We report on the development and testing of the building blocks of a possible compact heterodyne setup in the mid-infrared,
which becomes particularly relevant for flight instrumentation. The local oscillator is a Quantum Cascade Laser
(QCL) source at 8.6 μm operable at room temperature. The beam combination of the source signal and the local
oscillator will occur by means of integrated optics for the 10 μm range, which was characterized in the lab. In addition
we investigate the use of superlattice detectors in a heterodyne instrument. This work shows that these different new
components can become valuable tools for a compact heterodyne setup.
The THIS instrument (Tuneable Heterodyne Infrared Spectrometer) is a versatile heterodyne receiver with a sensitivity
close to theoretical prediction. It uses a Quantum Cascade Laser (QCL) as local oscillator and a HgCdTe
photo-voltaic detector as mixer. The IF-spectrum is analyzed by means of a new broadband Acousto-Optical Spectrometer
(AOS) with 3 GHz bandwidth and 1 MHz resolution. A dual sideband (DSB) system noise temperature has
been measured with 2300 K at 10 μm wavelength, which is only 60% above the quantum limit. The stability of the
system has been determined at an Allan variance minimum time of 50 seconds. Below this integration time the performance
is purely radiometric. Also, the frequency stability has been measured with 1 MHz rms error within several
hours. The quality of the instrument has been demonstrated by a few observing campaigns at the McMath-Pierce
observatory on Kitt Peak. Measurements of Winds on Mars and Venus have been carried out and molecular line signals
in sunspots have been detected. We propose to develop THIS as a second generation instrument for future astronomical
observations on SOFIA.
The Cologne spectrometer THIS (Tuneable Heterodyne Infrared Spectrometer) opens the mid-infrared wavelength region from 5 to 17 µm for ultra-high-resolution spectroscopy. With a current bandwidth of 14 km/sec and a frequency resolution of R=2*10<sup>7</sup> it is the only widely tuneable and transportable infrared heterodyne receiver. A Quantum-Cascade laser is used as local oscillator (LO). To provide optimum beam combination a Fabry-Perot ringresonator is used to superimpose the LO and the radiation. Frequency mixing is done by a Mercury-Cadmium-Telluride photomixer and spectral analysis is performed by an Acousto-Optical spectrometer. The system noise temperature is about three times the quantum limit giving THIS a sensitivity equivalent to CO<sub>2</sub>-laser based heterodyne systems. Various measurements at different ground based telescopes including the analysis of trace gases in Earth's atmosphere, observations of molecular features in sunspots, and detection of non-LTE CO2 emission from the atmosphere of Venus' have been performed and demonstrate the instrument's capabilities for astronomical observations at ground based telescopes and the stratospheric observatory SOFIA in the near future. Possible targets for future observations with THIS will be discussed.
We present the newly developed transportable setup of the Cologne Tuneable Heterodyne Infrared Spectrometer (THIS) designed for astronomical observations aboard the Stratospheric Observatory For Infrared Astronomy (SOFIA). With THIS a competitive tuneable heterodyne spectrometer for the mid-infrared is available that will allow measurements in a wide field of astronomical applications. Frequency tuneability over a wide range provided by the use of semiconductor lasers as local oscillators (LO) allows a variety of molecules in the mid infrared to be observed under very high frequency resolution. With the use of newly developed quantum-cascade lasers (QCL) a sensitivity close to the quantum limit will be in reach.