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. 2023 Feb 23;30:100467. doi: 10.1016/j.pacs.2023.100467

Highly sensitive photoacoustic acetylene detection based on differential photoacoustic cell with retro-reflection-cavity

Chu Zhang 1, Shunda Qiao 1, Yufei Ma 1,⁎
PMCID: PMC9982609  PMID: 36874591

Abstract

In this paper, a highly sensitive photoacoustic spectroscopy (PAS) sensor based on retro-reflection-cavity-enhanced differential photoacoustic cell (DPAC) is demonstrated for the first time. Acetylene (C2H2) was selected as the analyte. The DPAC was designed to effectively suppress noise and increase signal level. The retro-reflection-cavity consisted of two right-angle prisms was designed to reflect the incident light to realize four passes. The photoacoustic response of the DPAC was simulated and investigated based on the finite element method. Wavelength modulation and second harmonic demodulation technologies were applied for sensitive trace gas detection. The first-order resonant frequency of the DPAC was found to be 1310 Hz. The differential characteristics were investigated and the 2f signal amplitude for this C2H2-PAS sensor based on retro-reflection-cavity-enhanced DPAC had a 3.55 times improvement compared to the system without the retro-reflection-cavity. An Allan deviation analysis was performed to investigate the long-term stability of the system. The minimum detection limit (MDL) was measured to be 15.81 ppb with an integration time of 100 s

Keywords: Differential photoacoustic cell, Retro-reflection-cavity, Photoacoustic spectroscopy, C2H2 detection

1. Introduction

Acetylene (C2H2), a colorless, flammable and explosive gas, plays an important role in the fields of dissolved gas analysis for oil-immersed transformer and safety supervision in coal mine [1], [2]. When the concentration of C2H2 in the air exceeds the explosion limit of 2.3 %, it may cause an explosion and cause a safety accident [3]. Therefore, accurate measurement and real-time concentration monitoring of C2H2 is important in gas detection fields.

Among various optical detection techniques for trace gases [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], photoacoustic spectroscopy (PAS) technique, based on photoacoustic effect [14], [15], [16], is one of the most promising methods, which has the advantages of high detection sensitivity, high selectivity and real-time online monitoring [17], [18]. The photoacoustic effect refers to the fact that the target gas absorbs the modulated light and gas molecules undergo energy level transitions to the excited state. These molecules return to the ground state through non-radiative processes and result the heat energy produce. The heat energy can lead to local temperature and pressure increases, causing photoacoustic (PA) pressure wave. Meanwhile, a high-sensitivity acoustic wave transducer such as a condenser microphone or a quartz tuning fork [19], [20] is used to detect the photoacoustic signal. Quartz tuning fork as a detector has significant advantages such as small size, simple structure, high sensitivity, and highly quality factor [21], [22], [23]. But it also has the disadvantage of high operating frequency, usually greater than 10 kHz. Condenser microphone have the advantages of simple structure, good dynamic response and excellent temperature stability. The strength of the PA signal is proportional to the target gas concentration and light intensity.

As a booster of photoacoustic effect, photoacoustic cell (PAC) is an important element which affects the performance of PAS system significantly. Up to now, there are several common PACs, such as cylindrical PAC [24], spherical PAC [25], [26], T-type PAC [27], [28], Helmholtz PAC [29] and differential PAC (DPAC) [29], [30], [31]. Among them, the resonant DPAC can effectively avoid airflow noise, window noise and electromagnetic interference from the surrounding environment, and increase the signal level. Therefore, it greatly improves the detection signal-to-noise ratio (SNR) of the PAS system. The most common differential methods are mainly based on the opposite‐phase acoustic resonator‐enhanced PAS modality. Over the past few decades, a lot of work related to DPAC have been done. In 1999, Zeninari et al. presented a simple differential Helmholtz resonator for flow measurements [32], which enhanced the PA signal by a factor of 2. In 2016, Rouxel et al. presented two centimeter-sized photoacoustic cells. The minimum detection limits (MDL) of 92 ppbv was estimated for methane (CH4) [33]. In 2021, Yin et al. developed a PAS based ppb-level hydrogen sulfide (H2S) and carbon monoxide (CO) gas sensor [34], which combined a DPAC and a two-stage commercial optical fiber amplifier with a full output power of 10 W. The MDL of such a sensor were 31.7 ppb and 342.7 ppb for H2S and CO at atmospheric pressure, respectively. In 2022, Xiao et al. designed an ultra-sensitive all-optical PAS CH4 sensor [35], in which a near-infrared diode laser, fiber-optic microphones and a double channel differential T-type PAC were combined together. The MDL of 36.45 ppb was obtained with a 1 s integration time, which could be further improved to 4.87 ppb with an integration time of 81 s

