Flux-flow Oscillator Research Articles

Josephson tunnel junction microwave attenuator

A new element for superconducting electronic circuitry—a variable attenuator—has been proposed, designed, and successfully tested. The principle of operation is based on the change in the microwave impedance of a superconductor-insulator-superconductor (SIS) Josephson tunnel junction when dc biased at different points in the current-voltage characteristic. Both numerical calculations based on the Tien–Gordon theory and 70-GHz microwave experiments have confirmed the wide dynamic range (more than 15-dB attenuation for one stage) and the low insertion loss in the ‘‘open’’ state. The performance of a fully integrated submillimeter receiver circuit which comprises a flux–flow oscillator (FFO) as local oscillator, a superconducting variable attenuator, and a microwave SIS detector with tuned-out capacitance is also reported.

Superconducting millimeter wave oscillators and SIS mixers integrated on a chip

All-refractory material superconducting millimeter-wave oscillators have been designed and investigated experimentally with different superconductor-insulator-superconductor (SIS) mixers integrated on the same chip. Tested structures include a flux-flow oscillator (FFO) based on a long Josephson junction, a coupling section, and an SIS detector with tuned out junction capacitance. Coupling sections were designed as multistep microstrip quarter-wave impedance transformers. All junctions have been fabricated on the basis of a high-quality trilayer Nb-AlO/sub x/-Nb process. Microwave oscillations in the frequency range 75-500 GHz have been detected experimentally. The level of delivered power was estimated from the pumped I-V curve of the strongly coupled single junction detector. Coupled power levels higher than 0.1 mu W at 256 GHz were achieved. A spectral linewidth of the FFO of less than 1 MHz has been estimated experimentally. The first attempt to create an integrated receiver based on an FFO and an SIS array mixer integrated on the same chip was made in the 2-mm wavelength band.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Detection of mm and submm wave radiation from soliton and flux-flow modes in a long Josephson junction

The authors report on the coupling of millimeter and submillimeter waves from a long (3- mu m*350- mu m) Josephson junction to a small (2- mu m*2- mu m) superconductor-insulator-superconductor (SIS) junction by using a center-fed interdigital capacitor. Coupled power at around 100 GHz and 261 GHz was observed. Experiments confirm that the critical current density j/sub c/ of a long junction determines the fluxon motion. Coupled irradiation (>3nW) at around 100 GHz, which corresponds to resonant soliton oscillations in low j/sub c/ samples (250 A/cm/sup 2/ to 1000 A/cm/sup 2/), is detected by the SIS junction. The incident power to the SIS detector junction could be tuned with only little frequency pulling by adjusting the current bias of the oscillator. For j/sub c/>1000 A/cm/sup 2/, coupled radiation at around 100 GHz (about 3 nW) and 261 GHz (4.6 nW), from flux-flow oscillations, is also clearly seen. Flux-flow radiation at 350 GHz is also observed. It is concluded that a large j/sub c/ value is preferable for high-frequency flux-flow oscillators.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Multi-fluxon effects in long Josephson junctions

Experiments with fluxons moving in long Josephson junctions of different geometries are reviewed. A controllable way of introducing fluxons one by one into annular Josephson junctions has been realized. This allows a comparison with existing fluxon chain perturbation theory. Several new millimeter-wave stimulated regimes found in linear junctions are discussed. Large asymmetric zero-crossing steps of constant voltage have been observed in the I-V curves. These steps may have a potential application in Josephson voltage standards. Large narrowband radiation emitted from the flux-flow oscillator in the 62-77-GHz frequency range has been detected. >

Superconducting millimeter wave oscillators

The integrated superconducting mm wave oscillator have been designed and experimentally investigated. It consists of Flux Flow oscillator (FFO), coupling section and detector junction with tuned out capacitance. Oscillator is long Josephson junction with nonuniform current injection shunted by Mo resistor to obtain dissipation parameter α ∼ 0.5. Coupling section is designed as microstrip quarter-wave three step transformer with dc-block between oscillator and detector. All refractory material integrated circuits have been fabricated on the base of high quality Nb-AlOx-Nb junctions. The oscillations in the ranges 75 – 260 GHz have been experimentally observed. The values of the generated power have been calculated from the amplitudes of the quasiparticle and Shapiro steps on the I-V curve of the detector junctions. The detected power levels higher than 0.1 μW have been obtained. The linewidth value of FFO not more than 1 MHz has been experimentally estimated.

Design of the flux-flow type Josephson oscillator inductively coupled to a stripline impedance transformer

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 Zo smaller than the characteristic impedance of the coupling section Z1, 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 Z0=0.6 Z1, 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.

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

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.

Numerical calculation of the height of velocity-matching step of flux-flow type Josephson oscillator

The height of the velocity-matching step (VM step) of a flux-flow type Josephson oscillator is studied systematically by numerical simulations. It is defined by the phase velocity of the propagating fluxons equaling that of the electromagnetic wave in the junction structure. The height of VM step (γm) is an important parameter for the practical application of such a device. The results presented on γm are important data in order to save computer time in the simulation of electromagnetic properties, such as the output voltage, at the velocity-matching condition. The detailed simulations were performed using the perturbed sine-Gordon equation (PSGE) with a uniformly distributed bias current γ. The PSGE was solved numerically by using an implicit finite-difference scheme. The distributions of electromagnetic fields in an overlap, long Josephson junction in the flux-flow state are mapped. The parameters influencing γm are discussed. We show that γm increases almost linearly with an external magnetic field βe or with the quasiparticle damping α, while it is less dependent on surface loss β. γm increases slowly with junction length above a minimum length, and it levels out at long lengths. We also present an approximate expression for γm which fits numerical results well.

Inductive coupling of a flux-flow type Josephson oscillator to a stripline

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.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Millimeterwave emission from Josephson oscillator through thin film junction electrode

An experiment on the coupling of the flux-flow type Josephson oscillator to SIS detectors by inductive coupling scheme has been carried out. Radiated powers from the Josephson oscillator were detected by an array of Josephson tunnel junctions fabricated on top of the upper electrode of the oscillator. The thickness of the common electrode was made thin enough to allow inductive coupling through the electrode. The maximum value of the obtained voltage amplitude of the emitted wave from the oscillator was 1.7mV, which is in reasonable agreement with the theoretical one. The present method can be an efficient coupling scheme between the Josephson oscillator and external circuits.

Effects of junction geometry on performance of flux-flow type Josephson oscillator

Theoretical and experimental studies have been made of the effects of junction geometry on the performance of the flux-flow type Josephson oscillator. It is shown theoretically that the height and the slope of the current steps appearing in dc I-V curves of the junction, which determine the available power and the radiation linewidth of the oscillator, respectively, are dependent strongly upon self fields in the junction. In order to study the effects of junction geometries on the self fields, computer simulation and experiments have been carries out for an in-line junction, an overlap junction and an overlap junction with a small projection on one side. The best performance is obtained for the overlap junction with a moderate projection on side as expected theoretically.