Optical Time Domain Reflectometer Research Articles

Experimental research on a novel spring-shaped fiber-optic displacement sensor for settlement monitoring

In this paper an innovative spring-shaped fiber-optic displacement sensor (SSFODS) based on optical time domain reflectometer (OTDR) technology with simple construction and cheap cost was proposed for subgrade settlement monitoring. The sensor incorporated a simple spring-shaped fiber bending modulation that increased its sensitivity in bending, light source and detector. The sensing principle between the measured external displacement and bending loss of optical fiber was theoretically explained and its expression was also derived. Sensor calibration was conducted, which showed that the SSFODS was characterized by a large measurement range of 90 mm, minimum displacement resolution of 0.173 mm, maximum hysteresis error of 2.81% and maximum repeatability error of 8.42%, respectively. A soil drainage model test with the SSFODS was performed to study its performance in determining compression deformation of the soil. At last, a simple field monitoring application with two SSFODSs connected in series was conducted and the progressive compaction behavior of the fill slope was accurately captured by two SSFODSs. Additionally, it is possible to connect these SSFODSs in series on one optical link to realize quasi-distributed sensing monitoring. This capability supports a promising application of the sensor in geotechnical engineering for real-time monitoring of the stability of fill slopes, foundation pits and so on.

Reflectometric temperature measurement using a single-mode-multimode-single-mode" fiber optic structure

It is shown the "single-mode-multimode-single-mode" structure allows carrying out remote temperature measurement with optical time domain reflectometer. A multimode fiber length was 10 mm. We used the temperature range from 30 to 70oC and wavelengths of 1310 and 1550 nm. The total length of a single-mode fiber-optic line for reflectometric measurements was 20 km. Keywords: fiber optics, fiber optic sensors, optical fiber reflectometry, multimode interference.

Рефлектометрическое измерение температуры с использованием оптоволоконной структуры одномод-многомод-одномод"

It is shown the "single-mode-multimode-single-mode" structure allows carrying out remote temperature measurement with optical time domain reflectometer. A multimode fiber length was 10 mm. We used the temperature range from 30 to 70oC and wavelengths of 1310 and 1550 nm. The total length of a single-mode fiber-optic line for reflectometric measurements was 20 km.

Applicability of Optical Time Domain Reflectometer for Measuring Differential Modal Group Delay in Few-Mode Fibers

Applicability of Optical Time Domain Reflectometer for Measuring Differential Modal Group Delay in Few-Mode Fibers

Accurate Location of Fiber Cable Fault with OTDR

The paper reviews the factors limiting the accuracy of locating a fiber optic cable fault when using an optical time domain reflectometer (OTDR) and describes an error estimation method for typical use cases. The primary source of errors lies in the complex relationship between the length of the optical fiber (measured by OTDR), its routing, cable design depending on cable design and type of installation (i.e. duct, directly buried, aerial) as well as the spare lengths used for service purposes. The techniques which considerably improve the accuracy of the fault localization processes are presented, the importance of accurate documentation of the network and of referencing the fault location to the nearest splice instead of end of the line are discussed, as is the absence of cable helix factor in data sheets.

Improvement of Performance for Raman Assisted BOTDR by Analyzing Brillouin Gain Spectrum.

We propose a simplified partitioned Brillouin gain spectrum (BGS) analysis method to enhance the spatial resolution and measurement accuracy of a Brillouin optical time-domain reflectometer (BOTDR) assisted by a first-order Raman pump. We theoretically derive the mathematical model of the partitioned BGS and analyze the superposition process of sub-Brillouin signals within a theoretical spatial resolution range. We unified all the unknown constant parameters of the calculation process to simplify the partitioned BGS analysis method and the value of the uniform parameter is attained through the system test data and numerical analysis. Moreover, to automate data processing, the starting point of the temperature/strain change is determined by the first occurrence of the maximum Brillouin frequency shift (BFS), then the position where the partitioned BGS analysis method calculation begins is obtained. Using a 100 ns probe pulse and partitioned BGS analysis method, we obtain a spatial resolution of 0.4 m in the 78.45-km-long Raman-assisted BOTDR system, and the measurement accuracy is significantly improved. In addition, we achieve a strain accuracy of 5.6 με and a spatial resolution of 0.4 m in the 28.5-km-long BOTDR without Raman amplification.