The traditional DPAC mainly consists of two identical resonant cavities. When the modulated light passes through the optical windows on both sides of the cavity, the effective absorption path is equal to the length of the cavity. In order to further improve the detection performance of the DPAC based PAS sensor, the multi-pass cell was introduced to enhance the absorption path of the target gas. Zhao et al. combined the DPAC with a multi-pass cell to achieve a maximum of 30 times reflection and an effective absorption path of 4.9 m [36]. At an averaging time of 500 s, the MDL reached 0.6 ppb for CH4 in atmosphere condition. Li et al. developed a novel Helmholtz DPAC with gold-plated inner wall to achieve multiple reflections of the beam to obtain a strong PA signal [29]. A MDL of 177 ppb was obtained within 1 s integration time. However, the current studies on multiple reflection of DPACs have some shortcomings, such as the difficult adjustment of laser incident angle and poor anti-misalignment ability. Therefore, it is essential to design a novel multiple reflection DPAC that is easy to adjust and has good anti-misalignment ability.

In this paper, a highly sensitive C2H2-PAS sensor based on retro-reflection-cavity-enhanced DPAC is proposed for the first time. Two right-angle prisms were used to reflect the laser beam to pass through the DPAC for several times. The retro-reflection-cavity has the merits of easily optical adjustment and excellent anti-misalignment ability. With the help of finite element method, the model of retro-reflection-cavity-enhanced DPAC was built to simulate the acoustic characteristics. Finally, a ppb level detection sensitivity for this C2H2-PAS sensor was achieved with the multi-pass retro-reflection-cavity-enhanced DPAC configuration.

2. Experimental setup

2.1. Design of the DPAC

A novel double channel stainless-steel DPAC with retro-reflection-cavity is shown in Fig. 1. The DPAC consisted of two identical cylinder channels as acoustic resonators and two buffer volumes connect near both ends. The diameter and length of two acoustic resonators were 10 mm and 120 mm, respectively. The diameter and length of two buffer volumes were 35 mm and 40 mm, respectively. The inner surface of two acoustic resonators was polished to reduce the viscous damping of the gas. In addition, two calcium fluoride (CaF2) windows were mounted on both ends of the DPAC with retro-reflection-cavity. Two condenser microphones were used to separately detect the acoustic signals generated by the photoacoustic effect in the two acoustic resonators. In order to enhance the PA signal, a multi-pass retro-reflection-cavity consisted of two right-angle prisms was designed to reflect incident laser, which achieved four times reflection. The side length of the used two right-angle prisms were 14 mm and 20 mm, respectively.

Fig. 1.

Fig. 1

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Schematic design of the retro-reflection-cavity-enhanced DPAC.

For the DPAC, the excitation laser just passes through one channel to generate the standing acoustic wave. In addition, the phases of acoustic waves are reversed in the two acoustic resonators. The signals detected by the two condenser microphones (microphone 1 and microphone 2) were fed into a differential amplifier. As a result, the various in-phase background noise caused by window absorption and external environment is greatly suppressed, and the acoustic signals are doubled. The PA response of the DPAC was simulated and investigated based on the thermoviscous acoustics module of the finite element method as shown in Fig. 2. A uniform heat source with an amplitude of 1 W/m3 was placed inside one of the resonators to simulate a laser source. The acoustic field distributions of DPAC at the first-order resonant frequency is showed in Fig. 2(a). The maximum acoustic pressure was 0.0043 Pa at the antinode position, thus a pair of identical condenser microphones was placed at this position to detect the maximum acoustic wave signal. The frequency response of the two condenser microphones of the DPAC is shown in Fig. 2(b). The peak value of the first-order resonant frequency of the DPAC was about 1301 Hz. The phase of the DPAC is shown in Fig. 2(c). It can be seen that the phase difference of the acoustic pressure at the resonant frequency is about 180°.

Fig. 2.

Fig. 2

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(a) The acoustic pressure distribution of the designed DPAC. (b) Frequency response of the designed DPAC. (c) Phase of the designed DPAC.

