# A Dual-Gas Sensor Using Photoacoustic Spectroscopy Based on a Single Acoustic Resonator
## 1 Introduction  
Multi-gas analysis sensors have attracted considerable attention in recent years due to their significant applications in various fields of industry and science, such as environmental monitoring [1; 2], atmospheric research [3], medical diagnosis [4; 5] and industrial process control [6]. Various types of optical absorption spectroscopy--including photoacoustic spectroscopy [7; 8], tunable diode laser-absorption spectroscopy (TDLAS) [9; 10; 11; 12; 13] (that could also be combined with other hyperspectral imaging methods [14]) and cavity-enhanced absorption spectroscopy (CEAS) [15]--are commonly used for trace gas analysis due to their advantages of noninvasive direct measurement and high sensitivity. Multi-gas analysis using optical absorption spectroscopy requires either a broadband source (such as a light-emitting diode (LED)) [16], or widely tunable laser sources [17; 18] or a combined scheme using multi-laser sources for scanning the different absorption lines of gas species. An LED as a broadband light source is usually used in cavity-enhanced absorption spectroscopy for multi-species detection. [PERSON] et al. reported a transportable incoherent broadband CEAS instrument involving LED from 355 to 375 nm as a light source to realize a continuous measurement of HONO and NO\({}_{2}\) with an LoD of 0.6 and 2 ppbv, respectively [16]. [PERSON] et al. used an external-cavity quantum-cascade laser in a broadband range of 1225-1285 cm\({}^{-1}\) for the simultaneous measurement of CH\({}_{4}\), N\({}_{2}\)O, HDO and H\({}_{2}\)O with an LoD of 2.2 ppbv, 1.43 ppbv, 3.92 ppbv and 1.77 ppmv, respectively, based on wavelength modulation spectroscopy [19]. For coupling different narrow lasers as light sources, [PERSON] et al. reported a multi-gas TDLAS sensor equipped with an optic switch to alternatively measure methane, ethyne, ethene and ethane using four DFBlasers emitting at 1653.72, 1530.37, 1620.04 and 1679.06 nm [20]. For quartz-enhanced photoacoustic spectroscopy (QEPAS), [PERSON] et al. proposed a dual-gas QEPAS sensor based on two distributed feedback (DFB) lasers emitting at 1528.59 and 1368.60 nm as excitation sources, which are focused at two different positions between the quartz tuning fork prongs to excite two resonant modes for realizing simultaneous measurement of C\({}_{2}\)H\({}_{2}\) and H\({}_{2}\)O [21]. However, each technique has its own limitations for field measurement. The CEAS method is based on a high-finesse optical cavity that is composed of two relatively expensive high-reflectivity dielectric mirrors. The effective wavelength band of the CEAS method is limited by the spectrum band of the highly reflective dielectric mirrors [22]. In QEPAS, its measurement accuracy is easily affected by the beam quality [23]. The instrument performance of TDLAS highly depends on the background signal (not a zero-background approach) [24], and its measurement sensitivity is limited by the optical length in the optical cell (typically as a multi-pass cell). Photoacoustic spectroscopy (PAS) using a microphone is a well-established method for trace gas analysis because of its simplicity and wide dynamic range [25]. Photoacoustic spectroscopy relies on the generation of the acoustic wave by converting light energy into thermal energy [26]. The thermal wave variation results in an acoustic signal. An acoustic resonator is usually used to amplify the acoustic signal that is conventionally measured by a microphone [8,27]. Photoacoustic spectroscopy is a zero-background approach [28]. It is suitable for all broadband light sources. For realizing multi-gas analysis, several acoustic resonators or a mid-infrared broadband source are used in the PAS approach. [PERSON] et al. developed a multi-gas PA sensor based on three longitudinal acoustic resonators for the measurement of three trace gases of H\({}_{2}\)O at 1368.6 nm, CH\({}_{4}\) at 1651.0 nm and HCl at 1737.9 nm [7]. [PERSON] et al.