近日，來自中國科學院安徽光學精密機械研究所、先進激光技術安徽省實驗室、中國科學技術大學、法國濱海大學大氣物理化學實驗室聯合研究團隊發表了《基于釹鐵硼環形磁體陣列的雙中紅外波長法拉第旋轉光譜NOx傳感器》論文。

Recently, the joint research team from Anhui Institute of Optics and Fine Mechanics, HFIPS, Chinese Academy of Sciences, Advanced Laser Technology Laboratory of Anhui Province, University of Science and Technology of China, Laboratoire de Physicochimie de l′ Atmosph`ere, Universit´e du Littoral Cˆote d′ Opale, published an academic papers Dual mid-infrared wavelength Faraday rotation spectroscopy NOx sensor based on NdFeB ring magnet array.

 

氮氧化物（NOx，包括二氧化氮（NO2）和一氧化氮（NO））是對流層臭氧的重要前體，同時也影響羥基和過氧基自由基的濃度。大多數氣態化合物在被氧化和從空氣中去除或轉化成其他化學物質時，都會直接或間接接觸到NOx。在典型的羥基自由基水平下，NOx的壽命取決于季節和光化學反應速率，通常為幾小時。根據IPCC第六次評估報告，NOx的排放導致凈正向變暖，因為它既形成短期臭氧（變暖），又破壞環境甲烷（冷卻）。此外，NOx還導致酸沉降以及化學霧和氣溶膠的形成。由于NO和NO2在大氣光化學反應中起著核心作用，同時檢測它們有助于理解這兩種氣體的來源和去向，以及研究陸地生態系統與大氣之間的NOx交換通量。

 

Nitrogen oxides (NOx, the sum of nitrogen dioxide (NO2) and nitric oxide (NO)) are important precursors of tropospheric ozone, and they also influence the concentration of hydroxyl and peroxyl radicals. Most of the compounds that are oxidized and removed from the air or converted to other chemical species are in direct or indirect contact with NOx. At typical hydroxyl radical levels, the life time of NOx depends on the season and the photochemical reaction rate, which is typically a few hours. According to the IPCC sixth assessment report, the emis-sions of NOx result in net-positive warming from the formation of short-term ozone (warming) and the destruction of ambient methane (cooling). Additionally, NOx contributes to acid deposition and the formation of chemical smog and aerosols. Since NO and NO2 play a central role in atmospheric photochemical reactions, their simultaneous detection helps to understand the sources and sinks of these two gases, in addition to studying the NOx exchange fluxes between terrestrial eco-systems and the atmosphere. 

 

化學發光檢測（NO + O3 → NO2 + O2 + hν）是測量NOx的傳統方法。在通過化學發光反應（Mo + 3NO2 → MoO3 + 3NO）測量之前，NO2首先需要在高溫（~325°C）下轉化為NO。雖然這種方法被廣泛使用，但其他氧化氮化合物，如過乙酰亞硝酸酯（PAN）和硝酸（HNO3），可能會在測量NOx濃度時引起交叉干擾。同時，這種方法不能區分NO和NO2 。紅外吸收法也可用于測量NO和NO2。在這種方法中，通常需要通過轉化器將NO2還原為NO。由于NO和NO2是順磁分子，法拉第旋轉光譜（FRS）可以用作實現其高度敏感和選擇性檢測的潛在方法。FRS通過檢測氣態介質在縱向磁場中引起的光偏振狀態的變化，實現對物種濃度的高度敏感檢測。該方法通過測量光學色散實現氣體濃度的檢測，因此其動態測量范圍比基于比爾-蘭伯定律的吸收光譜（動態范圍上限≤10%）更大。FRS的另一個重要優勢是它對于順磁性物種（如水和二氧化碳）具有較強的抗干擾能力，從而使其具有高樣品特異性。

 

