† Corresponding author. E-mail:
Project supported by the Chinese Academy of Sciences (Grant Nos. GJJSTD20180003 and QYZDJ-SSW-SLH043), the National Key Basic Research and Development Program of China (Grant Nos. 2017YFA0304003 and 2018YFA0404701), the National Natural Science Foundation of China (Grant Nos. 11603081, 11673073, U1831202, and 11873099), and PICS projects between the CAS and the CNRS.
We report on the investigation of optimal bias region of a wide-band superconducting hot electron bolometer (HEB) mixer in terms of noise temperature performance for multi-pixel heterodyne receiver application in the 5-meter Dome A Terahertz Explorer (DATE5) telescope. By evaluating the double sideband (DSB) receiver noise temperature (Trec) across a wide frequency range from 0.2 THz to 1.34 THz and with a large number of bias points, a broad optimal bias region has been observed, illustrating a good bias applicability for multipixel application since the performance of the HEB mixer is uniquely determined by each bias point. The noise temperature of the HEB mixer has been analyzed by calibrating the noise contribution of all RF components, whose transmissions have been measured by a time-domain spectroscopy. The corrected noise temperature distribution shows a frequency independence relation. The dependence of the optimal bias region on the bath temperature of the HEB mixer has also been investigated, the bath temperature has limited effect on the lowest receiver noise temperature until 7 K, however the optimal bias region deteriorates obviously with increasing bath temperature.
Superconducting hot electron bolometer (HEB) mixers[1] have been widely used in ground-based[2] and space-based observatories[3] due to the advantages of high sensitivity and low local oscillator (LO) power requirement comparing to other THz detectors.[4–6] The use of multipixel receivers can improve the mapping speed of the telescope significantly compared with that with single pixel beam receivers. Superconducting HEB mixer array has and will be used in several telescopes such as GLT,[7] upGREAT,[8] GUSTO,[9] and FIRSPEX.[10] Since Dome A holds the best terahertz window on the earth,[11] 5-meter Dome A Terahertz Explorer (DATE5) is proposed by Purple Mountain Observatory and a 1 × 4 superconducting HEB mixer array has been chosen as the mixer for the 200 μm wavelength band.
Due to the extreme environment at Dome A, the DATE5 telescope will be unmanned. For the 1 × 4 beams heterodyne receiver array, a DC bias reuse technology will also be applied to reduce the complexity of the bias system. What is more, it is difficult to get a uniformed LO power distribution for each pixel,[12,13] and the operation point of each superconducting HEB mixer is uniquely determined by the absorbed LO power, DC bias voltage, and bath temperature. Therefore, it becomes important to study the noise temperature distribution of superconducting HEB mixers for the multipixel application in DATE5 telescope.
Previous research[11] has proved that Dome A has good transmittance at frequency beyond 7 THz. Considering the possible application of HEB mixer in far-infrared frequency range, we investigated the frequency dependence of the optimal bias region of a wideband superconducting HEB mixer.
The 0.2–2 THz superconducting HEB device consists of a NbN micro-bridge and a planar spiral antenna. The NbN micro-bridge, located at the center of the planar spiral antenna, has a width of 2 μm and a length of 0.2 μm. The thickness of the NbN film is 5.5 nm and is connected to the spiral antenna by two Ti/Au pads. Both micro-bridge and spiral antenna are fabricated on the high-resistivity silicon substrate.
Figure
The measured R–T and I–V curves of the HEB device are presented in Figs.
The double sideband (DSB) receiver noise temperatures of the 0.2–2 THz HEB mixer are measured at 0.2 THz, 0.5 THz, 0.85 THz, and 1.34 THz by the Y-factor method. The measurement system is illustrated in Fig.
The intermediate frequency (IF) output signal from the HEB mixer is amplified firstly by a low noise amplifier (LNA) and then a room temperature amplifier. The amplified IF power is then filtered by a bandpass filter (80 MHz) at 1.5 GHz and recorded by a square-law detector.
Figure
The DSB receiver noise temperatures have been measured at a large number of bias points at four frequencies, and are illustrated in Fig.
![]() | Table 1. Lowest uncorrected and corrected DSB receiver noise temperatures . |
![]() | Table 2. Loss of RF components. . |
The measured DSB receiver noise temperature Trec includes three parts: RF noise TRF, HEB mixer intrinsic noise Tmixer, and IF noise TIF, and can be expressed as
The IF noise TIF is mainly produced by the LNA and is normally very small (about 3–4 K). The conversion gain Gmixer of the HEB mixer is about –5 dB and is proved to be frequency independent.[14] Therefore we ignore the term TIF/Gmixer in Eq. (
The RF noise is produced by the RF optics in the receiver system which consists of a 25 μm Mylar beam splitter, a 1.5 mm HDPE window, a Zitex G104 infrared filter, an air path, a Si lens, and a planar spiral antenna. The equivalent input noise temperature Teq of each RF component with a gain G is given by
![]() | Fig. 7 (a) Simulated transmission of the air path in our lab by AM model. (b) Normalized coupling efficiency of spiral antenna by simulation and FTS measurement. |
After calibrating the RF noise and RF loss by Eqs. (
The receiver noise temperature of the superconducting HEB mixer has also been measured at 4 K, 5 K, 6 K, and 7 K to investigate the effect of the bath temperature on the optimal bias region. Figure
![]() | Fig. 9 Measured DSB noise temperature Trec in a two-dimensional graph of the bias voltage and current at 0.85 THz at different bath temperatures. |
The noise temperature distribution of a wide-band superconducting HEB mixer has been investigated for the multi-pixel application. A broad optimal bias region has been observed at 0.2 THz, 0.5 THz, 0.85 THz, and 1.34 THz. The HEB mixer noise temperature has been calculated at all frequencies by calibrating the noise contribution of the RF components. The corrected mixer noise temperature has the same distribution on bias point at all frequencies, a frequency independent optimal region can be observed. The bath temperature dependence has also been investigated, the bath temperature has limited effect on the lowest receiver noise temperature Trec until it raises to 7 K, however the optimal bias region deteriorates obviously with increasing bath temperature.
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