Recent Developments in MMW and Higher Frequency Spectrometers

Krupnov, in a review of the then current state of mZj-MMW spectroscopy in his home-institute, reflected on the absence of significant review papers in the preceding decade. He went on to describe the developments in high-fi equency MMW spectrometers and particularly the development of laboratory versions of synthesisers up to 500 GHz and spectrometers up to 1.5 THz, based aroimd their BWO designs. 

Detection of absorption was achieved by source FM and monitoring changes in the current when the Orotron was in a state of marginal oscillation using the usual phase-coherent detectors. The device under those conditions operated as a square law detector with sensitivity 1 AW . The resulting sensitivity of absorption was 3-5 X 10 m with a 1 Hz bandwidth receiver and they report analysis of CO, CH3OH, CH3COCH3 and OCS in admixture with water or air. 

Surin used the Orotron oscillator as a tuneable source of coherent MMW radiation to study the spectroscopy of SiH4 and ND3 in the 90-160 GHz range. The gas was introduced into a cell placed in a Fabry-Perot cavity 10. The absorption signals were detected from variation of the Orotron electron beam current. No phase or frequency lock schemes were necessary and resolution achieved was sufficient to resolve the lines Doppler profiles sensitivity was estimated as 3-5 X 10 m . 

The Orotron is a remarkable device, comprising no MMW semiconductors or cryogenically cooled bolometer, but possessing nonetheless better sensitivity than most cavity spectrometers. The penalties are the mechanical complexity, a magnet, and importantly, risk of total system failure if the interior became corroded or contaminated by the target anal3de. The last potential problem is not unique to an Orotron, however other designs also risk that catastrophe  

Vaks et alf have designed a novel spectrometric approach for gas analysis based on a BWO and low-Q absorption cell. The BWO spectral output was split between a reference cell and aim waveguide measurement cell. The gas in the 0.2 m pathlength waveguide reference cell was used to lock the BWO frequency to the spectral line at the 304 GHz OCS spectral line and the frequency modulated spectral source directed to the analytical cell. Detection was with a Schottky barrier diode mixer. 

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