A good understanding of input radio-frequency (RF) noise contribution is of particular importance to the development of high-sensitivity superconducting hot electron bolometer (HEB) receivers. In terms of simulation results based on the hot spot model, we found that the output noise of HEB mixers is nearly linearly decreased with the mixer's bath temperature when the HEB mixer is operated at its optimum DC-bias regime under appropriate local-oscillator signal pumping, and the effect of this decrease on the receiver output power can be well corrected. Thus, we used an intersecting lines technique to measure the input RF noise temperature of a quasi-optical superconducting NbN HEB receiver, and the measured input RF noise temperature of the HEB receiver is around 300 K at 850 GHz. For comparison, we also evaluated the losses and equivalent noise temperatures of the quasi-optical components in the RF signal path. The evaluated input RF noise temperature is found to be smaller than the measured one.
A good understanding of input radio-frequency (RF) noise contribution is of particular importance to the development of high-sensitivity superconducting hot electron bolometer (HEB) receivers. In terms of simulation results based on the hot spot model, we found that the output noise of HEB mixers is nearly linearly decreased with the mixer's bath temperature when the HEB mixer is operated at its optimum DC-bias regime under appropriate local-oscillator signal pumping, and the effect of this decrease on the receiver output power can be well corrected. Thus, we used an intersecting lines technique to measure the input RF noise temperature of a quasi-optical superconducting NbN HEB receiver, and the measured input RF noise temperature of the HEB receiver is around 300 K at 850 GHz. For comparison, we also evaluated the losses and equivalent noise temperatures of the quasi-optical components in the RF signal path. The evaluated input RF noise temperature is found to be smaller than the measured one.
In this paper, the performance of a quasi-optical NbN superconducting hot-electron bolometer (HEB) mixer, cryogenically cooled by a close-cycled 4-K refrigerator, is thoroughly investigated at 300, 500, and 850 GHz. The lowest receiver noise temperatures measured at the respective three frequencies are 1400, 900, and 1350 K, which can go down to 659, 413, and 529 K, respectively, after correcting the loss and associated noise contribution of the quasi-optical system before the measured superconducting HEB mixer. The stability of the quasi-optical superconducting HEB mixer is also investigated here. The Allan variance time measured with a local oscillator pumping at 500 GHz and an IF bandwidth of 110 MHz is 1.5 s at the dc-bias voltage exhibiting the lowest noise temperature and increases to 2.5 s at a dc bias twice that voltage.
In this paper, we report the performance of a quasi-optical NbN superconducting HEB (hot electron bolometer) mixer measured at 500 GHz. The quasi-optical NbN superconducting HEB mixer is cryogenically cooled by a 4-K close-cycled refrigerator. Its receiver noise temperature and conversion gain are thoroughly investigated for different LO pumping levels and dc biases. The lowest receiver noise temperature is found to be approximately 1200 K, and reduced to about 445 K after correcting the loss of the measurement system. The stability of the mixer’s IF output power is also demonstrated.
