引言
 

　　近日，來自山西大學激光光譜研究所、光學協同創新中心，-巴里大學和巴里理工大學跨校物理系波利森斯實驗室的聯合研究團隊發表了《Ppb級中紅外石英增強光聲傳感器，用于使用T型音叉調諧探測DMMP》論文。
 

　　Recently, the joint research team from State Key Laboratory of Quantum Optics and Quantum Optics Devices, Institute of Laser Spectroscopy, Shanxi University, Collaborative Innovation Center of Extreme Optics, PolySense Lab-Dipartimento Interateneo di Fisica, University and Politecnico of Bari published an academic papers Ppb-level mid-IR quartz-enhanced photoacoustic sensor for  DMMP detection using a T-shaped tuning fork.
 

　　項目背景
 

　　二甲基甲基膦酸酯(DMMP)被廣泛認為是最具代表性的模擬物，已開發并廣泛用于DMMP檢測的各種氣體分析技術。
 

　　氣相色譜(GC)和質譜(MS)分析可以高敏感地鑒定不同的有機磷化合物，但它們在原位監測方面具有幾個缺點，包括昂貴和耗時。此外，色譜分析必須由熟練的人員在專門的實驗室中進行，不適合小型化。相比，光聲光譜(PAS)是DMMP氣體水平監測最有前景的技術之一，因為它具有高靈敏度、選擇性和快速響應的優勢。作為PAS的一種變體，石英增強光聲光譜(QEPAS)技術自2002年首次報道以來迅速發展，其中超窄帶石英調諧叉(QTF)與兩個作為銳利共振聲學換能器的聲學微共振器(AmRs)在聲學上耦合，用于檢測聲音信號，而不是傳統的寬帶麥克風。與體積超過10 cm3的傳統光聲池相比，小體積的QTF更有利于DMMP檢測設備的小型化和快速響應。此外，QEPAS技術的顯著特點是激發波長的獨立性，這意味著可以使用相同的光譜聲學器測量具有不同特征吸收光譜的痕量氣體。DMMP在9–11.5 µm的中紅外區域顯示出強烈的光吸收特征，因此使用高性能中紅外量子級聯激光器(QCLs)可以在理論上實現高靈敏度的檢測。然而，中紅外QCL輸出光束通常具有較大的發散角，這使得將中紅外激光束耦合到具有300微米叉間距的QTF中成為巨大的挑戰，因為任何誤散射光束擊中QTF都會產生大的背景信號。
 

　　在本研究中，我們展示了一種基于定制T型QTF和中紅外量子級聯激光器(QCL)的小型化集成QEPAS DMMP傳感器。T型QTF的叉間距為0.8毫米，具有約15,000的高品質因數，避免了由誤散射光引起的背景信號，從而在ppb水平上獲得最佳檢測限。通過使用摻入DMMP的真實室外空氣對傳感器進行測試，以驗證其有效性。
 

　　BACKGROUND
 

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　　Dimethyl methylphosphonate (DMMP) is widely regarded as the most representative simulant and has been developed and extensively utilized in various gas analysis techniques for DMMP detection.
 

　　Gas chromatography (GC) and mass spectrometry (MS) analysis can identify the different organophosphorus compounds with high sensitivity, but they have several disadvantages for in situ monitoring, including being expensive and time-consuming. Moreover, the chromatographic analysis must be performed in a specialized laboratory by skilled personnel and is not suitable for miniaturization. Compared with the above techniques, photoacoustic spectroscopy (PAS) is one of the most promising techniques for sarin gas level monitoring in public places due to its benefits of high sensitivity, selectivity, and fast response. The quartz-enhanced photoacoustic spectroscopy (QEPAS) technique as a variant of PAS has rapidly developed since it was first reported in 2002, in which an ultra-narrowband quartz tuning fork (QTF) acoustically couples with two acoustic micro-resonators (AmRs) acting as a sharply resonant acoustic transducer to detect sound signals instead of conventional broadband microphones. Compared with the sizes of the conventional photoacoustic cell, which is more than 10 cm3, the small volume of QTF is more conducive to the miniaturization and rapid response of sarin or DMMP detection equipment. Besides, the remarkable feature of the QEPAS technique is the excitation wavelength independence, meaning that trace gases with different characteristic absorption spectra can be measured using the same spectrophone. Sarin and DMMP show strong optical absorption features in the mid-infrared region of 9–11.5 µm, so high detection sensitivity can be theoretically achieved using high-performance mid-infrared quantum cascade lasers (QCLs). However, the mid-infrared QCL output beam usually has a large divergence angle, which makes it a great challenge to couple a mid-infrared laser beam through a 300-μm prong-spacing QTF since any stray light hitting the QTF can cause a large background signal.
 

　　In this work, we demonstrate a miniaturized and integrated QEPASbased DMMP sensor, in which a custom T-shaped QTF and a midinfrared quantum cascade laser (QCL) are used. The T-shaped QTF has a prong spacing of 0.8 mm and a high-quality factor of ~ 15,000, avoiding the background signal caused by stray light, thus obtaining an optimal detection limit at the ppb level. The DMMP sensor was tested using real outdoor air mixed with DMMP to verify its effectiveness.
 

