Distributed Fiber Sensor Research Articles

Optomechanical time-domain reflectometry

Optical fibres constitute an exceptional sensing platform. However, standard fibres present an inherent sensing challenge: they confine light to an inner core. Consequently, distributed fibre sensors are restricted to the measurement of conditions that prevail within the core. This work presents distributed analysis of media outside unmodified, standard fibre. Measurements are based on stimulated scattering by guided acoustic modes, which allow us to listen where we cannot look. The protocol overcomes a major difficulty: guided acoustic waves induce forward scattering, which cannot be mapped using time-of-flight. The solution relies on mapping the Rayleigh backscatter contributions of two optical tones, which are coupled by the acoustic wave. Analysis is demonstrated over 3 km of fibre with 100 m resolution. Measurements distinguish between air, ethanol and water outside the cladding, and between air and water outside polyimide-coated fibres. The results establish a new sensor configuration: optomechanical time-domain reflectometry, with several potential applications.

Fast distributed Brillouin optical fiber sensing based on pump frequency modulation

We present a fast distributed Brillouin optical fiber sensor based on pump frequency modulation. The frequency shift of the probe is fixed, while the pump is fast scanned using an arbitrary waveform generator (AWG). Thus, the bandwidth requirement of the AWG is reduced to several hundred megahertz without any expense to the acquiring speed. Combining with differential pulse width pair (DPP) technology, a vibration measurement is implemented in a 230-m polarization-maintaining fiber with a sampling rate of 2.6 kHz and 50 cm spatial resolution. A vibration signal up to 31 Hz is identified successfully with a strain resolution of 15 µε.

Performance analysis of frequency shift estimation techniques in Brillouin distributed fiber sensors.

The performance of post-processing techniques carried out on the Brillouin gain spectrum to estimate the Brillouin frequency shift (BFS) in standard Brillouin distributed sensors is evaluated. Curve fitting methods with standard functions such as polynomial and Lorentzian, as well as correlation techniques such as Lorentzian Cross-correlation and Cross Reference Plot Analysis (CRPA), are considered for the analysis. The fitting procedures and key parameters for each technique are optimized, and the performance in terms of BFS uncertainty, BFS offset error and processing time is compared by numerical simulations and through controlled experiments. Such a quantitative comparison is performed in varying conditions including signal-to-noise ratio (SNR), frequency measurement step, and BGS truncation. It is demonstrated that the Lorentzian cross-correlation technique results in the largest BFS offset error due to truncation, while exhibiting the smallest BFS uncertainty and the shortest processing time. A novel approach is proposed to compensate such a BFS offset error, which enables the Lorentzian cross-correlation technique to completely outperform other fitting methods.

Optical Fiber Sensor Based In-Field Structural Performance Monitoring of Multilayered Asphalt Pavement

The durability, robustness, and long-term stability of optical-fiber-based sensors applied to practical engineering have always been challenging problems. Refer to the sensors embedded in asphalt pavements, the situation becomes serious and feasible sensors with enhanced function are in high demand. Therefore, an improved design to configure the quasi-distributed and distributed optical fiber sensors and FBG-based point sensors for monitoring the three-dimensional information of multilayered asphalt pavements is needed. The in-field data declare that the transversal, longitudinal, and vertical deformations of the tested urban asphalt pavement are mainly affected by temperature. The M-shape strain profile induced by heavy vehicles can decrease to the regular state in approximately 30 min after unloading. The tested asphalt pavement presents good structural performance to bear the tensile strain and permanent deformation. The high survival ratio and the good robustness of the proposed sensors against the harsh construction and operation environment validate the feasibility and reliability for the long-term monitoring. Improved design proposals on the construction scheme of asphalt pavement are also addressed to control the strain of the established asphalt concrete course in relatively low level.

