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Compact 1.6-1.9 THz local oscillator as stand-alone unit for GREAT

Authors:
M. Philipp, U.U. Graf, F. Lewen, D. Rabanus, A. Wagner-Gentner, P. Müller, J. Stutzki
Abstract:
The German Receiver for Astronomy at Terahertz Frequencies (GREAT) is a first generation PI instrument for the Stratospheric Observatory For Infrared Astronomy (SOFIA), developed by a collaboration between the MPIfR, KOSMA, DLR and the MPAe. GREAT is designed as a dual-channel, tri-band receiver, covering the frequencies of 1.6-1.9 THz, 2.4-2.7 THz and 4.7 THz. Each of the first three institutes named above contribute one heterodyne receiver channel, respectively. A later upgrade with an e.g. 1.4 THz channel will follow. The GREAT instrument is designed to observe with any two of the three frequency channels simultaneously and to be able to quickly switch between different flight configurations. This demands a high modularity and flexibility on our receiver concept. For this purpose, we must be able to exchange the cryostats containing the mixer devices, the optical benches and the local oscillator subsystems (L0). Especially the LO systems are built as stand-alone units and integrated into compact boxes. KOSMA contributes GREATs heterodyne receiver channel operating at a frequency of 1.6-1.9 THz. We present an overview of the 1.6-1.9 THz LO system, and a detailed description of its internal parts. Radiation of 1.9 THz is generated by means of a Backward Wave Oscillator (BWO) followed by a frequency tripler. The estimated output power at 1900 GHz is about 1µW. To correct the BW0's considerable elliptical output beam pattern we developed astigmatic off-axis mirrors to achieve maximum beam coupling into the tripler's feed horn antenna. All mirrors are manufactured in-house on a 5 axis CNC milling machine at KOSMA. To phase-lock the BWO we use a custom-made two-stage PLL system with a Gunn oscillator as intermediate LO. The PLL system is located in a small compartment in the front of the LO-box. Schottky mixers for 633 GHz have been manufactured at KOSMA. The required magnetic field of about 1.25 Tesla for the BWO-tube is provided by a permanent magnet. This solution makes the oscillator independent from high current electromagnets as well as helium infrastructure for superconducting magnets. The BWO-tube produces approximately 150 Watt of excess heat. We integrated a high efficiency chiller into the LO-box designed to dispose of this heat with a minimum additional power consumption and space requirement. The complete LO-system fits into a small box with 21cm × 24cm × 100cm in size and consumes only 230 Watts of power.
Categories:
Poster Session
Year:
2004
Session:
7
Full-text:
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Page Number(s):
248-254