Abstract— Superconducting NbN hot electron bolometer (HEB) mixers are so far the most sensitive detectors for\nheterodyne spectroscopy in the frequency range between 1.5 THz and 5 THz. To reach the ultimate receiver noise\ntemperatures in the high end of the THz range (3-6 THz), it is crucial to understand their fundamental noise contribution\nfrom different origins. With increasing frequency, the classical output noise contribution should remain unchanged, but\nthe quantum noise contribution is expected to play an increasing role [1].\nThis paper reports the first dedicated experiment using a single NbN HEB mixer at a number of local oscillator\nfrequencies between 1.6 to 4.3 THz to address and quantify the contribution of the quantum noise to the receiver noise\ntemperature.\nWe used a spiral antenna coupled NbN HEB mixer with a bolometer size of 2 μm×0.2 μm. In order to minimize\nuncertainties in the corrections of the optical losses, we use a vacuum hot/cold load setup [2] to eliminate the air loss, and\nan uncoated elliptical Si lens. Although other components, a 3 μm Mylar beam splitter and a QMC heat filter, also\nintroduce frequency dependent optical losses, they can be accurately calibrated. Furthermore, to reduce uncertainties in\nthe data, we measure Y-factors responding to the hot/cold load by fixing the voltage, but varying the LO power [2]. As\nLO, we use a FIR gas laser.\nWe measure the Y-factor at the optimal point at different frequencies by only varying LO frequencies, but keeping\nthe rest exactly the same. We obtain DSB receiver noise temperatures, which are 842 K (at 1.6 THz), 845 K (1.9 THz), 974\nK (2.5 THz) and 1372 K (4.3 THz). After the correction for the losses of the QMC filter and the beam splitter, the noise\ndata show a linear increase with increasing frequency.\nUsing a quantum noise model [1] for HEB mixers and using a criterion for which the classical output noise must be\nconstant at different frequencies, we analyze the results and find the excess quantum noise factor β to be around 2 and\nthat 24 % of the total receiver noise temperature at 4.3 THz (at the input of the entire receiver) can be ascribed to\nquantum noise. Clearly the quantum noise has a small but measurable effect on the receiver noise temperature at this\nfrequency.\nWe are still analyzing different alternatives of interpretation for the mismatch loss between the bolometer and\nthe spiral antenna.\n[1] E. L. Kollberg and K. S. Yngvesson, “Quantum-noise theory for terahertz hot electron bolometer mixers,” IEEE Trans.\nMicrowave Theory and Techniques, 54, 2077, 2006.\n[2] P. Khosropanah, J.R. Gao, W.M. Laauwen, M. Hajenius and T.M. Klapwijk, “Low noise NbN hot-electron bolometer mixer\nat 4.3 THz,” Appl. Phys. Lett., 91, 221111, 2007.\n
In this paper, the performance of a quasi-optical phonon-cooled NbN superconducting hot electron bolometer (HEB) mixer is thoroughly investigated. The variation of the receiver noise temperature and the conversion gain around 600 GHz with respect to DC bias voltage and LO pumping level has been measured. The lowest receiver noise temperature is 750 K and reduced to about 250 K after correcting the quasi-optical losses before the HEB mixer. We compared the measured results with the predictions according to one dimensional distributed hot spot model. A reasonable match between theoretical model and experiment is observed. In addition, the direct response of the HEB mixer (integrated with a log-periodic spiral antenna) is measured by using Fourier Transform Spectrometer (FTS). The stability of the receiver is also investigated by means of Allan variance time measurements performed with an intermediate frequency (IF) bandwidth of 40 MHz.
A quasi-optical superconducting NbN hot-electron bolometer (HEB) mixer is measured in the frequency range of 0.5–2.5 THz for understanding of the frequency dependence of noise temperature of THz coherent detectors. It has been found that noise temperature increasing with frequency is mainly due to the coupling loss between the quasi-optical planar antenna and the superconducting HEB bridge when taking account of non-uniform distribution of high-frequency current. With the coupling loss corrected, the superconducting HEB mixer demonstrates a noise temperature nearly independent of frequency.
The heterodyne mixing and direct detection performance of a superconducting NbN hot-electron bolometer (HEB) integrated with a log-spiral antenna have been thoroughly characterized. The corrected receiver noise temperature and IF gain bandwidth are approximately 800 K at 0.5 THz, 750 K at 0.85 THz and 3 GHz at its optimum bias point. In addition, both the receiver noise temperature and IF gain bandwidth were found insensitive to the bath temperature, while the bias point was fixed, in good agreement with those simulated with the hot-spot model taking account of the HEB's current-dependent resistive transition. Furthermore, the HEB's frequency response was measured by a Fourier transform spectrometer at different bias points and bath temperatures. The estimated noise equivalent power (NEP) was close to 10-13W/Hz0.5around the HEB's transition temperature.