Modern Millimeter-Wave Resonator Spectroscopy of Broad Lines

Abstract

Broad line millimeter-wave resonator spectroscopy is shown to have high sensitivity and fast and broad frequency scanning possibilities. Methods of absolute measurement of separate line, sample, and resonator losses are demonstrated and discussed. The primary radiation source (BWO) frequency is controlled digitally with minimal steps of a few hertz and switching time of 200 ns. An open Fabry–Perot resonator with a quality factor of ∼600 000 is used. A resonance-width measurement accuracy of 20 Hz is reached which corresponds to an absorption coefficient sensitivity limit of ∼4 × 10−9 cm−1 (0.0018 dB/km). In situ open atmosphere spectra records at 44–98 and 113–200 GHz are obtained in one experiment each. Lines of atmospheric oxygen and atmospheric water are observed. The water line at 183 GHz fits the Van Vleck–Weisskopf shape up to 20 half-widths from the line center within experimental accuracy. The dry-air broadening parameter for this water line is defined as 3.985(40) MHz/Torr. Atmosphere absorption in a 140-GHz window is measured. Results from other authors are compared. Possible applications of the modern broad line millimeter-wave resonator spectroscopy including real-time monitoring of open-air atmosphere and technological processes and measurement of absorption in thin dielectric films and conducting surfaces and metals are discussed.