Research On Difference Frequency Generation Mid-infrared Laser Source And Methane Gas Detection

Mid-infrared band covers the baseband absorption lines of a large number of gas molecules,and its absorption intensity is 2-3 orders of magnitude higher than that of near-infrared band.Therefore,the absorption spectroscopy technology based on mid-infrared laser can highly sensitively detect gas types,concentration,and other information.It also has important application values in environmental monitoring,deposit development,medical diagnosis,interstellar exploration,and other fields.Mid-infrared light source based on quasi-phase-matching(QPM)difference frequency generation(DFG)has become the first choice of atmospheric trace gas detection because of such advantages as workability at room temperature,narrow linewidth,and high output beam quality.In this paper,a mid-infrared DFG system was built based on MgO:PPLN crystals,and the output performance of the system was studied thoroughly.On this basis,a CH4 gas spectrum detection system was constructed.The specific contents of the study are as follows:1?With the theory of nonlinear optics being applied,together with the basic principles of DFG,a discussion was had about the commonly used phase matching techniques and an analysis was conducted on the influences of the acceptance bandwidth of crystal period,temperature,angle and wavelength on DFG/QPM.2?Continuous-wave tunable mid-infrared light source was obtained by employing 1060nm and 1550nm band narrow linewidth lasers as the fundamental light sources,and a 30.3?m periodic MgO:PPLN crystal as the nonlinear crystal.Analysis was carried out on effects of fundamental light power,crystal length,and focusing parameters on the conversion efficiency of the DFG system from theoretical and experimental aspects.In addition,a systematic study was conducted on crystal temperature and fundamental light wavelength tuning characteristics.According to the experimental results,the conversion efficiency of the DFG system was approximately 191?W/(W2·cm),the temperature acceptance bandwidth of MgO:PPLN crystal was approximately 4.2?,and the wavelength matching bandwidth of the pump light was approximately 5.8nm.The mid-infrared wavelength tuning range could be up to 390nm,and the power fluctuation of the system within 2 hours was less than 5%.3?The absorption spectra of CH4 molecule at 2999.01cm-1 were measured by scanning the wavelength of pump light based on direct absorption spectroscopy added with 10Hz triangular wave signal.According to the experimental results,the full width at half absorption peak of CH4 molecule was about 0.227cm-1,the relative error of which was smaller than the theoretical value in HITRAN database(0.252cm-1).It indicated that the mid-infrared DFG light source system we built featured excellent narrow linewidth characteristics and it was suitable for conducting high-sensitivity and high-resolution spectrum detection analysis on gas.