Fabry-Perot Effect Suppression in Gas Cells Used in THz Absorption Spectrometers. Experimental Verification

Area of Science:

• Spectroscopy
• Terahertz (THz) technology
• Optical physics

Background:

• Standard gas cells in absorption spectrometers use transparent windows that cause Fabry-Perot effects.
• These effects, due to wave reflections, significantly alter transmitted radiation intensity.
• Accurate absorption spectra measurements using Bouguer law require accounting for Fabry-Perot effects.

Purpose of the Study:

• To experimentally validate a method for reducing Fabry-Perot effects using an external movable window.
• To compare the efficiency of this method in amplitude modulation (AM) spectroscopy against frequency modulation (FM) spectroscopy.
• To assess the combined effectiveness of FM spectroscopy and the external window for minimizing Fabry-Perot interference.

Main Methods:

• Experimental setup utilizing a standard gas cell with an added external movable window.
• Amplitude modulation (AM) spectroscopy and frequency modulation (FM) spectroscopy were employed for terahertz wave measurements.
• Comparative analysis of spectral data obtained with and without the external window, and between AM and FM techniques.

Main Results:

• The external movable window method was experimentally validated for reducing Fabry-Perot effects in AM spectroscopy.
• Frequency modulation spectroscopy inherently reduced Fabry-Perot effects more than standard AM spectroscopy.
• The combination of FM spectroscopy and the external movable window provided the most effective reduction of Fabry-Perot effects.

Conclusions:

• The external movable window is a viable technique for mitigating Fabry-Perot effects in AM absorption spectroscopy.
• FM spectroscopy offers superior reduction of Fabry-Perot effects compared to standard AM spectroscopy.
• Integrating an external movable window with FM spectroscopy provides enhanced elimination of Fabry-Perot interference for precise spectral measurements.