基于中红外量子级联激光器和石英增强光声光谱的CO超高灵敏度检测研究

采用石英增强光声光谱(QEPAS)技术对CO痕量气体展开检测研究. 为了实现超高灵敏度探测, 采用输出波长为4.6 μm的新颖中红外高功率分布反馈量子级联激光器为光源, 实现了对CO气体基频吸收带的激发与测量. 在优化了调制深度、气体压强和提高了CO分子的振动-转动弛豫速率后, 获得了1.95 ppbv的优异探测极限. 在分析检测结果的过程中, 讨论了能级寿命对信号强度的影响, 并对QEPAS信号强度的表达式进行了修正.

Research on high sensitivity detection of carbon monoxide based on quantum cascade laser and quartz-enhanced photoacoustic spectroscopy

Quartz-enhanced photoacoustic spectroscopy (QEPAS) technology was invented lately. Therefore it is an innovative method for trace gas detection compared with other existing technologies. In this paper, trace gas detection for carbon monoxide (CO) based on QEPAS technology is demonstrated. In order to realize high sensitive detection, a novel mid-infrared, state-of-art 4.6 μm high power, continuous wave (CW), distributed feedback (DFB) quantum cascade laser (QCL) with single mode output is used as the laser exciting source. Therefore, the strongest absorption of fundamental frequency band of CO is achieved. Using the wavelength modulation spectroscopy and the 2nd harmonic detection, the influence of laser wavelength modulation depth on QEPAS signal level is investigated. Two important parameters of Q-factor and resonant frequency for quartz tuning fork as a function of gas pressure are measured. After optimization of the modulation depth of laser wavelength, the gas pressure of CO:N₂ gas mixture and the improving speed of the V-R relaxation rate through the addition of water vapor, a minimum detection limit (MDL) of 1.95 parts per billion by volume (ppbv) for CO at gas pressure of 500 Torr and modulation depth of 0.2 cm^-1 is achieved with a 1 sec acquisition time and the addition of 2.6% water vapor in the analyzed gas mixture. Finally, the influence of level lifetime of the targeted gas on QEPAS signal amplitude is investigated by comparison of CO QEPAS sensor performance using two different CO absorption lines of R(5) and R(6) located at 2165.6 cm^-1 and 2169.2 cm^-1respectively. The expression of the QEPAS signal amplitude is modified by adding the level lifetime parameter for a better precision.