Enhanced fire sprinkler system: Fiber optical monitoring of fire sprinkler heads

A novel fiber optical monitoring concept for real-time localization of activated fire sprinkler heads is presented and discussed in this work. The concept is implemented by a designed fiber optical sensor and standard telecom components, such as optical time-domain reflectometer (OTDR), single-mode glass fiber, couplers, and reflectors. The proposed sensor based on optic reflection and interrupting mechanism allows to monitor the current status of sprinkler head and to differentiate the sprinkler activation from unwanted interferences, for example, glass fiber breakage. According to the experiments and calculations, hundreds of sprinkler heads can be monitored by a single OTDR device that results in net system energy consumption well below 100 mW per sensor point. The proposed monitoring concept opens up new perspectives for applying fiber optical sensors in fire protection methods. It can be used as a self-contained sensor network or in combination with modern fire detection and alarm systems in a primary-secondary-configuration.

Wavelength-Tunable OTDR for DWDM-PON Based on Optimized Wavelet Denoising

In this letter, a wavelength-tunable Optical Time-Domain Reflectometer (OTDR) for Dense Wavelength Division Multiplexing Passive Optical Network (DWDM-PON) is proposed and experimentally demonstrated. An Integrated Tunable Laser Assembly (ITLA) serves as the light source permitting measurements of 80 DWDM channels in the C-band. Dither modulation is added to ITLA to depress coherent noise resulted from the narrow linewidth of the light source. Semiconductor Optical Amplifier (SOA) modulates the continuous light into pulsed light and meanwhile amplifies peak optical power of the pulsed light to 13 dBm. In addition, we optimize the wavelet denoising algorithm for further signal-noise ratio (SNR) enhancement. Dynamic range and spatial resolution of the proposed OTDR reach up to 16.2 dB and 2 m respectively. A DWDM-PON fiber link including an Arrayed Waveguide Grating (AWG) is measured to verify the system performances, and the OTDR profiles of different channels coincide with each other. The designed wavelength-tunable OTDR is shown to be appropriate for DWDM-PON.

On the measurement accuracy of coherent Rayleigh-based distributed sensors

The phase change of back-scattered light due to external perturbations is retrieved in coherent Rayleigh-based distributed sensors by estimating the frequency shift (FS) between the traces of different measurements. The uncertainty associated with the estimator, due to the presence of system noises, can lead to an inaccurate evaluation of the FS. Additionally, in coherent Rayleigh-based sensors, the calculation of the signal-to-noise ratio (SNR) from the jagged back-scattered intensity trace using the statistical estimators can cause an erroneous determination of the absolute value of the SNR. In this work, a method to accurately evaluate the non-uniform SNR caused by the stochastic variation of the back-scattered light intensity along the fibre is presented and validated. Furthermore, an analytical expression to evaluate the uncertainty in the FS estimation using one of the standard estimators, namely cross-correlation, is presented. A direct-detection frequency-scanned phase-sensitive optical time-domain reflectometer (φ-OTDR) is employed for the experimental verification of the expression as a function of two crucial system parameters: the SNR and the spatial resolution. The performance of various distributed sensing system configurations utilising cross-correlation for determining the FS occurring due to the external perturbations can be properly predicted hereafter with the aid of the analytical expression presented in this study.

Time attention analysis method for vibration pattern recognition of distributed optic fiber sensor

The distributed optical fiber sensing technology based on phase-sensitive optical time-domain reflectometer (Φ-OTDR) has the advantage of multi-point vibration monitoring simultaneously. To improve the robustness of the recognition system, a deep learning model is designed to extract time-frequency sequence correlation from signals and spectrograms. This lightweight plug-and-play convolutional neural network architecture achieves better performance with a lower number of extra parameters and computations. This study is verified on a vibration dataset containing eight different scenarios collected by a Φ-OTDR system, and the classification accuracy of 96.02% is achieved.

