根据乙烷气体分子在3.3 μm处的基频吸收特性, 使用中心波长为3.337 μm室温连续带间级联激光器(ICL)和有效光程为54.6 m密集光斑多通气体吸收气室(600 mL)研制了基于波长调制光谱技术(WMS)的乙烷传感器。 详细介绍了基于WMS和二次谐波(2f)探测技术的光谱吸收法气体检测原理, 给出了目标乙烷气体吸收线的遴选细节。 此项技术的使用减小了光功率漂移对系统的影响, 使得系统最低检测下限(MDL)和稳定性能得到提升。 结合原理框图, 通过光学和电学两个模块分别详细介绍了乙烷传感系统设计方案, 描述了自主研制的软、 硬件单元和商用仪器的使用及其型号供他人参考, 并给出传感器光学配置实物图。 而且, 为匹配激光波长调制与基于压力的吸收线宽, 对气压和调制深度进行优化, 研究了调制幅度对应2f信号峰值及调制幅度与调制深度的关系, 最终确定最优气压和调制深度分别为100 Torr和0.074 cm-1, 对应的调制信号幅度为~0.026 V。 此外, 基于优化后的气压和调制深度, 使用136.8 nmol·mol-1 乙烷标准气体进行了系统灵敏度估算。 详细介绍了ICL扫描调制信号、 锁相放大及数据采集单元的参数设置, 并给出示波器记录的扫描调制信号及2f信号波形图片。 通过对比DAQ采集的2f信号和背景噪声信号, 估算系统最低检测下限为33 nmol·mol-1。 最后, 使用9个不同浓度乙烷标准气体(20～400 nmol·mol-1)分别进行~5 min系统标定测试, 并列出了拟合曲线和拟合相关度等信息。 而且, 使用浓度为48 nmol·mol-1乙烷气体样品开展连续2 h系统稳定性测试并进行Allan-Werle 方差分析。 结果显示, 该系统工作稳定, 积分时间为4 s时, 乙烷气体检测灵敏度为~0.81 nmol·mol-1。 通过增加系统积分时间至63 s, 系统灵敏度可被提高至~0.36 nmol·mol-1。

According to the fundamental absorption properties of ethane (C2H6) near 3.3 μm, a mid-infrared C2H6 sensor based on a wavelength modulation spectroscopy (WMS) technique was developed using a room temperature, continuous-wave (CW) interband cascade laser (ICL) emitting at 3.34 μm and a dense multi-pass gas cell (600 mL) with a 54.6 m optical path length. The principle of gas detection using spectral absorption method based on wavelength modulation spectroscopy and two harmonic (2f) detection technology is introduced in detail. Selection details of the target ethane absorption line are also given. The use of this technology reduces the influence of optical power drift on the system, making the minimum detection limit (MDL) and stability performance of the system get promoted. Ethane sensing system is introduced in detail through optical and electrical modules combined with the scheme. The application of self-developed software and hardware units as well as commercial instruments and their model are described for the reference to others, and physical map of the sensor optical core is also given. Moreover, the pressure and modulation depth are optimized in order to match the wavelength modulation of laser and absorption linewidth based on gas pressure. The curves of the modulation amplitude corresponding to the peak value of 2f signals and the modulation depth corresponding to modulation depth are also drawn accordingly, and finally the appropriate pressure and modulation depth are determined to be 100 Torr and 0.074 cm-1, respectively. The corresponding modulation amplitude is ~0.026 V at that point. In addition, the work of system sensitivity estimation is conducted by using 136.8 nmol·mol-1 C2H6 standard gas based on the optimized air pressure and modulation depth. The parameters setting of ICL scanning and modulation signals, phase-locked amplification as well as data acquisition are introduced in details, and pictures recorded by oscilloscope are also given. In this case, the system MDL is estimated to 33 nmol·mol-1 by comparing 2f signal acquired by DAQ and background noise signal. Finally, the fitting curves and its correlation information are described by carrying out ~5 minute system calibration tests, respectively, by using 9 different C2H6 standard gases from 20～400 nmol·mol-1. Moreover, 2 hours system stability test was conducted by using 48 nmol·mol-1 C2H6 sample. The result shows that this system works steadily and a minimum detection limit (MDL) of ~0.81 nmol·mol-1 is achieved with a measurement time of 4 s. The MDL is further improved to 0.36 nmol·mol-1 with a measurement time of 63s, based on an Allan deviation analysis for the C2H6 sensor operation.