Fig. 1. Growth in fiber-optic hydrogen sensor publications since 2000 according to an enquiry (December 2020) in Ei Compendex

Fig. 2. Schematic of strain modulation M-Z interference hydrogen sensor^[40]

Fig. 3. (a) Schematic of strain modulation F-P interference hydrogen sensor^[42]; Schematic of F-P interference structure with Pd film (b) on the side^[58] and (c) on the end^[59]. PHF：Palladium based hydrogen sensitive film

Fig. 4. Schematic probe structure of optical fiber hydrogen sensor based on F-P interferometer formed by microcantilever^[39]

Fig. 5. (a) Schematic of strain modulation FBG hydrogen sensor^[45]; Schematic of FBG sensing area of (b) side polishing^[57], (c) cladding corrosion^[60], (d) taper^[61], and (e) grooving^[62]

Fig. 6. (a) Schematic of modulation optical fiber interferometer hydrogen sensor based on radial strain^[41]；(b) Enlarged view of PM-PCF sensor head

Fig. 7. (a) Schematic of typical refractive index modulation micro mirror hydrogen sensor^[47]; (b) Schematic of single optical path differential structure^[64]; (c) Schematic of dual optical path differential structure^[48]

Fig. 8. (a) Schematic of typical refractive index modulation evanescent field hydrogen sensor^[49]; (b) Schematic of D-type evanescent field hydrogen sensor^[50]

Fig. 9. (a) Schematic of typical refractive index modulation SPR hydrogen sensor; (b) Sketch for Pd metallic grating on the end-face of fiber^[69]

Fig. 10. (a) Schematic of typical refractive index modulation M-Z hydrogen sensor^[52]; Schematic of M-Z sensing area of (b) Core-offset fusion^[70], (c) taper^[33], (d) collapse fusion^[71], and (e) LPG^[73]

Fig. 11. Schematic of temperature modulation FBG hydrogen sensor. THF: Tungsten trioxide hydrogen sensitive material

Fig. 12. Principle of sensing technology based on stimulated Raman dispersion^[75]

Fig. 13. Magnetron sputtering coating equipment. (a) Magnetron sputtering coating machine; (b) Three independent target sources; (c) Optical fiber clamp