Prague Millimeter Wave High Resolution Spectrometer

     The Prague millimeter wave spectrometer was designed as an advanced semiconductor system. It means, in comparison with most of millimeter wave spectrometers abroad our system does not use the electron-beam device as a millimeter wave source (such as a backward wave oscillator). In addition to this,  advanced Schottky diodes working at room temperature are used as a detection part of the spectrometer instead of use usual a helium-cooled bolometer as a detector but. An advantage of this modern  set-up is a compactness and a lightness of the spectrometer, low energy consumption and mainly a needless of Helium cooling of system and high magnetic field.
    The semiconductor millimeter wave high resolution spectrometer (see Fig.1) covers a spectral range from 12 to 700 GHz. The basic microwave radiation is synthesized using a swept signal generator 83650 B (from Agilent) operating from 10 MHz up to 50 GHz. The signal generator is frequency stabilized by phase locked loop to an in-built frequency standard with relative accuracy 5∙10^-10 over 24 hours. The frequencies above 50 GHz are generated using a set of both passive and active multipliers (doublers, tripler, quadrupler and sextupler). A set of both broadband as well as high power semiconductor  amplifiers is used to ensure the sufficient radiation power before the frequency multiplication. By this manner the sufficient radiation output is ensured to allow high sensitivity as well as saturation measurements in the whole spectral extent. The Agilent signal generator makes possible both the amplitude and frequency modulations for the elimination of noise and the improvement sensitivity of the spectrometer. A set of the Schottky diodes is used as the non-cooled fast detectors in the whole spectral range. Our first experiments showed a high “breakability” of the detectors by accidental voltage pulses, electrostatic damages, etc. and therefore a construction of special protective bias boxes was necessary. The detection system is completed by a phase sensitive lock-in amplifier 7265 (from Perkin-Elmer).

    For increasing of the frequency stability of the generated millimeterwave signal the additional frequency standard in the form rubidium atom clock is used. This one has a relative accuracy 5∙10^-12 per day and its aging is constrained by phase locked loop to the absolute frequency standard – cesium atom clock in the GPS system.
    Measurements of spectra are performed in conventional free space glass cells with a total length between 150 and 280 cm . The optical path length can be doubled by using a roof top mirror and polarization grid (see Fig. 1). The rotational lines are studied close to the Doppler limit, however, in addition to this; some overlapping lines can be measured also under the saturation Lamb-dip conditions and reached a sub-Doppler resolution (see Fig. 2). The rotational lines are mostly measured using amplitude modulation since this modulation makes possible more subtle numerical evaluation of the spectral background and overlapping lines. In cases of very weak lines and sub-Doppler measurements, the frequency modulation is used in the second harmonic (see Figs 3-4). An accuracy of the well developed lines measured by the amplitude modulation and subjected to the numerical treatment is estimated to be better then 1 kHz. The accuracy of frequency modulated lines is generally lower; however, for the well-developed symmetric profiled lines (in case of the flat background) the accuracy of reading of transition frequencies is also close to 1 kHz.
    The mm-wave spectrometer is controlled by a computer program written in the LabVIEW development environment (National Instruments) that makes it possible to adjust, set and monitor all the measurement parameters, starting with the type of modulation (amplitude or frequency), the frequency step, the depth of modulation, the time constant, as well as, to collect the measured data.
    In the future, the sensitivity of the experimental setup in the cases of the low frequency spectra and the molecular radical studies will be enhanced by using Stark and Zeeman modulation.