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Terahertz Receiver Array Scans the Stars

Satellite-based astronomy conducted over the 1-to-5-THz range by the Herschel observatory satellite mission from 2009 through 2012 proved the effectiveness of using the far-infrared area of the frequency spectrum for radio astronomy. The Stratospheric Observatory for Infrared Astronomy (SOFIA) observatory is continuing that research into the astronomical use of infrared (IR) and terahertz wavelengths, with a large radio telescope on board a modified Boeing 747SP wide-body aircraft. The SOFIA observatory is a joint project of NASA and the German Aerospace Center, DLR.

By rising high enough in the stratosphere (about 41,000 ft. altitude), the aircraft and telescope are not blocked in the reception of IR or terahertz signal wavelengths from deep space. To enhance the reception of terahertz signals, researchers from a number of different institutions in Germany have developed low-frequency (LF) and high-frequency (LF) receiver arrays in a dual-polarization configuration for working with frequencies from 1.9 to 2.5 THz and at 4.745 THz. To minimize the effects of thermal noise, the arrays employ closed-cycle coolers to achieve cryogenic operating temperatures of 4.5 K or less.

The terahertz receivers are part of the SOFIA system’s upGREAT multiple-pixel heterodyne arrays for the two terahertz frequency ranges. To minimize noise, the mixers and low-noise amplifiers (LNAs) are cryogenically cooled to temperatures below 4.5 K. The intermediate-frequency (IF) processing section provides additional gain, equalization, filtering, and signal leveling closely matched to the requirements of the system’s digital spectrometer. The IF signals are brought to the cryostat output connectors by means of stainless-steel coaxial lines.

To achieve the 1.9 to 2.5 THz frequency range of the LF receiver array, solid-state multiplying chains have been used with lower-frequency sources to generate the required local oscillator (LO) signals for the superheterodyne receiver. Unfortunately, the other option, using signals from a quantum cascade laser (QCL), does not provide the LO power necessary for effective frequency conversion to IF. The LO sources for the receivers are supplied by high-frequency component supplier Virginia Diodes, covering an LO bandwidth of 1.882 to 1.920 THz with 20 μW LO output power. For both LF subarrays, two identical LO chains are used, one for each polarization. The outputs for both signal chains have orthogonal polarization and are combined via a wire grid outside the LO cooler.  

For use in this airborne system, the components including the cryocooler required flight testing and qualification to ascertain airworthiness. A number of commissioning flights were conducted, with positive results of performance and durability. Work continues in the development of LO sources for covering the full LF frequency range of 1.9 to 2.5 THz as part of the application of this terahertz space telescope for deep space exploration. 

See “First Supra-THz Heterodyne Array Receivers for Astronomy with the SOFIA Observatory,” IEEE Transactions in Terahertz Science and Technology, Vol. 6, No. 2, March 2016, p. 199.

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