the system power budget with a minimal insertion loss of 0.4 dB. The superiority of the suggested technique over real networks was verified with success by the Huawei labs’ infrastructure nodes in the simulation experiment results.

A hybrid distributed optical fiber vibration and temperature sensor based on optical Rayleigh and Raman scattering

A hybrid distributed optical fiber vibration and temperature sensing system is proposed and experimentally demonstrated. By the system structure of optical time domain reflectometer (OTDR) and wavelength division method, the Rayleigh and Raman scattering light in multi-mode fiber (MMF) are extracted respectively for fiber vibration event detection and hot spot monitoring. The vibration event causes the polarization state and phase of the optical signal propagating in MMF to change, which makes the OTDR trace become smooth from the vibration position to the fiber end, so that it provides a reliable way to locate the vibration events. And then, a data processing method that averages the data within the vibration position and at least 50 continuous positions behind is proposed to improve the signal to noise ratio (SNR) of the vibration signal and finally by fast Fourier transform (FFT), the vibration spectrum is accurately obtained. In addition, the backscattered Raman Stokes and anti-Stokes signals are separated by the wavelength division multiplexer (WDM) and then they are converted into voltage signals. By conventional temperature demodulation algorithm, the temperature information along the MMF is obtained with measurement accuracy of about 1.5 °C. The experimental results show that the proposed hybrid distributed vibration and temperature sensing system possess good performance and as it is compatible with MMF that has been laid on the power cable, it is of great significance for engineering application.

Study on the Efficiency of Temperature/Strain Measurement for Ultra-Long-Distance Optical Fiber Composite Overhead Power Transmission Lines

The coherent optical time domain reflectometer (COTDR) is a very important instrument for distributed temperature/strain measurement along the sensing fiber with a large dynamic range and high accuracy. The length of the sensing fiber and the temperature/strain measurement range limit the system performance, especially the measurement efficiency. So, a COTDR system is constructed, and the characteristics of the obtained coherent Rayleigh noise (CRN) are analyzed. Then, in consideration of the temperature/strain measurement range, accuracy, and time efficiency, the temperature/strain demodulation algorithm in noise conditions is studied. With different noise coefficients, the array length with 11, 21, 31, 51, and 101 independent frequency sweeping points are adopted to calculate the cross-correlation coefficients along a standard reference array with 301 independent frequency sweeping points. The results demonstrate that the array length has little influence on the signal processing time, but it can decide the measurement accuracy. To balance the system measurement efficiency and accuracy, it is inferred that, for a sensing fiber with a length of 100 km or more, the optimal independent frequency sweeping points are 101 and the trace average number is 1000.

Noise suppression for phase-sensitive optical time-domain reflectometer based on non-local means filtering

Phase-sensitive optical time-domain reflectometer (Φ-OTDR) has been widely used in various fields because of its unique advantages for long-haul measurements. In this paper, the result of Φ-OTDR is treated as a two dimensional spatial–temporal image which can be filtered by the technique of non-local means (NLM). To improve the calculation speed of NLM, fast NLM based on integral image is introduced. The noise suppression effect of three intrusion cases processed by fast NLM is studied by both simulations and experiments, including single-position intrusion, double-position intrusion and riding along the optical cable. Both peak signal-to-noise ratio (PSNR) and structural similarity (SSIM) in the area of image processing are used for quantifying the effects of NLM filtering. The noise suppression ratio (NSR) based on root-mean-square (RMS) is introduced and applied to evaluate the noise suppression of Φ-OTDR. After fast NLM processing, it can be obtained that the NSR could increase to more than 10 dB, the PSNR could increase by more than 12 dB and the SSIM also increase from 0.9361 to 0.9931. Parameters of both search window and smoothing control parameter are adjusted to investigate the regularities of above three metrics in three intrusion cases and the suitable NLM parameters with the optimal performance has been discussed in the field tests. Both simulations and field test results show that NSR, PSNR and SSIM increase to a constant level with the increase of search window. Both PSNR and SSIM increase at first and then decrease while NSR directly increases to a fixed level as the smoothing control parameter increases.

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

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

A Novel Distributed Vibration Sensor Based on Fading Noise Reduction in Multi-Mode Fiber.

Multi-mode fiber (MMF) is used in a polarization-sensitive optical time domain reflectometer (OTDR) for vibration event location and spectrum analysis. The vibration events acting on MMF are considered to be the optical polarization state and phase diversifying process for fading noise reduction. In addition, data averaging with continuous positions and the fast Fourier transform (FFT) method is proposed to extract the spectrum of the vibration events. In the experiment, the vibration events are loaded at the positions of 5.167 and 10.145 km, respectively, along MMF. The experimental results demonstrate that the vibration event can effectively diversify the optical polarization state and phase of the Rayleigh scattering light to make the averaged OTDR trace behind the vibration position converge rapidly, which helps to locate corresponding vibration events and extract the vibration spectrum. It is inferred that the new distributed vibration sensor shall have a lower false alarm rate, as it can greatly reduce the errors caused by randomness of the sensing light signals. Additionally, it also saves time in comparison with the method that analyzes the vibration spectra for all the positions along the fiber under test.

Reducing large errors in frequency-scanned phase-sensitive optical time-domain reflectometers using phase cross correlation.

The use of phase cross correlation is proposed to estimate the frequency shift of the Rayleigh intensity spectral response in frequency-scanned phase-sensitive optical time-domain reflectometry (φ-OTDR). Compared with the standard cross correlation, the proposed approach is an amplitude-unbiased technique that evenly weights all spectral samples in the cross correlation, making the frequency-shift estimation less sensitive to high-intensity Rayleigh spectral samples and reducing large estimation errors. Using a 5.63-km sensing fiber with 1-m spatial resolution, experimental results demonstrate that the proposed method highly reduces the presence of large errors in the frequency shift estimation, increasing the reliability of the distributed measurements while keeping the frequency uncertainty as low as approximately 1.0 MHz. The technique can be also used to reduce large errors in any distributed Rayleigh sensor that evaluates spectral shifts, such as polarization-resolved φ-OTDR sensors and optical frequency-domain reflectometers.

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.