Distributed fiber optic sensing (DFOS) technique is a promising and robust non-destructive testing tool that can seamlessly acquire environmental conditions over large scales. Therefore, it has found extensive applications in structural health monitoring. Its appeal for monitoring underground facilities lies in the intrinsic properties of the optical fiber, such as immunity to magnetic interference, small size, chemical inertia, etc. This paper provides a concise overview of DFOS applications in underground facility monitoring. Following a brief introduction to the working principle of the DFOS technique, various examples are provided to demonstrate how distributed fiber sensors contribute to monitoring underground facilities. The paper presents unpublished field test results with an emphasis in the energy sector, including monitoring gas storage facilities, geothermal reservoir exploration, and ground movement detection. Furthermore, the paper identifies several directions for enhancing the DFOS system.
Geometric phase measured per beat period in a Φ-OTDR based on coherent heterodyne detection is used to measure strain. Proposed method is robust to polarisation mismatch fading as a polarisation mismatch between interfering beams is not a hindrance to the measurement of the geometric phase. The Geometric phase is a function of the intensities of the interfering beams as well as the envelope of the beat signal. Its calculation does not require phase unwrapping and accordingly does not suffer the phase unwrapping errors. It is required to be equated with the traditionally measured phase by applying a scaling factor. The spatial resolution of the measured strain is reduced as it is calculated per beat period. Results are verified using a piezo-electric transducer inline a fiber-under-test.
Wavelength scanning coherent optical time domain reflectometer (WS-COTDR) is a good candidate to spatially resolve the environmental information at comparatively low frequency. This paper reveals it can also work as a traditional optical time domain reflectometer (OTDR) to identify Fresnel reflection by averaging the obtained signal over the wavelength scanning range. Simultaneous distributed vibration sensing and a traditional OTDR measurement are experimentally demonstrated using the WS-COTDR system.
In the last few years, the use of machine learning has emerged in the field of distributed fiber optic sensors as a promising approach to enhance their performance and provide new capabilities. In this study, we use machine le arning for simultaneous measurements of temperature and humidity in polyimide (PI)-coated optical fibers based on Brillouin Brillouin optical frequency domain analysis (BOFDA). Different non-linear machine learning algorithms are employed, namely polynomial regression, decision trees and artificial neural networks (ANNs), and their discrimination performance is benchmarked against that of the conventional linear regression. The performance is evaluated using leave-one-out cross-validation to ensure that the models are reliable and able to generalize well on new data. We show that nonlinear machine learning algorithms outperform the conventional linear regression and thus could pave the way towards simultaneous cost-effective tempera ture and humidity distributed sensing, which has the potential to find attractive new applications in the field of civil and geotechnical engineering, from structural health monitoring of dikes and bridges to subsea cables and long pipelines corrosion detection.
Humidity is one of principal environmental parameters that plays an important role in various application areas. Using measurement of strain induced in an optical fiber by a water swellable coating represents a promising approach for realization of distributed humidity sensing (DHS). In this work, humidity and temperature response of four different commercial PI-coated fibers and four tight-buffered (TB) fibers is investigated with the aim of evaluating their potential for development of DHS in context of water ingress sensor for high-voltage power cable splices. PI-coated fibers exhibited close-to-linear humidity and temperature response. While the temperature response is relatively coating-independent, magnitude of humidity response was broadly correlated to the relative fiber-to-coating thickness ratio. In contrast, both humidity and temperature response of TB fibers is strongly influenced by buffer type, with Leoni TB900L fiber with Hytrel buffer exhibiting largest humidity and temperature sensitivity. While the response of tight-buffered fibers is generally nonlinear, roughly three-times higher humidity response can be achieved with TB900L compared to the most sensitive PI-coated fiber. Using the TB fiber can be, therefore, advantageous for simpler water detection applications, such as one targeted in this study, when larger sensitivity is more important than the linear response of the sensor.
