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Applied Sciences

Superconducting Sub-Terahertz Oscillator with Continuous Frequency Tuning

М. Е. Парамонов, Lyudmila V. Filippenko, Fedor V. Khan, Oleg S. Kiselev, V. P. Koshelets

Published Sep 5, 2022• 2 citations
Publisher

Abstract

The development and approbation of a superconducting local oscillator based on a long Josephson junction made it possible to create a fully superconducting integrated receiver in sub-terahertz frequency range, which was successfully tested both on board a high-altitude balloon and in the laboratory. In order to expand the frequency range of a superconducting integrated local oscillator, it is necessary to ensure the continuous tuning of its frequency at an arbitrary bias current, including a so-called resonant mode regime. The resonant mode regime takes place for high-quality tunnel junctions with low leakage; in this regime, stable generation is possible only at Fiske steps, the distance in frequency between which is tens of GHz. A method for suppressing resonances has been proposed and implemented; this method is based on the introduction of normal metal layers into the region near the long Josephson junction. Modeling of the propagation of electromagnetic waves in the proposed integrated structure was carried out; experimental samples were fabricated, and their comprehensive study was performed. The complete suppression of resonances and the possibility of the continuous tuning of the frequency of a superconducting local oscillator in the range of 200–700 GHz have been demonstrated. The linewidth of the FFO radiation does not exceed 15 MHz over the entire frequency range, which makes it possible to implement the phase locked loop mode in an integrated receiver intended for spectral studies.

