Fig. 1. Schematic diagram of micromirror fiber optic hydrogen sensor^[23]

Fig. 2. (a) Response of fiber optic hydrogen sensors with 50 nm Mg₇₀Ti₃₀ as well as 30 nm Pd catalytic layer at 1% hydrogen concentration for different oxygen concentrations^[27]; (b) Dual optical path type micromirror fiber optic hydrogen sensor^[28]; (c) Response of alloy films before/after annealing at 2% hydrogen concentration^[29]; (d) Fiber optic micromirror hydrogen sensor based on Pd/Au alloy^[30]

Fig. 3. (a) Hydrogen sensing detection system based on WO₃-Pd₂Pt-Pt composite thin-film fiber optic micromirrors^[31]; (b) Variation of sensor response with increasing hydrogen concentration^[31]; (c) Reflective hydrogen sensing system based on polarization modulation^[32]

Fig. 4. Schematic diagram of the fiber optic hydrogen sensor based on Mach-Zehnder interferometer^[33]

Fig. 5. (a) Schematic of the Mach-Zehnder interferometer-based fiber-optic hydrogen sensor prepared by arc discharge; (b) Transmission spectra of the Mach-Zehnder interferometer hydrogen sensor at 0 (black) and 4% (red dashed) hydrogen concentrations^[34]; (c) Schematic of the MZI hydrogen sensor based on a Pd-coated air cavity^[35]

Fig. 6. (a) Schematic diagram of the fiber optic device; (b) Microscope picture of the sample; (c) Dip wavelength shifted as a function of H₂ concentration, and the inset shows the transmission spectra of the sensor at different H₂ concentrations^[36]; (d) Schematic diagram of the structure of the MZI hydrogen sensor with nanowires; (e) Sensing performance of the sensor at different H₂ concentrations^[37]

Fig. 7. Novel MZI hydrogen sensor based on nanopatterned Pd film^[38]

Fig. 8. Structure and sensing principle diagram of MZI hydrogen sensor in optical fiber^[39]

Fig. 9. (a) Schematic structure of MMF-SMF-MMF-based MZI hydrogen sensor; (b) Variation of MZI sensor coated with Pd-GNF resonance dipping wavelength with different concentrations of H₂^[40]

Fig. 10. (a) Schematic diagram of exogenous FPI fiber optic hydrogen sensor; (b) Variation of spectral peaks with switching between H₂ and N₂^[42]; (c) Schematic diagram of FP fiber optic hydrogen sensor prepared based on femtosecond laser micromachining; (d) Variation of reflectance spectra with the hydrogen concentration ^[43]; (e) Fiber optic FP sensor based on Pd/Y thin film^[44]

Fig. 11. (a) Schematic of the hydrogen sensor based on a fiber FP cavity with a suspended palladium film; (b) Schematic geometry of the bending properties of the palladium film; (c) Time response and wavelength shift of the sensor at different H₂ concentrations^[45]; (d) Hydrogen sensor based on a Pd-modified multilayer graphene composite thin-film fiber FP^[46]; (e) Schematic of the hydrogen sensor based on the graphene-Au-Pd cantilever; (f) Scanning of the surface of the cantilever electron microscopy (SEM) image; The inset shows the top view of the cantilever surface; (g) Side view of the sensor under the microscope; (h) Response spectra of the sensor at different hydrogen concentrations^[47]

Fig. 12. (a) Schematic of nano-photomechanical cavity structure based on Au/Pd composite layer; (b) Schematic of hydrogen sensing principle of fiber-optic nano-photomechanical cavity structure; (c) Durability and repeatability experiments of nano-photomechanical cavity with 6 nm Pd thin film for hydrogen concentration sensing^[48]; (d) Schematic of fiber-optic hydrogen sensor based on the principle of optical-force synergistic hydrogen detection; (e) Dynamic hydrogen concentration (0.5%-3.5%-0.5%) response of the sensor corresponding to different Pd nanofilm thicknesses, and the wavelength shifts of the sensors corresponding to different thicknesses at different hydrogen concentrations^[49]

Fig. 13. (a) Schematic diagram of hydrogen sensing system based on all-optical photothermal technology; (b) Schematic diagram of photothermal-assisted hydrogen permeation enhancement; (c) Time-response curves of fiber-optic hydrogen sensors at different laser powers^[50]; (d) Temperature distributions on the surface of the Pd membrane before and after 405-laser irradiation; (e) Influence of the photothermal effect on the equilibrium of the H-Pd reaction^[51]

Fig. 14. (a) Schematic of the process of preparing an FP interferometer using the H-O catalytic bonding technique; (b) Hydrogen response performance of a fiber optic FP interferometer based on a Pd nanofilm^[52]