We present a technique for distributed temperature gradient sensing in real-time along an optical fiber utilizing simple amplitude-based direct-detection coherent optical time domain reflectometry (C-OTDR) and a special sensing fiber. Our technique enables us to determine phase changes or low-frequency variations of the C-OTDR signal stemming from temperature variations. The distinct feature of the used sensing fiber is its structuring with equidistant strongly scattering dots. Consecutive pairs of these scatterers form the dominant local interferometers, effectively overwriting the otherwise highly nonlinear transfer function of common optical fiber. This enables a quasi-phase-resolved evaluation of perturbation responses originating from temperature changes at sensor positions between the scatterers. Using our method, we show the measurement of a nonlinear temperature transient from a heating process with a maximum temperature gradient of 0.8°C over 20 s and a total temperature increase of 28.4°C. This method requires almost no post-processing and can be used for simultaneous distributed vibration sensing (DVS) and quantification of local temperature gradients in a single fiber, e.g., for the use in condition monitoring of infrastructure or industrial installations.
We compare four different sensing solutions suitable for distributed fiber optic humidity sensing in perfluorinated graded-index polymer optical fibers (PFGI-POFs). Compared to silica fibers, polymer optical fibers offer advantageous benefits including significantly 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.
Solution doping of off-the-shelf plastic optical fibers (POFs) represents rather simple and cheap way for preparing custom cladding-doped POFs (CD-POFs) with short to medium lengths. CD-POFs are especially attractive for environmental sensing applications, but might be of interest for illumination task as well. In this work, the proposed doping technique is tested with three different commercial low-cost polymethyl methacrylate (PMMA) POFs; Eska CK-40 and Eska GK-40 from Mitsubishi Rayon and Raytela PGU-FB1000 from Toray. The aim of the work is to aid the selection of the most suitable fiber yielding highest optical quality of prepared CD-POFs. Firstly, the optimal doping times are determined for the individual fiber types using short fiber samples. Secondly, longer 10 m CD-POFs are prepared from all tested fibers using the optimized doping procedure. Finally, attenuation of pristine POFs and prepared CD-POFs is measured using optical time domain reflectometry in order to characterize the impacts of the doping on fiber optical properties. In addition, the importance of post-doping drying procedure for CD-POF optical performance is investigated as well. The results suggest that, although doping of all tested fibers is generally feasible, Eska CK-40 is the most suitable candidate with regard to the doping efficiency and fiber post-doping performance.
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.
Online condition monitoring of submarine power cables helps to avert failures and damages produced by mechanical impacts. We report, to our knowledge for the first time, on investigations regarding the feasibility of distributed fiber optic acoustic sensors based on C-OTDR, with the sensor fibers being embedded in the cable, to detect vibrations due to mechanical disturbances along the cable. We present first results of sensing experiments where acoustic signals are transmitted through water to simulate the corresponding submarine conditions. Furthermore, we show results evaluating the usefulness of fibre commonly embedded in existing power cable designs for our sensing purposes.
Sensitivity of two commercial perfluorinated (PF) polymer optical fibers (POFs) to gamma radiation was measured, compared and exploited for dosimetry purposes. Fiber radiation induced attenuation (RIA) was found to be highly wavelength dependent and pronounced towards the UV part of the spectrum. PF-POFs were shown to be highly radiation sensitive and RIA-based on-line radiation monitoring with sensitivity as high as 135 dBm-1/kGy was demonstrated at 460 nm with co-extruded PF-POF. In addition, thanks to low inherent attenuation in the near-infrared region, possibility of distributed radiation detection with PF-POFs was demonstrated for the first time, utilizing optical time domain reflectometry.
Measurement and data recording systems are important parts of a holistic Structural Health Monitoring (SHM) system. New sensor technologies such as fiber-optic sensors are often used; however, standards (or at least guidelines) are not yet available or internationally approved. This lack in standardization makes the acceptance of FOS technologies in complex SHM systems substantially difficult. A standard family for different FOS technologies is therefore being developed that should help to design SHM systems in an optimal way. International standardization activities take place in several standardization bodies such as IEC and ASTM, and within SHM societies such as ISHMII. The paper reports on activities in standardization of fiber-optic sensors, on results already achieved, and on newly started projects. Combined activities of fiber sensor experts and SHM experts from Civil Engineering are presented. These contributions should help owners of structures as well as developers of sensors and monitoring systems to select effective and validated sensing technologies. Using these standards, both parties find recommendations how to proceed in development of SHM systems to evaluate the structural behavior based on e.g. standardized fiber optic sensors, and to derive necessary measures, e.g. the optimal maintenance strategy.
