# Monolithic Double Resonator for Quartz Enhanced Photoacoustic Spectroscopy
## 1 Introduction  
Gas sensing is strongly demanding on sensitive, selective, and efficient techniques, to be implemented on laboratories experiments, environmental, medical, or industrial pur-poses. Since its invention in 2002 [1], Quartz Enhanced Photoacoustic Spectroscopy (QEPAS) demonstrated many advantages among techniques based on tunable sources and indirect gas absorption detection. Photoacoustic spectroscopy relies on light absorption, through a specific gaseous species, when an absorption line corresponds to the emitted wavelength of a source. The source can be a laser or a broader source such as a light emitting diode [2]. While the source intensity is modulated, the light absorption is modulated as well, and gives rise to a localized sound wave at the modulation frequency. This indirect photonic effect is called the photoacoustic effect, which has been exploited for years [3], using resonant acoustic cells and microphones for sound sensing. In QEPAS, the sound wave interacts with a mechanical transducer (a quartz tuning fork or QTF), leading to a piezoelectric current. Thanks to the very high quality factor of the QTF (~10,000 at atmospheric pressure), it is unnecessary to use any resonant acoustic cell, as done in classical microphone photoacoustics sensing. In order to enhance the pressure wave amplitude, some small tubes called micro-resonators (mR) can be added to the optical setup, leading to two main configurations (on-beam/off-beam). In the on-beam configuration [4], the resonator tubes are placed on both sides of the QTF plane, leaving a small air gap (<100 \(\upmu\)m) with the QTF prongs. In the off-beam configuration [5], the resonator tube is located on one side of the QTF, in the center alongside the prongs. On-beam QEPAS exhibits a good signal enhancement factor of around 30 [6, 7], but it requires a stringent optical alignment to avoid photothermal ef-fects, due to direct illumination of the QTF. Standard commercial QTF are not adapted to long wavelength sources such as quantum cascade lasers because of the small gap (0.3 mm) between the prongs, leading to the development of large custom QTF, first for THz sensing [8, 9], and their successful use in several applications. Despite a smaller signal enhancement, the off-beam configuration reduces optical constraints, gives more flexibilityto the setup developments, and can be competitive in terms of sensitivity with on-beam QEPAS. This configuration was studied theoretically [10] and successfully implemented in molecular spectroscopy, for many purposes [11; 12; 13; 14; 15]. The two QTF prongs move symmetrically and are best coupled with a cylindrical acoustic wave, when the longitudinal axis of the wave is located in between the two prongs. Both on-beam and off-beam satisfy this condition--one acoustic source symmetrically acting on the inner surface of both prongs. Alternatively, the QTF can be excited by applying a force on the external surface of both prongs, but it requires two acoustic sources. We rethought the design of the spectrophone to be able to excite the QTF from the outside of the two prongs. A comparable approach was realized at the same time by [PERSON] and [PERSON] [16; 17]. This design was based on a QTF-embedded, double-pass, off-beam spectrophone. Similar to the original off-beam mR [5], the spectrophone is made of two cylindrical mRs with a slit in the center. The QTF is embedded in the mRs, the slit is large enough to partly immerse the QTF prong inside the mR cavity. In this configuration, two acoustic sources (from the two mRs) were employed to excite the external surfaces of the prongs. The novelty of our approach relied on a monolithic double-resonator (double mR) in a T-shape configuration, 3D-printed (stereolithography = SLA) characterized, and modelized, thanks to our experience on single resonator evaluation [15]. In this paper, we pre-sent this new configuration, its design, characterization, and evaluation, using an open-air QEPAS configuration dedicated to ambient water vapor sensing with a 1392 nm DFB laser.