The sensing principle of a new type of crack sensor based on linear macro-bending loss of an optical fiber and its experimental investigation

For traditional distributed optical fiber sensors, the sensing principle is based on a complex relationship between the crack opening displacement (COD) and the macro-bending loss of the fiber, which introduces many difficulties in its application in practical engineering projects. To solve this problem, a simple linear relationship between the macro-bending loss of the fiber and the COD is established in this work by controlling the fiber winding shaft to ensure the bending radius remains constant. Based on this linear relationship, a crack sensing principle of optical fiber is proposed. To verify the feasibility of this crack sensing principle, a validation experiment was performed using three groups of fiber winding shafts with different diameters and three groups of working light sources with different wavelengths. The experimental results show that the macro-bending loss is linearly related to the COD. Based on the proposed crack sensing principle, and a new type of optical fiber crack sensor to monitor the COD of pre-existing cracks and contraction joints of structures is designed, and the performance is verified by an experiment.

Brillouin Frequency Shift of Fiber Distributed Sensors Extracted from Noisy Signals by Quadratic Fitting.

It is a basic task in Brillouin distributed fiber sensors to extract the peak frequency of the scattering spectrum, since the peak frequency shift gives information on the fiber temperature and strain changes. Because of high-level noise, quadratic fitting is often used in the data processing. Formulas of the dependence of the minimum detectable Brillouin frequency shift (BFS) on the signal-to-noise ratio (SNR) and frequency step have been presented in publications, but in different expressions. A detailed deduction of new formulas of BFS variance and its average is given in this paper, showing especially their dependences on the data range used in fitting, including its length and its center respective to the real spectral peak. The theoretical analyses are experimentally verified. It is shown that the center of the data range has a direct impact on the accuracy of the extracted BFS. We propose and demonstrate an iterative fitting method to mitigate such effects and improve the accuracy of BFS measurement. The different expressions of BFS variances presented in previous papers are explained and discussed.

基于分段保护的光纤分布式传感智能控制系统

基于分段保护的光纤分布式传感智能控制系统

Evaluation of interface shear behavior of GFRP soil nails with a strain-transfer model and distributed fiber-optic sensors

A glass fiber reinforced polymer (GFRP) material is a newly developed material for slope reinforcement. In this study, distributed optical fiber sensors are developed to characterize the interface shear stress–strain behavior in GFRP soil nails with a strain-transfer model. The GFRP bar with a diameter of 40 mm was used as a reinforcing element in a soil slope subjected to excavations. A newly distributed fiber optic sensors are developed to measure the axial strains of the GFRP reinforcement which was then inserted into a drilled hole together with pressure grouting. The calibration of distributed fiber optic sensors indicated that the strains along the GFRP bar can be accurately measured by the distributed sensors. An analytical strain-transfer model is developed and then verified with a finite element model. The comparisons indicate that the strain-transfer model could be able to obtain the shear strain and stress at the cement grout–soil interface from arbitrary strain of the GFRP bar. Finally, a field project study is conducted for the condition assessment of a GFRP soil nail with the interface shear stress at different excavation depths. The potential failure surface is identified at the position where the maximum shear stress occurs, that is, at the point around 4.5 m away from the soil nail head. Results in terms of the shear stress are able to detect the active and passive zones of the soil nail. A further analysis indicates that the health condition of the GFRP soil nail can be determined with the distributed fiber sensors and strain-transfer model.

Impact of the Fiber Coating on the Temperature Response of Distributed Optical Fiber Sensors at Cryogenic Ranges

The thermomechanical behavior of a standard single-mode fiber with different coating materials is theoretically analyzed under different temperature conditions. Results show that the thermal expansion/shrinkage of the fiber coating introduces an extra strain on the optical fiber and can modify its thermal response. Distributed fiber sensors based on coherent Rayleigh and Brillouin scatterings are employed to characterize the impact of different coatings on the temperature sensitivity. The standard coating with dual-layer demonstrates a little influence on the thermal response at room temperature due to the softness of primary coating, but it increases the temperature sensitivity by some 50% at ∼220 K as the primary coating becomes stiffer at low temperature. Optical fibers with aluminum and Ormocer coatings are also experimentally tested. All the measured results agree well with the theoretical analysis.

