Multiscale‐Designed Transparent Glass‐Ceramic Fiber for Optical Thermometry From Healthcare Monitoring to Chemical Processes

ABSTRACT Achieving precise and rapid temperature detection in extreme environments remains a formidable challenge for optical thermometry. Here, we report a multiscale‐designed Ba 2 LaF 7 : Yb 3+ /Er 3+ glass‐ceramic fiber that achieves relative sensitivities comparable to the highest reported and a fast thermal response. By integrating glass‐forming region analysis, crystallization kinetics control, and molecular dynamics simulations, we realized uniform Ba 2 LaF 7 nanocrystallization within a highly transparent (∼85%) oxide‐fluoride matrix. The fiber, fabricated via a melt‐in‐tube process using glasses with high thermal conductivity for both core and cladding, exhibits low optical loss and rapid thermal equilibrium. Owing to the optimized local structure and phonon environment, the sensor delivers a maximum relative sensitivity of 1.21% K −1 at 293 K and a temperature resolution below 0.5°C, values that are among the highest reported for rare‐earth‐doped glass‐based systems. It maintains excellent reproducibility over >10 heating‐cooling cycles, enabling stable and repeatable operation. Demonstrations in real‐time monitoring of human respiratory behavior and acid‐base neutralization reaction processes verify its robustness in dynamic and corrosive environments, showcasing its potential for applications ranging from healthcare monitoring to chemical processes. This work demonstrates a computation‐guided approach that links phase‐diagram analysis, microstructural design, and functional device fabrication, providing a pathway for developing high‐performance glass‐ceramic fiber sensors with broad applicability under specific conditions.