Optical Time Domain Reflectometry Technique Research Articles

Highly birefringent microstructured optical fiber for distributed hydrostatic pressure sensing with sub-bar resolution.

We demonstrate distributed optical fiber-based pressure measurements with sub-bar pressure resolution and 1 m spatial resolution over a ∼100 m distance using a phase-sensitive optical time-domain reflectometry technique. To do so, we have designed a novel highly birefringent microstructured optical fiber that features a high pressure to temperature sensitivity ratio, a high birefringence and a mode field diameter that is comparable to that of conventional step-index single mode fibers. Our experiments with two fibers fabricated according to the design confirm the high polarimetric pressure sensitivities (-62.4 rad×MPa-1×m-1 and -40.1 rad×MPa-1×m-1) and simultaneously low polarimetric temperature sensitivities (0.09 rad×K-1×m-1 and 0.2 rad×K-1×m-1), at a wavelength of 1550 nm. The fiber features a sufficiently uniform birefringence over its entire length (2.17×10-4 ± 7.65×10-6) and low propagation loss (∼3 dB/km), which allows envisaging pressure measurements along distances up to several kilometers.

Distributed temperature sensing system based on a densely spaced FBG array for small fire recognition

A distributed temperature sensing (DTS) system based on a densely spaced fiber Bragg grating (FBG) array is proposed in this work for small fire recognition. A large capacity identical FBG array with centimeter-scale grating spacing is employed to build a high-density sensor network, increasing the probability that the small fire will affect at least one FBG. To retrieve the sensing information, we exploit an FBG interrogation system based on optical time-domain reflectometry and swept laser techniques, together with a fiber section (FS) spectrum interrogation method. The sensing technique consists in scanning the sensing array by means of a low-coherence wavelength-tunable pulsed laser and in reconstructing and analyzing the returned spectra signals. The sensing performance of the proposed DTS system is studied by theoretical analysis and experimental demonstration, verifying the feasibility and superiority of the densely-spaced FBG array based DTS system for small fire recognition and location.

Sensing accuracy enhancement of long‐range distributed fibre‐optic temperature sensor using hybrid algorithm

This paper presents a long-range distributed fibre-optic temperature sensor with high sensing accuracy to monitor temperature using Brillouin scattering mechanism. Fourier deconvolution, Fourier wavelet regularised deconvolution, dichotomised singular value decomposition and hybrid algorithms were employed to measure the temperature of proposed sensor. Optical time domain reflectometry technique is used to extract the temperature profile for 70, 60 and 50 km sensing ranges. The improvement of temperature accuracy using hybrid algorithm is achieved for 70 km sensing distance. In order to extract temperature information, the numerical simulation process has been incorporated in the proposed long-range sensing system. Moreover, the temperature accuracy of the proposed system is obtained using Fourier deconvolution, Fourier wavelet regularised deconvolution, dichotomised singular value decomposition and hybrid algorithms for 70, 60 and 50 km sensing ranges. The simulation results show that the temperature accuracy is observed as 4.8°K using hybrid algorithm compared to Fourier deconvolution algorithm that shows temperature accuracy as 14°K at 70 km. In the proposed sensor, a spatial resolution of 20 m is observed for a sensing range of 70 km using hybrid algorithm. In addition, the denoising competence of the proposed hybrid algorithm is validated using 50 km optical time domain reflectometry backscattered trace observed experimentally. Further, a peak dynamic range improvement of optical time domain reflectometry backscattered trace as 3.91 dB is observed using hybrid algorithm.

Multipath distributed acoustic sensing system based on phase-sensitive optical time-domain reflectometry with frequency division multiplexing technique

We propose a multipath distributed acoustic sensing (MP-DAS) system based on phase-sensitive optical time-domain reflectometry (Φ-OTDR) and frequency division multiplexing (FDM) technique. With FDM introduced to Φ-OTDR, simultaneously monitoring of multipath vibration is realized with flexible functionality. In the proposed MP-DAS system, the parameters of each path can be adjusted individually according to the path's specific monitoring requirement. A dual path Φ-OTDR system with 1 km and 22 km measurement distance is demonstrated in the experiment. Through heterodyne detection and FDM technique, both amplitude and phase information are obtained for event locating and vibration sensing in each of the two paths, with a spatial resolution of 5 m and 10 m respectively. With changing optical power and pulse width of one path, its influence on the other path is discussed and analyzed. This proposed MP-DSA system based on FDM technique provides an efficient and cost-effective way for upgrading a conventional single DAS into an integrated DAS network, which shows great potential in many applications such as modern city civil structure monitoring with different demands.

