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Abstract
We report, what we believe to be, a novel miniaturized 3D-printed Y-type resonant photoacoustic cell (YRPAC) consisting of a frustum of cone-type buffer chamber and a cylindrical resonant chamber. The volume of the designed YRPAC is about 7.0 cm3, which is only about a half of the T-resonant photoacoustic cell (TRPAC). The finite element simulation of the sound field distribution of the TRPAC and YRPAC based on COMSOL shows that the photoacoustic signal is enhanced with the shape of the buffer chamber changing from the traditional cylinder to a frustum of cone. The photoacoustic spectroscopy (PAS) system, utilizing the YRPAC and TRPAC as the photoacoustic reaction units, a 1653.7 nm distributed feedback (DFB) laser as the excitation light source, a cantilever beam acoustic sensor as the acoustic sensing unit, and a high-speed spectrometer as the demodulation unit, has been successfully developed for high-sensitivity trace CH4 sensing. When the CH4 concentration is 1000 ppm, the 2f signal of YRPAC in the first-order resonance mode is 2.3 nm, which is 1.7 times higher than the 2f signal amplitude of TRPAC. The detection sensitivity and minimum detection limit for the PAS system are 2.29 pm/ppm and 52.8 parts per billion (ppb) at 100 s of averaging time. The reported YRPAC has higher sensitivity, smaller size, and faster response time compared to the conventional TRPAC, which can provide a new solution for PAS development.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Trace gas sensing plays an integral role in many real-world scenarios, e.g., coal mines, medical diagnostics, and environmental pollution [1–6]. Conventional methods for trace gas detection include electro-chemistry [7], semiconductor [8], and catalyst combustion [9]. These gas detection methods with one or more drawbacks such as low sensitivity and accuracy, high cost, and system complexity have prevented them from being widely used in many practical scenarios. The photoacoustic spectroscopy (PAS) based on absorption spectroscopy technology, offering ultra-fast response, ultra-high sensitivity as well as strong gas selectivity, has become a research hotspot for gas detection in recent years [10–17]. Typically, a classical PAS experimental setup consists of a light source for excitation light generation, a photoacoustic cell (PAC) for the photoacoustic effects site, and an acoustic sensor for photoacoustic signal detection [18]. In recent years, to improve the sensitivity of PAS systems, researchers have employed highly sensitive microphones, e.g., quartz tuning fork microphones [19–24] and cantilever beam microphones [25–30], instead of traditional condenser microphones, for PAS gas monitoring. In addition, the performance of the PAC, which is the reaction site of the photoacoustic effect, is vital to the measurement sensitivity in PAS systems. Usually, PACs are classified into resonant and non-resonant PACs. The resonant PAC is widely used in PAS gas detection because of its high sensitivity, good signal-to-noise ratio (SNR), low detection limit, and strong noise immunity compared with the non-resonant PAC [31]. Therefore, designing and optimizing the structure of the resonant PAC to improve the sensitivity of the photoacoustic system has received extensive attentions. Yin et al. proposed a dual-channel differential structure H-type resonant PAC (HRPAC), and combined it with two condenser microphones for atmospheric carbon dioxide monitoring. The minimum detection limit (MDL) was 54 parts per trillion (ppt) [32]. To further enhance the sensitivity of the PAS, Zhao et al. reported a differential multiple reflection resonant PAS technique [33], which realized the dual enhancement effect of excitation and detection of the photoacoustic signal. The experimental results showed that the MDL of the PAS for methane (CH4) gas reached 0.6 parts per billion (ppb), and the calculated normalized noise equivalent absorption (NNEA) coefficient was 1.79 × 10−10 cm−1·W·Hz−1/2. Although the MDLs of these optimized HRPACs based PAS system were on the order of ppb or even ppt, they also had several drawbacks, such as large size and slow response speed. Thus, it is difficult for HRPAC to achieve fast and highly sensitive monitoring of trace gases in small spaces. In 2019, Gong et al. showed a novel T-type resonant PAC (TRPAC) with higher sensitivity, larger pooling constants, and faster response compared to the HRPAC while reducing its size by half [34]. In 2022, Lu et al. presented a new scheme of all-optical differential PAS based on a dual-channel TRPAC, which was able to detect CH4 at the ppb level [35]. However, the volumes of the buffer chamber of HRPAC or TRPAC have to be an order of magnitude higher than the volume of the resonant cavity to achieve the same performance. Optimizing the volume of the buffer chamber is essential to obtain faster response time and has been rarely pursued.
