Study On Highly Sensitive Detection Technology Of Carbon Dioxide Based On Cavity Ringdown Spectroscopy

Environmental problems such as climate change and natural disasters caused by global warming have become global challenges faced by mankind today.A number of researches have found that man-made emissions of greenhouse gases are largely responsible for global warming.The United Nations Convention on Climate Change signed in 1992,the 1.5℃ temperature control target proposed by the Paris Agreement in 2015 and the "carbon peak and carbon neutrality" plan proposed by China in 2021 all demonstrate that mankind is deeply aware of the negative impact of greenhouse gas emissions.Carbon dioxide is one of the most important long-lived greenhouse gases,monitoring its concentration is a key aspect to control carbon dioxide emissions.The global average concentration of carbon dioxide is close to 420 ppm and the global average annual change is less than 2.5 ppm.Based on the above characteristics,carbon dioxide concentration monitoring requires highly sensitive gas detection technology.As a kind of optical sensing technology,cavity ring-down spectroscopy has the advantages of high resolution and high precision and has been widely used in the field of atmospheric environment monitoring.Based on cavity ring-down spectroscopy,the following contents were carried out in this thesis.Since the PDH and optical feedback frequency-locking methods commonly used in cavity ring-down spectroscopy suffer from problems such as poor robustness,difficulty in applying to small distributed feedback lasers or low sensitivity,a new type of frequency-locked cavity ring-down system,frequency modulation spectroscopy based frequency-locked cavity ring-down system,with the advantages of fast response and high sensitivity is proposed in the thesis on the basis of distributed feedback semiconductor lasers by combining frequency modulation spectroscopy and cavity ring-down spectroscopy technologies.The system could be divided into a frequency modulation spectroscopy locking module and a gas concentration measurement module,which were used to achieve the precise locking of the laser emission frequency to the centre frequency of the carbon dioxide absorption line and the fast and high-precision measurement of the carbon dioxide concentration,respectively.In addition,a dual-laser configuration was used in the system to avoid the drift of baseline ring-down time caused by environmental changes(e.g.,temperature,pressure)and the interruption of frequency locking during gas concentration measurement.Moreover,two sets of ringdown times at the frequency with and without gas absorption were used to calculate the carbon dioxide concentration,which improved the response time of the system to 1.5 s,reaching the international leading level.The effective enhancement of the stability of the laser output frequency leaded to a 32-fold increase in the standard deviation of the discriminating signal,4.3-fold increase in the standard deviation of the ring-down time and 5-fold increase in the detection sensitivity of the system,respectively.After investigating and optimizing the effects of the gas flow rate,the scanning rate of piezoelectric transducer and the polarization state of the detected light on the performance of the system,a minimum detectable absorption coefficient of 4.4×10-11 cm-1 was achieved at an optimum averaging time of about 5 s,corresponding to a minimum detectable carbon dioxide concentration of 78 ppb.Over a wide range of carbon dioxide concentrations,the system displayed a good linear response with a linear correlation coefficient of 0.99999.Finally,the system was compared with a highperformance commercial instrument(Picarro,G2401)by observing the concentration of carbon dioxide in the atmosphere and the relative deviation of the gas concentration values measured by the two devices was less than 1%.Secondly,in order to measure carbon dioxide in the field with high precision,a small gas sensor based on cavity ring-down spectroscopy with ultra-high sensitivity is designed in this thesis.The self-designed Fabry-Perot optical resonator with ultra-high fineness,the stable temperature and pressure control modules of the optical resonator and the low-power circuit modules guaranteed the high precision and miniaturization of the system.In addition,the designed matching algorithm between laser and cavity mode guaranteed the stable and fast acquisition of ring-down times.The measured number of ring-down times at a single cavity mode was 20 and the corresponding response time of the system was 10 s.After 30 minutes of operating temperature and pressure control modules,the temperature and pressure of the resonator gradually converged to the preset values.Moreover,the fluctuation of temperature and pressure in 24 hours was less than 0.07℃ and 15 Pa respectively.According to Allan variance analysis,the minimum detectable absorption coefficient of the system was 0.7×10-12 cm-1 under the optimum average time of 303 s and the corresponding minimum detectable carbon dioxide concentration was 1.6 ppb.Over a wide range of carbon dioxide concentrations,the linear correlation coefficient of the system’s response was 0.99994.Finally,the system performed observation of atmospheric carbon dioxide concentration for two days,and the relative deviation between the results and the monitoring data of commercial instrument(Picarro,G2401)was better than 6‰ after excluding the interference of human breath.