Study On The Cavity Ring Down Spectroscopy In Trace Gas Concentration Measurement

The Earth's atmosphere is the cradle of all lives,but because of the unrestrained activities of human beings,the atmospheric component of the Earth is undergoing dramatic changes.Especially in our country,due to the long-time neglect of the the link between economic development and environmental protection,environmental pollution,especially air pollution is particularly prominent.How to effectively monitor the concentration of trace gases and understand the physica and chemical processes in the air,and then control the air pollution effectively is of great significance.There are many drawbacks in traditional trace gas detection technologies,so optical detection method gradually become the mainstream of trace gas concentration detection technology because of its characteristics of non-contact,good real-time and high sensitivity.As a new type of spectroscopy detection method,cavity ring-down spectroscopy increase the effective absorption path through the optical resonant cavity and have a very high detection sensitivity.This paper systematically studied and realized the cavity ring-down spectroscopy technology,and complete the CO₂ concentration detection on this basis,the mainly work of this paper are as follows:Firstly,the the theoretical basis of the cavity ring-down spectroscopy system is studied,including the stability condition,the mode theory and so on,then an optical resonator with a very simple structure and low cost is designed,and then the CRDS system experimental platform is build based on the optical resonator.Secondly,the adjustment method of the CRDS system is introduced,and the mode coupling of the laser and the optical resonator is realized by scanning the laser frequency.A stable transmission signal is obtained and the cavity ring-down time is calculated,the equivalent absorption path of the system is estimated.Then the influence ofthe laserscanning frequency on the transmitted-signal is analyzed and the optimal scanning frequency is determined.The measurement of the gas concentration was completed by configuring different concentrations of CO₂ gas and calculating its ring-down time.Finally,the system are measured by measuring different concentrations of gas,and the system is optimized by Kalman filter algorithm.The results show that the experimental system can complete the measurement of CO₂ concentration correctly.