Abstract

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-μm 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 7 μm, 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-μm diameter and 10-mm length is used, the minimum detectable concentration of methane is less than the lowest explosive limit of 5%.

© 1987 Optical Society of America

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