| Summary: | Over decades, fiber-optic temperature sensors based on conventional single-mode fibers (SMF) have been demonstrated with either high linearity and stability in a limited temperature region or poor linearity and thermal hysteresis in a high-temperature measurement range. For high-temperature measurements, isothermal annealing is typically necessary for the fiber-optic sensors, aiming at releasing the residual stress, eliminating the thermal hysteresis and, thus, improving the high-temperature measurement linearity and stability. In this article, an annealing-free fiber-optic high-temperature (1100 °C) sensor based on a diaphragm-free hollow-core fiber (HCF) Fabry-Perot interferometer (FPI) is proposed and experimentally demonstrated. The proposed sensor exhibits an excellent thermal stability and linearity (R<sup>2</sup> > 0.99 in a 100–1100 °C range) without the need for high-temperature annealing. The proposed sensor is extremely simple in preparation, and the annealing-free property can reduce the cost of sensor production significantly, which is promising in mass production and industry applications.
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