Rayleigh Φ-OTDR based DIS system design using hybrid features and machine learning algorithms

There is an increasing interest among the researcher community, and industries on the design, and development of a combined distributed acoustic sensing, and a pattern recognition system to detect, and classify potentially dangerous intrusion events. In this work, we describe the design, and optimization of a distributed intrusion sensing system using Rayleigh-phase sensitive optical time domain reflectometry (Rayleigh Φ -OTDR) technique, and supervised machine learning algorithms. The proposed system can classify an intrusion along with the position of an intrusion caused along a single mode optical fiber. We have considered three different external intrusion events, such as a person walking, digging by pickaxe, and electrical drilling. After training and testing the data samples of the simulated intrusion events, we have achieved an average intrusion classification rate of 100% with a 10 dBm of input laser source power over a 25 km length of sensing fiber. The relevant simulated experiments are carried out using MATLAB 20.0 platform.

Long-distance fiber optic vibration sensing using convolutional neural networks as real-time denoisers

A long distance range over tens of kilometers is a prerequisite for a wide range of distributed fiber optic vibration sensing applications. We significantly extend the attenuation-limited distance range by making use of the multidimensionality of distributed Rayleigh backscatter data: Using the wavelength-scanning coherent optical time domain reflectometry (WS-COTDR) technique, backscatter data is measured along the distance and optical frequency dimensions. In this work, we develop, train, and test deep convolutional neural networks (CNNs) for fast denoising of these two-dimensional backscattering results. The very compact and efficient CNN denoiser "DnOTDR" outperforms state-of-the-art image denoising algorithms for this task and enables denoising data rates of 1.2 GB/s in real time. We demonstrate that, using the CNN denoiser, the quantitative strain measurement with nm/m resolution can be conducted with up to 100 km distance without the use of backscatter-enhanced fibers or distributed Raman or Brillouin amplification.

Distributed SOP tracking fault detection technique for MeerKAT radio telescope array sub-metre single mode fibre link health monitoring

Deployed short-run receptor fibre network (RFN) in the MeerKAT radio telescope array require frequent health assessment due to their harsh environmental exposures. The inherent launch and event dead-zone of optical time-domain reflectometry (OTDR) technique limits its application in the MeerKAT radio telescope RFN sub-metre fibre links. This work experimentally demonstrates the first distributed polarization-based precise fault detection technique based on state-of-polarization (SOP) change tracking, for health monitoring of sub-metre fibre links and passive/active optical/photonic components. A beat frequency signature due to interference of the local oscillator with Rayleigh backscattered and Fresnel reflected optical signal is exploited to monitor Birefringence evolution over a 0.35 m MeerKAT digitizer fibre loom. Utilizing the ultra-sensitive inherent SOP change functionality due to mode coupling between fast and slow eigenmodes, an SOP change tracking technique is developed offline with 0.050 SOP angle change detection precision at 125μm spatial resolution. Fault location performance of the developed SOP change tracking technique is then compared to classical optical frequency-domain reflectometry (OFDR) technique over a 0.28 m SMF fibre link. The developed SOP change tracking technique is adopted for precise fault location along a 0.35 m MeerKAT digitizer fibre loom due to its high sensitivity. This work offers a precisely accurate, stable and reproducible approach for location of propagation defect detection in sub-metre single mode optical fibre links.

Investigating Polymer Fibre Optics for Condition Monitoring of Synthetic Mooring Lines

Synthetic mooring lines are becoming a popular alternative to conventional chain mooring systems. For marine renewable energy devices, they have been considered as an enabling technology for this nascent sector, given their reduced costs and ease of deployment. However, the extreme operating environment has led to an increased interest in the ‘in-situ’ condition monitoring of these mooring lines. This paper considers the use of polymer fibre optic technology and the optical time domain reflectometry (OTDR) technique for the condition monitoring of synthetic mooring lines. To establish the operating envelope of the fibres, Polymethylmethacrylate (PMMA) polymer optical fibres are mechanically tested. Additionally, an OTDR is used to monitor fibres whilst under elongation using a tensile test machine, and the sensitivity of the system in monitoring strain is established. At the lowest strain rate, the average proportional limit and yield points of the fibres are found at 1.16% strain and 5.41% strain, respectively. Fatigue exposure of fibres up to 1.25% strain identifies no measurable effect on fibres’ proportional limit or yield point. The occurrence of significant creep is identified for fibres strained beyond 1.5%. The OTDR system is able to identify strains at and above 4%. The study identifies important criteria that should be considered in the integration of polymer optical fibres for mooring applications. Limitations are discussed and suggestions for progressing this technology are provided.

