Research On Gas Detection Based On Absorption Spectrum And Photothermal Effect

With the development of science and technology,trace gas detection technology is widely used in atmospheric monitoring,medical health,environmental monitoring,and industrial process control.The demand for the accuracy and stability of gas detection technology is increasing.Compared with other gas detection technologies,laser measurement technology has advantages of direct measurement,fast response speed and high sensitivity.Therefore,this article mainly take advantage of laser spectroscopy measurement technology in trace gas detection to build a semiconductor laser-based tunable absorption spectroscopy system（TDLAS）and a laser-induced photothermal spectroscopy system（LITES）,and carry out related problems of the system,the following were studied:（1）Compare,analyze and introduce the basic theories and experimental methods involved in gas detection.Investigate the research status of the TDLAS system and the LITES system to ensure the feasibility and effectiveness of this experiment;The CO₂ absorption spectrum was calculated according to the HITRAN database,and finally select the absorption line near wavenumber of 6330.82 cm^-1 for the measurement;Using Trace Pro optical path simulation software to simulate the construction of the optical path,and gives an admirably succinct account of the characteristics of different types of absorption cells.（2）A vertical cavity surface emitting laser（VSCEL）with a center wavelength of 1577.57nm was used as a light source,combined with a Herriot cell with a length of 20 m and a volume of 400 m L to build a TDLAS system.Firstly,the system is calibrated with standard concentration gas,and the linear relationship between the signal amplitude and the concentration of the gas to be measured is proved through experiments.By analyzing the data,the signal-to-noise ratio（SNR）of the system is about 55.56.To evaluate its stability,when the system works for 2 hours,the concentration fluctuates within the range of±0.056%,and the sensitivity is 0.14%.Finally,the Allan variance was measured and the best detection limit concentration is calculated to be about 310 ppm when the integration time of the system is about 180 s.Finally,through the system,the actual exhaled CO₂ of the human body is measured online,and the CO₂ gas concentrations in the human breathing stage are obtained,an accurate and efficient online measurement is realized.（3）The LITES system was built using a distributed feedback laser（DFB）and a quartz tuning fork（QTF）coupled with a fiber-coupled center wavelength of 1580.13 nm,and CO₂gas was used for experimental verification.First,the QTF sensor is tested by changing the laser power,and it is proved that the QTF response and the laser power have a good linear relationship within a certain range,and the SNR of the system is affected by the laser power.Then,the concentration calibration experiment was carried out through the sample gas,which proved that the concentration and the electrical signal in the system had a good linear relationship,and the SNR of the system was extracted to be about 26,and the minimum detectable concentration was about 0.28%.And then,by allowing the system to run continuously for 2 hours,the measured concentration fluctuation range of the LITES system does not exceed±0.35%,and it has good stability.Finally,the system stability is evaluated through Allan variance and found that the system is in the best performance.When the integration time is 140 s,the minimum detection limit concentration for CO₂ reaches to 33ppm.