Research On Gas Detection Based On Laser And THz Spectroscopy

With the continuous development of science and technology and the improvement of people's living standards,the demand for highly sensitive gas detection technology in many fields has gradually increased.The gas detection based on Tunable diode laser absorption spectroscopy?TDLAS?technology has the advantages of high sensitivity,high selectivity,high reliability,and fast response,it has a very broad application prospect.Terahertz time-domain spectroscopy can simultaneously obtain the amplitude and phase information of terahertz pulse signal of the sample to be measured,and has the advantages of high sensitivity,high signal-to-noise ratio,high resolution and wide spectral range,which is widely used in the field of trace gas detection.This article first introduces the basic theory of gas detection and two commonly used gas absorption cells.Based on the HITRAN database and spectral line selection principles,a spectral line at 6330.82 cm^-11 was selected to measure the CO₂.A TDLAS system was built by using a vertical cavity surface emitting laser?VCSEL?with a center wavelength of 1579 nm as the light source,combined with a Herriott absorption cell with 20 m effective optical path,built a set of real-time online monitoring device for human exhaled CO₂ spectral detection.The effect of pressure on the second harmonic signal was also investigates by experiment.By measuring CO₂ with different concentrations,a good linear relationship between the intensity of the second harmonic signal and the corresponding gas concentration was obtained.At the same time,the CO₂ concentration of 2.5%was measured as long as 1 hour.The system has good stability,and the minimum detection limit of CO₂ is 0.769‰.Allan deviation is used for analysis the stability,and the measurement accuracy can reach about 100 ppm when the integration time is 168 s.The online measurement of exhaled CO₂ was performed on two healthy volunteers during resting and exercise,different concentrations of CO₂ were obtained in the dead space and alveolar gas,and the exhaled CO₂ increased significantly with the subject's physical exercise.At the peak of exercise,the End-Tidal Carbon Dioxide?EtCO₂?concentration of the two volunteers increased by 58.58%and 108.61%,respectively,when compared with the resting state.Then the terahertz spectrum of acetone gas was studied based on terahertz time-domain spectroscopy.In this experiment,a 3 THz terahertz time-domain spectroscopy system is achieved by 1560 nm laser excitation and the sample characteristics was studied.The terahertz spectrum of 2 mm thick high-density polyethylene was first measured,and the transmittance of terahertz wave is as high as more than 85%.The terahertz absorption spectra of acetone gas at different concentrations are measured with the volatile acetone liquid at room temperature as the research object.The absorbance of acetone gas is linearly related to the concentration,and the characteristic absorption peak is located at 0.56 THz.The White-type absorption cell was applied to the terahertz time-domain spectroscopy system,and the terahertz spectrum of the acetone gas was measured.We found that the modified terahertz time-domain spectroscopy system increased the optical path of gas chamber by 10 times,and the detection limit was also increased.