Fig. 15. (a) Schematic of the H₂ sensor constructed by dual parallel FPIs; (b) Structural diagram of a single FPI; (c) Schematic cross-section of a single FPI head; (d) Reflectance spectra of the sensing-FPI at different H₂ concentrations^[53]; (e) Schematic of FP hydrogen sensing based on the Pd nano-cavity-array fiber; (f) Working schematic of the FP hydrogen sensor; (g) SEM characterization of the end face of the FP hydrogen sensor; (h) Reflectance spectra wavelength drift at different H₂ concentrations^[54]

Fig. 16. (a) Schematic structure of fiber optic hydrogen sensor based on FP combined with FBG^[55]; (b) Fiber optic FP hydrogen sensor with transverse offset structure based on SMF; (c) Spectra in air and its upper envelope; (d) Upper envelope spectra at different H₂ concentrations; (e) H₂ sensitivity fit^[56]

Fig. 17. (a) Cross-sectional view as well as side view of the Pd-coated polarization maintaining fibre; (b) Variation of the output transmission spectra of the sensor measured at room temperature with different hydrogen concentrations; (c) Relative wavelength shift and transmission change of the spectral dip for various hydrogen concentrations^[57]

Fig. 18. (a) Schematic diagram of Sagnca hydrogen sensor based on polarization-maintaining photonic crystal fiber; (b) Measured wavelength shift versus H₂ concentration for three samples with different lengths of polarization-maintaining photonic crystal fiber coating^[58]

Fig. 19. (a) Experimental setup of a single Sagnac interference loop hydrogen sensor system; (b) Relationship between envelope shift and hydrogen concentration^[59]

Fig. 20. (a) Schematic of the Pd-coated FBG hydrogen sensor; (b) Bragg wavelength of the FBG sensor as a function of hydrogen concentration^[61]

Fig. 21. Dual-fiber Bragg grating sensing configuration ^[63]

Fig. 22. Schematic diagram of the FBG hydrogen sensor with HAF^[65]

Fig. 23. (a) Structure of a side-polished FBG hydrogen sensor based on WO₃-Pd composite and schematic diagram of the sensing system^[66]; (b) Wavelength shift of etched fiber grating at different hydrogen concentrations^[67]; (c) Schematic diagram of a fiber grating hydrogen sensor encapsulated in flexible polypropylene material; (d) Hydrogen response of a fiber grating hydrogen sensor at different hydrogen concentrations^[68]

Fig. 24. Schematic of the hydrogen sensor configuration with two etched FBGs in cascade^[70]

Fig. 25. (a) Typical attenuation spectra of a 0.5 µm period fiber grating exposed to a mixture of pure nitrogen and 4% hydrogen at room temperature; (b) Typical attenuation spectra of a 400 µm period LPG exposed to a mixture of pure nitrogen and 4% hydrogen at room temperature^[73]; (c) Schematic diagram of the experimental setup of the hydrogen sensor for a single LPG; (d) Resonance of the sensor when sequentially exposed to 4% hydrogen and 100% nitrogen response at the peak position^[74]

Fig. 26. (a) Schematic structure of fiber optic SPR multilayer membrane sensor based on wavelength modulation^[78]; (b) Hydrogenation (red line) and dehydrogenation time (black line) of the sensor in 4% H₂^[79]; (c) Schematic structure of multilayer sensing with Ag/SiO₂/PdY multilayer stacks deposited on the fiber core^[80]

Fig. 27. Multilayer structure of heterocore fiber SPR hydrogen sensor^[81]

Fig. 28. Preparation process of alloy nanostructures and patterning transfer on fiber^[82]

Fig. 29. (a) Heterocore fiber LSPR hydrogen sensor based on Pd nanoparticles; (b) Expanded view of the real-time response of the heterocore fiber hydrogen LSPR sensor to optical loss variations, including the response and recovery at 0 and 4% H₂ concentrations^[83]; (c) Schematic diagram of the fiber optic localized surface plasmon resonance hydrogen detection system based on palladium-coated gold nanoparticles; (d) Sensor response at 0.8% to 4% hydrogen concentration

Fig. 30. (a) Swift-field fiber-optic hydrogen sensor^[86]; (b) Structure of swift-field fiber-optic hydrogen sensor based on Pd/WO₃ material^[87]; (c) Tapered fiber-optic hydrogen sensor based on Pd/GO nanocomposites; (d) Response characteristics of the tapered fiber-optic hydrogen sensor with Pd/GO nanocomposites in the range of 0.125%-2% H₂ concentration^[89]

Fig. 31. (a) Fiber optic swift field hydrogen sensor with fiber core mismatch structure; (b) Normalized response of the sensor with 10 nm thick Pd/Au film upon repeated exposure to pure nitrogen and 2% or 4% H₂ concentration^[91]; (c) Fiber optic Bragg grating swift field hydrogen sensor based on Pd nanoparticles; and (d) Variation of the intensity response of the sensor in the range of 1%-5% hydrogen concentration^[92]