Study Of Monitoring Method Of Environmental CO Based On Tunable Diode Laser Absorption Spectroscopy

With the rapid development of the human economy, the environmentalpollution and the ecological destruction are becoming increasingly serious andsudden environmental pollution incidents also occurred frequently. Prevention andcontrol of pollution to protect the environment is an important issue of the modernscience. The concentration change of carbon monoxide, one of the main airpollutants, is the important indicator of air quality. Tunable Diode LaserAbsorption Spectroscopy (TDLAS), with features of high selectivity, fast response,high sensitivity and high precision, has been widely applied in the realm ofpollution gas monitoring.The basic principle and method of tunable diode laser absorption spectroscopy(TDLAS) technique were analyzed, and the theory of modulation of tunable diodelaser spectroscopy based on Fourier analysis method was deduced. Thecharacteristics of Wavelength Modulation Spectroscopy (WMS) harmonics oftunable diode laser spectroscopy under sinusoidal frequency modulated and theinfluences of laser amplitude modulation were studied, and the background noisewas analyzed theoretically.The multi-pass reflection absorption cell is often employed in the air trace gasdetection to prolong the optical path length, to improve the sensitivity and toreduce the detection limit. A novel multi-pass absorption cell with easily adjustableoptical path was designed by employing the Herriott cell’s beam reflectionmechanism. The optical simulation analysis and the mechanical design werefinished. The stability condition of this optical system was worked out and thesimulated spot pattern was in good agreement with the experimental observation.Carbon monoxide was taken as the research and study object, and one set ofthe environmental CO continuous detection system was designed and installed forthe purpose of research and application of TDLAS technology combined with amulti-pass absorption cell. According to the characteristics of the second harmonicsignal with TDLAS, a method of the linear least-squares fitting used to deal with the harmonic signal based on the standard gas absorption spectrum was presented,and the concentrations of the carbon monoxide were obtained with high precision.In the end the feasibility of entire project was verified, and the detection limit wastested.