Biomedical subjects

R L Davidchack

Publications and source records attributed to R L Davidchack.

4 recordsLinked to original sources

PubMed2001

Regular dynamics of low-frequency fluctuations in external cavity semiconductor lasers.

It is commonly believed that the dynamics responsible for low-frequency fluctuations (LFF's) in external cavity semiconductor lasers is stochastic or chaotic. A common approach to address the origin of LFF's is to investigate the dynamical behavior of, and the interaction among, various external cavity modes in the Lang-Kobayashi (LK) paradigm. In this paper, we propose a framework for understanding of the LFFs based on a different set of fundamental solutions of the LK equations, which are periodic or quasiperiodic, and which are characterized by a sequence of time-locked pulses with slowly varying magnitude. We present numerical evidence and heuristic arguments, indicating that the dynamics of LFF's emerges as a result of quasiperiodic bifurcations from these solutions as the pumping current increases. Regular periodic solutions can actually be observed when (1) the feedback level is moderate, (2) pumping current is below solitary threshold, and (3) the linewidth enhancement factor is relatively large.

Journal Article↗

PubMed1999

Weighted-density approximation for general nonuniform fluid mixtures.

In order to construct a general density-functional theory for nonuniform fluid mixtures, we propose an extension to multicomponent systems of the weighted-density approximation of Curtin and Ashcroft [Phys. Rev. A 32, 2909 (1985)]. This extension corrects a deficiency in a similar extension proposed earlier by Denton and Ashcroft [Phys. Rev. A 42, 7312 (1990)], in that that functional cannot be applied to the multicomponent nonuniform fluid systems with spatially varying composition, such as solid-fluid interfaces. As a test of the accuracy of our functional, we apply it to the calculation of the freezing phase diagram of a binary hard-sphere fluid, and compare the results to simulation and the Denton-Ashcroft extension.

Journal Article↗

PubMed1999

Efficient algorithm for detecting unstable periodic orbits in chaotic systems.

We present an efficient method for fast, complete, and accurate detection of unstable periodic orbits in chaotic systems. Our method consists of an iterative scheme and an effective technique for selecting initial points. The iterative scheme is based on the semi-implicit Euler method, which has both fast and global convergence, and only a small number of initial points is sufficient to detect all unstable periodic orbits of a given period. The power of our method is illustrated by numerical examples of both two- and four-dimensional maps.

Journal Article↗

PubMed1999

Least squares acceleration filtering for the estimation of signal derivatives and sharpness at extrema.

A family of finite impulse-response (FIR) filters is derived which estimate the second derivative or "acceleration" of a digitized signal. The acceleration is obtained from parabolas that are continuously fit to the signal using a least squares optimization criterion. A closed-form solution for the filter coefficients is obtained. The general approach is computationally simple, can be performed in real-time, and is robust in the presence of noise. An important application of the method, that of measuring sharpness in biologic signals, is presented using the electroencephalogram (EEG) and electrocardiogram (EKG) signals as examples. Furthermore, the design method is extended to derive FIR filters for estimating derivatives of arbitrary order in digital signals of biologic or other origins.

Algorithms↗