Optical Time Domain Reflectometry Research Articles

Distributed Optical Fiber Sensing and Applications Based on Large-Scale Fiber Bragg Grating Array: Review

To achieve data-driven intelligence in engineering applications, the key requirements for distributed optical fiber sensor networks are large capacity, long distance, dense distribution, fast response, and high signal-to-noise ratio (SNR). Combining the traditional optical reflectometry techniques with the large-scale fiber Bragg grating (FBG) array fabricated on-line by a fiber drawing tower, multiplexing and demodulation of thousands of FBG sensors is successfully demonstrated, which greatly expands the application of FBG sensors to the field of distributed sensing. In this work, the fabrication, demodulation, and applications of large-scale FBG arrays are reviewed. Firstly, the on-line fabrication technology and process of large-scale FBG arrays are introduced. Secondly, the demodulation techniques including wavelength demodulation based on optical time-domain reflectometry (OTDR) and optical frequency-domain reflectometry (OFDR), as well as phase demodulation based on phase-sensitive OTDR are reviewed, where we mainly focus on the key parameters such as multiplexing capacity, demodulation speed, spatial resolution, and SNR. Last, the applications of the large-scale FBG arrays are briefly discussed.

Phase-sensitive optical time-domain reflectometry with improved signal-to-noise ratio and quantization noise tolerance using power-tunable local oscillator

Phase-sensitive optical time-domain reflectometry with improved signal-to-noise ratio and quantization noise tolerance using power-tunable local oscillator

Leakage channel outlet detection and diameter estimation for earth-rock dam using ROTDR

In the early stage of leakage in earth-rock dam, detection and quantification of the leakage channel outlet can provide an important reference for leakage hazard assessment and emergency response. In this paper, Raman Optical Time Domain Reflectometry (ROTDR) is used to detect the leakage point and a leakage channel outlet diameter estimation method is proposed. Firstly, the variation law between the sensing fiber of different length and ROTDR measurement results under the condition of different temperature difference is explored. Then, a model is constructed to simulate the leakage of earth-rock dam. The sensing fiber is deployed on the shallow layer of downstream slope and cushion of earth-rock dam to detect leakage. Finally, according to the measurement results, the leakage point is located and the diameter of leakage channel outlet is estimated. The estimated value is very close to the measured value. This study provides a useful reference for promoting the quantitative research of ROTDR in leakage detection.

Denoising algorithm of [formula omitted] -OTDR signal based on curvelet transform with adaptive threshold

In this paper, a denoising scheme based on curvelet transform is proposed to improve the signal-to-noise ratio (SNR) for vibration sensing in phase-sensitive optical time-domain reflectometry (Φ-OTDR) systems. The noise level can be estimated based on non-vibration images, which can be used to set the threshold for curvelet coefficients. We further optimize the threshold according to the amplitude distribution of the curvelet coefficients. In the experimental demonstration, when the optical pulse width is 100 ns, the SNR of location information of the 100 Hz vibration events can be increased by 6.93 dB, which proves the effectiveness of the proposed method.

Smart textile embedded with distributed fiber optic sensors for railway bridge long term monitoring

The Structural Health Monitoring (SHM) of large civil infrastructures has become a priority due to economic advantages and early detection of structural failure. Distributed Fiber Optics (DFO) sensors are becoming widely used for SHM due to their immunity to Electromagnetic interference and the ability to collect multiple data points from a single fiber cable. However, its reliability for a long monitoring period is still an area of interest. This paper studied the long-term performance of DFO embedded into textiles for faster installation procedures. The aim was to understand if the textile distributed fiber sensor experience any signal degradation after multiple weather seasons. To perform this investigation a railway bridge was used as a case study. The data was collected using Brillouin Optical Time Domain Reflectometry (BOTDR) and Brillouin Optical Time Domain Amplification (BOTDA). Additionally, a temperature compensation technique was investigated to remove the cross-relation between temperature and strain. The final results show the sensor’s effectiveness for long-term monitoring, where the data quality remained constant with an average standard deviation of 0.455 for the BOTDR and 0.208 for the BOTDA approaches.

Towards single-photon Brillouin optical time domain reflectometry.