Crosstalk Noise Suppressed for Multi-frequency ϕ-OTDR Using Compressed Sensing

The frequency division multiplexing (FDM) is an effective method to improve the detection ability of high-frequency disturbance in phase-sensitive optical time domain reflectometer (ϕ-OTDR). However, as the probe channels increase, crosstalk between channels will be generated. As a result, the recovery phase signal-to-noise ratio (SNR) will be deteriorated, which will reduce the detection capability of ϕ-OTDR. Therefore, a novel method for suppressing crosstalk noise by compressed sensing under multi-frequency ϕ-OTDR is proposed in this paper. The band-limited characteristics of vibration are deeply analyzed, and the frequency domain is chosen as the sparse domain. Noise without sparsity is discarded in the compression process. Furthermore, the noise cannot be recovered in the reconstruction process which uses the critical information containing the global property of phase signal. Consequently, the influence of crosstalk noise is reduced effectively. In experiments, compared with the restored signals without the proposed method, the average SNRs of the reconstructed 12 kHz sinusoidal vibration are respectively enhanced by 18.57 dB, 40.47 dB, and 41.67 dB in the sensing system with 6, 7, and 8 channels. In addition, the 24 kHz triangular wave signal and the disturbance composed of several different frequencies with random amplitudes can be detected and reconstructed. The proposed crosstalk noise suppression method provides a potential choice for ϕ-OTDR performance enhancement.

High SNR Φ-OTDR with Multi-Transverse Modes Heterodyne Matched-Filtering Technology.

Phase-sensitive optical time domain reflectometer (Φ-OTDR) has attracted attention in scientific research and industry because of its distributed dynamic linear response to external disturbances. However, the signal-to-noise ratio (SNR) of Φ-OTDR is still a limited factor by the weak Rayleigh Backscattering coefficient. Here, the multi-transverse modes heterodyne matched-filtering technology is proposed to improve the system SNR. The capture efficiency and nonlinear threshold are increased with multiple transverse modes in few-mode fibers; the incident light energy is permitted to be enlarged by a wider probe pulse by using heterodyne matched-filtering without spatial resolution being deteriorated. As far as we know, this is the first time that both multi-transverse modes integration method and digital heterodyne matched filtering method have been used to improve the SNR of Φ-OTDR simultaneously. Experimental results show that the noise floor is reduced by 11.4 dB, while the target signal is kept. We believe that this proposed method will help DAS find important applications in marine acoustic detection and seismic detection.

Optical OFDM Error Floor Estimation by Means of OTDR Enhanced by Front-End Optical Preamplifier.

Optical time-domain reflectometer (OTDR) enables simple identification and localization of a plethora of refractive and reflective events on a fiber link, including splices, connectors and breaks, and measuring insertion/return loss. Specifically, large enough OTDR dynamic range (DR) and thus high signal-to-noise-ratio (SNR) enable clear far-end visibility of longer fibers. We point out here that, under such conditions, the optical bit-error-rate (BER) floor is dominantly determined by reflective events that introduce significant return loss. This complements the OTDR legacy tests by appropriate optical BER floor estimation in the field. As high SNR implies inter-symbol interference as dominating error generating mechanism, we could apply the classical time-dispersion channel model for the optical BER floor determined by the root-mean-square (rms) delay spread of the actual fiber channel power-delay profile. However, as the high-SNR condition is not always fulfilled mostly due to insufficient DR, we propose here inserting a low-noise optical preamplifier as the OTDR front-end to reduce noise floor and amplify the backscattered signal. In order to verify the model for the exemplar test situation, we measured BER on the same fiber link to find very good matching between the measured BER floor values and the ones predicted from the OTDR trace.

