In terms of hydrogen sensing and detection, optical fiber hydrogen sensors have been a research issue due to their intrinsic safety and good anti-electromagnetic interference. Among these sensors, hydrogen sensors consisting of fiber grating coated with sensitive materials have attracted intensive research interests due to their good reliability and distributed measurements. This review paper mainly focuses on optical fiber hydrogen sensors associated with fiber gratings and various materials. Their configurations and sensing performances proposed by different groups worldwide are reviewed, compared and discussed in this paper. Meanwhile, the challenges for fiber grating hydrogen sensors are also addressed.

In this paper, WO₃-Pd₂Pt-Pt nanocomposite films were deposited on a single mode fiber as the hydrogen sensing material, which changes its reflectivity under different hydrogen concentration. The reflectivity variation was probed and converted to an electric signal by a pair of balanced InGaAs photoelectric detectors. In addition, the performance of the WO₃-Pd₂Pt-Pt composite film was investigated under different optical powers, and the irrigating power was optimized at 5 mW. With the irrigation of this optical power, the hydrogen sensitive film exhibits quick response toward 100 ppm hydrogen in air atmosphere at a room temperature of 25 °C. The experimental results demonstrate a high resolution at 5 parts per million (ppm) within a wide range from 100 to 5000 ppm in air. This simple and compact sensing system can detect hydrogen concentrations far below the explosion limit and provide early alert for hydrogen leakage, showing great potential in hydrogen-related applications.

Novel fiber Bragg grating (FBG) relative humidity (RH) sensors based on polyimide (PI) doped with graphene quantum dots (GQDs) are proposed and experimentally demonstrated. The effects of GQDs doping on the sensor performance were investigated. The sensitivity of the humidity sensor could be increased by 96.36% when the doping mass fraction of GQDs was optimized at 0.2%. The FBG humidity sensor based on PI-doped GQDs exhibited good linearity at different operating temperatures in the RH range of 10–90%. Moreover, the excellent linearity of the temperature sensitivity facilitates the temperature compensation of these sensors. GQDs-doped PI is an excellent moisture-sensitive material that significantly improves the sensitivity of FBG humidity sensors, making it a promising candidate for humidity sensing in various applications.

One of the technological challenges for hydrogen sensors is long-term stability and reliability. In this article, the UV-light irradiation was introduced into the hydrogen sensing process based on water photolysis effect of Pt/WO₃. Ascribing to that, fiber optic hydrogen sensor with Pt/WO₃ nanosheets as the sensing element was demonstrated with significantly improved performance of stability. Under UV irradiation, the hydrogen sensor exhibits higher sensitivity and resolution together with a smaller error range than that without UV irradiation. The enhanced performance could be attributed to the effective decomposition of water produced in the hydrogen sensing process due to the water photolysis effect of Pt/WO₃. The influence of the water on stability was evaluated using experimental results, and the UV irradiation to remove water was analysed by theoretical and FT-IR spectra. This work provides new strategy of UV-light irradiation to promote the long-term stability of hydrogen sensor using Pt/WO₃ as the sensing element.

A compact fiber optic hydrogen sensing system employing self-referenced configuration and controllable light heating technologies, is proposed and experimentally demonstrated with ultra-high sensitivity and wide dynamic range of hydrogen concentration detection. This sensing probe, consisting of nano WO ₃ -Pd ₂ Pt-Pt composite films and single mode fiber inscribed with two Bragg gratings, is principally based on the gasochromic effect of sensitive films. Signal-to-noise ratio of this system can be significantly enhanced with peak intensity of high reflective fiber Bragg grating (I ₁ , FBG1 written in flat wavelength range of light source) and background intensity (I ₂ , at a spectral distance of 1nm on the red side of FBG1) as reference intensity and sensing intensity respectively. A 980 nm laser and proportion integration differentiation (PID) controller were employed to ensure the hydrogen sensitive film working at an optimized temperature of 80 °C. Hydrogen concentration detection from 0.5 to 12000 ppm in air was achieved by utilizing this sensing system. Experimental results demonstrate the resolution of this hydrogen sensing system can reach 0.2 ppm within range of 1 to 1000 ppm hydrogen exposure. Moreover, this sensor can present a quick response time of less than one second towards 4000 ppm hydrogen in air. Furthermore, the stability of this sensor can be greatly enhanced with controllable optical heating system. The approach proposed in this work can greatly improve the performance of hydrogen sensor, which is very beneficial to its potential application in hydrogen-related facilities.

Quickly detecting hydrogen leakage is crucial to provide early warning for taking emergency measures to avoid personnel casualties and explosion accidents in hydrogen energy fields. Here, a compact optical fiber hydrogen sensing system with high sensitivity and quick response rate is proposed in this work. A laser diode (LD) and two photodetectors (PD) are employed as light source and optical signal transformation devices, respectively. This sensing system employs single-mode optical fiber deposited with WO₃-PdPt-Pt nanocomposite film system as sensing element. Under irrigating power of 6 mW, the sensing probe exhibits an ultra-fast response to hydrogen concentrations of 4000 ppm and 10,000 ppm, with response times of 0.44 s and 0.34 s, respectively. In addition, detection limit of 3 ppm can be achieved by using this sensing system. The sensor also shows good repeatability during hydrogen exposure of 3~10,000 ppm, demonstrating its great potential application for hydrogen leakage in hydrogen energy facilities.