Measurement of temperature and strain simultaneously with high spatial resolution for a long sensing range

This study presents a Brillouin distributed sensor (BDS) with high spatial resolution for the monitoring of temperature and strain simultaneously. Fourier deconvolution (FD), Fourier regularised deconvolution (FRD), and Fourier regularised denoising deconvolution (FRDD) algorithms have been explored to measure the Brillouin backscattered power using the optical time-domain reflectometry technique. The improvement of temperature and strain resolutions along with the spatial resolution using the FRDD algorithm is observed for 50 km sensing range. The numerical simulation process of the proposed sensing system has been simultaneously incorporated to extract temperature and strain information. The simulation results show that the temperature and strain resolutions are observed as 5°K and 138 μe, respectively, using FRDD compared with the FD algorithm which shows temperature and strain resolutions as 54°K and 1588 μe, respectively, at a 50-km distance. A high spatial resolution of 9 m is observed of the proposed BDS over the sensing range of 50 km using the FRDD algorithm. In addition, the denoising capability of the proposed FRDD algorithm is validated using the experimentally obtained Rayleigh backscattered signal.

High-resolution distributed strain sensing system for landslide monitoring

A novel fiber-optics landslide monitoring system is proposed to achieve distributed,high-sensitivity and high-resolution strain measurement. The whole system adopts Rayleigh backscatter-based coherent optical time-domain reflectometry (COTDR) technique to realize long-distance, real-time monitoring with a resolution of 0.1με and a spatial resolution of one meter.

Structural Health Monitoring Using Textile Reinforcement Structures with Integrated Optical Fiber Sensors.

Optical fiber-based sensors “embedded” in functionalized carbon structures (FCSs) and textile net structures (TNSs) based on alkaline-resistant glass are introduced for the purpose of structural health monitoring (SHM) of concrete-based structures. The design aims to monitor common SHM parameters such as strain and cracks while at the same time acting as a structural strengthening mechanism. The sensor performances of the two systems are characterized in situ using Mach-Zehnder interferometric (MZI) and optical attenuation measurement techniques, respectively. For this purpose, different FCS samples were subjected to varying elongation using a tensile testing machine by carefully incrementing the applied force, and good correlation between the applied force and measured length change was observed. For crack detection, the functionalized TNSs were embedded into a concrete block which was then exposed to varying load using the three-point flexural test until destruction. Promising results were observed, identifying that the location of the crack can be determined using the conventional optical time domain reflectometry (OTDR) technique. The embedded sensors thus evaluated show the value of the dual achievement of the schemes proposed in obtaining strain/crack measurement while being utilized as strengthening agents as well.

Analysis of Fiber-Optic Strain-Monitoring Data from a Prestressed Concrete Bridge

This paper presents data from fiber-optic strain monitoring of the Nine Wells Bridge, which is a three-span, pretensioned, prestressed concrete beam-and-slab bridge located in Cambridgeshire in the United Kingdom. The original deployment at the site and the challenges associated with collecting distributed strain data using the Brillouin optical time domain reflectometry (BOTDR) technique are described. In particular, construction and deployment issues of fiber robustness and temperature effects are highlighted. The challenges of interpreting the collected data as well as the potential value of information that may be obtained are discussed. Challenges involved with relating measurements to the expected levels of prestress, including the effects due to debonding, creep, and shrinkage, are discussed and analyzed. This paper provides an opportunity to study whether two commonly used models for creep and shrinkage, adequately model data collected in field conditions.

Experimental analysis of the temperature dependence of the Brillouin gain spectrum in short-length single-mode fiber

The Brillouin backscattered wave was examined in a short-length single-mode fiber at different ambient temperatures using a 1553-nm wavelength laser source. To obtain Brillouin scattering depending on the temperature change in a 95-m long single-mode fiber cable, the Brillouin optical time domain reflectometry (BOTDR) technique was used. In this technique, a 1553 nm wavelength signal was sent from one side of the test fiber. The FC/APC connector was attached to the other end of the test fiber. The temperature parameters of the single-mode fiber cables of different brands and types can be obtained using Brillouin scattering. The Brillouin frequency shift is found 0.93 MHz/$^{\circ}$C. Measured linear change demonstrated its use as the temperature sensor of Brillouin Scattering.

