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We demonstrate (and are the first to do so) 63 km Brillouin Optical Frequency-Domain Analysis (BOFDA) for temperature and strain monitoring using a 100 km fiber loop. The use of BOFDA for long-range applications can be considered a novel approach, as previous investigations focused on the utilization of Brillouin Optical Time-Domain Reflectometry and Analysis (BOTDR and BOTDA, respectively). At 51.7 km, a 100 m hotspot (37 °C) was detected without using distributed Raman amplification or image processing.
We present, to our knowledge for the first time, a 100-km Brillouin Optical Frequency-Domain Analysis (BOFDA) employing a 200-km fiber loop. Compared to our previous publication, enhanced sensor length, sensor accuracy and spatial resolution are presented. The performance improvements are achieved by applying distributed Raman amplification (DRA) and a digital high-pass filter. We report on temperature measurements over sensing distances of 75 km and 100 km both with a 12.5-m spatial resolution. Temperature changes of 5 °C have been measured along 75 km sensing fiber. A temperature change of 30 °C has been detected at 99.5 km.
We report on the development of a complete system for spatially resolved detection of critical soil displacement in river embankments. The system uses Brillouin frequency domain analysis (BOFDA) for distributed measurement of strain in silica optical fibers. Our development consists of the measurement unit, an adequate coating for the optical fibers and a technique to integrate the coated optical fibers into geotextiles as they are commonly used in dike construction. We present several laboratory and field tests that prove the capability of the system to detect areas of soil displacement as small as 2 meters. These are the first tests of truly distributed strain measurements on optical fibers embedded into geosynthetics.
A dynamic and quasi-distributed sensor principle for simultaneous measurement of length changes and optical power changes between reflection points in an optical fiber is presented. The technique is based on the incoherent optical frequency domain reflectometry (I-OFDR). Length change resolutions < 1 µm and measurement repetition rates up to 2 kHz can be achieved using standard single-mode and multi-mode optical fibers. Simultaneous length change and refractive index measurement as well as field test results showing the deformation of a masonry building under seismic load are presented. Promising fields of application for this technique are the structural health monitoring sector and chemical process control.
This paper presents new methods to calculate and interpret Rayleigh scattering profiles in polymer optical fibers (POF) recorded by optical time domain reflectometry (OTDR). In contrast to Silica glass optical fibers (GOF), POF experience inhomogeneous stress when being strained, resulting in small scattering centers which allow for a new distributed sensor based on POF. New methods to overcome issues like blurring due to modal dispersion, uncertainties due to inhomogenious losses in the fiber and signal degradation due to viscoelastic creep in the polymer, are presented, for the first time to our knowledge.
We present a novel technique for dynamic and simultaneous measurement of displacement and refractive indices at multiple reflection points in optical fibers. This quasi-distributed sensor is based on the incoherent optical frequency domain reflectometry (I-OFDR) technique and allows for m-resolution length change measurement and precise refractive index measurement. We show that the dynamic measurement ability and the simple sensor design allows for new applications in the field of structural health monitoring and chemical process control.
We present a novel technique based on incoherent optical frequency domain reflectometry (OFDR) to measure length changes quasi-distributed between reflection points in optical fibres. The technique enables length changes to be measured with a resolution better than 1 µm and allows for static and dynamic measurement capabilities up to 2 kHz. We demonstrate that dynamic measurements of multiple fibre sections can be conducted independently from each other with high precision. Due to the precise and dynamic measurement capabilities, the proposed sensor system is expected to open new fields of application, especially in the structural-health-monitoring sector. Possible applications are discussed in the paper.
A novel quasi-distributed long-gauge fiber optic strain sensor system for dynamic measurement
(2011)
We present a novel technique based on incoherent optical frequency domain reflectometry (OFDR) to measure length changes quasi-distributed between reflection points in optical fibres. The technique enables length changes to be measured with a resolution better than 1 µm and allows for static and dynamic measurement capabilities up to 2 kHz. We demonstrate that dynamic measurements of multiple fibre sections can be conducted independently from each other with high precision. Due to the precise and dynamic measurement capabilities, the proposed sensor system is expected to open new fields of application, especially in the structural-health-monitoring sector. Possible applications are discussed in the paper.
