A fiber-optic hydrogen gas sensor with low propagation loss

An optical fiber hydrogen gas sensor, whose clad region constitutes a gas sensing area, was developed. For gas leakage monitoring purposes, sensing devices with wider sensing region are highly desirable. The reported sensor has potential to detect the hydrogen leakage anywhere along the sensing fiber. The sensing principle was based on the absorption of the evanescent-wave interaction in the clad region composed of tungsten trioxide (WO3) and platinum (Pt) layer. This two-layered thin film was fabricated by electron beam (EB) deposition method. The WO3 layer was directly deposited on the quartz glass fiber core, and the Pt layer was deposited on top of the WO3 layer afterward. Hydrogen atoms are easily intercalated into WO3 causing the semitransparent WO3 film to be transformed to blue tungsten bronze. This color change resulted in increase of optical fiber propagation loss. The Pt layer serves as catalyst to stimulate the reaction. Proper thickness of the sensing layer was crucial to the ideal operation. Thicker WO3 layer results in higher sensitivity, with the sacrifice of the increase in propagation loss. The loss increase limits the practical sensing length. The optimal thickness of WO3 layer was determined to be 50 nm, whereas that of Pt catalyst was 5 nm. The propagation loss of this sensor device was lower than that of the device fabricated by sol–gel (SG) method, which was previously developed. The fast response speed and good reproducibility were also observed. The EB deposition method was effective to control optical properties of the sensor device with ease and highly promising technology for realizing distributed sensor devices.