Development And Application Of Dense-spot Pattern Herriott Gas Cell

Gas sensor is widely used in the fields of environment,industry,medicine,agriculture,and animal husbandry.Among commonly used gas detection technologies,Tunable Diode Laser Absorption Spectroscopy（TDLAS）has outstanding advantages of strong selectivity,high sensitivity,and good stability.The gas cell is the key optical module of the TDLAS system,which determines the performance of the sensor.In order to improve the optical path volume ratio of the gas cell and optimize the performance of the gas sensor,the following studies on dense-spot pattern Herriott gas cell were carried out in this thesis:According to the principle of infrared absorption spectrum,improving the absorption path per unit volume of gas cell is an important way to optimize the performance of the gas sensor.In this thesis,Tracepro and MATLAB were used to simulate Herriott gas cell.Optical parameters such as mirror spacing and incident position were optimized,and a variety of dense spot distributions and light path structures were obtained.The change rule of optical path structure was analyzed in detail.The optical path,optical path volume ratio,surface area and other parameters of the gas cell are quantified and compared.Based on simulation results of the optical path structure,a long optical path gas cell and a new small volume dense spot gas cell was designed.The long optical path gas cell has realized 432 reflections and 85.0 optical path volume ratio.Using the designed gas cell structure,optical path debugging experiments were carried out.The optical path of the gas cell was calibrated,and the optical path is 8.57m,which is consistent with the theory.The length,width and height of the new small volume dense spot pattern gas cell are 7.4cm,4.2cm,and 3.8 cm,the volume is 119 cm³ and the content volume is 38 m L.The optical path of the small volume gas cell was calibrated,and the measured optical path is 6.2 m.Combined with the wavelength modulation spectroscopy（WMS）technology,acetylene（C₂H₂）gas sensor system was built using two developed gas cells and their sensing performance was tested.The C₂H₂ sensor based on a long optical path gas cell used a distributed feedback（DFB）laser with a central wavelength of 1533 nm.According to the calculation results of Allan deviation,when the averaging time is 0.2s,the limit of detection of C₂H₂ is 0.02 ppm（part per million）,and when the averaging time is 63.8 s,the limit of detection of C₂H₂ is reduced to 1.4 ppb（part per billion）.The response time of the sensor is 94.7 s.The C₂H₂ sensor based on a small volume dense spot type gas cell used a DFB laser with a central wavelength of 1534 nm.According to the calculation results of Allan deviation,when the averaging time is 0.2 s,the limit of detection of C₂H₂ is 7.8 ppm,and when the averaging time is 94.7 s,the limit of detection of C₂H₂ is reduced to 0.32 ppm.The response time of the sensor is 3.8 s.The innovative points are as follows:A simulation model of a dense spot type Herriott multi-pass cell was bulit,and two types of gas cell with an optical path volume ratio of 85 and a volume of only 38 m L were developed.The function of the gas cell was verified through acetylene detection experiments.