Fig. 1. Schematic diagram of water concentration distribution of layered PI membrane

Fig. 2. Schematic diagram of FBG sensing

Fig. 3. Schematic diagram of measurement principle of PI film FBG concentration sensor

Fig. 4. Schematic diagram of manufacturing process of FBG concentration sensor

Fig. 5. Heating curing temperature curve of PI coating

Fig. 6. FBG sensor concentration experimental system

Fig. 7. Reflection spectra of layered PI film FBG at different concentrations

Fig. 8. Concentration characteristic curves of FBG in layered PI film

Fig. 9. Positive and negative stroke test results

Fig. 10. Concentration sensing characteristic curves of FBG in annular PI films with different layers

Fig. 11. Comparison diagram of wavelength drift