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We report on a fiber-coupled, quartz-enhanced photoacoustic spectroscopy (QEPAS) near-IR sensor for sequential detection of methane (CH 4 or C1) and ethane (C 2 H 6 or 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 CH 4 and C 2 H 6 detection, 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% CH 4 in N 2 and 1% C 2 H 6 in N 2 . 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 CH 4 in standard air has been investigated considering the effects of H 2 O, N 2 and O 2 molecules. No influence on the CH 4 QEPAS signal is expected when the water vapor concentration level present in air varies in the range 0.6-3%.

We report on a study of light-induced thermo-elastic effects occurring in quartz tuning forks (QTFs) when exploited as near-infrared light detectors in a tunable diode laser absorption spectroscopy sensor setup. Our analysis showed that when the residual laser beam transmitted by the absorption cell is focused on the QTF surface area where the maximum strain field occurs, the QTF signal-to-noise ratio (SNR) is proportional to the strain itself and to the QTF accumulation time. The SNR was also evaluated when the pressure surrounding the QTF was lowered from 700 Torr to 5 Torr, resulting in an enhancement factor of ∽4 at the lowest pressure. At 5 torr, the QTF employed as light detector showed an SNR ∽6.5 times higher than that obtained by using a commercially available amplified photodetector.