2.2. Experimental configuration

A schematic diagram of the PAS sensor system based on the multi-pass retro-reflection-cavity-enhanced DPAC is depicted in Fig. 3. According to the HITRAN 2016 database [37], a distributed feedback (DFB) diode laser with a central wavelength of 1530.37 nm (6534.37 cm−1) was adopted as excitation source, which corresponded to the strongest absorption coefficient of C2H2 in the near infrared (NIR) region. To cover the 6534.37 cm−1 C2H2 absorption line, a laser controller was used to change the laser temperature and injection current. In our experiment, the injection current of the laser was increased from 38 mA to 97 mA. When the laser temperature and current were set as 18 °C and 70 mA, respectively, the laser was tuned at this C2H2 absorption line. A fiber collimator with a focal length of 18.67 mm was used to collimate the output Gaussian laser beam into one acoustic resonator of the DPAC. Two calcium fluoride (CaF2) windows were mounted on both ends of the DPAC. Two condenser microphones were located to detect the acoustic signals generated by the photoacoustic effect in the two acoustic resonators separately. In order to enhance the optical absorption, the laser beam was reflected four times in the DPAC with retro-reflection-cavity. A bottle of 2 % C2H2:N2 standard gas and a bottle of pure nitrogen (N2) were diluted to produce different concentrations C2H2 gas, and two flow controllers were used to control the flow rate. Wavelength modulation spectroscopy and second harmonic (2 f) demodulation technologies were applied for sensitive trace gas detection. A dual channel function generator was used to generate a sawtooth wave to scan the laser central wavelength around the absorption line. A high-frequency sine wave was employed to modulate the laser wavelength, which was generated by the lock-in amplifier. Meanwhile, a high-level trigger signal was generated for the lock-in amplifier to demodulate the second harmonic. The modulation frequency was set as half of the DMPC resonance frequency. The integration time of the PAS system with multi-pass retro-reflection-cavity-enhanced DPAC was set as 250 ms. The electrical signals generated from the microphones were first processed by a differential amplifier and then fed to a lock-in amplifier to demodulate the 2 f signal. All experiments were carried out under normal temperature and pressure.

Fig. 3.

Fig. 3

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Schematic diagram of C2H2-PAS sensor based on retro-reflection-cavity-enhanced DPAC.

3. Results and discussion

The resonant PAC is an important part in the PAS based trace gas detection, and the resonant frequency is a key parameter of the resonant PAC. When the PAC is at the resonant frequency, the 2 f signal reaches a maximum. Therefore, the resonant frequency was measured firstly. A 2 % C2H2:N2 gas mixture was fed into the DPAC to measure 2 f signal at different modulation frequency. The experimental frequency response of the DPAC is shown in Fig. 4. The first-order resonant frequency (f0) of the DPAC was 1310 Hz, which is close the simulated value 1301 Hz. The ratio of the resonance frequency to the half-width value of resonance contour was estimated. Hence, the Q factor of the designed DPAC was determined to be 34.47. In the subsequent trace gas detection experiments, the modulation frequency of the C2H2-PAS sensor was set as 1310/2 = 655 Hz.

Fig. 4.

Fig. 4

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Frequency response of the designed DPAC.

In order to obtain the strongest 2 f signal, the wavelength modulation depth of the PAS system should be optimized. A 2 % C2H2:N2 gas mixture was filled into the DPAC without retro-reflection-cavity to measure the 2 f signal. Fig. 5 shows the relation of 2 f signal amplitude and modulation depth. It is indicated that the 2 f signal amplitude first increased and then decreased. When the modulation depth was 0.26 cm−1, the 2 f signal amplitude was the strongest. Therefore, this optimal laser modulation depth was adopted in the following.

Fig. 5.

Fig. 5

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Modulation depth of C2H2-PAS sensor.

The performance of the designed DPAC was investigated. The 2 f signal amplitude was measured when the single channel was used and the other channel was invalid by disconnecting the second the microphone. As shown in Fig. 6, the 2 f signal amplitude for the single channel DPAC and the double channel DPAC based PAS sensor system are 1.58 mV and 3.06 mV, respectively. It is shown that the 2 f signal level from the differential double channel basically doubled compared to that of the single channel PAS system, which confirmed that the designed DPAC had an ideal performance. The retro-reflection-cavity was added to the system to further improve the signal level. The 2 f signal amplitude of the C2H2-PAS sensor based on retro-reflection-cavity-enhanced DPAC was 10.87 mV, which had a 3.55 times improvement compared to the 2 f signal amplitude for DPAC based PAS sensor without retro-reflection-cavity. It can be found that the 3.55 times improvement is slightly lower than the designed 4 times, which probably resulted from the window absorption loss during light reflection.

Fig. 6.

Fig. 6

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2f signal for PAS sensor system based on single channel DPAC, DPAC, and DPAC with retro-reflection-cavity, respectively.

