Tapered acoustic resonator-based quartz-enhanced photoacoustic spectroscopy

In this paper, a quartz-enhanced photoacoustic spectroscopy (QEPAS) sensor based on a novel tapered acoustic resonator (TAR) was proposed for the first time, to our knowledge. The proposed structure not only enhances acoustic signals but also substantially reduces the demands on beam quality and optical alignment, leading to improved system applicability and stability. The structure features a large-opening design at one end, enabling low-loss beam coupling and significantly reducing optical power attenuation. To address the issue of weak signals caused by the high resonant frequency (<i>f</i>₀) of a commercial quartz tuning fork (QTF), a self-designed round-head QTF with a low <i>f</i>₀ of 9712.43 Hz was utilized as the acoustic detector. The TAR matching the <i>f</i>₀ of the QTF was designed through the finite element analysis (FEA) method. The integration of the QTF with the TAR enhances the acoustic signal generated by the photoacoustic effect in QEPAS, thereby amplifying the 2<i>f</i> signal. When the TAR was configured with a top radius of 1 mm, a bottom radius of 11 mm, and a height of 21.1 mm, its <i>f</i>₀ was determined as 9713 Hz, enabling effective acoustic field coupling with the QTF. When compared to the traditional QEPAS, the dual TAR-based methane (CH₄)-QEPAS achieved a 17.70-fold enhancement in signal-to-noise ratio (SNR). Allan deviation analysis of the dual TAR-based QEPAS system revealed that the minimum detection limit (MDL) could be improved to 0.96 ppm with an average time of 400 s.