In this paper, in order to obtain faster response time and higher sensitivity for trace gas sensing, a novel miniaturized 3D-printed Y-type resonant PAC (YRPAC) is reported. It is composed of a frustum of cone-type buffer chamber and a cylindrical resonant chamber, and its volume is smaller than that of the TRPAC. Based on finite element theory, the sound field distribution, frequency response, and characteristics of the YRPAC have been simulated in a thermal viscous acoustic physical field. The laser-based PAS system, utilizing the YRPAC and TRPAC as the photoacoustic reaction units, a 1653.7 nm distributed feedback (DFB) laser as the excitation light source, a cantilever beam acoustic sensor as the acoustic sensing unit, and a high-speed spectrometer as the demodulation unit, has been successfully developed for high-sensitivity trace CH4 sensing.
2. Fundamentals of PAS
Different from conventional absorption spectroscopy, PAS does not require the measurement of the transmitted light intensity of a light source passing through the substance to be measured. Instead, it uses an indirect method to detect the amount of light energy absorbed by the substance. When a light beam with an intensity of I0 passes through gas molecules to be measured, the light is absorbed by the gas molecules. According to the Beer-Lambert law, the light intensity after being absorbed is: [36]
where c is the volumetric concentration of the gas to be measured, l is the absorption optical range of the gas, α(v) is the absorption coefficient of the gas, which can be expressed as: where Ntol denotes the total molar concentration of the gas in mol/cm3. σ(v) denotes the absorption cross-section, which is equal to the product of the normalized line function g(v) and the intensity of the absorption line S. g(v) is used to describe the shape of the absorption spectral line in cm. The intensity of the absorption line S, which is an important physical measure of the ability of a molecule to absorb light, can be found through the HITRAN database [37], and it is measured in cm−1/(mol.cm−2). When the room temperature, and the pressure are T, and P0, respectively, the total molar concentration of the gas Ntol can be expressed as: where the Loschmidt constant NL = 2.488 × 1019 mol•cm−3•atm−1•K. The gas molecules that absorbed the light energy leap from the ground state to the excited state. Due to the instability of the molecules in the excited state, the majority of the excited molecules return to the ground state through nonradiative transition and release heat, thus generating a pressure wave that expands outward with the light source as the center. Since the pressure intensity is positively correlated with the measured gas concentration, the information on the concentration of the gas can be obtained by detecting the intensity of the pressure wave through the photoacoustic sensor.Assuming that the gas concentration to be measured is low, the expression for the amplitude of the photoacoustic signal SPA detected by the acoustic sensor can be expressed as [38]:
where C is the acoustic sensor sensitivity; Q is quality factor of the PAC, which is determined by modulation frequency, PAC structure, and individual measurement conditions; P represents the optical power; f is the modulation frequency of laser; V presents the volume of the PAC. From the above equation, it can be seen that as the volume of the PAC decreases, the amplitude of the photoacoustic signal increases. Therefore, the intensity of the photoacoustic signal can be effectively improved by optimizing the PAC.3. Design and simulation analysis of the YRPAC
