基于环形阵列永磁体的法拉第旋转光谱NO2传感器
 

　　NO2 Sensor Based on Faraday Rotation Spectroscopy Using Ring Array Permanent Magnets
 

　　近日，来自中国科学院安徽光学精密机械研究所、中国科学院沈阳应用生态研究所、中国科学技术大学、法国滨海大学的联合研究团队发表了一种基于法拉第旋转光谱的、采用环形阵列永磁体NO2传感器。
 

　　Recently, the joint research team from Anhui Institute of Optics and Fine Mechanics, HFIPS, Chinese Academy of Sciences, Institute of Applied Ecology, Chinese Academy of Sciences, University of Science and Technology of China, and Université du Littoral Côte d’Opale  published a NO2 Sensor Based on Faraday Rotation Spectroscopy Using Ring Array Permanent Magnets.
 

　　法拉第旋转光谱(FRS)通过检测沉浸在外部纵向磁场中的气体介质所引起的线偏振光偏振状态的变化，从而实现对顺磁分子的高选择性和高灵敏度检测。该光谱检测方法对水汽、CO2等抗磁性分子具有天然的免疫力，这使得其表现出高度的样品特异性。同时，由于采用了一对相互接近正交的偏振器极大抑制了激光噪声，因此法拉第旋转光谱具有非常高的检测灵敏度。
 

　　Farraday Rotational Spectroscopy (FRS) achieves highly selective and sensitive detection of paramagnetic molecules by detecting the changes in polarization state of linearly polarized light induced by the gas medium immersed in an external longitudinal magnetic field. This spectroscopic detection method exhibits inherent immunity to diamagnetic molecules such as water vapor and CO2, which results in a high degree of sample specificity. Additionally, the implementation of a pair of closely spaced orthogonal polarizers effectively suppresses laser noise, thus providing FRS with a very high detection sensitivity.
 

　　通常情况下，使用螺线管提供纵向磁场来产生磁光效应。然而，这种方法存在功耗过大和易受电磁干扰的缺点。研究团队提出了一种基于钕铁硼永磁体环形阵列和Herriott多次通过吸收池相结合的新型FRS方法。根据磁场的空间分布特性，使用14个相同的钕铁硼永磁体环以非等距形式组合，产生纵向磁场。在长度为380毫米的范围内，平均磁场强度为346高斯。宁波海尔欣光电科技有限公司为该项目提供了前置放大制冷一体型碲镉汞红外探测器(HPPD-B-08-10-150 K)，项目团队使用量子级联激光器以40毫瓦的光功率，针对最佳的441 ← 440 Q支氮氧化物跃迁(1613.25 cm–1，6.2 μm)。与Herriott多次通过吸收池耦合，积分时间为70秒，实现了0.4 ppb的最低检测限。实验结果也表明，低功耗FRS二氧化氮传感器有望发展成为一个稳健的现场可部署的环境监测系统。
 

　　Usually, a solenoid coil is used to provide a longitudinal magnetic field to produce the magneto-optical effect. However, such a method has the disadvantages of excessive power consumption and susceptibility to electromagnetic interference. The research team proposed a novel FRS approach based on a combination of a neodymium iron boron permanent magnet ring array and a Herriott multipass absorption cell is proposed. A longitudinal magnetic field was generated by using 14 identical neodymium iron boron permanent magnet rings combined in a non-equidistant form according to their magnetic field’s spatial distribution characteristics. The average magnetic field strength within a length of 380 mm was 346 gauss. HealthyPhoton Co.,Ltd provided an integrated TE-cooled mercury cadmium telluride (MCT) infrared detector with front-end amplification(HPPD-B-08-10-150 K) for this project. A quantum cascade laser was used to target the optimum 441 ← 440 Q-branch nitrogen dioxide transition at 1613.25 cm–1 (6.2 μm) with an optical power of 40 mW. Coupling to a Herriott multipass absorption cell, a minimum detection limit of 0.4 ppb was achieved with an integration time of 70 s. The low-power FRS nitrogen dioxide sensor proposed in this work is expected to be developed into a robust field-deployable environment monitoring system.
 

静态磁场法拉第旋转光谱传感装置 

　　Static magnetic field Faraday rotation spectral sensing device
 

　　海尔欣前置放大制冷一体型碲镉汞红外探测器(HPPD-B-08-10-150 K)

　　Integrated preamplifier and cryocooler type mercury cadmium telluride (MCT) infrared detector
 

环形阵列永磁体及其纵向磁场分布特征 

　　Circular array permanent magnets and their longitudinal magnetic field distribution characteristics
 

　　(a) 对于等距离的NdFeB永磁环阵列，模拟得到了中央纵向磁场的分布情况。
 

　　(b) 对于非等距离的NdFeB永磁环阵列，模拟得到了中央纵向磁场的分布情况(黑线)，并进行了实测(红线)。
 

　　(c) 示意图显示了Herriott腔和非等距离的NdFeB永磁环阵列的配置。
 

　　(a) Simulated distribution of the central longitudinal magnetic field for an equidistant NdFeB permanent magnet ring array;
 

　　(b) simulated (black line) and measured (red line) distributions of the central longitudinal magnetic field for a non-equidistant NdFeB permanent magnet ring array;
 

　　(c) schematic configuration of the Herriott cell and the non-equidistant NdFeB permanent magnet ring array.
 

法拉第旋转光谱信号及其信噪比与检偏器偏转角度的变化关系

　　The Relationship between FRS signal and its SNR and the Deflection Angle of the Polarizer
 

　　(a) 法拉第旋转光谱信号幅度
 

　　(b) SNR作为分析器角度α的函数
 

　　(a) FRS signal amplitude and
 

　　(b) SNR as a function of the analyzer angle α.