Chemiluminescence detection (NO+O3→NO2+O2+hν) is the con-ventional method for measuring NOx. NO2 first needs to be con-verted to NO at high temperature (~325 ?C) before it can be measured by chemiluminescence reaction (Mo+3NO2→MoO3+3NO). Although this method is more widely used, other oxidized nitrogen compounds, such as peroxyacetyl nitrate (PAN) and nitric acid (HNO3), can cause cross-interference in the measurement of NOx concentrations. Simultaneously, this method is non-selective in discriminating between NO and NO2. The infrared absorption method can also be used for NO and NO2 measurements. In this method, NO2 usually needs to be reduced to NO by the converter. As NO and NO2 are paramagnetic molecules, Faraday rotation spectroscopy (FRS) can be used as a po-tential method to achieve their highly sensitive and selective detection. FRS enables highly sensitive detection of species concentrations by detecting changes in the polarization state of light induced by a gaseous medium immersed in a longitudinal magnetic field. This method realizes the detection of gas concentration by measuring optical dispersion, so it has a higher dynamic measurement range than absorption spectroscopy (dynamic range upper limit ≤10%) based on Beer-Lambert law. Another significant advantage of FRS is that it is reasonably immune to diamagnetic species (e.g., water and carbon dioxide), which allows it to exhibit high sample specificity. 

 

大多數這些報道的FRS傳感器使用螺線管提供外部縱向磁場，從而導致能耗高和產生過多焦耳熱。產生目標磁場所需的高電流交流電路會產生不受控制的電磁干擾（EMI），通常會降低FRS傳感器的長期穩定性。此外，當前報道的FRS傳感器只能在吸收池中進行單組分測量，不能滿足復雜環境中同時進行多組分測量的需求。

 

Most of these reported FRS sensors use solenoid coils to provide an external longitudinal magnetic field, which makes them suffer from high power consumption and excessive Joule heat generation. The high-current alternating current circuit required to generate the target magnetic field produces uncontrolled electromagnetic interference (EMI), which usually deteriorates the long-term stability of the FRS sensors. In addition, the currently reported FRS sensors are only capable of single-component measurements in the absorption cell and cannot meet the demand for simultaneous multi-component measurements in complex environments. 

 

在本研究中，提出了一種新型的低能耗FRS傳感器，基于釹鐵硼（NdFeB）環形磁體陣列，實現在單個吸收池中同時檢測NO和NO2。分析了同軸雙波長赫里特吸收池（DWHC）的環形磁體陣列的磁場分布特性。使用兩臺室溫連續波中紅外量子級聯激光器（QCL），波長分別為5.33 µm（1875.81 cm−1）和6.2 µm（1613.25 cm−1），同時探測DWHC內的磁光效應。通過對激光波長進行高頻調制，有效抑制了1/f噪聲。優化了雙波長FRS NOx傳感器的性能，包括調制幅度、調制頻率、樣品氣壓和分析器偏置角。本研究提出的FRS傳感器為現場可部署的微量氣體檢測設備提供了理想解決方案。寧波海爾欣光電科技有限公司為此研究提供了HPPD-M-B 前置放大制冷一體型碲鎘汞（MCT）光電探測器，用以分別檢測2個激光束。

 

In the present work, a novel low-power FRS sensor based on a neodymium-iron-boron (NdFeB) ring magnet array was proposed to achieve simultaneous detection of NO and NO2 in a single absorption cell. The magnetic field distribution characteristics of a ring magnet array coaxial to a dual-wavelength Herriott cell (DWHC) were analyzed. Two room-temperature continuous wave mid-infrared quantum cascade lasers (QCL) with wavelengths of 5.33 µm (1875.81 cm−1) and 6.2 µm (1613.25 cm−1), respectively, were used simultaneously to probe magneto-optical effects within the DWHC. The 1/f noise was effectively suppressed by high-frequency modulation of the laser wavelength. The performance of the dual-wavelength FRS NOx sensor was optimized with respect to modulation amplitude, modulation frequency, sample gas pressure, and analyzer offset angle. The FRS sensor proposed in this work provides a preferable solution for field deployable trace gas detection equipment. The laser detected by two TEC-cooled mid-infrared thermoelectrically cooled mercury-cadmium- telluride (MCT) photodetectors (Healthy Photon, model HPPD-B- 10–150 K).

