Research Of Detection Technology Of Trace Gas Concentration By TDLAS

With the rapid development of our national industry, chemical substances produced by burning of coal and petroleum are in the form of exhaust and dust in the atmosphere. These will bring harmful effects to people's lives, production and health. Carrying out the monitoring studies of trace gases of pollution of the environment is urgent. In the testing field of air pollution, the traditional method use chemical methods, but these methods need a longer time for sampling, and the measurement results are vulnerable to interference of other gases. While spectroscopy methods in particular tunable diode laser absorption spectroscopy (TDLAS) has advantages of high fast, high sensitivity and high selectivity. Studying of tunable diode laser absorption spectroscopy and using it to monitor trace gas is of great significance.In this paper, the method of tunable diode laser absorption spectroscopy was studied, and the experimental system based on wavelength modulation method of TDLAS was designed. Following the physical meaning of the various devices of the wavelength modulation experimental system, the functional model of system is established, including the model of pure frequency modulation and combined with intensity modulation. Taking the absorption peak of water vapor at 1392.5nm under normal temperature and pressure condition as the researching object, and in this case the function of Lorentz line was used to simulate molecular absorption, the impacts of system parameters on harmonic signal were analyzed by using software, where paying more attention on the effect of the intensity modulation.The results showed that the second harmonic signal was the most suitable using for trace detection, the width of the second harmonic signal was broadened as the increasing of the modulation index. There was one value of modulation index which made amplitude of the second harmonic attain its maximum, the peak amplitude of second harmonic signal was directly proportional to the concentration of the vapor. Laser intensity modulation factors made the peak of second harmonic signal occur small offsetting, but the most obvious change was the asymmetry of the signal shape on both sides. Detection phase played an overall impact on the shape of the second harmonic signal. Due to the difficult explanation of the influence of the laser intensity modulation in experiment, this paper had a depth study of its causes.The performance of the experimental system of WMS is limited by detector noise, laser excess noise, laser sensitivity to optical feedback, injection current fluctuations, and optical interference fringes. These limitations will be analyzed later in more detail in this paper.