Simultaneous multi-parameter analysis of detonation field based on line fitting of wavelength modulated second harmonic signals

In the wavelength modulation method of laser absorption spectroscopy, the second harmonic signal contains numerous information about the flow field parameters. The fitting of the second harmonic line allows us to accurately obtain some of the important parameters in the flow field. In this paper, the second harmonic signal of the absorbed laser signal is obtained by simulation for the temperature, pressure, and gas concentration in the detonation field. A particle swarm optimization algorithm is then applied to obtain the optimal parameter solution for fitting the line, thus implementing an algorithm for the simultaneous analysis of multiple parameters using the second harmonic signal. When the appropriate modulation depth and parameters of the particle swarm optimization algorithm were selected, the relative error of fitting results for temperature and pressure were within 4.94% and 9.34%. This algorithm can be used to synchronize the measurement of temperature and pressure in the presence of large variations in the modulation index.