Study On Highly Sensitive Detection Technology Of Greenhouse Gases Based On Cavity Ringdown Spectroscopy

Climate and ecological changes caused by increased greenhouse gas emissions are the major issue facing the world today.With the deepening impact of global change,the realization of carbon peaking and carbon neutrality has become the consensus of the international community.The primary premise for realizing the "dual carbon "goal is to build my country’s carbon source-sink monitoring system,but the bottleneck lies in the lack of domestic greenhouse gas background detection equipment.Since the annual growth rate of the greenhouse gas background value is only between ppm and ppb,the amount of change in the monitoring process such as seasonal changes and gradient changes is even smaller.The development of high-sensitivity,high-precision,and long-term stable domestic testing equipment has become the development trend of my country’s future carbon monitoring system.Cavity Ring Down Spectroscopy(CRDS)is very suitable for high-precision monitoring of greenhouse gas concentrations in ambient atmosphere due to its high sensitivity,low cost and easy system integration.The paper focuses on the research and analysis of CRDS,which mainly includes the following contents:The first is to design and build the frequency matching CRDS measurement system.The frequency-locked spectral scanning method was proposed,which solved the problem that the laser frequency and the longitudinal mode of the resonator were easily lost when the environment changes,and realized the accurate and efficient matching of the laser frequency and the corresponding longitudinal mode.Developed the algorithm for concentration inversion,substituting the cavity loss into the molecular line fitting model without additional measurement of cavity ring-down time.The long-term stability of the system’s FSR was verified by continuously measuring the spectrum containing multiple absorption peaks for 500 minutes.Continuous measurement of cavity ring-down time,Allan variance results showed that the system can achieve the measurement sensitivity of 1×10-11 cm-1.Secondly,in response to the detection requirements of multiple greenhouse gases and their isotopes,the method for simultaneous measurement of multiple gases on the same device has been proposed.The time-division multiplexing cavity ring-down spectroscopy technique was proposed for the analysis of gas isotope abundance,which utilized the frequency response characteristics of high-resolution resonators.The dual-wavelength time-division resonance laser frequency control method was proposed,and the measurement accuracy of δ13CH4 abundance was 1.3‰.The wavelength division multiplexing cavity ring-down spectroscopy technique was proposed for the simultaneous measurement of multiple greenhouse gases,and the effectiveness of the method was verified by measuring five mixed gases of CH4,CO2 and CO with different concentrations.The two-component greenhouse gas measurement instrument was integrated based on the wavelength division multiplexing cavity ring-down spectroscopy method,and The new resonant cavity and optical structure were designed.The results of continuous measurement of standard gas showed that the standard deviation of CO2 measurement results in 1 hour was 0.025 ppm,and the standard deviation of 12 hours results rose to 0.084 ppm,and the overall change in measurement results was within 0.5 ppm,the standard deviation of the 1-hour results of CH4 measurements was 0.26 ppb,the standard deviation of the 12-hour results rose to 0.45 ppb,and the overall variation of the measurement results was within 2 ppb.Finally,in view of the problems of low longitudinal mode matching efficiency and low coupling efficiency between laser and resonator in the traditional CRDS technology,the optical feedback CRDS technology was developed.Aiming at the influencing factors of the optical feedback effect,the simulation calculation and analysis were carried out,and the optical feedback cavity ring-down measurement system was designed.Combined with modulation technology,the wavelength modulation feedback phase locking method is proposed,which can achieve locking for 120 s and can be regained within 100 milliseconds after the sudden loss of lock.Compared with the traditional cavity ring-down technology,the cavity-mode coupling efficiency can be improved by more than 20 times,and the Allan variance analysis results showed that the system can achieve the measurement sensitivity of 8 × 10-11 cm-1.The spectral measurement method of threshold comparison and voltage sweep was proposed,and the spectral measurement that the system can achieve within 1 s was verified by measuring the trace amount of CH4 gas at 1.8 ppm,the fluctuation of the measurement result was 0.0068 ppm.