# Monolithic Double Resonator for Quartz Enhanced Photoacoustic Spectroscopy
## 3 Experimental Evaluation  
The QTF was inserted in between the mRs, and the laser beam crossed the two mRs. The QEPAS signal was monitored as a function of the QTF position, in the y and z direction (Figure 6). The signal amplitude directly indicated the double mR-QTF coupling efficiency. In the z direction, the response grew significantly when the top of the prongs arrived in front of the slit and then decreased slowly as the lever became small. The response versus the y direction exhibited a mirror symmetry with the ordinate axis, and a maximum when y = 0, when the prong and the mR slit best overlapped.  
After the optimization of the QTF position, we studied the effect of each individual mR and compared it to the single off-beam configuration (with a single mR). To this end, we first measured the QEPAS signal as a function of the mirrors position. The mirrors were translated in the perpendicular direction with respect to the incoming beam, using a micrometric translation stage, thus changing the value of \(\delta\) (Figure 1a). At first, the laser beam only crossed the mR 1, giving a QEPAS signal maximum of about 20 mV (Figure 7). When the mirror was translated, the beam crossed the mR 2 and the signal rose to 40 mV, doubling the initial value of the signal. The signal presented a plateau over 200 \(\upmu\)m. The results could also be presented in terms of the QEPAS signal versus the laser current, with the typical 1\(f\) signal shape (Figure 7b). We added the signal obtained with the conventional off-beam, whose amplitude was about 30 mV. The amplitude difference between the off-beam and the misaligned monolithic double mR setup was associated with the mR resonance. In the off-beam configuration, the slit was considered almost open while in the double mR configuration, the slit should be considered as almost closed, thus changing the acoustic resonant frequency.  
Figure 5: Frequency response of a QTF in free space (**red**) and positioned inside the double mR (**black**).  
[PERSON] [16,17] made a thorough study on the effect of the length and the radius of the mR. The optimized spectrophone showed a Signal to Noise Ratio (SNR) gain of \(\approx\)20 and \(\approx\)40, without and with the right-angle mirror, respectively, which corresponded to a two-factor increase, in good agreement with the results obtained in this paper.  
We eventually estimated the normalized noise equivalent absorption (NNEA) on a 1f QEPAS signal in the double mR configuration (Figure 8a). We calculated the Allan-Werle deviation (Figure 8b). For the given relative humidity and ambient temperature, the absolute water vapor concentration in ambient air was 9900 ppmv (40% RH, 20 \({}^{\circ}\)C). The long-term drift appeared after a few seconds but the signal was really clean. The limit of detection was 2 ppmv in 1 s, giving an NNEA of 1.2 \(\times\) 10\({}^{-8}\) W\(\cdot\)cm\({}^{-1}\cdot\)Hz\({}^{-0.5}\).  
Figure 6: Evolution of the QEPAS signal when moving the QTF in the z (**a**) and y (**b**) direction. (**c**) Schematic of the mR and the QTF in the defined referential. The QTF is represented in the (0, 0, 0) position.  
Figure 7: (**a**) Effect of the mirror position on the QEPAS signal maximum. (**b**) QEPAS signal versus laser current when the beam is misaligned (**black**) or aligned (**red**) with the mR 2, and comparing it with the results from conventional off-beam (**blue**).  
The NNEA in this absorption range was comparable to a previously published paper [12] on water sensing, with a T-shape off-beam configuration. Even if the double mR 1f signal was clean, the long-term drift appeared sooner than in the QEPAS systems using closed cells. This value might be improved using a more stable configuration, with an enclosed gas cell.