Research On ?13?^C??18?^O Isotopes In CO₂ Measurement Technology Based On Off-axis Integrated Cavity Output Spectroscopy

Stable isotopes are widely used in geochemical,medical,ecological and other fields because of their unique indicative functions.These applications require accurate measurement of isotopic abundances.Traditional isotope abundance measurement techniques are mass spectrometry.After years of research and development,mass spectrometry has high sensitivity.However,mass spectrometry has many limitations,such as large volume,high price,samples need to be collected and pretreated.These factors have greatly limited the application of mass spectrometry.As a new type of isotope detection technology,laser absorption spectroscopy has many advantages,such as small volume,quick response,convenient operation,real-time online and so on.It has received extensive attention and developed rapidly.This paper mainly focuses on the measurement of CO2 gas and its isotopic abundance.Combined with the wavelength modulated off-axis integrated cavity output spectroscopy technology,the abundance of 13C and 180 in CO2 gas is detected.The research content and innovation are as follows:(1)For the absorption spectrum technology,the selection of spectral lines is very important and is also the primary task.There are many factors to consider in the selection of spectral lines,such as avoiding interference from other gases,suitable absorption lines and so on.Choosing the right spectral lines can greatly improve the detection results of the system.In this paper,CO2 and its isotopes are mainly detected and the absorption lines of the three groups of 16O12C16O,161O13C16O and 16O12C18O are analyzed,and the influence of temperature and pressure on the absorption line and absorption line is studied.The absorption spectrum is simulated by the database,and the location of the selected spectrum is finally determined.(2)A set of isotope abundance detection system based on wavelength modulation off-axis integrated cavity output spectroscopy is established.Study the performance index of related devices and select suitable components.Optical path system is an important part of absorption spectroscopy.To achieve narrower free spectral interval in the cavity,a Herriott type integral cavity is used in this system.The length of the integral cavity is 15.9 cm,and the spot is reflected back and forth on the two lenses 11 times to achieve a denser free spectral interval,so that more optical coupling can be integrated into the integration cavity.The optical path system is installed and adjusted,and the mode phenomenon is analyzed.The optical path distribution and the spot distribution on the mirror are simulated by optical simulation software.The volume of the integral cavity is smaller than 81mL,and the smaller volume guarantees the effect of gas replacement.On this basis,we compile related software to collect and process signals,and finally realize real-time detection of 13C and 18O isotope abundance in CO2 gas.(3)The influence of room temperature on the output wavelength of quantum cascade lasers is studied,and a method for stabilizing the output wavelength of lasers is proposed.The output of the laser is detected by the detector,and the drift is judged by comparing with the standard signal.According to the drift of output wavelength,the driving current of laser is changed by digital potentiometer.The drift of the laser output is eliminated by modifying the driving current.Finally,the influence of environmental temperature changes during the operation of quantum cascade lasers is reduced,which makes the system more widely applicable.(4)In addition to improving device performance and structural stability,signal processing is also a common method to improve the performance of the system.The selected band is at 4.3 m,and the output of the laser does not usually use optical fiber pigtail output.In this case,a part of the optical path will be exposed in the outer space,and the intracavity is longer than the external optical path,but the cavity belongs to the low pressure state,and the number density of the molecule in the cavity is far smaller than that of the external cavity.Therefore,changes in external space environment will affect base noise and measurement results.The signal is corrected by light intensity,and the characteristic of the signal is analyzed.A suitable small wave base function is selected to process the signal,and the background noise can be effectively suppressed to improve the measurement accuracy.