We investigate a novel distributed Brillouin optical time domain reflectometer (BOTDR) using standard telecommunication fibers based on single-photon avalanche diodes (SPADs) in gated mode, ν -BOTDR, with a range of 120 km and 10 m spatial resolution. We experimentally demonstrate the ability to perform a distributed temperature measurement, by detecting a hot spot at 100 km. Instead of using a frequency scan like conventional BOTDR, we use a frequency discriminator based on the slope of a fiber Bragg grating (FBG) to convert the count rate of the SPAD into a frequency shift. A procedure to take into account the FBG drift during the acquisition and perform sensitive and reliable distributed measurements is described. We also present the possibility to differentiate strain and temperature.

Speech Enhancement Based on Array-Processing-Assisted Distributed Fiber Acoustic Sensing

Distributed Fiber acoustic sensing (DAS) based on phase-sensitive optical time-domain reflectometry (Φ-OTDR) has attracted lots of attention due to unique advantages. In security monitoring, speech recognition and other relevant fields, DAS’s ability to acquire speech signals is a key indicator for the practical application. However, the speech signal has broadband and non-stationary characteristics, so the conventional filtering is not sufficient, and the speech enhancement algorithm based on the characteristics of the speech signal is needed; on the other hand, the acoustic sensitivity of conventional fiber layout is not high enough, resulting in poor signal quality. In this paper, speech enhancement based on array-processing-assisted DAS system is proposed to recover speech signal with high-fidelity. Pulse compression Φ-OTDR, combined with multichannel speech enhancement algorithm based on optimally modified log-spectral amplitude (OM-LSA) is used. As far as we know, this is the first time that the multichannel OM-LSA speech enhancement is proposed for DAS. Compared with the signal before processing, the speech signal processed by this method has a great improvement in the signal distortion, background noise intrusiveness and signal overall quality. In addition, other signal enhancement methods, such as wavelet denoising and beamforming are also analyzed and compared. Our scheme outperforms in above three aspects. This scheme does not need to obtain a large number of data sets in advance, and can be transplanted to other DAS systems, which has great significance for realizing high performance speech acquisition.

Time Expansion in Distributed Optical Fiber Sensing

Distributed optical fiber sensing (DOFS) technology has recently experienced an impressive growth in various fields including security, structural monitoring and seismology, among others. This expansion has been accompanied by a speedy development of the technology in the last couple of decades, reaching remarkable performance in terms of sensitivity, range, number of independent sensing points and affordable cost per monitored point as compared with competing technologies such as electrical or point optical sensors. Phase-sensitive Optical Time-Domain Reflectometry (ϕOTDR) is a particularly interesting DOFS technique, since it enables real-time monitoring of dynamic variations of physical parameters over a large number of sensing points. Compared to their frequency-domain counterparts (OFDR), ϕOTDR sensors typically provide higher dynamics and longer ranges but significantly worse spatial resolutions. Very recently, a novel ϕOTDR approach has been introduced, which covers an existing gap between the long range and fast response of ϕOTDR and the high spatial resolution of OFDR. This technique, termed time-expanded (TE) ϕOTDR, exploits an interferometric scheme that employs two mutually coherent optical frequency combs. In TE-ϕOTDR, a probe comb is launched into the fiber under test. The beating of the backscattered light and a suitable LO comb produces a multi-heterodyne detection process that compresses the spectrum of the probe comb, in turn expanding the detected optical traces in the time-domain. This approach has allowed sensing using ϕOTDR technology with very high resolution (in the cm scale), while requiring outstandingly low detection and acquisition bandwidths (sub-MHz). In this work, we review the fundamentals of TE-ϕOTDR technology and describe the recent developments, focusing on the attainable sensing performance, the existing trade-offs and open working lines of this novel sensing approach.

Detecting strain with a fiber optic cable on the seafloor offshore Mount Etna, Southern Italy

