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 CH
4
density in air, which is required for any practical CH
4
gas 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 CH
4
2\nu_{3}
band at 1.66 μm and up to nearly 12 km with the detection time of 30 s in the ν
2
+
2\nu_{3}
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.