High Performance Flexible Optical Fiber Sensor Enabled by Er ³⁺ /Yb ³⁺ Co‐Doped near Pure‐Green Upconversion Emission Phosphor for Implantable Temperature Sensing

ABSTRACT Er ³⁺ /Yb ³⁺ co‐doped upconversion (UC) emission phosphors play crucial roles for advancing implantable temperature sensing technologies. However, the fabrication of high‐performance optical fiber temperature sensor (OFTS) via these sensing materials remains a major challenge. Herein, we demonstrate a superior double‐cladding OFTS by utilizing a bright and stable near‐pure green UC phosphor NaLu(WO ₄ ) ₂ :Er ³⁺ /Yb ³⁺ as sensing material. Comprehensive investigations show that the near pure‐green UC emission mechanism is mainly attributed to the isolated distributed characteristics of Er ³⁺ (Yb ³⁺ ) ions and the relatively long distance between them in this system. The OFTS exhibits a high temperature measurement accuracy of 0.155°C at 303 K, excellent measuring repeatability and chemical stability, along with strong resistance to various physical interferences (excitation power fluctuation, mechanical vibration, etc.). Through non‐destructive implantation of the OFTS into a sodium‐ion pouch‐type battery, the battery's internal thermal states (temperature variation) during constant current (CC)/constant voltage charging discharging processes at different C‐rates are captured in real time. Moreover, we also get the temperature variation and distribution in vertical direction in an oil‐water system via mobile measurement of the OFTS. This study not only provides a high performance implantable OFTS but also shows new insights into the design of Er ³⁺ /Yb ³⁺ based sensing materials.