Tunable Heterodyne Receivers - A Promising Outlook for Future Mid-Infrared Interferometry.

Sketching the beam layout of the Cologne Tunable Heterodyne Infrared Spectrometer (THIS), the basic principle of operation of a heterodyne receiver is illustrated in Fig. 1 [1,2]. A local oscillator (LO) is precisely locked to a frequency close to the observing wavelength by the resonance of a Fabry-Perot diplexer, which itself is stabilised to a resonance of a special HeNe laser. The LO radiation with its known frequency is then superimposed onto the signal of the observed science source (or hot or cold calibration sources) and both get mixed via a HgCdTe (MCT) detector, what generates a beat frequency (intermediate frequency; IF) at a much lower (and therefore easier to process) frequency of about 1 GHz. This broadband low frequency signal, which still contains all the spectral information of the science source, can then be analysed using common broadband Acousto Optical Spectrometers (AOS). [ABSTRACT FROM AUTHOR]