A long period grating (LPG) mechanically imprinted in a microstructured polymer optical fibre (mPOF) is embedded in a carbon fibre reinforced polymer (CFRP) unidirectional laminate. Processing steps were established in order to ensure that mPOF does not suffer significant degradation. The strain and temperature responses of the embedded mPOF-LPG were then investigated.
For long-range measurements in geotechnical and industrial applications, distributed optical fiber sensors have become a tool of increasing importance throughout the past decade.
Classic deformation monitoring (performed by strain gauges etc.) and temperature monitoring (Pt100 and alike) deliver data from fixed, single spots of a structure; quasidistributed measurements (fiber bragg gratings) provide a Chain of discrete measurement points along a limited sensing length. In contrast, an opticai fiber connected to a device for distributed strain and temperature sensing (DTSS) will provide a continuous profile of strain and temperature - spatially resolved down to less than 1 m - over a ränge of several tens of kilometers.
In DTSS measurements, the nonlinear opticai effect of stimulated Brillouin Scattering (SBS) is employed: two light waves with a stable, tunable frequency offset are injected into opposite ends of the sensing fiber, where they will form a beat pattem, at which parts of the light will be scattered. By matching the frequency offset of the light waves to the propagation of acoustic fluctuation in the opticai fiber, a power transfer from one light wave to the other can be measured; since the acoustic propagation directly shifts with train and temperature of the fiber, these two quantities can be measured by tuning the light waves’ frequency offset.
Analytical model for ball-lensed couplers to calculate the insertion loss for each optical mode. The basis for the calculation are commercial perfluorinated polymer optical (PFGI-POFs) and silica based fibres (SOFs). The modal power distribution for given misalignments are presented as well as the reflection-based losses at all optical interfaces. Additionally, the ball-lensed interconnection between PFGI-POF and SOF was theoretically demonstrated.
Application of Intensity-Based Coherent Optical Time Domain Reflectometry to Bridge Monitoring
(2022)
Although distributed fiber sensing techniques have been widely used in structural health monitoring, the measurement results of bridge monitoring, particularly under destructive testing, have rarely been reported. To the best of our knowledge, this paper is the first report of distributed vibration measurement results, which we obtained during a three-day destructive test on an abolished bridge. A coherent optical time domain reflectometry (COTDR) was used to acquire the vibration information while the bridge was being sawed. The obtained signal was analyzed in time and frequency domain. Some characteristics of the sawing-induced vibration were retrieved by the short-time Fourier transform; the vibration exhibited several high frequency components within the measured range up to 20 kHz and all the components appeared in the same time slot. Some unexpected signals were also detected. Thorough analysis showed that they are quite different from the sawing-induced vibration and are believed to originate from internal damage to the bridge (probably the occurrence of cracks).
Application results of a dynamic technique for simultaneous measurement of length changes and optical power changes between multiple reflection points in an optical fiber are presented. The technique is based on incoherent optical frequency domain reflectometry (I-OFDR) and allows for measuring for example length changes and optical power changes quasi-distributed at repetition rates up to 2 kHz. Precise measurement with length change resolutions in the µm-range can be conducted using standard single-mode or multi-mode fibers. Previous results of dynamic refractive index change measurement and the use of polymer optical fibers for high-strain measurement are concluded and possible sources of measurement inaccuracies are discussed. Field test results with sensors installed on a masonry building during a seismic shake test are presented. The versatility and simplicity of this technique makes it potentially interesting for application in the structural health monitoring sector and chemical process control.
One of the unique advantages of polymer optical fibres (POF) is that they can be used to measure very high strain values up to 100 % and beyond exceeding the strain limits of silica fibre-based sensor principles. In this paper the distributed strain measurement capabilities of POF based on backscatter change evaluation are summarized and distributed backscatter measurement technologies are intro-duced. Application examples in the structural health monitoring (SHM) field are presented: a promising approach is the integration into technical textiles for high-strain measurement in earthwork structures and crack detection in buildings. The potential of POF for future applications in SHM such as distributed relative humidity sensing is discussed.
