Graphene-based Fiber-optic Fabry-pérot Interferometric Hydrogen Sensor

Hydrogen is widely used in the fields including aerospace,power,chemical industry and new energy as a high-efficiency clean energy with high combustion efficiency and pollution-free products.However,the hydrogen has a small molecular weight making its leakage difficult to be perceived.When the concentration of hydrogen in the air is in the range of 4%-75%,it will explode when exposed to flames or electrical sparks.Therefore,it is urgent to develop a hydrogen sensor with high sensitivity and fast response.Optical fiber sensors have attracted extensive research interests due to their inherent safety,compact structure,resistance to electromagnetic interference,and remote sensing capability.However,most of the current optical fiber sensors cannot realize both fast and sensitive detection of hydrogen.To address this issue,we proposed a graphene-based fiber-optic Fabry-Pérot interferometric hydrogen sensor.The sensor was fabricated by first cutting a piece of pure silica capillary spliced to the fiber end of a single-mode optical fiber and then transferring the palladium/graphene composite film to the end surface of the capillary.The cleaved capillary on the fiber end and the palladium/graphene composite film formed a Fabry-Pérot interferometer that was sensitive to hydrogen molecules.When the palladium/graphene composite film is exposed to hydrogen,the expansion of the palladium lattice causes the deformation of the composite film and changes the cavity length of the Fabry-Pérot interferometer.The change of the cavity length is reflected as the drift of the reflection spectrum of the interferometer.By detecting the amount of wavelength drift of the interference fringes of the reflection spectrum,the change of the hydrogen concentration can be acquired.The sensor with the graphene film of high mechanical strength and nanoscale thickness,can effectively transfer the hydrogen-induced stress in the palladium into the deflection of the palladium/graphene composite film for high hydrogen sensitivity.As sufficient sensitivity can be obtained using a palladium film with a thickness of only a few nanometers,the thin palladium can also effectively shorten the surface dissociation and diffusion time of hydrogen molecules,thereby improving the response speed.In the experiment,the sensor with a palladium/graphene composite film comprising of 5.6 nm-thick palladium and 3 nm-thick graphene demonstrated a minimum hydrogen detection limit of ? 20 ppm and a short response time of 18 s.The graphene-based fiber-optic Fabry-Pérot interferometric hydrogen sensor in this work combines nanometer-thick palladium/graphene composite films with an optical fiber interferometer to achieve both high sensitivity and fast response.With the advantages of high sensitivity,fast response,compact structure and intrinsic safety,the sensor structure can be exploited for different chemical/gas sensing applications after replacing the palladium with other functional materials in the future.