Recently, a collaborative research team from Key Laboratory of Opto-Electronic Information Acquisition and Manipulation of Ministry of Education and Laser Spectroscopy and Sensing Laboratory, Anhui University, and HealthyPhoton Technology Co., Ltd. published a research paper titled Measurements of line strengths for NO₂ near 6.2 μm using a quantum cascade laser spectrometer.

近日，來自安徽大學光電信息獲取與處理教育部重點實驗室、安徽大學激光光譜與傳感實驗室、寧波海爾欣光電科技有限公司的聯(lián)合研究團隊發(fā)表了一篇題為Measurements of line strengths for NO₂ near 6.2 μm using a quantum cascade laser spectrometer的研究論文。

Research Background研究背景

Nitrogen dioxide (NO₂) is a common pollutant that comes primarily from the emissions of burning fossil fuels, natural lightning, and microbial processes in soil. Atmospheric NO₂ contributes to the formation of ground-level ozone. It can cause photochemical smog and lead to increased acidity of rain. Continuous exposure to high NO₂ concentration may result in a wide variety of short- and long-term adverse health effects on the respiratory system of humans and animals. Therefore, developing a cost-effective and robust sensor system for NO₂ monitoring is crucial.

二氧化氮是一種常見的污染物，主要來自化石燃料燃燒排放、自然雷電以及土壤中的微生物過程。大氣中的NO₂有助于地面臭氧的形成，可能導致光化學煙霧，并導致雨水酸度增加。持續(xù)暴露于高濃度的NO₂可能對人類和動物的呼吸系統(tǒng)產生各種短期和長期的不良健康影響。因此，開發(fā)一種成本效益高、穩(wěn)健的NO₂監(jiān)測傳感器系統(tǒng)至關重要。

Many technical solutions have been developed for NO₂ detection. The chemiluminescence and wet chemical analysis are commonly used for NO₂ detection. However, these methods have a slow response time and suffer from low selectivity in discriminating between NO and NO₂, which limit their application. Optical methods based on absorption spectroscopy provide powerful access for trace gas analysis with extremely high sensitivity, selectivity, and fast response. Laser-based absorption spectroscopy techniques in mid-IR molecular fingerprint region are ideal for trace gas analysis because most atmospheric species have strong fundamental vibrational transitions in this spectral region, which allows highly sensitive and selective detection of trace gases. The commercial available continuous-wave (CW) quantum cascade lasers (QCLs) in the mid-IR spectral region have been widely used for developing spectroscopic techniques for quantitative analysis of NO₂.

已經開發(fā)了許多技術解決方案用于NO₂檢測。化學發(fā)光和濕化學分析通常用于NO₂檢測。然而，這些方法響應時間較慢，且在區(qū)分NO和NO₂時選擇性較低，限制了它們的應用。基于吸收光譜學的光學方法具有高度的靈敏度、選擇性和快速響應，為痕量氣體分析提供了強大的手段。基于中紅外分子指紋區(qū)的激光吸收光譜技術對于痕量氣體分析非常理想，因為大多數大氣成分在該光譜區(qū)域具有強烈的基本振動躍遷，從而實現(xiàn)對痕量氣體的高靈敏度和選擇性檢測。商業(yè)上可用的中紅外光譜區(qū)的連續(xù)波（CW）量子級聯(lián)激光器（QCLs）已廣泛用于發(fā)展NO₂ 的定量分析的光譜技術。

Experimental setup實驗設置

In this work, a mid-IR CW-QCL-based laser absorption spectrometer is constructed in our laboratory to revise the spectral region from 1629 cm^?1 to 1632 cm^?1. The schematic of the CW-QCL-based spectroscopic setup used to investigate the NO₂ absorption spectroscopy line parameters is shown in Fig. 1.

在這項工作中，我們在實驗室中構建了一臺基于中紅外CW-QCL的激光吸收光譜儀，以修訂波數從1629 cm^?1 到 1632 cm^?1的光譜區(qū)域。圖1顯示了用于研究NO₂ 吸收光譜線參數的基于中紅外CW-QCL的光譜設置的示意圖。

Fig. 1. Experimental setup of the CW-QCL based laser spectrometer.