Oceans cover more than 70 percent of the Earth's surface making it difficult and costly to deploy modern seismological instruments here. The rapidly expanding global network of submarine telecom cables offers tremendous possibilities for seismological monitoring using laser light. Recent pioneer studies have demonstrated earthquake detection using lasers in onland and submarine fiber optic cables. However, permanent strain at the seafloor has never before been measured directly as it happens. With this aim, we deployed a dedicated 6-km-long fiber optic strain cable, offshore Catania Sicily, in 2000 m water depth, and connected it to a 29-km long electro-optical cable for science use. We report here that deformation of the cable equivalent to a total elongation of 2.5 cm was observed over a 21-month period (from Oct. 2020 to Jul. 2022). Brillouin laser reflectometry observations over the first 10 months indicate significant strain (+25 to +40 microstrain) at two locations where the cable crosses an active strike-slip fault on the seafloor, with most of the change occurring between 19 and 21 Nov. 2020. The cause of the strain could be fault slip or seabottom currents. During the following 11 months, the strain amplitude increased to +45 to +55 microstrain, affecting a longer portion of the cable up to 500 m to either side of the first fault crossing. A sandbag experiment performed on the distal portion of the cable (3.2–6.0 km) starting Sept. 2021 demonstrates how the fiber optic cable deforms in response to an applied load and how the deformation signal partially dissipates over time due to the elastic properties of the cable. These preliminary results are highly encouraging for the use of BOTDR (Brillouin Optical Time Domain Reflectometry) laser reflectometry as a technique to detect strain at the seafloor in near real time and to monitor the structural health of submarine cables.

Phase correction based SNR enhancement for distributed acoustic sensing with strong environmental background interference

Fiber-optic distributed acoustic sensing (DAS) systems based on phase-sensitive optical time domain reflectometry (Φ-OTDR) measure acoustic waves by demodulating the phase variations of the Rayleigh backscattering (RBS) signals in a sensing optical fiber. However, in harsh environments, strong environmental background interference, coupled with the interference fading of the RBS, would introduce severe distortions in DAS signal that cannot be corrected or even can be worsened by phase unwrapping, thus resulting in low signal to noise ratio (SNR) and even undetectable signal. In this work, a novel method based on trend prediction is proposed to correct the distortions induced by phase unwrapping error around the fading points. The method is further validated in processing the data acquired from a field test performed in ocean environments using a home-built DAS system. By locating and erasing the distortion points, and then detrending, the acoustic signal buried in strong environmental background interference is retrieved with an SNR improvement greater than 10 dB. The results show that the proposed phase correction method can effectively enhance DAS's SNR for those challenging applications with strong background interference.

The Impact of Rayleigh Scattering in UWFBG Array-Based Φ-OTDR and Its Suppression Method.

Ultra-weak fiber Bragg grating (UWFBG) array-based phase-sensitive optical time-domain reflectometry (Φ-OTDR) utilizes the interference interaction between the reference light and the reflected light from the broadband gratings for sensing. It significantly improves the performance of the distributed acoustic sensing system (DAS) because the intensity of the reflected signal is much higher than that of the Rayleigh backscattering. This paper shows that Rayleigh backscattering (RBS) has become one of the primary noise sources in the UWFBG array-based Φ-OTDR system. We reveal the impact of the Rayleigh backscattering signal on the intensity of the reflective signal and the precision of the demodulated signal, and we suggest reducing the pulse duration to improve the demodulation accuracy. Experimental results demonstrate that using light with a 100 ns pulse duration can improve the measurement precision by three times compared with the use of a 300 ns pulse duration.

Dynamic fault monitoring method for passive optical network

We experimentally studied a passive optical network dynamic fault monitoring technology based on optical time domain reflectometry (OTDR), focused on various types of network structures with optical splitter (OS) devices, and analyzed the monitoring data with different lengths of fibers in front of the OS, such as 0, 14.5, and 50 km, respectively. Different from previous studies, we found that the new characteristics of echo curve can be used to realize the identification of different fault branches. We also make a detailed analysis on the feasibility of OTDR for multi-branch monitoring and the corresponding situation of instrument parameters. And the similar distance of the event situation is analyzed experimentally. We believe that these research data will provide effective technical reference for the intelligent management of optical networks in the current network.

300 km ultralong fiber optic DAS system based on optimally designed bidirectional EDFA relays

Optical fiber distributed acoustic sensing (DAS) based on phase-sensitive optical time domain reflectometry (φ-OTDR) is in great demand in many long-distance application fields, such as railway and pipeline safety monitoring. However, the DAS measurement distance is limited by the transmission loss of optical fiber and ultralow backscattering power. In this paper, a DAS system based on multispan relay amplification is proposed, where the bidirectional erbium-doped fiber amplifier (EDFA) is designed as a relay module to amplify both the probe light and the backscattering light. In the theoretical noise model, the parameters of our system are carefully analyzed and optimized for a longer sensing distance, including the extinction ratio (ER), span number, span length, and gain of erbium-doped fiber amplifiers. The numerical simulation shows that a bidirectional EDFA relay DAS system can detect signals over 2500 km, as long as the span number is set to be more than 100. To verify the effectiveness of the scheme, a six-span coherent-detection-based DAS system with an optimal design was established, where the cascaded acoustic-optic modulators (AOMs) were used for a high ER of 104 dB. The results demonstrate that the signal at the far end of 300.2 km can be detected and recovered, achieving a high signal-to-noise ratio of 59.6 dB and a high strain resolution of 51.8pε/Hz at 50 Hz with a 20 m spatial resolution. This is, to the best of our knowledge, a superior DAS sensing distance with such a high strain resolution.

