Heterodyne Method of Detection of Molecular Gas in the Terahertz Region Using the Beats Between Free Induction Decay Signals

The method of detection of molecular gas in the terahertz region using the free induction decay (FID) signal was tested. FID signal was recorded in real time after the pulse of free electron laser. To achieve linearity of the measured signal on the gas concentration, mixing with monochromatic reference FID was used. The beats of measured FID and reference FID were separated by a band-pass filter. It was shown that the energy of the beat signal can be equal to the absorbed energy. The method was tested for detecting methyl fluoride CH3F, hydrogen bromide HBr and methanol molecules. Hydrogen cyanide HCN was used as a source of reference FID.