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Abstract
A near-infrared (NIR) sub-ppm level photoacoustic sensor for hydrogen sulfide (H2S) using a differential Helmholtz resonator (DHR) as the photoacoustic cell (PAC) was presented. The core detection system was composed of a NIR diode laser with a center wavelength of 1578.13 nm, an Erbium-doped optical fiber amplifier (EDFA) with an output power of ∼120 mW, and a DHR. Finite element simulation software was used to analyze the influence of the DHR parameters on the resonant frequency and acoustic pressure distribution of the system. Through simulation and comparison, the volume of the DHR was 1/16 that of the conventional H-type PAC for a similar resonant frequency. The performance of the photoacoustic sensor was evaluated after optimizing the DHR structure and modulation frequency. The experimental results showed that the sensor had an excellent linear response to the gas concentration and the minimum detection limit (MDL) for H2S detection in differential mode can reach 460.8 ppb.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Hydrogen sulfide (H2S), is a colorless and flammable gas with high toxicity, primarily generated by volcanic eruptions and human production and living activities such as petroleum refining, landfill and livestock farming [1]. Evidences indicate that excessive exposure to low concentration of H2S can cause tearing of eyes, headache, respiratory tract injury and olfactory problems [2]. Inhalation of high concentration of H2S will cause coma, shock, brain damage and even paralysis of the respiratory nerve center, resulting in suffocation and death [3,4]. According to the regulations of the Occupational Safety and Health Administration (U.S. Department of Labor), the permissible exposure limit for H2S is 20 ppm over eight hours, and the maximal concentration exposure limit is 50 ppm over 10 minutes [5]. In addition to the dangerous effect on the human body, H2S can also lead to corrosion of pipelines and equipment, as well as air pollution [6]. Hence, there is an urgent need to develop a sensitive and efficient gas sensor for the detection of H2S.
Up to now, a variety of H2S trace gas sensors have been proposed based on different detection techniques, mainly including electrochemical sensors, semiconductor sensors, chromatography methods, colorimetric sensors and fluorescence methods [7–11]. However, most of these chemical-based sensors show cross sensitivity problem and minimal shell life [12]; gas chromatography requires expensive instruments and complex operations [13]. In addition, these sensors are unable to implement real-time monitoring [14]. Recently, trace gas sensors based on photoacoustic spectroscopy (PAS) make up for the deficiencies of these sensors and have received extensive attention due to their high detection sensitivity, selectivity, and real-time monitoring [13,15,16]. Thus, it is of great importance to develop photoacoustic sensors for sub-ppm level H2S detection.
PAS is a technique to detect the concentration of gas by analyzing the acoustic signal generated by gas absorbing light energy [17,18]. The detection system is mainly composed of light source, photoacoustic cell (PAC), acoustic sensor and signal processing module [19]. Among them, PAC is one of the crucial parts to determine the performance of the detection system. Until now, the structure of PAC has been extensively researched. The working mode of PAC can be divided into non-resonant mode and resonant mode. The sound pressure in the non-resonant PAC is uniform everywhere, and there is no amplification effect on the photoacoustic signal [20,21]. Resonant PAC can amplify the photoacoustic signal, which is extremely favorable for the enhancement of weak photoacoustic signals. As a result, resonant PAC is more widely used. Guo et al. presented a multi-mechanism collaboration enhancement photoacoustic spectroscopy analyzer (MCEPA) using a resonant PAC in SF6 background to realize acoustic resonance enhancement, obtaining a minimum detection limit (MDL) of 0.15 ppm for H2S when the integration time was 1 s [15]; Zhang et al. used a multi-pass PAC and achieved a MDL of 0.68 ppm for H2S with 10 s average time [22]; Chen et al. proposed a highly sensitive photoacoustic gas sensor based on fiber-optic Fabry-Perot (F-P) cavity, and obtained a MDL of 197.2 ppb for H2S in SF6 background with 1 s integration time [23]. However, the aforementioned PACs are all H-type PACs, which generally have large volume and require long ventilation time. They are not conducive to the implementation of miniaturization of the PAS gas detection system.
