N₂O Sensing Technique Based On 7.6 ?m Quantum Cascade Laser

In recent decades atmospheric greenhouse gases have contributed to climate forcing,which influences human and natural systems.The fundamental absorption band of trace gases in the most greenhouse is located in the mid infrared band,and its absorption line intensity is about two orders of magnitude higher than the near infrared band.The mid infrared quantum cascade laser,which combines high selectivity and high sensitivity laser absorption spectroscopy,can detect the fundamental absorption spectral in the mid infrared band.In this paper,the research on the high sensitivity detection of N2O gas based direct absorption spectroscopy,wavelength modulation spectroscopy and photoacoustic spectroscopy is carried out with the 7.6 ?m mid infrared QCL as the light source,and the results of various methods are analyzed and summarized,the future prospect is given finally.In this paper,nitrous oxide(N2O)is used as the main target gas and combined with N2O,H2O and N2O,CH4 for simultaneous detection of multi-component gases.Based on the HITRAN database,the absorption spectrum of gases in the tunable wavelength range of QCL is simulated and analyzed.The high sensitivity detection of these three gases in the atmosphere is achieved by selecting the N2O absorption line at 1307.66cm-1,the adjacent spectral lines of N2O and H2O near 1306.92cm-1,and the adjacent absorption spectral lines of N2O and CH4 near 1306.19cm-1.The main research results and innovation of thesis are as follows:(1)A direct absorption spectroscopy system based on the domestic independent research and developed QCL was established for detection of trace gases.A method for calculating direct absorption of gas concentration is proposed.The advantage of this method is as follows:wavelength-free,the influence of wavelength drift can be ignored and the system can be simplified.The high sensitivity detection of N2O,H2O and CH4 is achieved.(2)The simultaneous detection of N2O?H2O and N2O?CH4 gas based 7.6 ?m QCL combines with wavelength modulation absorption spectroscopy is carried out.The high sensitivity detection of 1.5 ppb is obtained.(3)The resonance photoacoustic spectroscopy system based on QCL is studied.Performance of the developed system was optimized and improved based on the traditional photoacoustic spectroscopy detection,dual beam enhancement method was applied to increase the effective optical power which effectively improve the detection sensitivity of the system.The system parameters are optimized and the systemperformance is tested under the optimal parameters.The results reveal that the detection limit of N2O is 37 ppb and atmospheric N2O gas can be monitored.