Measurement and Recognition Algorithm for Distributed Fiber Sensor

Measurement and Recognition Algorithm for Distributed Fiber Sensor

High Spatial Resolution Radiation Detection Using Distributed Fiber Sensing Technique

This paper studies perspectives on using optical fibers and distributed fiber sensing schemes to perform real-time-distributed gamma-ray radiation sensing with high spatial resolution. The radiation-induced optical property changes of aluminum-doped fibers were studied using cobalt-60 sources. The distributed optical loss of the aluminum-doped fiber was characterized using the Rayleigh backscattering optical frequency domain reflectometry (Rayleigh OFDR). The optical loss of unprotected fiber under various gamma dose rates remains linear up to 100 grays (Gy). Using the gamma radiation-sensitive fiber, the localized optical loss measured by the Rayleigh OFDR was used to map the accumulated gamma radiation dosage on the entire surface of the cylinder with a 1-cm spatial resolution. Using electrical cables as a ubiquitous sensor platforms for fiber sensor deployment, this paper explores the potential for multifunctional distributed fiber sensor by integrating distributed fiber temperature and gamma ionizing radiation sensors in electrical cables for multifunctional measurements to improve the safety of nuclear power systems at both the component and system levels. As sensors that can readily be embedded in a wide variety of materials and structures, radiation-sensitive fibers can be low-cost and highly flexible tool to gauge the performance degradation and longevity of materials and components used in the nuclear power systems.

Detecting cm-scale hot spot over 24-km-long single-mode fiber by using differential pulse pair BOTDA based on double-peak spectrum.

In distributed Brillouin optical fiber sensor when the length of the perturbation to be detected is much smaller than the spatial resolution that is defined by the pulse width, the measured Brillouin gain spectrum (BGS) experiences two or multiple peaks. In this work, we propose and demonstrate a technique using differential pulse pair Brillouin optical time-domain analysis (DPP-BOTDA) based on double-peak BGS to enhance small-scale events detection capability, where two types of single mode fiber (main fiber and secondary fiber) with 116 MHz Brillouin frequency shift (BFS) difference have been used. We have realized detection of a 5-cm hot spot at the far end of 24-km single mode fiber by employing a 50-cm spatial resolution DPP-BOTDA with only 1GS/s sampling rate (corresponding to 10 cm/point). The BFS at the far end of 24-km sensing fiber has been measured with 0.54 MHz standard deviation which corresponds to a 0.5°C temperature accuracy. This technique is simple and cost effective because it is implemented using the similar experimental setup of the standard BOTDA, however, it should be noted that the consecutive small-scale events have to be separated by a minimum length corresponding to the spatial resolution defined by the pulse width difference.

Resolving 1 million sensing points in an optimized differential time-domain Brillouin sensor.

A differential pulse-width pair (DPP) Brillouin distributed fiber sensor is implemented to achieve centimetric spatial resolution over distances of several kilometers. The presented scheme uses a scanning method in which the spectral separation between the two probe sidebands is kept constant, while the optical frequency of the pump is swept to scan the Brillouin spectral response. Experimental results show that this method avoids detrimental temporal distortions of the pump pulses, which in a standard implementation prevent the DPP method from operating over mid-to-long distances. Such a novel scanning procedure allows the resolving, for the first time in pure time-domain Brillouin sensors, of 1,000,000 sensing points, i.e., 1cm spatial resolution over 10km in a conventional acquisition time.

Investigations of Backscattering Effects in Optical Fibers and Their Influences on the Link Monitoring

In this paper, first, we employed the coupled-mode theory to investigate the backscattering power characteristics of Rayleigh backscattering (RB) and stimulated Brillouin scattering (SBS) both synchronously and precisely. We discovered how the RB and SBS power changed with pump powers and fiber lengths. The scattering power curves with different pump powers could be divided into three regions. The first was called “the linear region of RB,” in which the pump power was mainly converted into RB and the RB power grew linearly with pump powers. The second region was called “the stimulation region of Brillouin scattering,” in which the power was mainly converted into SBS. In “the gain-saturated region,” when the pump power was much higher than the SBS threshold, both the RB and SBS power were gain-saturated. We also obtained the power curves with different fiber lengths. The power curves were helpful to set up proper launch powers in practical engineering applications, such as optical time-dome reflectometry systems and distributed fiber sensors. Then, we verified our theoretical results by experiments. Finally, according to the measurements and numerical solutions of the coupled-mode equations, we discussed the application of backscattering and gave advice for pump power selections in different fiber link monitoring systems.

