近日，来自山东师范大学光学与光子器件技术重点实验室的联合研究团队发表了一篇题为 Open-path sensor based on QCL for atmospheric N2O measurement 的研究论文。
Recently, a collaborative research team from the Shandong Provincial Engineering and Technical Center of Light Manipulations & Shandong Provincial Key Laboratory of Optics and Photonic Device, School of Physics and Electronics, Shandong Normal University published a research paper titled Open-path sensor based on QCL for atmospheric N2O measurement.
简介
作为重要的温室气体之一，氧化亚氮（N2O）可能导致空气污染和全球变暖。N2O在大气中的寿命很长，更糟糕的是，其全球变暖潜力比二氧化碳高300倍。因此，开发一种快速、实时和高精度的气体传感器系统，用于检测大气中的N2O浓度水平，对于更好地理解全球变暖和气候变化至关重要。
调谐二极管激光吸收光谱学（TDLAS）在高灵敏度、选择性和快速响应领域广泛报道，并已被证明是实时检测N2O的可靠工具。基于波长调制光谱学（WMS）的TDLAS已被证明是提高检测灵敏度和降低电子噪声的良好方法。大多数传感器是封闭路径系统。这严重限制了在远程或露天研究中进行连续监测的实际适用性，并限制了测量的空间覆盖范围。为解决这一问题，本文开发了一种紧凑的开放光学路径气体传感器系统。
Introduction
As one of the important greenhouse gases, nitrous oxide (N2O), can give rise to air pollution and global warming. N2O has a long atmospheric lifetime, and worse its global warming potential is 300 times higher than carbon dioxide. Therefore, the development of a fast, real-time, and high-precision gas sensor system for detecting the atmospheric N2O concentration level is essential for the better understanding of global warming and climate changes.
Tunable diode laser absorption spectroscopy (TDLAS), as a versatile technique, has be widely reported for real-time analysis of gas compositions in the field of high sensitivity, selectivity, and fast response and it has been demonstrated as a dependable tool for real-time detection of N2O. Wavelength modulation spectroscopy (WMS) based TDLAS has been proved to be a good method for improving the detection sensitivity and reducing the electronic noise. Most of sensors are closed-path systems. This severely restricts the practical applicability of continuous monitoring in remote or open-field researches, and limits the spatial coverage of the measurements. To address this problem, in this paper, we develop a compact openoptical-path gas sensor system.
实验细节
基于QCL的开路N2O气体传感器的系统框架如图1所示。它主要由三部分组成：激光系统、光学元件和数据处理部分。激光系统由QCL、激光驱动器和信号发生器组成。光学部件具有检测光路和参考光路。数据处理部分包括数据采集、信号处理和显示模块。
The system framework of the open-path N2O gas sensor based on QCL is depicted in Fig. 1. It mainly consists of three parts: the lasersystem, the optical elements, and the data processing section. The laser-system consists of a QCL, a laser drive and a signal generator. The optical component has the detecting and reference optical paths. The data processing section includes the data acquisition, signal processing and display modules.
Fig. 1. The N2O sensor system schematic diagram.
宁波海尔欣光电科技有限公司为此项目提供了HPQCL-Q™ 标准量子级联激光发射头，QC750-Touch™ 量子级联激光屏显驱动器，HPPD-M-B 前置放大制冷一体型碲镉汞（MCT）光电探测器。
HealthyPhoton Technology Co., Ltd. , provided a QCL(HPQCL-Q™ ), a driver(QC750-TouchTM), a HgCdTe photodetector (HPPD-M-B) for this project.
HPQCL-Q™ QC750-Touch™ HPPD-M-B
在这项工作中，需要考虑N2O或其他物质（主要是水蒸气）的光谱吸收干扰，以减少它们对系统特异性和准确性的副作用。如图2(c)所示，根据HITRAN 2016数据库，已经模拟了N2O、CO和CO2的吸收线强度，范围从2020 ~ 2220 cm-1。幸运的是，N2O的独特基本振动带在波数为2200cm-1左右，远离水蒸气的吸收带。因此，室温下的QCL可以达到N2O的基本振动带，检测灵敏度为ppb级。考虑到灵敏度和成本，选择了中心波数为2203.73 cm-1的QCL来检测N2O。QCL的中心电流和温度分别设置为330 mA和36.0 °C。
Details
In this work, we need to take the spectral absorption interference of N2O or other substances (mostly water vapor) into consideration in order to reduce their side effects on the specificity and accuracy of the system. As depicted in Fig. 2(c), the absorption line intensity of N2O, CO and CO2 have been simulated from 2020 ~ 2220cm-1, according to the HITRAN 2016 database. Fortunately, the unique fundamental vibration band of N2O is around wavenumber of 2200cm-1, which is stay away from the absorption band of water vapor. Therefore, the N2O fundamental vibration band can be reached by room-temperature QCL, and the detection sensitivity is ppb level. Taking sensitivity and cost into consideration, a QCL emitting at center wavenumber of 2203.73 cm-1 was selected for detection of N2O. Of the QCL, the central current and temperature were set at 330 mA and 36.0 ◦C, respectively.
Fig. 2. (a): The relationship between the QCL emission wavenumber and drive current. (b): The dependence the QCL emission wavenumber and temperature. (c): The intensity distribution of absorption lines of N2O, CO and CO2 in the range of 2020 ~ 2220 cm-1.
结论
我们实现了用一种紧凑的开路气体传感器检测大气中的N2O。在这种传感器中，采用了波长调制光谱学与1f-归一化WMS检测策略，以提高检测灵敏度并消除光强度波动的影响。对20 ppm N2O标准气体进行了校准，标准偏差为0.011 ppm，表明具有高精度。对实验室N2O空气进行了连续7小时的测量，浓度的标准偏差低于1.5 ppb。我们使用Allan偏差分析得出，在1秒的积分时间下，N2O的检测限为1.1 ppb，而在最佳积分时间为95秒时，灵敏度可以提高到0.14 ppb。通过在自然环境中进行的为期两天的实时测量验证了所开发传感器系统的长期稳定性。得出的结果充分证明我们的开放光学路径气体传感器系统具有快速响应、良好稳定性、高灵敏度和卓越的精度。
在实际应用方面，该系统可用于检测农田和汽车尾气中的N2O排放。此外，我们认为通过更新具有不同发射波长的QCL，传感器系统还可以检测不同类型的微量气体。
Conclusions
A compact open-path gas sensor is developed for detection of atmospheric N2O. In such sensor, the wavelength modulation spectroscopy with 1f-normalized WMS detection strategy is employed to improve the detection sensitivity and eliminate the influence of light intensity fluctuations. The 20 ppm N2O standard gas is calibrated and the standard deviation is 0.011 ppm, which demonstrates it has high accuracy.
Laboratory N2O air is continuously measured for 7h and the standard deviation of the concentration is below 1.5 ppb. We use Allan deviation analysis to derive that N2O detection limit is 1.1 ppb at integration time of 1 s, and the sensitivity can be improved to 0.14 ppb at an optimal integration time of 95 s. The long-term stability of the developed sensor system is verified by a two-day real-time measurement in ambient air. The derived results well demonstrate our open-optical-path gas sensor system has a fast response, a good stability, a high sensitivity, and an excellent accuracy.
In terms of practical application, the developed system can be used to detect N2O emissions in farmland and automobile exhaust. Furthermore, we believe the sensor system can detect different types of trace gases, by updating QCLs with different emission wavelengths.
参考来源：
Open-path sensor based on QCL for atmospheric N2O measurement,
Results in Physics 31 (2021) 104909
https://doi.org/10.1016/j.rinp.2021.104909
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