Development And Application Of Multipass Cell For Infrared Gas Detection

Gas detection technology is widely used in atmospheric monitoring,industrial and agricultural production,energy exploitation,medical monitoring and so on.Gas detection is a means for people to expand their knowledge of substances,and able to detect all kinds of gases that are present in the environment but cannot be perceived by the human.Flammable gas has the risk of explosion in a confined space at a certain concentration level.Toxic gases harm physical health and even life safety without being noticed.Gas is also one of the important substances in biological reactions.The vital sign can be detected by detecting the human exhaled gas.Gas detection can effectively prevent gas explosions,poisoning and other safety accidents,and can also improve medical technology.Therefore,improving gas detection technology has extremely important effects on production and life.In this paper,two kinds of multi-pass cell with optical structure are proposed,and the gas detection systems are designed.Firstly,a multi-pass cell with three spherical mirrors is designed for laser.The mathematical model of three-mirror reflection was established by MATLAB software,and the optical path structure,spot distribution and shape,reflection times and optical path length were analyzed.The multi-pass cell of 11.0m was designed according to the simulation parameters,and the optical path length was verified by the gas calibration experiment.The mechanical structure is designed to fix the mirror and adjust the optical path.Using a Distribution Feedback?DFB?laser with a central Wavelength of 1.53?m,and combining with Wavelength Modulation Spectroscopy?WMS?technology,it is designed as an acetylene sensor,and a locked-in amplifier is used to extract second harmonics?2f?.The sensor was calibrated by gas experiment and its stability and limit of detection?LoD?were analyzed.According to the calculation of Allen's variance,the LoD of the system can reach 4.36ppmv within the averaging time of 0.5s.When the averaging time exceeds10s,the LoD will be less than 1ppmv.Secondly,a concentrating multi-pass cell is designed for the use of divergent light.Through the derivation of ABCD matrix,the law is obtained that the exit position and the incident position of the light are coincident.The multi-pass cell is independent of incident angle and solves the problem of divergence of thermal light.MATLAB software was used to build the mathematical model,the parameters of the optical path were obtained,and according to the simulation parameters,a multi-pass pool with optical path of 1.8m was designed.CO/CO₂ sensor is designed by using concentrating multi-pass cell,the constant current source driving circuit and digital locked-in amplifier circuit.The sensor was calibrated by gas experiment,and the LoD of the sensor was analyzed.The LoD of CO₂ sensor is lower than 10ppmv,and the LoD of CO sensor is²0ppmv.By comparing the gas experiments with different absorption intensities of CO and CO₂,the specific methods to improve the SNR are obtained:increasing the power of the light,optimizing the optical path structure and optimizing the filtering algorithm.Finally,the two types of sensors composed of multi-pass cell are summarized,and the problems existing in the sensors and the direction of further optimization are analyzed.The innovations of this paper:1.Designed a dense spot gas chamber for laser light source.2.Designed a converging multi-pass cell for divergent light.