Fiber-optic distributed sensing, employing the Brillouin effect, is already a commercially available measurement technique for the accurate estimation of the static strain/temperature fields along tens of kilometers with a spatial resolution of the order of a meter. Furthermore, relentless research efforts are paving the way to even much wider usability of the technique through recently achieved enhanced performance in each of its critical dimensions: measurement range has been extended to hundreds of kilometers; spatial resolution is of the order of a centimeter or less, signal to noise ratio has been significantly improved; fast dynamic events can be captured at kHz’s sampling rates; and a much better understanding of the underlying physics has been obtained, along with the formulation of figures of merit, and the preparation and early adoption of appropriate standards and guidelines. This paper describes the basics, as well as the state of the art, of the leading Brillouin interrogation methods, with emphasis on the significant progress made in the last 3 years. It also includes a short introduction to coding, which has proven instrumental in many of the recently obtained performance records.

[INVITED] State of the art of Brillouin fiber-optic distributed sensing

Lubricating oil conditioning sensors for online machine health monitoring – A review

In recent years, railway infrastructures and systems have played a significant role as a highly efficient transportation mode to meet the growing demand in transporting both cargo and passengers. Application of these structures in extreme environmental situation under severe working and loading conditions, caused by the traffic growth, heavier axles and vehicles and increase in speed makes it extremely susceptible to degradation and failure. In the last two decades, a significant number of innovative sensing technologies based on fiber optic sensors (FOS) have been utilized for structural health monitoring (SHM) due to their inherent distinctive advantages, such as small size, light weight, immunity to electromagnetic interference (EMI) and corrosion, and embedding capability. Fiber optic-based monitoring systems use quasi-distributed and continuously distributed sensing techniques for real time measurement and long term assessment of structural properties. This allows for early stage damage detection and characterization, leading to timely remediation and prevention of catastrophic failures. In this scenario, FOS have been proved to be a powerful tool for meticulous assessment of railway systems including train and track behavior by enabling real-time data collection, inspection and detection of structural degradation. This article reviews the current state-of-the-art of fiber optic sensing/monitoring technologies, including the basic principles of various optical fiber sensors, novel sensing and computational methodologies, and practical applications for railway infrastructure monitoring. Additionally, application of these technologies to monitor temperature, stresses, displacements, strain measurements, train speed, mass and location, axle counting, wheel imperfections, rail settlements, wear and tear and health assessment of railway bridges and tunnels will be thoroughly discussed.

A review of railway infrastructure monitoring using fiber optic sensors

The real remoteness of a distributed optical fiber sensor based on Brillouin optical time-domain analysis is considerably extended in this paper using seeded second-order Raman amplification and optical pulse coding. The presented analysis and the experimental results demonstrate that a proper optimization of both methods combined with a well-equalized two-sideband probe wave provide a suitable solution to enhance the signal-to-noise ratio of the measurements when an ultra-long sensing fiber is used. In particular, the implemented system is based on an extended optical fiber length, in which half of the fiber is used for sensing purposes, and the other half is used to carry the optical signals to the most distant sensing point, providing also a long fiber for distributed Raman amplification. Power levels of all signals launched into the fiber are properly optimized in order to avoid nonlinear effects, pump depletion, and especially any power imbalance between the two sidebands of the probe wave. This last issue turns out to be extremely important in ultra-long Brillouin sensing to provide strong robustness of the system against pump depletion. This way, by employing a 240 km-long optical fiber-loop, sensing from the interrogation unit up to a 120 km remote position (i.e., corresponding to the real sensing distance away from the sensor unit) is experimentally demonstrated with a spatial resolution of 5 m. Furthermore, this implementation requires no powered element in the whole 240 km fiber loop, providing considerable advantages in situations where the sensing cable crosses large unmanned areas.

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Extending the Real Remoteness of Long-Range Brillouin Optical Time-Domain Fiber Analyzers

In the past two decades Brillouin-based sensors have emerged as a newly-developed optical fiber sensing technology for distributed temperature and strain measurements. Among these, the Brillouin optical time domain reflectometer (BOTDR) has attracted more and more research attention, because of its exclusive advantages, including single-end access, simple system architecture, easy implementation and widespread field applications. It is realized mainly by injecting optical pulses into the fiber and detecting the Brillouin frequency shift (BFS), which is linearly related to the change of ambient temperature and axial strain of the sensing fiber. In this paper, the authors provide a review of new progress on performance improvement and applications of BOTDR in the last decade. Firstly, the recent advances in improving the performance of BOTDRs are summarized, such as spatial resolution, signal-to-noise ratio and measurement accuracy, measurement speed, cross sensitivity and other properties. Moreover, novel-type optical fibers bring new characteristics to optic fiber sensors, hence we introduce the different Brillouin sensing features of special fibers, mainly covering the plastic optical fiber, photonic crystal fiber, few-mode fiber and other special fibers. Additionally, we present a brief overview of BOTDR application scenarios in many industrial fields and intelligent perception, including structural health monitoring of large-range infrastructure, geological disaster prewarning and other applications. To conclude, we discuss several challenges and prospects in the future development of BOTDRs.

