Quasi-single-mode sapphire fiber Bragg grating for rapid transient temperature sensing in ultra-high-temperature combustion environments.

Sapphire fiber Bragg gratings (SFBGs) are suitable for temperature sensing in extreme environments, yet their dynamic response under rapid high-temperature perturbations remains insufficiently investigated. Here, we examine the rapid temperature-sensing performance of a quasi-single-mode SFBG in dynamic high-temperature environments. After annealing and calibration up to 1659 °C, a lightweight interrogation strategy based on pseudo-Gaussian fitting and local feature matching assisted by the K-nearest-neighbor (KNN) algorithm was introduced to reduce spectral-shape-induced uncertainty, achieving a mean absolute error of 0.56 °C and a maximum error below 1.92 °C. A thermal-response model was established to describe the dynamic heating process under CO2-laser excitation. The measured response time decreased from 420 ms to 200 ms as the laser power density increased, consistent with the model prediction. Further experiments in a simulated aero-engine exhaust flow showed that the sensor stably tracked flame-temperature variations under rapid high-temperature perturbations. These results verify the feasibility of quasi-single-mode SFBGs for rapid temperature sensing in ultra-high-temperature dynamic environments.