In this study, a signal processing approach for heterodyne Փ-OTDR and C-OTDR systems that can obtain external perturbation and its frequency content in a faster way is proposed. We can detect vibrations with the same or better SNR using this processing approach, only with a single step and fast calculation. Fig. 12 shows the comparison of the normalized measurement time for the gamma matrix method and the conventional method throughout the three experiments. The speed of processing using a gamma matrix is ~35%–50% faster compared to a conventional method in high frequency test (PZT), low frequency test (walking) and street monitoring test. The processing speed in low frequency test is normally a bit lower than the similar high frequency one, since we must use higher number of time frames. Also, in street test we have higher speed because we can select wider gauges.
Ausgewählte Anwendungsbeispiele faseroptischer Sensoren im Ingenieurbau und in der Geotechnik
(2007)
A novel DAS setup based on geometric phases in coherent heterodyne detection is applied for the first time to the characterisation of the Earth’s subsurface. In addition, an optimisation of the proposed setup in terms of its spatial resolution is also presented for the first time. The surface waves are generated by strong blasts of 25 kg of explosives at a dedicated test site. A 10 km dark fiber link in the vicinity of the test site connected to the test setup records the resulting strain signals. The spike-free and low-noise strain data thus obtained minimize post-processing requirements, making the setup a candidate for real-time seismic monitoring. An analysis of the dispersion characteristics of the generated surface waves is performed using a recently reported optimised seismic interferometric technique. Based on the dispersion characteristics, the shear wave velocities of the surface waves as a function of the depth profile of the Earth’s crust are determined using an optimised evolutionary algorithm.
Bond monitoring in temperature dependent applications using Brillouin optical time domain analyser
(2016)
Adhesive bond has to be evaluated for its integrity over a range of temperature. Adhesive is being used to bond the sensors with structures. There is no validated technique to test its performance. In this paper, we propose a Brillouin Optical Time Domain Analyzer (BOTDA) based methodology to detect temperature-induced adhesive bond failure below room temperature using distributed fiber optic sensor. The differential coefficient of thermal expansion of the structure and fiber sensor can lead to bond failure at low temperature. Optical fiber impregnated in the structure will experience differential temperature/strain due to debond of the adhesive. This leads to the frequency and amplitude decomposition of the Brillouin spectra. This is a good indication for real-time monitoring of the integrity of a bond.
We characterize the Brillouin gain spectra (BGS) in perfluorinated graded-index polymer optical fibers (PFGI-POFs) at 1.32 µm and 1.55 µm. Three kinds of PFGI-POFs with the same core diameter, but which are fabricated by different manufacturers, are tested. For all the PFGI-POFs, the Stokes power measured at 1.32 µm is higher than that at 1.55 µm due to the lower propagation loss, but significant differences in Stokes power are observed among the three. Based on the measurement obtained by the optical time-domain reflectometry, we show that the actual propagation loss of the PFGI-POFs plays a crucial role in observing BGS.
Phase-sensitive optical time domain reflectometry (φOTDR) is an excellent distributed fiber sensing technique and has been applied in various areas. Its noise is however never been comprehensively studied to the best of our knowledge. The different detection noise sources in such a sensing system are thoroughly investigated. The impacts of thermal noise, shot noise and the beat between signal and the amplified spontaneous emission from a pre-amplifier have been theoretically and experimentally demonstrated. Due to the random nature of the φOTDR signal, the detection noise demonstrates distinct features at different fiber positions in a single measurement. The theoretical analysis and the experimental result explicitly affirm most of the fiber sections, and the difference at some positions may be explained by ambient noise.