Using distributed optical fibre sensor to enhance structural health monitoring of a pipeline subjected to hydraulic transient excitation

Distributed optical fibre sensors have been applied in monitoring the structural health of water pipelines over the past decades. Most of these studies show that distributed optical fibre sensor is quite effective for static and quasi-static measurements and can be used for monitoring the condition of a pipeline. It is well known that pressure transients (water hammer) can occur in any pressurised pipeline system due to changes in the operating conditions. This article shows the ability of distributed optical fibre sensor to exploit these transient hydraulic pressures along the pipeline for assessing its condition. In this regard, this article aims to demonstrate the concept of utilising this natural stimulus to enhance the pipeline structural health monitoring with the distributed sensing. A total of three experimental case studies (multiple anomalies, material loss and leakage) along plastic pipes will then be reported using a water hammer as a transient excitation for the structural health monitoring applications using distributed optical fibre sensor. The results will show that the presence of a localised damage/anomaly monitored by the distributed optical fibre sensor is accentuated by the dynamic response of the pipe when subjected to water hammer.

A high performance distributed optical fiber sensor based on Φ-OTDR for dynamic strain measurement

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.

The application of distributed fibre sensors in determination of seepage in Derince Dam

The application of distributed fibre sensors in determination of seepage in Derince Dam

The Application of Distributed Fiber Sensors in Determination of Seepage in Derince Dam

Monitoring of civil infrastructure systems has emerged as a powerful tool for condition assessment of infrastructure performance. Structures could be monitored periodically from a central station located several kilometres away from the field with help of widespread use of modern telecommunication technologies. Fibre sensing technology is increasingly used in civil engineering applications. Distributed fibre optic sensing offers the ability to measure temperatures, strain, leakage, etc., at thousands of points along a single fibre. A case of seepage along plinth of concrete faced Derince Dam, located in Turkey, is measured continuously and presented in this paper while an application of distributed fibre sensor technology is used. The case helps to disclose potential leakage along the plinth in order to minimise any risk.

Applications of Brillouin Dynamic Grating to Distributed Fiber Sensors

In the process of stimulated Brillouin scattering (SBS), acoustic waves are generated by the interference of counterpropagating optical waves via electrostriction effect. These acoustic waves not only stimulate the Brillouin scattering process, but can also play a role of moving Bragg reflector for another optical wave when a proper phase-matching condition is satisfied between the interacting optical waves. Brillouin dynamic grating (BDG) represents this secondary role of the acoustic wave, which shows a unique reflection spectrum, called BDG spectrum. The BDG spectrum generally shows higher sensitivity to the change of physical variables, such as temperature and strain than ordinary Brillouin gain spectrum (BGS), and also provides information on the waveguide parameters, such as polarization or modal birefringence with high accuracy. This paper reviews the operation principle of BDG under various conditions and the research progresses on its application to distributed fiber sensors.

Strain transfer analysis of optical fiber based sensors embedded in an asphalt pavement structure

Asphalt pavement is vulnerable to random damage, such as cracking and rutting, which can be proactively identified by distributed optical fiber sensing technology. However, due to the material nature of optical fibers, a bare fiber is apt to be damaged during the construction process of pavements. Thus, a protective layer is needed for this application. Unfortunately, part of the strain of the host material is absorbed by the protective layer when transferring the strain to the sensing fiber. To account for the strain transfer error, in this paper a theoretical analysis of the strain transfer of a three-layered general model has been carried out by introducing Goodman’s hypothesis to describe the interfacial shear stress relationship. The model considers the viscoelastic behavior of the host material and protective layer. The effects of one crack in the host material and the sensing length on strain transfer relationship are been discussed. To validate the effectiveness of the strain transfer analysis, a flexible asphalt-mastic packaged distributed optical fiber sensor was designed and tested in a laboratory environment to monitor the distributed strain and appearance of cracks in an asphalt concrete beam at two different temperatures. The experimental results indicated that the developed strain transfer formula can significantly reduce the strain transfer error, and that the asphalt-mastic packaged optical fiber sensor can successfully monitor the distributed strain and identify local cracks.