Fiber optic measurement system

• Subject Area: Construction Engineering and Architecture
• Term: Funded in 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 558169066

To strengthen fundamental experimental research and applied research at the Department of Civil and Environmental Engineering, fibre-optic measurements are to be made possible using a high-resolution fibre-optic measuring system. The recording of stresses and strains in building components is one of the core requirements of experimental research in civil engineering and structural health monitoring. Until now, the detection of strains inside building components or soil bodies was either only possible at specific points using strain gauges or by accepting massive disturbances, e.g. using inclinometers, was subject to high uncertainties (as with ultrasound measurements or acoustic emission measurements) or was limited to strain fields on the surface, determined using digital image correlations. The method of distributed fibre-optic sensing (DFOS) allows high-resolution recording of strain and temperature fields. The measurement data can be recorded continuously along a sensor, an optical fibre, over measuring lengths of up to 100 m and, in the case of shorter measuring lengths, with resolutions down to the sub-mm range. In addition to the application on components (e.g. bridges, tunnels or glass facades), the flexible measuring fibres can also be inserted directly into structural systems or soil bodies without interference and thus allow new and innovative insights into the material behaviour of soils, steel, concrete, glass and the structural behaviour of bridges, traffic routes and sandwich panels, for example. The fibre optic interrogator, being subject of this application, enables the high-frequency readout and evaluation of DFOS measurements. The technology makes use of various optical phenomena, namely Rayleigh and Raman scattering of light transmitted through an optical fibre. Rayleigh scattering makes it possible to identify local changes in length between two successive measurements, which are observed as a frequency shift within the signal. From the resulting scattering profile, the strains along the fibre can be determined with very high resolution using optical backscatter reflectometers (the ‘fibre optic measurement system’). Raman scattering can be used to determine temperature changes along an optical fibre. Unless the measuring device requires the use of special measuring fibres from a pre-defined manufacturer, the optical fibres for DFOS are very cost-effective. They can be combined to form high-quality 3D sensors, enabling a wide range of innovative applications with low follow-up costs. Due to their flexibility and small size, the measuring fibres can also be applied in laboratory and field tests with virtually no macroscopic disturbance of the test specimen. The proposed device therefore represents a fundamental expansion of the experimental possibilities of the various working groups at the Department of Civil and Environmental Engineering of Technical University of Darmstadt.

• DFG Programme: Major Research Instrumentation
• Major Instrumentation: Faseroptisches Auslesesystem
• Instrumentation Group: 8710 Optische Längenmeßgeräte (außer 062-067 und Meßmikroskope 503)
• Applicant Institution: Technische Universität Darmstadt
• Leader: Professor Dr.-Ing. Hauke Zachert