Figure 1(a) depicts the schematic of the YRPAC, mainly composed of a collimator for expanding the working distance of the excitation light, two sealing caps for maintaining the airtightness, a photoacoustic cavity as a reaction site for generating photoacoustic effects, a shell and a cantilever beam microphone for sensing photoacoustic signals. Figure 1(b) and (c) show the mechanical diagrams of the Y-type photoacoustic cavity and T-type photoacoustic cavity, respectively. Unlike the TRPAC, the buffer chamber of the YRPAC is designed in the shape of a frustum of cone. The numerical model of YRPAC for different sizes of buffer chambers has been carefully evaluated using finite element analysis. All simulation models combine a pressure acoustic module and a thermo-viscous acoustic module, and the speed of sound is set to 343 m/s, and the heat source is set to one unity. Therefore, the YRPACs with the buffer chambers of top radius (r1 = 15 mm) × different bottom radii (r2 = 3-15 mm) × height (22 mm) and the resonant chamber of 3 mm (r3) × 22 mm (height) are simulated. Figure. 2(a) shows the simulated frequency responses of the YRPAC with different bottom radii (r2 = 3-12 mm) of the buffer chamber and the simulated frequency response of the TRPAC with a radius of 15 mm for the buffer chamber. It can be seen that the amplitudes of all simulated photoacoustic signals first increase and then decreases with the increasing of the frequency, where the frequency with the maximum amplitude is the resonance frequency. Besides, the photoacoustic signal at the first resonant frequency (FRF) decreases as the r2 of the buffer chamber decreases. In addition, the photoacoustic signal is strongest when the bottom radius of the buffer chamber is consistent with the radius of the resonant cavity. The amplitude of the photoacoustic signal of the YRPAC increases with the decreasing of r2, and the amplitude with a 3-mm radius (r2 = 3 mm) is about three times compared to that of the TRPAC at FRF. Meanwhile, the FRF of the YRPAC is about 200 Hz smaller than that of the TRPAC, which is useful for boosting the photoacoustic signal. Figure. 2(b) and (c) present the sound pressure distribution clouds inside the YRPAC and TRPAC at FRF, respectively. The photoacoustic signals at the ends of the resonant cavities of the two types of PACs are approximately 11.4 × 10−7 Pa, and 3.8 × 10−7 Pa, indicating that the YRPAC produces a larger resonant sound pressure at the end of the resonator. The strongest and weakest photoacoustic signals appear at the ends of the resonator and the buffer chamber, respectively. Therefore, the acoustic sensor for detecting the photoacoustic signal is placed at the end of the resonator to obtain higher gas detection sensitivity. Moreover, the YRPAC produces a larger resonant sound pressure at the end of the resonator compared to the TRPAC under the same initial sound pressure.
Fig. 1. (a) Structure diagram of the YRPAC. (b) The mechanical diagram of the Y-type photoacoustic cavity. (c) The mechanical diagram of the T-type photoacoustic cavity.
Fig. 2. (a) Simulated frequency responses of the YRPAC with different bottom radii (r2 = 3-12 mm) of the buffer chamber and the simulated frequency response of the TRPAC with a radius of 15 mm (r2 = 15 mm) for the buffer chamber. Simulated photoacoustic field distribution cloud maps of (b) the YRPAC (r1 = 15, r2 = 3, r3 = 3) and (c) the TRPAC (r1 = 15, r2 = 15, r3 = 3) at FRF.
Finally, through simulation analysis, it can be determined that the YRPAC includes a frustum of cone-type buffer chamber and a cylindrical resonant chamber. The buffer chamber has dimensions of 15 mm (r1) × 3 mm (r2) × 22 mm (height) and the resonant chamber has dimensions of 3 mm (r3) × 22 mm (height). The YRPAC is processed by 3D printing technology using a photosensitive resin (density = 1.3 g/cm), exhibiting manufacturing precision of 0.1 mm.
The transformation of the buffer chamber from a cylinder to a frustum of cone can be seen as the process of change from (a) to (d) shown in Fig. 3, with the total length of the buffer chamber fixed and the number of layers increased. Figure. 4 presents the simulated frequency responses with buffer chambers of 2 layers, 5 layers, 10 layers and 20 layers for the YRPAC, as well as the frequency response for the TRPAC, indicating that the photoacoustic signal of YRPAC at the FRF of the 20th layers is apparently stronger than the TRPAC. The photoacoustic signal is getting larger as the amounts of layers increases. Thus, the maximum sound pressure value is obtained when the buffer chamber is frustum of cone (r1 = 15, r2 = 3).
Fig. 4. The simulated frequency responses with buffer chambers of 2 layers, 5 layers, 10 layers and 20 layers for the YRPAC, as well as the frequency response for the TRPAC.
