Ralph Neubecker

• Darmstadt University of Applied Sciences

Image processing systems are used in a constantly increasing number of industrial applications, for instance in testing for quality control purposes. For users, it is of particular importance to be able to evaluate the system capability for a specific task. The capability of measuring image processing systems can very well be estimated in the frame...

The inspection of optical elements for surface imperfections is mostly based on subjective evaluation by human operators. Automatic inspection systems (AIS) may introduce advantages in term of reliability, reproducibility and cycle time. The potential and limits of a camera-based, high-resolution AIS for scratches and digs are discussed. One import...

Zusammenfassung Klassifizierende Bildverarbeitungssysteme sind weit verbreitet, z. B. in der industriellen Qualitätskontrolle. Heute gibt es keine etablierten Methoden, um deren Fähigkeit quantitativ zu erfassen. Hier wird eine Systematik vorgestellt, die sich an grundlegenden Bildverarbeitungsfunktionalitäten orientiert und daraus Kenngrößen für d...

Zusammenfassung Klassifizierende Bildverarbeitungssysteme sind weit verbreitet, z. B. in der industriellen Qualitätskontrolle. Heute gibt es keine etablierten Methoden, um deren Fähigkeit quantitativ zu erfassen. Hier wird eine Systematik vorgestellt, die sich an grundlegenden Bildverarbeitungsfunktionalitäten orientiert und daraus Kenngrößen für d...

We present a measurement technique that we call ‘edge-light’, which allows a very sensitive detection of cracks in solar wafers and cells. For polycrystalline material, we found to be able to detect much smaller cracks than possible with conventional equipment. The technique is based on dark-field illumination with the illuminating light entering t...

The measurement technique presented in this paper, provides a very sensitive detection of cracks in polycrystalline solar wafers and cells, combined with photoluminescence or electroluminescence measurements. The crack detection is based on illuminating light, entering the wafer (or cell) from the edges and being kept inside the bulk by total inter...

Under feedback extended nonlinear optical systems spontaneously show a variety of periodic patterns and structures. Control gives new insight into these phenomena and it can open the way for potential application of nonlinear optical structures. We briefly review methods to control localized states in single feedback experiments. Application of a F...

The experimental synchronization of a spatiotemporal disorder was presented. A liquid crystal light valve (LCLV) was used as optical nonlinearity, which provided with self-defocusing, saturable Kerr-type nonlinearity. The synchronization was investigated for different degrees of complexity of the spontaneous structures, which were analyzed using cr...

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 cryst...

Spatial synchronization effects are found in a nonlinear optical single-feedback system. If perfect hexagonal patterns are injected, several synchronization regimes are observed. For the coupling of spatiotemporally disordered structures, also indication for synchronization is found.

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 t...

A novel scheme for all-optical image processing is suggested, which is based on spontaneous pattern-formation processes. The spatiotemporal instabilities, connected with these self-organization effects, appear in many nonlinear optical systems and mostly obstruct the intended application. We propose to utilize the particular features of pattern for...

We report on the addressing and control of the lateral positions of optical spatial solitary structures in a single feedback experiment with a saturable Kerr nonlinearity. Solitary structures allow a locally self-confined switching between a dark background and a bright state. This binary character can be of use to all-optically route information i...

Defects play an important role in a number of fields dealing with ordered structures. They are often described in terms of their topology, mutual interaction and their statistical characteristics. We demonstrate theoretically and experimentally the possibility of an active manipulation and removal of defects. We focus on the spontaneous formation o...

The experimental characterization of pattern forming bifurcations is difficult, since the unstable solutions are generally not accessible in experiments. In nonlinear optics, a novel control scheme allows one to select and stabilize generic patterns and thus to track these solutions in parameter space. This Fourier space scheme is applied to a sing...

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 unfo...

Interest in spatial solitary structures originates from their characteristics to enable a locally self-confined switching between a dark background state and a bright spot. We report on the control and addressing of solitary structures in a single feedback experiment with a liquid crystal based reflective optically addressable spatial light modulat...

