An optical-fiber-based gas sensor for remote absorption measurement of low-level CH4gas in the near-infrared region

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Abstract
A long-distance low-loss silica optical-fiber link connected to a compact absorption cell was employed to realize a fully optical highly sensitive fiber gas sensor for low-level CH4gas in conjunction with a highly radiant InGaAsP light-emitting diode (LED) at 1.66 μm. By using a dielectric interference filter which replaces a monochromator to simplify the system configuration and operation, a 2-km-long silica fiber link was demonstrated to be capable of achieving reproducibly the detection sensitivity of nearly 700 ppm for CH4gas in air, i.e., 1.3 percent of the lower explosion limit of CH4density. This high sensitivity verifies a potential for major applications to strategic points within the environment, like industrial and mining complexes as well as urban and residential areas. This result also indicates that the purely optical gas-sensor system based on ultralow loss optical-fiber networks, incorporating LED's or laser diodes, can be extensively utilized for the real-time remote measurement and surveillance of a number of dangerous, explosive, and toxic gases in the near-infrared region.