A high-Q Fabry-Perot interferometer for water vapor absorption measurements in the 100 Gc/s to 300 Gc/s frequency range

The investigation of atmospheric losses in the far-infrared and submillimeter wavelength regions has been pursued from both ends of the spectrum, i.e., from the infrared using IR spectroscopic techniques, and from the microwave region using coherent, high resolution spectroscopic methods. This paper describes the design of a Fabry-Perot semi-confocal interferometer, and its application to the measurement of atmospheric gas losses using coherent sources in the 100 Gc/s to 300 Gc/s frequency range. The tunable interferometer exhibits Q's as large as 3 × 105at 300 Gc/s, permitting the measurement of loss with a sensitivity of tan δ= 10-8and an error of five percent or less.