Sensitive Detection of CH4 and CO2 Using Frequency-Division-Multiplexing Based Quartz-Enhanced Photoacoustic Spectroscopy
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A compact double-gas sensing system was reported for simultaneous sensitive CH4 and CO2 detection, this system combined quartz-enhanced photoacoustic spectroscopy (QEPAS) and frequency division multiplexing technique. Two continuous wave distributed feedback diode lasers operating at 1654 nm and 2004 nm were employed as the light sources. The photoacoustic signals were excited by simultaneously injecting sinusoidal modulation currents with independent frequencies near the resonant frequency of the QTF into the lasers，and were subsequently demodulated to obtain the corresponding second harmonic components of CH4 and CO2. The frequency-domain results showed no interference between the photoacoustic signals of the two gases. The relationships between CH4 and CO2 concentrations and the corresponding second harmonic signals have been calibrated, and linear correlation coefficients better than 0.994 were obtained. The system performance was further evaluated through Allan deviation analysis on CH4 and CO2 with given concentration. The minimum detection limits of CH4 and CO2 for the system were obtained as 0.58×10-6 and 1.32×10-6, corresponding to the normalized noise equivalent absorption coefficients (NNEA) of 7.2×10-9 cm-1W√Hz and 9×10-9 cm-1w√Hz, respectively.
本文报道了一款紧凑型双气体传感系统，可实现甲烷（CH4）与二氧化碳（CO2）的同时高灵敏检测。该系统融合了石英增强光声光谱（quartz-enhanced photoacoustic spectroscopy, QEPAS）与频分复用技术。系统采用两款工作波长分别为1654 nm与2004 nm的连续波（continuous wave, CW）分布反馈式二极管激光器作为光源。通过向激光器同时注入频率独立且接近石英音叉（quartz tuning fork, QTF）谐振频率的正弦调制电流，激发出光声信号，随后对信号进行解调以获取甲烷与二氧化碳对应的二次谐波分量。频域分析结果表明，两种气体的光声信号之间无交叉干扰。本研究已完成甲烷、二氧化碳浓度与对应二次谐波信号之间的定标实验，所得线性相关系数均优于0.994。通过对特定浓度的甲烷与二氧化碳进行阿伦方差分析，进一步评估了系统性能。该系统对甲烷与二氧化碳的最低检测限分别为0.58×10^-6和1.32×10^-6，对应的归一化噪声等效吸收系数（normalized noise equivalent absorption coefficients, NNEA）分别为7.2×10^-9 cm^-1·W·√Hz和9×10^-9 cm^-1·W·√Hz。
