Fig. 1. (a) Working mechanism of C₆₀-Lys film. When water molecules are adsorbed by C₆₀-Lys, the polarization intensity changes. (b) Configuration of the optical fiber humidity sensor. The transmission spectrum is affected by the polarity of C₆₀-Lys.

Fig. 2. (a) FTIR spectrum of C₆₀-Lys. (b) SEM image of the surface of TFBG. The inset is the dispersive energy spectrum (EDS) of the TFBG surface, which is evidence for the existence of C₆₀-Lys film on fiber. (c) Cutting face SEM image of the C₆₀-Lys film.

Fig. 3. (a) RH detection results by the FHS. The transmission spectra of FHS at the environment RH raising from 30% to 90% with the inset showing changes at the core mode. (b) Detailed variation of the cladding mode in the wavelength range from 1516 nm to 1520 nm. (c) The blue line is the linear fitting of intensity and RH at the cladding mode of 1518 nm. The red line is the linear fitting of wavelength and RH at the cladding mode of 1518 nm. (d) Comparison of cladding mode intensities at 1518 nm with increasing and decreasing RH.

Fig. 4. (a) Experimental setup of response time. Controlling the opening of the valve to change the air RH. (b) Results of change between 40% RH and 45% RH, and the response time and recovery time are calculated as 1.85 s and 1.58 s, respectively. (c) Results of change between 60% RH and 80% RH. The performance of the sensor is ultra-stable.

Fig. 5. Results of breath monitoring experiments. (a) Breath monitoring optical path. (b) Combination of the sensor and breathing tube. (c)–(e) Spectra at Test-1, Test-2, and Test-3 with different respiratory frequencies. (f)–(h) Response times of one breath at three tests. (i)–(k) FFT calculates the frequency of three tests.

Fig. 6. Comparison of the response time and recovery time of our FHS with different materials coated with optical fiber sensor.

Fig. 7. Stability test results of FHS within 7 days. The inset figure is the linear fit results.