A self-regulating method for the control of spontaneous instabilities of the transversal beam profile in nonlinear optical systems is experimentally realized. The control method is the high-dimensional analogue of classical negative feedback regulators. An all-optical implementation with its capabilities of parallel processing is essential for the...

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 orde...

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 d...

Summary form only given. Control techniques based on optical feedback of filtered Fourier components have been applied successfully to a wide range of experimental configurations in order to both manipulate stable and unstable spatial structures and to eliminate optical turbulence.

In a single-feedback setup with a Liquid Crystal Light Valve as optical nonlinearity, the formation of stationary hexagonal patterns can be observed. Under increase of pump power, the order of these regular patterns breaks down and a complex dynamics sets in. This transition is investigated for experimentally observed patterns in the self-focusing...

We show that a control technique, based on feedback filtered at a Fourier plane, can stabilize the spatio-temporally disordered output of a nonlinear optical system. We demonstrate this in an experiment with a LCLV feedback system and in a theoretical model. We stabilize the system and select square and roll patterns. The technique is non-invasive...

A generalization of autocorrelation functions is proposed, by which symmetry properties of spatial patterns can be measured. Such symmetry autocorrelation functions can in particular be useful for the quantitative characterization of the complexity of individual patterns obtained from experiments or numerical simulations. They allow a measurement o...

First Page of the Article

The use of liquid-crystal light valves (LCLV's) as nonlinear elements in diffractive optical systems with feedback leads to the formation of a variety of optical patterns. The spectrum of possible spatial instabilities is shown to be even richer when the LCLV's capability for polarization modulation is utilized and internal threshold and saturation...

We present a detailed analysis of transverse effects and pattern formation in bistable optical elements. The system we investigate consists of a Fabry-Perot resonator for the optical feedback element with a nematic liquid-crystal cell used as an optically nonlinear intracavity medium. On illumination with a cw-laser beam, the system causes the beam...

In a nonlinear optical single feedback system formation of symmetry broken patterns is observed. We make use of a liquid crystal light valve to experimentally simulate a self-defocusing Kerr-type nonlinearity. Beyond a certain threshold of the power of the input laser beam, bright spots appear and arrange themselves into a hexagonal formation. By f...

Optical patterns, forming in transversely extended nonlinear feedback systems, sometimes consist of ordered or disordered arrangements of many bright circular spots. We derive an approximate analytical description for a localized individual spot for the case of a diffusive Kerr nonlinearity. Each spot represents a Gaussian mode, which induces its o...

Presented are the first experimental results on transverse pattern formation with a liquid crystal light valve under optical feedback. The setup is reduced to the essential elements, retaining maximum symmetry. Formation of regular and perturbed hexagonal structures and optical turbulence is found. Analogy to an existing model, predicting such patt...

Optical phase singularities have been observed in a nonlinear Fabry-Pérot, with a nematic liquid crystal film serving as dispersive-nonlinear medium. The singularities appear as a concomitant phenomenon of low-complexity amplitude pattern formation. While creation and motion of the singularities is strongly related to the amplitude pattern, the lat...

Formation of transversal patterns and the appearance of phase singularities in nonlinear optical systems have attracted growingly interest in recent years [1, 2]. Up to now, most theoretical and experimental work has been performed for Maxwell–Bloch systems. However, it also turned out that these phenomena can be found in other nonlinear optical fe...

Structure formation, symmetry breaking, and optical vortices are typical spatial phenomena in NLO feedback systems, which have found growing interest in the last decade. The system we investigate consists of a Fabry–Perot resonator with planar mirrors spaced from 50 μm to some centimeters and an intracavity NLC film as an optically nonlinear medium...

We present some new experiments on self-diffraction in nematic liquid crystals and an extended static theoretical description of this effect. Our model gives some new insights and improvements to calculations carried out before. Using these calculations it was, e.g., possible to determine elastic constants in a pure optic way. For the static and dy...

Optical systems tend to evolve to more size and complexity, especially in the field of nonlinear optics. To obtain understanding and control of such systems, the best approach is to divide them into appropriate functional blocks and to investigate them separately. This leads to a system description in which each component is characterized by only a...