To address this problem, a differential Helmholtz resonator (DHR) is considered as a potential solution. PAS using DHR has been reported previously, for example Zeninari et al. employed a simple DHR structure in the PAS system for measuring flow and the photoacoustic signal was increased by a factor of 2 [24]. Parvitte et al. used a DHR combined with a near-infrared (NIR) diode laser in the PAS system for methane detection [25]. Alahmari et al. reported photoacoustic trace gas detection based on a DHR and achieved a noise equivalent detection limit of 22 ppmv for H2S under 1 bar nitrogen (N2) background with 1 s integration time [26]. Except for the merit of small volume, the DHR has the advantage of photoacoustic signal enhancement and is suitable for use in conjunction with divergent light sources, making it desirable to investigate [27].
In this paper, a DHR consisting of two resonant cavities and two capillaries was proposed to realize sensitive and stable photoacoustic H2S gas sensing. The commercial simulation software COMSOL Multiphysics was used to model and optimize the DHR. The influence of the parameters of the DHR on the resonant frequency and acoustic pressure distribution was simulated and analyzed. Then, the optimal structure of the DHR was finally determined. An H-type PAC with a similar resonant frequency was also modeled and simulated to compare the difference in performance between these two types of PACs. Finally, the experiment was implemented to analyze the performance of the designed differential Helmholtz-based H2S photoacoustic sensor, and a MDL of 460.8 ppb has been obtained for H2S detection.
2. Basic theory
PAS gas detection technology is based on the photoacoustic effect. The target gas molecules absorb the energy of the periodically modulated laser source, causing temperature and pressure change, thus exciting the sound wave [28,29]. A sensitive microphone can detect the acoustic signal and convert it into an electric signal. The detected photoacoustic signal S can be expressed as [30]:
where $\gamma $ is the adiabatic index of the measured gas, L is the length of the laser path through the gas, Q is the quality factor of the PAC, f is the modulation frequency, ${V_\textrm{c}}$ is the volume of the PAC, ${S_m}$ is the microphone sensitivity, ${P_\textrm{0}}$ is the excited laser power, and $\alpha $ is the gas absorption coefficient. In this paper, a DHR will be designed as the PAC to implement H2S gas detection.The major advantage of the Helmholtz resonator in photoacoustic measurement is the small cavity volume with a low resonant frequency [31,32]. Therefore, Helmholtz resonator with short ventilation time can make it possible to achieve miniaturization of PAC. A DHR has been designed for gas sensing, and its structure is shown in Fig. 1. It consists of two resonant cavities of length Lc and radius Rc and two capillaries of length Ln and radius Rn. The two gas vents in the center of the capillaries are the gas inlet and outlet, respectively. There are two SiO2 windows at both ends of two resonant cavities to maximize the NIR light through the excited cell and to ensure the sealing performance of the DHR. Unlike the H-type PAC, the sound amplification of the DHR is obtained by the oscillation of the gas volume in the channel. The gas filled in the capillaries periodically excites the compression of gas in one resonant cavity and the expansion of gas in the other, causing the two chambers to produce acoustic signals with opposite phase [33]. Two microphones are symmetrically embedded in the center of each resonant cavity to measure the signals from the two resonant cavities.
3. Simulation
Accurately understanding the acoustic pressure distribution in the PAC is particularly important for improving the performance of the PAS gas detection system. In this section, based on the finite element method, the commercial simulation software COMSOL Multiphysics was used to analyze the resonant frequency and acoustic pressure distribution of the DHR. The simulation model was built based on pressure acoustics and frequency domain physics. The gas in the DHR was pure N2, with a density value of 1.25 kg/m3 provided by the material library of the simulation software.
