## 1 Introduction  
The number of technological fields requiring in-situ and real time gas tracing is rapidly increasing and consequently the need for ad hoc sensing approaches for specific applications [1, 2]. For example, the operating conditions for gas sensors employed in environmental monitoring [3] are very different with respect to sensors implemented in breath analysis systems [4, 5] or integrated into pipelines for measuring natural gas composition from downhole [6, 7]. In fact, one of the most important challenges in gas sensing is the development of versatile sensors and modules capable of detecting different molecular species and operating in a wide range of pressures and temperatures, as well as in harsh environments. Extreme working conditions combined with life-threatening risks, such as intoxication and poisoning of the operators, triggered many remote sensing solutions like the employment of unmanned aerial vehicle (UAVs) [8; 9; 10]. Laser based spectroscopic techniques guarantee a high detection selectivity because of the narrow linewidths characterizing the laser source employed in the visible to the terahertz range, capable of exciting the target molecules while not being absorbed by the gas matrix [11; 12; 13; 14]. The spectral purity of the excitation source, combined with wavelength/amplitude modulation approaches and the efficiency of the optical detection techniques result in reaching detection sensitivity levels as low as parts per quadrillion [15; 16; 17]. Depending on the specific application, these unique characteristics can make laser-based sensors more suitable solutions for remote, fast, selective and sensitive trace gas detections relying on UAVs compared to electrochemical and semiconductor sensors. For instance, a non-dispersive infrared (NDIR) sensor was mounted on a UAV for environmental monitoring and proved to be able to detect CO\({}_{2}\) concentrations of 0.5% in air with a 10 s warm-up time [8]. Similarly, a tunable diode laser absorption spectroscopy (TDLAS) sensor was mounted on a UAV for remote detection of CH\({}_{4}\) clouds and sources localization [18]. Among the laser-based techniques, quartz-enhanced photoacoustic spectroscopy (QEPAS) offers unique advantages, such as the possibility to avoid the use of optical detectors, wavelength-independent detection, compact and robust sensor architectures [19]. The core element of a QEPAS sensor is the spectrophone composed of a quartz tuning fork (QTF) and a pair of micro-resonator tubes. In an on-beam configuration, the QTF is placed between two metallic tubes which are in turn aligned perpendicular to the QTF plane and set at an optimized distance [20]. The laser light is focused between the prongs of the resonator and forced to pass through both tubes without touching them, as otherwise a photothermal background noise affects the sensing performance [21]. In the wavelength modulation approach, the laser light is modulated at half of the resonance frequency of one of the QTF flexural modes and the QEPAS signal is demodulated at the resonance frequency. When the emission wavelength is resonant with an optical transition of the gas target, molecules experience an alternation of optical absorption and non-radiative relaxation processes, giving rise to a pressure wave, i.e., a sound wave. The pressure-induced prongs' vibrations generate a piezo-current proportional to the gas target concentration. The first QTF implemented in a QEPAS sensor was a standard 32.7 kHz QTF designed for timing applications [22]. This QTF has a quality factor of 10,000 in air that increases up to 100,000 in vacuum. Extremely robust and reliable QEPAS sensors implementing standard QTF-based acoustic detection modules (ADM) were demonstrated in literature [23; 24] and tested in real world applications [25], and also exploiting the single-mode beam delivery provided by optical fibers in the mid-IR [26; 27]. Targeted applications were leak detection [28], hydrocarbon detection [29], and environmental monitoring [30; 31].