Simulation of the impact of the cell interference on the THz gas absorption spectrum meashured by nonstationar spectroscopy methods

Abstract

Cell is the important part of the terahertz (THz) spectrometer designed to study the chemical composition of multicomponent gas mixtures, since it is used for supporting the optimal pressure of the gaseous sample. However, the reflection of electromagnetic waves from the cell windows leads to standing-wave interference, which can distort the measured spectrum. Mathematical modeling of the impact of the interference on the measured spectrum was carried out in the paper. The cell was treated as a Fabry-Perot cavity with infinite flat dielectric mirrors. Diffraction of electromagnetic waves, as well as the frequency dependences of the source power and the detector sensitivity, was taken out of the scope of the considered model. The two effects of the interference were described. The interference not only determined the slow frequency dependence of the cell transmission spectrum, but also affected the profile of the gas absorption lines reconstructed from the cell spectrum in accordance with the Bouguer-Lambert-Beer law. Both the processing of the measured spectrum and the setup calibration required for accurate quantitative measurements could be complicated by the effects. However, methods of modulation and non-stationary spectroscopy enabled to eliminate the first effect, they were possibly ineffective to eliminate the second one. Presumably, the detected distortion of the contour of the absorption lines was caused by a shift in the interference pattern and, accordingly, a change in the reflection coefficient of the cell due to the anomalous dispersion of the sample. The correlation of the distortions with the derivatives of the transmission spectrum of the empty cell was revealed. The obtained results could be used to develop algorithms for calibrating the measuring setup and processing experimental data.