Quantitative measurement of acetylene by using external-cavity tunable diode laser

An external-cavity tunable diode laser operating at near-IR region is used to detect acetylene with laser absorption spectroscopy. Quantitative measurement of gas concentration is obtained by applying low-frequency wavelength modulation to the tunable laser and by phase-sensitive-detection of the fundamental and second-harmonic signals using a lock-in amplifier. A single-sensor and a time-division multiplexed two-sensor system have been implemented and experimentally tested. The sensor systems use single path 25 mm absorption cells and have demonstrated sensitivity of 75 ppm for the single sensor system and 182 ppm for the multiplexed two- sensor system.