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Biomedical subjects

R Neubecker

Publications and source records attributed to R Neubecker.

8 recordsLinked to original sources
PubMed2004

Experimental synchronization of spatiotemporal disorder.

We report experimental evidence of a time-lag synchronization of spatiotemporal complexity. The experiments were performed on two unidirectionally coupled, nonlinear-optical systems of single-feedback type. Synchronization was investigated for different degrees of complexity of the spontaneous structures, which were analyzed with cross-correlation functions and mutual information. Numerical simulations yield comparable results and throw a light on the impeding role of spatial inhomogeneities.

Journal Article↗
PubMed2003

Manipulation and removal of defects in spontaneous optical patterns.

Defects play an important role in a number of fields dealing with ordered structures. We demonstrate theoretically and experimentally the possibility of an active manipulation of defects in terms of an externally induced motion. We focus on the spontaneous formation of two-dimensional spatial structures in a nonlinear-optical system, a liquid crystal light valve under single optical feedback. For a particular parameter setting, a spontaneously formed hexagonal intensity pattern contains several dislocation-type defects. A scheme based on Fourier filtering allows us to restore spatial order in a selectable part of the pattern. Starting without control, the controlled area is progressively expanded, such that defects are swept out of the pattern.

Journal Article↗
PubMed2003

Spatial synchronization of regular optical patterns.

We investigate an extended, nonlinear optical experiment, exhibiting the spontaneous formation of hexagonal patterns out of a stationary bifurcation. The system is exposed to a two-dimensional spatially periodic forcing, namely, static hexagonal patterns, under variation of their spatial periodicity. Parameters are the strength of the forcing and the distance to pattern forming threshold. The system response is quantitatively characterized with different methods. We observe several locking regimes, where the system is entrained by the forcing. Most of the locking regimes can be related to resonances between the different critical wave numbers and the forcing wave number or its spatial harmonics. One particular locking appears to result from two of these simple resonances in a kind of generalized order m:n synchronization. The width of the locking regimes increases with forcing strength, apparently representing a spatial analog of Arnold tongues.

Journal Article↗
PubMed2002

Spatial forcing of spontaneous optical patterns.

A nonlinear optical system, which spontaneously forms hexagonal patterns, is exposed to a weak, spatially modulated forcing. As forcing, stationary hexagonal patterns are used under variation of their transverse wave number. In the experiment, we observe a locking when the forcing wave number is close to one of the critical wave numbers of the unforced system. Outside the locking regimes, forcing provokes spatiotemporal disorder. The system response is characterized quantitatively with respect to its dynamics and to its spatial order.

Journal Article↗
PubMed2000

Experimental control of unstable patterns and elimination of spatiotemporal disorder in nonlinear optics

A method based on Fourier filtered control signals for stabilization of unstable patterns in a nonlinear optical single feedback system is experimentally realized. The successful stabilization of the homogeneous solution and different stationary periodic patterns far above the pattern forming threshold as well as the elimination of spatiotemporal disorder above a secondary instability has been achieved. The temporal evolution of the control signal verifies the underlying philosophy of noninvasive control that changes only the linear stability of the systems' solution and not the solution itself.

Journal Article↗
PubMed2000

Voronoi analysis of the breakdown of order in spontaneous optical spot patterns

In a nonlinear optical single feedback system, patterns of bright spots evolve spontaneously. Making use of the Voronoi construction, the breakdown of spatial order in experimental spot patterns under increase of the pump laser intensity is analyzed. The spot density is found to increase with the pump intensity. Our analysis shows that maximum order is maintained for a finite range of spot densities and starts to break down at a clearly detectable threshold value. The enhanced disorder with increasing density contrasts the behavior of simple hard-core systems. A separate analysis of the metrical properties of the Voronoi cells in ordered and disordered regions yields remarkably smaller nearest-neighbor distances in disordered regions. This suggests that the creation of new spots is a governing mechanism for the observed breakdown of order.

Journal Article↗
PubMed1977

Placenta accreta, increta, and percreta. A survey of 40 cases.

Forty patients with placenta accreta, increta, or percreta are presented. Clinical features revealed an average age of 29.5 years and an average parity of 3-2-1. Twenty-five had no antepartum complications. Nine were admitted with silent hemorrhage, of which 6 had a total placenta previa and 1 a low-lying previa. Postpartum hemorrhage occurred in 39% with an associated perinatal mortality of 25% and 1 maternal death. Histopathologic evaluations revealed the predominant factor to be an absent decidua. Etiologic in decidual deficiency was a previous cesarean section (12 patients). Therapy consisted of total abdominal hysterectomy in 38 patients.

Adolescent↗