Fig. 1. Structure of FBG and the transmitted, reflected, and output spectra^[45]

Fig. 2. Schematic of internal and external FBG sensors and their photo^[49]

Fig. 3. TC and FBG sensors^[50]. (a) Schematic of sensors on the battery surface; (b) schematic of the experimental device

Fig. 4. FBG, In-TC, and Ex-TC^［51］. (a) Temperature response curve at 0.5C; (b) temperature variation curve measured at 0.5C, 1C, and 2C cycles

Fig. 5. Temperature sensing based on special optical fibers^[55]. (a) Schematic; (b) fluorescence intensity of optical fiber conversion emission at different temperatures; (c) relationship between fluorescence intensity ratio and temperature

Fig. 6. Distributed optical fiber sensor^[60]. (a) Schematic of OTDR; (b) schematic of OFDR

Fig. 7. Cylindrical battery with DFOS^[63]. (a) Schematic; (b) discharge capacity of different batteries; (c) temperature changes under different charging and discharging rates

Fig. 8. 0.3C charge and discharge rate^[63]. (a) Current and voltage; (b) delta temperature measured by D1, D2, and D3 at 2 cm from the negative terminal; (c) delta temperature measured by D1, D2, and D3 at 4 cm from the negative terminal; (d) delta temperature measured by D1, D2, and D3 at 6 cm from the negative terminal; (e) temperature distribution curve at D1, D2, and D3 when the battery is fully discharged

Fig. 9. FBG sensor monitoring of strain in lithium ion soft pack battery^[65]. (a) Schematic; (b) strain signal during standstill after different SOC

Fig. 10. Influence of the position of FBG sensors on battery strain measurement. (a) (c) Schematic and physical diagram of the FBG sensor pasted on the battery anode; (b) (d) schematic and schematic diagram of battery anode implanted with FBG sensor^[66]

Fig. 11. Spectra at 0% and 100% SOC for different FBG sensors^[66]. (a) Attached;(b) implanted

Fig. 12. FBG sensor monitoring AFLMBS^[67]. (a) Constant current circulation curve; (b) strain signal; (c) derivative of strain vs time

Fig. 13. Structure of sensitivity enhanced optical FBG sensor^[68]. (a) Assembly and exploded view; (b) sensitivity enhanced structure

Fig. 14. Schematic of the hybrid sensor^[74]

Fig. 15. Fiber evanescent wave spectroscopy^[80]. (a) Schematic of the experimental setup; (b) normalized transmittance of optical fiber at different wavelengths and different SOC

Fig. 16. Light transmittance and electric potential of the flexible battery when cycling between 0% and 100% SOC^[81]

Fig. 17. Sensor signals during charging and discharging in three configurations^[82]

Fig. 18. Tilt Bragg grating^[84]. (a) Configuration and sensing principle; (b) spectral response to charge density

Fig. 19. Schematic of optical fiber local surface plasmon resonance sensing probe^[85]

Fig. 20. Schematic of the multi-point optical fiber sensor^[86]

Fig. 21. Schematic of refractive index sensor^[87]。(a) Conventional design of fiber Bragg grating; (b) self-compensated FBG

Fig. 22. Oxygen concentration measurement^[56]. (a) Schematic; (b) intensity variation curve of reflected phosphorescence at a certain point on the cathode during the charging and discharging process