基于7.6μm量子级联激光的光声光谱探测N_2O气体

近年来,气候变化对地球的生态环境产生严重影响,而大气温室气体在气候变化中具有重要的作用.一氧化二氮(N_2O)作为一种重要的温室气体,其浓度变化对大气环境产生重要影响,因此对其浓度的探测在大气环境研究中具有重要意义.本文开展了基于中国自主研发的7.6μm中红外量子级联激光的共振型光声光谱探测N_2O的研究,建立了N_2O光声光谱传感实验系统.此系统在传统的光声光谱探测的基础上优化改进,采用双光束增强的方式,增加了有效光功率,进一步提高了系统的探测灵敏度.探测系统以1307.66 cm~(-1)处的N_2O吸收谱线作为探测对象,结合波长调制技术对N_2O气体进行探测研究.通过对一定浓度的N_2O气体在不同调制频率和调制振幅的光声信号的探测,确定了系统的最佳调制频率和调制振幅分别为800 Hz和90 mV.在最优实验条件下对不同浓度的N_2O气体进行了测量,获得了系统的信号浓度定标曲线.实验表明,在锁相积分时间为30 ms时,系统的浓度探测极限为150×10~(-9).通过100次平均后,系统噪声进一步降低,实现了大气N_2O的探测,浓度探测极限达到了37×10~(-9).

Detection of nitrous oxide by resonant photoacoustic spectroscopy based on mid infrared quantum cascade laser

Atmospheric greenhouse gases have great influence on the climate forcing, which is important to human being and also for natural systems. Nitrous oxide (N₂O), such as carbon dioxide and methane, is an important greenhouse gas. It plays an important role in the atmospheric environment. Therefore, sensitive measurement of N₂O concentration is of significance for studying the atmospheric environment. In this paper, a photoacoustic spectroscopy (PAS) system based on 7.6 μm mid infrared quantum cascade laser combined with resonant PAS technique is established for the sensitive detection of N₂O concentration. The PAS has been regarded as a highly sensitive and selective technique to measure trace gases. Compared with laser absorption spectroscopy, the PAS offers several intrinsic attractive features including ultra-compact size and no cross-response of light scattering. In addition, the signal of PAS is recorded with low-cost wavelength-independent acoustic transducer. The performance of the developed system is optimized and improved based on the traditional photoacoustic spectroscopic detection. Dual beam enhancement method is used to increase the effective optical power which effectively improves the detection sensitivity of the system. The N₂O absorption line at 1307.66 cm^-1 is chosen as the target line, and an operation pressure of 50 kPa is selected for reducing cross-talking from H₂O absorption line. By detecting the photoacoustic signals of a certain concentration of N₂O at different modulation frequencies and modulation amplitudes, the optimal modulation frequency and modulation amplitude of the system are determined to be 800 Hz and 90 mV, respectively. Different concentrations of N₂O gas are detected under the optimized parameters, and calibration curve of the system, that is, the curve of photoacoustic signal versus concentration of N₂O is obtained, which shows good linearity. The experimental results show that the minimum detection limit of the system is 150 ppb at a pressure of 50 kPa with an integration time of 30 ms. The system noise can be further reduced by increasing the averaging time. A minimum detection limit of 37 ppb is achieved by averaging signals 100 times, and the signal of N₂O in the atmosphere is obtained.