1. Introduction

It is of great significance to develop trace gas sensing technique in the applications of fire alarm [1], combustion diagnosis [2], life science [3], electrical safety detection [4] and detection of toxic and flammable gases, as well as explosives [5]. Optical gas sensing technique is a widely used method for its advantages of high selectivity and sensitivity, non-invasive and real-time detection [6–8]. Quartz-enhanced photoacoustic spectroscopy (QEPAS) was reported in 2002 for the first time [9]. Based on the photoacoustic effect, which was discovered by A.G. Bell [10], the acoustic wave is excited when the modulated laser passes through the detected gas sample and causes optical absorption. A QTF (quartz tuning fork) is utilized as a transducer in QEPAS [11]. The laser is transmitted perpendicularly to the QTF plane and passed through the gap between QTF’s prongs. The acoustic wave generated by the gas between the prongs will cause QTF’s vibration, which is transformed to piezoelectric charge due to the piezoelectric effect of QTF [12]. The conduction of electric charge is realized by metal coating on the surface of QTF [13]. Due to the small volume of QTF, the gas detection unit of QEPAS is compact. Furthermore, QTF has narrow resonance band of about 2 Hz at atmospheric pressure, which can effectively suppress background noise [14–16]. With a merit of high sensitivity and selectivity, structure compactness and immunity to the ambient noise, QEPAS technique has been widely developed in detection of various trace gases [17–24].

Quartz-enhanced photothermal spectroscopy (QEPTS), another QTF based laser spectroscopy, was reported in 2018 by Ma et al. for the first time [25], which is also called light-induced thermoelastic spectroscopy (LITES) [26,27]. Different from QEPAS, the QTF in QEPTS is used as a photothermal detector. Due to the light-thermo-elastic conversion, laser partially absorbed by QTF is transformed to photothermal energy, which therefore leads to thermoelastic expansion and deformation [25–27]. If laser is modulated, the above thermoelastic expansion can bring mechanical vibration of QTF. In addition, due to the resonant characteristic, the mechanical vibration is resonant enhanced when the modulation frequency matches with the intrinsic frequency of QTF. Based on the piezoelectric effect, the vibration caused by the light-thermo-elastic conversion generates piezoelectric charge on the surface of QTF and is finally converted to piezoelectric signal. QEPTS is identified as a non-contact trace gas detection method since the QTF in QEPTS sensor has no need to be placed inside the detected gas [25], which is useful for trace gas detection under the extreme conditions, such as remote trace gas detection, standoff trace gas sensing and combustion diagnostics. QEPTS is widely studied since its invention [28–31].

Both QEPAS and QEPTS have advantages of low cost, compact size, high dynamic range, and no wavelength dependence. If multiple QTFs are utilized as photoacoustic and photothermal detectors respectively, then the advantages of QEPAS and QEPTS techniques can be combined and the sensor performance can be improved by adding the photoacoustic and photothermal signals. But up to now, for vast majority of trace gas sensors, detection of QEPAS signal and QEPTS signal are independent and the simultaneous detection of the two is lacked.

In this manuscript, a quartz-enhanced photoacoustic-photothermal spectroscopy (QEPA-PTS) technique for trace gas sensing is demonstrated. Two QTFs were employed in this trace gas technique to detect photoacoustic and photothermal signals, respectively. It makes full use of the laser passing through QEPAS system with no attenuation to excite QEPTS signal, realizing highly sensitive trace gas detection. Water vapor (H₂O) was adopted as the analyte to investigate the QEPA-PTS sensor performance.

