基于低功率分布反馈激光器和数字锁相技术的高灵敏光声气体传感器

光声信号强度与光功率成正比，然而，高功率激光光源存在功耗高、驱动控制电路复杂、低成本高质量的光源可选择范围窄等缺点，此类光源多集中在>6 μm波段，难以实现对基频吸收带位于2～6 μm波段的分子进行有效探测。而且，基于商用驱动控制仪器的光声气体传感器体积较大，不能满足多点连续移动监测工作的需要。利用输出功率为5.2 mW的分布反馈、带间级联激光器(ICL)和基于石英音叉的光声光谱探测方法，在3～4 μm波段实现了nmol·mol-1水平气体分子浓度测量。使用的ICL靶向乙烷(C₂H₆)基频吸收带的强吸收线2 996.88 cm-1。通过使用自主研制的数字锁相放大器及数字激光驱动控制方法，结合波长调制光谱技术，实现了高灵敏检测，同时，有效减小了系统体积并简化了数据获取和处理过程。首先，结合系统原理结构，顺次介绍了设计方案以及光、电等模块的设计细节。分析了目标气体及其临近干扰气体吸收谱线的模拟情况，以及不同气压对谱线展宽及重叠干扰的影响，最终确定系统工作气压为200 Torr。然后，通过对100～1 000 nmol·mol-1共6种浓度C₂H₆进行单周期光谱扫描测试分析，推断系统最低检测下限(MDL)<100 nmol·mol-1。对上述各浓度样品分别进行~10 min二次谐波(2f)信号峰值提取测试，系统线性性能良好，相关度为0.999 65，同时，明确了气体浓度与2f信号峰值的对应关系。最后，通过对氮气连续1小时测试得出系统噪声为~0.347 V，由此估算信噪比和系统灵敏度分别为~28.56和~40 nmol·mol-1。介绍的新型中红外C₂H₆传感器不仅实现了nmol·mol-1级测量，而且，使用自主研制的数字驱动和锁相放大器有效减小了系统体积，弥补了使用商用仪器占用体积大的不足，为将来实现小型化、移动式测量的目标奠定了一定基础。此外，对于功率消耗无限制的其他应用，可通过进一步完善和改进锁相和前置放大等模块的性能以及使用输出功率更高的光源进一步提高传感器灵敏度，并应用于更多场景。

High Sensitive Photoacoustic Gas Sensor Based on Low Output Power Laser and Digital Lock-in Technology

The intensity of photoacoustic signal is proportional to optical power. However, high power laser source has many disadvantages, such as high power consumption, complex driving and controlling circuit, the lasers with high quality and lower cost laser can't be attained easily. These kinds of laser source mostly focus on the band over 6 micron, which makes it difficult to detect the molecule in fundamental absorption band of 2～6 micron effectively. Moreover, photoacoustic gas sensors based on commercial driving and controlling instruments are more likely to have large volume, which can’t meet the needs of multi-points and continuous mobile monitoring. In this paper, nmol·mol-1 level measurement applied to molecule concentrations was realized in 3～4 micron band using low output power interband cascade laser (ICL) and a detection method which based on quartz tuning fork and photoacoustic spectroscopy. The ICL was used to target a strong ethane absorption ling at 2 996.88 cm-1 in its fundamental absorption band. High sensitive detection and system volume reduction were realized by using self-developed digital lock-in amplifier and digital laser driving and controlling method, as well as wavelength modulation spectroscopy technology. Moreover, data acquisition and processing process were simplified effectively. Firstly, system scheme and designing details of optical and electrical modules were introduced in sequence according to the system principle and structure. Simulations of target gas absorption and interference from other gases have been analyzed. Broadening and overlap conditions of absorption lines under different pressures have also been described. Working pressure of the system was determined to be 200 Torr finally. Secondly, the minimum detection limit (MDL) was deduced <100 nmol·mol-1 through performance test and analysis of one-circle spectrum scanning of ethane with 6 concentration levels (100～1 000 nmol·mol-1). The linear performance of this sensor was evaluated through ~10 min 2f peak value extraction test using aforementioned samples. The results indicated that the correlation was 0.999 65, and the relationship between gas concentration and the amplitude of 2f signal was clear. Finally, system noise was determined to be ~0.347 V by means of continuous 1 h test applied to nitrogen, thus the SNR and sensitivity were estimated to be ~28.56 and ~40 nmol·mol-1, respectively. The new mid-infrared C₂H₆ sensor introduced in this paper not only realized nmol·mol-1 level measurement but also greatly reduced the volume occupied by commercial instruments by use of digital driver and lock-in amplifier, which laid the foundation for realizing the goal of miniaturization and mobile measurement. In addition, for the applications with unlimited power consumption, sensor sensitivity can be further improved by using more powerful laser source or improving the performance of system modules such as lock-in and preamplifier, which can be applied in more fields.