The frequency-modulated mm-/submm-wave (FM-mmW) spectrometers
1) The radiation sources are either several Gunn diodes (covering the 75-134 GHz range) or passive multipliers (x2, x3, x6, x9) in cascade driven by the Gunn diode oscillators themselves (frequency range: 75 - 1600 GHz). The sources are phase-locked to the suitable harmonic of a frequency synthesizer referenced to an external rubidium frequency standard. The frequency modulation of the radiation is obtained by sine-wave modulating the reference signal of the wide-band Gunn synchronizer.
2) The radiation source is an active multiplier chain (VDI) that is driven by a synthesizer operating at centimetre wavelengths. Using a series of frequency multiplication stages, this set-up provides continuous coverage across the 82–900 GHz frequency range. Accurate frequency and phase stabilisation is achieved by providing the synthesizer with a 10MHz rubidium frequency standard.
The spectrometers are equipped with different absorption cells: one is equipped with two cylindrical hollow electrodes at both ends (DC discharge system) and is wounded with a plastic pipe for liquid nitrogen circulation; another cell is connected to a quartz reactor designed for Flash Vacuum Pyrolysis; a third cell is a quartz tube surrounded by a 30 cm long tubular pyrolysis oven. Schottky barrier detectors ( up to 500 GHz) or liquid-helium-cooled InSb bolometer (up to 3 THz) are used for detection in the different frequency ranges. The radiation is phase-sensitively detected by a lock-in amplifier tuned at twice the modulation frequency, so that the second derivative of the spectrum profile is displayed.
Under construction
