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α-MoO3 has been widely investigated as hydrogen sensing material due to its excellent electrical and optical properties. However, its irreversible structure change during reaction with hydrogen results in an obvious degradation of the sensor’s performance. In this paper, an ion intercalated method is proposed to improve the stability of fiber bragg grating (FBG) hydrogen sensor based on MoO3. Na+ and K+ ions are successfully intercalated into MoO3 nanobelts and the effect of the intercalated ions’ amount to sensing performance has been investigated. Compared to MoO3, ion intercalated MoO3 nanobelts exhibits significant improvement in stability and cycling performance. XRD and TEM analysis reveal that intercalated ions between the interlayers can reinforce the layer structure and inhibit the phase transition. This ion intercalation strategy may provide a new perspective to improve stability of sensors based on layer molybdenum trioxide and dichalcogenides.

Optical fiber hydrogen sensor has been a research issue due to its intrinsic safety and anti-electromagnetic interference in recent years. However, its sensitivity and stability still need to be improved to meet the demand of application. Here we present a novel fiber-optic hydrogen platform with significantly enhanced performance, especially for low hydrogen concentrations. The hydrogen sensing system, which consists of two Bragg gratings paired with high-low reflectivities and employs WO3-Pd2Pt-Pt composite film as sensing element, can detect hydrogen concentration down 20 ppm in air at ambient temperature of 25 °C. And the hydrogen sensor exhibits quick response rate and good repeatbility. Moreover, the sensor has good anti-humidity interference and selectivity during hydrogen response. Our work proposes a new concept to develop hydrogen sensing technology with greatly improved performance, which can significantly promote its potential application in various fields.

A simple, compact and highly sensitive hydrogen sensing device based on a polymer-filled hollow core fiber coated with Pt-loaded WO3/SiO2is demonstrated. The device is composed of a tiny segment of hollow core fiber (HCF) spliced to a standard single mode fiber (SMF) incorporated with a fiber Bragg grating (FBG). The HCF is filled with polymer and the inner air-gap near the interface between SMF and polymer forms a micro-cavity Fabry–Perot interferometer (FPI). With Pt-loaded WO3/SiO2 coating acting as the catalytic layer, hydrogen undergoes an exothermic reaction with oxygen in air and releases heat when the device is exposed to hydrogen, which induces local temperature change of FPI and hence leads to reflection spectrum shift of the proposed device. Here, the FBG is used to compensate the surrounding temperature effect and eliminate the temperature cross-sensitivity of the device. A wavelength shift of over 35 nm is obtained with a 242-μm-long polymer-filled HCF for 4% (vol%) H2 concentration, and the maximum sensitivity of 17.48 nm/% (vol%) H2 is achieved within the range of 0–4.0% (vol%) H2 in air. The sensor device also exhibits fast response time to hydrogen.

Pd–Y alloy has been widely studied for hydrogen separation because of its higher permeability to H than other Pd-based alloys, but the aging behavior in air atmosphere limits its application in fast hydrogen detection. An optical fiber bundle sensor was proposed here to determine the reflective response characteristics of an annealing-restored Pd–Y film. The 30-nm-thick Pd0.92–Y0.08 alloy film was deposited on a fused quartz substrate by co-sputtering and then placed in air aging condition at 20 °C for 1 month. After 200 °C thermal annealing, the response/recovery rate and signal value were monitored in hydrogen concentration ranging from 0.1% to 2% by the optical sensor system, while the microstructure of the film was characterized via atomic force microscopy and X-ray diffraction. The experimental results show that the response properties and crystallinity of the alloy film have achieved a great enhancement after the thermal annealing process, which suggests that H absorption and sensitivity of the film are restored. Attributing to the stress relaxation mechanism, a less than 5% zero drift is achieved under cyclic absorption/desorption of hydrogen. Compared to our previous work, the reflective value represented an approximately linear relationship to the hydrogen concentration. The stability and sensitivity of this annealing-restored film demonstrate the promising of Pd–Y alloy to fabricate economical and sensitive sensor for long term optical detection of hydrogen in low concentration.

