L. M. Pecora

Using multiple attractor chaotic systems for communication

In recent work with symmetric chaotic systems, we synchronized two such systems with one-way driving. The drive system had two possible attractors, but the response system always synchronized with the drive system. In this work, we show how we may combine two attractor chaotic systems with a multiplexing technique first developed by Tsimring and Suschick to make a simple communications system. We note that our response system is never synchronized to our drive system (not even in a generalized sense), but we are still able to transmit information. We characterize the performance of the communications system when noise is added to the transmitted signal.

Chaos and chaotic transients in yttrium iron garnet (invited)

Long‐lived chaotic transients are a prominent feature of the spin‐wave behavior of spheres of yttrium iron garnet (YIG) being perpendicularly pumped in the region of the first‐order Suhl instability. These transients may appear after a sudden increase in rf pumping power or during transitions between quasiperiodic auto‐oscillations. The transients, which result from the collision of a chaotic attractor with the basins of attraction of multiple stable quasiperiodic attractors, vary in lifetime by more than six orders of magnitude, from milliseconds to hours, as a function of the rf driving field. The average lifetimes of these transients fit an extended Grebogi–Ott–Yorke scaling law. Roughening the surface of the YIG sphere drastically changes the behavior of these transients.

Dual driving of magnetostatic modes in yttrium–iron–garnet film experiments

We study experimentally the behavior of magnetostatic wave modes in a rectangular yttrium–iron–garnet film placed in an in‐plane magnetic field and subject to multiple drive excitations. Patterned slotlines and coplanar waveguides are used to drive the film with microwave (2–4 GHz) excitations corresponding to magnetostatic wave modes. For a single drive, we observe linear transmission at low drive powers and saturation above the Suhl instability. For sufficiently large powers above the instability, the transmitted power through the film displays periodic low‐frequency (kHz) auto‐oscillations. A second microwave drive applied to the sample is used to excite additional spin‐wave modes that interact with those arising from the original excitation. Such dual‐drive experiments can probe the nature of spin‐wave mode interactions at high amplitudes.

Dynamic control of instability thresholds in yttrium-iron-garnet

Yttrium-iron-garnet (YIG) is an important material used in microwave limiters and other devices. In this work, we use dual microwave (1–4 GHz) drives to study the behavior of magnetostatic and spin-wave modes in YIG spheres and rectangular films. The samples are placed in a dc magnetic field and driven by cw and pulse-modulated microwave excitations at magnetostatic mode frequencies. A second microwave drive applied to the sample excites additional spin-wave modes that can interact with those arising from the original excitation and thereby affect the transmission characteristics at the primary frequency. For both films and spheres, we observe a significant decrease in transmission of the primary when the secondary frequency is tuned to approximately half that of the primary drive. This decrease is observed both in the steady state behavior and in the initial overshoot transient associated with pulse modulation of the primary excitation.

Magnetostatic modes and chaos in yttrium iron garnet films

Nonlinear interactions in yttrium iron garnet cause complex, low‐frequency signals seen during ferromagnetic‐resonance experiments. Several methods from nonlinear dynamics have been used to measure quantities such as dimensions, Lyapunov exponents, and others, to characterize complex behavior that is measured by ferromagnetic resonance experiments. It is found that these quantities may be more useful in understanding the interactions in yttrium iron garnet than more commonly measured quantities such as oscillation frequency. Very simple numerical models are also used to suggest which quantities are most important in characterizing these spin‐wave interactions.

Dynamics of transients in yttrium-iron-garnet.

Yttrium-iron-garnet (YIG) is an important technological material used in microwave devices. In this paper we use dual microwave (1-4 GHz) drives to study the dynamical bifurcation behavior of magnetostatic and spin-wave modes in YIG spheres and rectangular films. The samples are placed in a dc magnetic field and driven by cw and pulse-modulated microwave excitations at magnetostatic mode frequencies. A second microwave drive applied to the sample excites additional spin-wave modes that can interact with those arising from the original excitation and thereby affect the transmission characteristics at the primary frequency. We find a significant decrease in transmission of the primary when the secondary frequency is tuned to approximately half that of the primary drive. This decrease is observed both in the steady state behavior and in the initial overshoot transient associated with pulse modulation of the primary excitation. Results such as these are often treated by extending linear theory to include higher order interaction terms. Herein we present a simple dynamical model that reproduces results that qualitatively resemble the experimental data. (c) 1997 American Institute of Physics.

Inferences About Coupling from Ecological Surveillance Monitoring: Approaches Based on Nonlinear Dynamics and Information Theory

Some monitoring programs for ecological resources are developed as components of larger science or management programs and are thus guided by a priori hypotheses. More commonly, ecological monitoring programs are initiated for the purpose of surveillance with no a priori hypotheses in mind. No conceptual framework currently exists to guide the development of surveillance monitoring programs, resulting in substantial debate about program design. We view surveillance monitoring programs as providing information about system dynamics and focus on methods for extracting such information from time series of monitoring data. We briefly describe methods from the general field of nonlinear dynamics that we believe may be useful in extracting information about system dynamics. In looking at the system as a network of locations or components, we emphasize methods for assessing coupling between system components for use in understanding system dynamics and interactions and in detecting changes in system dynamics. More specifically, these methods hold promise for such ecological problems as identifying indicator species, developing informative spatial monitoring designs, detecting ecosystem change and damage, and investigating such topics as population synchrony, species interactions, and environmental drivers. We believe that these ideas and methods provide a useful conceptual framework for surveillance monitoring and can be used with model systems to draw inferences about the design of surveillance monitoring programs. In addition, some of the current methods should be useful with some actual ecological monitoring data, and methodological extensions and modifications should increase the applicability of these approaches to additional sources of actual ecological data.

Characterizing chaos in magnetostatic modes

The nonlinear behavior seen in ferromagnetic resonance experiments in yttrium iron garnet (YIG) has long proven difficult to model. Time series analysis techniques from nonlinear dynamics have been applied to characterize chaos seen in an experiment involving an YIG film. It is found that this characterization reveals possible shortcomings in some of the common models.

High-performance fiber optic systems for damage detection and structural health monitoring

This paper describes two systems that can monitor up to 64 fiber Bragg grating (FBG) strain gauges simultaneously and their use in structural health monitoring applications. One system directly tracks wavelength shifts and provides ~0.3 me sensitivity with data rates to 360 Hz. The second system uses an unbalanced Mach-Zehnder interferometer to convert wavelength to phase. It has a noise floor of ~5 ne/Hz1/2 and data rates to 10 kHz. The wavelength-based system was used in field tests on an all composite hull surface effects ship in the North Sea and on an Interstate highway bridge in New Mexico. The interferometric system has been used to demonstrate enhanced damage detection sensitivity in a series of laboratory experiments that rely on a novel data analysis approach based in nonlinear dynamics and state space analysis. The sensitivity of three of these novel damage detection methods is described.

Chaos starts to communicate

A laser is a device that exemplifies the notions of stability, coherence and purity. Yet it is now well known that the behaviour of a laser can easily become unstable and chaotic. While most research has been devoted to eliminating such behaviour and establishing lasers as uncontaminated light sources, the 1990s have seen increasing interest in the idea of using chaotic laser signals in communications.