Spectral characteristics of noisy Josephson flux flow oscillator

TL;DR

This paper investigates the spectral and voltage characteristics of a Josephson flux flow oscillator, revealing how chaotic signals and linewidth behaviors depend on junction length and boundary conditions through numerical simulations.

Contribution

It demonstrates that the chaotic regime is caused by wave reflections rather than internal fluxon modes, and shows how DLS transforms into Fiske steps with different linewidths.

Findings

Chaotic signals have extremely large linewidths at small bias currents.

DLS transforms into narrow-linewidth Fiske steps with longer junctions or proper waveguide matching.

Chaotic regime is caused by wave reflections, not internal fluxon modes.

Abstract

The current-voltage and spectral characteristics of a flux flow oscillator (FFO), based on a long Josephson junction, are studied. The investigations are performed in the range of small bias currents and magnetic fields, where the FFO radiates a quasi-chaotic signal with extremely large radiation linewidth, and the displaced linear slope (DLS) is observed at the current-voltage characteristic. By direct numerical simulation of the sine-Gordon equation it is demonstrated that for large lengths of the Josephson junction or in the case of finite matching of the FFO with external waveguide system, the DLS with extremely large linewidth is transformed into Fiske steps with very narrow linewidth. While there is the common belief that the chaotic regime of the FFO is due to excitation of the internal oscillation modes in the "soft" fluxon chain, it is demonstrated that this regime is inspired by multiple reflections of the traveling waves from junction ends.