Oral Presentation
The 1.5 THz Cartridge-type Multi-pixel Receiver for the Greenland Telescope (GLT)
Presenter: Yen-Ru Huang (ASIAA)
The Greenland Telescope (GLT) is a state-of-the-art 12-m sub-millimeter telescope for both single-dish and VLBI observations, and the site can provide excellent conditions to operate at very high radio frequencies up to 1.5 THz. For the 1.5 THz frequency band, we are developing an ALMA-type receiver cartridge based on 2 × 2 hot-electron-bolometer (HEB) mixer array to meet high mapping speed requirements, in which the front-end optics consists of a local oscillator (LO) beam path and a radio frequency (RF) path. For the LO beam path, cooled frequency multipliers are used in this module to boost the output power to about 30 μW at low temperature in the frequency range of 1.45-1.55 THz. The horn of LO module is fixed at the focal point of a 90 degree off-axis parabolic mirror on the 85 K plate, then the LO beam is divided into four sub-beams with a spacing of 18 mm through a power distributor module. After that the LO and RF beams are combined by a large-area beam splitter made of mylar films with 13 μm thickness, and then arrives at a four-pixel silicon lens with twin slot antenna (TSA). For the RF beam path, an additional four-pixel HDPE lens is located at 120 mm in front of the silicon lens to increase the beam waist size for fitting to the aperture parameters of the sub-reflector of GLT antenna. Then this design can match the optical parameters between Si/TSA and GLT antenna.
We have also successfully realized 1.5 THz HEB mixers by using NbN films with thicknesses of 3.0 nm, 4.0 nm, and 5.0 nm, and their receiver noise temperatures is about 1900 K, 1400 K, and 1100 K respectively. Up to the present day, the quasi-optical measurement, thermal analysis, and cryogenic testing of single-pixel engineering model are finished, but the fabrication and testing of four-pixel power distributor module is still ongoing. The power distributor module contains TE and TM mode polarizing beam splitters made of quartz and silicon slabs respectively, and these beam splitters are designed to make the transmission-to-reflection ratio equal to 1 and the power variation below 2.2% in the frequency range of 1.45-1.55 THz. In this presentation, we will report on design, assembly, thermal analysis, and performance testing of single-pixel and four-pixel receiver engineering model.
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