Thermal compensation of monolithic distributed fibre optic sensors: From the lab to the field

Distributed fibre optic sensing (DFOS) is gaining popularity as a diagnostic solution for short-term nondestructive testing (NDT) and long-term structural health monitoring (SHM). However, there are still challenges to be overcome, particularly with regard to long-term measurements under real field conditions, taking into account, among other things, temperature changes. These changes affect strain measurements through both thermal expansion and optical effects, and appropriate correction depends on the internal design of the sensor and the installation methods. Therefore, despite the fact that thermal compensation is one of the key prerequisites for correct data interpretation, there are no universal and clear procedures for DFOS. The lack of sufficient guidelines and proven applications is the main motivation for undertaking studies. The scope and novelty of the presented research lies in 1) the theoretical explanation of the thermal performance of monolithic DFOS sensors compared to the layered sensing cables; 2) the analysis and correction of strain data measured with monolithic sensors placed in a thermal chamber; 3) the verification of the laboratory results and the proposed approach in real field conditions: in a concrete footbridge deck and in a steel underground gas pipeline. All presented measurements were performed with high spatial resolution (mm order) Rayleigh-based interrogators under the control of conventional reference spot thermistors. The research demonstrated the linear thermal response of the monolithic sensors and confirmed their usefulness for distinguishing between length- and stressdependent strains. Finally, the thermal compensation approach was proposed and verified in real operating conditions.