适用于ppb量级NO_2检测的低功率蓝光二极管光声技术研究

在405 nm处基于低功率蓝光二极管光声技术探测ppb量级NO_2浓度系统,获取了NO_2有效吸收截面,探讨了水蒸气等气体的测量干扰,通过频率扫描拟合得到了1.35 kHz的谐振频率.采用内部抛光的铝制圆柱空腔作为光声谐振腔(内径为8 mm,长为120 mm),系统优化了腔体、窗片和电源等影响因素,分析了降低本底噪声、提高信噪比的方法,噪声信号可降至0.02μV.设计了两级缓冲结构,显著抑制了流量噪声的影响,提高了系统的稳定性.系统的标定梯度曲线经过线性拟合后的斜率为0.016μV/ppb, R~2为0.998,在60 s平均时间下,系统NO_2探测限为3.67 ppb(3σ).为了证实系统的测量结果,将其与二极管激光腔衰荡光谱系统同步对比测量大气NO_2浓度,二者线性拟合后的斜率为0.94±0.009,截距为1.89±0.18,相关系数为0.87,一致性较好.实验结果表明,该系统实现了ppb量级NO_2浓度的低成本在线探测,可用于NO_2浓度外场的实时检测.

Photo-acoustic technology applied to ppb level NO2 detection by using low power blue diode laser

Photo-acoustic technology based on a low power blue diode laser for measuring the ppb level NO₂ is presented in this paper. A low-cost NO₂ measurement system based on traditional photo-acoustic technology is established. The 405 nm blue diode laser with an external modulation is used as a light source. The central wavelength of the laser is 403.56 nm, the half-peak full width is 0.84 nm, and the power is 65.3 mW. The effective absorption cross section of NO₂ is obtained, and the interference of the water vapor and other trace gasisinvestigated. The resonant frequency is tested to be 1.35 kHz by frequency scanning fitting. An internally polished and coated poly tetra fluoroethylene aluminum cylindrical cavity is used as a photo-acoustic resonator (the inner diameter is 8 mm and the length is 120 mm). The influence factors caused by cavity parameters, optical windows and power supply are studied. The system is optimized to reduce background noise and improve signal-to-noise ratio. Then the noise signal is dropped to 0.02 \${\text{μV}}$\. An additional buffer chamber is integrated on the original buffer chamber to form a two-level buffer. The two-stage buffer structure significantly suppresses the effects of airflow noise and improves the system stability. The slope of the calibration curve of the system after linear fitting is 0.016 \${\text{μV/ppb}}$\, and R² is 0.998. The NO₂ detection limit of system is 2 ppb (3\$\sigma$\) with an average time of 60 s. To verify the results of the system, a diode laser cavity ring-down spectroscopy system (CRDS system, using a 409 nm the diode laser, with a system detection limit of 6.6×10^-1) is used to measure ambient NO₂ simultaneouslyon Lake Dong-Pu in western Hefei, Anhui Province, China. During the experiment, the measured NO₂ concentration ranges from 8 to 30 ppb, with an average concentration of 20.8 ppb. The results of two systems have good consistency:alinear fitting slope of 0.94 ±0.009, an intercept of 1.89 ±0.18 and acorrelation coefficient of 0.87. The experimental results show that the system can realize the low-cost on-line detection of the ppb level NO₂, and it can also be used for the real-time detection of NO₂ concentration field.