2. Experimental setup

The experimental schematic diagram of QEPA-PTS is depicted in Fig. 1. H₂O with a volume concentration of 1.01% was selected as the detected gas. As an excitation source, a fiber-coupled, distributed feedback (DFB), continuous wave (CW) diode laser with a wavelength of 1.39 µm and maximum output power of 22.5 mW was employed to target the 1.395 µm absorption line of H₂O. The laser diode operating temperature was 21.6°C. Because of the resonant feature of QTF, in order to obtain a response and minimize external noise of the QEPA-PTS system, wavelength modulation spectroscopy (WMS) based on the tunability and modulation capability of the DFB-CW laser was adopted [32]. The tuning of laser wavelength was realized by changing the loaded current on the diode laser and a modulation was executed by using a sinusoidal wave with high frequency to a direct current ramp with low frequency. The sinusoidal wave was at half of the QTF resonance frequency (f₀/2) to excite photoacoustic signal and photothermal signal, which were resonance enhanced. The direct current ramp with a frequency of 50 mHz was used to scan the entire absorption line of H₂O. In order to pass through the compact experimental system, the collimation of laser beam emitted from the pigtail was achieved by using a fiber collimator (FC). The modulated laser was directly injected to the QEPAS system firstly, where QTF1 with a resonance frequency of 30.72 kHz was used as a photoacoustic detector. As shown in Fig. 1, QTF has dimensions of 6 mm, 0.6 mm and 0.36 mm in height, prong width and thickness, respectively. Although QTF with resonance frequency of 32.768 kHz is used in most of QTF based trace gas sensors, a 30.72 kHz QTF was adopted because it has longer accumulation time compared to 32.768 kHz, which is advantageous for improving signal amplitude. The optimum incident position of laser beam of 0.7 mm from the top of QTF was used to generate the strongest photoacoustic signal [33]. The laser transmitted through the QEPAS system did not have power loss to ensure the strongest photothermal excitation. At a distance of 5 mm from QTF1, a lens L with focal length of 30 mm focused the laser into QEPTS system. QTF2 with a same resonance frequency of 30.72 kHz was employed as a photothermal detector. The overall optical path from FC to QTF2 surface was 40 mm. To generate the maximum photothermal signal, the laser was focused onto the root of the two prongs of QTF2 [26]. Second harmonic (2f) detection was adopted to detect the electrical signals. An adder was used to add the generated photoacoustic and photothermal signals. Finally, a lock-in amplifier was used to demodulate the 2f component. The internal sinusoidal reference signal of the lock-in amplifier was also output as a sinusoidal wave for modulation.

 

Fig. 1. Schematic diagram of QEPA-PTS sensor.

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3. Experimental results and discussions

The resonance frequency of QTF determined the modulation frequency of laser and the reference frequency of lock-in amplifier. To measure the parameters of QTFs, optical excitation method was adopted. In this frequency investigation experiment, output wavelength of laser was adjusted to 1.395 µm, which is the absorption peak of H₂O. Only a sinusoidal wave with variable frequency was utilized as the loaded current on the diode laser to realize the modulation of laser wavelength. At pressure of 760 Torr, the measurements were carried out. Scanning the excitation frequency, the output piezoelectric voltage as a function of input frequency was obtained. The experimental curve was fitted by Lorentzian function to calculate the resonance frequency f₁, f₂, f₃ and quality factors Q₁, Q₂, Q₃ for QTF1, QTF2 and QTF1+QTF2, respectively, where QTF1+QTF2 was the equivalent QTF for QEPA-PTS system. The frequency dependent peak-normalized Lorentzian fit response of different QTFs is shown in Fig. 2. The detailed parameters for different QTFs are listed in Table 1. In detail, resonance frequency f₁=30710.64 Hz, f₂=30710.29 Hz, f₃=30710.55 Hz and bandwidth Δf₁=2.33 Hz, Δf₂=2.01 Hz, Δf₃=2.26 Hz were obtained, respectively. According to the definition of Q = f/Δf, Q₁=13180.53 Hz, Q₂=15278.75 Hz, Q₃=13588.74 Hz were achieved, respectively. The above results showed the slight difference between QTF1 and QTF2 (the difference was less than 0.40 Hz), which was beneficial to the superposition of signals generated from QEPAS and QEPTS. f₃ is the equivalent resonance frequency for the whole system of QEPA-PTS. f₃/2 (30710.55/2 = 15355.28 Hz) was utilized as the modulation frequency in the following experiments.

 

Fig. 2. Normalized frequency response curves for different QTFs.

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Table 1. Measured parameters for different QTFs.

