Performance improvement of a near-infrared acetylene sensor system by reducing residual amplitude modulation

Abstract

A near-infrared acetylene (C2H2) sensor was experimentally demonstrated by using a tunable diode laser absorption spectroscopy (TDLAS) technique as well as a second-harmonic wavelength modulation spectroscopy technique. A near-infrared distributed feedback (DFB) laser was used as a light source, and an interference-free absorption line located at the vibration overtone band near 1.53 µm was selected for the detection of C2H2. A self-developed, open-reflective gas sensing probe with a 30 cm path length was adopted as the C2H2 absorption pool. In order to reduce the residual amplitude modulation (RAM) caused by wavelength modulation, a divider pretreatment module was introduced into the traditional dual-channel detection structure. The line shape distortion of the extracted 2f signal was eliminated by the reduction of RAM. Under general laboratory conditions (1 atm, 25 °C), a minimum detection limit (MDL) of 540 ppbv was achieved with an averaging time of 68 s while the MDL without reducing the RAM is up to 1.03 ppmv. A good linear relationship was observed between the amplitude of the 2f signal and the C2H2 concentration within the range of 50–2000 ppm. Long-term measurements were carried out to verify the stability of the system. Using an optical fiber to connect the DFB laser with the probe, the probe can be placed in a faraway field for long-distance, in situ measurement.