介绍了研制的一种基于共轴锁模腔增强吸收光谱技术的中红外甲醛气体检测系统。 系统采用了发射中心波长为3.6 μm的带间级联激光器为光源, 以高精度F-P谐振腔作为气体反应池, 通过激光在谐振腔内的多次反射极大地提高了有效吸收路径。 为了实现甲醛检测, 利用Pound-Drever-Hall(PDH)技术将激光频率和腔谐振频率锁定至波长为3 599.08 nm的甲醛吸收峰上。 实验发现, 谐振腔腔长容易受到外界环境的影响产生变化, 导致系统失去锁定, 产生测量误差; 为了抑制这一现象, 提高系统的准确性和抗干扰性, 采用了动态PDH锁定技术, 通过低频锯齿波信号对腔长进行小范围内的周期性调制, 使得腔谐振频率在目标气体吸收峰附近缓慢来回变化; 通过选择合适的扫描范围使得在扫描过程中激光与谐振腔保持频率锁定。 系统通过光电探测器采集谐振腔透射光强信号, 通过对腔透射信号进行拟合计算来确定甲醛浓度。 为了验证检测系统的有效性、 评估系统的性能, 采用质量流量计配备了6种不同浓度的甲醛气体样品并开展了甲醛吸收光谱测量实验、 系统标定实验和稳定性实验。 实验结果显示, 在0~10 mL·L^-1范围内, 腔透射信号拟合值与甲醛浓度之间呈现出良好的线性关系; 通过Allan方差分析得到当积分时间为1 s时系统检测下限为52.8 nL·L^-1, 积分时间为14 s时检测下限可以降至3.3 nL·L^-1。 此外, 通过增加谐振腔的腔镜反射率和腔长可以提高有效吸收路径, 进一步降低检测下限。 该系统灵敏度高、 响应速度快, 具有较好的抗干扰性和长期稳定性, 在痕量甲醛检测方面具有广阔的应用前景。

A mid-infrared formaldehyde gas detection system was developed based on the coaxial mode-locked cavity enhanced absorption spectroscopy technology. In order to realize the detection of formaldehyde, an interband cascade laser with a center emission wavelength of 3.6 μm was used as the light source, and a high-precision F-P resonator was used as the gas cell. The laser frequency was tuned by an electro-optic modulator and was locked to the cavity resonance frequency by the Pound-Drever-Hall (PDH) technology. In order to suppress the interference caused by the external environment and improve the accuracy and anti-interference of the system, a dynamic PDH locking technique was adopted. In this technique, the cavity length was modulated by a low-frequency saw-tooth signal, to realize the modulation of the cavity resonant frequency near the gas absorption peak. An appropriate scanning range should be selected to ensure the laser and the cavity keep locking during scanning. The formaldehyde concentration can be calculated by the amplitude of the cavity transmission signal. Experiments were carried out to evaluate the performance of the system. The absorption spectrum measurement experiment verified the effectiveness of the system. The system calibration experiment results show that the amplitude of the cavity transmission signal exhibits a good linear relationship with the formaldehyde concentration in the range of 0~10 mL·L^-1. The Allan analysis of variance shows that the system minimum detection limit is 52.8 nL·L^-1 when the integration time is 1 s which can be reduced to 3.3 nL·L^-1 at an integration time of 14 s. In addition, the system sensitivity can be further improved by increasing the effective absorption path of the resonant cavity. The system has high sensitivity, fast response speed, good anti-interference and long-term stability, making it have broad application prospects in the detection of trace formaldehyde.