Nd ³⁺ /Yb ³⁺ Co‐Doped Germanate Glass Fibers Toward Dual‐Mode Fluorescence Intensity Ratio and Lifetime Thermometry

ABSTRACT Fluorescence intensity ratio (FIR) thermometry is recognized for its fast response and high‐resolution characteristics; however, its performance is hindered by wavelength‐dependent absorption. Fluorescence lifetime sensing circumvents this limitation; however, it generally exhibits low temperature sensitivity. Here, this trade‐off is addressed through the integration of dual‐mode FIR and lifetime thermometry within a well‐designed Nd 3+ /Yb 3+ co‐doped germanate glass fiber. The glass matrix enables efficient up‐conversion emissions (750/810/890 nm) via Yb 3+ →Nd 3+ energy transfer, in addition to long‐lived NIR‐II emission at 1030 nm and compatibility with fiber preparation. A temperature sensitivity of 1.0% K −1 at 293 K is achieved in the FIR mode using thermally coupled transitions ( 4 F 7/2 / 4 F 5/2 → 4 I 9/2 ). Simultaneously, a Yb 3+ lifetime‐based sensitivity of 0.5% K −1 is obtained, effectively mitigating depth‐related inaccuracies associated with FIR measurements. We further show a demo from the designed materials to actual devices by fabricating a flexible fiber sensor composed of a GeO 2 ‐Nb 2 O 5 ‐BaO (GNB) glass core and polydimethylsiloxane (PDMS) cladding, which is employed for real‐time temperature monitoring within the range of 20–80 °C, exhibiting less than 1% hysteresis over five thermal cycles. These findings demonstrate a promising approach for depth‐tolerant, dual‐mode optical thermometry for accurate and real‐time temperature sensing in biomedical environments.