Research On Trace Gas Monitoring By Tunable Diode Laser Absorption Spectroscopy

At present environmental pollution has deteriorated the ecological balance and human health and safety more seriously. Carrying out the trace gas pollution monitoring studies has urgent significance. For gas monitoring, the method is to use the wet chemistry method that sampling at one point with long time and complicated process, so the system is difficult to update. Spectroscopy methods in particular tunable diode laser absorption spectrometry (TDLAS) has advantages of high fast, high sensitivity and high selectivity. Studying tunable diode laser absorption spectrometry and using it to monitor trace gas automatically is of great significance.In this paper, the method of tunable diode laser absorption spectroscopy was studied and be applied to the experimental technology for trace gases detection. The wavelength characteristics tuning with temperature and current of the semiconductor laser has been studied using Fourier Transform Infrared Spectrometer (FT-IR). For the large divergence angle of the laser output, a collimator has been designed and received the alignment result of 1 mrad. Designed and generated the modulation signal for the quasi-continuous semiconductor laser and the reference signal for the lock-in amplifier using a function generator, so the laser can be tuned automatically and continuously at large range. A TDLAS experimental system was built combined with multi-path cell and lock-in amplification technologies.By detecting different concentrations of methane when the laser worked at 15℃and 17.5℃, analyzed the pre-processing methods of spectral data and studied the characteristics of direct absorption signals and the second harmonic signals. The relationship between these signal values and the concentrations of gas was also studied and found that the second harmonic signal values has good linearity with concentrations. Through the measurement of low concentrations of methane, the system's sensitivity was analyzed, and acquired the detection limit of ppm for the order for measuring methane. This paper also analyzed the system noise sources.