Elliptical-tube off-beam quartz-enhanced photoacoustic spectroscopy
Zhijin Shang, Hongpeng Wu, Shangzhi Li, Frank K. Tittel, Lei Dong
Abstract
We propose an elliptical-tube off-beam quartz-enhanced photoacoustic spectroscopy (EO-QEPAS) method in which an elliptical tube is employed as an acoustic resonator, instead of a circular resonator in QEPAS, to match the stripe-like beam emitted from a high-power multimode laser diode (MLD). A lower noise level than that of conventional QEPAS is achieved due to the optimal matching between the elliptical resonator and the beam profile, hence resulting in a ∼3 times higher signal-to-noise ratio gain factor compared with the circular resonator. The parameters of the elliptical resonator are optimized, and a 1σ normalized noise equivalent absorption coefficient of 3.4 × 10−8 cm−1 W/Hz1/2 is obtained for dry NO2 detection at normal atmospheric pressure. EO-QEPAS paves the way for developing compact, cost-effective, and highly sensitive gas sensors based on the combination of MLDs and QEPAS.