4. Experimental system and results
4.1 Experimental setup
Figure 5 shows the schematic structure of the experimental system for CH4 gas sensing with the YRPAC, a circulator, a spectrometer, a super-luminescent emitting diode (SLED), a DFB laser, a computer, two mass flow controllers (MFCs). The SLED (center wavelength = 1550 nm, spectral width = 45 nm) is served as a broad-spectrum detection light source. The DFB laser (operating wavelength = 1653.7 nm, output power = 10 mW) is utilized as a pump light. The spectrometer (I-MON 256 OEM) used to acquire Fabry-Perot interferometric spectrum can reach a line scan rate of 35 kHz with 256 pixels and its principle can be summarized as the detection of intensities at different wavelengths by means of photodetectors. When the laser beam is directed through the collimator into the designed YRPAC, the CH4 gas absorbs the periodic modulated light and produces photoacoustic signal that causes the high-sensitivity cantilever microphone to vibrate periodically. The Fabry-Perot cavity length of the cantilever microphone is demodulated by the white-light interference (WLI) demodulation method. The broad-spectrum detection light from the SLED is projected into the cantilever microphone, and the interfering light is returned to the spectrometer through the circulator. The WLI demodulation algorithm is employed to process the Fabry-Perot interferometric spectrum detected by the spectrometer. The external square wave signal is phase-locked to the output signal of the direct digital synthesis (DDS) through a phase-locked loop (PLL) and synchronized to trigger the spectrum sampling. The demodulated photoacoustic signals are further processed by digital lock-in amplifier.
4.2 Experimental results
Under the same experimental conditions, the concentration of 1000 ppm CH4 is controlled by MFCs to purge the designed YRPAC and TRPAC at a gas flow rate of 20 standard cubic centimeter per minute (SCCM) for a period of time until the photoacoustic signal no longer increased then the PACs are sealed to test the experimental frequency response of the YRPAC-based and TRPAC-based photoacoustic sensors. The sine modulation frequency range of the DFB laser is set to 1200 and 2100 Hz, and the detection frequency range of two photoacoustic sensors from 2400 Hz to 4200 Hz is recorded utilizing the wavelength modulation spectroscopy (WMS) and second-harmonic detection (WMS-2f) technology [39]. The frequency responses of the two photoacoustic sensors based on YRPAC and TRPAC are displayed in Fig. 6, represented by blue and red curves, respectively. It clearly presents that the photoacoustic signal amplitudes of both photoacoustic sensors show two peaks in the range of modulation frequencies. The reason for the first signal peak is that the cantilever microphone also exhibits a photoacoustic signal peak at its FRF. In theory, the resonance frequencies at the first signal peak should be the same for two photoacoustic sensors, but the difference in the volume of the external cavities (YRPAC, TRPAC) can cause the FRF of the cantilever beam to change. The 2f signal peaks have maximum amplitudes of 2.3 nm (YRPAC) with a modulation frequency of 1770Hz, 1.34 nm (TRPAC) with a modulation frequency of 1852Hz, corresponding to half of the FRFs of YRPAC and TRPAC, respectively. The measurement results are in good agreement with the simulation studies. Furthermore, the 2f signal of YRPAC in the first-order resonance mode is 1.7 times higher than the TRPAC. This amplification is smaller than the simulation result (Fig. 3) due to the actual processing error, as well as the simplification of the damping and loss effects in the analytical model. Finally, considering the system stability, and sensitivity, the operating frequency of the experiment is set at 1770Hz, which corresponds to a detection frequency of 3540 Hz.
The sample gases (CH4) with different concentrations ranging from 50-1000 ppm obtained by MFCs are passed into the PAS to evaluate the linearity of the designed YRPAC-based and TRPAC-based sensors. The measured 2f-signal curves of different CH4 concentrations are drawn in Fig. 7. Furthermore, Fig. 8 depicts the 2f-signal peaks of the two sensors corresponding to different concentrations of CH4 (50-1000 ppm). By employing the method of linear fitting, the sensitivities of the YRPAC-based photoacoustic sensor and TRPAC-based photoacoustic sensor are 2.29 pm/ppm, and 1.35 pm/ppm, respectively. The R2 correlation coefficients of the two sensors for CH4 gas are beyond 0.99, indicating that the two sensors own approximately linear concentration response.
Fig. 7. The measured 2f-signal curves of different CH4 concentrations using (a) YRPAC-based sensor and (b) TRPAC-based sensor.