DOI
10.3390/app12178904

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The development and approbation of a superconducting local oscillator based on a long Josephson junction made it possible to create a fully superconducting integrated receiver in sub-terahertz frequency range, which was successfully tested both on board a high-altitude balloon and in the laboratory. In order to expand the frequency range of a superconducting integrated local oscillator, it is necessary to ensure the continuous tuning of its frequency at an arbitrary bias current, including a so-called resonant mode regime. The resonant mode regime takes place for high-quality tunnel junctions with low leakage; in this regime, stable generation is possible only at Fiske steps, the distance in frequency between which is tens of GHz. A method for suppressing resonances has been proposed and implemented; this method is based on the introduction of normal metal layers into the region near the long Josephson junction. Modeling of the propagation of electromagnetic waves in the proposed integrated structure was carried out; experimental samples were fabricated, and their comprehensive study was performed. The complete suppression of resonances and the possibility of the continuous tuning of the frequency of a superconducting local oscillator in the range of 200–700 GHz have been demonstrated. The linewidth of the FFO radiation does not exceed 15 MHz over the entire frequency range, which makes it possible to implement the phase locked loop mode in an integrated receiver intended for spectral studies.
A Superconducting Integrated Receiver (SIR) comprises in a single chip a planar antenna combined with a superconductor-insulator-superconductor (SIS) mixer, a superconducting Flux Flow Oscillator (FFO) acting as a Local Oscillator (LO) and a second SIS harmonic mixer (HM) for the FFO phase locking. In this report an overview of the FFO development and optimization is presented. In order to overcome temperature constraints and extend operation frequency of the fully Nb SIR we have developed and studied Nb-AlN-NbN circuits with a gap voltage Vg up to 3.7 mV and extremely low leak currents (Rj/Rn > 30). Continuous tuning of the frequency due to bending and overlapping of the Fiske steps and a possibility to phase lock the Nb-AlN-NbN FFO at any frequency in the range 350-750 GHz has been experimentally demonstrated. After optimization of the FFO design the free-running linewidth between 7 and 1.5 MHz has been measured in the frequency range 350 - 750 GHz, which allows to phase-lock from 50 to 95 % of the emitted FFO power. New designs of the FFO intended for further improvement of its parameters are under development, but even at the present state the Nb-AlN- NbN FFOs are mature enough for practical applications. These achievements enabled development of a 500 - 650 GHz integrated receiver for the Terahertz Limb Sounder (TELIS) project intended for atmosphere study and scheduled to fly on a balloon in 2008.
A superconducting integrated receiver (SIR) comprises all of the elements needed for heterodyne detection on a single chip. Light weight and low power consumption combined with nearly quantum-limited sensitivity and a wide tuning range of the superconducting local oscillator make the SIR a perfect candidate for many practical applications. For the first time, we demonstrated the capabilities of the SIR technology for remote operation under harsh environmental conditions and for heterodyne spectroscopy at atmospheric limb sounding on board a high-altitude balloon. Recently, the SIR was successfully implemented for the first spectral measurements of THz radiation emitted from intrinsic Josephson junction stacks (BSCCO mesa) at frequencies up to 750 GHz; linewidth below 10 MHz has been recorded in the high bias regime. The phase-locked SIR has been used for the locking of the BSCCO oscillator under the test. To extend the operation range of the SIR well above 1 THz, a new technique for fabrication of high-quality SIS tunnel junctions with gap voltage Vg up to 5.3 mV has been developed. Integration of a superconducting high-harmonic phase detector with a cryogenic oscillator opens a possibility for efficient phase locking of the sources with free-running linewidth up to 30 MHz that is important both for BSCCO mesa and NbN/MgO/NbN oscillators.
The combination of narrow linewidth and wide band tunability makes the Josephson flux flow oscillator (FFO) a perfect on-chip local oscillator for integrated sub-mm wave receivers for, e.g., spectral radio astronomy. The feasibility of phase locking the FFO to an external reference oscillator is demonstrated experimentally. A FFO linewidth as low as 1 Hz (determined by the resolution bandwidth of the spectrum analyzer) has been measured in the frequency range 270–440 GHz relative to a reference oscillator. This linewidth is far below the fundamental level given by shot and thermal noise of the free-running tunnel junction. The results of residual FFO phase noise measurements are also presented. Finally, we propose a single-chip fully superconductive receiver with two superconductor–insulator–superconductor mixers and an integrated phase-locked loop.
We report on the results of developing a single-chip superconducting integrated receiver which is the key element of the 500 -650 GHz spectrometer for the Terahertz Limb Sounder (TELIS) balloon project. TELIS is a collaborative European project to develop a three-channel heterodyne balloon-based spectrometer for measuring a variety of atmospheric constituents of the stratosphere. The Superconducting Integrated Receiver (SIR) comprises in one chip a planar antenna integrated with an SIS mixer, a superconducting Flux Flow Oscillator (FFO) acting as Local Oscillator (LO) and a second SIS harmonic mixer (HM) for the FFO phase locking. As a result of the FFO design optimization a free-running linewidth between 9 and 1.5 MHz has been measured in the frequency range 500 – 710 GHz resulting in phase-locking of 35 to 95 % of the FFO power correspondingly. A new generation of the SIR devices with improved FFO performance and optimized interface between FFO and SIS/HM has been developed and comprehensively tested; all required TELIS parameters were demonstrated. Phase–locked FFO operation over entire SIR channel frequency range has been realized, spectral resolution below 1 MHz has been confirmed by gas cell and CW signal measurements. An uncorrected double side band (DSB) noise temperature about 200 K has been measured with the phase-locked FFO. The intermediate frequency bandwidth 4-8 GHz has been realized. To ensure remote operation of the phase-locked SIR several procedures for its automatic computer control have been developed and tested. To overcome temperature constraints and extend operation frequency of the all-Nb SIR we have developed and studied Nb-AlN-NbN-Nb circuits with a gap voltage Vg up to 3.7 mV and extremely low leakage currents (Rj/Rn > 30). Based on these junctions integrated microcircuits comprising FFO and harmonic mixer have been designed, fabricated and tested; the radiation from such circuits has been measured at frequencies up to 700 GHz. Employment of NbN electrode does not result in the appearance of additional noise. For example, FFO linewidth as low as 1 MHz was measured at 600 GHz, that allows us to phase lock up to 92 % of the emitted by FFO power and realize very low phase noise about –90 dBc. Preliminary results demonstrated uncorrected DSB noise temperature of the Nb-AlN-NbN SIR below 250 K at frequencies of primary TELIS interest around 500 and 600 GHz. We expect that Nb-AlN-NbN-Nb device will be used for the first qualification TELIS flight that is scheduled on April 2007.
Comprehensive measurements of the flux flow oscillator (FFO) radiation linewidth are performed using an integrated harmonic SIS mixer; the FFO linewidth and spectral line profile are compared to a theory. An essential dependence of the FFO linewidth on frequency is found; a possible explanation is proposed. The results of the numerical solution of the perturbed sine–Gordon equation qualitatively confirm this assumption. To optimize the FFO design, the influence of the FFO parameters on the radiation linewidth is studied. A novel FFO design at a moderate current density has resulted in a free-running FFO linewidth of about 10 MHz in the flux flow regime up to 712 GHz, limited only by the gap frequency of Nb. This relatively narrow free-running linewidth (along with implementation of a wide-band phase locking loop system) allows continuous phase locking of the FFO in the wide frequency range of 500–710 GHz. These results are the basis for the development of a 550–650 GHz integrated receiver for the terahertz limb sounder (TELIS) intended for atmosphere study and scheduled to fly on a balloon in 2005. We report here also on the design of the second generation of the phase-locked superconducting integrated receiver chip for TELIS.
In this report an overview of the results on the development of a single-chip superconducting integrated receiver for the Terahertz Limb Sounder (TELIS) balloon project intended to measure a variety of stratosphere trace gases is presented. The Superconducting Integrated Receiver (SIR) comprises in one chip a planar antenna integrated with a superconductor-insulator-superconductor (SIS) mixer, a superconducting Flux Flow Oscillator (FFO) acting as local oscillator (LO) and a second SIS harmonic mixer (HM) for FFO phase locking. As a result of the FFO design optimization a free-running linewidth between 9 and 1.5 MHz has been measured in the frequency range 500-710 GHz resulting in phase-locking of 35 to 95% of the FFO power correspondingly. A new generation of the SIR devices with improved FFO performance and optimized interface between FFO and SIS/HM has been developed and comprehensively tested. As a result all required TELIS parameters were demonstrated. Phase-locked FFO operation over entire SIR channel frequency range has been realized, spectral resolution below 1 MHz has been confirmed by gas cell and CW signal measurements. An uncorrected double side band (DSB) noise temperature below 250 K has been measured with the phase-locked FFO. The intermediate frequency bandwidth 4-8 GHz has been realized. To ensure remote operation of the phase-locked SIR several procedures for its automatic computer control have been developed and tested.