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IEEE Transactions on Magnetics

Design and performance of flux-flow type Josephson oscillator coupled to a stripline

Keiji Yoshida, Keisuke Kudo, Keiji Enpuku

Published Mar 1, 1991• 4 citations
Publisher

Abstract

In order to realize efficient coupling between a flux-flow type Josephson oscillator (FFO) and an external stripline, the authors carried out experiments to investigate the inductive coupling scheme in a strong coupling regime. By adopting NbN films with large penetration depths as electrodes for strong coupling, they were able to realize coupling sections with a coupling coefficient alpha =M/ square root L/sub 1/L/sub 2/ as large as 0.7, with M, L/sub 1/, and L/sub 2/ being mutual inductance, self-inductances of, the FFO, and the stripline, respectively. Using this design for the coupling, the authors succeeded in detecting AC fields at frequencies around 150 GHz, which were emitted from FFO to the stripline via the coupling section.

DOI
10.1109/20.133930

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Journal of Applied Physics•Feb 15, 1988•70 citations
In order to realize efficient coupling between a flux-flow type Josephson oscillator (FFO) and an external stripline, the authors carried out experiments to investigate the inductive coupling scheme in a strong coupling regime. By adopting NbN films with large penetration depths as electrodes for strong coupling, they were able to realize coupling sections with a coupling coefficient alpha =M/ square root L/sub 1/L/sub 2/ as large as 0.7, with M, L/sub 1/, and L/sub 2/ being mutual inductance, self-inductances of, the FFO, and the stripline, respectively. Using this design for the coupling, the authors succeeded in detecting AC fields at frequencies around 150 GHz, which were emitted from FFO to the stripline via the coupling section.
A theoretical and experimental study of the coupling of the flux-flow-type Josephson oscillator (FFO) to an external stripline has been carried out to overcome the large impedance mismatch between them. For efficient coupling and impedance matching at millimeter and submillimeter wavelengths, the authors have proposed a scheme where the Josephson oscillator is inductively coupled to the stripline by a thin superconducting electrode. Theoretical modeling of the coupling section and the design of the stripline impedance-transformer have been carried out. In a preliminary experiment, the authors obtained a power of about 10/sup -7/ W emitted from the FFO into the stripline in the frequency range from 184 GHz to 193 GHz.>
In order to realize efficient coupling between a flux-flow type Josephson oscillator (FFO) and a stripline impedance transformer, the authors have made a theoretical study of the inductive coupling between FFO and a stripline. It is shown that by choosing the output impedance of the FFO Z o smaller than the characteristic impedance of the coupling section Z 1 , the frequency dependence of the output power at the end of the coupling section can be extremely flat over a large frequency band. A model simulation employing a NbN/Pbin Josephson junction and a Nb/NbN stripline demonstrates that in the case of optimum design Z 0 =0.6 Z 1 , the bandwidth of the coupling section, which is defined by the frequency range where the deviation is within +or-5%, is shown to be as large as 80% of the centre frequency.
In order to realize an efficient coupling between a flux-flow-type Josephson oscillator (FFO) and a stripline, we have carried out experiments to verify the mathematical model of the inductive coupling scheme between FFO and a stripline resonator in the frequency range between 50 GHz and 350 GHz. It is shown that the simulation using the proposed equivalent circuit for the inductive coupling scheme well explains the experimental results. The experimentally obtained center frequency and the bandwidth of the matching circuit were as large as 120 GHz and 40 GHz, respectively, which are also in reasonable agreement with those obtained in the simulation.
The authors have been developing an analog amplifier utilizing a flux flow phenomena in a Josephson line, i.e. the flux-flow-type Josephson amplifier (FFA). The FFA converts a signal current I/sub s/ to a voltage V/sub dc/ across the Josephson line, and the I/sub s/-V/sub dc/ characteristic determines the performance of the FFA, e.g. the gain and saturation level. Studies have been made to improve the I/sub s/-V/sub dc/ relation. It is shown that the transresistance can be improved significantly by reducing the width of the Josephson line and increasing the kinetic inductance of the line. The kinetic inductance can be increased by thinning the line electrode and by using an electrode with large London penetration depth, such as NbN. The transresistance obtained is as large as 6.8 Omega for NbN/Pb-alloy Josephson lines with a thickness of 5 nm and a width of 4 mu m. It is also shown that the linear operating range of the amplifier is as large as 1 mV.>
To develop an efficient local oscillator (LO) integrated with a SIS mixer operating above the gap frequency of Nb, we have investigated the radiation power P of NbN-based flux-flow-type Josephson oscillators (FFO's). The designed and fabricated chip incorporates FFO's, SIS power detectors (DET's), and their coupling circuits. Both FFO's and DET's consist of epitaxial NbN/AlN/NbN junctions with high critical current density J/sub C/ (15200 nW, enough for the optimum pumping of a SIS mixer with rf-resistance of 50 /spl Omega/, is coupled to DET's for 0.5-0.9 THz. The coupling bandwidth is larger than 20% of its central frequency. In the band, the radiation frequency is tuned by the control current through the FFO of 10-100 mA. The maximum coupling frequency of the present experiment is quantitatively agreed with the theoretical one. The peak power of 1.3 /spl mu/W is detected at 0.76 THz. The dissipated power in a FFO is smaller than 500 /spl mu/W, which is 10/sup -4/ times less than that of semiconductor sources. These results indicate the feasibility of NbN-based FFO's for a practical on-chip LO.
Power emission from a flux-flow-type Josephson oscillator with a thin-film coupling method is studied experimentally. The thickness of one electrode of the oscillator is made thin enough to allow electromagnetic fields generated by the oscillator to be emitted through the thin-film electrode. Radiation power from the oscillator is detected with superconductor-insulator-superconductor detectors, which are fabricated on top of the oscillator. Experimental results show that sufficient power can be obtained from the oscillator by using the thin-film coupling method when the thickness of the electrode is comparable to the London penetration depth. The results obtained are in reasonable agreement with theoretical predictions. The thin-film coupling method will be useful when the oscillator is connected to external circuits, such as an impedance matching circuit.