In order to design a suitable DHR, the influence of the parameters of the DHR (Rc, Lc and Ln) on the resonant frequency and acoustic pressure distribution of the system was simulated. The acoustic pressure at the center point of the resonant cavity was selected for analysis. The capillary radius Rn is usually relatively small, otherwise it would not be possible to stimulate the Helmholtz resonance mode [33]. In the following simulation, Rn was set to 3 mm. Figure 2(a) plots the resonant frequency and acoustic pressure for different Rc (Lc is 80 mm and Ln is 70 mm). The simulation results indicate that as Rc increases, both the resonant frequency and the acoustic pressure gradually decrease. The influence of Lc (Rc is 5 mm and Ln is 70 mm) and Ln (Rc is 5 mm and Lc is 80 mm) on the resonant frequency and acoustic pressure is also analyzed, and the results are shown in Fig. 2(b) and Fig. 2(c), respectively. From the results, it can be seen that as Lc increases, the resonant frequency decreases and the acoustic pressure increases correspondingly, and meanwhile, with Ln increasing, the resonant frequency decreases and the acoustic pressure increases slightly.
Fig. 2. (a) The simulation results of Rc in the range of 3 mm to 10 mm, 1 mm step; (b) The simulation results of Lc in the range of 50 mm to 100 mm, 10 mm step; (c) The simulation results of Ln in the range of 30 mm to 80 mm, 10 mm step.
If Rc is too small, the laser will easily irradiate the inner wall of the resonant cavity, resulting in large background noise. Therefore, Rc =5 mm was selected for the following simulation. Figure 3 plots the resonant frequency and acoustic pressure with different Lc and Ln when Rc = 5 mm. As shown in Fig. 3(b), the acoustic pressure increases with Lc and Ln becoming longer. However, the volume of the DHR increases and the resonant frequency of the DHR decreases accordingly, as Lc and Ln get longer. Considering the volume of the PAC and the 1/f noise, a DHR with Rc of 5 mm, Lc of 80 mm, Rn of 3 mm and Ln of 70 mm is selected. The frequency sweep results are shown in Fig. 4(a). The corresponding resonant frequency of the DHR is about 800 Hz and the volume is about 17 mL. In addition, it can be seen in Fig. 4(a) that the acoustic pressure at the gas inlet and outlet of the DHR was the smallest, which could avoid the influence of the gas flow noise to a certain extent. An H-type PAC with a similar resonant frequency was also modeled and simulated. The structure and frequency sweep results of the first-order longitudinal mode of the H-type PAC are shown in Fig. 4(b). The results show that the resonant frequency occurs at about 802 Hz and the volume is approximately 280 mL. Therefore, for a similar resonant frequency, the volume of the H-type PAC is 16 times larger than that of the DHR. Thus, the DHR-based PAS system has a much smaller volume and shorter ventilation time than the H-type PAC. The frequency and phase response of cell I and cell II of the DHR are shown in Fig. 5(a) and Fig. 5(b), respectively. It can be seen that the acoustic pressure amplitudes of the two cells are the same, whereas the phase difference is about 180°.
Fig. 3. (a) The relationship between resonant frequency and Lc; (b) The relationship between acoustic pressure and Lc.
Fig. 4. (a) The results of frequency sweep of the DHR. The resonant frequency occurs at about 800 Hz; (b) The results of frequency sweep of the H-type PAC. The resonant frequency occurs at about 802 Hz.
4. Detection system
4.1 Experimental setup of detection system
As shown in Eq. (1), the photoacoustic signal S is directly proportional to the gas absorption coefficient α. Therefore, to obtain a strong photoacoustic signal, it is essential to select a strong absorption line. Meanwhile, the effect of other unavoidable background gases in the air should also be considered. According to the HITRAN database, the absorption spectrum of H2S, H2O, and CO2 gas at 1 atm and 296 K is shown in Fig. 6(a). To avoid interference from other background gases, the absorption line at 1578.13 nm was selected to detect the concentration of H2S in the following experiments.
Fig. 6. (a) The absorption coefficients of several gases based on the HITRAN database; (b) Experimental measurement setup. MFC: Mass Flow Controller. DAQ: Data Acquisition.