Compensation of Phase Noise Impairments in Distributed Acoustic Sensors Based on Optical Pulse Compression Time-Domain Reflectometry

We introduce a method to compensate for the deleterious effects of the phase noise of the laser source on long-range distributed acoustic sensors (DAS) that implement optical pulse compression (OPC). Pulse compression can be used in coherent optical time-domain reflectometry (COTDR) sensors to extend the measurement range without compromising spatial resolution. In fact, OPC-COTDR sensors have demonstrated the longest measurement range to date in passive sensing links that do not require distributed amplification to compensate fiber attenuation. However, it has been found that the limited coherence of the laser source has a degrading effect on the actual performance enhancement that pulse compression can bring because it constrains the maximum duration of the compression waveforms that can be used and makes the use of lasers with extremely low phase noise necessary. We introduce a technique to compensate for the effects of phase noise on OPC-COTDR sensors so that they can demonstrate their full potential for long-range measurements using lasers with less stringent phase noise requirements. The method is based on sampling the phase noise of the laser with an auxiliary interferometer and using this information in a simple signal processing technique to mitigate its deleterious effect on the signal measured. We test our method in an OPC-COTDR sensor that uses 500- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mu$</tex-math></inline-formula> s linear frequency modulated pulses to demonstrate 100-km range measurements with 200 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\rm{p}\epsilon /\sqrt{\rm{Hz}}$</tex-math></inline-formula> of strain sensitivity at 2-m initial spatial resolution that becomes 10-m after applying the gauge length. To our knowledge, this is the longest compression waveform demonstrated to date in an OPC-COTDR sensor. Its use provides an extra 20-km range compared to previous demonstrations using laser sources of comparable linewidth. Furthermore, comparable performance is also demonstrated when using a laser source with an order of magnitude larger linewidth.

Self-Contained Reference Sensors to Reduce Nuisance Alarm Rate in φ-OTDR-Based Fence Intrusion Detection System

Nuisance alarm rate (NAR) is one of the key performance parameters in a phase-sensitive optical time domain reflectometry (φ-OTDR)-based fence intrusion detection system. Typically, the vibrations caused by ambient environmental conditions, such as heavy rain, strong wind, and passing vehicles, easily result in many nuisance alarms. Significant research efforts have been undertaken to suppress the NAR. In this paper, we propose to utilize short segments of the sensing fiber as reference sensors for significant reduction in the NAR in φ-OTDR for the first time, to the best of our knowledge. According to our field trial results, the proposed approach can reduce the NAR by more than 90%. The proposed approach is very simple, practical, and cost-effective, which can be easily integrated with the existing methods of reducing NAR and act as an additional level of decision-making algorithm for triggering alarms.

Quadrature Phase-Shift Keying Modulation With Random Coding Pulse for Long-Range ϕ-OTDR

Transient effect in erbium-doped fiber amplifier (EDFA) restricts the application of the phase-sensitive optical time-domain reflectometry (ϕ-OTDR) system with coding probe pulse (CPP) in long-distance vibration sensing. In this paper, a CPP modulation scheme based on phase coding is proposed to suppress the amplitude distortion after optical amplification. Quadrature phase-shift keying (QPSK), which has a high spectrum utilization efficiency, is designed on a I/Q modulator. The random sequence CPP is used in this system, because of the single coding sequence, the signal-to-noise ratio (SNR) can be improved without loss of the sensing bandwidth. Experimental results prove the effectiveness of the scheme. Compared to the conventional modulation scheme at a 50 mW pump power, the distortion coefficient of the CPP after amplification decreases from 57.67% to 9.23%, and the signal-to-interference ratio (SIR) increases from 13.89 dB to16.67 dB. Without distributed amplification, the proposed system successfully located the vibration at 85.652 km under the carrier period of 40 ns, and the SNR of the demodulated signal is up to 21.99 dB.

