Abstract:
The Fabry-Perot cavity in a miniature pulsed Fourier transform millimeter-wave spectrometer operating between 94 and 104 GHz is characterized in detail. The new device measures gas or volatile composition in situ and has a nominal volume of 12 cm3, which is 200 times smaller than cavities operating at comparable frequencies in laboratory gas spectrometers. Scans of mode amplitude are presented as a function of mirror spacing and transmitter frequency. Primary (TEM00) and secondary (TEM10) modes are both observed and are matched to an eigenmode calculation. The modes are well-behaved and have quality factors in the range of 1000–6000, which is a desirable compromise between field strength and mode width. Measurements of pulse bandwidth versus duration agree with time-bandwidth product predictions. Measurements of rotational transitions in N2O and CH3OH are plotted at various pressures and collisional broadening is resolved at mTorr pressures. Through these gas detections, we demonstrate that it is possible to significantly reduce the size of cavity spectrometers for in situ deployment. The new device opens new possibilities for molecular sensing in pollution monitoring, planetary science, and other fields.