Fig. 1. Schematic diagram and process of FBG hydrogen sensor preparation

Fig. 2. Reflection spectra of HT-FBG and T-FBG

Fig. 3. Hydrogen detection system

Fig. 4. Fiber surface morphology characterization at various stages of polydopamine-assisted electroless palladium plating process. (a)(e) For the bare fiber; (b)(f) for PDA-loaded fiber; (c)(g) for palladium-seeds fiber; (d)(h) for electroless palladium-plated fiber with sodium hypophosphite

Fig. 5. Response of SELP-60 s FBG hydrogen sensor with different thickness of polydopamine coating to hydrogen concentration from 0 to 4%

Fig. 6. Micrographs of the PDA coating surface at different deposition time on the fiber surface. (a) 0 min; (b) 10 min; (c) 20 min; (d) 30 min

Fig. 7. Response to 5% H₂ 3 cycles and surface morphology of FBG hydrogen sensor electrolessly plated with sodium hypophosphite and hydrazine hydrate. (a)(c)(d) With sodium hypophosphite; (b)(e)(f) with hydrazine hydrate

Fig. 8. Cross section morphology and thickness of the palladium film under different deposition time of hydrazine hydrate electroless palladium plating. (a) Cross section of the polydopamine coating without electroless palladium plating; (b)-(f) palladium membrane sections at deposition time of 60 s, 90 s, 120 s, 150 s, and 180 s

Fig. 9. Effect of HELP deposition time on palladium film thickness and sensor performance. (a) Relationship between HELP deposition time and palladium film thickness; (b) response of FBG hydrogen sensor to 5% hydrogen concentration under different deposition time; (c) variation trend of sensor sensitivity and response time with palladium film thickness

Fig. 10. HELP-120 s FBG hydrogen sensor response to hydrogen cycle and response of hydrogen with different concentrations. (a) Sensor response with time; (b) sensor response with hydrogen concentration

Fig. 11. Surface topography of FBG hydrogen sensor before and after coating with hydrophobic film. (a) Before coating; (b) after coating

Fig. 12. Effect of hydrophobic film coating layer number and relative humidity on sensor performance. (a) FBG hydrogen sensor with different hydrophobic film coating layers responds to 5% concentration hydrogen at 20%RH and 70%RH; (b) FBG hydrogen sensor coated with 15 layers of hydrophobic film responds to 5% hydrogen concentration under 20%RH-90%RH

Fig. 13. Temperature sensitive characteristics of HT-FBG and T-FBG and response of sensor before and after temperature compensation. (a) HT-FBG and T-FBG temperature calibration curves; (b) response of the FBG hydrogen sensor to 5% concentration H₂ before and after temperature compensation at 30-70 ℃