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.
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.
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.
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 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.
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.
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.
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.
KEYWORDS: Optical fibers, Multimode fibers, Single mode fibers, Signal attenuation, Scattering, Phase only filters, Temperature metrology, Structural health monitoring, Silica, Fusion splicing
We investigate the suitability of silica graded index multimode fibers (MMF) for distributed Brillouin sensing
in structural health monitoring, where the measurement range is limited by small bendings that appear during
the integration process of the sensing fibers into the structures. For the investigation of stimulated Brillouin
scattering (SBS) in MMF, we use an MMF connected on both ends to the SMF measurement setup by fusion
splices to ensure that only the fundamental mode is transmitted. The SBS spectra in MMF are recorded using
a 1319 nm single frequency (line width 5 kHz) laser. Results found for standard singlemode fibers and the
fundamental mode in multimode silica optical fibers are compared. We present the gain spectra showing the
dependence of frequency shift, attenuation and modal noise to both temperature and strain. The dependence
of the attenuation due to bending is shown. Finally, the perspective of the excitation of SBS in polymer optical
fibers is discussed against the background of our research on SBS in MMF.
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