High-Q Fabry-Perot Resonator for Nitric Oxide Absorption Measurements at 150 GHz

Abstract

A tunable high-Q Fabry-Perot resonator and related instrumentation which has been used to measure the shapes of nitric oxide rotational lines is described. The cavity was used around 150 GHz, but is suitable for any frequency above 120 GHz. The transmitter klystron is phase-locked to a tunable 15 MHz crystal standard. A sawtooth sweep is applied to this standard resulting in a linear-frequency sweep of the 150 GHz signal across the cavity resonance. The shape of the cavity response is a measure of the gas absorption. A beamsplitter is used to couple energy into the resonator. It is demonstrated that such resonators are well suited for certain microwave spectroscopy experiments.

The use of the Fabry–Perot interferometer for high resolution microwave spectroscopy

“…2. A similar setup of the Fabry-Perot resonator with the perpendicular coupling of electromagnetic waves was already introduced by French and Arnold [3] for frequencies around 150 GHz where this setup was used only for absorption measurements. In our paper, we describe potentialities of the Fabry-Perot spectrometer not only for the absorption experiments but above all for the emission measurements in a significantly lower frequency region.…”

“…The effective optical path length corresponding to the measured quality factors (according [3]) varies approximately from 40 up to 83 m from the intense absorption up to the evacuated resonator, respectively (see Fig. 3).…”

“…It should be mentioned, the method based on the coupling strength [6] was applied to derive the resonator response widths. This approach is different from conventional methods where this parameter is determined according to the 3 dB criterion [3] only.…”

“…It was necessary to avoid additional undesirable resonances inside of the resonator caused by inner surfaces of lenses (see Ref. [3]), which evokes a dummy increase of the quality factor. The optimal field distribution on the coupling foil and the position of the feeding antenna in front of the lens were also carefully sought [5].…”