Systematic investigation of annealing behavior of drawn PMMA fibers was performed. Dynamics of annealing process were measured under different environmental conditions by fiber longitudinal shrinkage monitoring. Process was found to follow stretched exponential decay function indicating heterogeneous nature of underlying molecular dynamics. Consequences of fiber annealing for climatic stability of polymer optical fiber-based sensors are discussed, emphasizing importance of fiber controlled post-fabrication annealing. Dependence of induced fiber shrinkage on annealing time, temperature and fiber drawing conditions is investigated and interpreted. In addition, strong influence of humidity on annealing process was observed, which was not recognized in previous studies. This further highlights role of fiber post-fabrication thermal treatment with respect to intended operating conditions.
We compare four different sensing solutions suitable for distributed fiber optic humidity sensing in per uorinated graded-index polymer optical fibers (PFGI-POFs). Compared to silica fbers, polymer optical fibers over advantageous beneffits including signifficantly higher break down strain, fracture toughness and humidity sensitivity. Various humidity-related effects in PFGI-POFs have been reported in the last years including measured attenuation and length changes as well as Brillouin frequency and Bragg wavelength shifts. The four aforementioned methods could serve as a basis for distributed and quasi-distributed humidity sensing and are described here closely with an emphasis on plausible cross effects to temperature and strain. The main focus of this paper lies on the comparison of four approaches with regard to method complexity, sensitivity to humidity, spatial resolution, real-time capability and effort to compensate for cross effects.
Condition monitoring of industrial infrastructures using distributed fibre optic acoustic sensors
(2017)
Distributed fibre optic acoustic sensing (DAS) can serve as an excellent tool for real-time condition Monitoring of a variety of industrial and civil infrastructures. In this paper, we portray a subset of our current Research activities investigating the usability of DAS based on coherent optical time-domain reflectometry (C-OTDR) for innovative and demanding condition Monitoring applications. Specifically, our application-oriented Research presented here aims at acoustic and vibrational condition monitoring of pipelines and piping systems, of rollers in industrial heavy-duty conveyor belt systems and of extensive submarine power cable installations, respectively.
We describe the fabrication of a polycarbonate (PC) micro-structured polymer optical fiber (mPOF) and the writing of fiber Bragg gratings (FBGs) in it to enable strain and temperature measurements. We demonstrate the photosensitivity of a dopant-free PC fiber by grating inscription using a UV laser. We further show that PC Bragg gratings can be extended up to at least 3% without affecting the initial functionality of the micro-structured fiber. The response of PC FBGs to temperature up to 125°C is also investigated. Polycarbonate has good mechanical properties and its high temperatureresistance might extend the range of application of polymeric FBGs.
This study discusses the development of advanced composite materials with sensing
capabilities obtained from the embedment of long period gratings in microstructured plastic
optical fibres (mPOF-LPG). The mPOF-LPG was embedded in a carbon fibre reinforced
polymer (CFRP) unidirectional laminate manufactured in autoclave. Processing steps were
established in order to insure that mPOF does not suffer significant optical and structural
degradation. The strain and temperature responses of the embedded mPOF-LPG were
investigated.
We report on the development of distributed fiber sensors and their integration into
geosynthetics as they are commonly used in the construction of dikes and dams for flood protection.
The sensing system uses Stimulated Brillouin Scattering (SBS) in silica optical fibers.
Using the Brillouin frequency analysis, the system is capable of detecting changes in both strain
and temperature along the optical fibers. The strain experienced by the optical fibers corresponds
to the deformation of the geosynthetics in case of a critical soil displacement. To satisfy
requirements of dike structures, the system will be optimized to detect soil displacement with a
spatial resolution of 5 m or less over a length in the range of 10 km. These rather modest requirements
leave space for optimization on resolution of the measurement values, measurement
time and cost efficiency.
To our knowledge, this is the first distributed fiber sensor using SBS that is especially designed
for the monitoring of soil displacement in dikes which is made possible by a novel method
for the integration of optical fibers into different types of geosynthetics.
