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Responding to a growing need for intelligent monitoring of critical geotechnical and civil infrastructures, new multifunctional geotextiles and geogrids with integrated optical fibers were developed and investigated within several research projects. Such smart two-dimensional geosynthetics intercorporating optical fibers as distributed sensors provide solutions both for soil reinforcement, erosion control, drainage or environmental protection and for cost-effective monitoring of critical mechanical deformations and temperature distribution in geotechnical and masonry structures at the same time. The integrated fiber optic sensors provide online information about the condition of the monitored structure and about the occurrence and localization of any damage or degradation for the purpose of preventing a total collapse.
In this paper, we present a study aimed at characterizing the optimal fiber optic components for Brillouin sensing in multimode fibers. For this purpose the use of single-mode and multimode circulators as well as couplers typically used in the Brillouin measurement setups was investigated. On the one hand the undesired coupling losses between conventional fiber optic measurement system components and a multimode sensor fiber can be overcome by replacing the singlemode components with their own multimode equivalents. On the other hand the use of multimode fiber optic circulators and multimode couplers affects the mode distribution of laser light which can impair the measurement signal backscattered in the multimode sensor fibers. In view of an increasing interest in high strain measurements using polymer optical fibers (POFs) as Brillouin-distributed sensors the investigation on Brillouin scattering effects in multimode fibers (MMFs) was performed on a low-loss perfluorinated graded-index polymer optical fiber (PFGI-POF). The obtained results were compared with those of a standard graded-index multimode (GI-MMF) silica glass optical fiber (GOF).
This study confirms the relevance of the adaptation of the measurement system components to the use of the multimode sensor fibers. In addition, due to mode coupling effects occurring in the tested POF itself, the results show differences in the yield of the components adaptation in the sensory implementation of the two kinds of the tested optical fibers.
Responding to a growing need for intelligent monitoring of critical geotechnical and civil infrastructures, new multifunctional geotextiles and geogrids with integrated optical fibers were developed and investigated within several research projects. Such smart two-dimensional geosynthetics intercorporating optical fibers as distributed sensors provide solutions both for soil reinforcement, erosion control, drainage or environmental protection and for cost-effective monitoring of critical mechanical deformations and temperature distribution in geotechnical and masonry structures at the same time. The integrated fiber optic sensors provide online information about the condition of the monitored structure and about the occurrence and localization of any damage or degradation for the purpose of preventing a total collapse. The optical fibers have a serious advantage over other kinds of sensors due to their fibrous nature, so they can be ideally processed like standard textile yarns.
The monitoring of extended geotechnical structures like dikes, dams, railways, embankments or slopes requires sensor technologies with measurement lengths of at least some hundred meters which can be realized by the use of silica fibers. However, the integration of silica fibers into geosynthetics during the manufacturing process experienced problems of sensor brittleness, low strain range not exceeding 2 % and bending-related attenuation increase optimized to the value of 1.5 dB/km. Unlike silica fibers, the integration of polymer optical fibers (POFs) into various geosynthetics was easily achieved free of bending losses enabling distributed high strain measurements up to 40 % using PMMA POFs. The first industrial product GEDISE based on a geogrid with integrated PMMA POFs is on the market.
Due to the five times lower attenuation of low-loss perfluorinated graded-index POFs (PFGI-POFs), compared to PMMA POFs, the measurement range can be extended to 500 m. The low-loss PFGI-POFs offer the possibility of implementation of Brillouin optical fiber frequency domain analysis (BOFDA) which will significantly improve the measurement accuracy and spatial resolution in comparison with existing POF-based distributed measurement methods like POF OTDR. The development of a distributed Brillouin system based on PFGI-POFs is in progress.
In this paper, we present a study aimed at characterizing the optimal fiber optic components for Brillouin sensing in multimode fibers. For this purpose the use of single-mode and multimode circulators as well as couplers typically used in the Brillouin measurement setups was investigated. On the one hand the undesired coupling losses between conventional fiber optic measurement system components and a multimode sensor fiber can be overcome by replacing the single-mode components with their own multimode equivalents. On the other hand the use of multimode fiber optic circulators and multimode couplers affects the mode distribution of laser light which can impair the measurement signal backscattered in the multimode sensor fibers. In view of an increasing interest in high strain measurements using polymer optical fibers (POFs) as Brillouin-distributed sensors the investigation on Brillouin scattering effects in multimode fibers (MMFs) was performed on a low-loss perfluorinated graded-index polymer optical fiber (PFGI-POF). The obtained results were compared with those of a standard graded-index multimode (GI-MMF) silica glass optical fiber (GOF).
This study confirms the relevance of the adaptation of the measurement system components to the use of the multimode sensor fibers. In addition, due to mode coupling effects occurring in the tested POF itself, the results show differences in the yield of the components adaptation in the sensory implementation of the two kinds of the tested optical fibers.
Distributed strain and temperature change detection using optical frequency domain reflectometry
(2017)
We propose the optical frequency Domain reflectometry (OFDR) technique based on intensity modulation frequency sweep measurement for coherent distributed sensing applications. By evaluating interferometric Rayleigh scattering changes along the fibre, strain and temperature changes can be detected with a sensitivity in the ne-range and 10 mK temperature resolution. Low Vibration frequencies and vibrations up to the kHz-range can be detected by differential power change evaluation in the spatial domain. This OFDR approach is a low-cost alternative to distributed vibration sensing based on C-OTDR up to distances of several kilometres.
Easy to handle and stretchable polymer optical fibres for sensor applications
Basic idea of stimulated Brillouin scattering
Brillouin threshold for PMMA-POF and PFGI-POF in comparison
Sensorial behavior of Brillouin scattering in PFGI-POF and silica SMF
Basics and classification of distributed Brillouin sensing methods
Basic idea of dynamic Brillouin sensing
We propose the use of alternating pulse wavelengths in a direct-detection coherent optical time domain reflectometry (C-OTDR) setup not only to measure strain and temperature changes but also to determine the correct algebraic sign of the change. The sign information is essential for the intended use in distributed mode shape analysis of civil engineering structures. Correlating relative backscatter signal shifts in the temporal/signal domain allows for measuring with correct magnitude and sign. This novel approach is simulated, experimentally implemented and demonstrated for temperature change measurement at a spatial resolution of 1 m.