High Performance Flexible Optical Fiber Sensor Enabled by Er3+/Yb3+ Co-Doped near Pure-Green Upconversion Emission Phosphor for Implantable Temperature Sensing

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.