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.
We introduce the optical frequency domain reflectometry (OFDR) technique based on intensity modulation frequency
sweep measurement for distributed disturbance measurement in optical fibres. By evaluating interferometric Rayleigh
scattering changes along the fibre, strain and temperature changes are detected with 100 n(epsilon) sensitivity and 10 mK
resolution. The vibration frequencies for low frequencies and up to the kHz-range can be obtained from power change
evaluation in the spatial domain. This novel OFDR approach is a low-cost alternative for distributed disturbance
measurement up to distances of several kilometres.
We have studied the influence of gamma rays on physical properties of different commercially available silica optical
fibers stepwise irradiated up to a total dose of 100 kGy. The detection of radiation-induced changes in silica glass offers
the possibility of using selected optical fibers as distributed radiation sensors. The measurements performed by us were
based on optical backscatter reflectometry and Brillouin distributed sensing. The measurement methods enable an
analysis of radiation-induced modification of the group refractive index and density of the optical fibers. The most
distinct physical effect observed by us concerns the increase of the optical attenuation with rising total radiation doses.
Quantitative measurement results indicate a crucial impact of fiber dopants on radiation-induced physical and sensory
characteristics of silica optical fibers affected by differences in fiber fabrication techniques. Based on the obtained
results, the suitability of distributed Brillouin sensing for dosimetry applications seems to be improved by modifying the
refractive index profile of the fiber core.
The sensitivity of low-loss perfluorinated polymer optical fiber (PF-POF) to gamma radiation is investigated for on-line
radiation monitoring purposes. The radiation-induced attenuation (RIA) of a commercial PF-POF based on Cytop material
is measured in the visible spectral region. The fiber RIA shows strong wavelength dependence with rapid increase towards
the blue side of the spectrum. The wide range of radiation sensitivities is available via careful selection of appropriate
monitoring wavelength. The accessible sensitivities span from 1.6 ± 0.2 dBm-1/kGy measured at 750 nm to
18.3 ± 0.7 dBm-1/kGy measured at 420 nm. The fairly high radiation sensitivity as well as its wide tunability makes the
fiber a promising candidate for a broad range of applications.
In this work, the potential of fiber Bragg gratings (FBGs) in low-loss perfluorinated polymer optical fibers (PF-POFs) is explored. The FBG is femtosecond-inscribed in a commercial multi-mode (MM) PF-POF based on Cytop polymer. Femtosecond inscription leads to creation of a highly saturated grating with a number of higher order reflection peaks visible throughout the visible and near-infrared spectral region. For 2 mm long FBG having a pitch of 2.2895 μm, a total of nine higher-order MM reflection bands are visible spanning from 1548 nm (4th order) to 520 nm (12th order). Strain sensitivity was measured for 6 peak bands in 500-900 nm region, where relatively low cost CCD based spectrometers and broadband LEDs are available. Strain sensitivity increases almost linearly with increasing initial peak wavelength, growing from 4.82 ± 0.02 nm/% measured for 12th order peak at 517 nm to 8.12 ± 0.04 nm/% measured for 7th order peak at 883 nm. These values correspond to roughly 20 % higher sensitivity than silica FBGs exhibit in this spectral range. The gratings in PF-POFs combine the higher strain sensitivity and low-loss operation while maintaining the mechanical advantages of polymer optical fibers. Therefore, they hold a high potential for considerable broadening of polymer optical fiber Bragg gratings application range.
In this paper, we present the first results regarding the on-line monitoring of gamma-ray exposure effects on a commercial multi-mode perfluorinated polymer optical fiber (PF-POF), type GigaPOF-50SR from Chromis Fiberoptics. Our focus was to evaluate on-line the radiation induced attenuation (RIA) over a wide spectral range (320 nm – 1700 nm), in order to assess the fiber’s radiation hardness and its possible use in radiation detection. An Ocean Optics QE65000 high sensitivity spectrometer and a StellarNet near-IR spectrometer were used to cover the spectral ranges 200 nm – 1μm and 900 nm – 1.6 μm, respectively. Electron paramagnetic resonance was used to monitor the recovery of the irradiation induced centers at room temperature. The study indicated that the optical fiber can be used as radiation monitor at low dose rates by measuring the attenuation in the UV, while higher dose rates irradiation can be observed by RIA monitoring at specific wavelengths in the visible spectral range.