 

 

 

(a) Schematic diagram of the dual mid-infrared wavelength FRS NOx sensor based on a NdFeB ring magnet array;

 

(b) Optical layout of the FRS NOx sensor.

 

 

thermoelectrically cooled mercury-cadmium- telluride (MCT) photodetectors (Healthy Photon, model HPPD-B- 10–150 K)

 

結論

本研究開發了一種基于NdFeB環形磁鐵陣列的雙中紅外波長FRS傳感器，用于同時檢測NO2和NO。在光學路徑長度為23.7米，積分時間為100秒的條件下，NO2和NO的檢測限分別為0.58 ppb和0.95 ppb。高頻激光波長調制與外部靜態磁場相結合，最大程度地減小了低頻噪聲對FRS信號的影響。基于有限元方法分析了使用的永磁體陣列的磁場分布特性，幫助確定與其耦合的吸收池長度。采用雙波長x赫里特吸收池放大兩種不同偏振光波長與氮氧化物分子之間的相互作用，從而實現了在單個吸收池內對兩種順磁分子的高度敏感檢測。本文提出的FRS NOx傳感器在大氣環境監測或生態系統NOx通量觀測等領域，具有進一步發展成為便攜式、可在實地使用的儀器的巨大潛力。

 

Conclusion

 

In this work, a dual mid-infrared wavelength FRS sensor based on a NdFeB ring magnet array was developed for the simultaneous detection of NO2 and NO. The detection limits for NO2 and NO were 0.58 ppb and 0.95 ppb, respectively, at an optical path length of 23.7 m and an integration time of 100 s. High frequency laser wavelength modulation was combined with an external static magnetic field to minimize the effect of low frequency noise on the FRS signal. The magnetic field distribution characteristics of the used permanent magnet array were analyzed based on the finite element method, which helped to determine the length of the absorption cell coupled to it. A dual-wavelength Herriott cell was used to amplify the interaction between two different wavelengths of linearly polarized light and nitrogen oxide molecules, thus achieving highly sensitive detection of two paramagnetic molecules within a single absorption cell. The FRS NOx sensor presented in this work shows great potential for further development into a portable, field-deployable instrument with applications in atmospheric environ-mental monitoring or ecosystem NOx flux observation. 

 

 

(a) Schematic diagram of a dual-wavelength Herriott cell (DWHC) with a NdFeB ring magnet array;

 

(b) Characteristics of the magnetic inductance line distribution around a NdFeB ring magnet array;

 

(c) Ray tracing results in a DWHC;

 

(d) Spot distribution on a concave mirror.

 

 

Optimization of laser modulation frequency for the dual mid-infrared wavelength FRS NOx sensor.

 

 

Optimization of laser modulation amplitude for the dual mid-infrared wavelength FRS NOx sensor.

 

 

(a), (b) Measured FRS NOx signal as a function of analyzer angle;

 

(c), (d) Calculated FRS NOx noise as a function of analyzer angle;

 

(e), (f) Calculated SNR as a function of analyzer angle.

 

 

(a), (b) FRS signals for different concentrations of NOx;

 

(c), (d) Linear dependence of FRS signal amplitude as a function of NOx concentration.

 

 

Allan deviation plot of the dual mid-infrared wavelength FRS NOx sensor.

 

 

Reference

 

Yuan Cao, Kun Liu, Ruifeng Wang, Guishi Wang, xiaoming Gao, Weidong Chen，Dual mid-infrared wavelength Faraday rotation spectroscopy NOx sensor based on NdFeB ring magnet array, Sensors & Actuators: B. Chemical 388 (2023) 133805

 

https://doi.org/10.1016/j.snb.2023.133805