基于光纤耦合宽带LED光源的Herriott池测量NO2的研究

NO2 measurement using fiber coupled broadband LED source combining a Herriott multi-pass cell

本文针对气溶胶吸收光声光谱仪需用较高浓度二氧化氮(NO2)进行标定的需求,开展了基于光纤耦合宽带LED光源的Herriott型多通池测量NO2的研究,解决了NO2的简便、快速和高精度测量问题.首先依据光线传输理论、仿真分析了Herriott型多通池,并采用优化的仿真结果设计了有效光程为26.1 m的光学多通吸收池,以增强吸收池内待测NO2气体的光吸收.针对LED光源的发光面、发散角大,常规准直的输出光难于在Herriott型多通池内来回传输的问题,本研究中将LED光源的输出光耦合进入一根单模光纤,然后用透镜准直后导入光学多通吸收池中,实现基于光学多通吸收池的宽带LED吸收光谱测量NO2浓度,最终实现了对NO2检测浓度极限1μmol/mol的预期设计值,对46μmol/mol的NO2测量结果表明,测量精度达到0.1%.最后开展了此NO2测量系统与气溶胶吸收光声光谱仪同时测量不同浓度NO2的观测研究,结果表明所测量NO2浓度与光声光谱信号呈现出很好的线性关系,线性度优于99.9%.基于宽带LED光源和Herriott型多通池的NO2测量系统,具有价格低廉、结构简单和易用的特点,可以满足NO2吸收法标定气溶胶吸收光声光谱仪的需求,也可用于化工领域对NO2的快速分析测量.

An NO2 sensor based on a fiber coupled broadband LED source with a Herriott multi-pass cell is developed and demonstrated for calibrating aerosol absorption photoacoustic(PA) spectrometer. At first, a Herriott multipass cell with an effective optical path of 26.1 m is designed based on the theory of light transmission, for increasing the light absorption of NO2 in the cell. It is difficult to obtain a high-quality beam of LED by using the conventional collimating method that enables the collimated output beam to transmit back and forth in the multi-pass cell, due to the large emitting surface and divergence angle of the LED. So, in the present work, the emission of the LED is coupled into a single model fiber, and then collimated by using a lens. The LED spectrum does not change before and after fiber coupling. The collimated beam with a central wavelength of 438.5 nm is coupled into the multi-pass cell. The output beam passing through the multi-pass cell is detected by using a spectrometer for retrieving the NO2 concentration. Finally, an expected concentration detection limit of 1 μmol/mol(3δ) is achieved within 1 s acquisition time and the signal-to-noise ratio(SNR) is 40. By analyzing the result measured with 46 μmol/mol NO2, a measurement precision of 0.1% is achieved. In order to calibrate the aerosol absorption PA spectrometer, the NO2 sensor and the aerosol absorption PA spectrometer based on 450 nm are used to measure different concentrations of NO2, simultaneously. The results show that the measured NO2 concentration has a good linear relationship with the PA spectrum signal, and the linearity is better than 99.9%. This good linear relationship further shows the feasibility and reliability of the NO2 sensor.The slope of calibration curve after normalizing the power is 0.95 nV/(mW·Mm-1). Using this calibration result,the PA signal measured with aerosol absorption PA spectrometer is transformed into the absorption coefficient.The developed NO2 measuring system based on a broadband LED light source and Herriott multi-pass cell has the advantages of low cost, simple structure and easy use. It can be used to calibrate the aerosol absorption PA spectrometer, and also to measure NO2 in industry.