All-optical remote monitoring of propane gas using a 5-km-long, low-loss optical fiber link and an InGaP light-emitting diode in the 1.68-μm region

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Abstract
We report the fully optical remote detection of low‐level propane (C3H8) gas realized by the scheme based on a long distance, very low‐loss silica optical fiber link connected to a compact absorption cell in conjunction with a high radiant InGaP light‐emitting diode at 1.68 μm. For this application, the near‐infrared absorption spectrum of propane was measured and studied to find very complicated bands around 1.69, 1.53, and 1.38 μm. This simple system, employing a 5‐km‐long silica optical fiber link, was demonstrated to be capable of achieving reproducibly the detection sensitivity less than 2.4 Torr for propane gas in air, i.e., about 14% of the lower explosion limit of propane density. This result verifies a large capability for major applications to various strategic points within the environment, such as industrial complexes as well as urban and residential areas, with considerably increased reliability and safety over the existing techniques.