Biomedical subjects

Parvez N Guzdar

Publications and source records attributed to Parvez N Guzdar.

5 recordsLinked to original sources

PubMed2006

Observation of chaotic itinerancy in the light and carrier dynamics of a semiconductor laser with optical feedback.

We report a direct experimental observation of chaotic itinerancy in simultaneous measurements of the light intensity and voltage fluctuations of a laser diode exhibiting low-frequency fluctuations. The distribution of trajectories leading up to (following) an intensity dropout is computed from the experiment and reveals the presence of itinerant mechanisms before (after) dropout initiation. A phase space reconstruction of the trajectory for the optimal path of motion illustrates sudden shifts between low-dimensional attractor ruins and is shown to correspond to simulations of the laser intensity and carrier number.

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PubMed2005

Intermittency in two-dimensional turbulence with drag.

We consider the enstrophy cascade in forced two-dimensional turbulence with a linear drag force. In the presence of linear drag, the energy wave-number spectrum drops with a power law faster than in the case without drag, and the vorticity field becomes intermittent, as shown by the anomalous scaling of the vorticity structure functions. Using previous theory, we compare numerical simulation results with predictions for the power law exponent of the energy wave-number spectrum and the scaling exponents of the vorticity structure functions zeta(2q). We also study, both by numerical experiment and theoretical analysis, the multifractal structure of the viscous enstrophy dissipation in terms of its Rényi dimension spectrum D(q). We derive a relation between D(q) and zeta(2q), and discuss its relevance to a version of the refined similarity hypothesis. In addition, we obtain and compare theoretically and numerically derived results for the dependence on separation r of the probability distribution of delta(r)omega, the difference between the vorticity at two points separated by a distance r. Our numerical simulations are done on a 4096 x 4096 grid.

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PubMed2005

Measurement of Hurst exponents for semiconductor laser phase dynamics.

The phase dynamics of a semiconductor laser with optical feedback is studied by construction of the Hilbert phase from its experimentally measured intensity time series. The Hurst exponent is evaluated for the phase fluctuations and grows from 0.5 to approximately 0.7 (indicating fractional Brownian motion) as the feedback strength is increased. A comparison with numerical computations based on a delay-differential equation model shows excellent agreement and reveals the relative roles of spontaneous emission noise and deterministic dynamics for different feedback strengths.

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PubMed2003

Hilbert phase analysis of the dynamics of a semiconductor laser with optical feedback.

The nonlinear dynamics of a semiconductor laser with an optical feedback is studied using Hilbert phase analysis, which reveals interesting facets of the nonlinear dynamical behavior, including the formation and interaction of external cavity modes with increasing external feedback. Here we report measurements on two illustrative cases with very different dynamics; the laser is first biased just near threshold, and then far above threshold. We observe 2pi phase jumps at intervals that are multiples of the external cavity round-trip time, indicating the interaction and transfer of energy between external cavity modes.

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PubMed2002

Influence of stochasticity on multiple four-wave-mixing processes in an optical fiber.

In this paper, we present the results of a computational study of the dynamical evolution of multiple four-wave-mixing processes in a single mode optical fiber with spatially and temporally delta-correlated phase noise. A generalized nonlinear Schrödinger equation (NLSE) with stochastic phase fluctuations along the length of the fiber is solved using the split-step Fourier method. A good agreement is obtained with previous experimental and computational results based on a truncated-ODE (ordinary differential equation) model, in which stochasticity was seen to play a key role in determining the nature of the dynamics. The full NLSE allows for simulations with high frequency resolution (60 MHz) and frequency span (16 THz) compared to the truncated-ODE model (300 GHz and 2.8 THz, respectively), thus enabling a more detailed comparison with observations. A physical basis for this hitherto phenomenological phase noise is discussed and quantified.

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