Nonlinear signal amplification in a 2D system operating in static and oscillatory regimes

We study the spectral response of a nonlinear system of coupled oscillator equations representing the overdamped two-junction (dc) superconducting quantum interference device (SQUID) loop; this system admits of static or oscillatory solutions for the autonomous case. In the presence of a weak time-dependent sinusoidal target signal and noise we find, in the regime of oscillatory or "running" solutions, an enhancement of the response (characterized by an output signal-to-noise ratio at the drive frequency) as a function of the intrinsic device parameters as well as externally controlled bias parameters that determine the nature of the long-time solutions. Modeling the device via a derived input-output transfer characteristic yields results in good agreement with recent experiments. This work offers a technique whereby the response of nonlinear devices with similar response characteristics may be optimized without directly adjusting the system noise. (C) 1999 Published by Elsevier Science B.V.