寧波海爾欣光電科技有限公司為此項目提供了激光發(fā)射器（QC-qube^TM）與驅動器（QC750-Touch^TM）。一個CW室溫QCL芯片被封裝在一個熱電（TE）制冷的光束整形包裝中，由一個集成的溫度和低噪聲電流控制器驅動。

QC-qube^TM

QC750-Touch^TM

A CW RT QCL chip is packaged in a thermoelectrically (TE) cooled beam collimation package (Q-qubeTM, HealthyPhoton Co., Ltd.), which is driven by an integrated temperature and low noise current controller (QC750-TouchTM, HealthyPhoton Co., Ltd.). The laser source is operating in the wavelength region from 1629 cm^?1 to 1632 cm^?1 without mode hops and has an average output power of 30 mW. The laser frequency is scanned across absorption lines using a triangular wave at a typical frequency of 100 Hz. The linewidth of the laser is approximately<10 MHz, and thus the broadening induced by the laser line profile can be neglected. The laser beam is initially collimated and sent through a sample cell with an optical path length of 29.6 cm. A wedged CaF2 window placed at the Brewster angle is used to avoid residual etalon fringes. The QCL output beam is combined with a visible red light (632.8 nm) by a ZnSe beam splitter to facilitate the optical alignments of the QCL output beam. The main beam that transmits through the sample cell is focused by a convex lenses into a TE-cooled, high-speed IR photovoltaic detector (PVI-4TE-6, Vigo, Poland) that can operate at RT. Therefore, the detector does not require liquid nitrogen cooling, simplifies the routine use of the system, and allows for longterm automated operation. Data are subsequently acquired using a data acquisition board card (National Instruments, USB 6259). The other part of the beam is coupled into an etalon, which is constructed with two ZnSe mirrors and has a free spectral range of 0.0163 cm?1.

激光源在1629 cm^?1到1632 cm^?1的波長范圍內工作，沒有模式跳變，并且平均輸出功率為30 mW。激光頻率通過三角波在典型頻率100 Hz下進行掃描。激光的線寬約為<10 MHz，因此可以忽略激光線型引起的展寬。激光束最初被準直，并通過一個光程為29.6 cm的樣品池。在Brewster角度放置的楔形CaF2窗口用于避免殘留的Etalon條紋。QCL輸出光束與可見紅光（632.8 nm）通過ZnSe分束鏡相結合，以便于對準QCL輸出光束的光學調整。透過樣品池的主光束通過凸透鏡聚焦到一個TE制冷的高速紅外光伏探測器，該探測器可以在室溫下操作。因此，探測器不需要液氮制冷，簡化了系統(tǒng)的常規(guī)使用，并允許長期自動化操作。數據隨后使用數據采集板卡進行獲取。光束的另一部分被耦合到一個Etalon中，該Etalon由兩個ZnSe鏡構成，自由光譜范圍為0.0163 cm^?1。

Fig. 2. DFB-QCL tuning features at different operating temperatures and operating currents.

Conclusion結論

In this study, a compact spectroscopic sensor based on a TE cooled RT CW-QCL was developed for trace NO₂ detection. The spectra of NO₂ and N2 mixtures with high resolution were detailedly investigated at RT (~296 K) and in the pressure range of 0–90 mbar. Absorption spectra were fitted with a standard Voigt profile. Accurate measurements of line intensities and N2 pressureinduced broadening coefficients for 43 lines of NO2 around 6.2 μm were performed. This spectral region is highly suitable for high sensitive detection of NO₂ concentration. Our results agree well with those given in the latest HITRAN16 database in terms of line strength. The experimentally spectroscopic parameters will be useful for upgrading our newly developed NO₂ gas sensor system for atmospheric trace gas monitoring and industrial process control. In addition, we hope that the results will be valuable to the spectroscopic databases of NO₂ molecule.

本研究中，我們開發(fā)了一款基于熱電制冷的室溫連續(xù)波量子級聯(lián)激光器（RT CW-QCL）的緊湊型光譜傳感器，用于痕量NO₂的檢測。在室溫（~296 K）和0-90毫巴的壓力范圍內，詳細研究了NO₂和N₂混合物的高分辨率光譜。吸收光譜采用標準Voigt輪廓進行擬合。對于約6.2微米附近的43條NO₂譜線，進行了線強度和N₂壓力誘導展寬系數的準確測量。這個光譜區(qū)域非常適合于對NO₂濃度進行高靈敏度檢測。我們的結果在譜線強度方面與最新的HITRAN16數據庫相當一致。實驗性的光譜參數將有助于升級我們新開發(fā)的用于大氣痕量氣體監(jiān)測和工業(yè)過程控制的NO₂氣體傳感器系統(tǒng)。此外，我們希望這些結果對于NO₂分子的光譜數據庫具有重要價值。

reference參考來源：

Measurements of line strengths for NO₂ near 6.2 μm using a quantum cascade laser spectrometer, Journal of Quantitative Spectroscopy & Radiative Transfer 250 (2020) 107047