　　實驗部分：檢測波長和光學激發源的選擇
 

　　強有力的靶向吸收帶對于DMMP檢測至關重要，因為實際應用需要具有亞百萬分之一靈敏度的傳感裝置。
 

　　由于其高輸出功率、緊湊性和窄的光譜線寬，QCLs在中紅外光譜區域已成為最多功能的半導體激發源。考慮到激發波長和激光源的大小，寧波海爾欣光電科技有限公司為該實驗提供了一個發射波長為9.5 µm，線寬為2 MHz的QCL激光器(QC-Qube 200831-AC712)作為DMMP-QEPAS傳感器的激發源，其輸出功率穩定性<2%，一個具有極低電流噪聲和溫漂的QCL激光器驅動電路(QC750-Touch™)，在室溫下操作，以穩定發射波長。通過激光驅動電路將QCL的溫度設定為25.5℃。如圖2所示，所使用的QCL激光器的輸出波長是驅動電流的函數，并且其波長調諧范圍落在所選吸收帶中(圖1中的綠色框區域)。圖2中繪制了QCL激光器的平均功率與驅動電流之間的線性關系，表現出良好的線性關系。此外，該激光源的小尺寸是一個顯著特點，外部尺寸約為300 cm3(65 × 65 × 70 mm3)，使激光源能夠實現緊湊的氣體傳感器。
 

　　Experimental Section
 

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　　A strong targeted absorption band is vital for DMMP detection because the practical application necessitates sensing devices with subparts-per-million sensitivities. Considering the excitation wavelength and size of the laser source, a QCL laser (Ningbo Healthy Photon Technology, QC-Qube 200831- AC712) with an emission wavelength of 9.5 µm and a linewidth of 2 MHz was employed as the excitation source of the DMMP-QEPAS sensor, which has an output power stability of < 2 %. The QCL laser driving circuit (Healthy Photon QC750-Touch™) with extremely low current noise and temperature drift operated at room temperature for stabilizing the emitting wavelength. The temperature of the QCL was set to 25.5 ℃ by means of the laser driving circuit. As shown in Fig. 2, the output wavelength of the QCL laser used is a function of the driving current and its wavelength tuning range fall in the selected absorption band (the green box area in Fig. 1). The linear relationship between the average power of the QCL laser and the driving current was plotted in Fig. 2, demonstrating good linearity. Moreover, the small size is a noticeable feature of this laser source, which has an outside dimension of ~ 300 cm3 (65 × 65 × 70 mm3), allowing the laser source to realize compact gas sensors.
 

QCL laser
 

　　HealthyPhoton, QC-Qube

 

QCL laser driving circuit
 

　　Healthy Photon, QC750-Touch™

 

　　Fig. 1. Absorption spectra of 1-ppm DMMP/N2 gas mixture (red) obtained by the FTIR spectrometer and absorption spectra of 300-ppm H2O (blue) and 5- ppm CO2 (orange) based on HITRAN database. Inset: DMMP absorption band in the range of 1040–1065 cm− 1 and wavelength tuning range of the used QCL laser.
 

　　Fig. 2. QCL emission wavelength and output optical power as a function of driving current in amplitude modulation operating mode with a duty cycle of 50 %.
 

　　結論
 

　　基于QEPAS的傳感器由于其波長獨立性具有很高的多功能性，這使得通過替換激光源可以檢測各種神經毒劑。在本研究中，開發了一種緊湊尺寸和可靠性能的ppb級QEPAS DMMP傳感器。選擇了9.56 µm的激發波長，這是DMMP吸收帶，不受H2O和CO2的干擾。優化了主要系統參數，包括激光激發功率、氣體壓力和調制頻率。最終，在0至1.5 ppm范圍內驗證了傳感器的線性，并在300毫秒的積分時間下實現了6 ppb的檢測限。我們使用真實室外空氣作為載氣檢測了500 ppb的DMMP，并獲得了與以零氣作為載氣時相同的信號幅度，從而驗證了傳感器的高選擇性。所開發的傳感器為在機場、鐵路車站、體育場館和港口等公共場所監測神經毒劑鋪平了道路。未來，可以引入時分復用技術，將多個連續可調諧中心波長的激光器耦合到傳感器系統中，從而提供廣泛的波長檢測范圍，實現對多種感興趣的神經毒劑的同時檢測。
 

　　Conclusions
 

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　　The QEPAS-based sensor has high versatility due to its wavelength independence, which makes it possible to detect various nerve agents by replacing the laser sources. In this work, a ppb-level QEPAS-based DMMP sensor was developed with a compact size and reliable performance for the first time. An excitation wavelength of 9.56 µm was chosen for the strongest DMMP band which is interference-free from H2O and CO2. The main system parameters, including the laser excitation power, the gas pressure, and the modulation frequency, were optimized. Finally, the sensor linearity was verified in the range of 0 − 1.5 ppm and a minimum detection limit of 6 ppb at an integration time of 300 ms was achieved. We detected 500 ppb DMMP with real outdoor air as the carrier gas and obtained the same signal amplitude as
 

　　that with zero air as the carrier gas, which verified the high selectivity of the sensor. The developed sensor paves the way for monitoring nerve agents in public places like airports, railroad stations, sports arenas, and ports. In the future, time division multiplexing technology can be introduced to couple multiple continuously tunable lasers with different center wavelengths into a sensor system, which would provide a broad wavelength detecting range, allowing for the simultaneous detection of several nerve agents of interest.
 

　　References
 

　　Ppb-level mid-IR quartz-enhanced photoacoustic sensor for DMMP detection using a T-shaped tuning fork, Sensors & Actuators: B. Chemical 390 (2023) 133937, https://doi.org/10.1016/j.snb.2023.133937