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18 December 1996 Open-path millimeter wave spectroscopy in the 225- to 315-GHz range
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This paper discusses the development of an open-path millimeter-wave spectroscopy system in the 225-315 GHz atmospheric window. The new system is primarily a monostatic swept-frequency radar consisting of a mm-wave sweeper, hot- electron-bolometer or Schottky detector, and trihedral reflector. The heart of the system is a Russian backward- wave oscillator (BWO) tube that is tunable over 225-350 GHz. A mm-wave sweeper has been built with the BWO tube to sweep the entire frequency range within 1 s. The chemical plume to be detected is situated between the transmitter/receiver and the reflector. Millimeter-wave absorption spectra of chemicals in the plume are determined by measuring swept- frequency radar signals with and without the plume in the beam path. Because of power supply noise and thermal instabilities within the BWO structure over time, the BWO frequencies fluctuate between sweeps and thus cause errors in baseline subtraction. To reduce this frequency-jitter problem, a quasi-optical Fabry-Perot cavity is used in conjunction with the radar for on-line calibration of sweep traces, allowing excellent baseline subtraction and signal averaging. Initial results of the new system are given for open-path detection of chemicals.
© (1996) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
Nachappa Gopalsami, Sasan Bakhtiari, and Apostolos C. Raptis "Open-path millimeter wave spectroscopy in the 225- to 315-GHz range", Proc. SPIE 2842, Millimeter and Submillimeter Waves and Applications III, (18 December 1996);

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