Fig. 1. Schematic of sensor

Fig. 2. Characterization test results of GO and RGO coated on the fiber. (a) Raman spectra of GO and RGO; (b) XRD spectra of GO and RGO; (c)(d) XPS spectra of GO and RGO. Power spectra of C—C bond (284.7 eV), C—O bond (286.9 eV), and surface adsorbed oxygen (532.65 eV) are shown

Fig. 3. SEM photo of cross section of RGO-covered microfiber

Fig. 4. Experimental setup

Fig. 5. Interference spectrum of transmitted light in sensor at 25 ℃ when RH is 65%

Fig. 6. Variation of interference spectrum of transmitted light in sensor with humidity at humidity rise stage. (a) 45%RH; (b) 95%RH; (c) 45%RH

Fig. 7. Wavelength of interference dip as a function of humidity. (a) Interference dip1; (b) interference dip2

Fig. 8. Responses of interference dip1 and dip2 to variation of temperature at circumstance humidity of 61%

Fig. 9. Comparison of transmitted light spectra of sensor before and after heating under different conditions. (a) 25 ℃, 65%RH; (b) 25 ℃, 95%RH

Fig. 10. Evolution of interference dip2. (a) Evolution of interference dip2 with increase of humidity; (b) evolution of interference dip2 with decrease of humidity; (c) wavelength of interference dip2 as a function of humidity

Fig. 11. Response of output optical power of sensor to human expiration at transmitted light wavelength of 1568 nm

Fig. 12. Fluctuations of wavelength of interference dip2 at different constant humidity