In order to verify the concentration response of the PAS sensor system based on retro-reflection-cavity-enhanced DPAC, the gas detection experiments were further investigated when the C2H2 concentration ranged from 50 ppm to 2 %. The 2 f signals of C2H2:N2 gas mixture at different concentrations is shown in Fig. 7(a), and the 2 f signal peak values as a function of C2H2 concentration is depicted in Fig. 7(b). Due to the R-square value of ≥0.99, the result of Fig. 7(b) shows that there is an excellent linear concentration response. The standard deviation of background noise of this C2H2-PAS sensor based on retro-reflection-cavity-enhanced DPAC was about 75.24 nV when pure N2 was filled into the DPAC. Hence, the MDL was calculated to be 138.43 ppb.

Fig. 7.

Fig. 7

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(a) 2f signals of C2H2:N2 gas mixture at different concentrations. (b) 2 f signal peak values as a function of C2H2 concentration.

To evaluate the long-term stability of this PAS sensor based on retro-reflection-cavity-enhanced DPAC, Allan deviation analysis was performed. Pure N2 was filled into the DPAC. The measured results are shown in Fig. 8. As the integration time increased, the Allan deviation showed a continuous downward trend similar to an inverse proportional function curve. Therefore, the MDL can be improved with a longer averaging time. It is observed that the designed system was stable within 100 s integration time. The MDL of the PAS sensor with retro-reflection-cavity-enhanced DPAC was improved to 15.81 ppb when the system integration time was 100 s

Fig. 8.

Fig. 8

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Allan deviation analysis for differential double channel with retro-reflection cavity PAS system.

4. Conclusion

In conclusion, a highly sensitive C2H2-PAS sensor based on retro-reflection-cavity-enhanced DPAC is proposed for the first time. The resonant DPAC can effectively suppress the noise and increase the signal level. Two right-angle prisms were used to construct the retro-reflection-cavity and reflect the laser beam to pass through the DPAC for four times. The retro-reflection-cavity has the merits of easily optical adjustment and excellent anti-misalignment ability. With the help of COMSOL Multiphysics, the DPAC was theoretically analyzed and designed. The diameter and length of two acoustic resonators were set to 10 mm and 120 mm, respectively. The side length of the used two right-angle prisms were chosen as 14 mm and 20 mm, respectively. A DFB diode laser was chosen as the excitation source to target the 6534.37 cm−1 C2H2 absorption line. The resonant frequency of the DPAC was measured as 1310 Hz, which is close the simulated value of 1301 Hz. The Q factor of the designed DPAC was calculated as 34.47. The optimal laser modulation depth was found to be 0.26 cm−1. The almost two times signal amplitude enhancement confirmed the designed the DPAC had an ideal performance. The 2 f signal amplitude for C2H2-PAS sensor based on retro-reflection-cavity-enhanced DPAC had a 3.55 times improvement compared to the system without the retro-reflection-cavity. The concentration response of the PAS system with retro-reflection-cavity-enhanced DPAC was investigated and an excellent linear relationship was obtained. When the integration time was 250 ms, the MDL of such system was 138.43 ppb. The MDL was improved to 15.81 ppb at an integration time of 100 s. The MDL can be further enhanced by increasing the reflection times and amplify the optical power.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work is supported by the National Natural Science Foundation of China (Grant No. 62275065, 62022032, 61875047 and 61505041) and Fundamental Research Funds for the Central Universities.

Biographies

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Chu Zhang received her B.S. degree in Qingdao University of Technology, China, in 2019. Then, she obtained a master's degree in control engineering from Northeastern University. She is now pursuing a PhD degree of physical electronics from Harbin Institute of Technology. Her research interest is photoacoustic cell design based on photoacoustic spectroscopy.

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Shunda Qiao received his B.S. degree in electronic science and technology from Yanshan university, China, in 2018. In 2020, he received his M.S. degree and began to pursue a PhD degree of physical electronics from Harbin institute of technology. His research interests include photoacoustic spectroscopy and its applications.

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Yufei Ma received his PhD degree in physical electronics from Harbin Institute of Technology, China, in 2013. From September 2010 to September 2011, he spent as a visiting scholar at Rice University, USA. Currently, he is a professor at Harbin Institute of Technology, China. He is the winner of National Outstanding Youth Science Fund. His research interests include optical sensors, trace gas detection, laser spectroscopy, solid-state laser and optoelectronics. He has published more than 100 publications and given more than 20 invited presentations at international conferences. He serves as associate editor for Optica Optics Express, SPIE Optical Engineering, Wiley Microwave and Optical Technology Letters and Frontiers in Physics. He also serves as topical editor for CLP Chinese Optics Letters and editorial board member for Elsevier Photoacoustics MDPI Sensors and Applied Sciences.

Data Availability

Data will be made available on request.

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Data Availability Statement

Data will be made available on request.

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