A Kramers-Kronig (KK) receiver is applied to a phase-sensitive optical time domain reflectometry based on direct detection. An imbalanced Mach-Zehnder interferometer with a 2× 2 coupler is used in sensing system to encode the phase information into optical intensity. The directly obtained signal is treated as the in-phase component, and the KK receiver provides the quadrature component by Hilbert transform of the obtained signal, so that the optical phase can be retrieved by IQ demodulation. The working principle is well explained, and the obtained phase variance is theoretically analyzed. The experiment demonstrates the functionality of the sensor and validates the theoretical analysis.
Industrial piping systems are particularly relevant to public safety and the continuous availability of infrastructure. However, condition monitoring systems based on many discrete sensors are generally not well-suited for widespread piping systems due to considerable installation effort, while use of distributed fibre-optic sensors would reduce this effort to a minimum. Specifically distributed acoustic sensing (DAS) is employed for detection of third-party threats and leaks in oil and gas pipelines in recent years and can in principle also be applied to industrial plants. Further possible detection routes amenable by DAS that could identify damage prior to emission of medium are subject of a current project at BAM, which aims at qualifying distributed fibre optic methods such as DAS as a means for spatially continuous monitoring of industrial piping systems. Here, first tests on a short pipe are presented, where optical fibres were applied directly to the surface. An artificial signal was used to define suitable parameters of the measurement system and compare different ways of applying the sensor.
A novel distributed acoustic sensing technique is proposed that exploits both phase and amplitude of the Rayleigh backscattered light to quantify the environmental variation. The system employs a wavelength-scanning laser and an imbalanced Mach-Zehnder interferometer to acquire the reflection spectra and the phase of the detected light, respectively. Fading-free and low-frequency measurements are realized via the crosscorrelation of the reflection spectra. The discrete crosscorrelation is used to circumvent the nonlinear frequency sweeping of the laser. Based on the phase of the backscattered light, it is possible to quantify fast environmental variations. The whole system requires no hardware modification of the existing system and its functionality is experimentally validated. The proposed system has the potential to monitor ground motion/movement at very low frequency band like subsidence around mining areas and at high frequency band like earthquakes and vibrations induced by avalanches.
We report on the successful application of distributed acoustic sensing (DAS) to the detection of partial discharge (PD). A detection limit of about 1 nC discharge magnitude was achieved for PD in a real-scale model of a high voltage termination. Dedicated ultrasonic fibre-optic transducers were interrogated using coherent optical time-domain Rayleigh backscatter reflectometry (C-OTDR). Random quadrature demodulation was employed for retrieving relevant acoustic information from the raw C-OTDR backscatter traces. To our knowledge, our results are a first-time demonstration that quasi-distributed fibre-optic acoustic sensing is a candidate technology for the acoustic partial discharge monitoring of power cable joints and terminations.
We present our research on the development of a Brillouin optical frequency-domain analysis (BOFDA) using a perfluorinated graded-index polymer optical fiber (PFGI-POF) as a sensing fiber. The described works include investigations both on the selection of the setup components with respect to mode coupling effects and on the impact of the humidity cross-sensitivity. The suitability of the developed POF-based BOFDA is proved by a distributed measurement on an 86 m long PFGI-POF recorded with the spatial resolution of 6 m.
We report on the development of a distributed fiber optical sensor system for structural health monitoring of largc-scale geotechnical and civil infrastructures. The system is based on Brillouin frequency domain analysis (BOFDA) for distributed measurement of strain and temperature along Standard silica optical fibers used as sensors. By additional incorporation of the fiber optic sensors in geosynthetics, two-dimensional sensors can be realized. Such smart geosynthetics can be used for reinforeerneut of geotechnical and masonry structures and the embedded fiber optic sensors can provide information about the condition of the structures and detect the presence of any damages and destructions in real time.
In this work, we present our results achieved in several research activities for development of distributed fiber optic radiation sensors using glass and polymer optical fibers. The findings show that both the measurement of the radiation-induced attenuation (RIA) along the entire sensing fiber and the accompanying change in the refractive index of the fiber core can be used for distributed radiation monitoring.