As shown in Fig. 6(b), the DHR-based H2S detection experiment system primarily included a NIR distributed feedback (DFB) laser, a DHR, a valve system, and a signal detection and processing unit. The designed DHR was manufactured using 3D printing technology with resin material. To improve the PAS system performance, the second harmonic wavelength modulation spectroscopy (2f-WMS) was employed in the experiments [34]. The DFB laser wavelength was modulated by a superimposed signal consisting of a ramp wave and a sinusoidal wave, both provided by a digital arbitrary waveform generator (Fluke, 294). The superimposed modulation signal was then fed into a current source (ILX Lightwave, LDX-3232) to drive the DFB laser. A temperature controller was attached to the DFB laser to avoid the central wavelength drift caused by temperature change. The DFB laser's operating current and temperature were set to 79.6 mA and 29.5°C, respectively, to ensure the center wavelength of 1578.13 nm. The output wavelength of the modulated laser ranges from 1578.01 nm to 1578.31 nm. The laser power was amplified to ∼120 mW by an erbium-doped optical fiber amplifier (Micro Photons, EDFA-L-B-30-1) in order to obtain a strong photoacoustic signal. Two microphones (BSWA, MPA416) were embedded in the DHR to detect the photoacoustic signal generated in the monitor cell and the reference signal generated in the excited cell. Finally, the signals detected by the microphones were sent to the lock-in amplifier (Stanford Research Systems, SR830) for second harmonic (2f) demodulation.
4.2 Experimental results and discussion
The resonant frequency is an important parameter of the PAC system, which is determined by the DHR. Due to the actual machining error, the structure dimension inconsistency exists between the designed model and the customized model of the DHR. Therefore, there is a deviation between the theoretical resonant frequency and the measured resonant frequency of the DHR. So, it is necessary to measure the resonant frequency of the DHR. During the experiment, the 300 ppm H2S/N2 gas mixture was injected into the detection system through the valve system to measure the photoacoustic signals at different modulation frequencies. The experimental results are shown in Fig. 7(a). It can be seen that the amplitude of the photoacoustic signal increases sharply as the modulation frequency approaches half of the resonant frequency of the DHR. The amplitude of the detected 2f signal is largest when the modulation frequency is 372 Hz. Thus, the actual resonant frequency of the customized DHR is 744 Hz. The quality factor Q of the DHR is calculated to be ∼27 by Q = f /Δf, where Δf is the full-width at half-maximum (FWHM) of the measured signal response curve plotted in Fig. 7(a). The existing machining error causes the difference between the simulated resonant frequency and the measured frequency of the DHR, and it is inevitable. According to our experiences and analysis, the maximum machining error of the DHR is 0.5 mm and the radius of the capillary Rn has the largest effect on the resonant frequency. Therefore, the resonant frequency of the DHR with Rn (which was set to 3 mm in the previous simulation) in the range of 2.5 mm∼3.5 mm was analyzed. The results plotted in Fig. 7(b) show that the resonant frequency of the DHR is 680 Hz∼914 Hz. So, the measured resonant frequency of the DHR is within the error range.
Fig. 7. (a) The signal response curve of the DHR obtained by experiment; (b) The resonant frequency of the DHR with Rn in the range of 2.5 mm∼3.5 mm.
The modulation depth can also affect the intensity of the photoacoustic signal, so optimizing the modulation depth is important [35]. The concentration of the H2S/N2 gas mixture used in this experiment was 300 ppm. The peak-to-peak values of the 2f signal for different modulation currents ranging from 8 mA to 19 mA were measured, as shown in Fig. 8. The maximum amplitude of the 2f signal appears when the modulation current is ∼17 mA. Thus, the optimal modulation current was set at 17 mA in the following experiments.
The PAS system was calibrated using different concentrations of H2S/N2 gas mixtures. Standard cylinders of pure N2 and 300 ppm H2S/N2 were connected to a two-channel valve system to generate H2S/N2 sample gas of different concentrations. The total gas flow rate was set to 200 sccm for gas exchange time reduction and flow noise suppression. The modulation frequency of the DFB laser was set at 372 Hz to measure the photoacoustic signals in the concentration range from 22.5 ppm to 90 ppm. Figure 9 depicts the relationships between the measured 2f signal amplitudes and H2S concentrations. The amplitudes of the 2f signal generated in cell I, cell II and differential mode at different concentrations are plotted in Figs. 9(a), 9(b) and 9(c), respectively. The results indicate that the system has an excellent linear response to the H2S concentration levels.