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Signal amplitude was characterized firstly by the dependence of wavelength modulation depth. Both the 2f signal of QTF1 and QTF2 were measured to show the relationship of 2f signal amplitude versus modulation depth, which is shown in Fig. 3. Both QTF1 and QTF2 obtained the strongest 2f signal when the modulation depth was 0.49 cm⁻¹, indicated the optimal modulation depth for the measurement of H₂O at normal temperature and pressure.

 

Fig. 3. The wavelength modulation depth dependence of QTF1 and QTF2.

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The measured 2f signal amplitudes of QTF1, QTF2 and QTF1+QTF2 for QEPAS, QEPTS and QEPA-PTS are shown in Fig. 4. Signal level of 9.34 µV, 88.27 µV, 95.25 µV for sensors using QTF1, QTF2 and QTF1+QTF2 were obtained, respectively. Compared with the measured 2f signal generated by one single QTF1 or QTF2, the superposition of photoacoustic and photothermal signals resulted in the improvement of measured 2f signal for QEPA-PTS system. Specifically, enhancement of 10.2 times to QEPAS and 1.1 times to QEPTS were obtained. However, due to the difference between QTFs, the response for QTF1 and QTF2 cannot be the strongest under the same modulation frequency. So, the theoretical sum (97.61 µV) of signal level of 9.34 µV for QEPAS system and signal level of 88.27 µV for QEPTS system was greater than the measured signal level of 95.25 µV for the QEPA-PTS system. A micro-resonator (mR) such as two tubes aligned perpendicular to QTF plane can significantly improve the acoustic signal, that even a record SNR improvement of 30 times was achieved [11]. However, scheme of mR was not adopted to enhance QEPAS system, because the experiment was to verify the rationality and feasibility of this simultaneous detection, regardless of the signal amplitude. As shown in Fig. 4, in this QEPA-PTS sensor, it can realize the ideal synchronization of the QEPAS and QEPTS signals. This is correct only if the signals generated by both approaches are perfectly in phase with each other, otherwise this assertion would be true only if the R (magnitude) component of the signal were taken, rather than the in-phase component X. Besides, this behavior may not occur with all molecules, especially for slow relaxer, or in every analyzable spectral range.

 

Fig. 4. Measured 2f signal for QTF1, QTF2 and QTF1+QTF2.

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The noise for sensors employing QTF1, QTF2 and QTF1+QTF2 was determined when the laser wavelength was adjusted far away from the absorption peak of water vapor. The measured noise for QEPAS, QEPTS and QEPA-PTS systems are shown in Fig. 5, respectively. By calculating the standard deviation, the obtained noise amplitudes for QTF1, QTF2 and QTF1+QTF2 were 31.81 nV, 66.46 nV, 70.24 nV, respectively. The noise amplitude for QEPTS system was higher than that for QEPAS, which was mainly attributed to the thermal noise, in accordance with QEPA-PTS system.

 

Fig. 5. Noise level determination.

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4. Conclusion

In this paper, a trace gas sensing method of QEPA-PTS technique is proposed. Compared with QEPAS and QEPTS, QEPA-PTS sensor realized the simultaneous detection of photoacoustic and photothermal signals. Two commercially available QTFs were utilized as the detectors for QEPA-PTS. The pair of QTFs was matched in resonance frequency with the difference less than 0.40 Hz, which was beneficial to obtain the strongest response. The laser passed through QTF1 and then injected to QTF2. In this process, the laser transmitted through the QEPAS system did not have power loss to ensure the strongest photothermal excitation. Water vapor with a volume concentration of 1.01% was selected as the analyte gas to investigate the QEPA-PTS sensor performance. Compared with QEPAS and QEPTS systems employing a single QTF, the superposition of QEPAS and QEPTS signals resulted in the enhancement of 2f signal for QEPA-PTS system. The detection performance of QEPA-PTS can be further improved by employing enough QTFs as detectors. In addition, the use of micro-resonator tubes can enhance the photoacoustic signal and the performance of the QEPTS can be drastically improved by increasing the optical pathlength.