Fiber-optic evanescent-wave methane-gas sensor using optical absorption for the 3392-μm line of a He–Ne laser

Gas sensing using evanescent waves of optical fibers is investigated. Methane gas is detected by means of its strong optical absorption of the 3.392-microm line of a He-Ne laser. A single fiber is used as both a sensor and an optical transmission line. The sensor has a small diameter, ranging from 1.8 to 7microm, made by heating and expanding a part of a step-index silica fiber. An evanescent wave of 5 to 40% of the total propagating power is generated outside the fiber. When a sensor fiber of 1.8-microm diameter and 10-mm length is used, the minimum detectable concentration of methane is less than the lowest explosive limit of 5%.