With the unprecedented development of green and renewable energy sources, the proportion of clean hydrogen (H₂) applications grows rapidly. Since H₂ has physicochemical properties of being highly permeable and combustible, high-performance H₂ sensors to detect and monitor hydrogen concentration are essential. This review discusses a variety of fiber-optic-based H₂ sensor technologies since the year 1984, including: interferometer technology, fiber grating technology, surface plasma resonance (SPR) technology, micro lens technology, evanescent field technology, integrated optical waveguide technology, direct transmission/reflection detection technology, etc. These technologies have been evolving from simply pursuing high sensitivity and low detection limits (LDL) to focusing on multiple performance parameters to match various application demands, such as: high temperature resistance, fast response speed, fast recovery speed, large concentration range, low cross sensitivity, excellent long-term stability, etc. On the basis of palladium (Pd)-sensitive material, alloy metals, catalysts, or nanoparticles are proposed to improve the performance of fiber-optic-based H₂ sensors, including gold (Au), silver (Ag), platinum (Pt), zinc oxide (ZnO), titanium oxide (TiO₂), tungsten oxide (WO₃), Mg₇₀Ti₃₀, polydimethylsiloxane (PDMS), graphene oxide (GO), etc. Various microstructure processes of the side and end of optical fiber H₂ sensors are also discussed in this review.

A high-sensitivity optical fiber magnetic field sensor based on a multi-Fabry-Perot interference (F-P) cavity in an etched multimode optical fiber (MMF) was proposed. The MMF was etched along the fiber axis and a hole with the length of about 250 µm formed in the MMF. The multi-F-P cavity in the MMF is a sandwich structure, which is composed of UV glue, magnetic fluid and UV glue. The refractive index and effective cavity length of the magnetic fluid cavity change with the changing of the external magnetic field, which will result in changes of the reflection spectra of the multi-F-P. Thus, the external magnetic field could be detected by the changes of spectra. Experimental results showed that the high magnetic field sensitivity of 299.7 pm/mT and 0.164 dB/mT were obtained in the range of 0∼8 mT weak magnetic induction intensity by using the wavelength and intensity demodulations, respectively. The proposed sensor shows the potential applications in the magnetic field measurement in the weak magnetic environment.

Highly sensitive reflective-type fiber hydrogen sensor based on an enlarged taper cascaded with a tilted fiber grating (TFBG) was proposed. Hydrogen-sensitive Au-Pd nanofilm was coated on the TFBG surface by magnetron sputtering means. By cascading an enlarged taper upstream to the TFBG, cladding modes and Ghost mode of the TFBG could be coupled into the core to form a reflective-type TFBG-based hydrogen sensor. As the hydrogen was absorbed by the Au-Pd nanofilm, the effective refractive index of the cladding of the TFBG will change accordingly, so as to the reflective spectra of the TFBG. The hydrogen concentration was obtained through the intensity changing of the ghost mode in the reflective spectra of TFBG. The proposed sensor showed a high sensitivity of 4.83 dB/% with a calculated LOD of 0.07% for the hydrogen concentration range from 0% to 0.7% at room temperature with the rapid response time of 26 s and recovery time of 40 s, respectively.