As a green and renewable energy source, hydrogen (H₂) has caught extensive attention due to its excellent combustion performance and non-polluting characteristics. However, H₂ is a volatile, flammable, and explosive gas that faces risks during its storage, transportation, and utilization. Therefore, the development of online sensors for accurately detecting hydrogen concentration is an effective way to prevent explosive accidents. Fiber-optic hydrogen sensors based on Fabry-Perot (F-P) interferometer have been extensively investigated because of their electromagnetic interference resistance, corrosion resistance, and easy integration. The detection performance of F-P cavity-type fiber-optic hydrogen sensors depends mainly on the characteristics of hydrogen-sensitive films composed of palladium (Pd) and substrate. The results show that the thinner thickness of the Pd leads to faster sensor response, and the thinner thickness of the substrate brings about higher sensor sensitivity. In practice, the thickness of the Pd layer can be controlled by the preparation process, while that of the substrate layer is limited by various factors such as the mechanical strength of the material, and it is difficult to both achieve the desired thickness and ensure the mechanical strength. We propose and fabricate a fiber-optic hydrogen sensor based on Pd-modified hexagonal boron nitride (hBN) films. With the nanoscale thickness and high mechanical strength of hBN, the sensor features high sensitivity, fast response, and excellent repeatability.

We put forward an F-P type fiber-optic hydrogen sensor based on Pd/hBN films. The Pd/hBN film and the fiber end facet act as two partially reflective mirrors, forming a low-fitness flexible F-P interferometer (Fig. 1). When exposed to H₂, the Pd/hBN film adsorbs and dissociates H₂ molecules. Subsequently, hydrogen atoms diffuse into the Pd film to form PdH_x, which results in the expansion of the Pd lattice and then the deformation of the Pd/hBN film (Fig. 2). The ultrathin Pd film promotes the rapid dissociation of H₂ molecules, and the ultrathin hBN film allows the Pd lattice expansion to be effectively converted into Pd/hBN film displacement, which can be easily measured by fiber-optic interferometry. Theoretical analysis and simulation studies show that the thinner thickness of the hBN film and the larger radius of support structure lead to higher sensor sensitivity (Fig. 4). However, the single-layer hBN film is prone to fracture during the transfer process, and finally a multilayer hBN film is employed as the substrate during sensor preparation (Fig. 5). The prepared sensor is small and compact, and the output spectrum has a free spectral range of 6.4 nm and an interference fringe contrast of 16 dB (Fig. 6).

A test platform is built in the laboratory for the test and calibration of the hydrogen sensor (Fig. 7). When the hydrogen volume fraction in the gas chamber is 0.10%, the sensor spectrum shifts toward the short wavelength, and the drift is 0.65 nm after 85 s. During hydrogen desorption, the sensor spectrum shifts toward the long wavelength, and the drift is 0.64 nm after 75 s (Fig. 8). The blue shift and red shift of the sensor spectrum are basically consistent, and the extremely high Young's modulus of hBN equips the Pd/hBN film with good rigidity to ensure that the sensor has no "response memory" problem. The total shift of the dip wavelength near 1537.5 nm is 2.9 nm when the hydrogen volume fraction rises from 0 to 0.50%, the sensitivity obtained by linear fitting is 0.58 pm/10^-6, and the detection limit of the sensor is measured to be 30×10^-6 (Fig. 9). In the three experiments with hydrogen volume fraction of 0.10%, the spectra of the sensor show the same trend, the corresponding blue shifts of the spectra are 0.65 nm, 0.66 nm, and 0.64 nm respectively, and the response time of the sensors is about 60 s for all three experiments (Fig. 10). This is due to the good rigidity and fatigue resistance of the hBN film, which ensures the repeatability of the sensor. The temperature sensitivity of the sensor is 91 pm/°C (Fig. 11), and the sensor preparation can be improved by bonding to reduce the effect of temperature on the sensor. Compared with other hydrogen sensors based on the Fabry-Perot interferometer, our sensor has a high detection sensitivity (Table 1).

A probe type fiber-optic hydrogen sensor with highly sensitive detection characteristics is proposed. The sensor is composed of a flexible Fabry-Perot interferometer with a nanometer-thick hBN as a Pd support film and a single-mode optical fiber. The mechanical and optical properties of the hBN film are discussed, and its significant technical advantages as an F-P interferometer reflective film in mechanical, optical, and hydrogen adsorption/desorption rates are pointed out. The structure of fiber-optic F-P cavity with Pd-modified hBN films as the reflective film is designed and its preparation is studied. Finally, experiments show that the sensor has the detection sensitivity of 0.58 pm/10^-6 in the range of hydrogen volume fraction 0.02%-0.50%, the response time of 60 s for the volume fraction of 0.10% hydrogen, and good repeatability. The compact and corrosion-resistant sensor has potential technical advantages in fields such as hydrogen detection in power transformer oil.