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 temperature resistance might extend the range of application of polymeric FBGs.
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.
Fiber-optic sensors are increasingly established in the sensor market. Their advantages have unquestionably been verified by numerous demonstrations to enhance the operational performance of aged structures or to monitor the structural behavior of safety-relevant structures or their components. However, there are some barriers in use due to a lack of extensive standardization of fiber-optic sensors. This leads very often to restraints in the user’s community. The paper shows the status in international standardization of fiber-optic sensors as well as current activities in leading institutions such as IEC and ISHMII and others with the purpose of providing relevant standards for a broader use of selected fiber-optic sensor technologies.
We propose and experimentally demonstrate for the first time to the best of our knowledge the feasibility of an integrated hybrid optical fiber sensing system that efficiently combines distributed Brillouin optical frequency-domain analysis (BOFDA) technique with FBG-based quasi-distributed approach offering the possibility for simultaneous distributed and point-wise measurements. The highly integrated proposed scheme employs Gaussian shaped, broadband and low reflectivity apodized FBGs with a common narrow-band optical source and shared receiving unit over the same sensing fiber. A single mode optical fiber is used for distributed sensing and a pair of FBGs is employed for simultaneous pointwise measurements.
Strain sensors used for structural health monitoring (SHM) must provide reliable measurement data during their entire service lifetime. To achieve this for fibre optic sensors integrated into composites, the integration of the sensor has to be adapted according to the process conditions. This paper describes the fabrication of thermoplastic composite samples with integrated distributed fibre optic sensors (DFOS) based on copper-nickel and polyimide coated silica optical fibres. The performance of these DFOS as SHM sensors is evaluated in terms of reliability by measurements derived from comparative measurements with resistance strain gauges and from fatigue tests with 10 million load cycles.
We present a measurement setup for combined quasi-distributed strain and dynamic point-wise vibration measurement using an incoherent optical frequency domain reflectometry (I-OFDR) setup in combination with extrinsic Fabry-Perot interferometers (EFPIs). Several EFPIs can be multiplexed and at the same time the strain along their supply fibres can be measured in a quasi-distributed manner. The setup is characterised and a demonstration of its general performance is given.
We propose a digital implementation of the incoherent optical frequency domain reflectometry (I-OFDR) technique for precise backscatter measurement and optical fiber sensing applications. Specific performance parameters of the I-OFDR are discussed and compared to an analog vector network analyzer-based I-OFDR system. Improved sensitivity, dynamic range and signal stability of the digital I-OFDR is presented and demonstrated by means of quasi-distributed length change measurement.
KEYWORDS: Sensors, Phase only filters, Heart, Skin, Personal protective equipment, Fiber optics sensors, Optical fibers, Temperature metrology, Luminescence, Temperature sensors
Within the EU project i-Protect fibre optic sensors for measuring heart rate, respiratory rate and skin temperature are being developed. The plastic optical fibre (POF) sensors are based on macrobending effects, photoplethysmography and fluorescence thermometry. The sensors and monitoring units are integrated into underwear and are communicating wireless via body area network and communication unit with the rescue command centre. All sensors can be removed from the underwear to allow washing the textile. First prototypes of the fibre optic sensors were tested in comparison to commercial reference sensors.
We propose a new scheme of Brillouin optical frequency-domain analysis (BOFDA) for distributed fiber-optic strain and temperature sensing, named linear-configured BOFDA (L-BOFDA). In conventional loop-configured BOFDA, light injection into both ends of a fiber under test (FUT) is needed for counter-propagation of pump and probe waves. In contrast, in L-BOFDA, pump and probe waves are injected into the same end of an FUT, and the probe wave Fresnelreflected (or mirror-reflected) at the other end of the FUT is exploited; thus, one-end access is feasible. We show proof-of-concept results of distributed strain measurement in a silica single-mode fiber with L-BOFDA, and discuss the signal-to-noise ratio degradation and the benefit in developing distributed Brillouin sensors based on polymer optical fibers.
