Study On CO₂ Detection System Based On Tunable Diode Laser Absorption Spectroscopy

In recent years, environmental pollution improvement are concerned by people all over the world, and the pollution monitoring is the precondition of environment protection and pollution control. Carbon dioxide is one of the main greenhouse gases, so it is very important to monitor it. The study of on-line carbon dioxide monitoring technique laid a solid theoretical and experimental foundation for the online monitoring other pollution gases.This thesis takes Beer-Lambert law as the theoretical foundation. By combining the theory of gas absorption spectrum and the line-type theories, the principle of gas concentration measuring based on tunable diode laser absorption spectroscopy was introduced. To get the high sensitivity of measuring gas concentration, the principles of wavelength modulation and second harmonic detection were deduced. These principles offer the theoretical foundation for the application of tunable diode laser absorption spectroscopy (TDLAS) to measure the gas concentration.In the thesis, the noise of the gas concentration measuring system based on TDLAS was analyzed. The influences on the line-width of gas absorption spectrum caused by air pressure and air temperature were researched. To make the system simple, real-time self-adjusting and accurate, a method of self-adjusting trace gas analyzing system based on TDLAS and its experimental system were devised.An experimental system was set up based on the detection theory. The experiments for characteristic testing of tunable diode laser were carried out. The second harmonic signal of carbon dioxide absorption spectrum was acquired. A method of the linear least-squares fitting used to deal with the second harmonic signal of the measured carbon dioxide based on the signal of high concentration was presented.The work realized the on-line monitor of the carbon dioxide, the experience were useful in developing on-line monitoring system for other pollution gases.