Long-distance simultaneous detection of methane and acetylene by using diode lasers coupled with optical fibers

Remote simultaneous detection of methane and acetylene by using two single-mode diode lasers emitting at 1.66 and 1.53 mu m, respectively, coupled with 4-km optical fibers has been demonstrated by laboratory simulations. The lasers were modulated at different frequencies to measure in real time the absorption of the two chemical species without interference. The gases could be detected quantitatively by the ratio of the phase-sensitive-detected fundamental and second-harmonic signals. When the absorption pathlength was 10 cm, the minimum detectable concentrations of methane and acetylene in air were 5 and 3 p.p.m., respectively, for a signal averaging time of 1 s.<>