Smart technical textiles containing fiber optic sensors have been developed and successfully demonstrated for the
purposes of the structural health monitoring in geotechnical and civil engineering as well as for healthcare monitoring in
the medical sector. The paper highlights the results achieved in this innovative field in the framework of several German
and European projects and shows selected examples of using such fiber-sensor-based technical textiles in different
geotechnical and medical applications.
We propose focused femtosecond laser irradiation to create stable and permanent scattering damage in large-core
polymer optical fibers. Possible sensing applications using optical time domain reflectometry are discussed and
presented. Experiments show that quasi-distributed fiber bend sensors and absolute temperature sensors can be realized
by evaluating backscatter intensities.
We present the status of our work on a tube shaped 3D deformation sensor based on continuously distributed fiber
optic sensing. A sensor tube of very large diameter is used to ensure high sensitivity while the transportability
and applicability are still comfortable due to our application strategy. The setup is designed to use different
measurement techniques in the applied optical fibers including Brillouin based measurements and interferometric
optical frequency domain measurements using an OBR 4400 from Luna Technologies.
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.
In this document a new distributed sensor based on Rayleigh scattering in polymer optical fibers (POF) is
proposed and first measurement results of the proposed sensor are shown. Different from Silica glas optical fibers
POF absorb high quantities of water resulting in a change of their molecular structure and thereby reducing the
present small scattering centers in areas of high humidity. The interdependence between scattering intensity and
relative humidity is being investigated in case of steady cycles as well as stepwise changes of humidity and in the
presence of moisture. A quantitative measure of humidity and scattering is presented.
We report on three respiration sensors based on pure optical technologies developed during the FP6 EU project
OFSETH. The developed smart medical textiles can sense elongation up to 3%, while maintaining the stretching
properties of the textile substrates for a good comfort of the patient. The sensors, based on silica and polymer fibre, are
developed for monitoring of patients during MRI examination. The OFSETH harness allows a continuous measurement
of respiration movements while all vitals organs are free for medical staff actions. The sensors were tested in MRI
environment and on healthy adults.
We investigated the sensing properties of a single mode Poly Methyl Methacrylate (PMMA) Microstructured Polymer
Optical Fibre (MPOF) with mechanically imprinted Long Period Grating (LPG). We measured the influence of strain to
the LPG wavelength which showed the viscoelastic nature of PMMA. We also measured the influence of temperature
and humidity to the LPG wavelength.
Technical textiles with embedded distributed fiber optic sensors have been developed for the purposes of structural
health monitoring in geotechnical and civil engineering. The distributed fiber optic sensors are based on Brillouin
scattering in silica optical fibers and OTDR in polymer optical fibers. Such "smart" technical textiles can be used for
reinforcement 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. Thus,
structural health monitoring of critical geotechnical and civil infrastructures can be realized. The paper highlights the
results achieved in this innovative field in the framework of several German and European projects.
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 ìm-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.
The potential impact of optical fiber sensors embedded into medical textiles for the continuous monitoring of the patient
during Magnetic Resonance Imaging (MRI) is now proved. We report how two pure optical technologies can
successfully sense textile elongation between, 0% and 3%, while maintaining the stretching properties of the textile
substrates for a good comfort of the patient.
Investigating influence of different patients' morphology as well as textile integration issues to let free all vitals organs
for medical staff actions, the OFSETH harness allows a continuous measurement of respiration movements.
For example, anaesthesia for MRI examination uses the same drugs as for any surgical procedure. Even if spontaneous
respiration can be preserved most of the time, spontaneous respiration is constantly at risk of being impaired by
anaesthetic drugs or by upper airway obstruction. Monitoring of the breathing activity is needed to assess adequate
ventilation or to detect specific obstruction patterns.
Moreover artefacts due to physiological motions induce a blooming effect on the MRI result. The use of synchronisation
devices allows reducing these effects. Positioned at certain strategic places according to the investigated organ, the
presented sensors could constitute an efficient and adapted solution for respiratory synchronisation of the MRI
acquisition.