Research on Optical Fiber Sensor Signal Processing Technology Based on Variational Mode Decomposition and Singular Spectrum Analysis

To better apply the phase-sensitive optical time-domain reflectometer (Φ-OTDR) in projects with complex environments, given the problem of the low signal-to-noise ratio of the Φ-OTDR signal, a variational modal decomposition (VMD) is proposed. Signal processing algorithm combined with singular spectrum analysis. First, the equalization optimizer (EO) optimizes the VMD. The VMD decomposes the preprocessed signal into a multi-layer eigenmode function (IMF), selects the IMF component according to the correlation coefficient, and after the SSA noise reduction process, the signal Reconstruction to realize the noise reduction processing of the Φ-OTDR signal. Through experiments, it is proved that the relative mean square error (RMSE) of the method in this paper is lower than that of EO-VMD, and the noise reduction performance is better.

Distributed Humidity Sensing in Concrete Based on Polymer Optical Fiber.

We present a preliminary investigation on distributed humidity monitoring during the drying process of concrete based on an embedded polymer optical fiber (POF). The water dissipated into the POF changes several properties of the fiber such as refractive index, scattering coefficient and attenuation factor, which eventually alters the Rayleigh backscattered light. The optical time domain reflectometer (OTDR) technique is performed to acquire the backscattered signal at the wavelengths 650 nm and 500 nm, respectively. Experimental results show that the received signal increases at 650 nm while the fiber attenuation factor clearly increases at 500 nm, as the concrete dries out. In the hygroscopic range, the information retrieved from the signal change at 650 nm agrees well with the measurement result of the electrical humidity sensors also embedded in the concrete sample.

Low-cost real-time fiber optic sensor for intrusion detection

PurposeThe purpose of this paper is to introduce a novel intensity-modulated fiber optic sensor for real-time intrusion detection using a fiber-optic microbend sensor and an optical time-domain reflectometer (OTDR).Design/methodology/approachThe proposed system is tested using different scenarios using person/car as intruders. Experiments are conducted in the lab and in the field. In the beginning, the OTDR trace is obtained and recorded as a reference signal without intrusion events. The second step is to capture the OTDR trace with intrusion events in one or multiple sectors. This measured signal is then compared to the reference signal and processed by matrix laboratory to determine the intruded sector. Information of the intrusion is displayed on an interactive screen implemented by Visual basic. The deformer is designed and implemented using SOLIDWORKS three-dimensional computer aided design Software.FindingsThe system is tested for intrusions by performing two experiments. The first experiment is performed for both persons (>50 kg) in the lab and cars in an open field with a car moving at 60 km/h using two optical fiber sectors of lengths 200 and 500 m. For test purposes, the deformer length used in the experiment is 2 m. The used signal processing technique in the first experiment has some limitations and its accuracy is 70% after measuring and recording 100 observations. To overcome these limitations, a second experiment with another technique of signal processing is performed.Research limitations/implicationsThe system can perfectly display consecutive intrusions of the sectors, but in case of simultaneous intrusions of different sectors, which is difficult to take place in real situations, there will be the ambiguity of the number of intruders and the intruded sector. This will be addressed in future work. Suitable and stable laser power is required to get a suitable level of backscattered power. Optimization of the deformer is required to enhance the sensitivity and reliability of the sensor.Practical implicationsThe proposed work enables us to benefit from the ease of implementation and the reduced cost of the intensity-modulated fiber optic sensors because it overcomes the constraints that prevent using the intensity-modulated fiber optic sensors for intrusion detection.Originality/valueThe proposed system is the first time long-range intensity-modulated fiber optic sensor for intrusion detection.

Sensitivity Improvement of Phi-OTDR by Fiber Cable Coils

We present a theoretical and experimental study in which we increased the sensitivity of a phase-sensitive optical time-domain reflectometer (phi-OTDR). This was achieved by constructing coils in the sensor cable, which increased the total amplitude of the impact on the fiber. We demonstrate this theoretically using the example of a phase-sensitive reflectometer model and practically in testing grounds with a buried nearby conventional sensor and a sensor with coils. The sensitivity increased 2.2 times. We detected 95% of events when using coils, versus 20% when using a straight cable. The suggested method does not require any modifications to the device.