Distributed twist sensing using frequency-scanning φ-OTDR in a spun fiber.

In this paper, a novel distributed twist sensor based on frequency-scanning phase-sensitive optical time-domain reflectometry (φ-OTDR) in a spun fiber is proposed and demonstrated. Owing to the unique helical structure of the stress rods in the spun fiber, fiber twist gives rise to the variation of the effective refractive index of the transmitting light, which can be quantitatively retrieved through frequency shift using frequency-scanning φ-OTDR. The feasibility of distributed twist sensing has been verified by both simulation and experiment. For proof of concept, distributed twist sensing over a 136 m spun fiber with a 1 m spatial resolution is demonstrated, and the measured frequency shift shows a quadratic fitting dependence on the twist angle. In addition, the responses of both clockwise and counterclockwise twist directions have also been explored and the experiment result indicates that the twist direction can be discriminated since the frequency shift directions are opposite in the correlation spectrum. The proposed twist sensor possesses some outstanding advantages, including high sensitivity, distributed twist measurement and twist direction recognition capability, etc., which is very promising for specific applications in industry, e.g., structural health monitoring, bionic robots, etc.

Coherent OTDR with large dynamic range based on double-sideband linear frequency modulation pulse.

Rayleigh scattering-based distributed optical fiber sensors with long sensing distance and large dynamic range are highly desired for application scenarios such as vehicle tracking, structure health monitoring, and geological survey. To enlarge the dynamic range, we propose a coherent optical time domain reflectometry (COTDR) based on double-sideband linear frequency modulation (LFM) pulse. By utilizing I/Q demodulation, both the positive and negative frequency band of the Rayleigh backscattering (RBS) signal can be properly demodulated. Consequently, the dynamic range is doubled without increasing the bandwidth of signal generator, photodetector (PD), and oscilloscope. In the experiment, the chirped pulse with 10 μs pulse width and 498 MHz frequency sweeping range is launched into the sensing fiber. Single-shot strain measurement is achieved over 5 km single-mode fiber with a spatial resolution of 2.5 m and a strain sensitivity of 7.5 pε/H z. A vibration signal with 3.09 με peak-to-peak amplitude (corresponding to 461 MHz frequency shift) is successfully measured with the double-sideband spectrum, which cannot be properly recovered with the single-sideband spectrum.

Dual-parameter fiber sensor based on mode-division demultiplexing BOTDR system

We present and experimentally validate a neoteric and cost-effective dual-parameter distributed fiber sensor based on a Brillouin optical time-domain reflectometry in a general four-mode graded-index few-mode fiber. The spontaneous Brillouin scattering signals of different modes in the sensing fiber can be obtained simultaneously by a low-loss mode selective photonic lantern, on account of the difference of mode refractive indexes of different modes. By characterizing and evaluating the Brillouin scattering spectrum widths and the Brillouin frequency shift to temperature and strain sensitivities of different modes, the performance of the dual-parameter distributed sensor can be optimized. The proposed sensor achieves reliable differentiation of temperature and strain on a 1 km general four-mode graded-index few-mode fiber, with the temperature and strain accuracy of 1.6 °C and 39.2 με, respectively.

Application Research of Optical Fiber Sensing Technology in Power Optical Fiber Communication Systems

The distributed optical fiber sensing system uses optical fiber as the sensing element and signal transmission medium at the same time, and adopts advanced Optical Time-domain Reflectometry (OTDR) technology to detect the changes of temperature and strain at different positions along the optical fiber, thus achieving a truly distributed measurement. Using the Micro-structured Optical Time-domain Reflectometry (M-OTDR) optical fiber sensing access network technology, combined with optical link components such as fault locators, can judge the operating status of optical fiber Composite phase cable (IOPPC) lines, and detect faults (such as breakpoints) in optical fiber communication location and diagnosis. The position of the lightning strike is determined by detecting the change of polarization state caused by the transmitted light in the optical fiber inside the IOPPC before and after a lightning strike, and the positioning error is less than 1%. By observing the Brillouin frequency shift chart, the ice state can be accurately identified and the positioning function can be realized. By monitoring the vibration frequency of the IOPPC cable, the line dancing phenomenon can be detected in time to prevent line faults caused by dancing. The online safety and health monitoring of the operating state of transmission lines are realized.