Quantitative millimetre wavelength spectrometry Part V: Observation of dispersive gas spectra with a millimetre wavelength confocal Fabry-Perot cavity spectrometer

We describe a novel spectroscopic measurement of gas samples which monitors the spacing between the mirrors of a resonant Fabry-Perot cavity and the consequential shift in its resonant frequency as a millimetre wavelength (MMW) source is tracked through spectral lines of the sample. In a simplified theoretical treatment, we show that this effect is accentuated by lowering the cavity quality factor, in complete contrast to what is observed for absorption signals that have been transmitted through the sample. Furthermore, the frequency shift caused by a sample depends only on its peak absorption coefficient and not on any other system parameters, offering a simple technique for the absolute calibration of such a spectrometer. Experimental results obtained for OCS, N2O, SO2 and H2O samples bear out these predictions, and lead to a proposal that may lessen the need for costly, high performance and low noise detectors to observe weak sample absorptions at millimetre wavelengths.