In situ behaviour of an instrumented ring subjected to incipient TBM steering around a curve

During the TBM tunnelling of curved tunnel sections with passively articulated shield tails, the concrete segmental rings are susceptible to ring deformation and structural damage. In this paper, the in situ loading condition and response of the CAM4 instrumented ring in the Crossrail’s Thames tunnel (CTT) during tail seal passage at incipient TBM steering around a left curve are examined in detail. The field data comprises selected TBM data, the distributed strain measurements of a Brillouin Optical Time Domain Reflectometer (BOTDR) and 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, 2021c) and compared with the outputs of the finite element modelling conducted alongside (Gil Lorenzo, 2021a, 2021b). It was found that the eccentric tail seal passage was the main cause of the highest lining pressures and pressure gradients detected at the early stages of tunnelling. The machine turned around the thrust symmetry axis so that the shield posture changed from vertical and upwards to left and inwards. The transverse jack forces near the CAM4 ring switched orientation from downwards to rightwards. The locking mechanism of opposing TBM transverse actions during the eccentric tail seal passage 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 moderate sealing pressure gradients and the effective ring coupling ensured by the high thrust loads translated into a very limited ring distortion. The top half of the unbedded ring behaved as an arch-like structure with the crown segment being supported by the neighbouring segments. The in situ hoop curvatures were the result of ring ovalisation developed under the sealing pressure gradients and, in the first TBM cycle, the segment interaction with the hydraulic jacks. A temporary intrados front longitudinal crack near the mid length of the outer springline segment was detected at the time of incipient machine turning. The crack was caused by the combination of the spalling stresses between ram shoes with the front hoop tensile stresses typical of segmental rings loaded at the rear. Initial in-plane angularities of longitudinal joints with contact at the segment front can be corrected by the sequential loading of the ring from rear to front. There was good agreement between the in situ and FE-computed deformational quantities.

Absolute spectral backscatter measurements of large-core multimode PMMA polymer optical fibers.

To our knowledge, we are the first to measure the absolute value of the backscattering coefficient of a standard 1 mm core-diameter, multimode (MM) step-index (SI) polymethylmethacrylate (PMMA) polymer optical fiber (POF) for the spectral range of 450 nm to 700 nm. Our optical time domain reflectometer (OTDR) setup consists of a femtosecond supercontinuum laser with an acousto-optical filter as a tunable light source with short pulses and a time-correlated single-photon counting system as a receiver with a high dynamic range. The backscattering coefficient is calculated from the ratio between the energy within the fiber end reflex and the distributed backscattering level. We also measured the spectral attenuation with our OTDR setup and compared it with a standardized measurement method. At the attenuation minima within the measured spectral range the backscattering level of a 1 ns pulse is about -46dB at 520 nm, -48dB at 570 nm, and -51 dB at 650 nm. We were also able to show by the observed wavelength dependence that Rayleigh scattering causes a majority of the scattering.

DETECTION OF DISPLACEMENT USING GLASS OPTICAL FIBER SENSOR WITH VARIOUS CONFIGURATION

The research has measured the landslide using a displacement sensor based on Glass Optical Fiber (GOF) and Optical Time Domain Reflectometer (OTDR). The sensor manufacture process consists of various materials, structures, configurations, diameters, and directions. The sensor is made using GOF with a single-mode structure and single mode-multimode-Singlemode (SMS) structure. Each system is given a variety of configurations, namely gamma, bowknot, and three loops configuration. Each arrangement is provided a variation of the indentation diameter in three displacement directions. The test results will be read as the power loss on the OTDR in dB units. The results obtained were then compared between two materials, two structures, and directions. Displacement resulting landslides and changes to the sensor, causing power losses, and affecting the characteristics of each sensor. We found the best measurement results with the sensitivity value of 0.241 dB/mm and the resolution value of 0.004 mm. The displacement sensor has been made for measuring landslide in the laboratory-scale simulation based on GOF and OTDR with easier fabrication, easy operation, high sensitivity, better resolution and can be connected to a computer