All-optical fiber-based remote sensing system for near infrared absorption of low-level CH4gas

An all-optical remote sensing system utilizing 5-20-km-long, low-loss silica optical fiber links has been developed for the real-time detection of combustible, explosive, and toxic gases in the near infrared region. With this fiber-optic system in conjunction with InGaAsP and InGaAs light emitting diodes (LED\´s) and a remotely located, compact absorption cell, we have realized a detection sensitivity of less than 25 percent of the lower explosion limit (LEL) of CH4density in air, which is required for any practical CH4gas sensor. This detection sensitivity was confirmed up to a 20-km-long silica optical fiber link with the detection time of 3 s in the CH4 band at 1.66 μm and up to nearly 12 km with the detection time of 30 s in the ν2+ band at 1.33 μm. Potential improvement of the sensitivity and detection coverage of the system, as well as its further extensive applications using high power near-infrared LED\´s or laser diodes (LD\´s) are also discussed.