A femtosecond time-resolved coherent anti-Stokes Raman spectroscopy thermometry for steady-state high-temperature flame

A simplified thermometry based on femtosecond coherent anti-Stokes Raman spectroscopy (fs-CARS) was proposed to measure the temperature of steady-state high-temperature flame. Nitrogen molecules were used as a temperature gage and the polarization beats appearing in the initial partial fs-CARS transients were used to access the temperature. As the contribution of all rotational levels to the CARS signal was summarized as the frequency-spread decay, the calculation complexity of extracting temperature from CARS transients can be significantly reduced with this method. In our experiments, the CARS transients of atmospheric N2 from room temperature to 1200 K were well fitted with this simplified principle, which proved the effectiveness of this method. The flame temperature of a butane Bunsen burner was practically measured using this fs-CARS thermometry, and the measured temperature was slightly higher than that given by a platinum-rhodium thermocouple. Since the intrusive thermocouple wire will influence the flame reacting flow, we suggest that the measurement result given by fs-CARS is more reliable. At present, this simplified thermometry is only suitable for the measurement of steady flow. Considering the practical diagnostic requirements of dynamic processes, the possibility of fast thermometry based on this method was also discussed.