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This study investigates the heat treatment effect on the Rayleigh scattering based distributed single-mode fiber optic temperature measurement. The temperature increment and sustained time at the peak temperature considered in the test were 20 ℃/30 ℃ and 90 min/60 min, respectively. Moreover, a finite element model was established to investigate the effect of the thermal expansion of the coatings on the optical fiber core strains and thus temperature sensitivity before the coatings soft and melt. A theoretical derivation for the fiber core strains by using Lame equations was performed to verify the accuracy of the finite element model. It is found that the one-time heat treatment successfully eliminates the hysteresis effect and stabilizes the Rayleigh scattering based distributed temperature measurement up to 1000 ℃ along the optical fiber regardless of the temperature increments. The sustain time at the peak temperature does not significantly affect the Rayleigh scattering measurement. Moreover, a unified Rayleigh frequency-temperature equation (with R2 more than 0.999) is fitted for fiber optic temperature measurement after the heat treatment with different temperature increments. In addition, the numerical fiber core strain at ambient temperature provides the upper limit as compared to the experimental results at high temperature, and the thermal-induced strains on the fiber core are smaller than 1 με. Therefore, the typical dual-layer coating effect on the temperature sensitivity of the Rayleigh frequency shift can be neglected at high temperature for civil engineering applications. Finally, parametric studies are performed to investigate the effect and sensitivity of material properties and geometric parameters on the fiber core strain, based on the validated finite element model. The present study is further promoting Rayleigh scattering based fiber optic temperature measurement application.
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