Quantitative millimetre wave spectrometry. Part I: Design and implementation of a tracked millimetre wave confocal Fabry-Perot cavity spectrometer for gas analysis

This paper describes the design criteria and construction of a compact Fabry-Perot cavity spectrometer with frequency modulation (FM) signal recovery, for use in the 70–220 GHz region. A solid state Gunn oscillator was frequency locked to a radiofrequency synthesiser and used as the primary source. The confocal cavity comprised two brass spherical mirrors, one made part of the sample chamber and the other attached to a precision translation stage with piezoelectric displacement. The sample chamber used a PTFE cup with a central thin window 30 mm diam. supported by the rest of the cup structure which also acted as a spatial filter to suppress non-axial modes. The high quality factor cavity (Q = 1 × 104) was maintained resonant with the source frequency by locking the detected FM signal in phase with the incident FM impressed upon the source, in a control loop which adjusted the cavity dimensions using the piezoelectric transducer. The first harmonic (2F) of the FM (F) was measured to recover the second derivative of the spectral line profile, as the source was scanned across it. Examples of spectra arising from 9 ppm HDO, 2.7 ppm OCS (in an excited vibrational state), HCH18O, (2000 ppm natural abundance), HCH17O (373 ppm natural abundance) and acrylonitrile at approximately 300 ppm, demonstrate the high sensitivity and versatility of the technique.