近日，來自山東師范大學光學與光子器件技術重點實驗室的聯合研究團隊發表了一篇題為 Open-path sensor based on QCL for atmospheric N₂O measurement 的研究論文。

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.

簡介

作為重要的溫室氣體之一，氧化亞氮（N₂O）可能導致空氣污染和全球變暖。N₂O在大氣中的壽命很長，更糟糕的是，其全球變暖潛力比二氧化碳高300倍。因此，開發一種快速、實時和高精度的氣體傳感器系統，用于檢測大氣中的N₂O濃度水平，對于更好地理解全球變暖和氣候變化至關重要。

調諧二極管激光吸收光譜學（TDLAS）在高靈敏度、選擇性和快速響應領域廣泛報道，并已被證明是實時檢測N₂O的可靠工具。基于波長調制光譜學（WMS）的TDLAS已被證明是提高檢測靈敏度和降低電子噪聲的良好方法。大多數傳感器是封閉路徑系統。這嚴重限制了在遠程或露天研究中進行連續監測的實際適用性，并限制了測量的空間覆蓋范圍。為解決這一問題，本文開發了一種緊湊的開放光學路徑氣體傳感器系統。

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.

實驗細節

基于QCL的開路N₂O氣體傳感器的系統框架如圖1所示。它主要由三部分組成：激光系統、光學元件和數據處理部分。激光系統由QCL、激光驅動器和信號發生器組成。光學部件具有檢測光路和參考光路。數據處理部分包括數據采集、信號處理和顯示模塊。

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.

寧波海爾欣光電科技有限公司為此項目提供了HPQCL-Q™ 標準量子級聯激光發射頭，QC750-Touch™ 量子級聯激光屏顯驅動器，HPPD-M-B 前置放大制冷一體型碲鎘汞（MCT）光電探測器。

HealthyPhoton Technology Co., Ltd. , provided a QCL(HPQCL-Q™ ), a driver(QC750-Touch™), a HgCdTe photodetector (HPPD-M-B) for this project.

HPQCL-Q™            QC750-Touch™             HPPD-M-B

在這項工作中，需要考慮N₂O或其他物質（主要是水蒸氣）的光譜吸收干擾，以減少它們對系統特異性和準確性的副作用。如圖2(c)所示，根據HITRAN 2016數據庫，已經模擬了N₂O、CO和CO₂的吸收線強度，范圍從2020 ~ 2220 cm^-1。幸運的是，N₂O的基本振動帶在波數為2200cm^-1左右，遠離水蒸氣的吸收帶。因此，室溫下的QCL可以達到N₂O的基本振動帶，檢測靈敏度為ppb級。考慮到靈敏度和成本，選擇了中心波數為2203.73  cm^-1的QCL來檢測N₂O。QCL的中心電流和溫度分別設置為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.

結論

我們實現了用一種緊湊的開路氣體傳感器檢測大氣中的N₂O。在這種傳感器中，采用了波長調制光譜學與1f-歸一化WMS檢測策略，以提高檢測靈敏度并消除光強度波動的影響。對20 ppm N₂O標準氣體進行了校準，標準偏差為0.011 ppm，表明具有高精度。對實驗室N₂O空氣進行了連續7小時的測量，濃度的標準偏差低于1.5 ppb。我們使用Allan偏差分析得出，在1秒的積分時間下，N₂O的檢測限為1.1 ppb，而在最佳積分時間為95秒時，靈敏度可以提高到0.14 ppb。通過在自然環境中進行的為期兩天的實時測量驗證了所開發傳感器系統的長期穩定性。得出的結果充分證明我們的開放光學路徑氣體傳感器系統具有快速響應、良好穩定性、高靈敏度和高精度。

在實際應用方面，該系統可用于檢測農田和汽車尾氣中的N₂O排放。此外，我們認為通過更新具有不同發射波長的QCL，傳感器系統還可以檢測不同類型的微量氣體。

參考來源：

Open-path sensor based on QCL for atmospheric N₂O measurement,

Results in Physics 31 (2021) 104909