Fig. 1. Schematic of the sensor. (a) Fabrication of the composite flexible fluorescent temperature sensing fiber; (b) the fiber sensor; (c) light conduction effect of the flexible sensor under a large degree of bending

Fig. 2. Energy level and conformation of UCNPs. (a) Schematic of UCNPs^[16]; (b) transmission electron microscope of UCNPs

Fig. 3. Emission spectra of composite flexible fiber sensor at different temperatures

Fig. 4. FIR fluorescence temperature measurement process^[4]

Fig. 5. Performance of flexible fiber sensor under temperature change. (a) Nonlinear fitting curve for temperature and FIR; (b) linear fitting curve of temperature and FIR from 30 ℃ to 100 ℃; (c) sensitivity curves of the sensor; (d) real-time matching of the sensor between detected temperature and actual temperature

Fig. 6. FIR values fluctuation and temperature resolution of the sensor at constant temperature. (a) Fluctuation of the sensor output under constant temperature of 36 ℃; (b) FIR response from 40 ℃ to 45 ℃ with a temperature gradient of 0.5 ℃ and the temperature is held for 30 s at each stage

Fig. 7. Repeatability of the sensor

Fig. 8. Bending interference resistance and pressure interference resistance of the sensor. (a) Response of sensor under different bending radius; (b) response of the sensor under different pressures

Fig. 9. Wind-blown and stretching interference test results of flexible optical fiber sensor. (a) FIR response of different blowing frequency at constant room temperature, the inset shows the test method; (b) FIR value fluctuation of the sensor in the whole process of 120% stretching at constant room temperature, the inset shows the state of the sensor during stretching process

Fig. 10. Underwater temperature response test and anti-interference of the sensor. (a) Temperature sensing under different water temperature gradients, the inset shows the test method under the water; (b) sensor response under constant temperature water bath perturbation