Microstructured Y3Al5O12 single-crystal fibers for high-sensitivity quasi-distributed ultrasonic thermometry based on acoustic anisotropy engineering

The rapid development of aerospace, nuclear energy, and advanced manufacturing has created a growing demand for temperature sensing in extreme environments. Ultrasonic temperature sensors (UTS) are widely used in high-temperature sensing due to their extreme operating temperature close to the melting point of the waveguide materials. In this work, YAG single-crystal fibers (SCF) with spatially distributed acoustic reflection microstructures have been successfully fabricated via the laser-heated pedestal growth (LHPG) method and employed as acoustic waveguides. Herein, anisotropic acoustic waveguide behaviors were revealed in YAG SCF, where the [110]-oriented YAG SCF demonstrates enhanced unit sensitivity with the S-wave polarization direction of [$1\overline{1}0$], primarily attributed to the lower acoustic velocity and the more substantial velocity variations with temperature. Furthermore, quasi-distributed ultrasonic temperature sensing in the range of 30-1800℃ has been achieved based on the [110]-oriented YAG SCF with two discrete sensing units, reaching the maximum unit sensitivities of 47.18 ns·℃^-1·m^-1 and an optimal temperature resolution of 5.04℃ at 1800℃. Superior acoustic waveguide characteristics, a wide working temperature range, and the positive temperature-dependent sensor performance suggest that the [110]-oriented microstructured YAG SCF is an ideal candidate for distributed high-temperature sensing in harsh environments.