Recent Advances in Brillouin Optical Time Domain Reflectometry

Phase-sensitive optical time-domain reflectometry (Φ-OTDR) has been widely used in various applications for its distributed measurement capability of dynamic disturbance along the entire length of sensing fiber. However, traditional Φ-OTDR cannot make high-precision measurement on the external disturbance-induced strain due to the randomly distributed position and reflectivity of scattering points within the optical fiber. In this paper, ultra-weak fiber Bragg grating (UWFBG) array has been proposed to generate strong and controllable reflections while providing acceptable insertion loss. Active laser frequency sweeping and unwrapping algorithms were included to set up a definite relationship between strain value and variation of interference power between neighboring reflection lights. With the assistant of UWFBG array, the capability of Φ-OTDR has been upgraded to be able to take high-precision measurement on strain. In the experiment, multipoint μϵ level dynamic strain variation has been fully captured at the end of a 5-km long sensing fiber with 2 m spatial resolution. The measured strain value was compared to the calibrated one obtained with Mach–Zehnder Interferometer method. These two values fit with each other quite well with a maximum deviation of 6.2 nϵ, which confirmed the validity and accuracy of the proposed method.

Improved Φ-OTDR Sensing System for High-Precision Dynamic Strain Measurement Based on Ultra-Weak Fiber Bragg Grating Array

Phase-sensitive optical time-domain reflectometry ( $\Phi $ -OTDR) has the ability to make a measurement on external disturbance dynamically along the sensing fiber by monitoring the interference fading pattern variance of the Rayleigh backscattering waveform through continuous probe pulse injection. However, even if there is no disturbance, the received $\Phi $ -OTDR trace will still show slow but noteworthy distortion due to light source frequency drifting (LSFD). This trace-to-trace slow distortion makes it challenging to capture low-frequency event. In this letter, an active compensation method based on laser frequency sweep and cross-correlation calculation has been proposed to suppress the influence of LSFD. With the proposed active compensation method, the tracking of LSFD can be realized, and the trace-to-trace distortion can be greatly suppressed. The experimental result has shown that a vibration event of 0.5 Hz has been identified successfully under mean frequency drifting speed of 1.68 MHz/s, which extends the application realm of the $\Phi $ -OTDR sensing system to quasi-static measurement.

Active Compensation Method for Light Source Frequency Drifting in $\Phi $ -OTDR Sensing System

A high-performance distributed optical fiber sensor based on phase-sensitive optical time-domain reflectometry (Φ-OTDR) for dynamic strain measurement has been proposed. In this newly designed system, ultraweak fiber Bragg gratings (UWFBGs) are sparsely embedded in the sensing fiber. Through an unbalanced 3 × 3 coupler structure, the reflected lights from two adjacent UWFBGs would be mixed with each other and then split into three parts for phase demodulation. Then, a novelty table-look-up scheme is introduced to precisely demodulate the frequency, amplitude, and phase of dynamic strain. Rayleigh back-scattering light is utilized to identify and locate the accurate position of vibration event between two adjacent UWFBGs. The experimental results show that the proposed system is capable of reconstructing the vibration with a linear intensity response of R 2 = 0.9986 and a fiber length variation sensitivity of 117 pm/(Hz)1/2. Meanwhile, a wide frequency response band from 50 to 2075 Hz is achieved with high signal-to-noise ratio above 56 dB.

A High Performance Distributed Optical Fiber Sensor Based on Φ-OTDR for Dynamic Strain Measurement

Brillouin scattering based optical fiber sensors (BOFS) have the unique advantages over other sensors such as long distance, fully distributed, and multi-parameter sensing. The progresses on the development of BOFS technology in Nanjing University are reviewed. The key technologies to make BOFS with ultra-long distance, high spatial resolution, and fast measuring speed are discussed and realized.

https://link.springer.com/content/pdf/10.1007%2Fs13320-010-0019-7.pdf

Development of Fully-Distributed Fiber Sensors Based on Brillouin Scattering

The overhead power transmission line, as an important component of equipment for long-distance power transmission, is threatened by icing and galloping, which will lead to equipment troubles and cause huge economic loss. Thus, there is a great need for on-line monitoring for transmission lines. Because the photoelectric composite cable, such as optical fiber composite overhead ground wire (OPGW) is widely used, it is possible to introduce distributed optical fiber sensors (DOFS) into transmission line status monitoring. Common schemes based on DOFS usually offer low sensing density and are hard to realize global awareness, while phase sensitive optical time domain reflectometry (Φ-OTDR), as a DOFS that has the characteristic of distributed sensing, is hoping to address the limitation. In this paper, by establishing mathematical models, proposing analytical method, and building demonstration devices to analyze the dynamic strain characteristics of overhead power transmission line, an Φ-OTDR based on-line monitoring scheme of transmission line status is presented and experimentally proved for the first time. The estimation error of sag is less than 5.8% on centimeter scale, and the estimation error of ice thickness is no more than 10.84% on sub-millimeter scale, which gives an accurate description of transmission line status and provides strong support for early warning of transmission line failures.

Yixin Zhang

Papers

Improved Φ-OTDR Sensing System for High-Precision Dynamic Strain Measurement Based on Ultra-Weak Fiber Bragg Grating Array

Active Compensation Method for Light Source Frequency Drifting in $\Phi $ -OTDR Sensing System

A High Performance Distributed Optical Fiber Sensor Based on Φ-OTDR for Dynamic Strain Measurement

Development of Fully-Distributed Fiber Sensors Based on Brillouin Scattering

Phi-OTDR Based On-Line Monitoring of Overhead Power Transmission Line