Fiber optic displacement sensor based on a double-reflecting OTDR technique

In this work, it is proposed a technique to implement an intensity sensor based on the generation of a double-reflecting (ghost) signal in optical time domain reflectometry (OTDR). The intensity sensor is supported by a singlemode-multimode-singlemode (SMS) fiber structure combined with a fiber loop mirror (FLM). The results of the displacement sensitivity show linear behavior for both the first-reflecting and double-reflecting signals with linear slopes of approximately −4.5 dB/mm and −6 dB/mm, respectively. The displacement resolution achieved is ∼0.28 mm. It is also found that the system is able to read periodic displacement variations in the millisecond time scale applied to the sensing head. © 2015 Wiley Periodicals, Inc. Microwave Opt Technol Lett 57:1312–1315, 2015

Signal to Noise Ratio (SNR) Enhancement Comparison of Impulse-, Coding- and Novel Linear-Frequency-Chirp-Based Optical Time Domain Reflectometry (OTDR) for Passive Optical Network (PON) Monitoring Based on Unique Combinations of Wavelength Selective Mirrors

We compare optical time domain reflectometry (OTDR) techniques based on conventional single impulse, coding and linear frequency chirps concerning their signal to noise ratio (SNR) enhancements by measurements in a passive optical network (PON) with a maximum one-way attenuation of 36.6 dB. A total of six subscribers, each represented by a unique mirror pair with narrow reflection bandwidths, are installed within a distance of 14 m. The spatial resolution of the OTDR set-up is 3.0 m.

OTDR reach extension for monitoring of high-loss optical links

Centralized monitoring of high-loss optical fiber links is a major concern for network operators. Despite the recurrent deployment of optical time domain reflectometry (OTDR) techniques, which have been progressively improved over the past few decades, proposed solutions still lack the required dynamic range for achieving long distances and, at the same time, high resolution. This is especially true for long-reach passive optical networks, where large losses due to fiber attenuation and high split ratios of passive splitters are commonplace. In this article, we present a novel technique that uses a semiconductor optical amplifier in existing repeater nodes in such a way that incoming OTDR pulses are synchronously amplified, whereas backscattered light is kept unaffected. This method is capable of improving the OTDR dynamic range in a completely transparent way to OTDR settings such as operating wavelength, pulse duration, and repetition rate. © 2014 Wiley Periodicals, Inc. Microwave Opt Technol Lett 56:974–977, 2014

A Fresnel reflection-based optical fiber sensor system for remote refractive index measurement using an OTDR

In this paper, we propose a Fresnel reflection-based optical fiber sensor system for remote refractive index measurement using the optical time domain reflectometry technique as an interrogation method. The surrounding refractive index from a long distance away can be measured easily by using this sensor system, which operates based on testing the Fresnel reflection intensity from the fiber-sample interface. This system is a simple configuration, which is easy to handle. Experimental results showed that the range of this measurement could reach about 100.8 km, and the refractive index sensitivities were from 38.71 dB/RIU to 304.89 dB/RIU in the refractive index (RI) range from 1.3486 to 1.4525.

Amplified OTDR Systems for Multipoint Corrosion Monitoring

We present two configurations of an amplified fiber-optic-based corrosion sensor using the optical time domain reflectometry (OTDR) technique as the interrogation method. The sensor system is multipoint, self-referenced, has no moving parts and can measure the corrosion rate several kilometers away from the OTDR equipment. The first OTDR monitoring system employs a remotely pumped in-line EDFA and it is used to evaluate the increase in system reach compared to a non-amplified configuration. The other amplified monitoring system uses an EDFA in booster configuration and we perform corrosion measurements and evaluations of system sensitivity to amplifier gain variations. Our experimental results obtained under controlled laboratory conditions show the advantages of the amplified system in terms of longer system reach with better spatial resolution, and also that the corrosion measurements obtained from our system are not sensitive to 3 dB gain variations.

Polymer Optical Fiber Sensors for Distributed Strain Measurement and Application in Structural Health Monitoring

Polymer optical fiber (POF) sensors have the unique ability to measure high strain distributed along the fiber up to 40% using the optical time-domain reflectometry (OTDR) technique. Both, standard PMMA fibers and perfluorinated (PF) graded-index (GI) POF are introduced and evaluated in potential use and applicability. Further, distributed length change measurement based on cross-correlation analysis of the characteristic fiber signature of PF POF is introduced. We conclude the advances in distributed POF strain sensors technology with respect to application in structural health monitoring. Special focus is on the sensor integration into technical textiles for health monitoring of geotechnical structures and masonry structures. Measurement results of sensor-equipped textiles in different model tests are presented, displacement of soil and cracks in retrofitted masonry structures are detected and evaluated.

Wavelength tunable optical time-domain reflectometry based on wavelength swept fiber laser employing two-dimensional digital micro-mirror array

We demonstrate a cost effective wavelength tunable optical time-domain reflectometry (OTDR) for wavelength division multiplexing passive optical networks (WDM-PON). In order to realize the unique wavelength tunable optical time-domain reflectometry (OTDR), the wavelength swept fiber laser was developed by a digital micro-mirror array device (DMD), and the correlation OTDR (COTDR) technique was used. We successfully detected the fault location at the remote node fibers with 20 m resolution and fast wavelength setting speed of ∼15 μs in conventional band (C-band).