Real-time Detection Of Small Molecule Hydrocarbon Based On CARS Spectroscopy

CARS(Coherent Anti-stokes Raman scattering) spectroscopy has the advantages of high speed and high sensitivity. Compared with the ordinary Raman spectroscopy, it also has the advantages of high spatial resolution, strong resistance to noise. Based on these advantages, CARS spectroscopy has been used to make transient analysis of composition and concentration in the combustion process. In this paper, Detailed theoretical analysis of CARS and experimental measurement of concentrations of methane, ethane, ethylene,propylene and other low carbon olefin by CARS are carried out. Such measurement can meet the need of analyzing the detailed components in the process of methanol to olefin(MTO).The nonlinear optical theory about CARS is given in theory, by listing the derivation of the CARS signal strength equation from the Maxwell’s equations, and a more detailed discussion is carried out on the third-order nonlinear susceptibility, which is the main factors influencing the CARS signal strength. A simplified theory is finally given to describe the relationship between the CARS signal and the concentration of the measured species, by ignoring the non-resonance effect.An experimental device for scanning the narrowband CARS spectroscopy is built experimentally, and the CARS spectroscopies of methane, ethane, ethene and propylene are scanned by this device. The relationship between CARS peaks and concentration of measured gas and background gases are discussed. A simplified model on the CARS spectroscopy of mixture of four kinds of gas is established. Based on the narrow CARS spectroscopy detection device, to the best of our knowledge, for the first time, a time-sharing narrowband CARS spectroscopy to detect multi-Raman active gases for quick online analysis of fluids in chemical industry and other fields is presented. Using this design,detection of concentration ratios of methane, ethylene and propylene is realized. The detection error of the measurement of olefin concentration ratios by narrow-band CARS is analyzed.