Research On Key Technologies Of Trace Gas Detection In Near Infrared Photoacoustic Spectroscopy

With the rapid development of national economy,the requirements of trace gas detection in various industries are increasing.Gas photoacoustic(PA)spectroscopy(PAS)detection technology is regarded as an ideal method for real-time and accurate measurement of various gases,because of its high detection sensitivity,fast response speed,strong anti-interference ability,and wide detection range,etc.Theoretical analysis was carried out on the principles of PAS,and the influence of composition structure of the PA cell on the PA signal was put forward.In order to improve the performance of the near-infrared PAS detection system,two kinds of PA cells were designed.Non-resonant PA cell has the advantages such as small volume and less gas sample,but the PA signal has no resonant enhancement,therefore the detection sensitivity of the non-resonant system is low.This thesis analyzed the basic principle of gas PAS,and the theoretical analysis results showed that improving the absorption optical path in PA cell can enhance the effective excitation light power,then the detection sensitivity of the system is improved.Based on the characteristics of the Herriott optical long-distance cell and the traditional non-resonant PA cell,the multi-reflection non-resonant PA cell was designed.The theoretical analysis by using optical simulation software ensured that the focal length of spherical mirror which composes of multi-reflection Herriott optical long-distance cell was 500 mm,the cavity length and cavity radius of non-resonant PA cell were 21 mm and 5 mm,respectively.The experimental results showed that the background noise of the near-infrared multi-reflection non-resonant PAS system was 2.95 ?V,and the detection sensitivity of acetylene gas was 0.292 mV/ppm,therefore the acetylene gas detection limit of the system was calculated to be 10 ppb.Resonant PA cell system has larger PA signal and can measure flowing gas,but it has the disadvantages of large volume,more gas sample and long response time.This thesis analyzed the influence factors of PA signals in resonant PA cell in details.The relationships between the geometric dimensions of the PA cell and the resonant frequency,the quality factor and the cell constant were simulated by numerical calculation.On the basis of traditional resonant PA cell,the semi-cavity first-order longitudinal resonant PA cell is proposed.The results of theoretical analysis showed that the semi-cavity resonant PA cell can reduce the resonant frequency of the PA cell and increase the amplitude of the PA signal by the same geometrical size of the PA cell.The semi-cavity resonant PA cell reduces the design requirements of cavity opening,which reduces the difficulty of processing and weakens the influence of external noise on the system.The experimental results show that the resonant frequency of the semi-cavity PA cell is 844 Hz,which is 40% lower than the traditional PA cell.The background noise of the system is 4.09 ?V,and the response of acetylene gas is 1.736mV/ppm,so the detection limit of the system can be calculated as 2.36 ppm.