Fig. 9. (a) Amplitudes of the photoacoustic signal of cell I at different concentrations of H2S/N2 gas mixture; (b) Amplitudes of the reference signal of cell II at different concentrations of H2S/N2 gas mixture; (c) Amplitudes of the differential signal at different concentrations of H2S/N2 gas mixture.
Figure 10(a) shows the measured 2f signals of 90 ppm H2S/N2 of cell I, cell II and differential mode. The background noise of cell I, cell II and differential mode are plotted in Fig. 10(b). The noise is determined when the laser wavelength is in the non-absorbing portion of the 2f signal spectrum. As shown in Table 1, the 1σ standard deviation of the noise and the signal-to-noise ratio (SNR) gain factor of cell I, cell II and differential mode are calculated. According to Table 1, the 1σ standard deviation of the noise of cell I is found to be 5.3 mV and the SNR is 59.8, resulting in a MDL of 1505.7 ppb for H2S detection. In cell II, the 1σ standard deviation of the noise is 3.7 mV and the SNR is 132.9, thus the MDL for H2S detection is calculated to be 677.0 ppb. The system performance parameters of the differential mode are also listed in Table 1, the 1σ standard deviation of the noise and the SNR are 4.1 mV and 195.3, respectively. Therefore, the MDL for H2S detection of the differential mode is 460.8 ppb. The results show that the system performance of the differential mode is better than that of the non-differential mode. This occurs because the signals generated in cell I and cell II are in opposite phase whereas the incoherent noises are in the same phase, allowing the DHR to enhance the signal and attenuate the incoherent noise at the same time [36]. The measured 2f signals of the differential mode at different concentrations are shown in Fig. 11.
Fig. 10. (a) The 2f signal of cell I, cell II and the differential mode; (b) The noise of cell I, cell II and the differential mode.
Table 1. Comparison of system performance parameters in different mode
Table 2 lists the performance of the system in comparison to other NIR PAS-based H2S sensors that have been developed. It can be seen that the DHR usually has a smaller volume with a low resonant frequency compared to the H-type PAC. Thanks to the detailed performance simulation of different DHR sizes, we selected the optimal structure of the DHR, which not only ensured the small volume of the PAC, but also had an appropriate resonant frequency to avoid the interference of 1/f noise. Experimental results show that the sensor presented in this paper makes it possible to realize the miniaturization of the PAS system and can also achieve the sub-ppm level H2S detection.
Table 2. System performance comparison with other developed NIR PAS-based H2S sensors
5. Conclusion
In this work, a NIR H2S photoacoustic sensor using a DHR as the PAC was investigated and demonstrated. The influences of the DHR parameters on the resonant frequency and acoustic pressure distribution were simulated and analyzed using the finite element simulation software COMSOL Multiphysics. Considering the volume of the PAC and the 1/f noise, the optimal structural size of the DHR was finally determined to be 5 mm in the resonant cavity radius, 80 mm in the resonant cavity length, 3 mm in the capillary radius and 70 mm in the capillary length. An H-type PAC with a similar resonant frequency was also modeled and simulated to compare the difference in performance between these two kinds of PACs. The simulation results show that the volume of the conventional H-type PAC was 16 times larger than that of the DHR for a similar resonant frequency. Finally, experiment was implemented to analyze the performance of the designed differential Helmholtz-based H2S photoacoustic sensor. The experimental results demonstrate that the sensor has an excellent linear response to the gas concentration and the MDL for H2S detection in differential mode can reach 460.8 ppb. The results in this report can provide a feasible route to realize the miniaturization of photoacoustic sensors.
Funding
National Natural Science Foundation of China (62005247, 62271451); National Key Scientific Instrument and Equipment Development Projects of China (62027816); Henan Provincial Key Science and Technology Research Project (162102210018); Zhengzhou Collaborative Innovation Major Project (18XTZX12008); Henan Provincial Science and Technology Research Project (222102210163).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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