Fiber-coupled quartz-enhanced photoacoustic spectroscopy system for methane and ethane monitoring in the near-infrared spectral range

Abstract

We report on a fiber-coupled, quartz-enhanced photoacoustic spectroscopy (QEPAS) near-IR sensor for sequential detection of methane (CH4or C1) and ethane (C2H6or C2) in air. With the aim of developing a lightweight, compact, low-power-consumption sensor suitable for unmanned aerial vehicles (UAVs)-empowered environmental monitoring, an all-fiber configuration was designed and realized. Two laser diodes emitting at 1653.7 nm and 1684 nm for CH4and C2H6detection, respectively, were fiber-combined and fiber-coupled to the collimator port of the acoustic detection module. No cross talk between methane and ethane QEPAS signal was observed, and the related peak signals were well resolved. The QEPAS sensor was calibrated using gas samples generated from certified concentrations of 1% CH4in N2and 1% C2H6in N2. At a lock-in integration time of 100 ms, minimum detection limits of 0.76 ppm and 34 ppm for methane and ethane were achieved, respectively. The relaxation rate of CH4in standard air has been investigated considering the effects of H2O, N2and O2molecules. No influence on the CH4QEPAS signal is expected when the water vapor concentration level present in air varies in the range 0.6-3%.