An in-line refractive-index sensor is proposed and demonstrated. The sensor is based on Fresnel reflection modulated by Fabry–Perot interference, which consists of a section of multi-mode fiber (MMF) tip coated with porous dielectric multilayer. The measure is conducted by monitoring the shift of the interference fringe, which is correlated to different refractive indices. When the external refractive index (RI) changes from 1.333 to 1.443, the RI sensitivity is 135.5 nm/RIU with good linearity. The miniature size, simple physical structure, small temperature dependence, linear response and ability for batch fabrication make the device attractive for chemical and biological sensing.

A novel method was proposed to enhance sensitivity of fiber Bragg grating (FBG) hydrogen sensor in this paper. As sensing element, Pd/Ni composite film was deposited on side-face of etched fiber Bragg grating and surface of polypropylene substrate. The atomic ratio of Pd and Ni in hydrogen sensitive film was controlled at 91:9. The sensitivity of FBG hydrogen sensor is significantly improved due to the much lower Young's modulus of polypropylene substrate. FBG hydrogen sensor has 146 pm wavelength shift towards 4% hydrogen, and the sensor shows linear response and good repeatability during hydrogen concentration test. FBG hydrogen sensor has good stability even after six months, which demonstrates its great potential in hydrogen leakage monitoring.

Pt-loaded WO 3 coatings with different constitutes were prepared by Sol–Gel method and deposited on fiber Bragg gratings (FBG) with enhanced temperature sensitivity. The composition, structure and micro morphology of Pt-loaded WO 3 coating were investigated by energy dispersive X-ray analysis (EDAX), X-ray diffraction (XRD) and scanning electron microscopy (SEM). Experimental results show that Pt-loaded WO 3 coating has highest sensitivity when the molar ratio of Pt:W is 1:5. At room temperature of 25 °C, FBG coated with highest sensitivity Pt-loaded WO 3 coating has 536 pm wavelength shift toward 10,000 ppm hydrogen, and hydrogen detection threshold of FBG hydrogen sensor can reach to 200 ppm. And ambient humidity has less effect on its performance. FBG hydrogen sensor shows good selectivity (no response to CO and CH 4 ) and repeatability during hydrogen response. The experimental results show FBG hydrogen sensor has continuous hydrogen monitoring ability, and the hydrogen sensor may have great potential for hydrogen's distributed measurement in air.

Pd/Ni composite film was deposited on the side-face of chemically etched fiber Bragg grating as sensing element by magnetron sputtering process. The composition, structure and micro morphology of Pd/Ni composite film were investigated by energy dispersive X-ray analysis (EDAX), X-ray diffraction (XRD) and scanning electron microscopy (SEM). The atomic ratio of Pd:Ni in Pd/Ni composite film is controlled at about 91:9. XRD result proves that the Pd/Ni composite film has good structural stability during hydrogen absorption and desorption process. During the hydrogen response experiment, central wavelength shift of the etched FBG increases linearly with the increase of hydrogen concentration. When the hydrogen concentration is 4% in volume ratio, wavelength shift of the etched FBG is about 60 pm, and the hydrogen sensor is not sensitive to methane. The experimental results show the sensor's hydrogen response is reversible, and the hydrogen sensor may have great potential for hydrogen's distributed measurement.

Fiber-optic hydrogen gas sensor using Pt catalyzed tungsten trioxide (WO3) was proposed and characterized in this paper. Pt-loaded WO3 coatings prepared with sol–gel method were deposited on Bragg fibers. Shift of fiber Bragg wavelength is observed with different hydrogen concentration. Experimental results showed that highest sensitivity existed when the weight ratio of Pt was 6.5% in the Pt-loaded WO3 coating. The response time of the hydrogen sensor was about 10 s.