Figure. 9(a) exhibits the results of the background noise analysis obtained by filling the YRPAC and TRPAC with pure nitrogen (99.99%), with the integration time and the modulation frequency of 1 s and 1770Hz, respectively. Under the conditions of this experiment, the noise levels of the YRPAC-based and TRPAC-based PAS systems for CH4 are 1.2 pm and 1.1 pm, respectively. Combining the sensitivities and noise levels of the YRPAC-based and the TRPAC-based PAS systems for CH4 obtained in Fig. 8 and Fig. 9(a), the MDLs for CH4 can be calculated as 524 ppb, and 888 ppb, respectively, which correspond to YRPAC and TRPAC. The Allan deviation [40–42], presented in Fig. 9(b), is applied to assess the sensitivity and stability of the YRPAC-based PAS system. It demonstrates that the Allan deviation curve roughly conforms to the t−1/2 relationship, showing that the YRPAC-based PAS is dominated by white noise during the test period. When the integration time of VLIA is 100 s, the Allan deviation shows that the corresponding MDL is 52.8 ppb, indicating that the PAS can realize high sensitivity sensing for CH4 gas.
Fig. 9. (a) The background noise of the photoacoustic sensors based on YRPAC and TRPAC with pure N2. (b) Allan-deviation of the photoacoustic sensors based YRPAC.
A comparison experiment for the cleaning times of the two PACs has been carried out. Firstly, 500 ppm CH4 is injected into both PACs for a period of time until the photoacoustic signals no longer increased, then the pure N2 is controlled by MFCs to flush the two PACs at a gas flow rate of 20 SCCM. The valves are closed and the photoacoustic signals are recorded with a time interval of one minute. Figure 10 exhibits the comparison of cleaning times for the two PACs. For TRPAC, the photoacoustic signal no longer decreases after 7 minutes, while the output of YRPAC becomes stable after 4 minutes. Hence, the designed YRPAC has a faster response time than the TRPAC.
5. Conclusions
In conclusion, a novel miniaturized 3D-printed YRPAC, including a frustum of cone-type buffer chamber and a cylindrical resonant chamber, has been reported. The finite element simulation of TRPAC and YRPAC sound field distribution based on COMSOL shows that the photoacoustic signal is gradually enhanced when the shape of the buffer cavity is gradually changed from a conventional cylinder to a frustum of cone-type, and the photoacoustic signal is maximized when the radius of the bottom of the frustum of cone-type is the same as the radius of the resonance cavity. The buffer chamber has dimensions of 15 mm (r1) × 3 mm (r2) × 22 mm (height) and the resonant chamber has dimensions of 3 mm (r3) × 22 mm (height). Thus, the volume of the designed YRPAC is only about 7.0 cm3, which is only about half the one of the TRPAC. The laser-based PAS system, utilizing the YRPAC and TRPAC as the photoacoustic reaction units, a 1653.7 nm DFB laser as the excitation light source, a cantilever beam acoustic sensor as the acoustic sensing unit, and a high-speed spectrometer as the demodulation unit, has been successfully developed for high-sensitivity trace CH4 sensing. The experimental results show that the 2f signal of YRPAC in the first-order resonance mode is 2.3 nm, which is 1.7 times higher than the 2f signal amplitude of TRPAC, with the CH4 concentration of 1000 ppm. The MDLs of YRPAC and TRPAC for CH4 are 524 ppb and 888 ppb, respectively, at 1 s of averaging time. The MDL for the YRPAC-based PAS system can reach up to 52.8 ppb at 100 s of averaging time according to the Allan deviation. Furthermore, a comparison experiment for the cleaning times of the two PACs shows that the response times of TRPAC and YRPAC are about 7 minutes and 4 minutes, respectively. Hence, the designed YRPAC has a faster response time than the TRPAC. The reported YRPAC provides a new approach for optimizing the PAS performance. Future work involves in analyzing the relationship between the length of the resonator and the buffer chamber, reducing the size of the PAC, etc., which can hopefully further improve the performance of the sensor with the characteristics of miniaturization and ultra-high sensitivity.
Funding
National Natural Science Foundation of China (62075025); Natural Science Foundation of Liaoning Province (2022-MS-127); Fundamental Research Funds for the Central Universities (DUT22JC17, DUT22LAB102, DUT22QN246).
Disclosures
The authors declare no conflicts of interest.
Data availability
The data that support the findings of this study are available within the article.
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