Abstract
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.
Highlights
Conventional optical time domain reflectometry (OTDR) has long been a standard tool in the telecommunication industry for distributed fault detection and optical fiber loss characterization
We propose the use of alternating pulse wavelengths in a direct-detection coherent optical time domain reflectometry (C-OTDR) setup to measure strain and temperature changes and to determine the correct algebraic sign of the change
To our knowledge for the first time, a distributed temperature and strain change measurement technique by using the spectral information of a dual-wavelength directdetection C-OTDR and correlating the delay of pulse wavelength-dependent power variations along the signal axis
Summary
Conventional optical time domain reflectometry (OTDR) has long been a standard tool in the telecommunication industry for distributed fault detection and optical fiber loss characterization. The resulting jagged appearance of such a coherent OTDR (C-OTDR) trace is constant for static fiber condition but changes locally as the temporal relation between scatterers is changed as a function of applied temperature or strain changes. This technique is, depending on its implementation, known as phaseOTDR /ɸ -OTDR or phase-sensitive OTDR and is used for distributed sensing over tens of kilometers. The remarkable performance of COTDR DVS has attracted increasing interest from the structural health monitoring (SHM) sector Our investigation targets this field, the monitoring of transportation infrastructure such as bridges for distributed vibration mode analysis and the assessment of structural conditions. The requirements on frequency bandwidth and strain resolution are lower for structural vibration analysis but the spatial resolution target of 1 m is more demanding than typically required for DVS applications
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