Single-ended Long Period Fiber Grating coated with Polystyrene Thin Film for Butane Gas Sensing

A safe detection of hydrocarbons is a major concern in industrial and public environment, due to their high volatility and flammability. In this work, we report the development and characterization of an in-fiber sensor for the detection of aliphatic hydrocarbon gas operating at room temperature, focusing the attention on the butane (C4H10). A Long Period Grating (LPG) was used as optical transducer, integrated with a nano-scale high refractive index sensitive overlay of atactic Polystyrene (aPS). Both LPG and aPS thin film were carefully designed to work in mode transition, in order to enhance the final device sensitivity. Moreover, a practical single ended probe was developed by integrating a mirror on the fiber tip. The sensor was tested with butane vapor concentrations up to 1.0 vol%, obtaining a maximum sensitivity of -2.2 nm/vol%, and demonstrating the possibility to detect concentrations as low as one tenth of the Lower Explosive Limit (LEL) of 1.8 vol%.