We investigated sensing properties of single mode poly methyl methacrylate (PMMA) microstructured polymer optical
fibres (MPOF) with mechanically imprinted long period gratings (LPG). After preparation of the MPOF end-faces the
samples were elongated with silica fibres. These samples were used to measure the influence of strain to the LPG
wavelength which showed the viscoelastic nature of PMMA. We also measured the influence of temperature and
humidity. The results show that MPOF LPGs are well suited for strain sensing. One MPOF LPG was stitched to a
textile. Using this textile we measured a simulated respiratory motion.
Terrorists and criminals more and more attack and destroy important infrastructures like routes, railways, bridges,
tunnels, dikes and dams, important buildings. Therefore, reliable on-line and long-term monitoring systems are required
to protect such critical infrastructures. Fiber optic sensors are well-suited for that. They can be installed over many
kilometers and are able to measure continuously distributed strain, pressure, temperature and further mechanical and
physical quantities. The very tiny optical fibers can be integrated into structures and materials and can provide
information about any significant changes or damages of the structures. These so-called smart materials and smart
structures are able to monitor itself or its environment. Particularly smart technical textiles with embedded fiber optic
sensors have become very attractive because of their high importance for the structural health monitoring of geotechnical
and masonry infrastructures. Such textiles are usually used for reinforcement of the structures; the embedded fiber optic
sensors provide information about the condition of the structures and detect the presence of any damages and
destructions in real time. Thus, critical infrastructures can be preventively protected. The paper will introduce this
innovative field and will present the results achieved within several German and European projects.
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.
This paper presents the latest advances in distributed strain sensing using perfluorinated (PF) polymer optical fibers
(POF). Compared to previously introduced PMMA POF strain sensors, PF POF have the advantage of lower loss and
therefore extended measurement length of more than 500 m at increased spatial resolution of 10 cm. It is shown that PF
POF can measure strain distributed up to 100 %. The characteristic backscatter signature of this fiber type provides
additional evaluation possibilities. We show that, by applying a cross-correlation algorithm to the backscatter signal, the
distributed length change can be measured along the fiber. We also present, to our knowledge for the first time,
incoherent Optical Frequency Domain Reflectometry (OFDR) in POF to measure distributed reflections and loss along
the fiber. The OFDR technique proves superior to existing OTDR techniques in measurement speed, resolution and
potential instrument costs.
This article demonstrates how spatial resolution of distributed Brillouin sensing systems can be significantly
enhanced by measuring the spatially resolved Brillouin gain spectra of an optical fiber in the frequency domain.
We employ a novel signal processing method to overcome the known problem of spectral broadening that occurs
at centimeter-range spatial resolutions. The method is based on an analytical description of stimulated Brillouin
scattering in optical fibers for the case of harmonically modulated optical signals. From this analysis, the artifacts
that degrade the measurement resolution were modeled; for the first time to our knowledge, undegraded gain
spectra could be restored by means of deconvolution techniques adopted from digital image processing.
Sascha Liehr, Philipp Lenke, Katerina Krebber, Monika Seeger, Elke Thiele, Heike Metschies, Berhane Gebreselassie, Johannes Christian Münich, Lothar Stempniewski
Fiber optic sensors based on polymer optical fibers (POF) have the advantage of being very elastic and robust at the
same time. Unlike silica fibers, standard PMMA POF fibers can be strained to more than 40% while fully maintaining
their light guiding properties. We investigated POF as a distributed strain sensor by analysing the backscatter increase at
the strained section using the optical time domain reflectometry (OTDR) technique. This sensing ability together with its
high robustness and break-down strain makes POF well-suited for integration into technical textiles for structural health
monitoring purposes. Within the European research project POLYTECT (Polyfunctional textiles against natural hazards)
technical textiles with integrated POF sensors, among others sensors are being developed for online structural health
monitoring of geotechnical structures. Mechanical deformation in slopes, dams, dikes, embankments and retrofitted
masonry structures is to be detected before critical damage occurs. In this paper we present the POF strain sensor
properties, reactions to disturbing influences as temperature and bends as well as the results of the different model tests
we conducted within POLYTECT. We further show the potential of perfluorinated graded-index POF for distributed
strain sensing with increased spatial resolution and measurement lengths.
