Calibration of a Heterodyne Dual Frequency Comb Laser Absorption Spectroscopy System

Abstract

The use of hydrogen combustion based scramjets for propulsion is one promising idea to drive hypersonic passenger transport. Even though these propulsion systems prevent CO 2 formation, the production of NO and H2 O during the combustion process may have other environmental effects. Therefore, the experimental determination of the production and the atmospheric release of NO and H2 O in hydrogen combustion is of interest for the study of scramjet models in the High Enthalpy Shock Tunnel Göttingen. A new laser absorption spectroscopy method uses quantum cascade lasers (QCLs) in the infrared region within μ-second range. The method is based on the creation of heterodyne beating by two frequency combs. But the measurement technique requires a further calibration and understanding of the measurement modes. By modulating current and temperature of the lasers and thus sweeping through them, the spectral point spacing is reduced, resulting in a higher resolution and better foundation for the detection of gas concentrations. This work presents experimental prerequisites and results from tests with a multipass absorption cell in internal and external set-up for testing the influence of optical fibers. The fibers as well as additional optics are essential to bring the light beam from the spectrometer towards an external test section and back to a detector.