Fig. 1. Schematic diagram of experimental setup (inset is sensing structure)

Fig. 2. Optical power distributions of XZ and YZ cross-sections of optical fiber sensing region. (a) XZ cross-section; (b) YZ cross-section

Fig. 3. XY cross-sectional optical power distributions of optical fiber sensing area. (a) XY cross-section at Z=0.1 cm; (b) XY cross-section at Z=0.2 cm; (c) XY cross-section at Z=3.1 cm; (d) XY cross-section at Z=3.165 cm

Fig. 4. SEM images of TCF coated with optimized copper ion imprinting film and FTIR of composite film. (a) SEM image of outside surface of TCF (inset is enlarged SEM image of outside surface of TCF); (b) partially enlarged SEM image of cross section of TCF; (c) FTIR of composite film

Fig. 5. Response spectra of the sensor without sensing film and with the optimized copper ion imprinted composite film

Fig. 6. Response spectra of sensors coated with different composite films and linear fitting curves of wavelength shift and logarithm of copper ion concentration. (a) Response spectra of sensor coated with CS/PVA composite film; (b) linear fitting curve of wavelength shift of sensor with CS/PVA composite film and logarithm of copper-ion concentration; (c) response spectra of sensor coated with copper-ion imprinted composite film; (d) linear fitting curve of wavelength shift and logarithm of copper-ion concentration for sensor with copper-ion imprinted composite film; (e) response spectra of sensor coated with optimized copper ion imprinting composite film; (f) linear fitting curve of wavelength shift and logarithm of copper-ion concentration of sensor with optimized copper-ion imprinting composite film

Fig. 7. Response time, selectivity, and pH stability of sensor coated with optimized copper ion imprinting composite film. (a) Response time; (b) selectivity; (c) pH stability

Fig. 8. Sensing mechanism diagram