We report on the development of a distributed sensor system for strain measurement using Brillouin optical
frequency domain analysis (BOFDA) in single-mode silica optical fibers. Our research aims at the application of
the sensor system in flood protection. The sensing fibers are embedded into the soil body of river dikes, where
they perform early detection of critical soil displacement. We present a BOFDA setup that performs strain
measurements with a spatial resolution better than 3 meters over a length of 2 km. Its accuracy is verified by
measurements on a calibrated strain profile as well as several laboratory tests that emulate the stressing of the
optical fibers by soil movement. It is shown that the BOFDA approach offers feasible solutions to known critical
issues of Brillouin sensing such as spectral broadening at high spatial resolution, digital filtering for enhancement
of the dynamic range, and fluctuations of the Brillouin gain due to birefringence.
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.
Fiber optic sensors based on polymer optical fibers (POF) take advantage of the high elasticity and high break-down
strain of POF. Because of their outstanding elastic properties, POF are well suited for integration into technical textiles
like geotextiles and medical textiles. Smart textiles with incorporated POF sensors, able to sense various mechanical and
physical quantities, can be realized. The integration of POF as a sensor into geotextiles for monitoring of displacement of
soil is very attractive since POF can be used for distributed strain measurement of strain values of more than 40 %. An
online monitoring of critical mechanical deformations of geotechnical structures like dikes, dams, slopes, embankments
as well as of masonry structures can be ensured. Medical textiles that incorporate POF sensors can control vital
physiological parameters like respiratory movement and can be used for wearable health monitoring of patients requiring
a continuous medical assistance and treatment. The biocompatibility of POF is an important criterion for selecting POF
as a medical sensor. The paper shows selected examples of using POF sensors for the mentioned monitoring purposes.
The potential impact of optical fibre sensors embedded into medical textiles for the monitoring of respiratory movements
in MRI environment is presented. Preliminary investigations of the feasibility of sensing respiration movements with a
POF OTDR are reported. In addition, a macro-bending sensor based on a periodic design is demonstrated and
successfully implemented using a narrow fabric production process. It allows monitoring human abdominal breathing
movements with a very simple monitoring set-up.
We have measured for the first time the shift of the Bragg wavelength and other parameters of fibre Bragg grating (FBGs) exposed to γ-radiation at different wavelengths up to a dose of 100 kGy. The results show that the FBG sensitivity to radiation increases from 820 nm to 1516 nm so that FBGs with higher Bragg wavelength, e.g. 1.55 μm, are good candidates for high dose radiation sensing: no saturation was observed up to 100 kGy and the measured wavelength shift was fairly independent on the radiation dose rate.
Fiber Bragg grating sensor arrays can be used to monitor the mechanical behavior of rotor blades of wind turbines. In order to investigate how stable and reliably work such sensors, different fiber Bragg gratings were embedded into textilereinforced composite. Long-term temperature and tensile (fatigue) tests have been carried out with composite structure specimens. This paper reports on selected results of the tests and discusses important reliability aspects with regard to the fiber design and the fiber material.
The Brillouin scattering in combination with frequency-domain analysis is a new sensing technique for the distributed measurement of temperature and strain. Comprehensive theoretical examinations and experimental results of distributed temperature and strain measurements demonstrate the feasibility of this new concept. In an experimental demonstration a spatial resolution of 3 m was achieved for an 11-km-long single-mode fiber.
Experimental and theoretical investigations demonstrate the possible performance of a distributed temperature sensor based on Brillouin optical time domain analysis. The Brillouin frequency, its thermal coefficient and the gain bandwidth were measured for several standard telecommunication fibers at 1.3 micrometers . Theoretical investigations show that especially for sensor fibers with lengths of several ten kilometers the powers of the pump and the stokes laser in a pump-probe configuration are very critical parmaters. The optimum power level and the resulting sensor performance are calculated for different fiber lengths. A possible sensor configuration is presented.
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