Calibration of Heterodyne Dual Frequency Comb Laser Absorption Spectroscopy for NO and H2O Detection

The hydrogen combustion process within the propulsion unit of wind tunnel scramjet models produces NO and H2O. A precise determination of the concentration of both species applying laser absorption spectroscopy is necessary to validate numerical simulations. A novel approach to obtain absorption spectra of both species in the infrared region within μ-second range is based on the beating analysis of two combined frequency combs generated with quantum cascade lasers. This technique allows multi-species measurements of absorption spectra. The concentrations of species can be inferred from characteristic absorption features. The generated frequency combs are within the 1740 ± 20 cm−1 wavenumber region which includes strong NO and H2O spectral lines, thus enabling an identification of these species. The method offers a current-tuning based sweeping measurement mode for an increased spectral resolution. An extended calibration of all possible measurement modes under different conditions is reported.