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Physical review. B, Condensed matter

Self-pumping effects and radiation linewidth of Josephson flux-flow oscillators

V. P. Koshelets, S. V. Shitov, А. В. Щукин, Lyudmila V. Filippenko, J. Mygind, Alexey Vladimirovich Ustinov

Published Sep 1, 1997• 100 citations
Publisher

Abstract

Flux-flow oscillators (FFO's) are being developed for integration with a SIS mixer for use in submillimeter wave receivers. The present work contains a detailed experimental study of the dc, microwave, and noise properties of ${\mathrm{N}\mathrm{b}\ensuremath{-}\mathrm{A}\mathrm{l}\mathrm{O}}_{x}\ensuremath{-}\mathrm{Nb}$ FFO's. A model based on the Josephson self-pumping effect is proposed for an explanation of the experimental current-voltage characteristics. A reliable technique based on harmonic mixing is used to determine the spectral linewidth of the radiation emitted by the integrated FFO's up to 600 GHz. Comprehensive measurements of the dependence of the linewidth on the dynamic resistance and the applied magnetic field have been performed. In the resonant regime a linewidth as small as 200 kHz is obtained at 450 GHz. The experimental data are compared with recent theoretical predictions.

DOI
10.1103/physrevb.56.5572

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Flux-flow oscillators (FFO's) are being developed for integration with a SIS mixer for use in submillimeter wave receivers. The present work contains a detailed experimental study of the dc, microwave, and noise properties of Nb\ensuremath−AlOx\ensuremath−Nb{\mathrm{N}\mathrm{b}\ensuremath{-}\mathrm{A}\mathrm{l}\mathrm{O}}_{x}\ensuremath{-}\mathrm{Nb}Nb\ensuremath−AlOx​\ensuremath−Nb FFO's. A model based on the Josephson self-pumping effect is proposed for an explanation of the experimental current-voltage characteristics. A reliable technique based on harmonic mixing is used to determine the spectral linewidth of the radiation emitted by the integrated FFO's up to 600 GHz. Comprehensive measurements of the dependence of the linewidth on the dynamic resistance and the applied magnetic field have been performed. In the resonant regime a linewidth as small as 200 kHz is obtained at 450 GHz. The experimental data are compared with recent theoretical predictions.
Presently Flux Flow Oscillators (FFOs) are being developed for integration with superconductor-insulator-superconductor (SIS) mixers intended for submillimeter wave integrated receivers. Here we report on a detailed study of the dc and microwave properties of Nb-AlO/sub x/-Nb FFOs. A model based on the Josephson self-pumping effect is proposed for explanation of the experimentally measured dc I-V curves (IVC). Extensive measurements of the spectral linewidth of the radiation emitted from a FFO have been performed up to 600 GHz using a new reliable technique based on harmonic mixing. The results are compared to recent theoretical predictions.
Presently the Josephson Flux Flow Oscillator (FFO) is the best on-chip integrated local oscillator in all-superconducting sub-millimeter wavelength receivers based on SIS mixers. Potential applications are spectral investigations in astronomy, atmospheric pollution, biophysics, etc. Theoretically, the linewidth of the emitted radiation from the FFO - as for all other Josephson oscillators - should be given by the well-known lumped junction expression containing only internal noise and quantities measured from the DC I-V characteristic. The FFO, however, differs from the other members of the Josephson oscillator family in needing also an external magnetic field bias. Several theoretical models have been proposed adding also external noise. Experimentally the linewidth may be fitted by a simple extension of the lumped junction expression including also the measured dynamic resistance of the magnetic bias current. We suggest a simple solution to this based on the assumption that the DC bias current e.g. due to geometrical asymmetries creates an additional contribution to the magnetic field in the junction.
A Josephson flux flow oscillator (FFO) is the most developed superconducting local oscillator for integration with an SIS mixer in a single-chip submm-wave receiver. Recently, using a new FFO design, a free-running linewidth /spl les/10 MHz has been measured in the frequency range up to 712 GHz, limited only by the gap frequency of Nb. This enabled us to phase lock the FFO in the frequency range 500-712 GHz where continuous frequency tuning is possible; resulting in an absolute FFO phase noise as low as -80 dBc at 707 GHz. Comprehensive measurements of the FFO radiation linewidth have been performed using an integrated SIS harmonic mixer. The influence of FFO parameters on radiation linewidth, particularly the effect of the differential resistances associated both with the bias current and the applied magnetic field has been studied in order to further optimize the FFO design. A new approach with a self-shielded FFO has been developed and experimentally tested.
The results of a detailed study of the microwave linewidth of Nb-AlO/sub x/-Nb flux flow oscillators (FFO) are presented. The dependence of the FFO linewidth on the junctions parameters has been measured by using an improved technique based on harmonic mixing in the frequency range 250-600 GHz. Experimental data are compared with theoretical estimates to evaluate the influence of the possible mechanisms responsible for the broadening of the FFO linewidth. The origins of the increased linewidth at the transition from the resonant to the "pure" flux-flow regime are discussed. The results of the linewidth measurements for the FFO locked to an external synthesizer via a wideband feedback loop are presented. The possibility of phase locking of the Josephson oscillator has been demonstrated experimentally. A FFO linewidth as low as 3.3 kHz (determined by the resolution bandwidth of the spectrum analyzer) has been measured at 310 GHz; it is far below the fundamental level given by shot and thermal noise of the free-running tunnel junction.
A reliable technique for wide band measurements of the spectral linewidth of superconducting oscillators integrated on-chip with superconductor-insulator-superconductor (SIS) detectors has been developed. The spectral linewidth of flux-flow oscillators (FFO) based on the unidirectional and viscous flow of magnetic vortices in a long overdamped Josephson tunnel junction was measured in the frequency range 250–580 GHz, and a linewidth as low as 200 kHz was obtained at 450 GHz. Also stable frequency locking of a FFO to very high (≤60th) harmonics of an external microwave reference source has been demonstrated. The proposed technique may improve the sensitivity, frequency resolution, and stability of the fully superconducting integrated submillimeter wave receiver.
Flux flow oscillators (FFO) have been experimentally investigated at frequencies up to 850 GHz. At 440 GHz the received power in an on-chip integrated SIS mixer is 5 /spl mu/W. The first experimental measurements of the FFO linewidth as a function of applied dc bias current and magnetic field have been performed both for two autonomous FFOs and with one of the FFOs injection-locked to a narrow-band external microwave source. By beating the two autonomous FFOs an integral spectral linewidth as low as 750 kHz is measured at 280 GHz. Mechanisms leading to a broadening of the FFO linewidth are discussed. Experimentally it is shown that the FFO can be used as harmonic generator.>